O-Level Biology · Secondary 3 and 4
Cell Structure and Organisation
Cell structure is the starting point of O-Level Biology. You learn what each part of a cell does, how plant and animal cells differ, and how cells become specialised and are organised into tissues, organs and systems. Almost every later topic, from osmosis to photosynthesis to respiration, is explained at the level of the cell, so a clear picture of the organelles here makes the rest of the course much easier to follow.
What the syllabus expects
- Identify the cell membrane, cytoplasm, nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi body, vacuoles, chloroplasts and cell wall in diagrams and photomicrographs
- State the function of each of these structures
- Compare the structure of a typical plant cell with a typical animal cell
- Relate the structure of specialised cells, such as red blood cells and root hair cells, to their functions
- Describe the levels of organisation from cells to tissues, organs and organ systems
- Calculate magnification and the actual size of a specimen from a drawing or photomicrograph
The organelles and what they do
A cell is the basic unit of life. The cell membrane is partially permeable, so it controls which substances enter and leave the cell. Inside it is the cytoplasm, a jelly-like fluid where many chemical reactions take place. The nucleus contains chromosomes made of DNA, which carry the instructions that control the cell's activities and are passed on when the cell divides. Mitochondria are the site of aerobic respiration, where energy is released from food for the cell to use. Ribosomes make proteins. Many ribosomes are attached to the rough endoplasmic reticulum, which transports the proteins they make. The smooth endoplasmic reticulum makes fats and steroids and helps to break down harmful substances. The Golgi body modifies and packages substances, such as enzymes, into vesicles that carry them to the cell membrane to be secreted. Vacuoles are fluid-filled spaces. In animal cells they are small and often temporary, while a plant cell has one large central vacuole filled with cell sap.
Plant cells and animal cells compared
Both plant and animal cells have a cell membrane, cytoplasm, nucleus, mitochondria, ribosomes, endoplasmic reticulum and a Golgi body. Plant cells have three extra features. The cell wall, made of cellulose, is fully permeable. It gives the cell a fixed shape and stops it from bursting when it takes in a lot of water. Chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis. Only cells that photosynthesise have them, so a root hair cell, which is underground, has none. The large central vacuole holds cell sap and keeps the cell firm, or turgid, when it is full of water. Animal cells have no cell wall, no chloroplasts and only small vacuoles, so their shape is more irregular. Animal cells store carbohydrate as glycogen granules, whereas plant cells store it as starch grains.
Specialised cells and levels of organisation
Cells become specialised so that their structure suits one job. A red blood cell has no nucleus, which leaves more room for haemoglobin to carry oxygen. Its biconcave disc shape gives a large surface area to volume ratio, so oxygen diffuses in and out quickly, and it is flexible enough to squeeze through narrow capillaries. A root hair cell has a long, narrow extension that increases the surface area for absorbing water and mineral ions. Its cell sap is usually more concentrated than the soil solution, so water enters by osmosis. Similar cells working together form a tissue, such as muscle tissue. Different tissues working together form an organ, such as the stomach or a leaf. Several organs working together form an organ system, such as the digestive system, and the organ systems together make up the whole organism.
Magnification and actual size
Cells are measured in micrometres (µm), and 1 mm = 1000 µm. Magnification tells you how many times larger the image is than the real object: magnification = image size ÷ actual size. Rearranging gives actual size = image size ÷ magnification. Both lengths must be in the same unit before you divide, so convert the length you measure with your ruler from millimetres into micrometres first. For example, if a cell is 20 µm long and its drawing is 40 mm long, convert 40 mm to 40 000 µm, then magnification = 40 000 ÷ 20 = ×2000. Magnification has no unit, but it is written with a times sign. Measure the exact line the question names, such as the length of the cell or the diameter of the nucleus, and measure to the nearest millimetre.
Worked example
A student draws a palisade mesophyll cell seen under a light microscope. The drawing is 45 mm long and is labelled ×1500. (a) Calculate the actual length of the cell in micrometres. (b) Name two structures in this cell that are not found in a human cheek cell, and state the function of each.
- Write the formula: actual size = image size ÷ magnification.
- Convert the image size to micrometres so the answer comes out in µm: 45 mm × 1000 = 45 000 µm.
- Divide: 45 000 µm ÷ 1500 = 30 µm.
- Check by working backwards: 30 µm × 1500 = 45 000 µm, which is 45 mm, the length of the drawing.
- For (b), note that a palisade cell is a plant cell and a cheek cell is an animal cell, so pick structures that only plant cells have.
- Give one function for each structure you name: the cellulose cell wall and chloroplasts are the clearest choices.
Answer: (a) 30 µm. (b) The cellulose cell wall, which gives the cell a fixed shape and stops it bursting when it takes in water. Chloroplasts, which contain chlorophyll to absorb light energy for photosynthesis. (The large central vacuole containing cell sap is also accepted.)
Common mistakes
- Saying the cell wall is partially permeable. It is the cell membrane that is partially permeable. The cellulose cell wall is fully permeable.
- Writing that all plant cells have chloroplasts. Root hair cells and other cells that receive no light have none, and this is a common exam trap.
- Dividing a length in millimetres by a length in micrometres. Convert both to the same unit first, or the answer will be out by a factor of 1000.
- Saying mitochondria make or produce energy. Energy cannot be made. Mitochondria release energy from food by aerobic respiration.
- Mixing up tissue and organ. A tissue is a group of similar cells doing one job, while an organ is made of several different tissues working together.
Exam tips
- When asked to compare plant and animal cells, write paired statements such as 'plant cells have a cellulose cell wall, animal cells do not'. A list of plant cell parts on its own does not show a comparison.
- In magnification questions, write the formula, show the unit conversion on its own line, and give the answer with the right unit: µm for actual size and a times sign for magnification.
- For specialised cells, link each structure to its function in one sentence using 'so' or 'which', for example 'a long root hair, which increases the surface area for absorbing water'.
Practice questions
1. Easy: State the function of the ribosomes and of the Golgi body.
Show answer
Ribosomes make proteins. The Golgi body modifies and packages substances, such as proteins made on the rough endoplasmic reticulum, into vesicles that carry them to the cell membrane for secretion.
2. Medium: A red blood cell has an actual diameter of 8 µm. In a photomicrograph its diameter measures 24 mm. Calculate the magnification.
Show answer
×3000. Convert to the same unit: 24 mm = 24 000 µm. Magnification = image size ÷ actual size = 24 000 ÷ 8 = 3000. Check: 8 µm × 3000 = 24 000 µm = 24 mm.
3. Harder: Explain how two features of a red blood cell make it efficient at transporting oxygen.
Show answer
It has no nucleus, so there is more space for haemoglobin, which combines with oxygen, and each cell can carry more oxygen. Its biconcave shape gives a large surface area to volume ratio, so oxygen diffuses in and out of the cell quickly. Each mark needs the feature and how it helps.
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