Cell Structure Notes

Edexcel GCSE Biology: Revision notes

Key facts

  • Animal cells have a nucleus, cell membrane, mitochondria and ribosomes; plant cells also have a cell wall, chloroplasts and a permanent vacuole.
  • Bacteria are prokaryotic: no nucleus, chromosomal DNA plus plasmids.
  • Sperm, egg and ciliated cells have structures adapted to their function.
  • Electron microscopes have much higher magnification and resolution than light microscopes.
  • magnification = image size ÷ actual size; convert units first.

Animal and plant cells

Animal and plant cells share a nucleus, membrane, mitochondria and ribosomes; plant cells have three extra structures.

Eukaryotic cells have a nucleus enclosing genetic material as chromosomes.

  • Nucleus: contains DNA and controls the cell.
  • Cell membrane: controls what enters and leaves.
  • Mitochondria: site of aerobic respiration.
  • Ribosomes: site of protein synthesis.

Plant cells also have a cell wall (cellulose, for strength), chloroplasts (chlorophyll, for photosynthesis) and a permanent vacuole (cell sap, keeping the cell turgid).

Cell wallChloroplastCytoplasmCell membraneRibosomeNucleusPermanent vacuoleMitochondrion
Plant cell: as an animal cell, plus a cell wall, permanent vacuole and chloroplasts.
RibosomeCytoplasmCell membraneNucleusMitochondrion
Animal cell with its sub-cellular structures labelled.

Which structure is the site of protein synthesis?

Bacterial cells

Bacteria are prokaryotic: smaller and simpler, with no nucleus or membrane-bound organelles.

Bacterial cells have chromosomal DNA (a single loop, free in the cytoplasm) and plasmid DNA (small extra circular loops). They also have a cell membrane, a cell wall and ribosomes, and may have a flagellum that rotates to move the cell.

Cell wallCell membranePlasmidCytoplasmChromosomal DNAFlagellumRibosome
Bacterial cell: no nucleus, DNA free in the cytoplasm, with a plasmid.

Prokaryotic (bacteria)

Genetic material:
Chromosomal DNA (single loop) and plasmids, free in cytoplasm
Membrane-bound organelles:
None
Ribosomes:
Present (smaller)

Eukaryotic (animal, plant)

Genetic material:
DNA enclosed in a nucleus
Membrane-bound organelles:
Nucleus, mitochondria (plants also chloroplasts)
Ribosomes:
Present (larger)

Where is the DNA in a bacterial cell?

Specialised cells

Each specialised cell has structures that suit its job.

A sperm cell reaches and fertilises an egg. An egg cell nurtures an embryo and prevents multiple fertilisation. Ciliated epithelial cells line the airways and are covered in cilia, hair-like structures that beat together to move mucus, with trapped particles and bacteria, away from the lungs.

Sperm cell

  • Acrosome: enzymes digest the egg membrane
  • Haploid nucleus: half the chromosomes
  • Many mitochondria: energy to swim
  • Tail: swims to the egg

Egg cell

  • Nutrients in the cytoplasm feed the embryo
  • Haploid nucleus
  • Membrane changes after fertilisation, stopping more sperm entering

Ciliated epithelial cell

  • Cilia beat in a coordinated way
  • Move mucus away from the lungs

Worked example

Explain how the sperm cell is adapted to its function.

What stops more than one sperm fertilising an egg?

Microscopes

Electron microscopes show far more detail than light microscopes, so we have found many more organelles.

Light microscopes use light and lenses. They showed cells, nuclei and chloroplasts. Electron microscopes use beams of electrons. They have much higher magnification and resolution (the ability to distinguish two separate points), revealing ribosomes, plasmids and the internal membranes of mitochondria and chloroplasts.

Many organelles were unknown before electron microscopy.

Light microscope

Uses:
Light and lenses
Magnification and resolution:
Lower
Can show:
Cells, nuclei, chloroplasts

Electron microscope

Uses:
Beams of electrons
Magnification and resolution:
Much higher
Can show:
Ribosomes, plasmids, internal membranes of mitochondria and chloroplasts

What is resolution?

Size and scale

Use the magnification formula, and convert all units to be consistent before calculating.

Cells are measured in very small units, so use standard form and unit prefixes: milli (m) =×10−3= \times 10^{-3}, micro (μ) =×10−6= \times 10^{-6}, nano (n) =×10−9= \times 10^{-9}, pico (p) =×10−12= \times 10^{-12}.

Rearrange the formula as needed: actual size = image size ÷ magnification; image size = actual size × magnification. To convert mm to μm, multiply by 1000. Estimations are used when an exact count is not practical.

  • Magnificationimage sizeactual size\dfrac{\text{image size}}{\text{actual size}}

Worked example

An image of a cell is 40 mm across and is drawn at ×2000 magnification. What is the actual size of the cell?

Image size 6 mm, actual size 0.02 mm. What is the magnification?

Core practical: microscopes

Stain the specimen, find it at low power, focus, increase magnification, then draw it.

Scientific drawings use clear single lines, no shading or colouring, labels drawn with straight lines that do not cross, plus a title and the magnification.

Total magnification is the eyepiece lens magnification multiplied by the objective lens magnification.

  1. 1

    Prepare the slide

    Add a stain such as iodine to increase contrast.

  2. 2

    Lowest power first

    Place the specimen on the stage and use the lowest magnification objective lens.

  3. 3

    Focus

    Coarse focus, then fine focus.

  4. 4

    Increase magnification

    Switch to a higher-power objective lens once focused.

  5. 5

    Draw

    Clear single lines, no shading, title and magnification.

Using a light microscope.
  • Total magnificationeyepiece × objective

Which objective lens do you use first?

Try an exam question

Explain how a sperm cell is adapted to its function.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.