B7- Mass Transport

Question 1

Explain what happens in digestion

Answer 1

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

Question 2

Describe the digestion of starch in mammals

Answer 2

● 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 3

Describe the digestion of disaccharides in mammals

Answer 3

● 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 4

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

Answer 4

● 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 5

Describe the digestion of proteins by a mammal

Answer 5

● 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 6

Suggest why membrane-bound enzymes are important in digestion

Answer 6

● 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 7

Describe the pathway for absorption of products of digestion in mammals

Answer 7

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

Question 8

Describe the absorption of amino acids and monosaccharides in mammals

Answer 8

● 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 9

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

Answer 9

● 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

Question 10

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

Answer 10

● Red blood cells contain lots of haemoglobin (Hb) - no nucleus, biconcave, high SA:V, short diffusion path
● Hb associates with / binds / loads O2at 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 11

Describe the structure of haemoglobin

Answer 11

● Protein with a quaternary structure
● Made of 4 polypeptide chains
● Each chain contains a Haem group containing an iron ion (Fe2+)

Question 12

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

Answer 12

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

Question 13

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

Answer 13

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 14

Describe evidence for the cooperative nature of oxygen binding

Answer 14

● A 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 15

What is the Bohr effect?

Answer 15

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

Question 16

Explain effect of CO2 concentration on the dissociation of oxyhaemoglobin

Answer 16

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

Question 17

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

Answer 17

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

Question 18

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

Answer 18

● 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 19

Curve shift left → Hb has higher affinity for O2

Answer 19

● 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

Question 20

Curve shift right → Hb has lower affinity for O2

Answer 20

● 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 21

Describe the general pattern of blood circulation in a mammal

Answer 21

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 ; deoxygenated returns to right

Question 22

Suggest the importance of a double circulatory system

Answer 22

● Prevents mixing of oxygenated/deoxygenated blood
○ So blood pumped to body is fully saturated with oxygen for aerobic respiration
● Blood (after being lower from lungs)
○ Substances taken to/removed from body cells quicker / more efficiently

Question 23

Name the blood vessels entering and leaving the heart and lungs

Answer 23

● 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 24

Name the blood vessels entering and leaving the kidneys

Answer 24

● Renal arteries– oxygenated blood → kidneys
● Renal veins–deoxygenated blood to vena cava from kidneys

Question 25

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

Answer 25

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

Question 26

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

Answer 26

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

Question 27

Atrial systole

Answer 27

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 ventricles

Question 28

Ventricular systole

Answer 28

●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

Question 29

Diastole

Answer 29

●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 30

Explain how the structure of arteries relates to their function

Answer 30

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 31

Explain how the structure of arterioles relates to their function

Answer 31

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 32

Explain how the structure of veins relates to their function

Answer 32

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 33

Explain how the structure of capillaries relates to their function

Answer 33

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 34

Explain the formation of tissue fluid

Answer 34

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 35

Explain the return of tissue fluid to the circulatory system

Answer 35

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 36

Suggest and explain causes of excess tissue fluid accumulation

Answer 36

- 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 37

What is a risk factor? Give examples for cardiovascular disease

Answer 37

- 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 38

Describe the function of xylem tissue

Answer 38

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

Question 39

Suggest how xylem tissue is adapted for its function

Answer 39

- 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 40

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

Answer 40

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 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 7. Water enters roots via osmosis

Question 41

Describe how to set up a potometer

Answer 41

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 42

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

Answer 42

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 (πr2) ◦ Multiply this by distance moved by bubble 4. Calculate rate of water uptake - divide volume by time taken

Question 43

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

Answer 43

● Carry out the above, change one variable at a time (wind, humidity, light or temperature) ○ Eg. 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 44

Suggest limitations in using a potometer to measure rate of transpiration

Answer 44

- 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 45

Light intensity Increases rate of transpiration

Answer 45

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

Question 46

Temperature Increases rate of transpiration

Answer 46

Water molecules gain kinetic energy as temperature increases So water evaporates faster

Question 47

Wind intensity Increases rate of transpiration

Answer 47

Wind blows away water molecules from around stomata Decreasing water potential of air around stomata Increasing water potential gradient so water evaporates faster

Question 48

Humidity Decreases rate of transpiration

Answer 48

More water in air so it has a higher water potential Decreasing water potential gradient from leaf to air Water evaporates slower

Question 49

Describe the function of phloem tissue

Answer 49

Transports organic substances eg. sucrose in plants

Question 50

Suggest how phloem tissue is adapted for its function

Answer 50

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 51

What is translocation?

Answer 51

- 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 52

Explain the mass flow hypothesis for translocation in plants

Answer 52

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 53

Describe the use of tracer experiments to investigate transport in plants

Answer 53

1. Leaf supplied with a radioactive tracer eg. CO2 containing radioactive isotope 14C 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 54

Describe the use of ringing experiments to investigate transport in plants

Answer 54

1. Remove / kill phloem eg. remove a ring of bark 2. Bulge forms on source side of ring 3. Fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem 4. Tissues below ring die as cannot get organic substances

Question 55

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

Answer 55

- 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?