What you'll learn
Every living thing is built from cells, and almost everything else in CSEC Biology depends on understanding them first. This topic covers the structure of plant and animal cells, the function of each organelle, the differences between plant, animal and bacterial cells, and how cells are organised into tissues, organs and systems. Examiners return to this material constantly, both as direct recall questions and as the foundation for questions on transport, respiration and photosynthesis. A candidate who can name an organelle but not say what it does will lose most of the available marks, so aim to pair every structure with its job.
Key terms and definitions
Cell — the basic structural and functional unit of all living organisms.
Organelle — a specialised structure within a cell that carries out a particular function.
Cell membrane — the thin, partially permeable layer surrounding the cell that controls what enters and leaves.
Cytoplasm — the jelly-like material filling the cell, in which the organelles are suspended and where many chemical reactions occur.
Nucleus — the organelle containing the chromosomes, which control the cell's activities.
Chloroplast — the organelle in plant cells containing chlorophyll, where photosynthesis takes place.
Vacuole — a fluid-filled sac; plant cells have one large permanent vacuole containing cell sap.
Cell wall — a rigid layer of cellulose outside the cell membrane of a plant cell, providing support.
Prokaryotic — describing a cell with no true nucleus, such as a bacterial cell.
Eukaryotic — describing a cell in which the genetic material is enclosed within a nucleus.
Tissue — a group of similar cells working together to perform one function.
Organ — a structure made of several tissues working together.
Core concepts
The animal cell and its organelles
An animal cell is bounded by a cell membrane, a partially permeable layer made largely of lipid and protein. It holds the cell together and, more importantly, controls which substances pass in and out. Inside the membrane lies the cytoplasm, a watery jelly in which dissolved substances move and in which the organelles sit.
The nucleus is usually the largest visible structure. It contains chromosomes made of DNA, which carry the instructions for making proteins and therefore control everything the cell does. A cell whose nucleus has been removed cannot divide and soon dies.
Mitochondria are the sites of aerobic respiration, releasing energy from glucose for the cell to use. Cells that need a great deal of energy — muscle cells, sperm cells, cells lining the small intestine — contain large numbers of them. This is a favourite examination link: when asked why a particular cell has many mitochondria, the answer is always about its high energy demand.
Ribosomes are extremely small structures where proteins are assembled. The endoplasmic reticulum is a system of membranes that transports these proteins through the cell.
The plant cell and its additional structures
A plant cell contains everything listed above, and three structures besides.
The cell wall lies outside the cell membrane and is made of cellulose. It is fully permeable, so it does not control entry and exit — a common misunderstanding. Its job is mechanical: it stops the cell bursting when water enters and gives the plant support. Note the distinction carefully, because questions often offer "controls what enters the cell" as a tempting wrong answer for the cell wall.
Chloroplasts contain the green pigment chlorophyll, which absorbs light energy for photosynthesis. They are found in cells exposed to light, such as those in the palisade layer of a leaf, and are absent from root cells, which never receive light.
The permanent vacuole is a large sac filled with cell sap, a solution of sugars and salts. When it is full of water it pushes outwards on the cell wall, keeping the cell firm, or turgid. A plant short of water has vacuoles that shrink, the cells become flaccid, and the plant wilts.
Comparing plant, animal and bacterial cells
Plant and animal cells are both eukaryotic: their DNA is enclosed in a nucleus. The differences are that plant cells have a cellulose cell wall, chloroplasts and a large permanent vacuole, while animal cells have none of these. Animal cells are usually irregular in shape; plant cells tend to be more regular because the cell wall holds a fixed form.
A bacterial cell is prokaryotic and differs more fundamentally. It has no nucleus — its DNA lies free in the cytoplasm — and no mitochondria or chloroplasts. It does have a cell wall, but one made of a different material from cellulose, and it may carry a flagellum for movement and small rings of DNA called plasmids. Bacterial cells are also very much smaller than plant or animal cells.
Specialisation and levels of organisation
Cells are not all alike. A specialised cell has a structure suited to one job. A root hair cell has a long, thin extension that increases the surface area for absorbing water. A red blood cell has no nucleus, leaving more room for haemoglobin to carry oxygen. A sperm cell has a tail for swimming and many mitochondria to power it. A palisade cell is packed with chloroplasts and sits near the upper surface of the leaf.
Specialised cells are arranged in a hierarchy. Similar cells form a tissue, such as muscle tissue or xylem tissue. Several tissues form an organ, such as the heart or a leaf. Organs working together form an organ system, such as the circulatory system, and the systems together form the organism. Learning this sequence in order — cell, tissue, organ, system, organism — is worth the small effort, because it is asked directly.
Specialisation has a cost as well as a benefit. A red blood cell that has lost its nucleus can carry far more haemoglobin, but it can no longer divide or repair itself, which is why red blood cells must be replaced continually. This trade-off is a useful point to raise in longer answers about why multicellular organisms need so many different cell types.
Using the microscope and calculating magnification
Cells are far too small to see unaided, so this topic is always examined alongside practical microscope work. A light microscope has two lenses that contribute to the total magnification: the eyepiece and the objective. Total magnification is the eyepiece magnification multiplied by the objective magnification, so a ×10 eyepiece used with a ×40 objective gives a total of ×400.
Magnification and size are linked by a single relationship: magnification = image size ÷ actual size. Rearranged, the actual size of a structure equals the image size divided by the magnification. The commonest error is mixing units, so convert everything to the same unit before dividing. One millimetre is 1 000 micrometres, and cell measurements are normally quoted in micrometres.
When preparing a slide, a thin specimen is needed so light can pass through it, and a stain such as iodine or methylene blue is used to make structures visible. The cover slip is lowered at an angle to avoid trapping air bubbles, which would otherwise be mistaken for cells. Candidates are often asked why a specimen must be thin: the answer is that light must pass through it for the structures to be seen.
Worked examples
Example 1. A student examines two cells under a microscope. Cell P has a cell wall, chloroplasts and a large central vacuole. Cell Q has none of these but does have a nucleus and mitochondria. Identify each cell and give one reason for each answer.
Cell P is a plant cell, because chloroplasts and a large permanent vacuole are found only in plant cells. Cell Q is an animal cell, because it has a nucleus, so it is eukaryotic rather than bacterial, but lacks the cell wall and chloroplasts that would make it a plant cell. Notice that the reason must name a structure — simply writing "it looks like a plant cell" earns nothing.
Example 2. Cells lining the small intestine contain far more mitochondria than cells in the outer layer of the skin. Suggest an explanation.
Cells lining the small intestine carry out active transport to absorb nutrients against a concentration gradient, and active transport requires energy. That energy is released by aerobic respiration in the mitochondria, so a cell doing a great deal of active transport needs many of them. Skin cells in the outer layer are largely dead or inactive and have a much lower energy demand.
Example 3. Explain why a plant left without water for several days becomes limp, referring to cell structure.
Water leaves the vacuoles of the cells, so the vacuoles shrink and no longer press outwards against the cell walls. The cells become flaccid rather than turgid, and because the firmness of the tissue depends on that outward pressure, the stem and leaves lose their support and the plant wilts.
Example 4. A cell appears 60 mm long in a drawing made at a magnification of ×400. Calculate its actual length in micrometres.
Rearranging the relationship gives actual size = image size ÷ magnification, so the actual length is 60 ÷ 400 = 0.15 mm. Converting to micrometres by multiplying by 1 000 gives 150 µm. Always state the unit; an unlabelled number is normally not awarded.
Example 5. Explain why a root hair cell has no chloroplasts but does have a long extension.
Root hair cells are underground and receive no light, so chloroplasts would be of no use and are absent. The long, narrow extension greatly increases the surface area in contact with soil water, which speeds up the absorption of water and dissolved mineral ions. This illustrates the general principle that a specialised cell's structure follows from the job it does in the position it occupies.
Common mistakes and how to avoid them
Confusing the cell wall with the cell membrane. The membrane is partially permeable and controls entry and exit; the wall is fully permeable and provides support. Every plant cell has both. If a question asks what controls what enters a plant cell, the answer is the membrane, never the wall.
Claiming animal cells have no vacuoles at all. Animal cells may contain small, temporary vacuoles. What they lack is the single large permanent vacuole characteristic of plant cells. Write "large permanent vacuole" and the distinction is safe.
Saying bacteria have no DNA. Bacteria have DNA; what they lack is a nucleus to enclose it. The same care applies to the cell wall — bacteria have one, but it is not made of cellulose.
Naming an organelle without its function. A question worth two marks almost always wants the structure and its job. "Mitochondria" alone scores one mark at best; "mitochondria, where aerobic respiration releases energy" scores both.
Assuming all plant cells contain chloroplasts. Root cells receive no light and have none. Examiners use this to test whether candidates have learned the reason behind the structure rather than a list.
Exam technique for Cell Structure and Function
Read whether a question asks for a difference or a similarity, and answer only what is asked. For differences, write a comparative sentence covering both cells: "a plant cell has a cellulose cell wall whereas an animal cell does not" earns the mark, while "plant cells have cell walls" leaves the comparison unstated.
When a question gives an unfamiliar cell and asks you to explain its structure, work from function backwards. Ask what the cell must do, then connect each named feature to that job. This pattern — feature, then purpose — is what the mark scheme rewards.
Where a table is provided for comparison, fill every cell of it. A blank box scores nothing even when the answer is obvious from the row above.
Finally, be precise with size. Cells are measured in micrometres, and bacterial cells are smaller than plant and animal cells by roughly an order of magnitude. If a question asks you to arrange structures by size, work from the cell down through the nucleus to the ribosome.
Quick revision summary
- All cells have a cell membrane, cytoplasm and ribosomes; plant and animal cells also have a nucleus and mitochondria.
- Plant cells additionally have a cellulose cell wall, chloroplasts and a large permanent vacuole.
- Bacterial cells are prokaryotic: no nucleus, no mitochondria, no chloroplasts, but DNA free in the cytoplasm and a non-cellulose wall.
- The membrane controls entry and exit; the wall gives support and is fully permeable.
- Mitochondria release energy by aerobic respiration — abundant in cells with high energy demand.
- Chloroplasts carry out photosynthesis and are absent from cells that receive no light.
- A full vacuole makes a cell turgid; water loss makes it flaccid and the plant wilts.
- Specialised cells have structures matched to their function: root hair, red blood cell, sperm cell, palisade cell.
- Levels of organisation: cell → tissue → organ → organ system → organism.
- Pair every structure with its function in your answers; a name alone rarely earns full marks.