Cell Biology
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Animal cell structure
Animal cells are eukaryotic — they have a nucleus and membrane-bound organelles.
These five structures are found in all animal cells — the starting point for any cell structure question.
Animal cell structure — Key Knowledge
- Nucleus contains genetic material/DNA, controls cell activities
- Cell membrane controls what enters and leaves the cell
- Cytoplasm jelly-like substance where most chemical reactions take place
- Mitochondria where aerobic respiration occurs — releases energy
- Ribosomes where protein synthesis occurs
Plant cell structure
Plant cells have all the parts of an animal cell plus three additional structures.
A common exam question is "name three things found in plant cells but not animal cells" — cell wall, permanent vacuole, chloroplasts.
Plant cell structure — Key Knowledge
- Cell wall made of cellulose — provides strength and support
- Permanent vacuole filled with cell sap — keeps the cell turgid
- Chloroplasts contain chlorophyll — where photosynthesis occurs, found in green parts only
Prokaryotic cells
Prokaryotic cells (bacteria) are much smaller than eukaryotic cells and have no true nucleus or membrane-bound organelles.
Prokaryotic cells are typically 1–5 µm; eukaryotic cells are typically 10–100 µm.
Prokaryotic cells — Key Knowledge
- No true nucleus DNA is a single loop free in the cytoplasm
- Plasmids small extra rings of DNA
- Cell wall not made of cellulose
- Flagellum tail-like structure for movement — not all bacteria have one
Cell specialisation
Cells become adapted for a particular function through specialisation. Their structure is related to their job.
For each specialised cell, link the structural feature to the function it performs.
Cell specialisation — Key Knowledge
- Sperm cell tail for swimming, many mitochondria for energy, acrosome with enzymes to penetrate egg
- Root hair cell long extension increases surface area for absorbing water and minerals
- Nerve cell long axon to carry impulses, branched endings for connections
- Red blood cell no nucleus for more haemoglobin, biconcave shape for large surface area
Cell differentiation
Differentiation is the process by which a cell becomes specialised for a particular function.
This is why plants can regrow from cuttings but animals cannot regenerate limbs.
Cell differentiation — Key Knowledge
- Most animal cells differentiate early in development and lose the ability
- Many plant cells retain the ability to differentiate throughout life
Microscopy
Light microscopes use light and lenses to magnify up to ~×2000. Electron microscopes use beams of electrons and can magnify up to ~×2,000,000 with much higher resolution.
Magnification = image size ÷ actual size
Higher magnification without higher resolution just gives a bigger blur — resolution is what reveals detail.
Microscopy — Key Knowledge
- Magnification how much larger the image is than the real object
- Resolution the ability to distinguish between two close points — electron microscopes have much higher resolution
- Standard form used for very small measurements, e.g. 1 µm = 10⁻⁶ m
Chromosomes
Chromosomes are long molecules of DNA found in the nucleus. In body cells, chromosomes are found in pairs.
The full set is restored at fertilisation when two gametes fuse.
Chromosomes — Key Knowledge
- 23 pairs of chromosomes in human body cells 46 total
- Gametes (sex cells) have half the number — 23 single chromosomes
The cell cycle and mitosis
The cell cycle is a series of stages that results in cell division. Mitosis is the stage where the nucleus divides.
Mitosis is used for growth, repair, and asexual reproduction. It produces genetically identical cells.
The cell cycle and mitosis — Key Knowledge
- Stage 1: cell grows, organelles increase, DNA replicates. Stage 2: mitosis — one set of chromosomes pulled to each end of the cell, nucleus divides. Stage 3: cytoplasm and cell membrane divide — two identical daughter cells produced
Stem cells
Stem cells are undifferentiated cells that can divide to produce many types of specialised cell.
Embryonic stem cells are more versatile but more controversial; adult stem cells avoid the ethical issues but are more limited.
Stem cells — Key Knowledge
- Embryonic stem cells can differentiate into any cell type
- Adult stem cells more limited — e.g. bone marrow produces blood cells
- Therapeutic uses treating paralysis, diabetes, replacing damaged tissue
- Ethical concerns use of embryos — some argue embryo is a potential life
Meristems
Meristems are regions in plants where stem cells are found — at the tips of roots and shoots.
This is why plants continue to grow and can produce new organs (leaves, flowers) throughout their life, unlike most animals.
Meristems — Key Knowledge
- Meristem tissue can differentiate into any type of plant cell throughout the plant's life
Diffusion
Diffusion is the spreading out of particles from an area of higher concentration to an area of lower concentration. It is a passive process — no energy is required.
Examples: oxygen diffusing into blood in the lungs, carbon dioxide diffusing out of cells.
Diffusion — Key Knowledge
- Net movement down the concentration gradient
- Passive process driven by random movement of particles, no energy needed
- Factors affecting rate: concentration gradient steeper = faster
- temperature higher = faster
- surface area larger = faster
- distance shorter = faster
Osmosis
Osmosis is the diffusion of water from a dilute solution (high water concentration) to a concentrated solution (low water concentration) through a partially permeable membrane.
Osmosis is a specific type of diffusion — it only refers to water moving through a membrane.
Osmosis — Key Knowledge
- Partially permeable membrane allows water molecules through but not larger solute molecules
- Passive process no energy required
Active transport
Active transport moves substances from a more dilute solution to a more concentrated solution — against the concentration gradient. It requires energy from respiration.
The opposite direction to diffusion — that's why it needs energy.
Active transport — Key Knowledge
- Against the concentration gradient from low to high concentration
- Requires energy from respiration in mitochondria
Active transport examples
Active transport is essential where substances must be absorbed against their concentration gradient.
Without active transport, plants couldn't absorb enough minerals and the gut couldn't fully absorb nutrients from food.
Active transport examples — Key Knowledge
- Root hair cells absorb mineral ions from soil — minerals are in lower concentration in the soil than in the root
- Gut cells absorb glucose from the intestine into the blood — even when glucose concentration is already higher in the blood