AS Biology F212: Smoking and Heart Disease

Cigarette smoke contains over 4000 different chemicals, many of them are toxic. Three main components posing a danger to human health: Tar, Carbon Monoxide and Nicotine.

Tar

• Tar coats the airways and alveoli
• it can cause allergic reactions which narrows the airways
• it also destroys the cilia on the surface
• mucus cannot be moved
• tar stimulates production of extra mucus
• bacteria get stuck in the mucus and multiply in the airways
• the smoke has an increased risk of infection

• chronic bronchitis
• this is the disease caused by the inflammation of the airways
• accompanied by mucus collecting in the lungs
• symptoms
• irritation
• continual coughing
• coughing up mucus
• risk of lung infection
• long term effects
• frequent infections damages the lining of lungs
• white blood cells are attracted
• have to make their way from blood to airways
• use enzymes to dissolve their way through
• elastase enzyme dissolved the elastic tissue
• airways no longer recoil properly
• bronchioles collapse trapping air
• alveoli burst due to pressure of trapped air 

• Emphysema
• disease caused by alveoli bursting
• lungs have less surface area
• so can do less gas exchange
• healthy lungs contains large quantities of elastic tissue, mostly made up of the protein elastin.
• this tissue stretches when we breathe and in and springs back when we breathe out.
• in emphysematous lungs, the elastin has become permanently stretched and the lungs no longer able to force out all the air from the alveoli
• the surface area of the alveoli is reduced and they sometimes burst
• as a result, little if any exchange of gases can take place across the stretched and damaged air sacs.
• symptoms
• shortness of breath - results from difficulty in exhaling air due to the loss of elasticity in the lungs
• if the lungs cannot be empties much of their air, then it is difficult to inhale fresh air containing oxygen so patient feels breathless
• the smaller the alveolar surface area leads to reduce levels of oxygen in the blood and so patient tries to increase oxygen supply by breathing more rapidly
• chronic coughs - consequence of lung damage and the body's effort to remove the damaged tissue and mucus that cannot be removed naturally because the cilia on the bronchi and  bronchioles has been destroyed
• bluish skin colouration is due to low levels of O2 in the blood as a result of poor gas diffusion in the lungs

• Asthma
• common allergen which can trigger an asthma attack: pollen, dust mites, mould, pet dander
• asthma is an example of localised allergic reaction
• it can also be triggered or made worse by a range of factors, including air pollutants e.g. sulphur dioxide) exercise, cold air, infection, anxiety and stress.
• one or more of these allergens causes white blood cells on the lining of the bronchi and bronchioles to release a chemical called histamine
• effects of histamine - the lining of these airways become inflamed
• the cells of the epithelial lining secret larger quantities of mucus and enter the airways
• the muscle surrounding the bronchioles contracts and so constrict the airways.
• symptoms:
• difficulty in breathing - due to constriction of bronchi and bronchioles, their inflamed linings and the additional mucus and fluid within them
• a wheezing sound when breathing is caused by air passing through the very constricted bronchi and bronchioles
• a tight feeling in chest - consequence of not being able to ventilate the lungs adequately because of constricted bronchi and bronchioles
• coughing - reflex response to obstructed bronchi and bronchiole in effort to clear them

Chronic obstructive pulmonary disease

• combination of diseases including emphysema, chronic bronchitis and asthma

Lung Cancer

• cancers are caused carcinogens
• cigarette smoke contains many carcinogens
• carcinogens are present in tar
• they enter the cells of the lungg tissue
• they mutate the DNA in their nuclei
• if the gene for the cell division is mutate then uncontrolled cell division takes place - this is cancer
• lung cancer can take up 20-30 years to develop to a size to cause problems
• symptoms
• coughing up blood
• persistent cough
• weight loss
• shortness of breath
• chest pains
• Nicotine and Carbon monoxide
• these two chemicals can be absorbed in the lungs and enter the blood
• here they cause cardiovascular disease
• Nicotine
• causes addiction
• causes release of adrenaline
• thus increased heart and breathing rate
• increased force of contraction of the heart
• constriction of arterioles
• hypertension
• increases stickiness of platelets
• chance of thrombus (clotting)
• increased blood cholesterol and LDLs
• Carbon monoxide
• combine with haemoglobin
• forms carboxyhaemoglobin
• haemoglobin has higher affinity for CO than O2
• this is irreversible
• reduces O2 carrying ability of haemoglobin
• decrease in oxygen transported
• tissues starved of oxygen
• also damages lining areteries

Cardiovascular Disease

• atherosclerosis
• nicotine and CO damages artery walls
• hypertension contributes to the damage
• this attracts phagocytes to repair the damage
• these encourage the growth of smooth muscles and deposition of fatty substances
• fatty deposits are laid down when repaired including cholesterol
• these atheromas form plaques
• these narrow artery
• also make it rougher
• this reduced blood flow
• thrombosis
• blood flows slower past plaques
• combined with stickier platelets from nicotine - blood clots can form
• these block narrow arteries
• this may result in formation of blood cot, or thrombus, in a condition aka thrombosis
• this thrombus may block the blood vessel, reducing or preventing the supply of blood to tissues beyond it
• the region of tissue deprived of blood often dies as a result of the lack of oxygen glucos and other nutrients blood normally provides
• embolism is when the thrombus is carried from its place of origin and blocks another artery
• coronary heart disease
• when the atherosclerosis occurs in the coronary arteries and reduces/stop oxygen supply to the cardiac muscle for respiration
• Angina: severe chest pain due to lack of oxygen
• Heart attack: death of part of the heart muscle due to a blockage of artery
• Heart failure: heart stops pumping due to lack of oxygen by a blockage of major coronary artery
• Risk factors
• age - older 
• sex - male
• diet high in saturated fats causes more LDLs
• plaque formation
• blood cholesterol 
• high salt intake
• hypertension
• low fibre
• obesity
• heavy alcohol consumption
• and more
• stroke
• death of part of the brain
• due to blood clot forming in artery supplying brain
• artery supplying brain bursts (aneurysm)
• aneurysm
• atheromas that lead to the formation of a thrombus also weaken artery walls
• these weakened points swell to form a balloon-like blood filled structure -  aneurysm
• frequently burst leasing to haemorrhage and therefore loss of blood to the region of the body served by that artery
• a brain aneurysm is know as a stroke

Cardiovascular diseases

• great cause of premature death
• CHD and stroke often result from atherosclerosis
• this can start in adolsescence

Treatment

• is expensive
• long term drug treatment to reduce blood pressure and cholesterol
• surgery
• therefore reduction in atherosclerosis through the reduction of risk factors is important

AS Biology F212: Vaccines

A vaccine is a preparation containing antigenic material (e.g. antigens)

Vaccination confers artificial active immunity. (Entry of antigenic material into the body stimulates an immune response, memory cells are produce)

Types of vaccination

1. Live vaccine e.g. Smallpox
   The live vaccine - less harmful pathogen but with similar antigens and can still be induce active immunity
2. Harmless vaccine e.g. measles, TB
   Harmless or attenuated version of the pathogen
3. Dead vaccine e.g. Cholera vaccine
   the pathogen has been killed either by exposure to or chemicals
4. Antigens e.g. hepatitis B
   Preparation of antigens from a pathogen
5. Toxoid vaccine e.g. tetanus
   vaccine consists of harmless form of toxin produced by the pathogen

Herd vaccination

• providing immunity to almost all of the population at risk
• with enough people immune the disease cant spread
• level of immunity needed varies depending on the disease (80-85% for small pox, 95% for measles)

Ring vaccination

• used when a  new case s reported
• vaccinate everyone in the immediate vicinity (house, village, or town)

AS Biology F212: Enzymes

Enzymes are proteins

• These are catalysts of biochemical reactions and speeds up these reactions so they occur at normal body conditions.
• All enzymes are globular proteins, therefore they are soluble.
• The molecule on which the enzyme acts is called the substrate.

Properties of enzymes

• They act as a catalyst
  They speed up a chemical reaction without being used up themselves
• They are high specific
  In general, one enzyme will only catalyse one particular reaction/substrate
• Enzymes are effective in small amounts
• The enzymes do not alter the nature of the products

Active site

• The active site is an area on the surface of the protein where the reaction occurs
• it has a very specific shape that is complementary to that of a substrate

Mechanism of enzyme action

• The enzyme and substrate combine to form a complex (the enzyme-substrate complex)
• The complex then break down to give products and release the enzyme for further use

Enzyme actions

Chemical reactions

• Particles in gases and solutions continually move about and will collide with each other.
• During a chemical reaction two reactants collide and combine to form an intermediate activated complex or transition state, which breaks down to form products.
• In all reactions a certain amount of energy is needed for the reactants to form the transition state
• This energy is known as the activation energy and without it chemical reactions cannot proceed
• The energy comes from maltose which can be boiled in acid - this provides the right conditions for maltose molecule to collide with water molecules energetically enough to hydrolyse

Activation energy

• Before a reaction can occur it must overcome an energy barrier by exceeding its activation energy
• Enzymes work by lowering this activation energy s that the reaction happens more readily
• Enzymes allow reactions to take place at lower temperatures (found in cells)

Lock & Key 

• This idea would explain why enzymes are specific
• The substrate (key) fits the shape of the enzyme (lock)
• Once the products have been made they no longer fit the active site and are released from the enzyme which can react with another substrate because its active site is available once more

Induced fit

• The lock and key model considers the enzyme to have rigid active site and a reaction would depend on the randomly moving substrate molecule entering the active sight in the right orientations
• This is unlikely to happen
• As the substrate collied with the active site - it changes the shape of the enzyme
• This allows the active site to fit more closely around the substrate
• Te substrate is held by the oppositely charged R-groups of the active site
• This is the enzyme-substrate complex
• The product is different shape to the substrate
• They do not fit the active site and are released
• The enzyme is free to catalyse further reactions

Enzymes and temperature

• Temperature and kinetic energy
• Molecules (liquid or gas) move around continually
• This random movement is due to kinetic energy
• Increasing temperature increase this energy
• This increases the frequency and force at which molecules collide
• Enzymes and kinetic energy
• Increased kinetic energy
• Increased number of collisions between active site and substrate
• Increased successful collisions = more enzyme substrate complexes
• Thus, increased rate of reaction
• Temperature
• Increasing temperature increases the rate of reaction up to the optimum temperature.
• Optimum temperature is the maximum rate of reaction
• After this rate of reactions begin to decrease
• Above the optimum the temperature rate of reaction decreases due to the breaking of the bonds that hold the enzymes tertiary structure in place
• This is caused by vibrations of the bonds
• The more that are broken the greater the chance the active site will change
• if this happens the enzyme wills top working - it can't function
• This irreversible change is called denaturation 
• Different enzymes have different optimum temperatures an therefore become denatured at different teoratures

Enzymes and pH

• pH is the measure of the Hydrogen ions concentration.
• pH7 is neutral, below is acidic and above is basic.
• The greater the concentration of hydrogen ions the more acidic it is.
• H+ are attracted to the negatively charged parts of molecules and repelled by positively charge parts.
• Proteins (enzymes) rely on interactions between positive and negative regions to maintain its 3D tertiary structure 
• The H+ can interfere with the hydrogen and ionic bonds which alters the tertiary structure - this causes a change in the shape of the active site then it will effect the rate of an enzyme controlled reaction
• Optimum pH
• This is the pH at which the rate of reaction is highest.
• Enzymes work in a fairly narrow range of pH.
• At this value the H+ concentration gives the tertiary structure its best shape
• The active site is most complementary to the substrate  
• An enzymes optimum pH will be related to the pH of the environment in which it is found
• e.g. Pepsin is a protein digesting enzymes found in the stomach and has an optimum pH of 2
• Trypsin is found in the small intestines which has a pH of 7

Effects of concentration

• No substrate = no enzyme-substrate complexes thus no reaction
• more substrate = more collisions = higher rate of reaction
• this continues until a maximum rate is reached =Vmax
• At this point all the active sites are occupied at any one point in time
• therefore increasing the substrate concentration has no further effect
• the enzymes concentration is a limiting factor
• Enzyme concentration increased > more active sites > more enzyme substrate complexes ? rate of reaction increases
• BUT, a point will be reached where all substrate molecules are occupying active sites
• the substrate is sad to be a limiting factor
• if the substrate is in excess (concentration is high) then the rate of reaction can increase
• the substrate is no longer a limiting factor

Initial reaction rate

• The highest rate of reaction will be seen when the enzyme and substrate are first mixed
• As tie asses the substrate concentration will decrease and thus the rate of reaction ill decrease
• The frequency of collisions will decrease
• The highest reaction rate is known as the initial rate

Inhibition

• Some substance can prevent or slow down the action of enzymes
• These are called inhibitors
• Inhibition can be competitive or non competitive 
• Competitive
• Have a similar structure to the substrate
• prevents the substrate from entering
• level on inhibition depends on the relative concentration of substrate and inhibitor
• this because the substrate and inhibitor are competing for active sites
• Non-competitive
• These do not compete for the active site
• they attach at the region away from the active site
• this changes the tertiary structure of the enzyme
• this causes the active site shape to change
• the substrate no longer fits
• enzyme substrate complex cannot form
• If there are enough inhibitor molecules present to bind to all the active site then the reaction will stop.
• Increasing the substrate concentration will have no effect
• Reversible and irreversible
• both competitive and non competitive enzymes can be reversible or irreversible
• this depends on the whether the inhibitor unbinds with the enzyme or not
• if it stays bound then it's irreversible inhibiton and the enzyme can no loner be used

Cofactors

• Non-protein substance needed for enzymes to catalyse a reaction
• e.g. coenzymes, prosthetic groups, inorganic ion cofactors. 
• Coenzymes
• small
• organic
• non-protein
• binds to active site
• takes part in the reaction and are changed like the substrate 
• can be recycled and used again
• Vitamin B3
• This helps the body break down carbohydrates and fats releasing energy
• Needed for the eznyme pryuvate dehydrogenase to function properly
• This is an enzyme needed n respiration
• A disease called pellagra results from absent from diet
• diarrhoea, dermatitis, dementia, death
• Prosthetic groups
• this is a coenzyme that is permanent part of an enzyme
• they contribute to the 3D shape and tare therefore vital to the function
• e.g. Carbonic anhydrase as a zinc based group
• Inorganic ion cofactors
• these can increase the reaction rate
• they combine with either the substrate or enzymes
• it makes the formation of enzyme substrate complex easier
• this is due to its affects the charge distribution and sometime shape of the eznyme

Interfering with enzymes

• Cystic fibrosis
• affects on respiratory system
• also blocks the passage of digestive enzymes produces by the pancreas into the gut
• leads to digestive difficulties
• tablets are prescribed containing enzymes packaged in an acid resistant coat
• Ethylene glycol poisoning
• this product is found in antifreeze
• it is not poisonous but when ingested it gets broken down in the liver
• carried out by the enzyme alcohol dehydrogenase
• this produces oxalic acid which is extremely toxic and can lead to death 
• treatment: large doses of ethanol is given which causes severe alcohol intoxication
• the ethanol is a competitive inhibitor of alcohol dehydrogenase
• Snake venom
• mixture of toxins and enzymes
• Phosphodiesterase interferes with the working of the preys heart
• the inhibitor of the enzyme acetyl cholinesterase - involved in nerve transmission = paralysis
• Antibiotics and bacterial resistance
• some bacteria are resistant to antibiotics because of the mutation in their DNA which results in them being able to produce enzymes that inactivate antibiotics
• many produce an enzyme (beta lactamase) that breaks down penicillin
• this resistance can be passed on when they replicate
• HIV treatment
• this is treated with chemicals that inhibit protease enzymes
• this prevents the virus from replicating
• they are often competitive inhibitors

AS Biology F212: Biological molecules:Proteins

Metabolism is the total of all the biochemical reactions taking place in the cell of an organism.

There  are two types of metabolic reactions:

1. Catabolism: breaking down larger molecules into smaller molecules. e.g. digestion.
2. Anabolism: building up smaller molecules into larger ones. e.g. photosynthesis. 

Nutrients

• Carbohydrates, fats, proteins are macronutrients - they are are needed in large quantities.
• Vitamins and minerals are micronutrients - they are needed in small quantities.
• Water Makes up 70% of a cell
• Fibre*

Use of chemical groups in the body

• Carbohydrates
  Energy storage and supply, structure (e.g. cellulose)
• Proteins
  Structure, transport, enzymes, antibodies, hormones
• Lipids
  Membranes, energy supply, thermal insulation, protective layers, electrical insulation in neurones, some hormones
• Vitamin and minerals
  Form part of some larger molecules, takes part in some metabolic reactions, acts as co-enzymes
• Nucleic Acid
  Information molecules, carries instruction for life
• Water
  Takes part in  many reactions, support in plants, solvent for most metabolic reactions, transport

*Fibre is a carbohydrate, it does not provide nutrients nor energy, it adds bulk to the diet making it easier for gut muscles to push food along - lower risks of constipation and intestinal cancer.

Proteins

Amino acids

• Proteins are made up of individual molecules (monomers) called amino acids.
• They are joined to forma longer chain called polypeptide - which can be combined to form a protein.
• Polypeptides and proteins are therefore polymers.
• There are 20 different amino acids.

• All amino acids contains: Carbon, Hydrogen, Oxygen and Nitrogen.
• In addition, some contains sulphur.

Structure

• They have an amino group (NH2) and a carboxyl group (COOH)
• These roups along with a H atom are attached to a central carbon atom called the alpha-carbon.
• The alpha-carbon atom has an R group attached to it.
• This R group is a variable group - it's different in each of the twenty amino acids.

Where do animals get their amino acids?

• Animals need proteins in their diet.
• These are digested to amino acids and used to produce proteins.
• Excess amino acids can not be stored as the amino group makes them toxic and thus it is removed my deamination in the liver.

Where do plants get their amino acids?

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

Formation of a dipeptide

• Amino cids can link together by forming a peptide bond
• A peptide bond is formed when the carboxyl group of one amino acid combines with the amino group of another with the elimination of H2O
• this is called a condensation reaction
• When two amino acids are joined by a peptide bond they form  dipeptide

Making polypeptides and proteins

• Polypeptides and proteins are synthesised on the ribosomes - protein synthesis.
• This process uses mRNA.
• This puts amino acids together in the right order.
• Different mRNA molecules make different proteins.

Levels of Structures

• Primary structure
  This is the sequence of amino acids in a polypeptide molecule.
  The sequence of amino acids is vital because it determines the ultimate shape of the protein and therefore its function.

• Secondary structure
  This refers to the regular arrangement of the polypeptide chain.
  The alpha-helix is where the polypeptide chain is loosely coiled in a regular spiral.
  The beta-pleated sheet is were the polypeptide chains are more extended than in the alpha helix.
  The C=O and N-H groups from the peptide bond regions are held near to each other so that they for many hydrogen bonds.
  These hold the coils of the alpha-helix and the beta-pleated sheets together and make it a stable structure.

• Tertiary structure
  This is the further folding of the secondary structure which gives it a 3D compact shape.
  It depends on the properties of the variable R-groups in the polypeptide chain.

• R-group bonding
  Disulphide bond - the amino acid, cysteine, contains sulphur. where two cysteines are found close together a covalent bond called disulphide bond forms between chains.
  Ionic bonds - occurs between oppositely charged R groups
  Hydrogen bonds - occur between some R groups - these can be easily broken.
  Hydrophobic/philic interactions - hydrophobic amino acids will be most stable if they are held together with water excluded. Hydrophilic amino acids end to be found on the outside of globular proteins.

• Quaternary structure
  Consists of two or more different polypeptide chains which are held together by bonds between the R groups. 

• Globular proteins
• 3D feature: rolls up to form balls (compact)
• Soluble in water - hydrophilic
• Metabolic
• Examples: Enzymes, plasma proteins, hormones, antibodies
• Fibrous proteins
• Forms fibres (long)
• Insoluble - hydrophobic
• Structural
• Examples: Collagen, keratin

The effect of heat 

• Heat increases kinetic energy in a molecule. - causing molecules to vibrate and breaking some bonds maintaining tertiary structure.
• If enough heat is applied the structure can unravel and the protein can no longer function - denatured.
• Even when cooled it will not take on its original shape/arrangement,

Protein hydrolysis

• Protein breakdown is catalysed by enzymes.
• These enzymes are known as protease enzymes.
• Hormone regulation: hormones need to be broke down so that their effect is not permanent.
• Ageing: skin loses elasticity and becomes wrinkled due to inability to rebuild the protein collagen.

• Collagen
• Fibrous protein
• 3 polypeptide chains
• Twisted triple helix
• polypeptides held together by hydrogen bonds between chains
• this forms a collagen fibril
• many fibrils forms a fibre
• Haemoglobin
• 4 polypeptide chains, 2 alpha and 2 beta
• Each one also has an iron prosthetic group attached
• Globular therefore soluble 

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,

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: 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.