A2 Biology: The endocrine system

Endocrine uses hormones as its signalling molecules.

Hormones: are molecules that are released by endocrine glands directly into the blood. They act as messengers, carrying a signal from the endocrine gland to a specific target organ or tissue.

• Blood circulation is used to transport hormones
• Transported throughout the body
• Endocrine glands are ductless and release hormones straight into the capillaries running through them

ENDOCRINE gland: is a gland that secretes hormones directly into the blood. They have no ducts.

EXOCRINE gland: is a gland that secretes molecule into a duct that carries the molecules to where they are used e.g. salivary glands.

Adrenaline

• Adrenaline is an amino acid derivative, they cannot enter the target cell. Adrenaline receptor on outside of the plasma membrane ha a shape complimentary to the shape of the adrenaline molecule

Adenyl cyclase

• Adenyl cyclase is an enzyme associated with receptor for many hormones, including adrenaline. It is found on the inside of the plasma membrane.
• Adrenaline receptor is associated with this enzyme.

Action of adrenaline

• Adrenaline in blood binds to a specific receptor
• Adrenaline is first messenger
• Activate adenylcyclase enzyme which converts ATP to cyclic AMP
• cAMP is second messenger inside the cell
• cAMP causes effect inside cell by activating enzymes

Adrenal medulla

• Cells make and release adrenaline hormone in response to stress
• Most cells have adrenaline receptors
• Effect is to prepare for activity

Effects of adrenaline

• Relax smooth muscles in  bronchioles
• Increase stroke volume of heart and increase heart rate
• Causes vasoconstriction to raise blood pressure
• Stimulate glycogen to convert to glucose 
• Dilate pupils
• Increase mental awareness
• Inhibit action of gut and cause body hair to react

Target cells must have specific receptors as hormones binds to receptor and activates a process inside the cell receptors must be specific so the hormone only bind to the correct cells, receptors and hormones have shapes complementary to each other.

A2 Biology: KIDENYS

The kidney functions:

• blood filtration
• selective reabsorption via active transport and passive absorption

• human kidneys are about 12 cm long 7 cm wide
• They are covered by a layer of fat and are part of the urinary system

• Blood enters a kidney through the renal artery and leaves through the renal vein.
• Excretory products are removed from the blood and are collected in the form of urine.
• Urine collects in the central part of the kidney called the pelvis.
• Urine passes from each kidney to the bladder along the ureter tube.
• The outer darker region is called the cortex and the inner and lighter region is called the medulla.

The Nephron

• The nephron is the functional unit of the kidney
• It makes up the bulk of its structure
• There are about 1 million in each kidney
• At one end of each nephron is a cup shaped Bowman's capsule (in the cortex)
• This encloses a dense network of capillaries called the glomerulus
• This capsule leads into a tubule, first part coiled aka the proximal convoluted tubule
• Then it leads to a U shape loop of Henle
• This leads to another coiled section called the distal convoluted tubule
• These join to forma  collecting duct and many of these lead though the medulla and coverage of the renal pelvis where they empty into the ureter which takes urine to the bladder

Function of the nephron

• The kidney works by the processes of ultrafiltration and reabsorption
• The fluid parts of the blood are filtered into the capsular space and the resulting fluid flows along the tubules
• As it does so useful substance are reabsorbed back into the bloodstream

Ultrafiltration 

• Blood is brought to each glomerulus by an afferent arteriole and it leaves via the efferent arteriole. The afferent is wider in diameter than the efferent which results in a relatively high hydrostatic pressure of blood in the glomerular capillaries. This pressure tends to force the fluid part of blood into the Bowman's capsule lumen.

Barrier

• the barrier between the blood in the capillar and the lumen of the Bowman's capsule consists of three layers:

• Endothelium of the capillary there are pored between the calls hat plasma and dissolved  molecules can pass through.

•  Basement membrane; this is a fine mesh of collagen fibres and glycoproteins. These act as a filter preventing any molecule with a mass greater than 69 000 from passing through. This mean most plasma proteins and blood cells are held in the capillaries. 

•  Podocytes - have specialised shape. They have finger like projections called major and minor processes. These ensure gaps between the cell that fluid can pass through into the Bowman's capsule

Blood contains: Digested food, white blood cells, urea, platelets, hormones, plasma proteins, carbon dioxide, oxygen and red blood cells.
Plasma contains; Carbon dioxide, glucose, amino acids, proteins, minerals, etc.

Selective reabsorption

• As the filtrate flow along the tubules, its composition is altered.
• Reabsorption occurs in both the proximal and distal convoluted tubules.
• Water is also reabsorbed form the collecting ducts.
• Most reabsorption occurs in the proximal convoluted tubule. (85%)

Adaptions for efficient reabsorption

• Epithelial cells of the proximal convoluted tubule have a large surface area due tot he presence of the microvilli on both inner and outer surfaces.
• They also have many mitochondria which can supply energy for active absorption.
• The inner membrane contains special co-transporter proteins that transport glucose or amino acids in association with sodium ions, from the tubule into the cell. This is facilitated diffusion.
• The outer membrane contains sodium-potassium pumps that pump sodium ions out of the cell and potassium ions into the cell.

1. This sodium potassium pumps remove sodium ions from the cells lining in the proximal convoluted tubule.
2. This reduces the concentration of sodium ions in the cell cytoplasm.
3. Sodium ions enter the cells long with glucose or amino acids by facilitated diffusion.
4. As the concentration of glucose an amino acids rise inside the cell, they diffuse out of the cell into the tissue fluid
5. from the tissue fluid they diffuse into the blood and are transported away
6. the reabsorption of sodium, glucose and amino acids reduced the water potential of the cells and increase the water potential of the filtrate
7. this means water will enter the cells and be reabsorbed into the blood by osmosis.

The loop of Henle

• Role of loop of Henle is to produce a low water potential in the tissue of the medulla.
• This will ensure that even more water can be reabsorbed from the fluid in the collecting duct

Ascending Limb

• at base of ascending limb, sodium and chloride ions diffuse out into the tissue fluid. 
• Further up, the ascending limb of the loop of Henle pumps out sodium and chloride ions by active transport.
• This movement makes the tissue fluid surround the Loop of Henle more concentrated 
• Water does not move out of these ions because the wall of the ascending limb is quite thick and is impermeable to water.

The descending limb

• The descending limb is permeable to water and solutes. As the filtrate passes down the descending limb water moved out by osmosis.
• Sodium and chloride ions move in by diffusion.
• The fluid within the descending limb therefore becomes more and more concentrated as it flows towards the bottom of the loop,

A2 Biology: Nervous system + coordination

• The ability of living things to respond to changed in the environment is known as irritability or sensitivity. 
• A change in energy levels in the environment which causes a response by an organsim is called a stimulus.

Receptors - detects the stimulus. E.g. eyes, ears and muscle stretch receptors.

Effectors - brings about the response. E.g. muscles and glands.

Coordinating system - to link together the receptors and effectors

The central nervous system (CNS) - made up of brain and spinal cord.

The peripheral nerves - Made up of the nerves which link the CNS to the receptor and effectors.

Receptors - specialised cells or organs they detect change in our surrounding. They are energy transducers that convert from one form of energy into another. Each is adapted to detect changes in a particular form of energy.

Receptor cells

• These trigger action potentials
• They convert energy from one form into electrical impulses in a neurone

Nerve cells

• The nervous system is made up of millions of nerve cells.
• The entire nervous system consists of two main types: neurones and neuroglia.

Neurones + Neuroglia

• Neurones are cells which are adapted to carry nerve impulses
• Neurones show considerable variation in size and shape but have the same basic structure
• Neuroglia which include Schwann cells, are cells which provide structural and metabolic support to neurones. 

Each neurones consists of:

• a cell body - containing the nucleus surrounded by granular cytoplasm
• cytoplasmic processes which branch from the cell body - a sing axon and one or more dendrites.
• The axon conducts impulses away from the cell body either to other neurones or to effectors, such as muscles.
• dendrites are highly branched processes which carry impulses from specialised receptors or from adjacent neurones with which they form synapses

Synaptic knobs

• The axon ends in synaptic knobs, these are where the neurone meets other nerve
• Large numbers of mitochondria and vesicles containing transmitter substances

Types of Neurones

• Motor neurones carry an action potential from the CNS to an effector such as a muscle or gland.
• sensory neurones carry the action potential from a sensory receptor to the CNS.
• relay neurons connect sensory neurones and motor neurones.

Myelinated neurones

• In the mammalian peripheral nervous system, specialised Schwann cells surround most axons. this process encloses the axon in a spiral layer of Schwann cell membrane. The overlapping pushes the nucleus and cytoplasm to the outside layer
• The covering formed by Schwann cell i referred to as the myelin sheath and axons which are covered in this way are myelinated.
• Between adjacent Schwann cells, there are short gaps where the axon is not covered by myelin. These gaps are known as the nodes of Ranvier and are at intervals of 1-3 mm along the axon.
• Around one third of neurones of the peripheral nervous system are myelinated.
• Neurones have specialised channel proteins in their cell surface membranes. They are specific to either Na+ or K+. They also possess a gate that can open or close the channel.

• They are known as voltage gated channel proteins. when open, the permeability of that membrane to the specific ion is increased; when closed the permeability is reduced. The channels are usually kept closed.

•  The myelin sheath is an insulating layer of fatty material. Sodium and potassium ions cannot diffuse through this layer.

• The larger the diameter of the axon the faster the speed it can conduct.
• In vertebrates the myelin sheath surrounding the axon increases the transmission speed
• when an action potential arrives at one node of Ranvier it sets up an electric current between this node and the next node.
• Nodes of Ranvier: The small uncovered areas of the axon between the Schwann cells Impulses jump from node to node.

Saltatory conduction

• Action potential appears to jump from one node to the next.
• This type of conduction is called salutatory conduction and results in the speeding up f the transmission of impulses They carry signals over long distances, the longest neurone in the human body is about 1m in length. 

Sodium Potassium pump

• They also have carrier proteins that  are able to actively transport Na+ out of the cell and K+ into the cell. For every 3 Na+ that goes out 2K+ goes in.
• This is called Na-K pump and it requires ATP.
• These maintain the potential difference cross the membrane, the membrane is polarised. 

Polarised

• A polarised membrane is one that has a potential difference across it
• It is the resting potential when there is no impulse

A nerve impulse

• A nerve impulse is created by altering the permeability of the nerve cell membrane to these ions.
• The movement of ions creates a change in potential difference (charge_ across the membrane. This is called depolarisation.

Synapses

• There are small gaps bettween neurones when they meet (approx 20 nm)
• These gaps are called synaptic clefts
• The gaps of the two ends of the neurones near it make up a synapes
• The signals is passed on by a chemical
• Action potential arriving causes release of a transmitter sbstance into the cleft
• Transmitter substace diffuses across the cleft
• Set up action potential in the membrane of the second neurone

1. An action potential arrives
2. The membrane depolarises. Ca2+ channels open and Ca2+ enter the neurones
3. Ca2+ causes synaptic vesicles containing neurotransmitter to fuse with pre synaptic membrane
4. Neurotransmitter i released into the synaptic cleft
5. Neurotransmitter binds with receptors on the postsynaptic membrane. Cation channels open. Na+ flow through the channels.
6. The membrane depolarises and initiated an action potential
7. When released the neurotransmitter will be taken up across the pre synaptic membrane or it can diffuse away and be broke down.

• action potential reaches the synaptic knob
• voltage gated calcium ion channels in its membrane open to allow calcium ions diffuse into the knob
• Calcium ions cause synaptic vesicles, which contain a neurtransmitter to move to and fuse with presynaptic membrane
• Acetylcholin molecules are released by excocytosis and diffuse across the synaptic cleft
• —Acetylcholine molecules (have a specific shape to the receptor molecules) bind to receptor sites onthe sodium ion channels in the post-synaptic membrane, causing them to open.—
• Sodium ions diffuse in through the open channels in the post-synaptic membrane creating a generator potential or excitatory postsynaptic potential 
• —This depolarises the membrane. If sufficient generator potentials combine, then the potential across the post-synaptic membrane reaches the threshold potential and a new action potential is created in the post-synaptic cell.

Action potential

• When a neurone is not conducting an impulse it is said to be in the resting state
• The inside of the axon membrane has a negative charge relative to the outside
• This potential difference is called the resting potential and the membrane is polarised

Resting potential

• Na - positively charged Na+ are about ten times more concentrated on the outside of the membrane than they are inside of it.
• This is due to the activity of a sodium pump.
• Three sodium ions move for every two potassium ions that move in

K+

• K+ become more concentrated on the inside of the membrane
• but because the membrane is more permeable to K+ than Na+ some may move back out
• these are actively pumped back into the axon
• those that remain do so because they will be attracted back in the overall negative charge inside.

• So the differences in the concentrations of ions are due to the permeability of the axon membrane to these ions.
• in the resting state the permeability of the membrane to potassium is relatively high due to the presence of protein channels or gates in the membrane which allow K+ to pass through
• however there are no gates which allow the negatively charged organic ions to pass through so these remain trapped on the inside

• As a result the interior of the cell is maintained at a negative potential compared to the outside
• The membrane is said to be polarised and the potential difference across the cell surface membrane is about -60 mV (resting potential)

1. Resting potential: Na+ and K+ special voltage gated ion channels are closed and the  neurone is polarised  = negative inside
2. Some Na+ channels open (due to change in shape of protein itself makes protein more permeable to Na+) which allows some Na+ to diffuse into the neurone down the diffusion gradient.
3. Causing the inside of the neurone to become less negative: the membrane depolarises it reaches a threshold value of -50mV
4. Voltage gated sodium channels open and many sodium move in. The cell becomes positively charge inside compared to outside. Potential difference across the membrane is now +40 mV
5. Na+ channels now close and K+ gated channels now open
6. K+ diffuse out of the neurone down the electrochemical gradient so making the inside of the neurone less positive (more negative) again. The neurone is repolarised
7. So many K+ leaves axon that potential difference becomes even more negative than normal resting potential = hyperpolarised
8. the original potential differences is restored and the cell returns to its resting state

All or Nothing

Generator potentials in the sensory receptor are depolarisations of the cell membrane. A small polarisation will have no affect on the voltage gated channels unless the depolarisation is large enough to reach threshold potential it will open some nearby by voltage gated channels. This causes a large influx of Na+ and depolarisation reaches +40mV which is an action potential. Once this value is reached the neurone will transmit an action potential because many voltage gated sodium ions channels open. The action potential is self perpetuating once it starts are one point in a neurone it will continue along to the end of the neurone. This action potential does not vary in size nor intensity.

*Threshold point

1. Stimulus
2. Depolarisation
3. Action potential
4. Repolarisation
5. Refractory period - After an action potential the Na and K are in the wrong places. The Na+ and K+ ions which have diffused in/out of the cell are moved by actve transport (sodium-potassium pump) Until the Na+ and K+ are in their original positions no new action potential can be sent.
6. Resting potential

Voltage gated ion channels

• The gates on the channels further along the neurone membrane are operated y changes in the voltage across the membrane
• The movement of sodium ions along the neurone alters the potential difference cross the membrane
• When the potential difference cross the membrane is reduce the gates open
• This allows sodium ions to enter the neurone at a point further along the membrane, the action potential ha moved along the neurone.

A2 Biology: Photosynthesis

• Takes place in green plants
• Light energy from te sun is trapped and concerted into a cheicl energy which can be storeed in molecules of carbohydrate.
• 6CO2 + 6H2O --light energy--> C6H12O6 + 6O2
• The leaf is the main photosynthetic organ.
• The chloroplast is the organelle where photosynthesis is carried out.

Chloroplast features

• Chloroplast envelope - double membrane permeable to glucose, CO2, O2, and some ions
• Ribosome
• Circular DNA
• Thylakoid membranes (stack = graum) - contains chlorophyll for photosynthesis
• Starch grain - insoluble carbohydrate storage product of photosynthesis
• Stroma - matrix of chloroplast
• Lipid droplet - energy store made from sugars during this process

Chloroplast structure

• Outermembrane is permeable to small ions, the inner is less permeable.
• Transport proteins allow movement across.
• Inside the chloroplast is a system of membranes running through a colourless, structureless substance - matrix aka stroma.

Stroma

• gel-like substance which contains enzymes for photosynthesis.
• Site of the light independent stage of photosynthesis (Calvin cycle)
• Other structure found in the stroma of chloroplasts are starch grains, DNA, lipid droplets and ribosomes.

Thylakoids

• membrane system is the site of the light dependant stage of photosynthesis.
• membrane system consists of any flattened fluid filled sacs called thylakoids.
• forms stacks called grana at intervals
• membrane provides a large surface area covered with photosynthetic pigments, enzymes and electron carriers.
• The grana are interconnected with intergranal thylakoids/lamellae

Photosynthetic pigments

• photosynthetic pigments absorb light of certain wavelengths and reflects others.
• They are found in funnel shaped structure called photosystems which harvest lights and are situated in the thylakoid membranes.
• These include chlorophylls and the carotenoids, carotene and xanthophyll. 

Chlorophyll

• Two types: chlorophyll a (P680) and chlorophyll a (P700)
• Both absorb red light but at slightly different wavelengths and appear yellow/green.
• Found at the centre of the photosystems and are known as primary pigment reaction centre.

Chlorophyll and photosystems

• Chlorophyll a (p700) is found in photosystem I and has a peak absorption of 700 nm
• Chlorophyll a (P680) is found in photosystem II and has a peak absorption of 680 nm.

Breakdown of photosynthesis.

Two main reactions

1. Light Dependant Reaction - Produces energy from the photons (solar power) in form of ATP and NADPH - takes place in thylakoid membrane

2. Light Independent Reaction/Calvin Cycle/Carbon Fixation - Uses energy ATP and NADPH from light reaction to make glucose. - takes place in stroma]

LIGHT DEPENDENT STAGE

3 major events:

• Photolysis of water
• Light harvesting
• photophosphorylation

Photolysis of Water

• In photosystem II there is an enzyme in presents of light that can split water into protons, electrons and oxygen. This is called photolysis.
• H2O   2e- + 2H+  + 1/2O2
• Oxygen is lost through the stomata into the air
• Electrons replaces those lost by the chlorophyll during energy tansduction
• Protons are used in chemiosmosis to produce ATP and reduce the coenzyme NADP (both needed for light independent stage)

Light harvesting

• 2 distinct light harvesting systems: PS.I and PS.II
• This is where all the light energy is harvested by the chlorophylls is transferred to the few chlorophyll molecules t the centre of the two reaction centres.
• These are known as primary pigment reaction centre molecule and absorb wavelength of 700 nm in PSI and 68 in PS.II

PHOTOSYSTEM I

Cyclic phosphorylation

• Excited electrons pass to an electron acceptor and back to the chlorophyll molecule from which they were lost.
• No photolysis of water and no generation of reduced NADP.
• Small amounts of ATP made by chemiosmosis.
• This may be used in the light dependant reaction or y the guard cells to open the stomata.

PHOTOSYSTEM I & II

Non cyclic-phosphorylation

• PS.II contains chlorophyll a molecules called p680
• after excitation these molecules  by light, a pair of e- are ejected from primary pigment centre
• these pass to electron carriers and the energy released is used to synthesis ATP by chemiosmosis.

• PS.I contains chlorophyll a p700
• after excitation of these molecules by light, e- are rejected from primary pigment reaction centre
• These e- along with protons (H+) join NADP (coenzyme) and it is reduced
• The e- from PS.II will now replace e- lost by PS.I
• e- from photolysis of water replace the electrons lost by PS.II
• Protons from photolysis of water takes part in chemiosmosis to make ATP and the join with NADP

Photophosphorylation 

• Energy is released a electrons pass along the chain of electron carer in the thylakoid membrane.
• this pumps proton across the thylakoid membranes into the thylakoid space where they accumulate
• a proton gradient is formed across the membrane and protons flow down their gradient through channels associated with ATP synthase enzymes.
• this flow is called chemiosmosis. it produces a force that join ADP and Pi to make ATP. 
• kinetic energy from the proton flow is converted to chemical energy in the ATP
• this formation of ATP and occurs through the processes of cyclic and non-cyclic phosphorylation

LIGHT INDEPENDENT STAGE

• Takes place in the stroma of chloroplast
• aka Calvin Cycle
• products of light dependent stage are used
• CO2 is the source of carbon for the production of organic molecules.

Calvin Cycle

• CO2 (enters via stomata) combines with ribulose bisphosphate (RuBP) (5C) to make 2 molecules of glycerate 3-phosphate (GP) (2 x 3C)
• The enzyme RUBISCO catalyses this reaction
• CO2 has now been fixed
• GP is phosphorylated and reduced to form triose-phosphate (TP) using AT and reduced NADP from the light dependent stage
• 5/6 f TP molecules are recycled back to 3 molecules of RuBP which fixes more CO2 in a cycle. The rest of the TP is used to make other compounds such as lipids, amino acids or carbohydrates.

A2 Biology: Anaerobic respiration

• Occurs in the absence of oxygen
• Only consists of glycolyis
• therefore takes place in the cytoplasm
• Net ATP made: 2
• Produces lactate (in animals) or ethanol (in plants) as a waste product.

Only glycolysis occurs as oxygen is the final electron acceptor; without it the ETC cannot function. The reduced NAD is not re-oxidised (lose H) and ergo there is an absence NAD for the Krebs cycle to function. Fungi, such as yeast, use ethanol fermentation and animals use lactate fermentation.

LACTATE FERMENTATION

• Pryuvate (3C) -----lactate dehydrogenase----> Lactate (3C)
                      Reduced NAD -- NAD -- glycolysis
• During this process pryuvate is acting a an aternativ H acceptor
• Lactate is carried away from the muscles to the liver. It is converted back to pryuvate when oxygen is present again.
• Muscle fatigue is not caused by the lactate but the drop in the pH that results from its formation, buffers in muscle help prevent this.

ALCOHOLIC FERMENTATION

• Pryuvate (3C) --pryuvate decarboxylase--> Ethanal (2C) --ethanal dehydrogenase--> Ethanol (2C)
•                                                                  gives out CO2         reduced NAD -> NAD

A2 Biology: Celluar Respiration

Cellular respiration is the process ofusing enrgy in complex organic molecules to produce ATP

Glucose + Oxygen > Carbon Dioxide + Water + ATP

Metabolic reactions that build large molecules are called anabolic and those that break large molecules are called catabolic.  We require energy for: active transport, endocytosis and excocytosis, replication of DNA and movement.

ATP = Adenosine Triphosphate

 ATP is formed during a condensation reaction between ADP and Pi (it is phosphorylated)  The enzyme responsible for this is ATP synthase. The energy to produce the bond that is formed comes from the breakdown of organic substrate (such as glucose) during cellular respiration. ATP provides your cells with energy without it we would be dead.

• We get energy from ATP by breaking down the high-energy bonds between the last two phosphates in ATP.
• When ATP reaches the site of energy using processes in the cell it is hydrolysed back to ADP.
• When hydrolysed to ADP + Pi it yield 30.6kJ or energy per mol. This means energy is available to the cell in small manageable amounts.
• Respiration in which glucose is completely oxidised to form carbon dioxide using oxygen is called aerobic respiration.
• The second and third phases of aerobic respiration takes place in the mitochondria and so it is necessary to recall the structure of a mitochondrion.

The mitchondria matrix is the site of link reaction and Krebs cycle. The inner cristae membrane is the site of the electron transport chain (glycolysis takes place in the cytoplasm)

Phosphorylation - addition of an inorganic phosphate gorup.

Hydrolysis - the splitting of large molecules into smaller molecules through the addition of water.

Oxidation - this is when the substrate donates hydrogen to a hydrogen accepted (dehydrogenation)

Reduction - this is when a hydrogen accepter gains a hydrogen.

Coenzymes are needed during glcolysis, the link reacton and the Krebs cycle, H atoms are removed from the substrte molecules in the oxdation reachtion. These eactions are catalysed by dehydrogenase enzymes. 

This occurs in the mitochondria during respiration and the chloroplast during photosynthesis. 

Stages of respiration:

Glycolysis (cytoplasm)

The link reaction (matrix of mitochondria)

Krebs cycle (matrix of the mitchondria)

Oxidative phosphorylation (cristae of mitochondria)

GLYCOLYSIS

• This occurs int he cytoplasm of all cells. t is a series of reactions, each catalysed by a different enzyme. 
• It works under anaerobic conditions (without oxygen)
• Glucose 6(C) -*-> Glucose 6-phosphate (6C) --> Fructose 6-phosphate (6C) -*->Fructose 1,6-bisphosphate (6C) (Hexose 1,6-bisphosphate)

• (-*-> = ATP > ADP) 

•  Hexose 1,6 bisphosphate splits into 2 Triose phosphate (3C)
• Triose phosphate ----NAD>2xreduced nAD------ADP>2xATP----> Intermediate compound (3C)
• Intermediate compound (3C) ----ADP>ATP----> Pryuate (3C)

Glycolysis: One molecule of glucose produces:

Net gain: 2 ATP (4 ATP in total)

2 reduced NAD

2 molecules of pryuvate

THE LINK REACTION

• Pryuvate diffuses into the matrix of the mitochondria
• 3 stages:
• Decarboxylation of pryuvate by pryuvate decarboxylase
• Dehydrogenation (oxidation) of pryuvate by pryuvate dehydrogenase to form acetate
• Coenzyme A combines with the acetate to give two molecules of Acetyl CoA - enters Krebs cycle

KREBS CYCLE

• The carbon doxide is produced by decarboxylation
• The NAD and FAD are reduced to dehydrogenation during the conversion of citrate into oxaloacetate
• ATP is produced by substrate level phosphorylation where the energy taken directly from a substrate is used to add a phosphate group to ADP.

=6x reduced NAD

=2x reduced FAD

=4x CO2

=2x ATP 

Electron transport chain (ETC)

A number of reduced NAD and FAD have been produced during the first three phases of aerobic respiration  In the ETC these molecules are re-oxidised (lose H) by dehydrogenase enzymes. It takes place in the cristae of mitochondria.

• Each electron carrier is an enzyme, each associated with a coenzyme.
• The coenzyme can accept e- (become reduced) or donate e- (become oxidised)
• They are oxidoreductase enzymes
• Electron pass along the chain of carriers losing energy
• Some coenzymes use this energy to pump protons H+ from matrix to intermembranous space.
• The electrons from each reduced NAD can pump H+ into the space.
• Protons (H+) accumulate in the intermembranous space and build up a proton gradient (high conc)
• Stalked particles allows H+ to pass through them which generates ATP
• Each reduced NAD generates 2.6 molecules of ATP

Chemiosmosis: the process when protons flow down the proton gradient through the ATP synthase enzymes from intermembrane space into the matrix.

• The force of this flow rotates the enzyme and allows ADP + Pi join to form ATP
• aka oxidative phosphorylation - formation of ATP by adding a phosphate group to ADP in the presence of oxygen which is the final electron acceptor
• Oxygen is the final hydrogen acceptor
• when electrons are released from the chain they combine with the H+ ions to form hydrogen.
• The hydrogen then combines with oxygen to form water, so it is the final H or e- acceptor.
• There this stage will only take place in aerobic conditions.

AS Biology: The Heart

The coronary circulation

• Cardiac muscle in the heart wall needs a good supply of blood supply to provide nutrients and oxygen for contraction. This is achieved by the presence of a dense capillary network that received blood from the right and left coronary arteries.

The Cardiac cycle - the complete contraction and relaxation of the heart is a single heartbeat.

• Systole =  period of contraction.
• Diastole = period of relaxation. (This is longer than systole)
• Blood flows from an area of high pressure to an area of low pressure unless the blow is blocked by a valve.
• Pressures are lower on the right as there is more muscle on the left side as the blood has to travel further.

1.  The atria and ventricles are in diastole.
2. Blood in the veins flows into the atria.
3. This increases the pressure inside the empty atria as they fill.
4. some blood goes into the open atrioventricular vales into the relaxed ventricles below
5. Both the atria contract and blood passes down the ventricles.
6. The atrioventricular calves open due to blood pressure.
7. 70% of the blood flows passively down to the ventricles so the atria do not have to contract a great amount. 
8. The aria relax
9. The ventricle walls contract, forcing blood out
10. the pressure of the blood forces the atrioventricular valves to shut
11. the pressure of the blood opens the semi-lunar valves
12. blood passes into the aorta and pulmonary arteries
13. The ventricles relax
14. pressure in the ventricle falls below that in the arteries
15. blood under high pressure in the arteries causes semi-lunar vales to shut. 
16. during diastole all the muscle in the heart relaxes
17. Blood from the vena cava and pulmonary veins enter the atria
18. Cycle starts again

Control of heart rate

The mechanical work in pumping blood is carries out by the cardiac muscles in the walls of the four heart chambers aka cardiac cycle. Cardiac muscle has certain feature that are distinct in the other types of muscles. It contracts rhythmically without any nervous stimulation - it is myogenic.

The initiation of this rhythm comes from a patch of muscle fibres in a small part of the right atrium. This is called the sino-atrial node S.A.N and is also known as the pacemaker. - from the pacemaker  waves of electrical activity spread out rapidly over both atria. Each wave contracts the atria muscle forcing blood in the atria through the ventricular vales into the ventricles.

The atrioventricular septum between the atria and the ventricles does not conduct the cardiac impulse from the pacemaker. however, there is another specialised group (node) of cardiac muscle cells in the wall of the right atrium. this node is called atrioventricular node A.V.N and it picks up the atrial impulse and transmit it along a bundle to modified cardiac muscle fibres in the interventricular septum. When the impulses reach the apex of the heart, it spreads rapidly up the ventricular walls in a  network of conductive fibred called purkinje fibres. Impulses causes heart to contract. 

Blood must be put under pressure as:

• it enables the blood to reach all the cells in all parts of the body.
• it takes deoxygenated blood to the lungs to enable gas exchange  it delivers certain molecules e.g. oxygen and glucose.
• it removes waste material such as CO2 and urea. 

Electrocardiogram ECG

• Electrical impulses in the heart originate in the sinoatrial node and travel through the intrinsic conduction system to the heart muscle.
• The impulses stimulate the myocardial muscle fibres to contract and induce systole.
• The electrical waves can be measured at selectively places electrodes on the kin.
• electrodes on different sides of the heart measure the activity of different parts of the heart muscle. An ECG displays the voltage between pairs of these electrodes. 
• Displays indicate the overall rhythm of the heart and weaknesses in different parts of the heart muscle.
• it is the best way to measure and diagnose abnormal rhythm of the heart.

AS Biology: Lipids

Energy

• Lipids are an important source of energy in animals as they are also energy stores.
• They are well suited to this function because they are compact and insoluble.
• They are found as lipid droplets in the cytoplasm.
• When lipids are oxidised to release energy what is released. - metabolic water and is useful to organisms especially those that live in very dry conditions

In mammals such of the body lipid is found under the skin in adipose tissues where it prevents excessive heat loss. Lipids in plant seeds and fruits also provides thermal insulation against cold environmental conditions and also prevents moisture loss. 

Lipids also provides electrical insulations around neurones. Subcutaneous fat is also found around delicate body organs and gives protection against mechanical damage. It also gives buoyancy to some organisms. Some hormones are also lipids as well as all biological membranes.

Structure

Simple lipids are made up of glycerols and fatty acids

Fatty acids

• A fatty acid consists of a carboxyl group attached to a hydrocarbon chain. A fatty acids that contains the maximum number of hydrogen atoms that can be attached to the carbon atoms is called saturated fatty acid.
• Fatty acids that contain a double bond connecting two carbon atom are called unsaturated fatty acids because they do not contain the maximum number of hydrogen atoms. 
• Polyunsaturated is when there is more than one double bond present.

Double bonds

• The C=C bond changes the shape of the chain. It makes the liquid more fluid.
• Lipids with many unsaturated fatty acids are often oils. Those with mainly saturated fatty acids are more likely to be fats.

Triglycerides

• The most common lipid are known as fats and oils. Animals are usually fats and plants are usually oils. 
• Triglycerides are made up of three fatty acids join to one glycerol. if they are solid at room temperature they are fats and if they are liquid at room temperature they are oils.
• They are a good source of energy because they have a lot of bonds that could be broen down to release energy via respiration.
• They are good energy stores as they can hold a lot of energy in a small space.
• being hydrophobic means they also don't affect the water potential.

Phospholipids

• Phosphate molecules attract water (hydrophilic)
• It consists of a hydrophilic head which interacts with water and hydrophobic tail which orients itself away from water but mixes with lipid.
• When phospholipids are suspended in water they can form a variety of structures. 
• Phospholipids are the main components of membranes. They forma double membrane around the cell due to the hydrophobic interactions.

Cholesterol

• Cholesterol is a type of lipid but it isn't formed from fatty acids and glycerol.
• It regulates the stability and fluidity of membranes by sitting between phospholipids fatty acids tails as it is also hydrophobic.
• some hormones are made from cholesterol including oestrogen and testosterone.
• The lipid nature of these hormones allows them to pass through the phospholipid bilayer to reach cell contents. Vitamin D is also made from it.
• Too much cholesterol can be deposited in the wall of blood vessels causing atherosclerosis.
• In bile, produced by the liver, stored in the gall bladder, cholesterol can stick together forming gall stones.

AS Biology: Mammalian transport system

Artery

• Thick walls with muscles present
• a lot of elastic tissues
• Small lumen
• no valves except in the pulmonary artery and aorta
• ablt to constrict
• not permeable
• carries blood FROM heart 
• carries oxygenated blood
• withstands high pressure
• blood moves in pulses

Vein

• Thinner wall muscles present
• some elastic tissues
• larger lumen
• has semi lunar vales throughout
• cannot constrict
• not permeable
• carries blood TO heart
• carries deoxygenated blood
• low pressure
• no pulses

Capillary

• Thinnest wall with no muscles present
• no elastic tissue
• larger lumen
• no vales
• cannot constrict
• permeable
• carries blood to ad from the heart
• carries both oxygenated and deoxygenated blood
• pressure in between veins and arteries
• no pulses

AS Biology: Proteins

Amino acids

• Proteins are made of monomers called amino acids.
• These monomers join together to forma long chain = polypeptide.
• Polypeptides can be cominted to form a protein. Polypeptides and proteins are polymers.
• There are twenty biologicall important amino acids.
• All amino acids (and so proteins) contain Carbon, Hydrogen, Oxygen and Nitrogen (some contains sulphur)

• Proteins are polymers of amino acids and are made up of a Amino group (NH2) , a carboxyl group (COOH), and the central carbon (α-carbon), hydrogen and a variable group.

Animals needs proteins in their diets. these are digested to amino acids and used to prouce proteins. Excess amino acids cannot be stored and their amino group makes them toxic. This is removed by deamination in the liver.

Plants make the amino acids they need. They use nitrate from the soil to produce amino groups. These are added tot he organic groups made from photosynthesis.

Aminos acids can link together by forming peptide bonds. a peptide bond is formed when the carboxyle group of one amino acids combine with the elimination of water. Therefore it is a condensation reactions. When to amino acids are joining by a peptide bond they form a dipeptide.

• Many amino acids can joing to form a polypeptide chain (series of condensation reactions) = polymerisation. 
• Polypeptides and proteins are synthesised on ribosomes - protein synthesis. It uses mRNA which puts the amino acids together in the right order- different mRNA molecules make different proteins.

Primary structure

This is the sequence of amino acids in a polypeptide molecules.

The sequence of amino acid is important as it determines the shape of the protein and ergo the function.

Secondary structure

This is a regular arrangement of polypeptide chains. The alpha-helix is where the polypeptide chain is loosely coiled in a regular spiral. in the beta-pleated sheet the polypeptide chains are more extended n than alpha helix.

Tertiary structure

This si further folding of the secondary structure which gives a compact 3D shape. It depends on the properties of the different R-groups in the polypeptide chain.

Collagen is a fibrous protein, it has three polypeptide chains and is twisted into a triple helix, polypeptides held together by hydrogen bonds between chains, this forms a collagen fibril, many fibrils form a fibre.

Haemoglobin has 4 polypeptide chains, 2 alpha and 2 beta. Each one has an iron prosthetic group attatched to it. It is a globular protein so it is soluble.

AS Biology: Cell Membrane permeability - beetroot

• Betalain is the pigment that gives beetroot its colour. 
• The pigment is found in the vacuole of beetroot cells.
• The pigment has to pass through tonoplast and cell surface membrane to get out of the cell.
• Soaking beetroot in water of different temperature will give you different results (shades of colour)

Beetroot pigment is more likely to leak at higher tempeartures because ineasing remperatures:

• give molecules more kinetic energy
• phospholipids move more quickly at high temperatures, making membrane more fluid, leading to its breakdown and the leakage of the pigment
• proteins are denatured by the heat so no longer effectively control what enters leaves through the membrane
• Ethanol can dissolve phospholipids and so can act like a detergent

AS Biology: Human Ventilation System: Gas Exchange

Gas Exchange

An efficient gas exchange system has:

• a larger surface area 
• a short diffusion distance
• a large diffusion gradient

Adaptations for efficient gas exchange

• Large surface area - individual alveoli are small (approx. 100-300 µm) yet surface area for gas exchange is about 70m^2
• Permeable barrier to exchange - the plasma membranes that surround the thin cytoplasm of the cells form the barrier to exchange - allows exchange of O2 and CO2.
• Short diffusion distance - the alveolus walls and capillary walls are one cell thick and are made upp of squamous epithelium. Capillaries are close contact with the alveolus wall and are so narrow that the RBC are squeezed against the walls.
• Diffusion gradient - this is maintained by the ventilation of the lungs and movement of the blood through the capillaries in the lungs.

Surfactant

• A thin layer of moisture lines the alveoli, it evaporates when we breathe out.
• The lungs must produce this substance called surfactant to reduce cohesive forces between water molecules - without it alveolus would collapse due to cohesive forces between water and air sacs.
• alveoli must be kept open for their extensive surface area used for gaseous exchange
• surfactant reduces the surface tension by occupying the space between the watery film and alveolar membrane.

Features

• A large surface area provided by many alveoli - increases the rate of diffusion of gases
• A good blood supply due to dense work of capillaries - capillaries ensure the gradient for the diffusion of gases is maintained.
• Thin surface layer (one cell thick) - shorter diffusion distance  greater rate of diffusion
• Partially permeable to respiratory gases - allows few movement of gases across alveoli walls
• Ventilation mechanism - ensure fresh oxygenated air is drawn into lungs to maintain the diffusion gradient

Blood reaching the alveoli has a low concentration of oxygen and high concentration of carbon dioxide than the alveolar air; and so there is a concentration gradient which helps the diffusion of O2 and CO2 in opposite direction. As blood flows past an alveolus, oxygen diffuses into it and carbon dioxide out of it - by that time blood leaves the alveolus it has the same concentration of oxygen and carbon dioxide as the alveolar air. Each pulmonary capillary is very narrow so that RBC are slowed as thy pass the capillaries allowing more time for diffusion.

Ventilation in lungs

• Ventilation is when air is constantly moving in and out of the lungs. 
• Intrapulmonary pressure - pressure within the lungs.
• The lungs are not muscular so pressure changed are achieved indirectly

• When you breath in and out you ribcage moves up and out, your abdomen moves in and out.
• Nerve impulses from the brain causes the diaphragm and external intercostal muscles to contract; he diaphragm flattens/ribcage moves up and out.
• Increasing the volume of the thorax and decrease pressure inside the lungs makes air move down pressure gradient from outside into the lungs. 
• Air pressure in lungs is reduces.
• Air enter the lungs - whenever air pressure inside the alveoli is reduced below atmospheric pressure. 

Breathing in

• Diaphragm contract and moves downwards at the same time the intercostal muscles contract and move the ribcage up and out. The volume inside the thorax increases,
• The increase in volume causes the pressure to drop. The pressure in the chest is lower than the atmospheric pressure outside. Air is forced down the through trachea into the lungs.

Breathing out

• Diaphragm relaxes and moves upwards - at the same time intercostal muscles relax and ribcage fall down and inwards. The volume in the thorax decreases - the decrease in volume causes the pressure to rise.
• The pressure in the chest is higher than the atmospheric pressure outside.
• Air is forced up out of the trachea and out of the mouth.

Pulmonary ventilation

Pulmonary ventilation (dm^3 min^-1) = tidal volume (dm^3) x ventilation rate (min^-1)

The tidal volume is the volume of air breathed in at each breath during normal, relaxed, rhythmical breathing and it's about 0.5dm^3.

Ventilation rate is the number of breaths taken in one minute. Normally about 12-20 breaths in a healthy adult.

Lung capacity

• the change in lung volume can be analysed by using a spirometer.
• this uses an oxygen filled chamber floating over a water bath. The lid of the chamber is hinged t one side.
• when a person breathes in and out of the apparatus the lid moves and these movements of the lid correspond exactly to changed in the volume of air held in the lungs.
• CO2 released is removed by passing expired air through soda lime before returning to its main chamber.
• A recording pen draws a trace on a rotating drum in response to movements of floating chamber.

Tidal volume is the volume of air moved in and out of the lungs with each breath when you are at rest.

Vital capacity - largest volume of air that can be moved into and out of the lungs in any one breath.

Residual volume - volume of air that always remains in the lungs even after the biggest possible exhalation.

Breathing during exercise muscles cells use up more oxygen an produce increased amounts of carbon dioxide. The lungs and heart have to work harder to supple the extra oxygen and removed the carbon dioxide. the breathing rate and depth of increases. heart rate also increases in order to transport oxygenated blood to the muscles.

So during exercises

• muscle cells respiration increases (more O2 is used and CO2 produced)
• increasing level of CO2 made is detected by the brain and a signal is sent to the lungs to increase breathing rate.
• breathing rate and volume of air in each breath increases meaning more gaseous exchange is taking place.
• the brain also signals the heart to beat faster to pump blood to the lungs for efficient gaseous exchange.
• more oxygenated blood gets to the muscle and CO2 is removed. 

AS Biology: Human ventilation system

• Lungs are the gas exchange organs in humans and other mammals.
• Lungs are a pair of lobed structured made up of many highly branched tubules called bronchioles which  had tiny air sacs called alveoli.
• They fill most of the space inside the thorax which is bounded by the rib cage, sternum and muscular diaphragm.
• A system of tubes takes air into and out of the lungs.
• This consists of the nasal cavity were the air is filtered, warmed and moistened  continues across the pharynx to the trachea

The trachea

• The trachea is supported and prevented from collapsing by C-shaped ring of cartilage in its walls. The walls of the trachea are lined with ciliated epithelial cells and goblet cells.
• The goblet cells produce mucus to trap dirt particles and bacteria from the air breathed in. The cilia moves this mucus together with the trapped particles up to the throat. The mucus is then transported down to oesophagus to the stomach. 

Bronchi and bronchioles

• At the base the trachea divides into a left and right bronchus. These are similar in structure to the trachea. Both of the bronchi divide into smaller tubes, which continues to subdivide to eventually form narrow tube called bronchioles.
• Bronchioles have muscle in their walls, which allows them to constrict and control the flow of air in and out of the alveoli. The alveoli are well supplied with blood capillaries.
• Bronchioles are narrower than bronchi. Larger ones have some cartilage but smaller ones don't.
• The wall is mostly smooth muscles and elastic fibres, the smallest bronchioles have clusters of alveoli at their ends.

Tissue

• Trachea and bronchi have a similar structure, they differ only in size as the bronchi are narrower.
• Most of the wall contains c-shaped rings  in the cartilage.
• On the inside surface of the cartilage is a layer of glandular tissues, connective tissues, elastic fibres, small muscles and blood vessels.
• The inner lining is an epithelium layer that has two types of cells. Most cells have ciliated epithelium  among this are goblet cells. 

Cartilage

• Supports he trachea and bronchi - keeps them open. Prevents collapse during inhalation. C-shaped, flexible and allowed neck to move without constricting airways.

Smooth muscles

• Can contract and constrict the airway - this makes airway narrower which can restrict air flow - could be harmful if there's harmful substances in the air. It's not a voluntary act, some people have allergic reactions causing bronchioles to constrict making it difficult to breathe. (One of the cause: asthma)

Elastic fibres

• when smooth muscles contract and narrow the airways it cannot reverse the change - when it relaxes the elastic fibred recoils to their original shape and size
• Q.A. Antagonistic means that they work against each other. The smooth muscle contracts to narrow the lumen of the bronchioles. As this happens, the elastic fibres are deformed. When the muscle relaxes, the elastic fibres recoil against their original shape and extend the muscle fibres again.

Goblet cells

• These secrete mucucs
• Mucus traps tiny particles in the air e.g bacteria and ergo reduces chances of infections.

Ciliated epithelium

• Cilia move in a synchronised pattern to move mucus up the airway to the back of the throat. Mucus is swallowed and the acidity in the stomach kills the bacteria.

AS Biology: Movement of Water

The Casparian strip

The Casparian strip blocks the apoplast pathway to ensure that water and dissolved nitrate ions have to cross the cell membrane which is done by the transporter proteins Nitrate cane be actively transported into the xylem which lowers the water potential of the xylem and water follows by osmosis.

• The endodermis around the xylem is aka starch sheath which contains starch and uses it as its source of energy.
• The endodermis consists of special cells that have waterproof strip in their walls. - The Casparian strip.
• This strip block the apoplast pathway and ergo water is forced into the symplast pathway.
• The endodermal cells moves minerals by active transport from the cortex to the xylem. - decreases water potential in the xylem by osmosis.
• This reduces the water potential in the cells just outside the epidermis.
• This sets up a water potential gradient across the whole cortex.
• Ergo, water is moved along the symplast pathway from the root hair cells across the cortex and into the xylem - at the same time water an move through the apoplast pathway across the cortex

Movement up the stem

The force that pulls water up the stem of a plant is the evaporation of water from leaves - a process called transpiration. Water molecule evaporates from the leaves, hough the tiny openings called stomata on the surface of a leaf.

• As water move into the xylem by osmosis this pushes the water already present up the xylem. Root pressure can push water a few metres up a stem, but cannot account for movements over great distances.
• Capillary action - the same forces that hold water molecules together also attract the molecules of the side of the xylem vessel - adhesion. These forces can pull up the sides of a vessel.
• Transpiration pull -  water evaporates from leaves as a result of transpiration. Water molecules form hydrogen bonds between one another so they stick together aka cohesion.. Water forms a continuous, unbroken pathway across the mesophyll cells in the leaf and down the xylem. As water evaporates from the mesophyll cells into he leaf  into the air spaces beneath the stomata, more molecules of water draws up as a result of cohesion. Water is then pulled up the xylem as a result of transipation pull. This puts xylem under tension (cohesion tension theory) The lignified xylem vessels prevent collapse under pressure. 

AS Biology: Plants: Xerophytes

Plants in different habitats have different adaptations:

• Mesophytes: plants adapted to a habitate with adequate water
• XEROPHYTES: plants adapted to dry habitat
• Halophytes: plants adapted to a salty habitat
• Hydrophytes: plants adapted to a freshwater habitat

XEROPHYTES ADAPTATIONS

• Thick cuticle - stops uncontrolled evaporation though leaf cells
• Small leaf surface area - less surface area for evaporation and transpiration
• Low stomata density - smaller surface area for diffusion
• Sunken stomata, stomatal hairs, rolled leaves - maintains humid air around stomata e.g. marram grass
• Extensive root - maximise water uptake
• Spines - protect from animals

Sunken stomata - creates a local humidity, decreases exposure to air currents; moist air is trapped here in the diffusion pathway and reduces evaporation rate

Rolled leaves: traps moist air so reducing transpiration. Plus, smaller surface area of lead is exposed to the drying effects of the wind.

Stomata on inside of the rolled leaf creates local humidity/decreases exposure to air currents because water vapour evaporates into air space rather than atmosphere. e.g. marram grass Fewer stomata decreases transpiration as this is where water is lost.

Marram grass

Marram grass possesses:

Rolled leaves leaf hair and sunken stomata. These adaptation make it resistant to dry conditions and of course sand dunes which drain very quickly and retain very little water.

OCR, June 03, Q3.

Some plants, such as cacti, inhabit dry areas. These plants of dry areas are known as xerophytes. Reduction of water loss by the process of transpiration/evaporation can be achieved by employing a variety of adaptations. In some species the leaves are needle-like, which reduces the surface area to volume ratio, whilst in others the epidermis is covered by a thick layer of waxy cuticle. In order to conserve the greatest amount of water, many species shut their stomata during the day,

AS Biology: Plant cells and water

Water potential

• Pure water has a water potential of 0 however cells have negative water potential when there are more solute particles than water molecules therefore cells have higher water potential when there are less solute particles and more water molecules
• The plant cells becomes TURGID when water enters by osmosis, vacuole sweels and pushes against the cell wall. Plant cell is FLACCID when the water is lost from the cell and the vacuole shrinks hence the cell loses its shape

Water can travel via three different pathways:

1. Apoplast pathway - water moves through the cell wall

• The cellulose cell wall has many water filled spaces between the cellulose molecule, water can move through these spaces and between cells. The water does not pass through any plasma membranes.

2. Symplast pathway - water moves through the plasma membrane into the cytoplasm

• Water enters the cytoplasm via the plasma membrane. The plasmodesmata allows the movement of water from one cell to the next.

3. Vacuolar pathway - water moves through the vacuole

• This is similar to symplast pathway but the water can now enter and pas through the vacuoles as well.

Water uptake from soil

• The roots have root hair cells, to increase the surface area to absorb minerals by active transport.
• This lowers the water potential in the roots so water moves into the root hair cells by osmosis. 
• Water enter root hair cell by osmosis - minerals are actively transported into xylem. (water moves into xylem my osmosis.) 

Solute can enter the xylem by going through cell membrances - the Casparian strip blocks apoplastic route (outside cells) - water cannot pass between the cell or through cell walls - it must pass into cytoplasm or into symplast pathway

Water moves up the stem due to root pressure, transpirational pull, capillary action

• Transpiration pull - as water molecules are removed from the xylem, more water molecule are pulled to replaced them aka transpirational pull.

Mass flow of water also relies on the properties of water

• Cohesion - the water molecules tend to stick together
• Adhesion - the water molecules also stick the the inside of the xylem vessel
• the drawing of continuous column of water up to xylem vessel is aka cohesion-tension theory

AS Biology: Plants' transport system: Xylem and Phloem

Plants need a transport system as every cell of a multicellular plant needs a regular supply of water and nutrients. Cells inside the plant would not be able to receive enough nutrients and water to survive simply by diffusion.

Plants require:

• Carbon dioxide for photosynthesis
• Oxygen for aerobic respiration
• Organic nutrients for growth

PHLOEM transports sugars from the leaves  - it's also for amino acids. - they can move upwards or downwards.

XYLEM transports water, minerals up from roots. 

Vascular tissue is distributed throughout the plant and it helps with the plant transport. Xylem and phloem are found together in vascular bundles which also contains other tissues. It helps transport water from toots to leaves via the stem. The xylem and phloem run the entire length of the plant from the roots to the midrib and veins of the leaf.

Xylem vessels are empty tube shaped cells/ Their cytoplasm has been removed by the plant and their walls are strengthened and thickened with lignin. The lignin strengthens the tubes and help support the plant by giving rigidity to the xylem. Minerals from the soil are also carried in the xylem, they are needed by the plants in many of its chemical reactions.

Features:

• Wall thickened by lignin prevents collapse under tension and adhesion to lignin
• Hollow tubes means that there is less resistance to flow
• No end walls so there's a continuous columns so there is less resistance to flow
• Pits inside the walls allows lateral movement
• Narrower the lumen the higher water will rise by capillarity
• Stacked end to end develops as a continuous water filled column; allows tension to pull water up

Phloem (sieve) tubes carry sugar around the plant. Phloem cells are alive and have a cytoplasm unlike hollow xylem vessels. It's is made up of two types of cells: sieve tubs and companion cells.

Sieve tubes: the ends walls of the tube cells have pores which dissolved sucrose is transported from cell to cell. They have sieve plates at the end with pores so sugar can get through. they have no nucleus, the cytoplasm is controlled by companion cell nucleus. The vacuoles of the tube are joined and sugary sap flow along them.

Companion cells - proves the energy for the sieve tube cells. The nuclei tends to be large to compensate for the lack of nucleus in the sieve tube.

Features:

• Both cells are living which allows active processes
• Plasmodesmata (connections between sieve tube and companion cell) allows exchange between cells.
• Companion cell have many mitochondria to make energy and a nucleus to control functions both cells.
•  Sieve tubes have little cytoplasm and elongated cells so there's less resistance of fluid flow
• Sieve plates allow material through, it also joins end to end to provide continuous tubes.
• Sieve tubes are bi-directional which allows sugar to go to sink or it can travel either direction.

AS Biology: Mitosis - Cell Cycle

CELL CYCLE

Interphase - Normal state of all cells. Chromosomes are not yet visible. During this stage cells carries out synthesis (growth) of cytoplasm and organelles such as mitochondria, enzymes and it increases in size. DNA replication also takes place in this stage so each chromosome consists of a pair of chromatids.

Prophase - Chromosomes become visible as chromatin fibres shorten and thicken by spiralisation. This condensation of chromosomes takes place. Once chromosomes are clearly visible they can be seen to consist of two chromatids. They have the same size and has an identical DNA base sequence.

Late prophase - 2 chromatids are twisted around one another and joined together by a centromere. The centrioles migrate to oppose ends of the cell and mictotubules develop to form fibres. The fibres make up a structure aka spindle. The spindle runs from pole to pole but is broadest at equator. Nucleolus disappears and finally the nuclear membrane breaks down.

Metaphase - Chromosomes line up at the equator of the spindle. Thy become attached to the spindle at their centromeres and line up across the equator

Anaphase - The centromeres divide into two and the spindle fibres pull the daughter centromeres apt. The separated chromatids are pulled along behind the centromeres. once separated the sister chromatids should now be called chromosomes and are now drawn to opposite poles.

Telophase - Chromosomes reach poles of the cell's spindle. The nuclear membrane reforms around each of the two groups of chromosomes and the nucleoli re-appears. The spindle fibres disintegrate and centrioles replicate  Prophase coiling sequence is reversed so that as the chromosomes uncoils and lengthen they cannot be seen clearly.

CYTOKINESIS - Telophase leads into cytokinesis (division of cytoplasm) so that daughter cells are formed. Cytokines differs in plant and animal cells. Animals undergo cleavage by constriction of the cytoplasm and furrowing the plasma membrane in plants a cell plate forms across the equator.

(Plants) - In animals most cells are capables f cytokinesis  whereas in plants only special cells aka meristems can divide in this way. They are found at the root and shoot tips. Meristem tissues are responsible for the growth of the whole organism. Plants cells that do not have centrioles the tubulin protein threads are made in the cytoplasm

(Animals) - In animals cells cytokinesis starts from outside working inwards to the cell membrane but in pants cells it starts with the formation of the cell plate where the spindle equator was. New cell mebrane and new cell was material is laid down along this cell plate.

Yeast cells undergo cytokinesis by producing a small bud that nips of the cell, in a process called budding.

ROLL OF MITOSIS

• Asexual reproduction e.g. propagation in plants
• Growth - of multicelluar organsims by producing extra cells. 
• Replacement - e.g. RBC and skin cells are replaced by new ones.
• Repair - damaged cells that needs to be replaced by identical new ones.

Features of mitosis: chromosome number is maintained there is no change in genetic material.

AS Biology: Mitosis - DNA

DNA

• The nucleus includes long thread structures called chromosomes, which consists of DNA wrapped around proteins called histones.
• Specific lengths of DNA in chromosomes are called genes
• individual chromosomes are not visible when  cell is not dividing but the chromosomal material stains darkly and can be seen (chromatin) so DNA + hisones = chromatin.
• The human body calls have 46 chromosomes in each nucleus (diploid = 2n - because it contains two sets of chromosomes)
• In humans each of the parent contributes 23 chromosomes when the zygote is formed during sexual reproductions when the gametes fuse.
• The eggs and sperm have only one set of chromosomes
• This is called the haploid number (n)

• Chromosomes can be arranged in pairs, one of each front he parents are paired. The two members of each pair are identical in appearance.
• The chromosome pairs are therefore described as homologous and make up a homologous pair.
• Before mitosis, DNA replication takes place.
• Every chromosomes has a pair of sister chromatids which is joined together by a centromere.

• Mitosis produces two identical cells - it produces two genetically identical daughter cell that are the same as the parent cell