Cell structure, function and reproduction

Cells: the unit of life in the sea

Every living thing in the sea is made of cells. A bacterium, a diatom or a dinoflagellate is a single cell that does everything it needs to stay alive, while a fish or a whale is made of millions of cells that have become specialised for different jobs. The cell is the smallest unit that can carry out all the processes of life, which is why this topic comes before classification: growth, photosynthesis, respiration and reproduction all happen inside cells.

Marine science uses cell structure as an identification tool. A sample of green surface water under a microscope may be full of single-celled diatoms with box-like walls, or dinoflagellates, or bacteria, and the structures each cell has tell you which of these you are looking at and what part it plays in the food web. A cell with chloroplasts makes its own food; a cell without them must obtain food from something else.

Your syllabus limits the structures you can be asked about. For plant and animal cells there are seven: cell wall, cell membrane, nucleus, cytoplasm, chloroplast, mitochondria and vacuole. For a bacterial cell there are only three: cell wall, cell membrane and cytoplasm. Learn these lists exactly, because anything else you may have met in a biology course - ribosomes, plasmids, a flagellum - is outside this specification and cannot earn you marks here.

Plant and animal cells: what they share and where they differ

A plant cell and an animal cell are built on the same plan. Both are surrounded by a thin cell membrane, both contain cytoplasm and a nucleus, and both contain mitochondria. When a question asks for similarities, those four structures are the answer, and you should write 'both have' rather than making two separate lists.

Plant cells have three structures that animal cells do not. The cell wall is a tough layer outside the cell membrane, chloroplasts are small green bodies in the cytoplasm, and there is a large permanent vacuole filled with cell sap that takes up much of the centre of the cell. A cell with a wall and chloroplasts is a plant cell; an animal cell has neither.

Shape follows from the wall. A plant cell is usually regular, often drawn roughly rectangular, because the rigid wall holds it in shape. An animal cell has only the flexible membrane, so it is more variable and often rounded or irregular, and it can look squashed or folded when it is prepared on a slide. Shape is a clue in a diagram, but it is never the mark - the structures are.

• In both: cell membrane, cytoplasm, nucleus, mitochondria.
• Only in plant cells: cell wall, chloroplasts, a large permanent vacuole.
• Never in an animal cell: a cell wall or a chloroplast.
• Plant cells have all seven structures, so there is nothing on this list that a plant cell lacks.
• Animal cells are usually smaller and more irregular in shape because they have no wall to hold them firm.

The three structures only plant cells have

The cell wall is a strong, rigid layer that lies outside the cell membrane. In plants and green algae it is made of cellulose, and it is freely permeable, so water and dissolved substances pass through it without being selected. Its job is support and protection: it gives the cell its shape and stops it bursting when water enters by osmosis.

A chloroplast is a small body in the cytoplasm containing the green pigment chlorophyll, which absorbs light energy for photosynthesis. Chloroplasts are concentrated in the cells of parts that are exposed to light, which in a seagrass is the leaf rather than the root, and in seaweeds is the frond.

The large permanent vacuole is a space filled with cell sap, a solution of water with dissolved sugars and salts. It occupies much of the centre of a mature plant cell and pushes the cytoplasm out towards the membrane. Because the sap inside is more concentrated than seawater, water enters and presses the vacuole against the wall, keeping the cell firm or turgid - which is what holds a seagrass leaf out flat in moving water.

The function of each structure

Function questions are marked on the job the structure does and why that matters to the cell, not on what it looks like. 'The nucleus is a dark round body near the centre' describes an appearance and scores nothing; 'the nucleus contains the genetic material that controls the activities of the cell' scores the mark.

The two outer layers are the pair students most often confuse. The cell membrane is the living, partially permeable layer that controls what enters and leaves the cell, while the cell wall is the dead, fully permeable strengthening layer outside it. Every cell in this topic has a membrane; only plant cells and bacteria have a wall.

The two energy structures are the second pair to keep apart. Chloroplasts absorb light energy and use it to build glucose from carbon dioxide and water, so they only make sense in a cell that photosynthesises. Mitochondria release energy from nutrients such as glucose by respiration, so every active cell needs them, including animal cells that never photosynthesise.

The bacterial cell

A bacterial cell is much simpler and much smaller than a plant or animal cell, typically only 1 to 5 micrometres across. It has just three of the structures on your list: a cell wall, a cell membrane and cytoplasm. It has no nucleus, so its genetic material is a single loop of DNA lying free in the cytoplasm, and it has no chloroplasts and no mitochondria.

That absence of a nucleus is the single most important fact about a bacterium, and it has consequences for everything else. With no membrane-bound structures inside it, the chemical reactions of the cell happen in the cytoplasm and at the cell membrane, and the enzymes for respiration are found there rather than in mitochondria. Some bacteria do photosynthesise - the marine cyanobacteria are an example - but they carry out the reactions on folded membranes in the cytoplasm, not in chloroplasts.

The bacterial cell wall does the same job as the plant wall - support and protection - but it is not made of cellulose. Some bacteria carry extra structures such as a slime capsule, one or more flagella for movement, or small rings of DNA called plasmids; these are not on your list, and if a diagram shows one you are not expected to name it, but you must not mistake it for one of the three structures you do need.

• Cell wall: rigid outer layer, supports and protects the cell; not cellulose.
• Cell membrane: controls what enters and leaves the cell, and carries the enzymes for respiration.
• Cytoplasm: where the chemical reactions take place and where the DNA lies.
• No nucleus: the genetic material is free in the cytoplasm, which is why a bacterium is not a plant or animal cell.
• No chloroplasts and no mitochondria: photosynthesis and respiration happen on membranes and in the cytoplasm instead.

Identifying cells in diagrams and images

You will be shown a drawing, a light micrograph or a photograph and asked to identify structures, or to decide which type of cell you are looking at. Work from the outside inwards and make a decision at each step, rather than staring at the whole image.

First look at the outer boundary. One thin line is a cell membrane on its own, which means an animal cell. Two lines - a thick outer wall with a membrane just inside it - means a plant or bacterial cell. Next look for a nucleus: a large, darker, rounded body is present in plant and animal cells and completely absent from bacteria. Then look inside the cytoplasm: small green bodies are chloroplasts, a very large clear space taking up much of the cell is the vacuole, and small scattered rod-shaped bodies are mitochondria.

In photographs you also have colour and depth to help you. Chloroplasts are green, the nucleus is denser and darker than the cytoplasm, and the vacuole is clear and appears empty. Size is the last clue: bacteria are far smaller than plant and animal cells, so a field of tiny cells with no visible nuclei is almost certainly bacterial.

• Do not label the wall as the membrane: if two lines are drawn, the outer one is the wall and the inner one is the membrane.
• Not every round body is a nucleus - a vacuole in cross-section can look circular, but it is clear and much larger.
• Not every green cell is a plant: cyanobacteria are green and are bacteria, and the give-away is that they have no nucleus.
• Read the label line, not the shape: irregular cells are usually animal, but shape alone never earns the mark.

Asexual reproduction

Asexual reproduction is reproduction in which a single organism produces exact copies of itself. There is one parent, no sex cells are involved and no fertilisation takes place, so the offspring are genetically identical to the parent: they are clones.

It happens in several ways, and you should be able to describe at least two. In binary fission a single-celled organism grows, copies its genetic material and divides into two identical cells; this is how bacteria and single-celled algae such as diatoms increase. In budding a small outgrowth forms on the parent, develops into a miniature copy and either breaks away or stays attached, which is how a coral colony adds polyps. In fragmentation a piece breaks off and grows into a whole new organism, as in some seaweeds and sea anemones, and in vegetative spread a seagrass sends out rhizomes that produce new shoots.

The consequences follow from the definition. Asexual reproduction is fast, needs only one organism, and can produce large numbers of offspring in a short time, which suits a species living in conditions that already suit the parent. The weakness is that there is no variation: every offspring is identical, so a disease, a predator or a change in temperature that harms one harms them all.

• One parent, no sex cells, no fertilisation, offspring genetically identical to the parent.
• Binary fission: bacteria and diatoms; one cell divides into two.
• Budding: coral polyps and Hydra; a new individual grows out of the parent.
• Fragmentation: some seaweeds and sea anemones; a broken piece becomes a new organism.
• Vegetative spread: seagrass rhizomes produce new shoots that are clones of the parent plant.

Sexual reproduction

Sexual reproduction is reproduction in which organisms produce male and female sex cells, which fuse to produce offspring with characteristics from both parents. The sex cells are called gametes: the male sex cell is the sperm, which is small and motile, and the female sex cell is the egg, which is larger and carries a food store. Their fusion is fertilisation, and the single cell it produces is a zygote, which divides again and again to build the offspring.

Because each sex cell carries genetic material from the parent that made it, the offspring inherits a mixture from both parents. It is therefore not identical to either parent and not identical to its siblings - which is where the variation in a population comes from. Two parents are usually needed, although some organisms can produce both types of sex cell themselves.

In the sea most invertebrates and most fish release their eggs and sperm into the water and fertilisation happens outside the body, which is why so many eggs are produced and so few survive. Corals take this to an extreme: colonies of the same species spawn together on the same few nights, releasing bundles of eggs and sperm into the water at once so that fertilisation is likely. Marine mammals and marine reptiles fertilise internally instead, inside the female's body.

• Two parents (usually), each contributing one sex cell.
• Male sex cell: sperm - small, with a tail, and motile.
• Female sex cell: egg - larger, and containing a food store for the developing offspring.
• Fertilisation: the fusion of the two sex cells, producing a zygote.
• Offspring have characteristics from both parents, so they vary - they are not exact copies.

Asexual and sexual reproduction side by side

The two methods are easiest to hold in your head as a comparison, because exam questions often ask you to compare them rather than describe them separately. Keep the pairs straight: one parent against two, no sex cells against sex cells, no fertilisation against fertilisation, exact copies against offspring that vary.

Most marine organisms are not restricted to one method. A coral colony grows larger by asexual budding, producing a colony of genetically identical polyps, and then reproduces sexually by spawning so that its offspring can drift away and colonise new reef. Seagrass spreads across a sediment bed by asexual rhizomes and also produces flowers and seeds. The asexual route holds and expands ground that is already occupied; the sexual route produces variation and disperses into new ground.