• Revision:OCR AS Biology - Cell Structure

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Magnification and Resolution

Magnification: Degree to which the image is larger than the object itself Resolution: Degree to which it is possible to distinguish between two objects, the higher the resolution the greater the detail.


Light Microscope Maximum magnification of x1500 Maximum resolution of 200nm

Transmission Electron Microscope Maximum magnification of x500 000 Maximum resolution of 0.20nm

Scanning Electron Microscope Maximum magnification of x250 000 Maximum resolution of 0.20nm

Staining and Sectioning

Staining: Chemicals that bind on or in the specimen, making it easier to be seen under the microscope.

Sectioning: Some tissue such as Brain is embedded in wax, making it easier to cut.

Linear Magnification

It is possible to work out the magnification of a image, or the actual size of a cell with this formula A = I/M

Actual Size = Image Size over Magnification rearranging this formula can give the Image size; I = A X M

                                                    the Magnification; M = I/A

--Ieatcheeseyo 19:22, 19 November 2011 (UTC)--Ieatcheeseyo 19:22, 19 November 2011 (UTC)

Eukaryotic Organelles

  • Rough endoplasmic reticulum: Contains ribosomes which are the site of protein synthesis
  • Smooth endoplasmic reticulum

m: Carries out synthesis of lipids

  • Golgi apparatus: A stack of membrane bound vesicles. Packages macromolecules for transport around the cell
  • Mitochondria: A double membrane bound organelle which is involved in aerobic respiration. Inner membrane forms folds called cristae to increase surface area of membrane. An the cristae, glucose is combined with oxygen to produce ATP.
  • Lysosomes: Contains hydrolytic enzymes which in a white blood cell, digest bacteria. Enzymes are contained in the lysosomes to prevent necrosis (cell death)
  • Chloroplasts: Contain chlorophyll. Have a double outer membrane. Within the stroma there are other membrane structures called grana where photosynthesis takes place
  • Plasma membrane: A phospholipids bilayer containing proteins. These proteins include receptors, pores and enzymes. Responsible for controlled entry of water and minerals
  • Nuclear envelope: A double membrane around the nucleus containing muclear pores which allow exchange between the nucleus and the cytoplasm
  • Centriole: A hollow cylinder about 0.4µm long formed from a ring of microtubules which are used to grow the spindle fibres used in nuclear division
  • Nucleus: Contains DNA responsible for the individual characteristics of each cell. DNA is similar in all cells but depending on which type of cell it is, some genes maybe turned on or off. Division of the nucleus proceeds cell division
  • Nucleolus: Inside the nucleus. Produces ribosomes which leave the nucleus to take positions on the rough endoplasmic reticulum
  • Cilia: Move in a co-ordinated manner, each slightly out of phase with its neighbour so substances around the cell are made to move.

Comparing prokaryotic cells with eukaryotic cells


  • Average diameter = 0.5-20µm
  • Circular DNA lies free in cytoplasm
  • Naked DNA
  • Slightly smaller ribosomes (18nm)
  • No ER
  • Very few organelles none of which are surrounded by membrane
  • Cell wall present


  • Often up to 40µm and 1000-10000 x the volume of eukaryotes
  • DNA not circular and contained in nucleus
  • DNA associated with protein forming chromosomes
  • Slightly larger ribosomes (22nm)
  • ER
  • Many organelles, many are bounded by a membrane
  • Cell wall only present in plant cells

Types of tissue

  • Squamous epithelium: Individual cells are smooth, flat and very thin. The cells fit together to provide a smooth, low-friction surface. E.g. alveoli – thinness allows rapid diffusion
  • Cilliated epithelium: A single layer of tall cells all reaching to the basement membrane. Sometimes also contains cilia. E.g. lining of bronchus


  • Tissue: A collection of cells which is specialised to perform one or more particular functions. Cells can be all of one type (e.g. parenchyma in plants, squamous epithelium in animals) or of mixed type (e.g. xylem and phloem in plants, bone in animals)
  • Organ: Part of the body which is formed from more than one tissue and forms a structural and functional unit (e.g. leaves in plants, liver in animals)


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