3: Organisms exchange substances with their environment

Question 1

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

Answer 1

1. Large(r) organisms have a small(er) surface area:volume (ratio); OR Small(er) organisms have a large(r) surface area:volume (ratio);
2. Overcomes long diffusion pathway OR Faster rate of diffusion;

Question 2

How does oxygen move through the insect? [4]

Answer 2

1. Oxygen diffuses in through the open spiracles;
2. Spiracle closes;
3. Oxygen moves through the trachea into the tracheoles;
4. Oxygen delivered directly to the respiring tissues;

Question 3

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

Answer 3

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);
4. 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);
5. Body can be moved (by muscles) to move air so maintains diffusion / concentration gradient for oxygen / carbon dioxide;

Question 4

Describe how the structure of the insect gas exchange system:
* provides cells with sufficient oxygen
* limits water loss.

Explain your answers. [6]

Answer 4

1. Spiracles, tracheae, tracheoles;
2. Spiracles allow diffusion (of oxygen)
   OR (Oxygen) diffusion through tracheae/tracheoles;
3. Tracheoles are highly branched so large surface area (for exchange);
4. Tracheole (walls) thin so short diffusion distance (to cells) OR Highly branched tracheoles so short diffusion distance (to cells) OR Tracheoles enter cells so short diffusion distance;
5. Tracheole permeable to oxygen/air;
6. Cuticle/chitin/exoskeleton (impermeable) so reduce water loss;
7. Spiracles (can) close so no/less water loss OR Spiracles have valves so no/less water loss;
8. Hairs around spiracles reduce water loss;

Question 5

Describe and explain the advantage of the counter-current principle in gas exchange across a fish gill. [3]

Answer 5

1. Water and blood flow in opposite directions;
2. Maintains diffusion/concentration gradient of oxygen OR Oxygen concentration always higher (in water);
3. (Diffusion) along length of lamellae/filament/gill/capillary;

Question 6

A fish uses its gills to absorb oxygen from water. Explain how the gills of a fish are adapted for efficient gas exchange. [6]

Answer 6

1 Large surface area provided by many lamellae over many gill filaments;
2 Increases diffusion/makes diffusion efficient;
3 Thin epithelium/distance between water and blood;
4 Water and blood flow in opposite directions/countercurrent;
5 (Point 4) maintains concentration gradient (along gill)/equilibrium not reached;
6 As water always next to blood with lower concentration of oxygen;
7 Circulation replaces blood saturated with oxygen;
8 Ventilation replaces water (as oxygen removed);

Question 7

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

Answer 7

1. Named structures – trachea, bronchi, bronchioles, alveoli;
2. Above structures named in correct order OR Above structures labelled in correct positions on a diagram;
   Breathing in
3. Diaphragm (muscles) contract and diaphragm flattens;
4. External intercostal muscles contract and ribcage pulled up/out;
5. (Causes) volume increase and pressure decrease in thoracic cavity (to below atmospheric pressure);

Breathing out
6. Diaphragm (muscles) relaxes and diaphragm moves up; 7. External intercostal muscles relax and ribcage moves down/in;
8. (Causes) volume decrease and pressure increase in thoracic cavity (to above atmospheric pressure);

Question 8

Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf. (4)

Answer 8

1. (Carbon dioxide enters) via stomata;
2. (Stomata opened by) guard cells;
3. Diffuses through air spaces;
4. Down diffusion gradient;

Question 9

Explain why plants grown in soil with very little water grow only slowly [2]

Answer 9

1. Stomata close;
2. Less carbon dioxide (uptake) for less photosynthesis/glucose production;

Question 10

Describe the process of starch digestion [5]

Answer 10

(salivary/pancreatic) Amylase;
Starch to Maltose;
Maltase;
Maltose to glucose;
Hydrolysis;
Glycosidic bonds

Question 11

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

Answer 11

1. Micelles contain bile salts and fatty acids/monoglycerides;
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);
3. Fatty acids/monoglycerides absorbed by diffusion; REJECT Micelle enters cell
4. Triglycerides (re)formed (in cells); (Smooth Endoplasmic Reticulum)
5. (Golgi) Vesicles move to cell membrane;

Question 12

Describe the role of micelles in the absorption of fats into the cells of the ileum. [5]

Answer 12

1. Micelles include bile salts and fatty acids;
2. Make the fatty acids (more) soluble in water;
3. Bring/release/carry fatty acids to cell/lining (of the ileum);
4. Maintain high(er) concentration of fatty acids to cell/lining (of the ileum);
5. Fatty acids (absorbed) by diffusion;

Question 13

Describe the role of enzymes in the digestion of proteins in a mammal. [4]

Answer 13

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;

Question 14

The action of endopeptidases and exopeptidases can increase the rate of protein digestion. Describe how. [2]

Answer 14

1. Exopeptidases hydrolyse peptide bonds at the ends of a polypeptide/protein AND endopeptidases hydrolyse internal peptide bonds within a polypeptide/protein;
2. More ‘ends’ OR More surface area;

Question 15

Describe and explain two features you would expect to find in a cell specialised for absorption. [4]

Answer 15

1. Folded membrane/microvilli so large surface area (for absorption);
   Accept ‘brush border’ for ‘microvilli’.
2. Large number of co-transport/carrier/channel proteins so fast rate (of absorption) OR
   Large number of co-transport/carrier proteins for active transport OR
   Large number of co-transport/carrier/channel proteins for facilitated diffusion;
3. Large number of mitochondria so make (more) ATP (by respiration) OR
   Large number of mitochondria for aerobic respiration OR
   Large number of mitochondria to release energy for active transport;
4. Membrane-bound (digestive) enzymes so maintains concentration gradient (for fast absorption);

Question 16

Describe the absorption of glucose (Cotransport) [4]

Answer 16

1. Sodium ions actively transported from ileum cell in to blood;
2. Maintains / forms diffusion gradient for sodium to enter cells from gut (and with it, glucose);
3. GLucose enters cell down the sodium ion gradient
4. Glucose enters by facilitated diffusion with sodium ions;

Question 17

Describe the mechanism for the absorption of amino acids in the ileum. [5]

Answer 17

1. Facilitated diffusion of amino acid (into cell when higher concentration in lumen);
2. Co-transport;
3. Sodium ions actively transported from cell to blood/capillary/tissue fluid;
4. Creating sodium ion concentration/diffusion gradient;
5. Facilitated diffusion of amino acid into blood/capillary;

Question 18

In large cells of U. marinum, most mitochondria are found close to the cell-surface membrane. In smaller cells, the mitochondria are distributed evenly throughout the cytoplasm. Mitochondria use oxygen during aerobic respiration.

Use this information and your knowledge of surface area to volume ratios to suggest an explanation for the position of mitochondria in large U. marinum cells. [2]

Answer 18

1. Large(r) cells have small(er) surface area to volume ratio;
2. (Takes) longer for oxygen to diffuse (to mitochondria) OR Less/no oxygen diffuses (to mitochondria) OR Diffusion distance/pathway is long(er);

Question 19

Describe the relationship between size and surface area to volume ratio of organisms. [1]

Answer 19

As size increases, ratio (of surface area to volume) decreases;

Question 20

Breathing out as hard as you can is called forced expiration. Describe and explain the mechanism that causes forced expiration. [4]

Answer 20

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

Question 21

Describe how oxygen in the air reaches capillaries surrounding alveoli in the lungs. Details of breathing are not required. [4]

Answer 21

1. Trachea and bronchi and bronchioles;
2. Down pressure gradient;
3. Down diffusion gradient;
4. Across alveolar epithelium.
   Capillary wall neutral
5. Across capillary endothelium

Question 22

Amoebic gill disease (AGD) is caused by a parasite that lives on the gills of some species of fish. The disease causes the lamellae to become thicker and to fuse together.

AGD reduces the efficiency of gas exchange in fish. Give two reasons why.

Answer 22

1. (Thicker lamellae so) greater / longer diffusion distance / pathway;
2. (Lamellae fuse so) reduced surface area;

Accept: reduced SA:VOL

Question 23

Describe two adaptations of the structure of alveoli for efficient gas exchange.

Answer 23

1. Thin walls or squamous epithelia is one cell thick
2. (Total) surface area is large;

Question 24

Explain a property of iron ions that enables these ions to carry out their role in red blood cells. [2]

Answer 24

1. (Is) charged/polar OR (Is) part of haem(oglobin);
2. (So) binds/associates/loads (with) oxygen OR (So) forms oxyhaemoglobin OR (So) transports oxygen;

Question 25

Describe how haemoglobin normally loads oxygen in the lungs and unloads it in a tissue cell. [6]

Answer 25

• Oxygen combines (reversibly) to produce oxyhaemoglobin;
• each haemoglobin molecule/ one haemoglobin may transport 4 molecules of oxygen;
• high partial pressure of oxygen / oxygen tension / concentration in lungs;
• haemoglobin (almost) 95% / 100% saturated;
• unloads at low oxygen tension(in tissues);
• presence of carbon dioxide displaces curve further to right / increases oxygen dissociation;
• allows more O2 to be unloaded;
• increase temp/ acidity allows more O2 to be unloaded;
• low pO2 / increase CO2 / increase term / increase acid occur in vicinity of respiring tissue;

Question 26

Explain how oxygen in a red blood cell is made available for respiration in active tissues. [4]

Answer 26

• CO2 (increased) respiration;
• (increased) dissociation oxygen from haemoglobin;
• Low partial pressure in tissues/plasma;
• Oxygen diffuses from r.b.c. to tissues;

Question 27

The oxygen dissociation curve of the foetus is to the left of that for its mother. Explain the advantage of this for the foetus. [3]

Answer 27

• Foetal haemoglobin has Higher affinity / loads more oxygen;
• At low/same/high partial pressure/pO2;
• Oxygen moves from mother/to fetus;

Question 28

Explain how oxygen is loaded, transported and unloaded in the blood. [5]

Answer 28

• Haemoglobin carries oxygen / has a high affinity for oxygen / oxyhaemoglobin;
• In red blood cells;
• Loading/uptake/association in lungs at high p.O2;
• Unloads/ dissociates / releases to respiring cells/tissues at low p.O2;
• Unloading linked to higher carbon dioxide (concentration);

Question 29

Binding of one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind.

Explain why.
[2]

Answer 29

1. Binding of first oxygen changes tertiary / quaternary (structure) of haemoglobin; [conformational shift caused]
2. Creates / leads to / uncovers second / another binding site OR Uncovers another iron / Fe / haem group to bind to;

Question 30

Describe and explain the effect of increasing carbon dioxide concentration on the dissociation of oxyhaemoglobin. [2]

Answer 30

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

Question 31

Explain how an arteriole can reduce the blood flow into capillaries. [2]

Answer 31

1. Muscle (layer) contracts;
2. Constricts/narrows arteriole/lumen;

Question 32

Describe how the heart muscle and the heart valves maintain a one-way flow of blood from the left atrium to the aorta. [6]

Answer 32

1. Atrium has higher pressure than ventricle (due to filling / contraction);
2. Atrioventricular valve opens;
3. Ventricle has higher pressure than atrium (due to filling / contraction);
4. Atrioventricular valve closes;
5. Ventricle has higher pressure than aorta;
   Points 1, 3, 5, and 7 must be comparative: eg higher
6. Semilunar valve opens;
   Marks 2, 4, 6, 8 given in the correct sequence can gain 4 marks
7. Higher pressure in aorta than ventricle (as heart relaxes);
8. Semilunar valve closes;
9. (Muscle / atrial / ventricular) contraction causes increase in pressure;

Question 33

Describe how the movement of the diaphragm leads to air movement into the lungs [4]

Answer 33

1. Diaphragm contracts and flattens.
2. Volume of lungs increases.
3. Pressure inside the lungs is lower than
   atmospheric pressure.
4. Air moves into the lungs.

Question 34

Describe and explain how the lungs are adapted to allow rapid exchange of oxygen between air in the alveoli and blood in the capillaries around them. [4]

Answer 34

• Many alveoli/ alveoli walls folded provide a large surface area;
• Many capillaries provide a large surface area (So) fast diffusion;
  Alveoli or capillary walls/ epithelium/ lining are thin/ one cell thick / short distance between alveoli and blood;
• Flattened/ squamous epithelium (So) short diffusion distance/ pathway / (So) fast diffusion;
• Ventilation / circulation; So Maintains a diffusion / concentration gradient (So) fast diffusion;

Question 35

Describe the gross structure of human gas exchange system and how we breathe in and out [6]

Answer 35

• Named structures – trachea, bronchi, bronchioles, alveoli
• Above structures named in correct order
• Breathing in – Diaphragm contract and external intercostal muscles contract
• Volume increases and pressure decreases in thoracic cavity.
• Breathing out – Diaphragm relaxes and internal intercostal muscles contract
• Volume decrease and pressure increase in thoracic cavity

Question 36

Describe the pathway taken by an oxygen molecule from an alveolus to the blood [2]

Answer 36

1. (Across) alveolar epithelium;
2. Endothelium of capillary;

Question 37

Explain why death of alveolar epithelium cells reduces gas exchange in human lungs. [3]

Answer 37

1. Reduced surface area;
2. Increased distance for diffusion;
3. Reduced rate of gas exchange;

Question 38

Arteries and arterioles take blood away from the heart.

Explain how the structures of the walls of arteries and arterioles are related to their functions. (6)

Answer 38

**Elastic tissue **
1 Elastic tissue stretches under pressure/when heart contracts;
2 Recoils/springs back;
3 Evens out pressure/flow;
**Muscle **
4 Muscle contracts;
5 Reduces diameter of lumen/vasoconstriction/constricts vessel;
6 Changes flow/pressure;
**Epithelium **
7 Epithelium smooth;
8 Reduces friction/blood clots/less resistance

Question 39

The thickness of the aorta wall changes all the time during each cardiac cycle.
Explain why. (5)

Answer 39

1. (Aorta wall) stretches;
2. Because ventricle/heart contracts / systole / pressure increases;
3. (Aorta wall) recoils;
4. Because ventricle relaxes / heart relaxes /diastole / pressure falls;
5. Maintain smooth flow / pressure;

Question 40

Suggest two ways the student could improve the quality of his scientific drawing [5]

Answer 40

1. Only use single lines/do not use sketching (lines)/ensure lines are continuous/connected;
2. Add labels/annotations/title;
3. Add magnification/scale (bar);
4. Draw all parts to same scale/relative size;
5. Do not use shading/hatching;

Question 41

Describe two precautions the student should take when clearing away after the dissection. [2]

Answer 41

1. Carry/wash sharp instruments/scalpel by holding handle OR Carry/wash sharp instruments by pointing away (from body)/down;
2. Disinfect instruments/surfaces;
3. Disinfect/sanitise hands OR Wash hands with soap (and water);
4. Put organ/gloves/paper towels in a (separate) bag/bin/tray to dispose;

Question 42

Describe how tissue fluid is formed and how it is returned to the circulatory system. (6)

Answer 42

Formation
1. High blood / hydrostatic pressure / pressure filtration;
2. Forces water / fluid out;
3. Large proteins remain in capillary;

Return
4. Low water potential in capillary / blood;
5. Due to (plasma) proteins;
6. Water enters capillary / blood;
7. (By) osmosis;
8. Correct reference to lymph;

Question 43

Heat from respiration helps mammals to maintain a constant body temperature.
Use this information to explain the relationship between the surface area to volume ratio of mammals and the oxygen dissociation curves of their haemoglobins. (5)

Answer 43

1. Smaller mammal has greater surface area to
   volume ratio;
2. Smaller mammal/larger SA:Vol ratio more
   heat lost (per unit body mass);
3. Smaller mammal/larger SA:Vol ratio has
   greater rate of respiration/metabolism;
4. Oxygen required for respiration;
5. (Haemoglobin) releases more oxygen / oxygen released more readily / haemoglobin has lower affinity;

Question 44

If the dissociation curve is to the LEFT…..

Answer 44

• Loads MORE oxygen
• at LOWER partial pressures
• Haemoglobin has HIGHER affinity (for oxygen)

Question 45

If the dissociation curve is to the RIGHT….

Answer 45

• Unloads MORE oxygen
• for MORE aerobic Respiration so more ATP
• More heat energy released
• at same partial pressures
• Haemoglobin has LOWER affinity (for oxygen)

Question 46

Explain why water moves up the plant stem. [3]

Answer 46

1.   Water evaporates/is transpired out of leaves through stomata;

2.   Water potential gradient/lower water potential creates tension/pulls up water
OR Omosis creates tension/pulls up water; OR Osmosis creates negative pressure.

3.   Hydrogen bonds/cohesion/adhesion maintains ((continuous) column;

Question 47

Describe the cohesion-tension theory of water transport in the xylem [5]

Answer 47

1. Water lost from leaf because of transpiration / evaporation of water (molecules) / diffusion from mesophyll / leaf cells through stomata; 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); (negative pressure)
4. Water molecules cohere / ‘stick’ together by hydrogen bonds;
5. (forming continuous) water column;
6. Adhesion of water (molecules) to walls of xylem;
7. Accept Ψ