Cambridge O Level Biology · Syllabus 5090 · Cells
Cell
What is Cell?
The smallest structural and functional unit of a living organism. A cell is enclosed by a cell membrane that controls the movement of substances into and out of it, contains cytoplasm in which many enzyme-controlled reactions take place, and carries genetic information as DNA which controls the cell's activities and the production of proteins.
This definition is part of the Cells chapter in Cambridge O Level Biology.
Cell in context
Animal, plant and bacterial cells all share a cell membrane, cytoplasm and ribosomes, but they differ in what else they contain. An animal cell also has a nucleus and mitochondria. A plant cell adds a cellulose cell wall, a permanent sap vacuole and, in its photosynthetic cells only, chloroplasts. A bacterial cell has a cell wall that is not made of cellulose, circular DNA lying free in the cytoplasm and sometimes plasmids, but no nucleus, no mitochondria and no chloroplasts. In every cell the membrane controls what enters and leaves; a cell wall only supports, because it is fully permeable.
A cell is the smallest structural and functional unit considered in this syllabus. Cell structures are not labels to be memorised in isolation: each one performs work that keeps the cell, and therefore the whole organism, alive. Topic 1 asks you to identify those structures in animal, plant and bacterial cells, to state what each one does, to obtain your own evidence with a light microscope, to record that evidence as a biological drawing, to convert between image size and actual size using magnification, and to explain how specialised cells are built into tissues, organs, organ systems and organisms.
Common mistakes with Cell
- “The cell wall controls what enters the cell.” Repair The cell membrane controls exchange; the wall supports. The wall is fully permeable, so it cannot select anything.
- “A tissue is many cells.” Repair A tissue is a group of similar cells working together to perform a function. Both halves are needed.
- “All plant cells have chloroplasts.” Repair Only photosynthetic cells do. A root hair cell has none, because no light reaches it. An onion epidermal cell has none either.
- “Mitochondria make energy for the cell.” Repair Mitochondria are the site of aerobic respiration, which releases energy from nutrient molecules. Energy is never made.
- Listing features when the question said explain. Repair Use the FLE chain: feature, then what it physically causes, then what the cell can therefore do. “Many mitochondria” is a feature; “so energy is released for active transport” is the mark.
- Writing two separate lists when the question said compare. Repair Put both organisms in the same sentence: “A plant cell has a nucleus, whereas a bacterial cell does not.”
Examiner tips on Cell
- Read the “Plant” column carefully. Two of its entries are conditional, not absolute. Chloroplasts occur in photosynthetic cells, and the entry for the animal column is no large permanent vacuole rather than a flat “no vacuole”. Precision in these two cells of the table is worth real marks.
- A quick sanity check that costs five seconds. Cells are of the order of micrometres and images on a page are of the order of millimetres, so a magnification should come out in the hundreds or thousands. If your magnification is 0.4, you divided before converting. If an “actual size” comes out larger than the image, you multiplied when you should have divided.
- Two cells, one idea. Whenever a cell has to absorb or exchange something, look for a shape that raises surface area: the long thin projection of a root hair cell, and the biconcave dish of a red blood cell. If you can spot which problem a shape is solving, you can usually work out the function of a specialised cell you have never seen before.
Questions students ask about Cell
What is the difference between the cell wall and the cell membrane?
The cell membrane is the thin, partially permeable boundary present in every cell, and it controls the movement of substances into and out of the cell. The cell wall is a rigid layer of cellulose outside the membrane of a plant cell; it supports the cell and stops it bursting when water enters by osmosis. The wall is fully permeable, so it cannot select what passes through — writing “the cell wall controls what enters” loses the mark.
Do bacterial cells have DNA if they have no nucleus?
Yes. A bacterial cell has a circular molecule of DNA lying free in the cytoplasm, and sometimes small extra circles of DNA called plasmids. What it lacks is a nucleus to enclose that DNA, together with mitochondria and chloroplasts. It still has a cell membrane, cytoplasm, ribosomes and a cell wall, although that wall is not made of cellulose.
How do you calculate magnification?
Magnification = image size ÷ actual size, and both lengths must be in the same unit before you divide. Convert first: 1 mm = 1000 micrometres, so an image 48 mm wide of a cell 120 micrometres wide gives 48 000 ÷ 120 = ×400. Because magnification is a ratio of two lengths, the units cancel and it has no unit — write ×400, never “400 micrometres”.
Why do only some plant cells have chloroplasts?
Chloroplasts contain chlorophyll and are the site of photosynthesis, so they are found only in plant cells that receive light, such as the palisade mesophyll cells packed just below the upper surface of a leaf. A root hair cell has none because no light reaches it underground, and an onion epidermal cell has none either. “All plant cells have chloroplasts” is a common error.
What is the difference between a tissue and an organ?
A tissue is a group of similar cells working together to perform a particular function; “many cells” is not enough, because the cells must be similar and must work together. An organ is a structure made of several different tissues working together for one function: the heart contains cardiac muscle, nervous tissue, blood vessels and epithelium, all serving the job of pumping blood. The full order is organelle → cell → tissue → organ → organ system → organism.
Do mitochondria make energy for the cell?
No. Mitochondria are the site of aerobic respiration, which releases energy from nutrient molecules; energy is never made. Cells that do a great deal of work, such as muscle cells, sperm cells and root hair cells, contain many mitochondria because they need a large amount of energy released. In an exam, write “releases energy” rather than “makes energy” or “produces energy”.
How do I explain how a specialised cell is adapted to its function?
Chain three steps: name the feature, say what it physically causes, then state what the cell can therefore do. “A root hair cell has many mitochondria” is only a feature and earns nothing on its own; adding “so more energy is released for active transport of mineral ions from the soil” completes the explanation. Listing features when the question says explain is one of the commonest ways to lose marks in this topic.