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 spiracles;
2. Spiracle closes;
3. Oxygen diffuses 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 rate of 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;
4. Triglycerides (re)formed (in cells SER);
5. 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. [2]

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) [3]

Answer 16

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

Question 17

Describe the mechanism for the absorption of amino acids in the ileum [4]

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

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

Answer 18

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

Question 19

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 19

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 20

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

Answer 20

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 21

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 21

• 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 22

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

Answer 22

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

Question 23

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;
3. Reduced rate of gas exchange;

Question 24

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 24

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;
**Endothelium **
7 Endothelium smooth;
8 Reduces friction/blood clots/less resistance;

Question 25

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

Answer 25

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. **Maintains smooth flow / pressure**;

Question 26

Suggest two ways the student could improve the quality of his scientific drawing [2]

Answer 26

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 27

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

Answer 27

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 28

State two safety precautions when using a scalpel.

Answer 28

1. Cut onto a hard flat surface 2. Cut away from the body 3. Use a sharp blade

Question 29

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

Answer 29

**Formation ** 1. High blood / hydrostatic pressure / pressure filtration; 2. Forces water / fluid out (of fenestrations/ capillary pores); 3. Large proteins remain in capillary; **Return** 4. Lower water potential in capillary / blood; 5. Due to (plasma) proteins; 6. Water enters capillary / blood; 7. (By) osmosis; 8. (Correct reference to) lymph removes excess water;

Question 30

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 30

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; (Haemoglobin) releases more oxygen / oxygen released more readily / haemoglobin has lower affinity;

Question 31

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

Answer 31

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 32

What is the equation for Cardiac Output?

Answer 32

CO = SV X HR units = dm3 min-1

Question 33

How do you calculate the Heart rate?

Answer 33

60 _______________________________________ the time of one cardiac cycle in seconds

Question 34

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

Answer 34

* 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 35

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

Answer 35

* CO2 (increased) respiration; * (increased) dissociation oxygen from haemoglobin; * Low partial pressure in tissues/plasma; * Oxygen diffuses from red blood cells to tissues;

Question 36

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 36

* HbF hasHigher affinity / loads more oxygen; * At low/same/high partial pressure/pO2; * Oxygen moves from mother/to fetus;

Question 37

Explain how oxygen is loaded, transported and unloaded in the blood. (6)

Answer 37

* 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 38

Binding of one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind. Explain why. [2]

Answer 38

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 39

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

Answer 39

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

Question 40

Explain how water enters xylem from the endodermis in the root and is then transported to the leaves. (6)

Answer 40

**(In the root) ** 1. Casparian strip blocks apoplast pathway / only allows symplast pathway; 2. Active transport by endodermis; 3. (Of) ions/salts into xylem; 4. Lower water potential in xylem / water enters xylem by osmosis /down a water potential gradient; **(Xylem to leaf) ** 5. Evaporation / transpiration (from leaves); 6. (Creates) cohesion / tension / H-bonding between water molecules / negative pressure; 7. Adhesion / water molecules bind to xylem; 8. (Creates continuous) column of water

Question 41

Root pressure moves water through the xylem. Describe what causes root pressure. (4)

Answer 41

1. Active transport by endodermis; 2. ions/salts into xylem; 3. Lowers water potential (in xylem); 4. (Water enters) by osmosis;

Question 42

Name a factor that can affect transpiration

Answer 42

Light (intensity) / temperature / air movement / humidity;

Question 43

Give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot. [2]

Answer 43

1. Seal joints / ensure airtight / ensure watertight; 2. Cut shoot under water; 3. Cut shoot at a slant; 4. Dry off leaves; 5. Insert into apparatus under water; 6. Ensure no air bubbles are present; 7. Shut tap; 8. Note where bubble is at start / move bubble to the start position;

Question 44

Describe the mass flow hypothesis for the mechanism of translocation in plants. [5]

Answer 44

1. In source/leaf **sucrose** is 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;

Question 45

Describe the transport of carbohydrate in plants. [5]

Answer 45

1. Sucrose actively transported into phloem (cell); OR Sucrose is co-transported/moved with H+ into phloem (cell); 2. (By) companion/transfer cells; 3. Lowers water potential (in phloem) and water enters (from xylem) by osmosis; 4. ((Produces) high(er) (hydrostatic) pressure; OR (Produces hydrostatic) pressure gradient; 5. Mass flow to respiring cells OR Mass flow to storage tissue/organ; 6. Unloaded/removed (from phloem) by active transport;

Question 46

Explain how the Atrio Ventricular valve maintains a unidirectional flow of blood. [2]

Answer 46

1. Pressure in (left) atrium is higher than in ventricle causing valve to open; OR (When) pressure above valve is higher than below valve it opens; ’2.Pressure in (left) ventricle is higher than in atrium causing valve to close; OR (When) pressure in below valve is higher than above valve it closes;

Question 47

Explain the importance of the xylem being kept open as a continuous tube. [3]

Answer 47

1. (Allows unbroken) **continuous column of water** OR (So) no barrier to (water) movement; 2.** Cohesion from H bonds** **between** (all) **water** (molecules) OR Cohesion from (polar) attraction between (all) water (molecules); 3. Evaporation/**transpiration creates tension **(in column) OR Water moves from xylem (into cells) creates tension OR (To) pull up water creates tension (in xylem);