Topic 2

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?

Answer 2

Divide surface area (size length x side width 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

1. Spiracles = pores on surface that can open / close to allow diffusion
2. Tracheae = large tubes full of air that allow diffusion
3. 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 ψ 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 ψ 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

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

Question 13

Parallel flow:

Answer 13

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

Question 14

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

Answer 14

● 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 15

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

Answer 15

● 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 16

Xerophyte

Answer 16

Xerophyte = plant adapted to live in very dry conditions eg. Cacti and marram grass

Question 17

Describe the gross structure of the human gas exchange system

Answer 17

Trachea
Bronchi
Bronchioles
Alveoli (air sacs)
Capillary network surrounding alveoli

Question 18

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

Answer 18

● 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 19

Describe how gas exchange occurs in the lungs

Answer 19

● Oxygen diffuses from alveolar air space into blood down its concentration gradient
● Across alveolar epithelium then across capillary endothelium
Carbon dioxide opposite

Question 20

Explain the importance of ventilation

Answer 20

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

Question 21

Explain how humans breathe in

Answer 21

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

Question 22

Explain how humans out

Answer 22

1. Diaphragm relaxes → moves upwards
2. External intercostal muscles relax, internal
   intercostal muscles may contract → 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 23

Suggest why expiration is normally passive at rest

Answer 23

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

Question 24

Suggest how different lung diseases reduce the rate of gas exchange

Answer 24

● 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 25

Suggest how different lung diseases affect ventilation

Answer 25

● 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 26

Suggest why people with lung disease experience fatigue

Answer 26

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

Question 27

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 27

● 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 28

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

Answer 28

● 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 29

Explain the difference between correlations and causal relationships

Answer 29

● 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 30

Explain what happens in digestion

Answer 30

● Large (insoluble) biological molecules hydrolysed to smaller (soluble) molecules ● That are small enough be absorbed across cell membranes into blood

Question 31

Describe the digestion of starch in mammals

Answer 31

● Amylase (produced by salivary glands / pancreas) hydrolyses starch to maltose ● Membrane-bound maltase (attached to cells lining ileum) hydrolyses maltose to glucose ● Hydrolysis of glycosidic bond

Question 32

Describe the digestion of disaccharides in mammals

Answer 32

● Membrane-bound disaccharidases hydrolyse disaccharides to 2 monosaccharides: ○ Maltase - maltose → glucose + glucose ○ Sucrase - sucrose → fructose + glucose ○ Lactase - lactose → galactose + glucose ● Hydrolysis of glycosidic bond

Question 33

Describe the digestion of lipids in mammals, including action of bile salts

Answer 33

● Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets ● This increases surface area of lipids for increased / faster lipase activity ● Lipase (made in pancreas) hydrolyses lipids (eg. triglycerides) → monoglycerides + fatty acids ● Hydrolysis of ester bond

Question 34

Describe the digestion of proteins by a mammal

Answer 34

● Endopeptidases - hydrolyse internal (peptide) bonds within a polypeptide → smaller peptides ○ So more ends / surface area for exopeptidases ● Exopeptidases - hydrolyse terminal (peptide) bonds at ends of polypeptide → single amino acids ● Membrane-bound dipeptidases - hydrolyse (peptide) bond between a dipeptide → 2 amino acids ● Hydrolysis of peptide bond

Question 35

Suggest why membrane-bound enzymes are important in digestion

Answer 35

● Membrane-bound enzymes are located on cell membranes of epithelial cells lining ileum ● (By hydrolysing molecules at the site of absorption they) maintain concentration gradients for absorption

Question 36

Describe the pathway for absorption of products of digestion in mammals

Answer 36

Lumen (inside) of ileum → cells lining ileum (part of small intestine) → blood

Question 37

Describe the absorption of amino acids and monosaccharides in mammals

Answer 37

Co-transport: ● Na + actively transported from epithelial cells lining ileum to blood (by Na + /K + pump) ● Establishing a conc. gradient of Na + (higher in lumen than epithelial cell) ● Na + enters epithelial cell down its concentration gradient with glucose against its concentration gradient ● Via a co-transporter protein ● Glucose moves down a conc. gradient into blood via facilitated diffusion

Question 38

Describe the absorption of lipids by a mammal, including the role of micelles

Answer 38

● Micelles contain bile salts, monoglycerides and fatty acids ○ Make monoglycerides and fatty acids (more) soluble in water ○ Carry / release fatty acids and monoglycerides to cell / lining of ileum ○ Maintain high concentration of fatty acids to cell / lining ● Monoglycerides / fatty acids absorbed (into epithelial cell) by diffusion (lipid soluble) ● Triglycerides reformed in (epithelial) cells and aggregate into globules ● Globules coated with proteins forming chylomicrons which are then packaged into vesicles ● Vesicles move to cell membrane and leave via exocytosis ○ Enter lymphatic vessels and eventually return to blood circulation