Mass Transport in Animals and Plants – Lecture Review

Question-and-answer flashcards covering key concepts in animal and plant mass transport, circulatory physiology, and related practical techniques.

What structural features of red blood cells maximise space for haemoglobin and surface area to volume ratio?

They have no nucleus and a biconcave shape.

Where does haemoglobin load (associate with) oxygen, and why?

At gas-exchange surfaces such as the lungs where the partial pressure of oxygen (pO₂) is high.

How many oxygen molecules can one haemoglobin molecule carry, and where do they bind?

Four oxygen molecules, one at each haem group.

Why does haemoglobin unload (dissociate) oxygen at respiring tissues?

Because the partial pressure of oxygen is low, giving haemoglobin a lower affinity for oxygen.

Describe the quaternary structure of haemoglobin.

It consists of four polypeptide chains, each with a haem group containing an Fe²⁺ ion.

What causes the S-shaped (sigmoid) oxyhaemoglobin dissociation curve?

Cooperative binding: binding of the first O₂ changes haemoglobin’s tertiary/quaternary structure, making additional O₂ binding easier.

Give one piece of evidence for cooperative oxygen binding from the dissociation curve.

A small increase in % saturation at low pO₂ but a rapid increase at higher pO₂ as additional O₂ binds more easily.

What is the Bohr effect?

An increase in CO₂ concentration lowers blood pH, reducing haemoglobin’s affinity for O₂ and shifting the dissociation curve to the right.

Why is the Bohr effect advantageous during exercise?

It causes more rapid unloading of O₂ where CO₂ is high, supporting faster aerobic respiration and greater ATP production.

How can different haemoglobins adapt organisms to low-oxygen environments?

Their dissociation curves shift left, giving a higher O₂ affinity so O₂ loads more readily at lower pO₂ (e.g., high-altitude species, foetuses).

State one reason a small or very active animal might have haemoglobin with a right-shifted curve.

To unload O₂ more readily at tissues with high metabolic rate where O₂ demand is high.

Outline the path of blood in the mammalian double circulatory system (names of sides only).

Right heart → lungs → left heart → body → right heart.

Why is a double circulatory system important for mammals?

It prevents mixing of oxygenated/deoxygenated blood and allows high-pressure delivery of O₂-rich blood to tissues.

Name the vessel that carries deoxygenated blood from the body to the heart.

The vena cava.

Which vessels take oxygenated blood from the lungs to the heart?

Pulmonary veins.

Why is the left ventricle wall thicker than the right?

It must contract with greater force to generate higher pressure to pump blood around the whole body.

During atrial systole, which valves are open and why?

Atrioventricular valves are open because atrial pressure exceeds ventricular pressure.

State the equation for cardiac output.

Cardiac output = stroke volume × heart rate.

How is heart rate calculated from cardiac-cycle data?

Heart rate (bpm) = 60 ÷ length of one cardiac cycle (s).

When do semilunar valves open?

When ventricular pressure exceeds pressure in the aorta or pulmonary artery.

Which structural feature of arteries helps maintain high blood pressure during diastole?

Thick elastic tissue that recoils as ventricles relax.

What role does smooth muscle in arterioles play?

Contraction causes vasoconstriction, narrowing the lumen and reducing blood flow to capillaries.

Give two adaptations of capillaries for efficient exchange.

One-cell-thick endothelium for short diffusion distance and an extensive capillary bed for large surface area.

Why do veins contain valves?

To prevent backflow of blood at low pressure.

Explain how tissue fluid forms at the arteriole end of a capillary.

High hydrostatic pressure forces water and dissolved substances out of the capillary into surrounding tissues.

How is most tissue fluid returned to the circulatory system at the venule end?

Lower hydrostatic pressure and low water potential inside capillary draw water back in by osmosis; excess enters lymph vessels.

Give one cause of oedema (excess tissue fluid).

High blood pressure increases outward hydrostatic pressure, causing more fluid to leave and less to return.

Define a risk factor and give two for cardiovascular disease.

A lifestyle aspect or environmental/biological factor linked to increased disease rate; examples: smoking, high saturated-fat diet.

List two safety precautions when using a scalpel in dissection.

Cut away from the body on a hard surface and carry the scalpel with the blade protected/pointing down.

What ethical consideration should be made when dissecting animals?

Use animals already killed humanely for meat, to avoid killing solely for dissection.

Outline how to prepare a temporary plant tissue mount for light microscopy.

Place drop of water on slide, add thin specimen section, stain (e.g., iodine), lower coverslip at an angle without air bubbles.

State two rules of scientific drawing for biology.

Use clear, continuous lines without shading, and include a labeled magnification scale.

What is the main function of xylem tissue?

To transport water and mineral ions from roots to leaves.

How does lignin in xylem walls aid its function?

Provides strength, prevents collapse under tension, and limits water loss.

Summarise the cohesion-tension theory in one sentence.

Transpiration creates tension pulling a continuous column of cohesive water molecules up the xylem, aided by adhesion to the vessel walls.

Describe two steps taken when setting up a potometer to ensure no air enters the xylem.

Cut the shoot underwater at a slant and insert it into the apparatus while submerged in water.

How is transpiration rate calculated using a potometer?

Measure distance an air bubble moves in a set time, calculate volume (πr² × distance), then divide by time.

Predict the effect of increased wind intensity on transpiration rate and explain why.

Rate increases because wind removes water vapour around stomata, steepening the water-potential gradient.

State the transport function of phloem tissue.

Translocation of organic substances such as sucrose between sources and sinks.

Why do sieve tube elements lack a nucleus and many organelles?

To maximise space for flow of assimilates through the lumen.

How do companion cells provide energy for translocation?

They contain many mitochondria for ATP production to fuel active transport of sucrose.

Outline one key step at the source in the mass flow hypothesis.

Sucrose is actively transported into sieve tubes, lowering their water potential so water enters by osmosis, raising hydrostatic pressure.

What is a tracer experiment, and what isotope is commonly used?

Using a radioactive substance (e.g., ¹⁴C in CO₂) to track movement of assimilates through the plant.

What result in a ringing experiment shows sugars move in the phloem?

A sugar-rich bulge forms above the removed ring of bark (phloem), and tissues below die from lack of sugars.

Give one question scientists consider when evaluating evidence for the mass flow hypothesis.

Does the evidence show movement is from high to low hydrostatic pressure?

Differentiate between transpiration and the transpiration stream.

Transpiration is loss of water vapour from leaves; the transpiration stream is the continuous flow of water through the plant.

Why is it incorrect to say 'oxygen bonds to haemoglobin'?

Oxygen binds (associates) to haemoglobin; it does not form covalent bonds.

Identify the pressure relationship that closes atrioventricular valves.

When ventricular pressure exceeds atrial pressure.

Why does a narrow lumen help arteries maintain high pressure?

It increases resistance to flow, helping sustain pressure generated by the heart.

Explain why low plasma protein concentration can lead to oedema.

Water potential in capillaries is not as low, reducing osmosis back into capillaries at the venule end, so more tissue fluid remains outside.

State one limitation of using a potometer to estimate transpiration rate.

Water uptake measured may not equal water lost because some water is used in photosynthesis or for turgor.

What common mistake involves confusing the two mass-transport hypotheses?

Mixing up the mass flow hypothesis (phloem, sucrose) with the cohesion-tension hypothesis (xylem, water).

Which blood vessel supplies oxygenated blood to the heart muscle itself?

Coronary arteries.

Give an example of an organism whose haemoglobin curve is left-shifted and explain why.

Foetus – needs to load O₂ from maternal blood where pO₂ is relatively low.

What happens to haemoglobin saturation at a given pO₂ when CO₂ concentration rises?

% Saturation decreases because the dissociation curve shifts to the right (Bohr effect).

Name two structural features of veins that help return blood to the heart despite low pressure.

Wide lumen to reduce resistance and valves to prevent backflow.

How do arterioles differ from arteries in elastic tissue, and why?

Arterioles have a thinner elastic layer because pressure surges are lower farther from the heart.

What variable should be kept constant when testing environmental effects on transpiration with a potometer?

All variables other than the one under investigation, e.g., keeping light, temperature, or humidity constant where not the test variable.

Why does water potential of mesophyll cells decrease during transpiration?

Evaporation removes water, raising solute concentration and lowering water potential, drawing water from the xylem.

Describe one adaptation of xylem pits.

Pits allow lateral movement of water between adjacent xylem vessels.

What does the term 'source' mean in translocation?

A plant region that produces or releases sucrose, e.g., photosynthesising leaves.

Why might movement in a ringing experiment not be due to gravity?

Because sugars accumulate above the ring regardless of plant orientation, indicating a phloem-mediated process.