2hTransport

Simple, unicellular organisms

A single-celled organism such as Amoeba or a bacterium relies entirely on diffusion to move oxygen, carbon dioxide, food molecules and waste in and out of the cell. No heart, no blood, no vessels — and it works fine. Two reasons:

• Large surface area to volume ratio. The whole cell surface membrane is available for exchange, and there is only a tiny volume of cytoplasm to supply.
• Short diffusion distance. No part of the cell is far from the surface, so substances reach the centre quickly enough to keep the cell alive.

Add to that a low metabolic rate overall (a small organism has small total demands) and diffusion alone meets its needs.

Multicellular organisms

As an organism gets bigger, its volume increases faster than its surface area, so the surface area to volume ratio falls. That causes two problems at once:

So multicellular organisms need:

• specialised exchange surfaces (lungs, small intestine, root hairs, leaves) to get substances into the organism, and
• a transport system to carry substances between the exchange surface and every cell — the blood circulatory system in humans, and xylem and phloem in flowering plants.

Common mistake

Don't just say "it's too big". State the biology: surface area : volume ratio is too small and the diffusion distance is too great, so diffusion alone would be too slow to supply the cells.

Examiner tip. Watch the command word. Describe = say what happens; Explain = give the reason why. If a question says "Explain why a large organism needs a transport system", a description of the blood system scores nothing — you must link small surface area:volume ratio and long diffusion distance to diffusion being too slow.

Flowering plants have two separate transport tissues. Mixing them up is the easiest mark to lose in this sub-topic.

Xylem (2.54)

Water and dissolved mineral ions are absorbed by the roots and carried up through the xylem to the stem, leaves and flowers. The water is used for:

• photosynthesis in the leaves
• keeping cells turgid, which supports the plant
• replacing water lost by transpiration

Mineral ions such as nitrate ions are needed by the plant to make amino acids, and therefore proteins; magnesium ions are needed to make chlorophyll.

Phloem (2.53)

The leaves make glucose by photosynthesis. It is converted to sucrose for transport. The phloem carries sucrose and amino acids from the leaves to:

• growing regions (shoot tips, root tips, developing fruits and seeds) where they are used for respiration and to build new cells
• storage organs (roots, tubers, bulbs) where sucrose is stored, often as starch

The movement of dissolved food through phloem is called translocation.

Root hair cells (2.55) [separate Biology only — not in Science (Double Award)]

A root hair cell is an epidermal cell of the root with a long, thin extension into the soil.

• The hair gives a large surface area for absorbing water and mineral ions.
• The cell wall is thin, so the diffusion/osmosis distance is short.
• There are no chloroplasts — there is no light underground, so photosynthesis is impossible.

How water is absorbed: the soil water is a dilute solution (few dissolved solutes). The cytoplasm and cell sap of the root hair cell are a more concentrated solution (lots of dissolved sugars and ions). Water therefore moves into the root hair cell by osmosis, from the dilute solution to the more concentrated solution, across the partially permeable cell surface membrane. It then passes from cell to cell across the root and into the xylem.

Mineral ions are usually at a lower concentration in the soil than inside the root, so they are taken in by active transport, which requires energy from respiration (root hair cells contain many mitochondria).

Examiner tip. Don't transfer plant mineral needs to humans. Plants absorb nitrate ions to make amino acids and proteins; humans get their amino acids from digested protein in food, not from nitrates. Examiners regularly see 'humans need nitrates for protein' — it scores zero. Also, in osmosis questions always say that water has moved (into or out of the cells), not that the sugar or dye has moved.

[All of this note is separate Biology only — not in Science (Double Award).]

What transpiration is (2.56)

Transpiration is the evaporation of water from the surface of a plant.

Water evaporates from the surfaces of the mesophyll cells inside the leaf, and the water vapour then diffuses out through the stomata. This loss of water pulls more water up the xylem from the roots — the transpiration stream.

Factors affecting the rate (2.57)

Practical: investigating transpiration from a leafy shoot (2.58)

Apparatus: a potometer (capillary tube with a scale, connected to a leafy shoot via rubber tubing, with a reservoir/syringe to reset the bubble).

Setting up

1. Cut the shoot under water and assemble the potometer under water, so no air bubbles enter the xylem.
2. Dry the leaves, check all joints are airtight (use petroleum jelly on seals).
3. Let the shoot equilibrate for a few minutes, then introduce one air bubble into the capillary tube.

What you measure

• The distance the air bubble moves along the scale in a set time (e.g. mm per minute). Water uptake is used as a measure of the rate of transpiration.
• Repeat three times for each condition and take a mean; reset the bubble with the reservoir between readings.

Changing the conditions

Control variables: the same shoot (same number, size and species of leaves), the same time interval, and all the other three factors kept constant while you change one.

Expected result: bubble moves fastest in hot, dry, windy, bright conditions; slowest in humid, still, cool, dark conditions.

Common mistake

The potometer measures water uptake, which is assumed to equal water lost. Say this if asked for a limitation — some water is used in photosynthesis and to keep cells turgid.

Examiner tip. If a question asks you to plan or devise an investigation, you must write a plan — apparatus, what you change, what you measure and how, what you keep the same, repeats. Examiners report candidates who instead simply describe the effect of the factor and score almost nothing. Equally, if asked to explain the effect of wind or humidity, name the concentration gradient of water vapour — 'it dries the leaf' is not enough.

Composition of the blood (2.59)

Plasma (2.60)

Plasma transports:

• carbon dioxide — from respiring tissues to the lungs to be excreted
• digested food — glucose and amino acids from the small intestine to the liver and body cells
• urea — made in the liver from excess amino acids, carried to the kidneys to be excreted in urine
• hormones — from glands to their target organs
• heat energy — from warm organs (liver, muscles) around the body, helping to keep body temperature even

Red blood cell adaptations (2.61)

• Biconcave disc shape — large surface area to volume ratio for fast diffusion of oxygen in and out.
• No nucleus — more room for haemoglobin, so more oxygen can be carried.
• Contains haemoglobin — combines with oxygen in the lungs to form oxyhaemoglobin, and releases oxygen in respiring tissues where the oxygen concentration is low.
• Small and flexible, so they can squeeze through capillaries.

The immune response (2.62)

Two different white blood cells — do not mix them up:

Antibodies attach to the antigens on the pathogen's surface, causing pathogens to clump together and marking them for destruction by phagocytes.

Common mistake

Examiners repeatedly report candidates saying phagocytes produce antibodies. They do not. Phagocytes ingest; lymphocytes release antibodies.

Vaccination (2.63) [separate Biology only — not in Science (Double Award)]

A vaccine contains dead, weakened or harmless forms of the pathogen (or its antigens).

1. The antigens are recognised by lymphocytes, which produce antibodies — this is the primary response: slow, and only a small quantity of antibody.
2. Crucially, memory cells are also made and remain in the body.
3. If the same pathogen later infects you, the memory cells recognise its antigens and the secondary response occurs: antibodies are produced sooner, faster and in greater quantity.
4. The pathogen is destroyed before it can make you ill — you are immune.

The examiner-approved answer must include memory cells and all three of sooner / faster / greater quantity.

Blood clotting (2.64) [separate Biology only — not in Science (Double Award)]

When a blood vessel is damaged, platelets collect at the wound and trigger a series of reactions in which the soluble protein fibrinogen is converted into insoluble fibrin. The fibrin forms a mesh of fibres that traps red blood cells, forming a clot, which dries to a scab.

The clot:

• prevents blood loss
• prevents the entry of micro-organisms (pathogens) into the body

Examiner tip. Two guaranteed mark-losers here. (1) Never write that phagocytes make antibodies — examiners flag this every series. (2) On vaccination, don't stop at the primary response or at 'antibodies are released'. State that memory cells are produced and that on re-infection antibodies are made sooner, faster and in greater quantity.

Structure of the heart (2.65)

Four chambers, divided into left and right by the septum:

• Right atrium ← receives deoxygenated blood from the vena cava
• Right ventricle → pumps blood to the lungs through the pulmonary artery
• Left atrium ← receives oxygenated blood from the pulmonary vein
• Left ventricle → pumps blood to the whole body through the aorta

Valves stop backflow so blood flows one way only:

• Atrioventricular valves (tricuspid on the right, bicuspid on the left) between atria and ventricles
• Semilunar valves at the base of the pulmonary artery and aorta

The heart muscle itself is supplied with oxygen and glucose by the coronary arteries.

How it functions:

1. Atria fill with blood and contract, pushing blood into the ventricles.
2. Ventricles contract; atrioventricular valves close (the 'lub'), semilunar valves open, blood is forced into the pulmonary artery and aorta.
3. Ventricles relax; semilunar valves close (the 'dub') preventing backflow.

Why the left ventricle wall is thicker than the right: it must generate a higher pressure to pump blood all round the whole body, whereas the right ventricle only pumps to the lungs, which are close by and would be damaged by high pressure. This is a double circulation — blood passes through the heart twice for each circuit of the body.

Heart rate: exercise and adrenaline (2.66)

During exercise, muscles respire more, so they need more oxygen and glucose delivered and more carbon dioxide removed. Heart rate increases (and each beat is stronger), so blood flows faster and these substances are delivered/removed more quickly. Rising carbon dioxide levels in the blood are detected and the heart rate is increased in response.

Adrenaline is a hormone released by the adrenal glands when you are frightened, angry or excited. It is carried in the plasma to the heart, where it increases heart rate and the force of contraction, preparing the body for 'fight or flight' by delivering more oxygen and glucose to the muscles.

Coronary heart disease (2.67)

CHD occurs when the coronary arteries become narrowed by fatty deposits, reducing blood flow to the heart muscle, so it receives less oxygen and glucose.

Risk factors that increase the chance of developing CHD:

• diet high in saturated fat and cholesterol
• diet high in salt (raises blood pressure)
• smoking
• high blood pressure
• lack of exercise / obesity
• stress
• increasing age, being male, and genetic (inherited) factors

Blood vessels (2.68)

General structure of the circulation (2.69)

Learn the vessel names for each organ:

The pulmonary artery is the only artery carrying deoxygenated blood, and the pulmonary vein the only vein carrying oxygenated blood — arteries are defined by direction (away from the heart), not by oxygen.

What each organ removes:

• Lungs excrete carbon dioxide.
• Kidneys excrete urea (and excess water and salts) — made by the liver.

Examiner tip. Examiners report a common error: writing that carbon dioxide is excreted by the kidneys. It is not — the kidneys remove urea; carbon dioxide leaves at the lungs. Also, if asked to explain why the left ventricle wall is thicker, don't just describe it as thicker: link it to generating higher pressure to pump blood around the whole body.