AQA GCSE Biology contents

Unit 1 · Cell Biology

Eukaryotes, Prokaryotes & Cell Structure

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Eukaryotes and prokaryotes

  • Plant and animal cells are eukaryotic: they have a cell membrane, cytoplasm and genetic material enclosed in a nucleus
  • Bacterial cells are prokaryotic: much smaller, with cytoplasm and a cell membrane surrounded by a cell wall, and genetic material that is not in a nucleus: a single DNA loop plus one or more small rings of DNA called plasmids
  • Scale: an animal cell is about 10 to 100 μm across; a bacterium about 1 to 2 μm; so a bacterium is about 100 times smaller than an animal cell, two orders of magnitude
Prefix Meaning In metres
centi (cm) one hundredth 1×10−21 \times 10^{-2}
milli (mm) one thousandth 1×10−31 \times 10^{-3}
micro (μm) one millionth 1×10−61 \times 10^{-6}
nano (nm) one billionth 1×10−91 \times 10^{-9}
  • Each step down the table is ×1000\times 1000: 1 mm == 1000 μm; 1 μm == 1000 nm

Animal and plant cells

Structure Function Animal Plant Bacterial
Nucleus contains the genetic material (DNA) that controls the cell yes yes no
Cytoplasm where most chemical reactions happen yes yes yes
Cell membrane controls what enters and leaves the cell yes yes yes
Mitochondria where aerobic respiration releases energy yes yes no
Ribosomes where proteins are made (protein synthesis) yes yes yes
Chloroplasts contain chlorophyll; absorb light for photosynthesis no yes no
Permanent vacuole filled with cell sap; keeps the cell rigid no yes no
Cell wall made of cellulose in plants; strengthens the cell no yes yes (not cellulose)
Plasmids small rings of DNA carrying extra genes no no yes
  • Algal cells have the same structures as plant cells, including a cellulose cell wall

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plant and bacterial cellsanimal cellcell membranecontrols what goes in and outcytoplasmwhere most reactions happennucleuscontains the DNAmitochondria — aerobic respirationribosomes — protein synthesisplant cellvacuolecell wallmade of cellulose — keeps the shapeeverything the animal cell has as well:nucleus, cytoplasm, membrane,mitochondria and ribosomeschloroplastscontain chlorophyll for photosynthesispermanent vacuolefilled with cell sapbacterial cellprokaryotic — no nucleuscell wallribosomes(smaller than in animal and plant cells)a single loop of DNA,free in the cytoplasmplasmids — small rings of extra DNAAnimal and plant cells are eukaryotic: the genetic material is inside a nucleus.Bacterial cells are prokaryotic: much smaller, with no nucleus, and no mitochondria or chloroplasts.
An animal cell, a plant cell and a bacterial cell side by side, each with every structure labelled: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes; plus chloroplasts, vacuole and cell wall in the plant cell; and cell wall, single DNA loop and plasmids in the bacterium

  • Estimating: a nucleus fills roughly a tenth of a cell's width; a mitochondrion is about a micrometre long, so tens of them fit in one cell. Use estimates when a question wants a rough size or a comparison, not an exact measurement