4/5/6 Mark Qs Organisms Exchange Substances With Their Environment

Question 1

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

Answer 1

1. Named structures – trachea, bronchi, bronchioles, alveoli;
   a. 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;
   a. Reject mp1 if structures from other physiological systems are named but award mp2 if the correct structures are in the correct order.
   b. Breathing in – diaphragm contracts and external intercostal muscles contract;
3. (Causes) volume increase and pressure decrease in thoracic cavity (to below atmospheric, resulting in air moving in);
   a. For thoracic cavity accept ‘lungs’ or ‘thorax’.
   b. Reference to ‘thoracic cavity’ only required once.
4. Breathing out - Diaphragm relaxes and internal intercostal muscles contract;
   a. Accept diaphragm relaxes and (external) intercostal muscles relax and lung tissue elastic (so recoils).

Question 2

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

Answer 2

1. Micelles contain bile salts and fatty acids/monoglycerides; Ignore other correct components of micelles
2. Make fatty acids/monoglycerides (more) soluble (in water) OR Bring/release/carry fatty acids/monoglycerides to cell/lining (of the iluem) OR Maintain high(er) concentration of fatty acids/monoglycerides to cell/lining (of the ileum);
   a. Accept lipid/fat for fatty acid/ monoglyceride
3. Fatty acids/monoglycerides absorbed by diffusion;
   a. Reject if absorbed by facilitated diffusion Ignore if micelles themselves are being absorbed
4. Triglycerides (re)formed (in cells);
   a. Accept chylomicrons form
5. Vesicles move to cell membrane;
   a. Accept exocytosis for ‘vesicles move’

Question 3

Describe the role of enzymes in the digestion of proteins in a mammal. (4)

Answer 3

1. (Reference to) hydrolysis of peptide bonds;
2. Endopeptidase act in the middle of protein/polypeptide OR Endopeptidase produces short(er) polypeptides/ increase number of ends;
3. Exopeptidases act at end of protein/polypeptide OR Exopeptidase produces dipeptides/amino acids;
4. Dipeptidase acts on dipeptide/between two amino acids OR Dipeptidase produces (single) amino acids;
   o Accept chain/chain of amino acids/peptide for polypeptide
   o Accept digest/breakdown/ break for ‘act’
   o Mark points 2, 3 and 4 reject answers where substrate or product is incorrect eg ‘Endopeptidase produces dipeptides’
   o Ignore references to source and location of enzymes

Question 4

Tissue fluid is formed from blood at the arteriole end of a capillary bed. Explain how water from tissue fluid is returned to the circulatory system. (4)

Answer 4

1. (Plasma) proteins remain;
   a. Accept albumin/globulins/fibrinogen for (plasma) protein
2. (Creates) water potential gradient OR Reduces water potential (of blood);
3. Water moves (to blood) by osmosis;
4. Returns (to blood) by lymphatic system;

Question 5

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

Answer 5

1. Water lost from leaf because of transpiration / evaporation of water (molecules) / diffusion from mesophyll / leaf cells; OR Transpiration / evaporation / diffusion of water (molecules) through stomata / from leaves;
2. Lowers water potential of mesophyll / leaf cells;
3. Water pulled up xylem (creating tension);
4. Water molecules cohere / ‘stick’ together by hydrogen bonds;
5. (forming continuous) water column;
6. Adhesion of water (molecules) to walls of xylem;
   a. Accept Ψ or WP#

Question 6

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

Answer 6

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.
6. Accept starch

Question 7

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

Answer 7

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 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. Capillary wall neutral
6. Across capillary endothelium / epithelium

Question 9

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

Answer 9

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