Exchange

1. It is easier and more accurate to measure mass when shapes are irregular.
2. It causes less distress to the animal.
3. Fewer measurements and calculations are involved.

1. It must have a short diffusion pathway to cells.
2. It must have a surface permeable to these resources.

1. As SA:V increases, metabolic rate increases.
2. As a higher SA:V means more heat will be lost from the surface of the organism.
3. This means a higher rate of respiration is needed to release heat to maintain body temperature.

1. Tracheoles have thin walls so short diffusion distance to cells.
2. There are a large number of tracheoles and they are highly branched, causing a large surface area for diffusion and a short diffusion distance to cells.
3. The tracheoles enter muscle tissue, meaning diffusion can occur directly into cells, creating a short diffusion path.
4. Tracheae provide tubes full of air for fast diffusion into insect tissues.
5. Tracheal fluid in the end of the tracheoles that moves out into tissues during exercise so faster diffusion through the air to the gas exchange surface.Note: this also causes a large surface area for gas exchange. I’d also mention the role lactic acid plays in this process.
6. The body can be moved by muscles to move air so it maintains a diffusion / concentration gradient for oxygen / carbon dioxide.Note: I’d mention that it is the thorax and abdomen that does this pumping. I also believe a similar point could be made by air sacs maintaining a diffusion gradient for oxygen when the spiracles are closed.Note: references to thin membranes were not accepted and you were capped at 2 marks if you mentioned blood.

1. Oxygen is used in aerobic respiration in cells.
2. This establishes an oxygen concentration gradient.
3. This causes oxygen to diffuse in.

1. The chitin exoskeleton prevents evaporation.
2. The spiracles can close to prevent water loss.
3. The hairs around the spiracles prevent water loss.

1. It contains many, highly folded lamellae, creating a large surface area for gas exchange.
2. The thin, flattened epithelium of the lamellae creates a short diffusion pathway between the water and blood.
3. They have a dense capillary network, constantly supplying the lamellae with deoxygenated blood and removing oxygenated blood, maintaining a high concentration gradient.
4. The countercurrent system maintains a concentration gradient along the whole length of the lamellae. Bonus point: Fish maintain a constant flow of water through their mouths and into the gills, maintaining a constant oxygen concentration gradient across the gill filaments.

1. Water and blood flow in opposite directions.Note: this is basically always the first mark.
2. Blood is always passing water with a higher oxygen concentration.Note: this is a rare mark but appeared on one MS so mention it.
3. This maintains a concentration gradient throughout the entire length of the gill filament, meaning diffusion occurs throughout the length of the gill filament.Note: the ‘entire length of the gill filament’ is a key piece of wording you must include.

1. More active fish have a higher respiratory rate.
2. This means they use more oxygen per unit mass.

1. Fish have 1 atrium and 1 ventricle, whereas mammals have 2 atria and 2 ventricles.
2. In fish, blood does not return to the heart after being oxygenated, but it does in mammals.
3. Fish only have one vein carrying blood towards the heart, whereas mammals have the vena cava and pulmonary vein.
4. Fish have a single circulatory system, mammals have a double circulatory system.Note: All of these points basically describe the same thing, but each has its own mark in the question. This shows the importance of breaking down concepts and redescribing them as much as possible in the exam to make sure you are getting as many marks as possible.

1. The diaphragm contracts and flattens.
2. The external intercostal muscles contract and the internal intercostal muscles relax.
3. This increases thorax volume,
4. Decreasing the pressure inside of the lungs.
5. This causes air to move into the lungs down a pressure gradient.Note: It is key you say ‘down’ a pressure gradient, not just ‘along’ one.

1. It will mean less CO₂ gets exhaled out of the lungs, so more will remain in the lungs.
2. This causes a reduced concentration gradient between the alveoli and blood.
3. This causes slower movement of CO₂ out of the blood, so more remains in the blood.

1. The muscular walls of the brionchioles contract.
2. The walls of the brionchioles also secret more mucus.
3. This reduces the diameter of the airways.
4. Therefore airflow is reducedNote: Not many students know you actually need to know this.

1. It travels from the trachea, to the bronchi, then bronchioles.
2. Down a pressure gradient.
3. It then travels across the alveolar epithelium.
4. Into the capillary endothelium.
5. Down a diffusion gradientNote: always say ‘down’ a gradient, not just ‘along’ or ‘via’.

1. It is one cell thick.
2. It is made of flattened cells.
3. It is permeable, allowing diffusion of oxygen and carbon dioxide.Note: Do NOT say it has a thin cell membrane/ wall, this probably isn’t true and wouldn’t be the main cause of the short diffusion distance even if it were.

1. It would decrease the rate as the diffusion distance would be longer.
2. This causes a slower rate of diffusion.Note: I mainly made this note to highlight the ‘slower’ wording. When explaining why the rate of diffusion/ gas exchange is lower, always say ‘slower diffusion’ or ‘slower movement of gas out the blood’.

1. There would be less surface area available for gas exchange.
2. There would be a longer diffusion distance due to scar tissue.

1. They hydrolyse peptide bonds.
2. Endopeptidases act in the middle of the polypeptide.
3. Exopeptidases act on the ends of the polypeptide (producing dipeptides or amino acids)
4. Dipeptidases act on dipeptides, producing single amino acids.Note: Don’t forget to mention the role of dipeptidases.Note: I’d also mention the endopeptidases creating a large surface area for the exopeptidases to act on.

1. The lipase enzyme.
2. Digests ester bonds between glycerol and fatty acids.
3. In hydrolysis reactions via the addition of water.

1. It creates smaller droplets of lipids so increases surface area.
2. This causes a faster rate of lipase activity and hydrolysis.

1. They contain bile salts, fatty acids, and monoglycerides. (This wasn’t really related to the question but was on the mark scheme, so I think it’s probably a good thing to always mention in questions about micelles).
2. They make fatty acids and monoglycerides soluble in water, allowing them to be transported through the digestive system.
3. They then break down close to the cells of the ileum, maintaining a higher concentration of the substance outside the ileum lining, allowing the monoglycerides and fatty acids to diffuse (via simple diffusion) into the cell.

1. The fatty acids and monoglycerides are absorbed into the ileum via simple diffusion.
2. Triglycerides are reformed in cells.
3. Golgi vesicles then move them to the cell membrane.Note: sometimes they are also modified inside the golgi apparatus into things like glycolipids.

1. The golgi apparatus modifies and processes the triglycerides.
2. By combining them with proteins.
3. They are then packaged for release for exocytosis in golgi vesicles.

1. Exopeptidases hydrolyse peptide bonds at the ends of the polypeptides.
2. Endopeptidases hydrolyse the internal peptide bonds within a polypeptide, creating more ‘ends’ and surface area for the exopeptidases to work on.

1. The walls of the atria contract, decreasing their volume and increasing the pressure of the blood inside of them.
2. This causes the pressure behind the atrio-ventricular valves to be greater than the pressure in front of them, causing them to open, causing blood to flow from the atria into the ventricles.
3. This causes the volume of blood inside the ventricles to increase and the pressure of the blood to increase slightly too.

1. The walls of the ventricles contract, decreasing their volume and increasing the pressure of the blood inside of them.
2. This causes the pressure of the ventricles to rise above the pressure in the atria, causing the atrio-ventricular valves to close.
3. The pressure of the ventricles also rises above the pressure in the arteries (aorta or pulmonary artery), causing the semi-lunar valves to open and blood to be pumped out of the arteries.

1. Neither the atria nor ventricles contract.
2. The pressure in the ventricles falls below that of the pressure in the arteries, causing the semi-lunar valves to close.
3. As blood flows from the veins into the atria, their pressure increases to above that of the ventricles, causing the atrioventricular valves to open and blood to flow passively from the atria into the ventricles.Note: so technically the atrioventricular valves open during diastole, but I’d also talk about them opening during atrial systole too.

1. The ventricles have thicker, more muscular walls.
2. This makes their contractions stronger.

1. Oxygenated and deoxygenated blood mixing.
2. Which causes there to be a lower concentration of oxygen present in the blood being pumped around the body

1. It travels through the renal vein to the vena cava to the right atrium.
2. Then through the atrio-ventricular valve to the right ventricle.
3. Then through the semi-lunar valve to the pulmonary artery to the lungs then to the pulmonary vein.
4. Then from the left atrium to the left ventricle through the atrio-ventricular valve.
5. Then to the aorta through the semi-lunar valve, then to the coronary artery.

1. The thickness of the walls.
2. The width of the lumen.

Note: arteries will have thicker walls and narrower lumen than veins.

1. Its elastic tissue stretches under pressure and recoils when pressure decreases, evening out and maintaining blood pressure.
2. Its thick walls allow it to withstand the high pressure created by the heart.
3. Its narrow lumen helps it to maintain high blood pressure.
4. Its endothelium is smooth to reduce friction.

1. Its elastic tissue allows stretching and recoil to smooth out flow of blood and maintain pressure, as it stretches when the ventricles contract and recoils when the ventricle relaxes.
2. Its thick, muscular wall withstands high pressure and prevents bursting.
3. Its protein coat prevents the walls splitting.
4. Its smooth endothelium reduces friction.
5. The semilunar valve prevents backflow.

1. It smooths blood flow and maintains blood pressure.
2. By stretching as a result of high blood pressure, then recoiling.

1. Muscle in their walls can contract to reduce the diameter of the lumen by constricting it.
2. This allows them to control the volume of blood flow to specific organs.

1. They contain valves to prevent the backflow of blood.
2. They contain wide lumen to allow a high volume of blood flow. The wide lumen also decreases resistance to blood flow by causing less (relative) friction on the blood.
3. Their walls are thin with little muscle and elastic tissue as they do not carry blood at high pressures so the thick walls and elastic recoil aren’t needed.

1. Their endothelium is one cell thick and is the only tissue in their walls, creating a short diffusion distance.
2. Their walls contain pores, allowing small molecules to be forced out to create tissue fluid.
3. Their lumen is very narrow, slowing blood flow, allowing more time for diffusion.

1. A larger lumen
2. Causes less friction in the blood vessels.

1. Greater blood supply to the tissue.
2. Brings more oxygen and glucose for respiration.
3. And more amino acids for protein synthesis.
4. This allows for a faster rate of cell repair and mitosis.

1. At low partial pressures, saturation doesn’t increase much with increased pO2 as the first oxygen molecule is quite slow to bind to haemoglobin.
2. When the first oxygen molecule binds, it distorts the haemoglobin molecule and causes conformational changes in its shape, increasing haemoglobin’s affinity for oxygen and making it easier for the second and third oxygen molecules to bind, causing saturation to rise steeply with increased pO2. This is called positive cooperation.
3. At high pO2, the saturation stops increasing rapidly as pO2 increases and the curve levels off because it is difficult for the fourth oxygen molecule to bind to haemoglobin.

1. The binding of the first oxygen changes the tertiary/ quaternary structure of haemoglobin.
2. This uncovers another binding site/ another haem group for oxygen to bind to.

1. Increased CO₂ from respiration decreases the pH of the blood.
2. This causes oxygen to unload more readily due to its decreased affinity for haemoglobin.
3. This provides more oxygen for aerobic respiration at the muscles.Note: always use the wording of ‘more’ for these types of questions (and any other type of question where something is already happening and the change allows it to happen more).

1. Add a scale.Note: don’t say ‘add magnification’ if it is not a microscope image.
2. Add labels/ annotations.
3. Do not shade.
4. Draw all parts to the same scale.
5. Only use single lines and ensure they are all connected.

1. Carry by the handle with the blade pointing downwards or protected.
2. Make sure it is sharp.
3. Cut away from yourself on a hard surface.

1. Carry sharp instruments by holding their handle and carry them facing downwards.
2. Disinfect instruments and surfaces.
3. Wash hands with soap.Note: always say 'with soap'.

1. A high hydrostatic pressure at the arterial end of the capillary forces small molecules like water, ions, and glucose out.
2. Plasma proteins remain in the blood as they are too large to exit the capillary.Note: It is important you get the idea that the proteins are ‘too large’ to exit in there.
3. These plasma proteins create a low (more negative) water potential inside of the blood at the venule end of the capillary.
4. This causes water to reenter the blood via osmosis.
5. Excess tissue fluid is drained by the lymphatic system.Note: It then eventually returns to the blood.Note 2: Always mention ‘excess’ tissue fluid being drained.

1. A high blood pressure creates a high hydrostatic pressure inside the capillary.
2. This increases the outward pressure at the arterial end of the capillary and reduces the inwards pressure at the venule end of the capillary.Note: Notice the terminology of ‘outward’ and ‘inward’ pressure - it is vital you use this terminology when talking about why tissue fluid is formed/ reformed.
3. This means more tissue fluid is formed and less is reabsorbed.

1. Blood plasma is inside the capillaries and tissue fluid is outside.
2. Blood plasma contains large proteins, tissue fluid doesn’t.
3. Blood plasma often has a higher concentration of glucose than tissue fluid.

1. They contain many stomata which allow gas diffusion.
2. They contain air spaces in the mesophyll which creates a large surface area for diffusion in and out of cells.
3. They have a large SA:V which increases rate of diffusion.
4. They have thin leaves which allows a short diffusion distance.

1. Water is lost from the leaf due to evaporation of water through the stomata.
2. This lowers the water potential of the leaf cells.
3. The water potential gradient creates tension, pulling water up the xylemNote: Always include the idea of a water potential gradient.
4. Water molecules cohere together by hydrogen bonds.
5. Forming a continuous water column.
6. The water molecules adhere to the walls of the xylem.

1. More stomata open.
2. Increase in transpiration/ more water gets evaporated.
3. Increase in tension.
4. More water pulled up xylem due to cohesion tension.

1. Fewer leaves means fewer stomata.
2. This causes less evaporation and transpiration.
3. This means less tension is created.
4. This means water uptake is slower.

1. Hairs trap water vapour, decreasing the water potential gradient.
2. Stomata in pits cause water vapour to be trapped, decreasing the water potential gradient.
3. Their curled leaves cause water vapour to be trapped, decreasing the water potential gradientNote: it was important you talked about ‘water vapour’ and the ‘decreased water potential gradient’ for all three of these points.
4. They have a thick waxy cuticle, increasing the distance for evaporation/ diffusion of water.
5. They have fewer stomata, reducing evaporation out of the leaf cells.

1. The sieve plates in between the sieve tubes lack nuclei, allowing the solutes to flow freely through them.
2. The sieve tubes have few organelles and their cytoplasms are at the edge, allowing more room for easier flow.
3. They have thick walls which resist pressure.

1. At the source, sucrose is actively transported into the phloem.Note: I’d always say sucrose, as it was required on one mark scheme.
2. By the companion cells.
3. This lowers the water potential inside the sieve tube, causing water to enter via osmosis (from the xylem)
4. This causes an increase in (hydrostatic) pressure at the source which causes mass movement and mass flow from the source to the sink down a pressure gradient.
5. The sugars are then used for respiration or converted into starch at the sink (roots), where they are unloaded via active transport.Note: I would also mention this increasing water potential, causing water to exit via osmosis, causing pressure to decrease, maintaining a pressure gradient from source to sink.

1. No growth of roots/ no respiration in cells at the sink.Note: therefore if the phloem are removed and roots still grow at the sink, this is evidence against the hypothesis. Side note: Always state what the source and sink is in these types of questions.
2. A bulging above where the phloem are removed due to a build up of water and solutes.Note: therefore an absence of this is evidence against the hypothesis.

1. The movement of the solutes down the stem could be due to gravity.
2. Often the role of osmosis cannot be supported/ observed as there is no water flowing through the xylem in the stems.
3. There is no photosynthesis producing sugars at the source, meaning the way translocation would naturally occur isn’t being accurately replicated.

1. Seal the joints (with vaseline) to ensure its airtight.
2. Cut the shoot underwater (o prevent air entering.
3. Cut the shoot at a slant to maximise the surface area for water uptake.
4. Dry off the leaves to allow water to evaporate out of them.
5. Insert the plant into the apparatus underwater to prevent air entering.
6. Ensure no air bubbles are present.
7. Move the air bubble to the start position with the syringe.

1. To prevent air entering.
2. So a continuous water column can form.

1. The jelly is waterproof, meaning it prevents water from leaking out and exiting in any way that isn’t transpiration.
2. It all ensures all joints are sealed, preventing air from entering and creating unwanted air bubbles.

1. Water is used for support by the plant.
2. Water is used in photosynthesis.
3. Water is produced during respiration.
4. Water is used in hydrolysisNote: In the context of the potometer practical, it could also be because the apparatus aren’t sealed.