Cell Biology
Cell structure, transport, cell division, and stem cells.
In this topic
Cell Structure
All living organisms are made of cells. There are two main types: eukaryotic cells (plants and animals) and prokaryotic cells (bacteria).
Animal cells contain: cell membrane, cytoplasm, nucleus, mitochondria, and ribosomes.
Plant cells have all of the above plus: cell wall, permanent vacuole, and chloroplasts.
Prokaryotic cells (bacteria) are much smaller and simpler. They have: cell membrane, cytoplasm, ribosomes, a single DNA loop (not in a nucleus), and sometimes plasmids and a flagellum.
Key Points
- Nucleus contains DNA — controls cell activities
- Mitochondria are the site of aerobic respiration
- Ribosomes are where proteins are synthesised
- Chloroplasts contain chlorophyll for photosynthesis
- Cell wall provides structural support in plant cells
Example Questions
2Name two organelles found in plant cells but not in animal cells.
Cell wall and chloroplasts (also accept permanent vacuole).
[2 marks]
3Explain why muscle cells have many mitochondria.
Muscle cells need a lot of energy for contraction. Mitochondria carry out aerobic respiration to release energy (ATP), so having many mitochondria means more energy can be produced.
[3 marks]
2State two differences between prokaryotic and eukaryotic cells.
1) Prokaryotic cells have no nucleus (DNA is free in the cytoplasm), while eukaryotic cells have a nucleus. 2) Prokaryotic cells are much smaller (typically 1-5μm vs 10-100μm).
[2 marks]
Microscopy
Microscopes allow us to see cells and structures too small for the naked eye.
Light microscopes: magnification up to ×1500, resolution of 200nm. Can view living specimens.
Electron microscopes: magnification up to ×2,000,000, resolution of 0.2nm. Cannot view living specimens.
Magnification = image size ÷ actual size To convert: 1mm = 1000μm, 1μm = 1000nm
Key Points
- Resolution = ability to distinguish between two close points
- Higher resolution = more detail visible
- Standard form is used for very small measurements
- Always show units in calculations
Example Questions
2A cell has an actual size of 50μm. Under a microscope it appears 25mm. Calculate the magnification.
Convert to same units: 25mm = 25,000μm. Magnification = 25,000 ÷ 50 = ×500
[2 marks]
Specialised Cells
As organisms develop, cells differentiate to become specialised for particular functions.
Key specialised cells: - Sperm cell: streamlined head, acrosome with enzymes, many mitochondria, long tail (flagellum) for swimming - Nerve cell (neuron): long axon for carrying impulses, branched endings (dendrites) for connections, myelin sheath for insulation - Red blood cell: biconcave disc shape (large surface area), no nucleus (more room for haemoglobin), contains haemoglobin to carry oxygen - Root hair cell: long projection increases surface area for water absorption - Xylem cell: hollow, no end walls, reinforced with lignin for water transport
Key Points
- Structure is related to function
- Most animal cells differentiate early and permanently
- Plant cells can differentiate throughout life
- Stem cells retain the ability to differentiate
Example Questions
3Explain how red blood cells are adapted for their function.
Biconcave disc shape gives a large surface area to volume ratio for efficient gas exchange. No nucleus means more space for haemoglobin to carry oxygen. Flexible shape allows them to squeeze through narrow capillaries.
[3 marks]
Transport in Cells
Substances move in and out of cells by three main methods:
1. Diffusion: the net movement of particles from an area of higher concentration to an area of lower concentration. It is passive (no energy required).
2. Osmosis: the diffusion of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution.
3. Active transport: movement of substances against the concentration gradient (from low to high concentration). Requires energy from respiration.
Key Points
- Diffusion is passive — no energy needed
- Osmosis is specifically about water through a membrane
- Active transport requires energy (ATP from mitochondria)
- Examples: O₂ diffuses into blood, glucose absorbed in gut by active transport
Example Questions
3Explain why root hair cells use active transport to absorb mineral ions.
The concentration of mineral ions is higher inside the root hair cell than in the soil solution. Therefore, mineral ions must be moved against the concentration gradient, which requires energy from respiration (active transport).
[3 marks]
3A piece of potato is placed in a concentrated sugar solution. Explain what will happen to the potato cells.
Water will move out of the potato cells by osmosis, from the less concentrated solution inside the cells to the more concentrated sugar solution outside. The cells will become plasmolysed (the cell membrane pulls away from the cell wall) and the potato will become flaccid.
[3 marks]
Cell Division
Cells divide for growth, repair, and reproduction.
Mitosis: produces two identical daughter cells with the same number of chromosomes as the parent cell. Used for growth and repair.
The cell cycle: 1. Interphase: cell grows, DNA replicates, organelles increase 2. Mitosis: nucleus divides 3. Cytokinesis: cytoplasm divides
Meiosis: produces four genetically different daughter cells, each with half the number of chromosomes. Used to produce gametes (sex cells).
Key Points
- Human body cells have 46 chromosomes (23 pairs)
- Mitosis: 2 identical cells, same chromosome number
- Meiosis: 4 different cells, half the chromosome number
- Cancer is uncontrolled mitosis
Example Questions
2State two differences between mitosis and meiosis.
1) Mitosis produces 2 daughter cells; meiosis produces 4. 2) Mitosis produces genetically identical cells; meiosis produces genetically different cells. 3) Mitosis maintains chromosome number; meiosis halves it.
[2 marks]
3Explain why meiosis is important for sexual reproduction.
Meiosis halves the chromosome number, so when two gametes fuse at fertilisation, the full chromosome number is restored. It also introduces genetic variation through independent assortment and crossing over, which is important for evolution.
[3 marks]