Topic 3

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

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

Answer 2

Easier / quicker to find / more accurate because irregular shapes

Question 3

What is metabolic rate? Suggest how it can be measured

Answer 3

● 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 4

Explain the relationship between SA:V and metabolic rate

Answer 4

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 5

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

Answer 5

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 6

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

Answer 6

● 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 7

Describe the tracheal system of an insect

Answer 7

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 8

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

Answer 8

● 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)
○ As fluid is removed, air fills tracheoles
○ So rate of diffusion to gas exchange surface increases as diffusion is faster through air

Question 9

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

Answer 9

● 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 10

Explain how the gills of fish are adapted for gas exchange

Answer 10

● 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 11

Counter current flow:

Answer 11

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 12

What would happen if in a fish it were parallel flow

Answer 12

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

Be able to identify:
Trachea
Bronchi
Bronchioles
Lungs
Capillary network
Alveoli

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

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

Answer 19

Inspiration
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 20

Explain how humans breathe out

Answer 20

Expiration
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 21

Suggest why expiration is normally passive at rest

Answer 21

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

Question 22

Suggest how different lung diseases reduce the rate of gas exchange

Answer 22

● 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 23

Suggest how different lung diseases affect ventilation

Answer 23

● 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 24

Suggest why people with lung disease experience fatigue

Answer 24

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

Question 25

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

Answer 25

● 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 26

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

Answer 26

● 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 27

Explain the difference between correlations and causal relationships

Answer 27

● 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 28

Explain what happens in digestion

Answer 28

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

Question 29

Describe the digestion of starch in mammals

Answer 29

● 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 30

Describe the digestion of disaccharides in mammals

Answer 30

● 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 31

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

Answer 31

● 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 32

Describe the digestion of proteins by a mammal

Answer 32

● 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 amino acids in dipeptides → 2 amino acids ● Hydrolysis of peptide bond

Question 33

Suggest why membrane-bound enzymes are important in digestion

Answer 33

● 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 34

Describe the pathway for absorption of products of digestion in mammals

Answer 34

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

Question 35

Describe the absorption of amino acids and monosaccharides in mammals

Answer 35

Co-transport: 1 ● Na+ actively transported from epithelial cells lining ileum to blood (by Na+/K+ pump) ● Establishing a concentration gradient of Na+ (higher in lumen than epithelial cell) 2 ● Na+ enters epithelial cell down its concentration gradient with a monosaccharide or amino acid against its concentration gradient ● Via a co-transporter protein 3 ● Monosaccharide or amino acid moves down a concentration gradient into blood via facilitated diffusion

Question 36

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

Answer 36

● Bile salts combine with monoglycerides and fatty acids to form micelles ○ Micelles make monoglycerides and fatty acids (more) soluble in water ○ Micelles carry fatty acids and monoglycerides to cells lining the ileum, where they break down to release them ○ This maintains a high concentration of fatty acids and monoglycerides near cells lining the ileum ● Monoglycerides / fatty acids are absorbed (into epithelial cell) by diffusion (as they’re 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 fuse with it, releasing chylomicrons via exocytosis ○ Chylomicrons enter lymphatic vessels and eventually return to blood circulation

Question 37

Describe the role of red blood cells & haemoglobin (Hb) in oxygen transport

Answer 37

● Red blood cells contain lots of Hb ○ No nucleus & biconcave → more space for Hb, high SA:V & short diffusion distance ● Hb associates with / binds / loads oxygen at gas exchange surfaces (eg. lungs) where partial pressure of oxygen (pO2) is high ● This forms oxyhaemoglobin which transports oxygen ○ Each can carry four oxygen molecules, one at each Haem group ● Hb dissociates from / unloads oxygen near cells / tissues where pO2 is low

Question 38

Describe the structure of haemoglobin

Answer 38

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

Question 39

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

Answer 39

Areas with low pO2 - respiring tissues ● Hb has a low affinity for oxygen ● So oxygenreadily unloads / dissociates with Hb ● So % saturation is low Areas with high pO2 - gas exchange surfaces ● Hb has a high affinity for oxygen ● So oxygenreadily loads / associates with Hb ● So % saturation is high

Question 40

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

Answer 40

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 41

Describe evidence for the cooperative nature of oxygen binding

Answer 41

● 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 oxygen to bind

Question 42

What is the Bohr effect?

Answer 42

● Effect of CO2 concentration on dissociation of oxyhaemoglobin ● Oxyhaemoglobin dissociation curve shifts right

Question 43

Explain the effect of CO2 concentration on the dissociation of oxyhaemoglobin

Answer 43

1. Blood CO2 increases eg. due to increased rate of respiration 2. This lowers blood pH (more acidic) 3. Reducing Hb’s affinity for oxygen as its shape / tertiary / quaternary structure changes slightly 4. So more / faster unloading of oxygen to respiring cells at a given pO2

Question 44

Describe evidence for the Bohr effect

Answer 44

At a given pO2 %, the saturation of Hb with oxygen is lower

Question 45

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

Answer 45

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

Question 46

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

Answer 46

● Different types of Hb are made of polypeptide chains with slightly different amino acid sequences ● Resulting in different tertiary / quaternary structures / shapes ● So they have different affinities for oxygen

Question 47

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

Answer 47

Oxyhaemoglobin dissociation curve shifts left Hb has a 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 Oxyhaemoglobin dissociation curve shifts right Hb has a 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)