Cell Structure Notes
AQA GCSE Biology: Revision notes
Key facts
- Animal and plant cells are eukaryotic: they have a nucleus and other membrane-bound organelles.
- Plant cells also have a cell wall, a permanent vacuole and chloroplasts.
- Prokaryotic cells (bacteria) have no nucleus: DNA is free in the cytoplasm, with extra loops called plasmids.
- Magnification = image size ÷ actual size, with both in the same units.
- Electron microscopes have higher magnification and resolution than light microscopes.
Animal and plant cells
Animal and plant cells share a nucleus, cytoplasm, membrane, mitochondria and ribosomes; plant cells add a cell wall, vacuole and chloroplasts.
Both are eukaryotic cells.
- Nucleus: contains DNA and controls the cell's activities
- Cytoplasm: where most chemical reactions happen
- Cell membrane: controls what enters and leaves
- Mitochondria: where aerobic respiration releases energy
- Ribosomes: where protein is made
Plant cells also have a cell wall of cellulose for support, chloroplasts for photosynthesis and a permanent vacuole of cell sap.
Animal cell
- Cell wall: absent
- Chloroplasts: absent
- Vacuole: small or absent
Plant cell
- Cell wall: present
- Chloroplasts: present
- Vacuole: large, permanent
Which structure is found in a plant cell but NOT in an animal cell?
Prokaryotic cells
A prokaryotic cell has no nucleus and is much smaller; its DNA lies free in the cytoplasm, with extra loops called plasmids.
Prokaryotic cells such as bacteria are much smaller than eukaryotic cells (typically 0.5 to 5 µm compared with 10 to 100 µm).
- The genetic material is not in a nucleus: a single DNA loop lies free in the cytoplasm.
- Plasmids are small extra DNA loops, often carrying genes such as antibiotic resistance.
- Many have a cell wall (not cellulose), a flagellum for movement and a slime capsule.
- They reproduce by simple division, which is fast.
Prokaryotic
- No nucleus
- No mitochondria or chloroplasts
- Plasmids present
- Very small
Eukaryotic
- DNA in a nucleus
- Membrane-bound organelles
- No plasmids (normally)
- Larger
Where is the DNA found in a bacterial cell?
Structure fits function
Cells have the structures their job needs: more mitochondria where more energy is used, more chloroplasts where more photosynthesis happens.
Different cells have different jobs, so structures are present or abundant only where needed. This is adaptation.
- Muscle, nerve and sperm cells have many mitochondria because they use a lot of energy.
- Leaf cells have many chloroplasts for photosynthesis.
- Root cells have no chloroplasts, since there is no light underground, but have mitochondria for active transport of minerals.
For a sperm cell: it swims to reach an egg, which needs energy from aerobic respiration, so it needs many mitochondria.
Root cell
- Chloroplasts: few or none
- Mitochondria: many
- No light underground; energy for active uptake of minerals
Leaf cell
- Chloroplasts: many
- Mitochondria: present
- Photosynthesis happens here
Why do root hair cells contain many mitochondria but few or no chloroplasts?
Microscopes and magnification
Magnification is image size divided by actual size; convert both to the same unit first.
Start on the lowest power, focus with the coarse knob, then the fine knob. At higher power use only the fine focus.
Units: 1 mm = 1000 µm; 1 µm = 1000 nm. Always convert to the same unit and state the units: write ×100, not 100.
When drawing cells, draw only what you see, use label lines (not arrows) and keep the drawing large.
- Magnification
- 1
Lowest power
Start with the lowest magnification objective lens
- 2
Slide
Place the slide on the stage and secure with clips
- 3
Coarse focus
Bring the object into rough focus
- 4
Fine focus
Achieve a sharp image
- 5
Higher power
Rotate to a higher-power objective lens, then fine focus only
Worked example
A cell is 500 µm long but its image is 5 cm long. Calculate the magnification.
- 1
Convert 5 cm to µm: 5 cm = 50 mm = 50 000 µm.
- 2
Magnification = 50 000 µm ÷ 500 µm.
A cell has an actual length of 0.02 mm and its image is 4 mm long. What is the magnification?
Electron microscopes
Electrons have a much shorter wavelength than light, so electron microscopes have higher resolution and magnification.
Resolution is the ability to tell two close points apart. A light microscope is limited by the wavelength of light, so it cannot show detail smaller than about 0.2 µm and magnifies up to about ×1500.
An electron microscope uses electrons, which have a much shorter wavelength, so it magnifies over ×1 000 000 and shows fine detail such as ribosomes and the inside of mitochondria. But it is expensive, needs skilled operators, and samples are dead.
Light microscope
- Magnification up to about ×1500
- Resolution about 0.2 µm
- Can view living cells
- Colour images
- Cheap and portable
Electron microscope
- Magnification over ×1 000 000
- Resolution about 0.1 nm
- Samples must be dead
- Black and white images
- Expensive, skilled operator
Why can an electron microscope show more detail than a light microscope?
Try an exam question
Explain why electron microscopes show more detail of sub-cellular structures than light microscopes. Give one disadvantage of using an electron microscope.
[4 marks]
- [1]Electron microscopes have a higher resolution.
- [1]Because electrons have a much shorter wavelength than light.
- [1]So they can distinguish points that are closer together / show structures such as ribosomes.
- [1]Disadvantage: any one of expensive; samples must be dead; black and white images; skilled operator needed.
That's the notes covered.
Carry on to the next subtopic.