TOPIC 3 MARK SCHEME SHIT

Question 1

Explain why hydrostatic pressure falls from arteriole end to venule end of capillary (1)

Answer 1

Loss of water/loss of fluid/friction against capillary lining

Question 2

Explain why water potential of blood plasma is more negative at venule end than arteriole end of capillary (3)

Answer 2

1. Water has left the capillary
2. Proteins in blood too large to leave capillary
3. Increasing/giving higher conc of blood proteins and thus wp

Question 3

Stomata close when lights are off: explain advantage of this to the plant (2)

Answer 3

1. Water is lost through stomata
2. Prevents/reduces water loss
3. Maintains water content of cells

Question 4

Describe how proteins are digested in the human gut (4)

Answer 4

1. Hydrolysis of peptide bonds
2. Endopeptidases break polypeptides into smaller peptide chains
3. Exopeptidases remove terminal amino acids
4. Dipeptidases hydrolyse dipeptides into amino acids

Question 5

Describe the advantage of the Bohr effect during intense exercise (2)

Answer 5

1. Increases dissociation of oxygen
2. For aerobic respiration at the tissues/muscles/cells

Question 6

The oxygen dissociation curve for haemoglobin shifts to the right during vigorous exercise. Explain the advantage of this shift. (3)

Answer 6

1. lowER affinity for oxygen/releases MORE oxygen/oxygen is released quickER/oxygen dissociates/unloads MORE readily
2. to muscles/tissues/cells
3. for high/rapid respiration

Question 7

Explain how the shape of an [animal] is an adaptation to living in a cold environment (2)

Answer 7

1. small SA:V ratio
2. reduces heat loss/more heat retained

Question 8

Describe how oxygen in the air reaches capillaries surrounding alveoli in the lungs. Details of breathing are NOT required (4)

Answer 8

1. Trachea and bronchi and bronchioles
2. Down pressure gradient
3. Down diffusion gradient
4. Across alveolar epithelium
5. Across capillary endothelium/epithelium

Question 9

One theory of translocation states that organic substances are pushed from a high pressure in the leaves to a lower pressure in the roots. Describe how a high pressure is produced in the leaves (3)

Answer 9

1. Water potential becomes lower
2. Water enters phloem by osmosis
3. Increased volume (of water) causes increased pressure

Question 10

Describe and explain the mechanism that causes forced expiration (4)

Answer 10

1. Contraction of internal intercostal muscles
2. Relaxation of diaphragm muscles/of external intercostal muscles
3. Causes decrease in volume of chest/thoracic cavity
4. Air pushed down pressure gradient

Question 11

Explain how an asthma attack caused the drop in the mean FEV (forced expiratory volume - vol of air breathed out in 1 sec). (4)

Answer 11

1. Muscle walls of bronchi/bronchioles contract
2. Walls of bronchi/bronchioles secrete more mucus
3. Diameter of airways reduced
4. Flow of air reduced

Question 12

Why are mitochondria in larger unicellular organisms closer to the cell-surface membrane, compared to smaller unicellular organisms where mitochondria are distributed evenly throughout the cytoplasm? (2)

Answer 12

1. Large(r) cells have small(er) surface area to volume ratio
2. Longer for oxygen to DIFFUSE (to mitochondria)

Question 13

Explain the advantage for larger animals of having a specialised system that facilitates oxygen uptake (2)

Answer 13

1. Large(r) organisms have a small(er) surface area:volume ratio OR converse
2. Overcomes long DIFFUSION pathway/faster DIFFUSION

Question 14

Why do fish obtain oxygen from systems developed to the outside of the body (protruding), while humans obtain oxygen from systems developed to the inside of the body (alveoli)? (2)

Answer 14

1. Water has low(er) oxygen partial pressure/concentration (than air)
2. So (system on outside) gives large surface area (in contact with water) OR so (system on outside) reduces diffusion distance (between water and blood)

Question 15

Use your knowledge of surface area to volume ratio to explain the higher metabolic rate of a mouse compared to a horse (3)

Answer 15

Mouse:
1. (Smaller so) larger surface area to volume ratio
2. More/faster heat loss (per gram/in relation to body size)
3. (Faster rate of) respiration/metabolism releases heat

Question 16

Describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels. (5)

Answer 16

1. Micelles contain bile salts and fatty acids/monoglycerides
2. Make fatty acids/monoglycerides (more) soluble (in water)
3. Fatty acids/monoglycerides absorbed by diffusion
   - REJECT FACILITATED DIFFUSION
   - ignore if micelles themselves are being absorbed
4. Triglycerides (re)formed (in cells)
   - accept chylomicrons form
5. Vesicles move to cell membrane
   - accept exocytosis for “vesicles move”

Question 17

The photograph shows a fire-breather creating a ball of fire. Fire-breathers do this by blowing a fine mist of paraffin oil onto a flame. Some of this mist can be inhaled and may eventually lead to fibrosis. People who have been fire-breathers for many years often find they cannot breathe out properly. Explain why. (2)

Answer 17

1. Loss of elasticity / elastic tissue / increase in scar tissue;
   Accept elastin
2. Less recoil;

Question 18

Describe and explain how the countercurrent system leads to efficient gas exchange across the gills of a fish. (3)

Answer 18

1. Water and blood flow in opposite directions
2. Maintains concentration/diffusion gradient/equilibrium not reached/water always next to blood with a lower concentration of oxygen
3. Along whole/length of gill/lamellae

Question 19

Give two features of leaves of xerophytes (excluding SA, # of stomata and leaf thickness). Explain how each features reduces water loss. (2)

Answer 19

1. Thick(er) cuticle
   so
   increase in diffusion
   distance / slower (rate of) diffusion;
   Feature and explanation needed for each mark.
   Reject other features not related to leaves.
   Reject features related to water storage.
   ‘Cuticle’ alone is insufficient (all leaves have a cuticle).
   Reject suggestion of ‘less’ diffusion, for idea of ‘slower diffusion’, an idea of rate is required.
2. Hairs on leaves
   so
   reduction in air movements / increase in humidity / decrease in water potential gradient;
3. Curled leaves
   so
   reduction in air movements / increase in humidity / decrease in water potential gradient;
4. Sunken stomata
   so
   reduction in air movements / increase in humidity / decrease in water potential gradient.

Question 20

Describe and explain one feature of the alveolar epithelium that makes the
epithelium well adapted as a surface for gas exchange. Do not refer to surface area or moisture in your answer. (2)

Answer 20

Mark in pairs: 1 and 2 OR 3 and 4
1. Flattened cells
OR
Single layer of cells;
Reject thin cell wall/membrane
Accept thin cells
Accept ‘one cell thick’
2. Reduces diffusion distance/pathway;
3. Permeable;
4. Allows diffusion of oxygen/carbon dioxide;
Ignore gas exchange

Question 21

Tidal volume is the volume of air inhaled and exhaled during a single breath when a person is resting. The tidal volume in a person with emphysema is reduced compared with the tidal volume in a healthy person.
Suggest and explain how a reduced tidal volume affects the exchange of carbon dioxide between the blood and the alveoli. (3)

Answer 21

1. Less carbon dioxide exhaled/moves out (of lung)
   OR
   More carbon dioxide remains (in lung);
2. (So) reduced diffusion/concentration gradient (between blood and alveoli);
3. Less/slower movement of carbon dioxide out of blood
   OR
   More carbon dioxide stays in blood;

Question 22

Describe and explain the mechanism that causes lungs to fill with air. (3)

Answer 22

1. Diaphragm (muscle) contracts and external intercostal muscles contract;
   Ignore ribs move up and out
2. (Causes volume increase and) pressure decrease;
3. Air moves down a pressure gradient
   Ignore along
   OR
   Air enters from higher atmospheric pressure;

Question 23

Particulate matter is solid particles and liquid particles suspended in air. Polluted air contains more particulate matter than clean air.
A high concentration of particulate matter results in the death of some alveolar epithelium cells. If alveolar epithelium cells die inside the human body they are replaced by non-specialised, thickened tissue.
Explain why death of alveolar epithelium cells reduces gas exchange in human lungs. (3)

Answer 23

1. Reduced surface area;
2. Increased distance for diffusion;

Accept description of efficient gas exchange in
healthy alveolar epithelium as long as reference made to the damaged tissue changing this.

3. Reduced rate of gas exchange;

Question 24

Use your knowledge of gas exchange in leaves to explain why plants grown in soil with very little water grow only slowly. (2)

Answer 24

1. Stomata close;
2. Less carbon dioxide (uptake) for less photosynthesis/glucose production;
   ‘Less’ only required once.
   Reject ‘no photosynthesis’ but accept ‘carbon dioxide can’t enter so less photosynthesis’.
   Ignore oxygen for respiration but reject oxygen for photosynthesis.
   Ignore less water for photosynthesis.
   Accept only correct chemical formulae.
   For ‘glucose’ accept named product of photosynthesis eg triose phosphate, TP, amino acid, lipid.

Question 25

Explain how one feature of an alveolus allows efficient gas exchange to occur. (2)

Answer 25

1. (The alveolar epithelium) is one cell thick; Reject thin membrane 2. Creating a short diffusion pathway / reduces the diffusion distance;

Question 26

Describe the gross structure of the human gas exchange system and how we breathe in and out. (6)

Answer 26

1. Named structures – trachea, bronchi, bronchioles, alveoli; Reject mp1 if structures from other physiological systems are named but award mp2 if the correct structures are in the correct order. 2. Above structures named in correct order OR Above structures labelled in correct positions on a diagram; Reject mp1 if structures from other physiological systems are named but award mp2 if the correct structures are in the correct order. 3. Breathing in – diaphragm contracts and external intercostal muscles contract; 4. (Causes) volume increase and pressure decrease in thoracic cavity (to below atmospheric, resulting in air moving in); For thoracic cavity accept ‘lungs’ or ‘thorax’. Reference to ‘thoracic cavity’ only required once. 5. Breathing out - Diaphragm relaxes and internal intercostal muscles contract; Accept diaphragm relaxes and (external) intercostal muscles relax and lung tissue elastic (so recoils). 6. (Causes) volume decrease and pressure increase in thoracic cavity (to above atmospheric, resulting in air moving out); For thoracic cavity accept ‘lungs’ or ‘thorax’. Reference to ‘thoracic cavity’ only required once. If idea of thoracic cavity is missing or incorrect, allow ECF for mark point 6.

Question 27

Explain three ways in which an insect’s tracheal system is adapted for efficient gas exchange. (3)

Answer 27

1. Tracheoles have thin walls so short diffusion distance to cells; 2. Highly branched / large number of tracheoles so short diffusion distance to cells; 3. Highly branched / large number of tracheoles so large surface area (for gas exchange); 4. Tracheae provide tubes full of air so fast diffusion (into insect tissues); 5. 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; OR Fluid in the end of the tracheoles that moves out (into tissues) during exercise so larger surface area (for gas exchange); 6. Body can be moved (by muscles) to move air so maintains diffusion / concentration gradient for oxygen / carbon dioxide; 1. Do not accept unqualified references to thin membranes. Max 2 if any reference to blood Ignore references to spiracles 5. Accept ‘water’ for fluid. Accept ‘cells’ and ‘tissues’ as interchangeable words.

Question 28

Explain two ways in which the structure of fish gills is adapted for efficient gas exchange. (2)

Answer 28

1. Many lamellae / filaments so large surface area; 2. Thin (surface) so short diffusion pathway; 1 & 2 must each have a feature and a consequence

Question 29

Explain how the counter current mechanism in fish gills ensures the maximum amount of the oxygen passes into the blood flowing through the gills. (3)

Answer 29

1. Water and blood flow in opposite directions; 2. Blood always passing water with a higher oxygen concentration; 3. Diffusion gradient maintained throughout length (of gill) OR Diffusion occurs throughout length of gill OR If water and blood flowed in same direction equilibrium would be reached;

Question 30

Describe the role of micelles in the absorption of fats into the cells lining the ileum. (3)

Answer 30

1. Micelles include bile salts and fatty acids; Ignore other correct components of micelles. 2. Make the fatty acids (more) soluble in water; For 'fatty acids' accept fats / lipids. 3. Bring/release/carry fatty acids to cell/lining (of the ileum); For ‘fatty acids’ accept fats/lipids. 4. Maintain high(er) concentration of fatty acids to cell/lining (of the ileum); 5. Fatty acids (absorbed) by diffusion; Reject if absorbed by facilitated diffusion Ignore if micelles themselves are being absorbed. Ignore references to monoglycerides.

Question 31

Explain the advantages of lipid droplet and micelle formation. (3)

Answer 31

1. Droplets increase surface areas (for lipase / enzyme action); 2. (So) faster hydrolysis / digestion (of triglycerides / lipids); 3. Micelles carry fatty acids and glycerol / monoglycerides to / through membrane / to (intestinal epithelial) cell; 1. Context is important 1. Reject micelles increase surface area 2. Ignore ‘breakdown’ 3. Ignore ‘small enough’ 3. Accept description of membrane 3. Reject any movement through membrane proteins

Question 31

How is the Golgi involved in the absorption of lipids? (3)

Answer 31

1. Modifies / processes triglycerides; 2. Combines triglycerides with proteins; 3. Packaged for release / exocytosis OR Forms vesicles; Ignore ‘processes and packages’ unqualified 1. Reject synthesises triglycerides 2. Accept ‘forms / are lipoproteins’

Question 32

Explain how the pressure in the ventricle is related to blood flow in the aorta (2)

Answer 32

1. Ventricle pressure rises THEN blood starts to flow into aorta because pressure causes (aortic/semilunar) valve to open 2. Ventricle pressure starts to fall SO blood falls

Question 33

Explain how pressure in the ventricle is related to thickness of the ventricle wall (2)

Answer 33

1. Thickness of wall increases BECAUSE ventricle (wall) contracts 2. Contraction CAUSES increase in pressure

Question 34

Explain how hypertension leads to accumulation of tissue fluid (3)

Answer 34

1. High blood pressure = high hydrostatic pressure 2. Increases outward pressure from (arterial) end of capillary/reduces inward pressure at (venule) end of capillary 3. (So) more tissue fluid formed/less tissue fluid is reabsorbed. Allow lymph system unable to drain tissues fast enough

Question 35

Other than causing slow blood flow, explain two advantages of capillaries being narrow (2)

Answer 35

1. Short pathway / short distance between blood and outside of capillary 2. Large SA (of blood) in contact with walls of capillaries 3. Fast exchange / fast diffusion / fast osmosis

Question 36

What factor limits the minimum internal diameter of the lumen of a capillary?

Answer 36

Width / size / diameter of blood cell

Question 37

Suggest why the aorta has a greater rise and fall in blood pressure compared to small arteries (3)

Answer 37

1. Aorta is close / directly linked to the heart / ventricle / pressure is higher / is high 2. (Aorta has) elastic tissue 3. (Aorta has) stretch / recoil

Question 38

Explain how cardiac output can remain the same even when resting heart rate has decreased (2)

Answer 38

1. cardiac output = stroke volume x heart rate 2. stroke volume increases

Question 39

Explain how water from tissue fluid is returned to the circulatory system (4)

Answer 39

1. (Plasma) proteins remain 2. (Creates) water potential gradient / reduces water potential (of blood) 3. Water moves (to blood) via osmosis 4. Returns (to blood) by lymphatic system

Question 40

Explain how an arteriole can reduce the blood flow into capillaries (2)

Answer 40

1. Muscle contracts 2. Constricts / narrows arteriole / lumen

Question 41

Which blood vessel carries blood at the lowest pressure?

Answer 41

VENA CAVA NOT CAPILLARIES

Question 42

An increase in exercise intensity produces an increase in volume of CO2 produced. However, pCO2 breathed out in experiment didn't show a large increase. Suggest ONE physiological change that would cause this, and explain how it would allow for the removal of the increase in vol CO2 produced. (2)

Answer 42

1. Increase in breathing (rate) 2. Similar/same pCO2 per breath, but more breaths OR 1. Increase in tidal volume 2. Similar/same pCO2 per breath, but increased volume per breath

Question 43

EPO is a performance enhancing drug that increases % of RBCs in blood. Suggest how overuse of EPO can increase risk of heart attack (2)

Answer 43

1. EPO causes blood to thicken 2. Could block coronary arteries/slows blood flow/could cause clots

Question 44

Describe & explain effect of increasing [CO2] on dissociation of oxyhaemoglobin (2)

Answer 44

1. Increases/more oxygen dissociation/unloading OR decreases haemoglobin's affinity for O2 2. (By) decreasing (blood) pH/increasing acidity

Question 45

Explain how AV valves maintain a unidirectional flow of blood (2)

Answer 45

1. Pressure in atrium higher than in ventricle causing valve to open 2. Pressure in ventricle is higher than in atrium causing valve to close

Question 46

Explain why binding of one molecule of oxygen to haemoglobin makes it easier for a 2nd oxygen molecule to bind (2)

Answer 46

1. Binding of first oxygen changes tertiary / quaternary (structure) of haemoglobin 2. Create / leads to / uncovers second / another binding site

Question 47

Explain the role of the heart in the formation of tissue fluid (2)

Answer 47

1. Contraction of ventricle(s) produces HIGH blood/hydrostatic pressure 2. (This) forces water (and some dissolved substances) out (of blood capillaries)

Question 48

Describe the mass flow hypothesis for the mechanism of translocation in plants (4)

Answer 48

1. In source/leaf, sugars actively transported into phloem 2. By companion cells 3. Lowers water potential of sieve cell/tube and water enters by osmosis 4. Increase in pressure causes mass movement (towards sink/root) 5. Sugars used/converted in root for respiration for storage (accept starch)

Question 49

One theory of translocation states that organic substances are pushed form a high pressure in the leaves to a lower pressure in the roots. Describe how a high pressure is produced in the leaves (4)

Answer 49

1. Sucrose actively transported 2. Water potential becomes lower 3. Water enters phloem by osmosis 4. Increased volume (of water) causes increased pressure

Question 50

Use your understanding of transpiration and mass flow to explain why phloem pressure is reduced during the hottest part of the day (3)

Answer 50

1. High (rate of) transpiration/evaporation 2. Water lost through stomata / (high) tension in xylem 3. (Causes) less water movement from xylem to phloem / insufficient WP in phloem to draw water from xylem

Question 51

Explain why water moves up the xylem (3)

Answer 51

1. Water evaporates/is transpired 2. Water potential gradient/lower WP creates tension/pulls up water Accept negative pressure for tension 3. Hydrogen bonds/cohesion/adhesion maintains column

Question 52

Describe the cohesion-tension theory of water transport in the xylem (5)

Answer 52

1. Water lost from leaf due to transpiration/evaporation/diffusion from mesophyll/leaf cells 2. Lowers WP of mesophyll/leaf cells 3. Water pulled up xylem 4. Water molecules cohere/“stick” together by hydrogen bonds 5. (forming continuous) water column 6. Adhesion of water to walls of xylem