AS Biology F212: Disease

Health is a state of mental, physical and social wellbeing - NOT just absence of diesease.
If one is in good health they are:

• able to carry out normal mental and physical tasks
• well fed with a balanced diet
• usually happy with a positive outlook
• suitably housed with proper sanitation
• well integrated into society
• free from diseases

What is disease?

• Disease is a departure from good health caused by malfunction of the mind or body
• symptoms can be physical, mental or social
• those caused by living organisms are called infectious diseases - usually physical 

Parasites and Pathogens

• Parasites
• Live on (external) or in (internal) another living thing (host)
• they cause harm to the host by taking nutrients
• they may live all or part of their life on the host
• they can overburden the host and make it more susceptible to secondary infections
• Pathogens
• this is an organism that causes disease
• they live by taking nutrition from their host and cause damage in the process
• includes a wife range of bacteria, fungi, protoctist and viruses

• Bacteria - cholera
• caused by bacterium Vibrio cholerae
• it is a water-borne disease and spreads through contaminated water, food or shell fish
• they act on the walls of the small intestine causing diarrhoea, dehydration and weakness.
• Tuberculosis
• aka TB is an infectious disease that can affect any part of the body although it is usually found n the lungs as these are the first site of infection
• it kills approx 2 million people each year - more than any other infectious disease
• tuberculosis is caued by one of two species of rod shaped bacteria
• Mycobacterium tuberculosis
• Mycobacterium bovis
• Fungi - Athletes foot and ringworm
• Caused by Tinea fungus
• many different species that causes these diseases
• they live in the skin
• cause redness and severe irritation
• Viruses - TMV and HIV
• TMV - tobacco mosaic virus - affects plants
• HIV - human immunodeficiency virus - affects humans 
• also more common diseases such as colds and flu.
• take over the cells genetic machinery and organelles to allow it to reproduce
• Protosctist
• Amoeboid dysentery 
• Malaria
• enter cells and feed on the contents as they grow

TRANSMISSION OF DISEASE

For a micro-organism to be considered a pathogen they must

• travel from one host to another
• gain entry to the host's tissues
• reproduce
• resist the defences of the host
• cause damage to the host's tissues

Forms of transmission

• by the means of a vector (carrier)
• by physical contact
• by droplet infection

• Malaria
• caused by eukaryotic Plasmodium
• Plasmodium falciparum is the most common
• spreads by the female Anopheles mosquito
• these feed on blood
• the parasite lives in the red blood cells and feed on the haemoglobin
• HIV/AIDs
• The HIV virus enters the body and may remain inactive
• This is known as being HIV positive
• once active, the virus attacks and destroys T helper cells in the immune system
• these cells help to prevent infection
• their destruction reduces the person's ability to resist infection
• EFFECT
• you are unable to defend yourself against any pathogen that enters your body
• these are known as opportunistic infections
• it is the effect of these that eventually kills as person with HIV
• AIDS stands for acquired immune deficiency syndrome
• Transmission
• Exchange of bodily fluids such as blood-to-blood contact
• unprotected sex
• unscreened blood transfusions
• use of unsterislised surical equipment
• sharing hypodermic needles
• accidents such as 'needle-stick'
• across the placenta or during childbirth
• from mother to baby during breast feeding
• Tuberculosis (TB)
• TB is an infectious disease that can affect any part of the body although it is usually found in the lungs as these are the first site of infection
• Pulmonary tuberculosis is spread through the air by droplets containing the bacteria, released into the air when infected individuals cough, sneeze or even talk.
• normally takes close contact with an infected person over a period of time rather than a causal meeting in the street to transmit the bacteria
• Symptoms (droplet infection)
• the symptoms of pulmonary tuberculosis initially include a persistent cough, tiredness, and loss of appetite that leads to weight loss.
• As the disease develops, fever and coughing up of blood may occur
• Risks
• some groups are at greater risks of contracting TB than others.
• people who are in close contact with infected individuals over long period e.g. living and sleeping in overcrowded conditions
• work or reside in long term care facilities where relatively large numbers of people live close together e.g. old people's home, care homes, hospital or prisons
• people from countries where TB is common
• have reduced immunity such as
• the very young or very old
• those with AIDS
• people with other medical conditions that make the body less able to resist disease e.g. diabetes, or lung disease such as silicosis
• those undergoing treatment wit immunosuppressant drugs eg. following a transplant surgery
• the malnourished
• alcoholics or injecting drug-users
• the homeless

The World Health Organisation (WHO)

• WHO states that good health is a human riht
• poor health causes a lot of suffering
• Ill health has an economic cost as a result of medical provision and loss of productivity
• worldwide, many peopl have no access to the basic requirements for good health
• contributing factors
• poverty
• lack of proper shelter
• lack of purified water
• poor nutrition
• poor hygiene
• lack of government investment
• poor/inadequate education on disease, it causes and transmission
• civil unrest or warfare

• Malaria
• kills around 3 million people peryear
• about 300 million are affected worldwide
• it is limited to were the vector (Anopheles mosquito) can survive
• 90% of sufferers live in sub-Saharan Africa
• It is difficult to control
• mosquito
• resistant to insecticides/pesticides/chemicals used for control
• build up in food chains/kill predators
• breeds quickly/very common/lays many eggs
• breeds in small bodies of water/inaccessible places
• especially in rainy seasons
• difficult drain/spray/cover
• difficult to encourage everyone to use netswide range of increasing because of climate change
• rests and hides in houses
• Plasmodium
• side effects of (e.g. anti-malarial) drugs/people don't take drugs long enough (think they are well but not)
• many strains and species
• resistant to drugs
• inside red blood cells or liver cells
• antigen concealment
• dormant/in body for a long time/symptomless carriers/long incubation
• different stages in life cycle in the body
• no vaccine/difficult to develop vaccine
• people lose immunity if malaria is eradicated
• HIV/AIDS
• Spreading in pandemic proportions (is an epidemic of infectious disease that is spreading through human populations across a large region)
• 45 million people living with HIV/AIDS at the end of 2005 (>half in sub-Saharan Africa)
• 5 million newly infected each year
• at the end of 2006 30 million died as result of HIV/AIDS related disease
• Epidemiology
• = patterns in the occurrence of the disease in the human population 
• it identifies the cause of a disease
• the risk factors associated with the disease
• determines the incidence of a disease
• determines the prevalence of the disease as well as the mortality and morbidity
• study how quickly it's spreading
• identify countries/part of population at risk
• identify a disease as endemic, epidemic or pandemic
• ENDEMIC: always present in a population
• EPIDEMIC: spreading rapidly to a lot of people over a large area
• PANDEMIC: a worldwide epidemic
• they target education programmes at people at most risks
• target advertisements to raise the awareness
• target screening programmes to identify individuals at risk
• provide specialist healthcare in certain areas
• provide vaccination programme for the major disease
• targeting research to find cures for the major disease

AS Biology F212: DNA and RNA

DNA

The DNA needs to be stable and must be replicated accurately so daughter cells have the same genetic makeup.

When a cell divides into two,, the chromosomes have to replicate so hat each other the new cells still contains the original number of chromosome, e.g. 46 chromosomes in humans.

Because each chromosome consists of DNA, the DNA molecule has to REPLICATE. The parent cell produces a set of chromosomes identical to its own set. DNA replicates semi-conservatively.

Semi-conservative replication:

• Each parent strand acts as a template for a new strand
• Each new DNA double helix would then have one parent strand and one new strand 

Stages of DNA replication

• The whole  of the DNA molecule uncoils
• The DNA molecule unzips as the hydrogen bonds between the organic bases break
• The bases are now exposed
• In the nucleus individual DNA nucleotides are activated
• The bases of the free activated DNA nucleotides pair up with the complementary exposed bases on each original DNA strands
• The process of complementary base pairing ensures that C-G, A-T
• Covalent bonds form between the phosphate of one of the nucleotide and the sugar of the next to seal the sugar-phosphate backbone
• The whole process is controlled by the enzyme DNA POLYMERASE
• this continues all along the length of the DNA molecule until two DNA molecules are produced
• Both original strands are copied to give 1 old and 1 new strand.
• Two identical copies f the DNA are produced by the semi conservative method of replication

RNA

• RNA is found in three forms, (mRNA, rRNA, tRNA)

Difference between RNA and DNA

• (RNA
• DNA)

• RNA - found in the nucleus and cytoplasm
• DNA- found in the nucleus (small amounts in mitochondria and chloroplasts)
• Contains bases A, C, G, U
• Contains bases, A, C, G, T
• RNA molecule consists of a single strands
• DNA molecule consists of two strands running in opposite directions, twisted together to form a double helix
• Contains the pentose sugar ribose
• Contains the pentose sugar deoxyribose.

How DNA and RNA works together to produce a protein

The sequence of bases on DNA codes for particular polypeptide/protein molecules.

TRANSCRIPTION

• Part of the DNA uncoils and unzips 
• The hydrogen bonds break between the complementary base pairs
• The bases are exposed
• Only 1 of the DNA strand is used
• The particular sequence of bases for that gene form the template
• RNA nucleotide align next to the DNA template strand
• They join up individually using complementary base pairing, A with U, C with G
• A strand of mRNA is made when the backbone for the nucleotides are joined together
• DNA zips back up

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: 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.

A2 Biology: Respiration - The Krebs Cycle

The Krebs cycle takes place in the mitochondrial matrix. This stage takes place in aerobic respiration.

1. Acetate (carried from the Link reaction by coenzyme A) joins with Oxaloacetate (4C) to form Citrate (6C). CoA is the released to collect more acetate.
2. Citrate is decarboxylated (CO2 is removed) and dehydrogenated (2H is removed) to form a 5C compound. 2H is accepted by a molecule NAD which becomes reduced NAD.
3. 5C compound is also decarboxylated and dehydrogenated to form a 4C compound and a molecule of reduced NAD.
4. 4C compound is changed to another 4C compound and during this change a molecule of ADP is phosphorylated to produce a molecule of ATP. (Substrate-level phosphorylation).
5. The 4C compound then changes into another 4C compound and 2H is removed and accepted by coenzyme FAD which becomes reduced.
6. The other 4C compound go through dehydrogenation  and oxaloacetate is regenerated and another molecule of NAD is reduced. 

There needs to be two turns round the cycle for each molecule of ATP

To conclude: 

• 6 Reduced NAD are produced
• 2 Reduced FAD are produced
• 4 molecules of CO2 are produced
• and 2 ATP are produced

A2 Biology: Homeostasis: Control of blood glucose concentration

If blood glucose concentration is too high it is detected by beta cells 

• Beta cells (in islets of Langerhans) then secretes insulin into the blood
• They target hepatocytes, muscles cells and other cells such as brain that have membrane bound receptors for insulin. Insulin binds to these receptors
• Adenyl cyclase is activated in each cell and converts ATP to cAMP
• this activates a series of enzyme reactions 
• Glucose is converted into glycogen for storage/lipids
• or used in respiration
• glucose leave cells through specific channels

If blood glucose concentration is too low

• This change is detected by alpha cells (in islets of Langerhans)
• Alpha cells secrete the hormone glucagon
• Targets cells are heptocytes which posses specific receptors for glucagon
• glycogen is converted to glucose - glycogenolysis
• glucose is made from amino acids and lipids - gluconeogenesis 

A2 Biology: Kidney Failure

If the kidney fails - unable to remove urea - not going through osmoregulation

3 main causes:

• Diabetes mellitus
• hypertension
• infections

Use of dialysis

Dialysis removes waste, salts and excess fluids from the blood by passing over a dialysis membrane. Dialysis membrane is partially permeable and contains the perfect plasma, so it has right amount of glucose, water and other substances but not urea. Urea is passed through into the dialysis membrane by diffusion.

Methods of control:

• Haemodialysis - blood from vein is passed through a machine which contains  artificial dialysis membrane and returns back into the blood. Heparin is added to act as an anticoagulant.

• Peritoneal dialysis - filters the body's abdominal membrane. A permanent tube is implant into the abdomen and dialysis solution is poured in to fill up the space between abdomen wall and organs. Several hours later this solution is drained from the abdomen.

• Kidney transplant - is a major surgery where a kidney from a healthy willing relative donor or a healthy kidney from a deceased person is implanted in and is attached to the blood supply and bladder. The old kidney remains in the body unless it is cancerous or likely to cause an infection. The patient would have to take immunosuppressants for life as the new organ would be treated as a foreign object and this would prevent it from being rejected.

A2 Biology: Respiration - Oxidative phosphorylation

-On inner mitochondrial membrane known as the cristae

• Has a series of electron carriers = electron transport chain
• Each one has a haem group (co factor) which contains iron, Fe.
  (If it's Fe2+ it is reduced ~ gained e- | If it's Fe3+ it's oxidised ~ loss e-)

1. NADH releases H to the first carrier
2. First carrier passed H to the second carrier
3. H now splits into H+ and e- and only the e- is passed on to the net carrier
4. H+ is pumped into the inter membrane space 
5. There is now a high concentration of H+ in intermediate slate = protein motive force/electrochemical gradient
6. H+ diffuses through the stalked particle which acts as a channel protein of hydrogen ions back into the matrix
7. H+ are back in matrix they recombine with their e- to form an Hydrogen atom.
8. H combines with O2 (which we breathe in) to form H2O hich is a waste product of respiration

• Enzyme involved - cytochrome oxidase
• Oxidose phosphorylation is making ATP using oxygen therefore involves electron transport chain
• Chemiosmosis - making ATP as H+ diffuse through stalked particle

A2 Biology: Respiration - Glycolysis

Glycolysis

• Happens in the cytoplasm of all cells
• It can take place in anaerobic or aerobic conditions.
• Glucose (6C sugar) is broken down to 2 molecules of pryuvate (3C compound)

Stage 1. Phosphorylation

• Glucose (6C sugar) - very stable - needs to be activated to be split
• An ATP molecule is hydrolysed - phosphate group attached to glucose at carbon 6
• Glucose > Fructose 6 phosphate
• Another ATP is hydrolysed - phosphate group attached to fructose 6 phosphate at carbon 1
• Activated - fructose 1,6 bisphosphate aka hexose, 1,6-bisphosphate
• Energy from hydrolysed ATP activates the sugar and prevents it from going out of the cell

2. Splitting hexose 1,6 bisphosphate

• each molecules of hexose 1,6 bisphosphate is splt into 2 x triose phosphate 

3. Oxidation of triose phosphate

• 2H atoms are removed from triose phosphate (involves dehydrogenase enzymes)
• These are helped by NAD which is a H acceptor - combines with H and becomes 'reduced NAD'
• 2 ATP formed - substrate level phosphorylation

4. Conversion triose phosphate > pryuvate

• 4 enzyme-catalysed reactions convert each phosphate to a pryuvate
• during this, 2 molecule of ADP are phosphorylysed to 2 ATP

So overall, 2 molecules of ATP (=net amount - 4 have been made but 2 are used again)

There are also 2 molecules of reduced NAD and 2 pryuvate which ill be used in next stage of respiration

A2 Biology: Osmoregulation in collecting duct

• The Loop of Henle is where most water is reabsorbed.
• The longer the loop of Henle the more water is reabsorbed from the filtrate - back in the blood.

E.g. A desert rat would have a longer loop of Henle than a horse or a beaver as it requires more water to be reabsorbed and conserve to help it survive in the heat. A beaver would have a smaller loop of Henle in comparison to the horse as it lives in water.

When you are THIRSTY 

• it is detected by osmoreceptors in the hypothalamus
• the osmoreceptors shrink due to a decrease in water potential in the blood
• it then synapses with neurosecretory cell - terminal bulb (which is equivalent to synaptic knobs) and fires off an action potential
• it releases vesicles that contains the hormones Antidiuretic Hormones (ADH) into the blood in the posterior pituitary gland
• ADH in blood targets cells forming walls of collecting duct
• causing a series of enzyme reactions including cyclic AMP
• vesicles with water channels aka aquaporins fuse with the cell membrane makin collecting duct more permeable to water
• the collecting duct passes through medulla (salty tissue fluid) so water pulls out of the filtrae by osmosis to be reabsorbed in blood.
• this produces a small quantity of concentrated urine
• water back in the blood  blood water potential is restored back to normal
• this process switches off = negative feedback

My revision notes: A2 Biology: Urea + Ornithine cycle + detoxification of alcohol

Deamination is the process where ammonia (NH3) is produced. Ammonia is very soluble and highly toxic. It also produces an organic keto acid which is then used in respiration. Ammonia is converted into Urea which is less soluble and less toxic. Urea travels in the blood and is transported to the kidneys where it is filtered out of the blood and stored into the bladder until released from the body.

The ornithine cycle

2NH + CO2 > CO(NH2)2 + H2O

ammonia + carbon dioxide > urea + water

Detoxification of alcohol

Alcohol can be bad for us but it also contains chemical potential energy which could be used in respiration.

• Alcohol is broken down b hepatocytes by the action of enzyme ehtanol dehydrogenase
• In result of this ETHANAL is made
• it is then dehydrogenated again by ethanal dehydrogenase
• Which produces ethanoate (acetate)
• combined with coenzyme A to form acetyl coenzyme A which enters respiration.
• Ethanol > Ethanal > Ethanoic acid > Acetyl coenzyme A

My revision notes: A2 Biology: EXCRETION + Kidney functions

Excretion - Removal of toxic waste products formed by metabolic reactions.

CO2 - from respirations

• Dissolves in H2O to form Carbonic acid >  lowers pH of blood and denatures enzymes
• Detected - by chemoreceptors/aorta > sends and impulse to the medulla bongata > increase rate and depth of breathing. If thi falls below the pH of 7.35 > find it difficult to breathe = respiratory acidosis. 
• Carbonic acid: CO2 + H2O > H2CO3
• H2CO3  >(dissolves in)> H+  +  HCO3- (hydrogen carbomate)
• H+  +  Hb (haemoglobin) > HHb (Haemoglobinic acid) 
• Which leads to Hb not being able to load enough O2 ad therefore reduce amount of O2 that Hb can load

Removal of urea by the kidney

KIDNEY FUNCTIONS

• Removes urea from blood to urine
• maintin water  potential of blood
• remove exceess ions etc

Different sections of the nephtons have different functions.

Ultrafiltration - takes place in the Bowman;s capsule

Blood contains - Red blood cells (RBC), white blood cells (WBC)

                        -Plasma, which contains - amino acids, glucose, H2O, CO2, O2, hormones, urea

Ultrafiltration filters the blood (plasma) but the RBC and WBC are too big to be filtered out. 

The capillary is made of endothermic cells which is very thing and flat an have tessellated margins. 

In the nephron, the afferent arteriole has a wider diameter than the efferent arteriole which means the blood pressure in the glomerulus (knot of capilleries in the nephron) increaser - so all the small molecule sin the blood (amino acids, urea, co2, h2o, o2 etc) gets forced out of the blood capillaries BUT blood cells and large plasma proteins are too large to be forced out.

The filtrate (plasma without plasma proteins) is forced out into the bowman's capsule. 

There is only a basement membrane is the only true separation between endothelial cells of the glomerulus ad the bowman's capsule

The kidney 

•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 a tube called the ureter.

•The outer darker region of the kidney is the cortex and the inner, lighter region is called the medulla.

My revision notes: A2 Biology: Control of Heart Rate

All cells, e. muscle cells have different requirements, depending on the activity e.g. exercising - muscle cells require an increase in oxygen, glucose and removal of carbon dioxide.

The average heart rate is 70 beats / min
SAN controls this with no need of neurone impulses

If you exercise you need to increase:

• Heart rate
• strength of contraction
• stroke of volume  volume of blood pumped / beat

When you exercise you produce more CO2 - which dissolves to form carbonic acid - lowers pH

> This is detected by chemoreceptors in carotid arteries (going to head and aorta)

> They send an impulse to the cardiovascular centre which in turn sends an impulse down an accelerator nerve to SAN 

> So heart beats faster and stronger

Why are the chemoreceptors in the head and aorta?

Carotid artery is going to the brain ensures that the blood is not acidic

Going for aorta as its the biggest artery as it has the most amount of blood passing through

If you eercise you got receptors in muscles > gets stretched > impulse sends to cardiovascular neurone > SAN  > Increase in heart rate

Stop eercising > decrease in CO2/muscle receptors not stretched > impulse vagus nerves > SAN > slows heart rate back to normal