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Biology · Cell Structures

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Biology · OCR Paper 1

Cell Structures

Eukaryotic and Prokaryotic Cells

Bacteria do have DNA. It is just not shut inside a nucleus.

  • Eukaryotic cell A cell whose genetic material is enclosed in a nucleus, such as an animal or plant cell.
  • Prokaryotic cell A cell with no nucleus, whose genetic material lies free in the cytoplasm, such as a bacterium.
  • Nucleus The part of a eukaryotic cell that contains the genetic material and controls the cell's activities.
A rod-shaped bacterium labelled with cell wall, cell membrane, cytoplasm, a single loop of DNA, a plasmid, ribosomes and a flagellum.
A bacterium is a prokaryotic cell: there is no nucleus.

Eukaryotic and prokaryotic cells compared

  • Eukaryotic cell (animal or plant) Genetic material is enclosed in a nucleus; Larger, often 10 to 100 µm across; Has a cell membrane, cytoplasm, mitochondria and ribosomes
  • Prokaryotic cell (bacterium) No nucleus: a single loop of DNA lies in the cytoplasm; Much smaller, often 1 to 5 µm long; Has a cell membrane, cytoplasm and ribosomes, with a cell wall outside

Animal and Plant Cells

Structure always fits function: a cell has more of whatever its job needs most.

  • Sub-cellular structure A structure inside a cell, such as the nucleus or a mitochondrion.
  • Mitochondria Structures where aerobic respiration takes place, releasing energy for the cell.
  • Ribosome A tiny structure where proteins are made.
A labelled animal cell showing the nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
An animal cell, with its main sub-cellular structures labelled.

Animal cells and plant cells compared

  • Animal cell Nucleus, cytoplasm, cell membrane, mitochondria, ribosomes; No cell wall, so the shape can change; No chloroplasts, so it cannot photosynthesise
  • Plant cell Has all of those structures too; Cellulose cell wall gives a fixed shape; Chloroplasts in the green parts, for photosynthesis

Microscopy and Magnification

Magnification makes an image bigger. Resolution makes it clearer.

  • Magnification How many times bigger an image is than the real object.
  • Resolution The ability to see two points that are close together as separate. Also called resolving power.
  • Light microscope A microscope that uses light and glass lenses to magnify a specimen.
Two panels showing that a light microscope blurs two close dots into one, while an electron microscope shows them as two separate dots.
The same two structures: only the electron microscope has the resolution to separate them.

Light microscope and electron microscope compared

  • Light microscope Magnifies up to about ×1500; Resolution about 0.2 µm (200 nm); Specimen can be alive, and colour stains can be used
  • Electron microscope Magnifies up to about ×2 000 000; Resolution about 0.2 nm, about 1000 times better; Specimen is dead and the image is black and white

Using a light microscope (core practical)

  • Prepare the slide Put a thin piece of tissue, such as onion skin, on a slide, add…
  • Clip it on the stage Place the slide on the stage and secure it with the clips.
  • Choose the lowest power Turn the nosepiece to the ×4 objective lens.
  • Focus Looking from the side, use the coarse focus wheel to raise the…