3.3 Organisms exchange substances with their environment

Question 1

Describe the relationship between the size and structure of an organism and its surface area to volume ratio (SA:V)

Answer 1

• As size increases, SA:V tends to decrease
• More thin / flat / folded / elongated structures increase SA:V

Question 2

How is SA:V calculated? Use an example

Answer 2

Divide surface area (size length x side with x number of sides) by volume (length x width x depth)

Question 3

Suggest an advantage of calculating SA:mass for organisms instead of SA:V

Answer 3

Easier / quicker to find / more accurate because irregular shapes

Question 4

What is metabolic rate? Suggest how it can be measured

Answer 4

• Metabolic rate = amount of energy used up by an organism within a given period of time
• Often measured by oxygen uptake → as used in aerobic respiration to make ATP for energy release

Question 5

Explain the relationship between SA:V and metabolic rate

Answer 5

As SA:V increases (smaller organisms), metabolic rate increases because:
- Rate of heat loss per unit body mass increases
- So organisms need a higher rate of respiration
- To release enough heat to maintain a constant body temperature ie. replace lost heat

Question 6

Explain the adaptations that facilitate exchange as SA:V reduces in larger organisms

Answer 6

1. Changes to body shape (eg. long / thin)
   • Increases SA:V and overcomes (reduces) long diffusion distance / pathway
2. Development of systems, such as a specialised surface / organ for gaseous exchange e.g. lungs:
   • Increases (internal) SA:V and overcomes (reduces) long diffusion distance / pathway
   • Maintain a concentration gradient for diffusion eg. by ventilation / good blood supply

Question 7

Explain how the body surface of a single-celled organism is adapted for gas exchange

Answer 7

• Thin, flat shape and large surface area to volume ratio
• Short diffusion distance to all parts of cell → rapid diffusion eg. of O2 / CO2

Question 8

Describe the tracheal system of an insect

Answer 8

• Spiracles = pores on surface that can open / close to allow diffusion
• Tracheae = large tubes full of air that allow diffusion
• Tracheoles = smaller branches from tracheae, permeable to allow gas exchange with cells

Question 9

Explain how an insect’s tracheal system is adapted for gas exchange

Answer 9

• Tracheoles have thin walls
- So short diffusion distance to cells
• High numbers of highly branched tracheoles
- So short diffusion distance to cells
- So large surface area
• Tracheae provide tubes full of air
- So fast diffusion
• Contraction of abdominal muscles (abdominal pumping) changes pressure in body, causing air to move in / out
- Maintains concentration gradient for diffusion
• Fluid in end of tracheoles drawn into tissues by osmosis during exercise (lactate produced in anaerobic respiration lowers water potential of cells)
- Diffusion is faster through air (rather than fluid) to gas exchange surface

Question 10

Explain structural and functional compromises in terrestrial insects that allow efficient gas exchange while limiting water loss

Answer 10

• Thick waxy cuticle / exoskeleton → Increases diffusion distance so less water loss (evaporation)
• Spiracles can open to allow gas exchange AND close to reduce water loss (evaporation)
• Hairs around spiracles → trap moist air, reducing water potential gradient so less water loss (evaporation)

Question 11

Explain how the gills of fish are adapted for gas exchange

Answer 11

• Gills made of many filaments covered with many lamellae
- Increase surface area for diffusion
• Thin lamellae wall / epithelium
- So short diffusion distance between water / blood
• Lamellae have a large number of capillaries
- Remove O2 and bring CO2 quickly so maintains concentration gradient

Question 12

What is the counter current flow?

Answer 12

1. Blood and water flow in opposite directions through/over lamellae
2. So oxygen concentration always higher in water (than blood near)
3. So maintains a concentration gradient of O2 between water and blood
4. For diffusion along whole length of lamellae

If parallel flow, equilibrium would be reached so oxygen wouldn’t diffuse into blood along the whole gill plate.

Question 13

Explain how the leaves of dicotyledonous plants are adapted for gas exchange

Answer 13

• Many stomata (high density) → large surface area for gas exchange (when opened by guard cells)
• Spongy mesophyll contains air spaces → large surface area for gases to diffuse through
• Thin → short diffusion distance

Question 14

Explain structural and functional compromises in xerophytic plants that allow efficient gas exchange while limiting water loss

Answer 14

Xerophyte = plant adapted to live in very dry conditions eg. Cacti and marram grass
• Thicker waxy cuticle
- Increases diffusion distance so less evaporation
• Sunken stomata in pits / rolled leaves / hairs
- ‘Trap’ water vapour / protect stomata from wind
- So reduced water potential gradient between leaf / air
- So less evaporation
• Spines / needles
- Reduces surface area to volume ratio

Question 15

Describe the gross structure of the human gas exchange system

Answer 15

Nose + mouth → Trachea → Bronchi → Bronchioles → Alveoli → Alveolar epithelium → Capillary endothelium → Blood

Question 16

Explain the essential features of the alveolar epithelium that make it adapted as a surface for gas exchange

Answer 16

• Flattened cells / 1 cell thick → short diffusion distance
• Folded → large surface area
• Permeable → allows diffusion of O2 / CO2
• Moist → gases can dissolve for diffusion
• Good blood supply from large network of capillaries → maintains concentration gradient

Question 17

Describe how gas exchange occurs in the lungs

Answer 17

• Oxygen diffuses from alveolar air space into blood down its concentration gradient
• Across alveolar epithelium then across capillary endothelium

(Carbon dioxide = opposite)

Question 18

Explain the importance of ventilation

Answer 18

• Brings in air containing higher conc. of oxygen & removes air with lower conc. of oxygen
• Maintaining concentration gradients

Question 19

Explain how humans breathe in and out (ventilation)

Answer 19

Inspiration (breathing in)
1. Diaphragm muscles contract → flattens
2. External intercostal muscles contract, internal intercostal muscles relax (antagonistic) → ribcage pulled up / out
3. Increasing volume and decreasing pressure (below atmospheric) in thoracic cavity
4. Air moves into lungs down pressure gradient

Expiration (breathing out)
1. Diaphragm relaxes → moves upwards
2. External intercostal muscles relax, internal intercostal muscles may contract (forced expiration) → ribcage moves down / in
3. Decreasing volume and increasing pressure (above atmospheric) in thoracic cavity
4. Air moves out of lungs down pressure gradient

Question 20

Suggest why expiration is normally passive at rest

Answer 20

• Internal intercostal muscles do not normally need to contract
• Expiration aided by elastic recoil in alveoli

Question 21

Suggest how different lung diseases reduce the rate of gas exchange

Answer 21

• Thickened alveolar tissue (eg. fibrosis) - increases diffusion distance
• Alveolar wall breakdown → reduces surface area
• Reduce lung elasticity → lungs expand / recoil less → reduces concentration gradients of O2 / CO2

Question 22

Suggest how different lung diseases affect ventilation

Answer 22

• Reduce lung elasticity (eg. fibrosis - build-up of scar tissue) - lungs expand / recoil less
- Reducing volume of air in each breath (tidal volume)
- Reducing maximum volume of air breathed out in one breath (forced vital capacity)
• Narrow airways / reduce airflow in & out of lungs (eg. asthma - inflamed bronchi)
- Reducing maximum volume of air breathed out in 1 second (forced expiratory volume)
• Reduced rate of gas exchange → increased ventilation rate to compensate for reduced oxygen in blood

Question 23

Suggest why people with lung disease experience fatigue

Answer 23

Cells receive less oxygen → rate of aerobic respiration reduced → less ATP made

Question 24

Suggest how you can analyse and interpret data to the effects of pollution, smoking and other risk factors on the incidence of lung disease

Answer 24

• Describe overall trend → eg. positive / negative correlation between risk factor and incidence of disease
• Manipulate data → eg. calculate percentage change
• Interpret standard deviations → overlap suggests differences in means are likely to be due to chance
• Use statistical tests → identify whether difference / correlation is significant or due to chance
  > Correlation coefficient → examining an association between 2 sets of data
  > Student’s t test → comparing means of 2 sets of data
  > Chi-squared test → for categorical data

Question 25

Suggest how you can evaluate the way in which experimental data led to statutory restrictions on the sources of risk factors

Answer 25

• Analyse and interpret data as above and identify what does and doesn't support statement • Evaluate method of collecting data - Sample size → large enough to be representative of population? - Participant diversity eg. age, sex, ethnicity and health status - representative of population? - Control groups → used to enable comparison? - Control variables eg. health, previous medications → valid? - Duration of study → long enough to show long-term effects? • Evaluate context → has a broad generalisation been made from a specific set of data? • Other risk factors that could have affected results?

Question 26

Explain the difference between correlations and causal relationships

Answer 26

- Correlation = change in one variable reflected by a change in another - identified on a scatter diagram - Causation = change in one variable causes a change in another variable - Correlation does not mean causation → may be other factors involved

Question 27

Describe the role of red blood cells and haemoglobin in oxygen transport

Answer 27

- Red blood cells contain lots of haemoglobin (Hb) - no nucleus, biconcave, high SA:V, short diffusion path - Hb associates with / binds / loads O2 at gas exchange surfaces where partial pressure of O2 (pO2) is high - This forms oxyhaemoglobin which transports O2 (each can carry 4O2 - one at each Haem group) - Hb dissociates from / unloads O2 near cells / tissues where pO2 is low

Question 28

Describe the structure of haemoglobin

Answer 28

• Protein with a quaternary structure • Made of 4 polypeptide chains • Each chain contains a Haem group containing an iron ion (Fe^2+)

Question 29

Describe the loading, transport and unloading of oxygen in relation to the oxyhemoglobin dissociation curve

Answer 29

Areas with low pO2 (respiring tissues): • Hb has a low affinity for O2 • So O2 readily unloads / dissociates with Hb • So % saturation is low Areas with high pO2 (gas exchange surfaces): • Hb has a high affinity for O2 • So O2 readily loads / associates with Hb • So % saturation is high

Question 30

Explain how the cooperative nature of oxygen binding results in an S-shaped (sigmoid) oxyhaemoglobin dissociation curve

Answer 30

1. Binding of first oxygen changes tertiary / quaternary structure of haemoglobin 2. This uncovers Haem group binding sites, making further binding of oxygens easier

Question 31

Describe evidence for the cooperative nature of oxygen binding

Answer 31

• At low pO2 as oxygen increases there is little / slow increase in % saturation of Hb with oxygen - When first oxygen is binding • At higher pO2 as oxygen increases there is a big / rapid increase in % saturation of Hb with oxygen - Showing it has got easier for oxygens to bind

Question 32

What is the Bohr effect?

Answer 32

Effect of CO2 concentration on dissociation of oxyhaemoglobin → curve shifts to right

Question 33

Explain effect of CO2 concentration on the dissociation of oxyhaemoglobin

Answer 33

1. Increasing blood CO2 eg. due to increased rate of respiration 2. Lowers blood pH (more acidic) 3. Reducing Hb's affinity for oxygen as shape / tertiary / quaternary structure changes slightly 4. So more / faster unloading of oxygen to respiring cells at a given pO2 How the curve provides evidence for this: at a given pO2 % saturation of Hb is lower

Question 34

Explain the advantage of the Bohr effect (eg. during exercise)

Answer 34

More dissociation of oxygen → faster aerobic respiration / less anaerobic respiration → more ATP produced

Question 35

Explain why different types of haemoglobin can have different oxygen transport properties

Answer 35

• Different types of Hb are made of polypeptide chains with slightly different amino acid sequences • Resulting in different tertiary / quaternary structures / shape → different affinities for oxygen

Question 36

Explain how organisms can be adapted to their environment by having different types of haemoglobin with different oxygen transport properties

Answer 36

Curve shift left → Hb has higher affinity for O2 • More O2 associates with Hb more readily • At gas exchange surfaces where pO2 is lower • Eg. organisms in low O2 environments - high altitudes, underground, or foetuses Curve shift right → Hb has lower affinity for O2 • More O2 dissociates from Hb more readily • At respiring tissues where more O2 is needed • Eg. organisms with high rates of respiration / metabolic rate (may be small or active)

Question 37

Describe the general pattern of blood circulation in a mammal

Answer 37

Closed double circulatory system - blood passes through heart twice for every circuit around body. 1. Deoxygenated blood in right side of heart pumped to lungs; oxygenated returns to left side 2. Oxygenated blood in left side of heart pumped to rest of body; deoxygenated returns to right

Question 38

Suggest the importance of a double circulatory system

Answer 38

• Prevents mixing of oxygenated / deoxygenated blood - So blood pumped to body is fully saturated with oxygen for aerobic respiration • Blood can be pumped to body at a higher pressure (after being lower from lungs) - Substances taken to / removed from body cells quicker / more efficiently

Question 39

Draw a diagram to show the general pattern of blood circulation in a mammal, including the names of key blood vessels

Answer 39

Vena cava → RA → RV → Pulmonary artery → Lungs → Pulmonary vein → LA → LV → Aorta → → Rest of body → Vena cava Aorta → Renal artery → Kidneys → Renal vein → Vena cava

Question 40

Name the blood vessels entering and leaving the heart and lungs

Answer 40

• Vena cava - transports deoxygenated blood from respiring body tissues → heart • Pulmonary artery - transports deoxygenated blood from heart → lungs • Pulmonary vein - transports oxygenated blood from lungs → heart • Aorta - transports oxygenated blood from heart → respiring body tissues

Question 41

Name the blood vessels entering and leaving the kidneys

Answer 41

- Renal arteries - oxygenated blood from aorta to kidneys - Renal veins - deoxygenated blood to vena cava from kidneys

Question 42

Name the the blood vessels that carry oxygenated blood to the heart muscle

Answer 42

Coronary arteries - located on surface of the heart, branching from aorta

Question 43

Label a diagram to show the gross structure of the human heart (inside)

Answer 43

Pulmonary artery Aorta Vena cava Pulmonary vein Semilunar valve Semilunar valve Atrioventricular valve Atrioventricular valve

Question 44

Suggest why the wall of the left ventricle is thicker than that of the right

Answer 44

- Thicker muscle to contract with greater force - To generate higher pressure to pump blood around entire body

Question 45

Explain the pressure & volume changes and associated valve movements during the cardiac cycle that maintain a unidirectional flow of blood

Answer 45

Atrial systole • Atria contract → volume decreases, pressure increases • Atrioventricular valves open when pressure in atria exceeds pressure in ventricles • Semilunar valves remain shut as pressure in arteries exceeds pressure in ventricles • So blood pushed into ventrices Ventricular systole • Ventricles contract → volume decreases, pressure increases • Atrioventricular valves shut when pressure in ventricles exceeds pressure in atria • Semilunar valves open when pressure in ventricles exceeds pressure in arteries • So blood pushed out of heart through arteries Diastole • Atria & ventricles relax → volume increases, pressure decreases • Semilunar valves shut when pressure in arteries exceeds pressure in ventricles • Atrioventricular valves open when pressure in atria exceeds pressure in ventricles • So blood fills atria via veins & flows passively to ventricles

Question 46

Explain how graphs showing pressure or volume changes during the cardiac cycle can be interpreted, eg. to identify when valves are open / closed

Answer 46

SL valves closed • Pressure in [named] artery higher than in ventricle • To prevent backflow of blood from artery to ventricles SL valves open • When pressure in ventricle is higher than in [named] artery • So blood flows from ventricle to artery AV valves closed • Pressure in ventricle higher than atrium • To prevent backflow of blood from ventricles to atrium AV valves open • When pressure in atrium is higher than in ventricle • So blood flows from atrium to ventricle

Question 47

Describe the equation for cardiac output

Answer 47

Cardiac output (volume of blood pumped out of heart per min) = stroke volume (volume of blood pumped in each heart beat) x heart rate (number of beats per min)

Question 48

How can heart rate be calculated from cardiac cycle data?

Answer 48

Heart rate (beats per minute) = 60 (seconds) / length of one cardiac cycle (seconds)

Question 49

Explain how the structure of arteries relates to their function

Answer 49

Function - carry blood away from heart at high pressure - Thick smooth muscle tissue → can contract and control / maintain blood flow / pressure - Thick elastic tissue → can stretch as ventricles contract and recoil as ventricles relax, to reduce pressure surges / even out blood pressure / maintain high pressure - Thick wall → withstand high pressure / stop bursting - Smooth / folded endothelium → reduces friction / can stretch - Narrow lumen - increases / maintains high pressure

Question 50

Explain how the structure of arterioles relates to their function

Answer 50

Function - (division of arteries to smaller vessels which can) direct blood to different capillaries / tissues • Thicker smooth muscle layer than arteries - Contracts → narrows lumen (vasoconstriction) → reduces blood flow to capillaries - Relaxes → widens lumen (vasodilation) → increases blood flow to capillaries • Thinner elastic layer → pressure surges are lower (as further from heart / ventricles)

Question 51

Explain how the structure of veins relates to their function

Answer 51

Function - carry blood back to heart at lower pressure • Wider lumen than arteries → less resistance to blood flow • Very little elastic and muscle tissue → blood pressure lower • Valves → prevent backflow of blood

Question 52

Explain how the structure of capillaries relates to their function

Answer 52

Function - allow efficient exchange of substances between blood and tissue fluid (exchange surface) • Wall is a thin (one cell) layer of endothelial cells → reduces diffusion distance • Capillary bed is a large network of branched capillaries → increases surface area for diffusion • Small diameter / narrow lumen → reduces blood flow rate so more time for diffusion • Pores in walls between cells → allow larger substances through

Question 53

Explain the formation of tissue fluid

Answer 53

At the arteriole end of capillaries: 1. Higher blood / hydrostatic pressure inside capillaries (due to contraction of ventricles) than tissue fluid (so net outward force) 2. Forcing water (and dissolved substances) out of capillaries 3. Large plasma proteins remain in capillary

Question 54

Explain the return of tissue fluid to the circulatory system

Answer 54

At the venule end of capillaries: 1. Hydrostatic pressure reduces as fluid leaves capillary (also due to friction) 2. (Due to water loss) an increasing concentration of plasma proteins lowers water potential in capillary below that of tissue fluid 3. Water enters capillaries from tissue fluid by osmosis down a water potential gradient 4. Excess water taken up by lymph capillaries and returned to circulatory system through veins

Question 55

Suggest and explain causes of excess tissue fluid accumulation

Answer 55

• Low concentration of protein in blood plasma - Water potential in capillary not as low → water potential gradient is reduced - So more tissue fluid formed at arteriole end / less water absorbed at venule end by osmosis • High blood pressure (eg. caused by high salt concentration) → high hydrostatic pressure - Increases outward pressure from arterial end AND reduces inward pressure at venule end - So more tissue fluid formed at arteriole end / less water absorbed at venule end by osmosis - Lymph system may not be able to drain excess fast enough

Question 56

What is a risk factor? Give examples for cardiovascular disease

Answer 56

• An aspect of a person's lifestyle or substances in a person's body / environment • That have been shown to be linked to an increased rate of disease • Examples - age, diet high in salt or saturated fat, smoking, lack of exercise, genes

Question 57

Describe the function of xylem tissue

Answer 57

Transports water (and mineral ions) through the stem, up the plant to leaves of plants.

Question 58

Suggest how xylem tissue is adapted for its function

Answer 58

• Cells joined with no end walls forming a long continuous tube → water flows as a continuous column • Cells contain no cytoplasm / nucleus → easier water flow / no obstructions • Thick cell walls with lignin - provides support / withstand tension / prevents water loss • Pits in side walls → allow lateral water movements

Question 59

Explain the cohesion-tension theory of water transport in the xylem

Answer 59

1. Leaf - 1. Water lost from leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata - 2. Reducing water potential of mesophyll cells - 3. So water drawn out of xylem down a water potential gradient 2. Xylem - 4. Creating tension (negative pressure' or 'pull') in xylem - 5. Hydrogen bonds result in cohesion between water molecules (stick together) so water is pulled up as a continuous column - 6. Water also adheres (sticks to) to walls of xylem 3. Root - 7. Water enters roots via osmosis

Question 60

Describe how to set up a potometer

Answer 60

1. Cut a shoot underwater at a slant → prevent air entering xylem 2. Assemble potometer with capillary tube end submerged in a beaker of water 3. Insert shoot underwater 4. Ensure apparatus is watertight / airtight 5. Dry leaves and allow time for shoot to acclimatise 6. Shut tap to reservoir 7. Form an air bubble → quickly remove end of capillary tube from water

Question 61

Describe how a potometer can be used to measure the rate of transpiration

Answer 61

Potometer estimates transpiration rate by measuring water uptake: 1. Record position of air bubble 2. Record distance moved in a certain amount of time (eg. 1 minute) 3. Calculate volume of water uptake in a given time: • Use radius of capillary tube to calculate cross-sectional area of water (πr^2) • Multiply this by distance moved by bubble 4. Calculate rate of water uptake - divide volume by time taken

Question 62

Describe how a potometer can be used to investigate the effect of a named environmental variable on the rate of transpiration

Answer 62

• Carry out the above, change one variable at a time (wind, humidity, light or temperature) - E.g. set up a fan OR spray water in a plastic bag and wrap around the plant OR change distance of a light source OR change temperature of room • Keep all other variables constant

Question 63

Suggest limitations in using a potometer to measure rate of transpiration

Answer 63

• Rate of water uptake might not be same as rate of transpiration - Water used for support / turgidity - Water used in photosynthesis and produced during respiration • Rate of movement through shoot in potometer may not be same as rate of movement through shoot of whole plant - Shoot in potometer has no roots whereas a plant does - Xylem cells very narrow

Question 64

Suggest how different environmental variables affect transpiration rate

Answer 64

Light intensity - Increases rate of transpiration • Stomata open in light to let in CO2 for photosynthesis • Allowing more water to evaporate faster • Stomata close when it's dark so there is a low transpiration rate Temperature - Increases rate of transpiration • Water molecules gain kinetic energy as temperature increases • So water evaporates faster Wind intensity - Increases rate of transpiration • Wind blows away water molecules from around stomata • Decreasing water potential of air around stomata • Increasing water potential gradient so water evaporates faster Humidity - Decreases rate of transpiration • More water in air so it has a higher water potential • Decreasing water potential gradient from leaf to air • Water evaporates slower

Question 65

Describe the function of phloem tissue

Answer 65

Transports organic substances eg. sucrose in plants

Question 66

Suggest how phloem tissue is adapted for its function

Answer 66

1. Sieve tube elements - No nucleus / few organelles → maximise space for / easier flow of organic substances - End walls between cells perforated (sieve plate) 2. Companion cells - Many mitochondria → high rate of respiration to make ATP for active transport of solutes

Question 67

What is translocation?

Answer 67

• Movement of assimilates / solutes such as sucrose • From source cells (where made, eg. leaves) to sink cells (where used / stored, eg. roots) by mass flow

Question 68

Explain the mass flow hypothesis for translocation in plants

Answer 68

1. At source, sucrose is actively transported into phloem sieve tubes / cells 2. By companion cells 3. This lowers water potential in sieve tubes so water enters (from xylem) by osmosis 4. This increases hydrostatic pressure in sieve tubes (at source) / creates a hydrostatic pressure gradient 5. So mass flow occurs - movement from source to sink 6. At sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs

Question 69

Describe the use of tracer experiments to investigate transport in plants

Answer 69

1. Leaf supplied with a radioactive tracer e.g. CO2 containing radioactive isotope C-14 2. Radioactive carbon incorporated into organic substances during photosynthesis 3. These move around plant by translocation 4. Movement tracked using autoradiography or a Geiger counter

Question 70

Describe the use of ringing experiments to investigate transport in plants

Answer 70

- Remove / kill phloem eg. remove a ring of bark - Bulge forms on source side of ring - Fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem - Tissues below ring die as cannot get organic substances

Question 71

Suggest some points to consider when interpreting evidence from tracer & ringing experiments and evaluating evidence for / against the mass flow hypothesis

Answer 71

- Is there evidence to suggest the phloem (as opposed to the xylem) is involved ? - Is there evidence to suggest respiration / active transport is involved? - Is there evidence to show movement is from source to sink? What are these in the experiment? - Is there evidence to suggest movement is from high to low hydrostatic pressure? - Could movement be due to another factor eg. gravity?