Mass transport

In a nutshell

Most cells in a large organism are too far from an exchange surface to be supplied by diffusion alone, so substances are moved over long distances in bulk by mass transport: blood in animals, and water and sugars in plants.

This subtopic covers how oxygen is carried on haemoglobin, how the heart and blood vessels move blood and form tissue fluid, and how water (xylem) and sucrose (phloem) move through a plant.

Assumed knowledge: Surface area to volume ratio, Proteins, Transport across cell membranes.

Core content

Haemoglobin and the transport of oxygen

Haemoglobin is a protein with a quaternary structure: four polypeptide chains, each with a haem group that contains an Fe²⁺ ion.

Each Fe²⁺ binds one O₂ molecule, so one haemoglobin molecule carries up to four O₂ molecules. When it is carrying oxygen it is called oxyhaemoglobin.

The haemoglobins are a group of chemically similar molecules found in many organisms, not a single molecule.

Two words carry the marks:

• Loading (associating): haemoglobin binds oxygen. This happens where partial pressure of oxygen (pO₂) is high, e.g. at the gas exchange surface (lungs).
• Unloading (dissociating): haemoglobin releases oxygen. This happens where pO₂ is low, e.g. at respiring tissues.

Affinity is how strongly haemoglobin attracts and holds oxygen. High affinity means it loads readily and holds tightly; low affinity means it unloads readily.

The oxyhaemoglobin dissociation curve

The curve shows the percentage saturation of haemoglobin against the partial pressure of oxygen. It is S-shaped (sigmoid).

Read it as loading and unloading:

• At the high pO₂ in the lungs, haemoglobin has a high affinity for oxygen and is almost fully saturated (loads oxygen).
• At the low pO₂ in respiring tissues, haemoglobin has a lower affinity and unloads oxygen for use in respiration.

The steep middle of the curve matters: over the range of pO₂ found in tissues, a small fall in pO₂ causes a large fall in saturation, so a lot of oxygen is unloaded exactly where respiration is happening.

The cooperative nature of oxygen binding

The sigmoid shape is caused by cooperative binding.

The binding of the first oxygen changes the tertiary and quaternary structure of haemoglobin. This uncovers another binding site and makes the binding of further oxygen molecules easier.

That is why the curve is shallow at first (the first O₂ is hard to load) and then steepens (the next molecules load easily).

Still don't get it? · cooperative binding

Imagine a stiff four-seater sofa where the seats are folded shut. The very first person has to shove hard to prise a seat open and sit down. But the frame is all connected, so when they sit, their weight tugs the whole frame and pops the next seat open a little. Now the second person sits down more easily, and that opens the third, and so on. The first is a struggle; the rest are easy.

Now the exam version. Haemoglobin has four O₂ binding sites (one per haem group). The first O₂ binds with difficulty. When it does, it changes the tertiary and quaternary structure of the whole molecule, which uncovers another binding site and makes the next O₂ bind more easily. This is cooperative binding, and it is what gives the dissociation curve its S-shape.

To score the marks you must say the shape changes and that this uncovers/reveals another binding site. Just naming it "positive cooperativity" earns nothing.

The Bohr effect

Actively respiring tissues release more CO₂. The effect of this on oxygen unloading is the Bohr effect.

The causal chain (this is an "explain", so every link is a mark):

1. Respiring tissues produce more CO₂.
2. CO₂ forms carbonic acid, which releases H⁺ ions, lowering the pH.
3. The lower pH changes the shape of haemoglobin, reducing its affinity for oxygen.
4. So the dissociation curve shifts to the right: at the same pO₂, haemoglobin is less saturated and more oxygen is unloaded to the respiring tissue.

The advantage: where respiration is fastest (most CO₂), the most oxygen is released, matching supply to demand.

Different haemoglobins in different animals

Many animals are adapted to their environment by having different types of haemoglobin with different oxygen transport properties. The rule to learn is the direction of the shift:

• Curve shifted left = higher affinity for oxygen. Suits an animal living where pO₂ is low, so haemoglobin still loads oxygen despite the low pO₂ (e.g. an organism in a low-oxygen environment; fetal haemoglobin, which must load oxygen from the mother's blood at the placenta).
• Curve shifted right = lower affinity for oxygen. Suits an animal with a high metabolic rate, so haemoglobin unloads oxygen readily to keep up with respiration (e.g. a small mammal with a large surface area to volume ratio and high respiration rate).

The general pattern of circulation in a mammal

Mammals have a double circulatory system: blood passes through the heart twice for each complete circuit of the body.

• Pulmonary circulation: right side of the heart to the lungs and back.
• Systemic circulation: left side of the heart to the body and back.

Returning to the heart between the two circuits raises the blood pressure again before blood is sent to the body, giving faster, more efficient delivery.

You are required to name only these vessels:

The coronary arteries branch from the aorta and supply the heart muscle itself with oxygenated blood.

The gross structure of the human heart

• Atria are thin-walled: they only pump blood a short distance into the ventricles.
• Ventricles are thick-walled: they pump blood out of the heart.
• The left ventricle wall is thicker than the right ventricle wall, because it must generate a higher pressure to pump blood around the whole body (systemic circulation); the right ventricle pumps only to the nearby lungs, at lower pressure.
• The septum separates oxygenated and deoxygenated blood.
• Valves keep blood flowing one way (unidirectional flow).

The cardiac cycle

The cardiac cycle is one heartbeat, in three stages. Throughout, a valve opens or closes purely because of a pressure difference: it opens when pressure behind it is greater, and closes when pressure in front is greater.

1. Atrial systole: atria contract, pushing blood into the ventricles (AV valves open, semilunar valves closed).
2. Ventricular systole: ventricles contract. Ventricular pressure rises above atrial pressure, so AV valves close; it then rises above aortic pressure, so semilunar valves open and blood is forced into the aorta and pulmonary artery.
3. Diastole: all chambers relax. Ventricular pressure falls below aortic pressure, so semilunar valves close; atria fill and pressure rises above ventricular pressure, so AV valves open and the ventricles fill again.

The pressure graph is the graph AQA examines most in this topic. Read the valve events off the crossovers:

• Where the ventricle line rises above the atrium line, the AV valve closes.
• Where the ventricle line rises above the aorta line, the semilunar valve opens (blood is ejected).
• Where the ventricle line falls below the aorta line, the semilunar valve closes.
• Where the atrium line rises above the ventricle line, the AV valve opens (filling).

The right side shows the same pattern but reaches a lower maximum pressure, because the right ventricle wall is thinner.

Arteries, arterioles and veins

Every feature is learned as structure linked to function:

Note the vocabulary: elastic tissue stretches and recoils (never "contracts"); only muscle contracts and relaxes.

Capillary beds and tissue fluid

A capillary bed is a dense network of capillaries, and it is the exchange surface between blood and cells. Substances are not exchanged directly with cells: they pass out of the capillary to form tissue fluid, which bathes the cells.

Formation (at the arteriole end):

1. Blood arrives at high hydrostatic (blood) pressure.
2. This pressure forces water and small molecules (glucose, oxygen, amino acids, ions) out of the capillary.
3. Large plasma proteins are too big to leave, so they stay in the capillary.

Return (at the venule end):

4. The plasma proteins left behind give the blood a lower (more negative) water potential than the tissue fluid.

5. Hydrostatic pressure has now fallen (fluid has left, and the blood is further from the heart).

6. So water re-enters the capillary by osmosis, down the water potential gradient.

7. Excess tissue fluid drains into the lymphatic system and is eventually returned to the blood.

Still don't get it? · how tissue fluid forms and returns

Think of a garden hose with tiny pinholes, lying in a sponge. Near the tap the water pressure is high, so water squirts out of the holes into the sponge. Further along the hose the pressure has dropped, and because the hose water is now "saltier" (it kept its big dissolved bits), it actually draws some of that sponge water back in. Anything left in the sponge slowly drains away down a separate gutter.

Now the biology. Near the arteriole end, high hydrostatic pressure forces water out of the capillary; the big plasma proteins stay behind. That lost water becomes tissue fluid around the cells. At the venule end, the proteins that stayed behind have given the blood a lower water potential, and the hydrostatic pressure has dropped, so water moves back in by osmosis. The leftover fluid drains into the lymphatic system.

Two phrases win or lose the marks: it is water that moves (not "tissue fluid" or "plasma"), and it returns by osmosis down a water potential gradient.

Cardiovascular disease: interpreting risk factors (data skill)

The spec here is a skill, not a mechanism to recall. You should be able to:

• Interpret data linking a risk factor (something that increases the probability of getting a disease, e.g. high blood pressure, smoking, a high-fat diet, age, genetics) to the incidence of cardiovascular disease.
• Distinguish correlation from causation: a correlation (two things changing together) does not prove that one causes the other; a third factor may be involved.
• Evaluate conflicting evidence: compare sample sizes, whether other variables were controlled, and whether differences are significant (e.g. do the error bars / standard deviations overlap?).

Required practical 5: dissection of a mass transport system

Required practical 5 is the dissection of an animal or plant gas exchange or mass transport system, or an organ from one (commonly a mammalian heart).

• Safety, specific to the practical: cut on a board with the scalpel blade facing away from fingers; wear gloves and wash hands, because raw tissue can carry pathogens. (Do not give generic lab rules like "tie hair back".)
• Identify the external features (thicker left side, aorta, vena cava, pulmonary artery, pulmonary vein, coronary arteries), then cut open to see the chamber walls and the valves.
• A biological drawing uses sharp continuous pencil lines, no shading, ruled label lines that do not cross, and a scale or magnification.

Xylem and the transport of water

Xylem is the tissue that transports water and mineral ions from the roots up the stem to the leaves. Xylem vessels are adapted for this:

• Dead, hollow cells with no end walls, forming a continuous column of water.
• No cytoplasm or organelles, so water flow is not obstructed.
• Walls thickened with lignin, which supports the vessel and stops it collapsing under tension.
• Pits (non-lignified gaps) let water move sideways between vessels.

The cohesion-tension theory

This explains how water is pulled up the xylem. Teach it as an ordered chain:

1. Transpiration: water evaporates from mesophyll cells and diffuses out of the leaf through the stomata, down a water potential gradient.
2. This lowers the water potential in the leaf, so water is pulled out of the xylem, creating tension (negative pressure) in the xylem.
3. Water molecules are held together by hydrogen bonds (cohesion), so they form a continuous column: as water leaves the top, the whole column is pulled up.
4. Adhesion of water molecules to the xylem walls also helps the column move up.

Do not say "reduced pressure": the mark word is tension (negative pressure). Do not describe hydrogen bonds as "strong".

Transpiration and the potometer (data skill)

A potometer measures the rate of water uptake by a shoot (a close estimate of transpiration rate). Key steps and why:

• Set up and cut the shoot under water, and seal joints, so no air enters the xylem and breaks the water column.
• Record how far the air bubble moves in a set time.
• Assumption: water uptake is taken as equal to transpiration, though some water is used in photosynthesis and for support.

Transpiration rate increases with higher temperature (more kinetic energy, faster evaporation and diffusion), lower humidity and higher wind speed (both steepen the water potential gradient out of the leaf), and higher light intensity (stomata open).

Phloem and translocation

Phloem is the living tissue that transports organic substances (mainly sucrose). Its movement from source (where sucrose is made or released, e.g. leaves) to sink (where it is used or stored, e.g. roots) is called translocation, and it can occur in both directions in the plant.

• Sieve tube elements: the conducting cells, joined end to end by perforated sieve plates; they have little cytoplasm and no nucleus, so sap flows freely.
• Companion cells: packed with mitochondria to supply ATP for the active transport of sucrose into the phloem.

The mass flow hypothesis

This explains translocation as a pressure-driven flow:

1. At the source: sucrose is actively transported (loaded) into the sieve tubes via the companion cells.
2. This lowers the water potential in the sieve tube, so water enters by osmosis (from the xylem), raising the hydrostatic pressure at the source.
3. Sucrose is removed at the sink, raising the water potential there, so water leaves and hydrostatic pressure falls.
4. The difference gives a hydrostatic pressure gradient, and the phloem sap moves by mass flow down it, from source to sink.

Evidence for transport in plants

The spec requires you to interpret tracer and ringing experiments and evaluate the evidence.

• Ringing: removing a ring of outer bark (phloem), leaving the xylem, causes sugars to build up above the ring (swelling), showing sugars are transported in the phloem.
• Radioactive tracers: a plant given radioactive ¹⁴CO₂ makes radioactive sucrose; autoradiography shows the label moving in the phloem, confirming translocation.

Evidence questioned: if translocation were simple mass flow, all solutes should move at the same rate and in one direction per tube; but different solutes can move at different rates and sometimes in opposite directions, so some argue additional active processes are involved.

Worked examples

Model calculation: cardiac output. One cardiac cycle lasts 0.8 s and the stroke volume is 70 cm³. Calculate the cardiac output in dm³ min^-1.

Step 1: heart rate is the number of cycles per minute.

$heart\ rate = \frac{60}{0.8} = 75\ \text{beats min}^{-1}$

Step 2: cardiac output is stroke volume multiplied by heart rate.

$cardiac\ output = stroke\ volume \times heart\ rate = 70 \times 75 = 5250\ \text{cm}^3\ \text{min}^{-1}$

Step 3: convert to dm³ min^-1 (divide by 1000). The final answer, with units, is:

$cardiac\ output = 5.25\ \text{dm}^3\ \text{min}^{-1}$

If instead you are given cardiac output and heart rate, rearrange to $stroke\ volume = \dfrac{cardiac\ output}{heart\ rate}$.

Model long-answer: "Explain how tissue fluid is formed and how it is returned to the circulatory system." (This is the point examiners report as most often lost; each numbered line is a separate marking point.)

1. At the arteriole end the hydrostatic pressure of the blood is high.
2. This forces water and small molecules out of the capillary, forming tissue fluid.
3. Large plasma proteins remain in the capillary.
4. These proteins give the blood a lower (more negative) water potential than the tissue fluid.
5. At the venule end the hydrostatic pressure has fallen, so water re-enters the capillary by osmosis, down the water potential gradient.
6. Excess tissue fluid drains into the lymphatic system and is returned to the blood.

Common exam mistakes

• Writing that the coronary arteries supply "the whole body"; they supply the heart muscle with oxygenated blood.
• Saying elastic tissue in artery walls "contracts". Elastic tissue stretches and recoils; only muscle contracts and relaxes. Saying the aorta wall gets thicker "with increasing pressure" is also wrong.
• For arterioles, muddling the terms: it is the muscle that contracts and the arteriole (lumen) that narrows. Answers that say muscles "constrict" or arterioles "contract, stretch or recoil" lose the mark.
• Explaining the capillary's thin wall only as "a short diffusion pathway" without saying it is the pathway between the blood and the surrounding tissues; the fuller link is needed at AS.
• On the cardiac cycle graph, stating "pressure and flow go up together" instead of explaining valve events by pressure differences (a valve opens when pressure behind it exceeds pressure in front).
• Describing tissue-fluid build-up as "more tissue fluid is forced out". It is more water/fluid forced out; "tissue fluid" and "plasma" are rejected here.
• Saying oxygen "diffuses" from one side of the heart to the other; blood moves by mass flow from high to low pressure.
• Cooperative binding: just naming "positive cooperativity" or saying haemoglobin "changes shape to make binding easier" (which only restates the question). You must say binding changes the tertiary/quaternary structure and uncovers another binding site.
• On a dissociation curve, calling a rightward shift an "increase in affinity". A shift right means lower affinity (more oxygen unloaded); it must be linked to the partial pressure of oxygen, not just "oxygen levels".
• For cohesion-tension, vague answers like "water is pulled up by cohesion-tension" or water is "sticky", without saying cohesion is hydrogen bonding forming a continuous column and tension is negative pressure. "Reduced pressure" is not accepted for tension; "strong hydrogen bonds" is rejected.
• In the phloem, saying glucose is transported (it is sucrose), that phloem is made of dead cells (xylem is dead; phloem is living), or that sugars move by cohesion-tension (that is water in the xylem).
• Saying mitochondria in companion cells "make/produce energy"; they release energy / produce ATP by respiration for active transport.
• Xerophyte and potometer traps: "hairs trap water" (it is water vapour), "concentration gradient" (it is water potential gradient), and thinking water is produced in photosynthesis or used in respiration (it is the other way round).

Key definitions

• Haemoglobin - a protein with a quaternary structure (four polypeptide chains, each with a haem group containing an Fe²⁺ ion) that transports oxygen.
• Affinity for oxygen - how strongly haemoglobin attracts and binds oxygen.
• Partial pressure of oxygen (pO₂) - the pressure contributed by oxygen in a mixture of gases; a measure of oxygen concentration.
• Cooperative binding - the binding of the first oxygen molecule changes the tertiary and quaternary structure of haemoglobin, uncovering another binding site and making the binding of further oxygen molecules easier.
• Bohr effect - at higher carbon dioxide concentrations the affinity of haemoglobin for oxygen is reduced (the dissociation curve shifts to the right), so more oxygen is unloaded at a given partial pressure of oxygen.
• Cardiac cycle - one complete heartbeat: atrial systole, ventricular systole and diastole.
• Cardiac output - the volume of blood pumped by the heart per minute; cardiac output = stroke volume × heart rate.
• Tissue fluid - the fluid surrounding cells, formed when the high hydrostatic pressure at the arteriole end forces water and small molecules out of the capillary (large plasma proteins remain behind).
• Cohesion - the attraction between water molecules caused by hydrogen bonding, forming a continuous column of water.
• Tension - the negative pressure created in the xylem as water is pulled up by transpiration.
• Adhesion - the attraction between water molecules and the walls of the xylem vessels.
• Transpiration - the loss of water vapour from a plant by evaporation from the leaves and diffusion through the stomata.
• Translocation - the transport of organic substances (mainly sucrose) in the phloem from source to sink.
• Mass flow hypothesis - sucrose is actively loaded into the phloem at the source, lowering water potential so water enters by osmosis and raises the hydrostatic pressure; sap then flows by mass flow down a hydrostatic pressure gradient to the sink.
• Source - a part of the plant that loads sucrose into the phloem. Sink - a part that removes sucrose from the phloem.

Specification

• I can describe haemoglobin as a protein with a quaternary structure and state the role of haemoglobin and red blood cells in transporting oxygen.
• I can explain the loading, transport and unloading of oxygen using the oxyhaemoglobin dissociation curve.
• I can explain the cooperative nature of oxygen binding (the change in shape after the first oxygens makes further binding easier).
• I can explain the effect of carbon dioxide concentration on oxygen unloading (the Bohr effect).
• I can explain how animals with different types of haemoglobin are adapted to their environment.
• I can describe the general pattern of circulation in a mammal and name the coronary arteries and the vessels of the heart, lungs and kidneys.
• I can describe the gross structure of the human heart and explain the pressure and volume changes and valve movements of the cardiac cycle.
• I can relate the structure of arteries, arterioles and veins to their functions.
• I can describe the structure of capillaries and explain the formation of tissue fluid and its return to the circulatory system.
• I can analyse data on the cardiac cycle and on risk factors for cardiovascular disease, and recognise the difference between correlation and causation.
• I can describe xylem and explain the cohesion-tension theory of water transport.
• I can describe phloem and explain translocation by the mass flow hypothesis.
• I can interpret tracer and ringing experiments and evaluate the evidence for and against the mass flow hypothesis.

Related notes

• Surface area to volume ratio
• Gas exchange
• Digestion and absorption
• Transport across cell membranes
• Proteins
• Water