2.2.1 Exchange surfaces and surface area : volume ratio

Surface area to volume ratio

1. Every substance an organism needs has to cross a surface to get in, and every waste substance has to cross a surface to get out.
2. The amount of surface available for that exchange is the surface area, and the amount of living material that has to be supplied is the volume.
3. The surface area to volume ratio sets one against the other and is written $SA:V$.
4. When an organism grows larger its surface area increases with the square of its length while its volume increases with the cube of its length.
5. For a cube of side $l$ the surface area is $6l^2$ and the volume is $l^3$, so the ratio is $\frac{6l^2}{l^3}=\frac{6}{l}$.
6. That expression gets smaller as $l$ gets bigger, which is why a small organism has a large ratio and a large organism has a small ratio.
7. A large ratio means there is a great deal of surface serving each unit of living material, so substances can be supplied across that surface quickly enough.
8. A small ratio means there is very little surface serving each unit of living material, so the outer surface on its own cannot supply the cells inside.
9. Shape matters as well as size, because a flattened or folded body has a larger ratio than a compact body of the same volume.
10. That is why a flatworm can be several centimetres long and still survive without a transport system, since it is only a fraction of a millimetre thick.

Calculating the ratio

1. Work out the total surface area first by adding the area of every exposed face.
2. A cube of side $l$ has six identical faces of area $l^2$, so its surface area is $6l^2$.
3. A cuboid of sides $a$, $b$ and $c$ has surface area $2(ab+ac+bc)$.
4. Work out the volume next, which is $l^3$ for a cube and $abc$ for a cuboid.
5. Divide one by the other using $$ratio=\frac{surface\; area}{volume}$$
6. Give the answer either as a single number or in the form $n:1$, so a value of $3$ is written $3:1$.
7. Both quantities must use the same length unit, because an area in ${cm}^2$ divided by a volume in ${mm}^3$ gives a meaningless figure.
8. Comparing two organisms only works when both ratios are worked out the same way round and in the same units.

Why diffusion alone is not enough

1. Diffusion moves particles down a concentration gradient and needs no energy from the organism.
2. Diffusion is fast when the surface area is large, when the gradient is steep and when the diffusion distance is short.
3. The time diffusion takes rises far more steeply than the distance, so doubling the distance more than doubles the time.
4. In a single-celled organism such as an amoeba, no part of the cytoplasm is more than a fraction of a millimetre from the surface, so oxygen reaches every part in a moment.
5. In a large animal the innermost cells sit tens of centimetres from the body surface, and diffusion over that distance would take days rather than seconds.
6. A large organism also has far more respiring cells, so its total demand for oxygen and glucose is much greater.
7. Demand rises with volume while supply across the outer surface rises only with surface area, so demand outgrows supply as size increases.
8. Waste products build up for the same reason, because carbon dioxide and urea also have to travel further to reach a surface where they can leave.
9. That mismatch is why a multicellular organism needs specialised exchange surfaces and a transport system rather than relying on its outer surface.

Features of an efficient exchange surface

1. A large surface area lets more particles cross at the same time, and it is produced by folding, branching or flattening the surface.
2. A thin surface, usually a single layer of flattened cells, keeps the diffusion distance short so particles cross in seconds.
3. A steep concentration gradient is maintained by keeping the medium moving on one side and the blood moving on the other.
4. A good blood supply carries absorbed substances away and delivers waste substances, which is what stops the gradient falling to zero.
5. A moist surface lets gases dissolve before they diffuse across the membrane.
6. In the lungs the alveoli provide the area, their walls are one cell thick and capillaries are pressed directly against them.
7. In the small intestine the villi and microvilli fold the lining into a vast area for absorbing digested food.
8. In a fish the gill filaments are stacked into thin plates washed continuously by water flowing in the opposite direction to the blood.
9. In a plant the root hair cells push the root surface out into the soil, and the air spaces inside a leaf give a large moist surface for gas exchange.

The need for a transport system

1. An exchange surface brings substances into the body, but those substances still have to reach cells that may be a long way off.
2. A transport system carries them in a moving liquid, so the long part of the journey is covered by mass flow rather than by diffusion.
3. Mass flow moves substances many thousands of times faster than diffusion over the same distance.
4. In a mammal that system is the circulatory system, which pumps blood through vessels that pass within a fraction of a millimetre of almost every cell.
5. The final short step out of a vessel and into a cell is still made by diffusion, but the distance is now tiny, so it takes only a moment.
6. In a flowering plant the transport tissues are the xylem and the phloem, which run continuously through the roots, stem and leaves.
7. Every transport system therefore does two jobs: it moves substances quickly over long distances and it delivers them close enough for diffusion to finish the journey.

2.2.2 Substances transported in organisms

Substances that must be transported

1. Every cell in a large organism needs a steady supply of raw materials and a steady removal of the waste its reactions produce.
2. The substances a transport system moves in an animal are oxygen, carbon dioxide, water, dissolved food molecules, mineral ions and urea.
3. Each one is carried between an exchange surface, where it enters or leaves the body, and the cells that use or produce it.
4. Some are carried in solution in the liquid part of the blood, and oxygen is carried bound to a protein inside a specialised cell.
5. The direction each substance travels depends on where it enters the body and where it is needed, so oxygen and food travel inwards while carbon dioxide and urea travel outwards.
6. The quantity of each substance an organism moves depends on what that organism requires, so an active mammal shifts far more oxygen and glucose per kilogram than a slow-moving one.

Oxygen and carbon dioxide

1. Oxygen is needed by every respiring cell because aerobic respiration releases far more energy from glucose than anaerobic respiration does.
2. Oxygen enters the body across the lungs, diffusing from the air in the air sacs into the blood in the capillaries wrapped around them.
3. It is then carried to the respiring tissues, where it diffuses out of the blood and into the cells.
4. Carbon dioxide is the waste product of aerobic respiration and travels in the opposite direction, from the tissues to the lungs.
5. Carbon dioxide has to be removed because it dissolves in water to form a weak acid, and a build-up would lower the pH of the blood and damage enzymes.
6. Most of it is carried dissolved in the liquid part of the blood as hydrogencarbonate ions, and it diffuses out into the air in the lungs and is breathed out.
7. A plant transports these two gases very little, because the air spaces inside its leaves and stem reach almost every cell directly.

Water

1. Water is the solvent in which every other transported substance dissolves, so nothing else can be carried without it.
2. Cytoplasm is mostly water, and the chemical reactions of a cell take place between substances dissolved in it.
3. In a mammal water is absorbed from the small intestine and the large intestine and carried in the blood to every tissue.
4. Water also spreads heat around the body, which is how a warm-blooded animal keeps a steady internal temperature.
5. In a plant water is absorbed by the roots, carried up to the leaves and used as a raw material for photosynthesis.
6. Water inside plant cells also presses outwards on the cell walls, and that pressure is what holds a soft young stem upright.

Dissolved food molecules

1. Large food molecules cannot be transported, so digestion breaks them into small soluble molecules that dissolve and travel in the blood.
2. Starch is digested to glucose, which every cell needs as the fuel for respiration.
3. Proteins are digested to amino acids, which cells reassemble into the particular proteins they need for growth and repair.
4. Lipids are digested to fatty acids and glycerol, which are used to build membranes and are stored as an energy reserve.
5. These products are absorbed across the wall of the small intestine and carried away in the blood, glucose and amino acids straight into the capillaries of the villi.
6. A plant makes its own glucose in the leaves and moves it as sucrose, together with amino acids, to wherever it is used or stored.

Mineral ions

1. Mineral ions are needed in small quantities but are essential, because without them particular molecules cannot be built at all.
2. In a mammal, iron ions are needed to make haemoglobin, so a shortage lowers the amount of oxygen the blood can carry.
3. Calcium ions and phosphate ions are needed to build bone and teeth, and calcium ions are also needed for muscles to contract and blood to clot.
4. Sodium ions and potassium ions carry the electrical signals along nerve cells.
5. Mineral ions are absorbed in the intestine, dissolve in the liquid part of the blood and are carried to wherever they are needed.
6. A plant takes its mineral ions from the soil solution, and nitrate ions are needed to turn glucose into amino acids and so into protein.
7. A plant short of nitrate ions grows poorly and stays small, and a plant short of magnesium ions cannot make chlorophyll and its leaves turn yellow.

Urea

1. Amino acids cannot be stored, so any the body does not need are broken down in the liver.
2. The nitrogen-containing part is removed and converted first to ammonia, which is highly toxic, and then to the far less toxic urea.
3. Urea dissolves in the liquid part of the blood and is carried from the liver to the kidneys.
4. The kidneys filter it out of the blood and it leaves the body dissolved in urine.
5. Urea must be transported continuously because a rising concentration in the blood would poison the tissues.
6. A plant makes no urea, because it takes in only the nitrogen it needs as nitrate ions rather than eating protein it must break down.

Matching supply to demand

1. The quantity of each substance an organism has to transport depends on how fast its cells are respiring.
2. A resting mammal respires slowly, so oxygen and glucose are delivered at a modest rate and carbon dioxide is removed at a modest rate.
3. During hard exercise the muscles respire many times faster, so all three rates rise together.
4. The body meets that demand by breathing faster and more deeply and by making the heart beat faster and more strongly.
5. A sprinting animal reaches a point where oxygen cannot be delivered fast enough, and the muscles then respire anaerobically for a short time.
6. That limit on delivery is why an all-out sprint can be held for only a short burst, whatever the animal.

Substances transported in plants

1. A plant transports water and mineral ions upwards from the roots to the leaves.
2. It transports dissolved sugars and amino acids from the leaves to the roots, the growing tips, the flowers and the fruits.
3. The two routes are separate tissues running side by side, and each carries its load in one characteristic direction.
4. Gases are the exception, because oxygen and carbon dioxide move through the air spaces between the cells rather than through a transport tissue.

2.2.3 The human circulatory system

The double circulatory system

1. The human circulatory system is made of the heart, the blood vessels and the blood they carry.
2. It is a closed system, which means the blood stays inside vessels the whole way round instead of washing freely through the body cavity.
3. It is also a double system, because blood passes through the heart twice for every complete circuit of the body.
4. The first circuit is the pulmonary circulation, which runs from the right side of the heart to the lungs and back to the left side.
5. The second circuit is the systemic circulation, which runs from the left side of the heart to the rest of the body and back to the right side.
6. The two circuits are joined in series, so all the blood leaving the lungs passes through the heart again before it goes anywhere else.
7. The right side of the heart handles only deoxygenated blood and the left side handles only oxygenated blood, and the two never mix.
8. Keeping them apart means the blood sent to the tissues is always fully loaded with oxygen.

Why a mammal needs two circuits

1. Blood loses a great deal of pressure while it is squeezed through the narrow capillaries of an organ.
2. In a single circulatory system, such as a fish has, blood passes through the heart only once per circuit and travels straight from the gill capillaries to the body.
3. By the time it leaves the gills its pressure is already low, so it flows slowly to the rest of the body.
4. A double system returns the blood to the heart after the lungs, and the left side raises its pressure again before it is sent to the body.
5. That second push makes the blood flow much faster, so oxygen and glucose are delivered and carbon dioxide removed at a much higher rate.
6. Mammals need that higher rate because they are warm-blooded and respire quickly all the time in order to keep their body temperature steady.
7. The pulmonary circuit is kept at a lower pressure than the systemic circuit, because the thin walls of the lung capillaries would be damaged by a high pressure.
8. A double system therefore gives each circuit the pressure that suits it, which a single system cannot do.

The arrangement of the main vessels

1. Deoxygenated blood returns from the body in the vena cava and enters the right atrium.
2. It passes into the right ventricle and is pumped out through the pulmonary artery to the lungs.
3. Oxygenated blood returns from the lungs in the pulmonary vein and enters the left atrium.
4. It passes into the left ventricle and is pumped out through the aorta to the rest of the body.
5. The pulmonary artery and the pulmonary vein are the two exceptions to the usual rule, because this artery carries deoxygenated blood and this vein carries oxygenated blood.
6. The naming rule that always holds is the direction of travel, so an artery leaves the heart and a vein returns to it whatever blood it carries.
7. Branches of the aorta carry blood to each organ, so the hepatic artery supplies the liver and the renal artery supplies the kidneys.
8. Blood leaves those organs in the matching veins, the hepatic vein and the renal vein, which join the vena cava.
9. Blood carrying absorbed food from the small intestine travels to the liver in the hepatic portal vein, so the liver can adjust the glucose and amino acid levels before that blood reaches the rest of the body.
10. The coronary arteries branch from the base of the aorta and supply the heart muscle itself, which cannot take oxygen from the blood passing through its chambers.
11. Within every organ the artery divides into narrower and narrower vessels until it becomes a network of capillaries, where exchange with the cells takes place.
12. Those capillaries then join up into wider and wider vessels that become the vein leaving the organ.

Working with the gas exchange system

1. The circulatory system and the gas exchange system meet at the alveoli, and neither system is any use without the other.
2. Air is drawn down the trachea, along the bronchi and bronchioles, and into the millions of alveoli that fill each lung.
3. Each alveolus is wrapped in a dense network of capillaries carrying blood from the pulmonary artery.
4. That blood has a low oxygen concentration and a high carbon dioxide concentration, because it has just come back from the respiring tissues.
5. Oxygen therefore diffuses from the air in the alveolus into the blood, and carbon dioxide diffuses from the blood into the air.
6. The circulation keeps that exchange going by carrying oxygenated blood away, which stops the concentrations on the two sides becoming equal.
7. Breathing does the same job on the other side, refreshing the air in the alveoli so its oxygen concentration stays high.
8. Between them the two systems hold a steep concentration gradient across a wall only two cells thick, which is why exchange is so rapid.

2.2.4 The heart and blood vessels

Structure of the heart

1. The heart is a double pump built almost entirely from cardiac muscle, a tissue that contracts and relaxes rhythmically for a lifetime without becoming fatigued.
2. It has four chambers: a right atrium and a right ventricle on one side, and a left atrium and a left ventricle on the other.
3. The septum is a solid wall of muscle down the middle that keeps oxygenated and deoxygenated blood completely separate.
4. The atria have thin walls because they only push blood a few centimetres down into the ventricles below them.
5. The ventricles have much thicker walls because they have to generate enough pressure to drive blood out of the heart altogether.
6. The right ventricle wall is moderately thick because it pumps only as far as the lungs, which sit alongside the heart.
7. Its pressure is deliberately kept low, because the thin capillary walls in the lungs would burst under a high pressure.
8. The left ventricle wall is the thickest of all, roughly three times the thickness of the right, because it pumps blood all the way round the body.
9. More muscle produces a more forceful contraction, which produces a higher blood pressure and so a faster flow to the tissues.
10. Four vessels are attached at the top: the vena cava into the right atrium, the pulmonary artery out of the right ventricle, the pulmonary vein into the left atrium and the aorta out of the left ventricle.
11. The cardiac muscle itself is supplied with oxygen and glucose by the coronary arteries that run across the outside of the heart.

The valves of the heart

1. A valve is a flap of tissue that opens in one direction only, so blood can pass forwards but cannot slip back.
2. Each valve opens when the pressure behind it is higher than the pressure in front of it, and snaps shut when that difference reverses.
3. The atrioventricular valves sit between each atrium and the ventricle below it.
4. The one on the right has three flaps and is called the tricuspid valve.
5. The one on the left has two flaps and is called the bicuspid valve, or the mitral valve.
6. Both are anchored to the ventricle wall by tough tendons that stop the flaps turning inside out when the ventricle contracts hard.
7. The semilunar valves sit at the base of the aorta and the base of the pulmonary artery, each made of three half-moon pockets.
8. They stop blood that has just been forced into an artery falling back into the ventricle as it relaxes.
9. Without valves the heart would push blood back and forth in the same place and no net flow would leave it.

How one heartbeat works

1. Blood flows into the two atria from the vena cava and the pulmonary vein while the heart is relaxed.
2. Both atria then contract together, raising the pressure inside them so the atrioventricular valves are pushed open.
3. Blood passes down into the ventricles, which fill and stretch.
4. Both ventricles then contract, and the pressure inside them rises sharply.
5. That pressure forces the atrioventricular valves shut and forces the semilunar valves open.
6. Blood is driven out of the right ventricle into the pulmonary artery and out of the left ventricle into the aorta.
7. The ventricles then relax, the pressure inside them falls below the pressure in the arteries and the semilunar valves close.
8. The two sets of valves closing are what produce the two sounds of a heartbeat.

Arteries

1. An artery carries blood away from the heart, and that blood leaves at high pressure in surges.
2. The wall is thick so that it can withstand that pressure without bursting.
3. The wall contains a great deal of muscle and elastic fibres, so it stretches as each surge arrives and recoils afterwards.
4. That stretching and recoil smooths the flow between beats and keeps the blood moving forwards, and it is what you feel as a pulse.
5. The lumen is narrow compared with the thickness of the wall, which helps to keep the pressure high.
6. An artery has no valves along its length, because the pressure behind the blood is high enough to keep it flowing one way.

Veins

1. A vein carries blood back towards the heart, and by this stage the blood is at low pressure and flowing steadily rather than in surges.
2. The wall is thin with little muscle or elastic tissue, because there is no high pressure to resist.
3. The lumen is wide, which reduces the resistance to flow so that low-pressure blood still returns quickly.
4. Veins contain valves along their length, which is the clearest structural difference from an artery.
5. Those valves stop blood flowing backwards, which matters most in the legs where blood has to travel upwards against gravity.
6. Many veins run between skeletal muscles, and each time those muscles contract they squeeze the vein and push blood on towards the heart.

Capillaries

1. A capillary links the smallest arteries to the smallest veins and is the only vessel where substances are actually exchanged with the tissues.
2. Its wall is one cell thick, giving the shortest possible diffusion distance between the blood and the surrounding cells.
3. Its lumen is so narrow that red blood cells pass through in single file, which presses them against the wall and shortens the distance further.
4. Capillaries are extremely numerous and branch through every tissue, so no cell is far from a supply.
5. That branching also gives a very large total surface area for exchange.
6. Blood flows slowly through a capillary bed, which allows time for diffusion to take place.
7. The wall is also slightly leaky, so plasma can seep out to bathe the tissues and white blood cells can squeeze through it.

2.2.5 Red blood cells and plasma

What blood is made of

1. Blood is a tissue, because it is a group of cells working together, but its cells are carried in a liquid rather than held in a solid block.
2. Just over half of blood by volume is plasma, and the rest is made up of cells and cell fragments.
3. The four components are plasma, red blood cells, white blood cells and platelets.
4. Red blood cells and plasma are the two components that carry out the transport work of the blood.
5. Red blood cells vastly outnumber the others, which is why whole blood looks red rather than straw-coloured.
6. All of them are made in the bone marrow, the soft tissue inside the larger bones.

How a red blood cell is built

1. A red blood cell has no nucleus, which frees the whole of the inside of the cell for carrying oxygen.
2. That space is filled with haemoglobin, so a single cell holds hundreds of millions of these oxygen-binding molecules.
3. The cell is a biconcave disc, which means it is dented inwards on both flat faces.
4. That shape gives a larger surface area than a sphere of the same volume, so oxygen can diffuse in and out over a bigger area.
5. It also makes the cell thin through the middle, so no haemoglobin molecule sits far from the surface and the diffusion distance stays short.
6. The cell is small, roughly $7\mu m$ across, and flexible, so it can bend and squeeze single file along a capillary narrower than itself.
7. Being pressed against the capillary wall as it passes shortens the distance oxygen has to diffuse to reach the tissue.
8. Having no nucleus means the cell cannot repair itself or divide, so it survives only about three months and is replaced continuously.

Carrying oxygen

1. Oxygen dissolves only slightly in water, so plasma on its own could not carry enough of it to keep a mammal alive.
2. Haemoglobin solves that problem by binding oxygen instead of dissolving it, which raises the amount blood can carry many times over.
3. In the lungs the oxygen concentration is high, so haemoglobin binds oxygen and becomes bright red oxyhaemoglobin.
4. The reaction is written as $$haemoglobin+oxygen\rightleftharpoons oxyhaemoglobin$$
5. The double arrow matters, because the binding is reversible and the oxygen has to be released again.
6. In a respiring tissue the oxygen concentration is low, so oxyhaemoglobin breaks apart and releases oxygen into the cells.
7. The blood turns the darker red of deoxygenated blood as it does so.
8. Each haemoglobin molecule contains iron, which is why a shortage of iron in the diet lowers how much oxygen the blood can carry and leaves a person tired and short of breath.

What plasma carries

1. Plasma is about nine-tenths water, and water is an excellent solvent, so a great many substances dissolve in it and travel together.
2. Being a liquid, plasma can be pumped, so everything dissolved in it is carried by mass flow rather than relying on diffusion.
3. Plasma carries carbon dioxide from the respiring tissues to the lungs, mostly as hydrogencarbonate ions.
4. It carries urea from the liver to the kidneys.
5. It carries the soluble products of digestion, such as glucose and amino acids, from the small intestine to the liver and then to every tissue.
6. It carries mineral ions, hormones, antibodies and the proteins that make blood clot.
7. It also carries the blood cells and the platelets themselves, which are suspended in it rather than dissolved.
8. Plasma distributes heat as well as substances, moving warmth from busy organs such as the liver and the muscles to cooler parts of the body.
9. Because water needs a great deal of energy to change temperature, plasma can absorb heat without the body temperature swinging about.

The other cells in the blood

1. White blood cells are larger than red blood cells and, unlike them, each one keeps its nucleus.
2. They defend the body against pathogens rather than transporting substances, so they are not adapted for carrying oxygen.
3. Some engulf and digest pathogens, while lymphocytes produce antibodies that stick pathogens together.
4. Because a white blood cell can change shape, it can squeeze out through a capillary wall and reach an infection in the surrounding tissue.
5. Platelets are small fragments of larger cells and have no nucleus of their own.
6. They gather at a wound and trigger the reactions that turn a soluble plasma protein into a mesh of insoluble fibres, and that mesh traps red blood cells to form a clot.
7. A clot stops blood loss and seals the skin against pathogens while the wound heals.

2.2.6 Uptake of water and mineral ions by plants

The root hair cell

1. A plant takes in all its water and all its mineral ions through the roots, not through the leaves.
2. The absorbing surface is the epidermis, the single outer layer of cells covering the root.
3. Many of those epidermal cells grow a long, narrow extension out into the soil, and a cell with such an extension is a root hair cell.
4. The hair is an outgrowth of one cell, so it has no cross wall and no nucleus of its own.
5. The long, thin shape gives a very large surface area, so a great many water molecules and ions can enter at the same time.
6. It also pushes between the soil particles into gaps a smooth root surface could never reach, so it touches far more of the water film clinging to those particles.
7. The wall and the membrane are thin, which keeps the diffusion distance for water and ions extremely short.
8. The cell contains many mitochondria, because moving mineral ions inwards needs a continuous supply of energy from respiration.
9. It has a large permanent vacuole holding cell sap, a concentrated solution of sugars and salts that keeps the inside of the cell more concentrated than the soil water.
10. It has no chloroplasts, because it lives underground where there is no light to photosynthesise with.
11. Root hairs are found only in a band a few millimetres behind the growing tip, and each one lives for a matter of days before being replaced as the root grows on into fresh soil.

Taking up water by osmosis

1. Soil water is a very dilute solution, so it has a high concentration of water molecules and only a trace of dissolved salts.
2. The cell sap inside the root hair cell is a concentrated solution, so it has a lower concentration of water molecules.
3. The cell membrane is partially permeable, so water molecules can pass through it but the dissolved solutes largely cannot.
4. Water therefore moves into the cell by osmosis, down a water potential gradient from the dilute soil solution to the concentrated cell sap.
5. Osmosis is a passive process, so it needs no energy from respiration.
6. Water then passes from that cell into the next along the same kind of gradient, moving steadily across the root towards the centre.
7. The gradient is maintained because water is drawn out of the centre of the root and up the stem, so the cells nearer the middle never become saturated.
8. Uptake slows and then stops if the soil dries out or is heavily salted, because the soil solution becomes as concentrated as the cell sap and the gradient disappears.
9. In very salty soil water moves out of the root cells instead of in, which is why a plant watered with sea water dies.

Taking up mineral ions by active transport

1. Mineral ions such as nitrate, magnesium and potassium are present in the soil solution at very low concentrations.
2. Their concentration inside the root hair cell is already higher than their concentration outside it.
3. Diffusion would therefore move them out of the cell rather than in, because diffusion only runs down a concentration gradient.
4. The cell instead uses active transport, which moves ions against the gradient, from the lower concentration to the higher one.
5. Carrier proteins in the cell membrane pick up a particular ion on the outside, change shape and release it on the inside.
6. That change of shape needs energy released by respiration, which is why the cell is packed with mitochondria.
7. Each carrier protein handles one kind of ion, so the plant can take up the particular ions it needs and leave the rest.
8. Anything that stops respiration therefore stops mineral uptake, so a waterlogged soil with no oxygen leaves a plant short of nitrate even though the nitrate is there.
9. Water uptake and ion uptake happen through the same cell at the same time, but by two different processes, one passive and one requiring energy.

2.2.7 Transpiration and translocation

Transpiration

1. Water travels up a plant in a continuous stream, and almost all of it leaves again as vapour from the leaves.
2. Inside a leaf the cells are loosely packed with air spaces between them, and every cell surface facing those spaces is wet.
3. Water evaporates from those wet cell walls, so the air spaces fill with water vapour.
4. The air inside the leaf therefore holds far more water vapour than the air outside it.
5. Water vapour then diffuses out through the stomata down that concentration gradient, and this loss is transpiration.
6. Transpiration is therefore two steps: evaporation inside the leaf, then diffusion out of it.
7. As water leaves a mesophyll cell, that cell draws replacement water from the cell next to it, and so on back to the nearest xylem vessel.
8. That pull is passed all the way down the water in the xylem to the roots, producing the transpiration stream.
9. The column of water holds together because water molecules attract one another, an effect called cohesion, so pulling at the top drags the whole column upwards.
10. The plant spends no energy on this, because the energy that drives the evaporation comes from the sun.
11. Transpiration is useful as well as unavoidable: it delivers water for photosynthesis, carries dissolved mineral ions up to the leaves, cools the leaf as water evaporates, and keeps cells firm with water so soft stems stay upright.

Stomata and guard cells

1. A leaf is covered by a waxy cuticle that is waterproof, so gases and water vapour cannot pass freely through the surface.
2. The stomata are the controlled openings through it, and the singular of the word is stoma.
3. Each stoma is a pore held between two sausage-shaped guard cells.
4. Carbon dioxide diffuses in through the open pore for photosynthesis, oxygen diffuses out, and water vapour is lost at the same time.
5. Most stomata are on the lower surface of a broad leaf, which is shaded and cooler, so less water evaporates than would be lost from the hot upper surface.
6. Guard cells are the only epidermal cells of a leaf that contain chloroplasts, and they photosynthesise in the light.
7. The wall of a guard cell is thicker on the side facing the pore and thinner on the outer side, and that uneven thickness is what makes the pore open.

Opening and closing a stoma

1. In the light the guard cells take in water by osmosis and become turgid, which means swollen and firm.
2. The thin outer wall stretches more easily than the thick inner wall, so each cell curves away from the pore.
3. The two cells bow apart and the stoma opens, letting carbon dioxide in while the plant is photosynthesising.
4. In the dark the guard cells lose water, become flaccid and straighten, so the two cells meet and the stoma closes.
5. Closing at night costs the plant nothing, because there is no photosynthesis in the dark and so no need for carbon dioxide.
6. Stomata also close during the day when the plant is short of water, which saves the plant even though photosynthesis stops.

The cost of opening the stomata

1. A plant cannot let carbon dioxide in without letting water vapour out, because both use the same pore.
2. Transpiration is therefore an unavoidable consequence of gas exchange rather than a process the plant carries out on purpose.
3. Keeping the stomata open maximises photosynthesis but risks losing water faster than the roots can replace it.
4. Closing them saves water but starves the leaf of carbon dioxide, so photosynthesis and growth stop.
5. Guard cells settle that compromise minute by minute, opening the pores when water is plentiful and shutting them when it is not.

Translocation

1. Photosynthesis makes glucose in the leaves, but every other part of the plant needs it too.
2. Glucose is converted to sucrose for the journey, because sucrose is soluble but far less reactive, so it can be moved without being used up on the way.
3. The movement of sucrose and amino acids through the phloem is called translocation.
4. It runs from a source, where sugars are loaded in, to a sink, where they are taken out.
5. A photosynthesising leaf is the usual source, and growing root tips, flowers, developing fruits and storage organs are the usual sinks.
6. At a sink the sugar is respired for energy, built into cellulose for new cell walls, or stored as insoluble starch.
7. Translocation moves in either direction, upwards to a growing shoot and downwards to a root, depending on where the sinks are at the time.
8. The same part can switch role through the year, so a potato tuber is a sink while it is filling in summer and a source when it sprouts in spring.
9. Loading sugar into the phloem needs energy from respiration, so translocation is an active process, unlike transpiration.
10. That is why translocation slows or stops when a plant is starved of oxygen, while the transpiration stream keeps flowing.

2.2.8 Xylem and phloem

Xylem

1. Xylem carries water and dissolved mineral ions from the roots to the stem and leaves, and it carries them in one direction only, upwards.
2. A xylem vessel begins as a column of living cells, which then die and lose their cytoplasm, nucleus and organelles.
3. The end walls between them break down completely, so the whole column becomes one continuous hollow tube running the length of the plant.
4. An empty tube with no cross walls offers almost no resistance, so water flows through it far faster than it could pass from cell to cell.
5. The walls are thickened with lignin, laid down in rings and spirals along the inside of the tube.
6. Water in the xylem is under tension because it is being pulled from above, so without that thickening the tube would collapse inwards like a drinking straw sucked too hard.
7. Lignin is also waterproof, so water is not lost sideways on the way up.
8. Gaps in the thickening called pits let water leave the vessel where it is needed, at the roots and in the leaves.
9. The tubes are narrow, which helps the column of water hold together as it is pulled upwards.
10. Because it is dead and hollow, xylem uses no energy at all, and the pull that drives the flow comes from evaporation at the leaves.
11. Lignified xylem also gives the plant support, and the accumulated xylem of a tree trunk is what we call wood.

Phloem

1. Phloem carries dissolved sugars and amino acids, and it carries them in both directions, to whichever parts of the plant need them.
2. Unlike xylem, phloem is made of living cells, because moving sugars requires energy.
3. The transport cells are sieve tube elements, joined end to end into a long tube.
4. Their end walls are not removed but perforated with pores, forming sieve plates that let the sugary sap flow from one cell into the next.
5. A sieve tube element loses most of its contents, including its nucleus, leaving only a thin layer of cytoplasm around the edge so the tube stays as clear as possible.
6. With no nucleus it cannot control itself, so each one is paired with a companion cell lying alongside it.
7. The companion cell keeps its nucleus and is packed with mitochondria, so it supplies the energy that loads sugar into the tube.
8. The two are linked by fine strands of cytoplasm through the wall between them, so materials pass freely from one to the other.
9. Phloem walls carry no lignin, which is why phloem gives the plant no support and why it can be crushed easily.

Where the two tissues lie in the plant

1. Xylem and phloem run side by side through the whole plant in strands called vascular bundles.
2. Running together means water reaches the sugar-loading cells and the two systems can exchange water between them along the way.
3. In a root the vascular tissue lies in the centre, which anchors the root against the pull of the plant being tugged upwards.
4. In a stem the bundles are arranged in a ring near the outside, where they resist bending in the same way as the walls of a tube.
5. In each stem bundle the xylem lies on the inside and the phloem on the outside.
6. In a leaf the bundles form the midrib and the network of veins, which carry water out to the mesophyll and carry sugars away from it.
7. The veins branch finely enough that no photosynthesising cell is far from a supply, and they also stiffen the thin blade of the leaf.
8. Because the phloem sits outside the xylem in a stem, stripping a ring of bark from a tree removes the phloem and starves the roots while the leaves still receive water.

2.2.9 Environmental factors and water uptake

Water uptake and transpiration rate

1. Water is pulled up a plant because water is being lost from the leaves, so anything that changes the rate of loss changes the rate of uptake at the roots.
2. Over ninety-nine parts in a hundred of the water a plant takes in is lost again as vapour, and only a trace is used in photosynthesis.
3. Water uptake is therefore treated as a good measure of the transpiration rate, although the two are not exactly equal.
4. Every environmental factor works through one of two routes: it changes how fast water evaporates inside the leaf, or it changes how steep the concentration gradient of water vapour is between the leaf and the air outside.
5. A third route exists only for light, which also changes how many stomata are open and so how much escape route there is.
6. The rate is calculated as a volume of water taken up divided by the time taken, so $$rate=\frac{volume\; of\; water\; taken\; up}{time}$$
7. Rates are usually given in ${cm}^3$ per minute or ${cm}^3$ per hour, and a rate can also be worked out from a distance moved along a scale of known cross-sectional area.

Light intensity

1. Increasing the light intensity increases the rate of water uptake.
2. Light causes the guard cells to become turgid, so the stomata open.
3. Open stomata give a larger total area for water vapour to diffuse out through, so more escapes each second.
4. In the dark the stomata close, so transpiration falls to a trickle through the waxy cuticle and water uptake almost stops.
5. Brighter light is usually warmer light as well, so a lamp placed close to a shoot raises the temperature at the same time and the two effects add together.
6. Once every stoma is fully open, raising the light intensity further has little extra effect, so a graph of rate against light intensity levels off.

Air movement

1. Increasing the air movement increases the rate of water uptake.
2. In still air the water vapour leaving the stomata collects just outside them as a layer of damp air.
3. That damp layer makes the concentration of water vapour outside the leaf almost as high as inside it, so the gradient becomes shallow and diffusion slows.
4. Moving air blows that layer away and replaces it with drier air.
5. The gradient is therefore kept steep, and water vapour keeps diffusing out quickly.
6. A gentle breeze produces a large increase, while making an already windy day windier produces much less extra effect, because the damp layer has already gone.
7. In very strong wind some plants close their stomata to protect themselves from drying out, and the rate then falls again.

Temperature

1. Increasing the temperature increases the rate of water uptake.
2. Warmer water molecules have more kinetic energy, so more of them have enough energy to escape from the wet cell walls into the air spaces.
3. Evaporation inside the leaf therefore speeds up and the air spaces hold more water vapour.
4. That raises the concentration inside the leaf, which steepens the gradient out through the stomata.
5. Warmer molecules also diffuse faster once they are in the air, so they move away from the pore more quickly.
6. All three effects push in the same direction, which is why temperature usually has the largest effect of any single factor.
7. At very high temperatures a plant short of water closes its stomata, and the rate then falls even though it is hot.

Humidity

1. Increasing the humidity decreases the rate of water uptake, which makes it the one factor here that works in the opposite direction.
2. Humid air already holds a great deal of water vapour, so the concentration outside the leaf is close to the concentration inside it.
3. The gradient across the stomata is therefore shallow, and water vapour diffuses out slowly.
4. In dry air the outside concentration is very low, the gradient is steep and vapour leaves quickly.
5. Covering a plant with a clear plastic bag traps the vapour it releases, raises the humidity around it and slows its water loss, which is why a cutting is kept in a bag while it grows roots.

When uptake cannot keep up

1. On a hot, bright, windy day water can be lost from the leaves faster than the roots can pull it out of the soil.
2. The cells then lose water, the pressure inside them falls and they stop pressing outwards against their walls.
3. A soft stem is held upright by that pressure, so once it is lost the plant droops and wilts.
4. Wilting reduces further loss, because the drooping leaves hang together and shade one another and the stomata close.
5. Closing the stomata also stops carbon dioxide entering, so photosynthesis and growth stop while the plant is wilted.
6. A wilted plant recovers when it is watered, unless it has been left so long that the cells are damaged beyond repair.

2.2.10 Using a potometer

What a potometer measures

1. Water vapour leaving a leaf cannot be seen or collected easily, so transpiration is measured indirectly by measuring how much water the plant takes in.
2. A potometer does exactly that, by holding a cut leafy shoot in a sealed column of water and watching that column move.
3. The simplest form is a bubble potometer: a shoot sealed into rubber tubing that is joined to a horizontal capillary tube marked with a scale.
4. A single air bubble is introduced into the capillary tube, and as the shoot draws water up, the bubble is pulled along behind it.
5. The distance the bubble travels in a given time is therefore a direct measure of the volume of water the shoot has taken up.
6. A reservoir with a tap, or a syringe, is fitted so the bubble can be pushed back to the start of the scale for the next reading.
7. The whole apparatus has to be airtight and watertight, because any leak lets the bubble move for reasons that have nothing to do with the plant.
8. Strictly it measures water uptake rather than water loss, because a small amount of the water taken in is used in photosynthesis and held inside the cells.
9. That difference is under one part in a hundred, so uptake is treated as a good estimate of the transpiration rate.

Calculating the rate of water uptake

1. When only a comparison between conditions is needed, the rate can be quoted straight from the scale as $$rate=\frac{distance\; moved}{time}$$
2. That gives an answer in millimetres per minute, which is enough to say which condition produced faster uptake.
3. For a true volume, the bubble sweeps out a cylinder of capillary tube, so the volume taken up is $$V=\pi r^{2}d$$
4. In that equation $r$ is the internal radius of the capillary tube and $d$ is the distance the bubble has moved.
5. The rate of water uptake is then $$rate=\frac{V}{t}$$ usually written in ${mm}^3$ per minute or converted to ${cm}^3$ per hour.
6. The conversion needed is $1{cm}^3=1000{mm}^3$, and both lengths in the volume equation must be in the same unit before multiplying.
7. Comparing two shoots fairly means dividing by the total leaf area, because a shoot with more leaves will always take up more water.

Measuring water loss as a change in mass

1. A second way to follow transpiration is to weigh the plant, because water leaving as vapour makes the plant lighter.
2. A whole potted plant is used with its pot and soil sealed inside a plastic bag, so the only water leaving is the water lost through the leaves.
3. The plant is weighed on a balance at the start and at set intervals, and each mass is recorded to the same number of decimal places.
4. The change in mass is the final mass minus the starting mass, so a loss of water gives a negative value.
5. Because plants differ in size, results are compared as a percentage change in mass rather than as a raw number of grams.
6. The equation is $$percentage\; change=\frac{change\; in\; mass}{starting\; mass}\times 100$$
7. The same equation is used for a percentage gain, and the sign of the answer says which has happened.
8. The mass method is simpler to set up than a potometer but far less sensitive, so it needs hours rather than minutes to give a measurable change.