Cell structure Notes

Oxford AQA IGCSE Biology: Revision notes

Key facts

  • Animal cells have a cell membrane, cytoplasm, nucleus, mitochondria and ribosomes.
  • Plant cells also have a cellulose cell wall, a large vacuole and chloroplasts.
  • Eukaryotic cells have a membrane-bound nucleus; prokaryotic cells (bacteria) do not.
  • Bacterial DNA is a circular chromosome in the nucleoid, with small plasmids.
  • Magnification =image sizeactual size= \dfrac{\text{image size}}{\text{actual size}}; convert to the same units first.

Animal cell structures

Each structure in an animal cell has a function: the membrane controls entry and exit, the nucleus controls the cell, mitochondria release energy and ribosomes make protein.

The cytoplasm is a gel-like substance where chemical reactions happen, and it contains the organelles. The nucleus is surrounded by a nuclear membrane.

Name each structure and give its function. Do not just say "mitochondria make energy": they are the site of aerobic respiration, releasing energy as ATP.

RibosomeCytoplasmCell membraneNucleusMitochondrion
An animal cell.

Cell membrane

  • Controls what enters and leaves
  • Selectively permeable

Nucleus

  • Controls the cell's activities
  • Contains DNA

Mitochondria

  • Site of aerobic respiration
  • Releases energy as ATP

Ribosomes

  • Site of protein synthesis
  • Free or on rough endoplasmic reticulum

Where does aerobic respiration take place?

Plant cell structures

Plant cells have everything an animal cell has, plus a cell wall, a large vacuole and chloroplasts.

The cell wall is made of cellulose and sits outside the cell membrane. The large central vacuole stores water and keeps the cell firm. Chloroplasts contain chlorophyll for photosynthesis.

Animal cells lack these because animals are mobile and take in food, rather than making it.

Cell wallChloroplastCytoplasmCell membraneRibosomeNucleusPermanent vacuoleMitochondrion
A plant cell.

Cell wall

  • Cellulose
  • Support and rigidity
  • Plants have no skeleton

Vacuole

  • Stores water
  • Keeps the cell firm (turgor)
  • Prevents wilting

Chloroplasts

  • Site of photosynthesis
  • Contain chlorophyll
  • Plants make their own food

Which structure is found in a plant cell but not an animal cell?

Eukaryotes and prokaryotes

The defining difference is the nucleus: eukaryotic cells have a membrane-bound nucleus, prokaryotic cells do not.

Eukaryotic cells (animals, plants) have a nucleus and organelles. Prokaryotic cells (bacteria) have no nucleus, and their DNA lies free in the cytoplasm.

Be precise: say a membrane-bound nucleus with linear chromosomes versus a circular chromosome in the nucleoid region.

Cell wallCell membranePlasmidCytoplasmChromosomal DNAFlagellumRibosome
A prokaryotic cell: no membrane-bound nucleus, with DNA free in the cytoplasm.

Eukaryotic

Nucleus:
Present (membrane-bound)
Organelles:
Present
Cell wall:
Cellulose in plants; none in animals
Size:
10 to 100 μm
DNA:
Linear chromosomes
Ribosomes:
80S

Prokaryotic

Nucleus:
Absent
Organelles:
Absent
Cell wall:
Peptidoglycan
Size:
1 to 10 μm
DNA:
Circular chromosome and plasmids
Ribosomes:
70S

Which cell has no membrane-bound nucleus?

The bacterial cell

A bacterial cell has a cell wall of peptidoglycan, a circular chromosome in the nucleoid, plasmids, 70S ribosomes and sometimes flagella.

The cell wall is made of peptidoglycan. The single circular chromosome lies in the nucleoid, with no membrane around it. Plasmids are small loops of DNA that often carry genes such as antibiotic resistance.

Flagella rotate to propel the cell. Bacteria have no nucleus, mitochondria or chloroplasts.

Cell wallCell membranePlasmidCytoplasmChromosomal DNAFlagellumRibosome
A bacterial cell.

What are plasmids?

Microscopy and magnification

Magnification is image size divided by actual size. Convert both to the same units before dividing.

Prepare a specimen, place it on the stage and focus with the objective lenses (usually ×4, ×10 or ×40). Draw clearly and record the magnification.

1 m = 1000 mm; 1 mm = 1000 μm; 1 μm = 1000 nm. Multiply to go to a smaller unit; divide to go to a larger one.

  1. 1

    Prepare

    a fresh or stained specimen

  2. 2

    Focus

    on the stage using the objective lenses

  3. 3

    Draw

    clearly, labelling the visible structures

  4. 4

    Record

    the magnification used

Using a light microscope
  • Magnificationimage sizeactual size\dfrac{\text{image size}}{\text{actual size}}

Worked example

A cell has an actual width of 50 μm. In the image it is 20 mm wide. Calculate the magnification.

From → to

  • m to mm
  • mm to μm
  • μm to nm
  • μm to mm

Rule

  • × 1000
  • × 1000
  • × 1000
  • ÷ 1000

A cell is 0.2 mm long. What is that in μm?

Try an exam question

A plant cell has an actual width of 40 μm. In a photograph it is 12 mm wide. (a) Calculate the magnification. (b) Name the structure in plant cells where photosynthesis happens.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.