Unit 1: cell structure

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  • Created by: rachael
  • Created on: 06-11-12 18:22

Eukaryotic and Prokaryotic

Eukaryotic cells are complex and include all animal and plant cells.

Eukaryotic cells 

  • Larger cells
  • DNA is linear
  • Nucleus present - DNA in the nucleus
  • No cell wall (animals), cellulose cell wall (plants) chitin cell wall in plants
  • Many organelles including mitochondria
  • Larger ribosomes

Prokaryotic cells are smaller and simpler e.g. bacteria.


  • Extremely small cells
  • DNA is circular
  • No nucleus - DNA free in cytoplasm
  • cell wall made from polysaccharide
  • few organelles, no mitochondria
  • small ribosomes
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Microscopes - light and electron


  • uses light
  • lower resolution than electron max of 0.2 micrometres
  • maximum magnification x1500


uses electrons to form an image, they have higher resolution than light microscopes so give more detailed images but are much more expensive. There are two kinds:

Transmission electron microscope (TEM)

  • Produces high resolution images: magnification x500000
  • only on thin specimins

Scanning electron microscope

  • 3D image is produced
  • high resolution 0.1nm - detailed images
  • low magnification of x100000
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Cell membranes - different components

Phospholipids: form a barrier to dissolved substanes. Head is hydrophilic, tail is hydrophobic so they arrange in a bilayer, with the heads towards the water on either side of the membrane. The centre of the bilayer is hydrophobic so doesn't allow water soluble molcecules through it .

Cholesterol: gives stability to the membrane. Its present in all cell membranes (except bacterial cell membranes). It fits between the phospholipids, it binds to the hydrophobic tails, causing them to pack closer together, making the membrane less fluid and more rigid.

Proteins: control what enters and leaves the cell. Some form channels in membrane, these allow small or charged particles through. Carrier proteins transport molecules and ions across the membrane by active transport and facilitated diffusion. Also acts as receptor molelcules in cell signalling. when a molecule binds to the protein,a chemical reaction is triggered inside the cell.

Glycolipids and Glycoproteins:stablilise the membrane by forming hydrogen bonds with surrounding water molecules. Sites where drugs, hormones and antibodies bind. Act as receptors for cell signalling and also are antigens.

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Gas exchange - SA:V; things needed, alveoli

Smaller animals have higher Surface area:volume ratio. 

Large organisms have small surface area:volume ratio which means they cannot diffuse needed nutrients through the skin but have to a specialised surface for diffusing nutrients.

Things needed for efficient gaseous exchange:

  • large surface area - lungs have alveoli which has large surface area.
  • thin which provide a short diffusion pathway - lungs have alveolar epithelium and capillary endothelium are only one cell thick giving a short diffusion pathway.
  • maintain a steep concentration gradient of gases across exchange surface - in the lungs all alveoli have good blood supply from capillaries and breathing in and out refreshes air in the alveoli.

Gas exchange between a capillary and alveolus -(each alveolus has its own blood supply)

  • Oxygen diffuses out of the alveoli ands in to the haemoglobin in the blood
  • Carbon Dioxide diffuses into the alveoli from the blood, crossing the cappilary endothelium and the alveolar endothelium.

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