Cell structure, movement of substances, gas exchange and osmoregulation

The organelles of a typical cell

All living organisms are formed of units called cells, and every cell has the same basic organisation. Each one is separated from its immediate environment by the cell surface membrane, and inside it the cytoplasm contains the genetic material and a set of membrane-bound structures called organelles. The same organelles turn up again and again in marine organisms: the gill cells of a mussel, a cell in a seagrass leaf and a white blood cell in a tuna all have a nucleus, mitochondria and a membrane, even though they do very different jobs.

The organelles work as a production line. The nucleus holds the DNA and so controls which proteins the cell makes; the ribosomes are the machines that actually assemble those proteins; the rough endoplasmic reticulum is the network of membranes the new proteins are threaded into and carried along; the Golgi body modifies and packages them into vesicles; and the vesicles fuse with the cell surface membrane to release their contents outside the cell, or to deliver membrane proteins into it. Once you can follow that route, the function of each organelle is easy to remember.

Plant cells differ from animal cells in three structures. A plant cell has a cell wall of cellulose outside the membrane, one large permanent vacuole filled with cell sap that keeps the cell firm, and chloroplasts for photosynthesis. Animal cells have none of these, which is why an animal cell can change shape and squeeze through a capillary while a plant cell cannot, and why a plant cell does not burst when it takes in water.

The cell surface membrane and the fluid mosaic model

The membrane is far too thin to see with a light microscope, but its structure explains everything it does. It is built from phospholipids, each of which has a hydrophilic head containing a phosphate group and two hydrophobic tails made of fatty acids. In water the molecules arrange themselves tail to tail, with the heads facing outwards into the watery environment on both sides, and this double layer is the phospholipid bilayer.

The bilayer is the reason membranes are selectively permeable. Very small molecules such as oxygen and carbon dioxide, and non-polar substances, dissolve in the lipid and pass straight through. Water molecules are small enough to pass between the phospholipids.

Ions and larger polar molecules such as glucose and amino acids are repelled by the hydrophobic interior and cannot cross the bilayer on their own. They can only get through with the help of proteins.

Proteins are scattered through the bilayer like tiles in a mosaic, some sitting on the surface and some spanning the whole width, and because both the phospholipids and the proteins can move sideways, the model is described as fluid. Channel proteins form water-filled pores through the membrane, so small ions and water-soluble molecules can pass through.

Carrier proteins bind to a specific substance on one side and change shape to carry it across to the other, and they are also the proteins that use ATP to pump substances against a concentration gradient. This combination of a fluid bilayer and a shifting mosaic of proteins is the fluid mosaic model.

• Phospholipid: hydrophilic phosphate head, two hydrophobic fatty acid tails.
• Bilayer: tails inward, heads outward, so both surfaces are compatible with water.
• Selectively permeable: oxygen, carbon dioxide and other small non-polar molecules cross freely; ions and large polar molecules cannot cross the lipid and need proteins.
• Channel protein: a water-filled pore through the membrane, used by small ions and water-soluble molecules.
• Carrier protein: binds a specific molecule, changes shape and releases it on the other side; can move substances down the gradient, or use ATP to move them against it.
• Fluid: the phospholipids and proteins move laterally within the membrane.
• Mosaic: the proteins are embedded in the bilayer in a patchy, scattered pattern.

Microscopes, magnification and interpreting cell images

Cells were discovered with a light microscope, which shines light through a stained specimen. A light microscope is cheap, it can be used on living cells, and it magnifies up to about 1500 times, but its resolution is limited to about 200 nm, so mitochondria and ribosomes appear only as faint dots or not at all.

The electron microscope uses a beam of electrons instead of light, which gives a much shorter wavelength and a resolution small enough to show the internal membranes of an organelle. Electron micrographs are always black and white, because the image is formed by electrons rather than light.

All magnifications are worked out from the same relationship: magnification = image size / actual size. The formula is rearranged for whatever the question gives you - actual size = image size / magnification, and image size = actual size x magnification - and you must convert the two sizes into the same unit before you divide. There are 1000 µm in a millimetre and 1000 nm in a µm, and mixing these up is the single most common arithmetic error in the paper.

Reading an unfamiliar micrograph is a skill of its own, because you may be shown a cell from an organism you have never studied. Work from the largest, most obvious features inwards: a thick rigid outer layer means a cell wall, so the cell is a plant or algal cell; a stack of parallel membranes with dots on them is rough ER; oval bodies with internal folds are mitochondria; a large pale area bounded by a single membrane is a vacuole. Then use the scale bar, or the stated magnification, to check that the sizes are sensible.

When you make your own drawing, use a sharp pencil, draw continuous unbroken lines with no shading, keep the proportions correct, and add straight label lines that do not cross, with the labels written outside the drawing. Give the drawing a title and state the magnification.

• magnification = image size / actual size - the same formula written three ways.
• 1 mm = 1000 µm; 1 µm = 1000 nm.
• A scale bar gives the actual size directly: measure the bar, then measure the structure, and divide.
• Plant cell clues: cell wall, chloroplasts, large permanent vacuole, regular shape.
• Animal cell clues: no wall, irregular shape, small or no vacuole, many mitochondria if the cell is active.
• Drawing rules: pencil, continuous lines, no shading, correct proportions, straight uncrossed label lines, title and magnification.

Diffusion, facilitated diffusion and active transport

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, and it happens because the particles are in constant random motion. It needs no energy from the cell, so it is a passive process. The rate depends on the steepness of the gradient, the distance or thickness the particles must cross, the surface area available, the temperature and the size of the particles.

In a marine organism, oxygen diffusing from sea water into a gill and carbon dioxide diffusing out of it are both simple diffusion.

Facilitated diffusion is still passive, and it still moves substances down their concentration gradient, but it uses a channel or carrier protein in the membrane. It is needed for substances that cannot dissolve in the phospholipid bilayer - ions such as sodium and chloride, and polar molecules such as glucose and amino acids. Each protein is specific to one substance or one group of substances, and because there is a limited number of them, the rate levels off at high concentrations when every protein is occupied.

Active transport moves substances against their concentration gradient, from a lower concentration to a higher one. It uses carrier proteins and it requires metabolic energy in the form of ATP from respiration, so it is the only one of the four processes that can make a gradient steeper rather than letting it run down. It is how a gill cell takes up the last traces of an ion from very dilute water, and how marine fish pump excess salts out of their bodies against a gradient.

Osmosis and water potential

Water potential is a measure of the relative number of water molecules in a solution and their freedom to move. It is given the symbol psi and measured in units of pressure, usually kilopascals. Pure water has a water potential of zero. Dissolving any solute in water lowers the water potential, because the solute particles get in the way of the water molecules and reduce their freedom to move, so every solution has a negative water potential and the more concentrated the solution, the more negative it is.

Osmosis is the net movement of water molecules from a region of higher water potential (less negative, closer to zero) to a region of lower water potential (more negative), through a partially permeable membrane. It is a passive process, a special case of diffusion that applies to water only. Water therefore always moves towards the more concentrated solution, and the size of the movement depends on the size of the difference between the two water potentials.

In a plant cell, water entering the vacuole pushes the cytoplasm and membrane outwards against the cell wall. The wall pushes back, and the cell becomes turgid, which is what keeps leaves and algal fronds firm. Water leaving has the opposite effect: the vacuole shrinks, the cytoplasm and membrane pull away from the cell wall, and the cell is plasmolysed.

In an animal cell there is no wall to push back, so a cell in a solution of higher water potential swells and may burst, and a cell in a solution of lower water potential shrinks and becomes distorted.

Surface area to volume ratio and the need for exchange surfaces

As a shape gets bigger, its surface area increases in proportion to the square of its length, but its volume increases in proportion to the cube. The result is that surface area to volume ratio falls as size increases: a cube 1 cm on each side has a ratio of 6:1, and a cube 4 cm on each side has a ratio of only 1.5:1. The same arithmetic applies to organisms, and it is the single most important idea in the whole of this topic.

The surface is where substances are exchanged, and the volume is the living tissue that needs supplying. A small organism therefore has a large surface for every unit of tissue it has to feed, and the distance from its surface to its centre is short, so diffusion alone can supply its oxygen and remove its carbon dioxide.

A large organism has the opposite problem: a small surface for a large volume, and a long diffusion distance to the middle of the body. Diffusion through the body surface becomes far too slow, and the organism needs two things - a specialised exchange surface with a large area and a thin barrier, and a transport system to carry gases between that surface and the tissues.

Shape matters as well as size. An organism that is long, flat and thin, such as a ribbon-shaped worm or a flattened frond of seaweed, keeps a short diffusion distance from the water to every cell even when it is large overall. An organism that is compact and rounded, such as a tuna, cannot, which is why the tuna needs gills with a huge surface area and a circulation to match.

Every efficient exchange surface shares the same four features: a large surface area, a thin barrier so the diffusion distance is short, a moist, permeable surface, and a steep concentration gradient maintained by ventilation and by the blood carrying substances away.

• Cube: surface area = 6 x length²; volume = length³; ratio = 6 / length.
• Sphere: surface area = 4 x pi x r²; volume = 4/3 x pi x r³; ratio = 3 / r.
• Cylinder (closed): surface area = (2 x pi x r²) + (2 x pi x r x h); volume = pi x r² x h.
• Always calculate both values and divide the surface area by the volume, then write the answer as a ratio to 1.
• Agar block experiment: cubes of different sizes in acid, or in a dye, show that the smallest block changes colour throughout first, because it has the largest surface area to volume ratio and the shortest diffusion distance to its centre.

Gas exchange: simple diffusion, pumped ventilation and ram ventilation

Aerobic respiration needs oxygen and produces carbon dioxide, and both gases have to cross the body surface, in one direction or the other. This exchange is gas exchange. The gases move by diffusion, so they move down their concentration gradients: oxygen is used up inside the organism, which keeps the oxygen concentration in the tissues lower than in the water, and carbon dioxide is produced, which keeps its concentration higher inside than outside.

Anything that maintains those gradients speeds the process up.

Simple diffusion across the whole body surface is enough for small, thin marine organisms with a large surface area to volume ratio and a low demand for oxygen. A coral polyp is the classic example: it is small and its tissues are thin, so every cell is close to the sea water and no cell is more than a short distance from the surface; it is sessile, so its oxygen demand is low and fairly constant; and it has no gills, no blood system and no ventilation mechanism. Water movements over the colony, helped in some species by beating cilia, keep the water around the polyp refreshed so that the gradients do not fade.

Pumped ventilation is used by fish that can afford to stop moving. A grouper is an ambush predator that lies still on the reef for long periods, then makes a short, explosive strike. It draws water in through the mouth by expanding the buccal cavity, closes the mouth, and then raises the floor of the mouth and squeezes the opercula so that the water is pushed over the gills.

Because the pump works whether or not the fish is moving, the grouper can ventilate while stationary, which is exactly what its hunting method requires. The cost is that pumping uses energy, and the volume of water moved per minute is limited.

Ram ventilation is used by fish that never stop swimming. A tuna swims with its mouth open, and the forward movement forces water through the mouth and over the gills, so no pumping muscles are needed and the faster the fish swims, the more water passes over the gills. Adult tuna cannot ventilate by pumping effectively, so they are obligate ram ventilators and must keep swimming all their lives, even when resting.

This suits a fast, continuous cruiser in the open ocean with a very high oxygen demand, and it is paired with a large gill surface area, thin gill lamellae and a powerful circulation. The water and the blood flow in opposite directions across the gill lamellae, which keeps a diffusion gradient along the whole length of the gill rather than only at one end.

Osmoregulation: conformers, regulators, euryhaline and stenohaline

Sea water contains about 35 g of dissolved salts per kilogram, roughly 3.5%, mostly sodium and chloride ions with smaller amounts of magnesium, sulfate, potassium and calcium. Because it is so concentrated, sea water has a fairly low (negative) water potential.

The body fluids of most marine invertebrates have almost the same concentration as sea water, so there is little or no water potential gradient between the animal and the sea, and water does not tend to flood in or drain out. That is why many marine species can survive without regulating their water content at all.

A species whose body fluid concentration simply follows the concentration of the surrounding water is an osmoconformer. A marine mussel is the example: its body fluids are isosmotic with sea water, it does not spend energy holding them at a different concentration, and its cells tolerate the water that moves in or out when the tide brings fresher or saltier water over it.

A species that holds its body fluids at a different concentration from the surrounding water, by active transport, is an osmoregulator. A tuna is the example: its body fluids are far more dilute than sea water, at roughly a third of the concentration, so it is constantly losing water by osmosis and gaining salts, and it must spend energy correcting both.

Salinity tolerance is a separate idea from regulation. Stenohaline species tolerate only a narrow range of salinity, and most marine organisms are stenohaline, because their cells and enzymes are adapted to a nearly constant internal concentration - the tuna is an example. Euryhaline species tolerate a wide range of salinity - the marine mussel is one, which is why it can live in estuaries, and the salmon is the other, because it spends part of its life in fresh water and part in the sea.

A species can therefore be a euryhaline osmoconformer (the mussel) or a euryhaline osmoregulator (the salmon), and those two labels are not interchangeable.

Osmoregulation in the salmon. Salmon hatch in fresh water, migrate to sea to feed and grow, and return to fresh water to spawn, so they must reverse their physiology twice. In fresh water the body fluids are more concentrated than the surrounding water, so water enters by osmosis, mainly across the gills.

The salmon therefore drinks little, produces large volumes of dilute urine, and actively takes up sodium and chloride ions from the water across its gill cells. In sea water the situation is reversed: the body fluids are more dilute than the sea, so water is lost by osmosis.

The salmon now drinks sea water, absorbs the water in its gut, excretes the surplus ions actively across its gills, and produces only a small volume of urine, so that water is conserved. The switch between the two sets of mechanisms happens during the migration, when the gills and kidneys remodel.