S2 Transport Systems in Plants & Humans

• A multi-cellular organism is built from many cells.
• Only the thin outer layer of cells touches the surroundings; the cells deep inside do not.
• Those inner cells still need oxygen and food in, and carbon dioxide and waste out.
• In a very small organism this happens by itself — substances diffuse in from outside and reach every cell quickly.
• Once the body grows large, that shortcut fails: the distance from the surface to the inner cells is too great.
• A transport system is then needed: vessels through which a fluid is pushed, carrying dissolved substances from where they are taken in to where they are used.

• Diffusion is the passive spreading of particles from a region of higher concentration to one of lower concentration.
• It is how substances cross the short distances inside a cell, and it costs the organism nothing.
• But the further a particle travels, the longer the journey takes: quick across a few micrometres, hopeless across tens of centimetres.
• Two changes work against it as an organism grows.

• Doubling a body's size roughly doubles the distance from the surface to the cells at its core.
• The time needed to diffuse that distance rises far more steeply, because a particle wanders at random rather than travelling in a straight line.
• Interior cells would run short of oxygen long before enough of it drifted in.

• As a body grows, its volume increases faster than its surface area.
• A large organism therefore has less exchange surface spread over more living tissue.
• The surface can no longer take in enough for the whole body.

• Human blood never leaves the vessels: it is pushed round a closed set of tubes.
• During one complete journey round the body it is driven through the heart twice — a double circulation.
• It consists of two linked circuits, the pulmonary and the systemic.

• Splitting the work brings a pressure advantage: blood sent to the lungs travels only a short way, so a gentle push is enough — hence the right ventricle's modest muscle.
• That blood then returns to the heart and is pumped out again with fresh force for the much longer trip to the body.
• With a single pump and one loop it would reach the tissues too slowly and at too low a pressure.

• The heart is a muscular organ lying in the chest, tilted slightly to the left, with the ribcage in front of it for protection.
• It is roughly the size of its owner's closed hand.
• It contracts and relaxes without rest, typically 60 to 100 times a minute at rest.
• Each contraction drives blood out into the vessels; each relaxation lets the chambers refill.

• Four chambers sit inside. The upper two are the atria (singular: atrium); the lower two, larger ones are the ventricles.
• An atrium's wall is thin, because it only pushes blood down into the ventricle directly below it.
• A ventricle's wall is much thicker, because it drives blood out of the heart altogether.

• A thick muscular sheet, the septum, runs down the middle of the heart and seals the left chambers off from the right.
• Oxygen-rich blood is confined to the left side and oxygen-poor blood to the right, so the two never mix.
• Without the septum the tissues would receive a diluted supply of oxygen.

• A valve is a flap of tissue that lets blood pass one way but snaps shut if blood tries to move backwards.
• The heart holds four of them, and they turn its repeated squeezing into a steady one-way flow instead of a sloshing back and forth.

• The atrioventricular valves are anchored by the chordae tendineae, cord-like tendons joining the flaps to the papillary muscles on the floor of each ventricle.
• Like the strings of a parachute, they stop the flaps being turned inside out into the atria when the ventricles contract hard.
• Veins contain valves too, keeping slow-moving blood from sinking backwards.
• Arteries have none, since their blood already travels forwards under high pressure.

• Tracing the path of blood in the correct order is a standard examination task, so learn it as one chain rather than as separate facts.

• As a story: blood that has given up its oxygen to the body's cells drains through the vena cava into the right atrium, which passes it through the tricuspid valve into the right ventricle.
• That ventricle contracts, the tricuspid valve shuts, and the semi-lunar valve in the pulmonary artery opens, sending blood to the lungs to release carbon dioxide and take up oxygen.
• Oxygen-rich blood returns through the pulmonary veins to the left atrium, which passes it through the bicuspid valve into the left ventricle.
• When that ventricle contracts, the semi-lunar valve at the entrance to the aorta opens and blood is forced out at high pressure to the whole body.

• The vessels that deliver it are the arteries, branching repeatedly until they become the narrow capillaries threading between the body cells.
• The finest are so narrow that blood cells pass through in single file.
• At these walls oxygen and digested food leave the blood for the cells, and carbon dioxide and waste enter it.
• Blood then flows into veins, which return it to the heart to begin the cycle again.

• Blood does not wander freely through the tissues: it is kept inside a closed, branching network of tubes.
• Each kind of tube in that network is shaped for the job it has to do, and three types are named in the syllabus.
• An artery is a vessel that carries blood away from the heart.
• A vein is a vessel that carries blood back towards the heart.
• A capillary is the fine vessel that runs between the body cells, and it is the only place in the network where substances are exchanged.

• Blood leaves the heart under a strong push, so the artery receives it while the pressure is at its greatest. Two features of the artery match that demand.
• The wall is thick and elastic, containing muscular and elastic tissue that stretches as the surge of blood passes through and then springs back — this keeps blood moving onward between heartbeats instead of flowing in a single violent spurt.
• The lumen (the central channel through which blood flows) is small, so blood is squeezed through a narrow opening and pressure is kept up rather than allowed to fall away.

• Because the heart itself keeps driving blood forward, an artery needs no valves along its length.
• The pressure inside an artery is high, and this is also why an artery that is cut bleeds in a rapid, pulsing jet.

• The blood in most arteries is oxygenated — it has collected oxygen in the lungs and is on its way to supply the body.
• The one exception is the pulmonary artery: it is an artery by direction of flow (it leaves the heart) but carries deoxygenated blood, because its destination is the lungs, where the blood is to pick oxygen up.

• By the time blood reaches a vein it has travelled through capillaries and has lost most of the pressure given to it by the heart.
• A vein is therefore built for gentle flow rather than for withstanding force.
• Its wall is thin and less elastic, since there is little pressure to contain.
• It has a large lumen, which allows blood to return easily without meeting resistance.

• Low pressure creates a problem: blood moving sluggishly uphill towards the heart could slip backwards.
• Valves solve it — flaps of tissue lining the inside of the vein, pointing in the direction of the heart.
• Blood travelling the right way pushes them open and passes; any blood starting to flow the wrong way presses the flaps shut and is stopped.
• Squeezing of the veins by the surrounding muscles as the body moves also helps to push blood along.

• Most veins carry deoxygenated blood, but the pulmonary vein is again the exception: it returns oxygenated blood from the lungs to the heart.

• For a substance to move between blood and a body cell, three conditions help: a large total surface area for exchange, a very short distance across which the substance must travel, and blood that moves slowly enough to give up what it carries.
• The capillary answers all three.

• Capillaries branch so profusely that no cell in the body lies far from one, which gives an enormous total surface area.
• Their wall is only one cell thick, so the gap that a substance must cross is extremely short.
• Their lumen is narrower than the width of a single red blood cell, so the cells are forced into single file and the flow slows down.
• The wall is also permeable, letting dissolved substances pass through it.

• Capillaries have no valves, and the pressure within them is low.
• Blood enters a capillary bed from a small artery branch and leaves it through a small vein branch, having delivered some of its load and collected waste in exchange.

• What exchange needs is a blood supply lying immediately beside the cells it serves, with as little material between the two as possible.
• That is exactly how a capillary is arranged: it presses close to the body cells and its wall is a single cell thick.
• The distance a substance must cover is therefore kept to a minimum, and exchange happens quickly and efficiently.

• Oxygen and digested food pass out of the blood in the capillary, through the capillary wall into the tissue fluid that bathes the cells, and from there into the body cells themselves.
• In the opposite direction, carbon dioxide and other waste products released by the cells move into the tissue fluid and then pass into the capillary, to be carried away.
• Both movements take place by diffusion, down a concentration gradient.

• Blood is best described as a liquid tissue: it is a fluid, yet it is a body tissue in its own right, made up of several components working together.
• Four components are named in the syllabus — plasma, red blood cells, white blood cells and platelets.
• Plasma forms a little more than half of the blood volume, and the cells and cell fragments together make up the remainder, suspended in the plasma.

• Plasma is the pale yellow liquid part of blood, and it forms a little more than half of the blood volume.
• Its main role is transport: it is the medium in which dissolved substances are carried from one part of the body to another.
• Among the materials it carries are digested food substances from the small intestine, excretory products such as urea and carbon dioxide, hormones, soluble proteins and mineral salts.

• Because these substances dissolve in plasma, they can be moved wherever they are needed while the blood is circulating.
• The cells of the blood are themselves carried along in the plasma.

• A red blood cell is built for one purpose: carrying oxygen to every part of the body.
• It can do this because it contains haemoglobin, a red pigment that combines with oxygen in the lungs and releases it in the tissues.
• Red blood cells are manufactured in the bone marrow.

• Its biconcave shape — a disc dented on both faces — gives a large surface area in proportion to its volume, so oxygen can diffuse in and out rapidly.
• It has no nucleus and very few other organelles, which leaves the maximum space for haemoglobin.
• It also lacks mitochondria, which matters because a cell that respired would use up the very oxygen it is supposed to deliver.
• Finally, the cell is very small and flexible, so it can bend and squeeze through capillaries narrower than itself.

• White blood cells defend the body rather than transport materials.
• They differ from red blood cells in several visible ways: each one has a nucleus, is colourless because it contains no haemoglobin, is larger, and may be roughly spherical or irregular in outline.
• Their number rises when the body is invaded by foreign material.

• Their defence takes two forms.
• Some white blood cells engulf foreign microorganisms, a process called phagocytosis. Detecting a foreign body such as a pathogen or cell debris, the cell changes shape and flows around it, taking it into a vesicle inside itself, where enzymes break it down; the cell then either uses or expels the remains.
• White blood cells that die in this work accumulate as pus.
• Other white blood cells, the lymphocytes, produce antibodies — proteins that recognise and attach to particular foreign microbes so that they can be dealt with.
• A person whose supply of one type of white blood cell is far below normal falls ill easily, because that type can no longer deal with the infections it normally guards against.

• Platelets are not whole cells but fragments of cells, each a small packet of cytoplasm enclosed by a membrane.
• They are smaller than red blood cells, have no nucleus, and yet remain metabolically active because they contain mitochondria and can release energy.

• Their work is to begin the clotting process.
• When a vessel is cut, platelets gather at the damaged point and set off the reactions that produce a clot.
• The clot seals the wound, which limits how much blood is lost and blocks the route by which pathogens or other foreign bodies could enter.
• Blood left to stand separates into plasma and cells; platelets remain with the cell fraction.

• Between them, plasma and red blood cells move a wide range of materials around the body.
• Blood delivers oxygen, bound to haemoglobin in the red cells, and carries digested food substances absorbed from the small intestine.
• It removes carbon dioxide from the respiring tissues and takes excretory products such as urea to the organs that will get rid of them.
• It carries hormones, released by glands, to the organs they act on, along with mineral salts and soluble proteins.
• It also distributes the heat produced by respiration, helping to keep body temperature steady.

• A tiny plant can rely on substances drifting in and out of its cells by diffusion alone, because every cell sits close enough to the outside for the distance to be short.
• A plant that grows tall cannot work that way: a leaf at the top of the shoot is metres away from the roots, and diffusion is far too slow to move water that far before the leaf runs dry.
• Multi-cellular plants therefore need a transport system: a set of internal channels that carries substances quickly between the part that makes food and the parts that need it.

• Two movements have to happen at once, in opposite directions.
• Water and mineral salts are absorbed by the roots from the soil and must travel upward to the shoots, leaves and flowers.
• Sugar made in the green leaves during photosynthesis must travel to wherever it is used or stored — growing tips, stems, roots, fruits and seeds.
• A rose bush in the garden, a mango tree, and a fern all solve this the same way: with long tubes that run through the whole plant body.

• Plants build their internal plumbing from two tissues, each specialising in one cargo.
• The xylem carries water and mineral salts.
• The phloem carries food.

• Running side by side, the two form a continuous, hollow network that threads from the root tips up through the stem, out into every leaf, and on into flowers and fruits.
• Where xylem and phloem lie next to each other inside a stem or leaf they are together called a vascular bundle.
• Because a stem has to stay upright while the plant grows taller, the bundles also serve a structural role, stiffening the shoot so it does not flop over.

• A xylem vessel is a column of dead cells.
• As each cell matures it loses its living contents — the nucleus, the cytoplasm and the cell membrane all break down — so that only the tough outer cell wall is left.
• The end walls of neighbouring cells also disappear, so the dead cells join into one open tube rather than a chain of closed boxes.

• These two changes give the xylem one long, unobstructed channel from root to leaf.
• A column of water can move through it with very little friction, which is exactly what a tall plant needs.
• The walls are thickened with lignin, a hard, waterproof material that keeps the tube from being squeezed shut as water is pulled through it, and that also gives the plant its rigidity.

• The phloem is built from sieve tube elements stacked end to end.
• Each element keeps its cell wall but has lost most of its internal contents, including its nucleus, which leaves the interior fairly clear.
• Where two elements meet, the end wall is perforated with tiny holes like the holes of a sieve — a sieve plate — so dissolved food can flow from one element straight into the next.

• Packed alongside each sieve tube element is a companion cell.
• Companion cells keep their nucleus and cytoplasm and are packed with mitochondria, the sites of respiration.
• They stay alive and act as the control centres for the sieve tubes beside them, supplying the energy needed to load food into the phloem.

Reading the two tissues as a pair makes the division of labour obvious.

• A root hair is a long, narrow outgrowth from an outer cell of the root.
• It pushes out into the film of water between soil particles.
• Because it is so thin and so numerous it multiplies the surface area across which the root can absorb water and dissolved mineral salts; a single root tip may carry many thousands of them.

• Inside a root hair cell, the cell sap holds a high concentration of sugars and dissolved mineral salts; the soil solution around it is far more dilute.
• Water therefore crosses the partially permeable cell membrane from the dilute soil solution into the more concentrated cell sap by osmosis.
• Mineral salts are absorbed as well, usually by active transport, which uses energy from respiration to move ions into the cell even when the concentration inside is already higher than outside.
• Diffusion also carries some ions inward whenever a concentration gradient happens to favour it.

Water does not jump straight from soil to xylem; it steps from cell to cell. Trace the route:

1. Water in the soil enters a root hair cell by osmosis.
2. From that cell it passes into the neighbouring cell closer to the centre of the root, again by osmosis, because each successive cell has a slightly more concentrated sap.
3. The movement continues cell by cell across the root until the water reaches a xylem vessel.
4. Water then travels up the xylem vessels in the stem as a continuous column.
5. In the leaf, water leaves the xylem and enters the surrounding mesophyll cells by osmosis, still moving from one cell to the next.
6. Inside the leaf, the air spaces between the mesophyll cells are saturated with water vapour. Some of the water reaching the cell surfaces evaporates into these spaces.
7. The water vapour then diffuses out of the leaf to the outside air through the stomata, the pores in the leaf surface.

Steps 6 and 7 together are what we call transpiration: the loss of water vapour from the plant, chiefly through the stomata of the leaves.

• No pump drives water up a plant. Instead three effects add up, and the last of them does most of the work in a tall shoot.

• The root cells surrounding the xylem vessels actively move ions into them.
• That raises the concentration inside the xylem, water follows by osmosis, and the extra water pushes the column above it a little way upward.
• Root pressure is real but weak — it can only lift water a short distance.

• Water molecules are attracted to the lignin-lined walls of a very narrow tube and creep up its sides.
• This attraction, together with the clinging of water molecules to one another, drags a thin film of water upward.
• The narrower the tube, the higher the film climbs — but in a xylem vessel of realistic width, this effect alone cannot carry water more than a few metres.

• This is the dominant mechanism. As water vapour escapes from the leaves, the leaf cells left behind become more concentrated, so water moves into them from the xylem by osmosis.
• Removing water from the top of the xylem column creates a suction that pulls the whole column upward.
• Because the water molecules hold tightly to one another, the pull is transmitted all the way down to the roots, like drawing on a long straw.
• Water lost at the leaf therefore tugs the next mouthful up from below, and the soil supplies the root hairs to replace it.

• Transpiration happens mainly at the leaves, and mainly through the stomata.
• Each stoma is a gap in the leaf surface bordered by two guard cells.
• When the guard cells swell with water the gap opens; when they lose water it closes.
• Stomata are found mostly on the lower surface of a leaf, where they are shaded and less exposed to moving air.

• An open stoma is a two-way doorway: it lets carbon dioxide in for photosynthesis and lets water vapour and oxygen out.
• That is why transpiration is high in daylight, when photosynthesis is running and the stomata are open, and falls sharply at night, when they close.
• Transpiration also cools the leaf and keeps the water column in the xylem moving, so it is not simply a loss to the plant.

• If a plant loses water faster than its roots can replace it, the cells lose their firmness and the shoot droops — the plant wilts.
• Once wilting is severe and prolonged, the plant may die.

The rate rises whenever water vapour is removed from the air spaces inside the leaf more quickly, or when the gradient between the moist air inside the leaf and the drier air outside becomes steeper.

• The transport of dissolved food through the phloem is called translocation.
• Sugar made by photosynthesis in the leaves is loaded into the sieve tubes, and the companion cells supply the energy for that loading.
• From the leaves it is carried to any part of the plant that needs it — the growing shoot tip, a developing fruit, a swelling root — or to a place where it is stored, such as a root or a stem.

• Much of the sugar leaves the leaf as sucrose, a soluble form that is convenient to carry.
• Where the sugar arrives and either is used for growth or is stored, it is unloaded from the phloem into the surrounding cells.

The mark scheme is written against the command word, not against how much you wrote. Read the word before you read the question.

At 2.00 p.m. on a hot, windy afternoon, a pupil is asked to find out how much water a tomato plant growing in a pot loses. The soil surface in the pot is covered with a sheet of plastic so that water can only leave through the plant, and the pupil records the mass of the whole pot every half an hour. It falls steadily. Meanwhile, her classmate cycles hard around the school garden; his leg muscles use more oxygen and produce more carbon dioxide than when he is resting.

(a) Name the plant tissue that carries water up from the roots to the leaves, and state one feature of this tissue that allows the water to move upwards quickly. (2 marks)

(b) Explain how water is taken in at the roots of the tomato plant. (2 marks)

(c) Explain why the mass of the pot falls more quickly on this hot, windy afternoon than it would on a cool, still evening. (2 marks)

(d) Explain how oxygen reaches the cyclist’s leg muscles, and how the carbon dioxide they produce leaves them. Use the term diffusion in your answer. (2 marks)

(e) Both the tomato plant and the cyclist need a transport system, but a single-celled organism such as an Amoeba does not. Explain why. (2 marks)

Model answers (written as a pupil should write them, with the reasoning a marker looks for):

1. (a) The tissue is the xylem. It is suited to carrying water quickly because its cells are hollow and joined end to end with no end walls (cross walls) and with lignin in the walls, so the cells form a continuous tube; there is therefore little resistance to the flow of water up the plant. (1 mark for xylem; 1 mark for feature plus the reason it speeds water up — the feature alone earns nothing.)
2. (b) The root hair cell is a long, thin outgrowth of an epidermal cell, which gives the root a large surface area for absorption. Its vacuole contains cell sap with sugars and mineral salts dissolved in it, so the water potential inside the root hair cell is lower than that of the dilute solution in the soil. Water therefore enters the root hair cell by osmosis, through its partially permeable cell membrane, down a water potential gradient; it then passes from cell to cell by osmosis until it reaches the xylem. (1 mark for the lower water potential in the cell than in the soil solution; 1 mark for osmosis through the partially permeable membrane / movement to the xylem.)
3. (c) On a hot afternoon the water in the leaf has more kinetic energy, so it evaporates from the surfaces of the leaf cells into the air spaces inside the leaf faster. The wind blows away the humid air around the stomata, so the concentration of water vapour outside the leaf stays low and the concentration gradient between the air spaces and the outside air remains steep. Water vapour therefore diffuses out of the stomata more quickly, so transpiration is faster and the mass of the pot falls more quickly. (1 mark for heat giving the water molecules more energy to evaporate / diffuse; 1 mark for wind removing humid air so the concentration gradient stays steep.)
4. (d) The muscles need more oxygen, so the blood flowing through the capillaries in his legs arrives with a higher concentration of oxygen than the muscle cells have. Oxygen therefore diffuses from the blood, through the thin one-cell-thick capillary wall, into the muscle cells, down a concentration gradient. In the same way, carbon dioxide is produced faster in the muscles, so its concentration inside the cells is higher than in the blood; the carbon dioxide diffuses out of the muscle cells into the blood, which carries it away. (1 mark for oxygen diffusing from blood to muscle cells down a concentration gradient; 1 mark for carbon dioxide diffusing from the cells into the blood.)
5. (e) In a large multi-cellular organism, most cells are far from the body surface or from the soil, so substances would have to travel a long way. Diffusion is only fast enough over very short distances, so diffusion alone cannot supply oxygen and food to the inner cells quickly enough or remove waste products quickly enough. Both organisms therefore need a transport system to move substances over long distances: the plant has the xylem and phloem, and the cyclist has the circulatory system, with the heart, blood vessels and blood. (1 mark for distance too great / diffusion too slow for a large organism; 1 mark for diffusion being fast enough for a single-celled organism because of its large surface area to volume ratio.)