AQA A Level Biology Unit 3B

Mass flow theory

1. Sucrose diffuses down a concentration gradient from the photosynthesising cells to the companion cells by facilitated diffusion. 2. Hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP. 3. The hydrogen ions diffuse down a concentration gradient through carrier proteins into the sieve tube elements. 4. Sucrose molecules are transported along with the hydrogen ions in co-transport. 5. The sieve tubes have a lower and more negative water potential as sucrose is actively transported from photosynthesising cells. 6. Water moves from the xylem into the sieve tubes by osmosis creating a high hydrostatic pressure. 7. At the respiring sink cells sucrose is used during respiration or converted to starch. Sucrose is actively transported into the sink cells from the sieve tubes which lowers the water potential. 8. Water moves into the respiring cells from the sieve tubes by osmosis. The hydrostatic pressure is lowered. There is a mass flow of sucrose solution down the hydrostatic pressure gradient in the sieve tubes.

Diastole

1. The atrioventricular valves open when the pressure within the atria exceeds the pressure within the ventricles. 2. The relaxation of the ventricle walls causes them to recoil and reduces the pressure within the chamber. 3. The pressure within the ventricles is lower than that in the aorta and pulmonary artery causing the semi-lunar valves to close.

Atrial systole

1. The contraction of the atrial walls reduces the volume of the chamber. 2. The pressure is higher in the atria than the ventricles. 3. Blood is forced through the atrioventricular valves.

Ventricular systole

1. The contraction of the ventricular walls reduces the volume of the chamber. 2. The pressure is higher in the ventricles than the aorta and pulmonary vein. 3. Blood is forced though the semi-lunar valves.

Formation of tissue fluid

1. The loss of tissue fluid from the capillaries reduces the hydrostatic pressure inside them. 2. The hydrostatic pressure is lower than that of the tissue fluid at the venous end. 3. The tissue fluid is forced back into the capillaries by the higher hydrostatic pressure outside them. 4. The plasma loses water and contains proteins, therefore is has a lower water potential than the tissue fluid. 5. Water leaves the tissue by osmosis down a water potential gradient.

Cohesion-tension theory

1. Water evaporates from mesophyll cells due to heat from the sun leading to transpiration. 2. Water molecules form hydrogen bonds between one another and hence tend to stick together. This is known as cohesion. 3. Water forms a continuous, unbroken column across the mesophyll cells and down the xylem. 4. As water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata more molecules of water are drawn up behind it as a result of cohesion. 5. A column of water is pulled up the xylem as a result of transpiration. This is known as the transpiration pull. 6. The transpiration pull puts the xylem under tension as there is a negative pressure.

Atheroma formation

1. When the endothelium is broken white blood cells and lipids from the blood clump together under the lining to form fatty streaks. 2. Over time more white blood cells, lipids and connective tissue build up and hard to form a fibrous plaque. 3. The plaque partially blocks the lumen of the artery and restricts blood flow which causes the blood pressure to increase.

Positive cooperativity

A smaller increase in the partial pressure of oxygen is needed to bind the second oxygen molecule.

Tissue fluid

A solution of glucose, amino acids, fatty acids and ions.

Aneurysm

An aneurysm is a swelling of the artery. It starts with the formation of atheromas. 1. Atheroma plaques damage, weaken and narrow the arteries which increases the blood pressure. 2. When blood travels through a weakened artery at high pressure, it may push the inner layers of the artery though the outer elastic layer to from an aneurysm. 3. The aneurysm may burst, causing a haemorrhage.

Arteriole structure

Arterioles carry blood under lower pressure than arteries to capillaries from the arteries. • The muscle layer is relatively thicker than in arteries. The contraction of the muscle layer allows constriction of the lumen of the arteriole. This restricts the flow of blood and therefore controls the movement into capillaries that supply the tissues with blood. • The elastic layer is relatively thinner than in arteries because blood pressure is lower.

Pulmonary vein

The blood vessel that is connected to the left atrium and brings oxygenated blood from the lungs.

Aorta

The blood vessel that is connected to the left ventricle and carries oxygenated blood to the body.

Vena cava

The blood vessel that is connected to the right atrium and brings deoxygenated blood from the body.

Pulmonary artery

The blood vessel that is connected to the right ventricle and carries deoxygenated blood to the lungs.

Coronary arteries

The blood vessels which supply the heart with oxygen.

Artery structure

The function of arteries is to transport blood rapidly under high pressure from the heart to the tissue. The overall thickness of the artery wall is great so that it resists the vessel from bursting under pressure. • The muscle layer is thick compared to veins which means arteries can be constricted and dilated to control the volume of blood passing through. • The elastic layer is relatively thick compared to veins because it is important that blood pressure is kept high if blood is to reach the extremities of the body. The elastic all is stretched during systole and recoils during diastole. The stretching and recoiling action helps to maintain high pressure and smooth pressure surges created by the beating of the heart. • Arteries do not contain valves as the blood is under constant high pressure.

Myocardial infarction

The heart muscle is supplied with blood by the coronary arteries. The blood contains oxygen needed by heart muscle cells to carry out respiration. When a coronary artery becomes blocked the heart muscle is cut off from its blood supply and therefore heart attack is caused.

Partial pressure of oxygen

The measure of oxygen concentration. The greater the concentration of oxygen the higher the partial pressure.

Transpiration

The movement of water through the xylem vessels and its evaporation from the leaves.

Translocation

The passive process by which organic molecules and mineral ions are transported in the phloem.

Association

The process by which haemoglobin binds with oxygen in the lungs where there is a high partial pressure.

Dissociation

The process by which haemoglobin releases its oxygen in the tissues where there is a low partial pressure.

Oxygen affinity

The tendency of a molecule to bind with oxygen. Haemoglobin can change its affinity for oxygen depending on the partial pressure of oxygen in the environment.

Stroke volume

The volume of blood pumped per heartbeat.

Cardiac output

The volume of blood pumped per minute.

Thrombosis

Thrombosis is the formation of a blood clot. It starts with the formation of atheromas. 1. An atheromatous plaque can rupture the endothelium of an artery. This damages the artery wall and leaves a rough surface. 2. Platelets and fibrin accumulate at the site of damage and form a thrombus. 3. The blood clot can cause a complete blockage of the artery.

Vein structure

Veins transport blood at a slow rate and under low pressure from the capillaries in tissues to the heart. The overall thickness of the wall is small because the low pressure reduces the risk of bursting. The walls of veins can be flattened easily which aids the flow of blood within them. • The muscle layer is relatively thin compared to arteries because veins carry blood away from the tissues, and therefore the constriction and dilation cannot control the flow of blood to cells. • The elastic layer is relatively thin compared to arteries because the low pressure of blood within the veins will not cause them to burst. The pressure is too low to create a recoil action. • Veins contain pocket valves to ensure that blood does not flow backwards due to the low pressure. When body muscles contract, veins are compressed, pressuring the blood within them. The valves ensure that this pressure directs the blood in only towards the heart.

Bohr effect

• Carbon dioxide is acidic in solution therefore the pH of the blood in respiring tissues is lowered. • The tertiary structure and therefore the overall shape of haemoglobin is altered. • The affinity of haemoglobin for oxygen is reduced therefore the ability to dissociate is increased. • The increased carbon dioxide concentration shifts the oxygen dissociation curve to the right.

Smoking

• Carbon monoxide combines with haemoglobin and reduces the amount of oxygen transported in the blood, which reduces the amount of oxygen available to tissues. If the heart muscle does not receive enough oxygen it can lead to a heart attack. • Nicotine stimulates the production of adrenaline, which increases heart rate and raises blood pressure. The substance makes the blood platelets stick together which leads to a greater risk of thrombosis. • Smoking decreases the amount of antioxidants in the blood that are important for protecting cells from damage.

Evidence supporting the cohesion-tension theory

• During the day when transpiration is at its greatest, there is more tension and negative pressure in the xylem. This pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter. • At night when transpiration is at its lowest, there is less tension in the xylem and therefore the diameter of the trunk increases. • If a xylem vessel is broken the tree can no longer draw up water. This is because the continuous column of water is broken and therefore the water molecules can no longer stick together. • When a xylem is broken, water does not leak out. As the xylem is under tensions and not pressure, water does not leak out when it is broken.

Risks of cardiovascular disease

• High blood pressure increases the risk of damage to the artery walls. Damaged walls have an increased risk of atheroma formation causing a further increase in blood pressure. • A diet high in salt increases the risk of cardiovascular disease because it increase the risk of high blood pressure • High blood cholesterol increases the risk of cardiovascular disease as cholesterol is one of the main constituents of the fatty deposits that form atheromas. • A diet high in saturated fat is associated with high blood cholesterol levels due to increased LDL intake.

Ultrafiltration

• High hydrostatic pressure at the arterial end of the capillaries causes tissue fluid to move out of the blood plasma. • The outward pressure is opposed by the hydrostatic pressure of the tissue fluid outside the capillaries and the lower water potential of the blood due to plasma proteins. • The pressure is only enough to force small molecules out of the capillaries, leaving cells and proteins in the blood.

Tracer experiments

• If a plant is grown in an atmosphere containing ¹⁴CO₂ the ¹⁴C isotope will be incorporated into the sugars produced during photosynthesis. • The radioactive sugars can then be traced as they move within the plant using autoradiography. Thin cross-sections of the plant stem are taken and placed on X-ray film. • The film is darker where it has been exposed to the radiation produced by the ¹⁴C in the sugars. The darker regions correspond to where phloem tissue is in the stem.

Evidence supporting the mass flow hypothesis

• Sap is released from the sieve tubes when they are cut which suggests that there is high pressure within the tubes. • The concentration of sucrose is higher in leaves than in the roots. • The downward flow in the phloem occurs in daylight but ceases when leaves are shaded or at night. • Increases in sucrose levels in the leaf are followed by similar increases in the phloem. • Metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem. • Companion cells possess many mitochondria and therefore readily produce ATP.

Structure of the heart

• The atria are thin-walled and elastic and therefore can stretch when they collect blood. • The ventricles have a thicker muscular wall than the atria as they contract to pump the blood out of the heart. • The left ventricle is thicker than the right as it pumps blood around the body and therefore needs to build up higher pressure by contraction.

Evidence opposing the mass flow hypothesis

• The function of the sieve plates is unclear as they would seem to hinder mass flow. It has been suggested that they may have a structural function which helps to prevent the tubes from bursting under pressure. • Not all solutes move at the same rate. The mass flow theory suggests that they would move at the same rate. • Sucrose is delivered at the same rate to all regions rather than going faster to the ones with the lowest sucrose concentration which the mass flow theory would suggest.

Ringing experiments

• The outer layers of the stem are removed. The region of the stem above the missing ring of tissue will swell. • Samples of the liquid that has accumulated in the swollen region are found to be rich in sugars and other dissolved organic substances. • Some non-photosynthetic tissues in the region below the ring are found to wither or die, while those above the ring continue to grow. • If the xylem were the tissue responsible for translocation sugars would not accumulate above the ring nor would tissues below it die.

Haemoglobin molecules

• The quaternary structure of haemoglobin molecules consists of four polypeptide chains. • Each chain is associated with a haem group and Fe²⁺ ion. • A single haemoglobin molecule can carry four oxygen molecules.

Oxygen dissociation curve

• The shape of the haemoglobin molecules makes it difficult for the first oxygen molecule to bind to one of the sites on its four polypeptide subunits. At low oxygen concentrations, little oxygen binds to haemoglobin. • The binding of the first oxygen molecule changes the quaternary structure of the haemoglobin molecule which makes it easier for the other subunits to bind to oxygen. • It takes a smaller increase in the partial pressure of oxygen to bind the second oxygen molecule. Where the curve is steep a small change in pO₂ causes a big change in the affinity of haemoglobin for oxygen. • With the majority of the binding sites occupied after the third oxygen molecule, it is less likely that a single oxygen molecule will find an empty site to bind to.

Capillary structure

• The walls consist of mostly of the endothelium lining layer. This makes capillaries very thin and therefore reduces the diffusion pathway for materials. • Capillaries are numerous and highly branched which provides a large surface area for exchange. • The narrow diameter and permeate tissues which means that no cell is far from a capillary and therefore there is a short diffusion pathway. • Red blood cells are squeezed flat in the narrow lumen, bringin them closer to the cells to which they supply oxygen. This further reduces the diffusion pathway. • There are spaces between the endothelial cells that allow white blood cells to escape in order to deal with infections within tissues.

Evidence supporting translocation in the phloem

• When the phloem is cut an organic solution flows out. • Plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in the phloem. • The removal of a ring of the phloem from around the whole circumference of a stem leads to the accumulation of sugars above the ring.