AS Biology: Unit F211: Cells, Exchange and Transport

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Unit 1.1.1: Cell Structure

The Light Microscope

Magnification is the degree to which the size of an image is larger than the object itself.

  • Light microscopes are capable of magnifying up to x1500

Resolution is the degree to which it is possible to distinguish between two objects that are very close together.

  • The higher the resolution, the greater the detail you can see. The maximum resolving power of light is 200nm. 
  • This means if two objects are closer than 200nm, they will be seen as one object.
  • Limitation = the magnitude of the wavelength of light - two objects can only be distinguished if light waves pass between them. 
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Light microscopes: How they work

How it works

  • Use a number of lenses to produce an image that can be viewed directly at the eyepieces. 
  • Light passes from a bulb under the stage, through a condenser lens, through the specimen & is focused through the objective lens before going to the eyepiece. 
  • Four objective lenses are present and can be rotated to change the magnification. Lenses are usually: x4, x10, x40 and x100.
  • Eyepiece lens magnifies this image again, usually x10.

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Staining and Sectioning in Light Microscopes

Staining and Sectioning in Light Microscopes

A lot of biological material is not coloured, some may distort when you try to cut it. Overcome by:

  • Chemicals called "coloured stains" bind to chemicals in the specimen, allowing it to be seen. 
  • Some stains bind to specific cell structures. 
  • E.g. staining DNA Dark Red with Acetic Orcein; & staining bacterial cell walls violet using Gentian. 
  • Staining refers to any process that helps to reveal or distinguish different features. 
  • Sectioning: specimens are embedded in wax, followed by thin sections being cut without distorting the structure of the specimen. 
  • This is useful for making sections of soft tissue: E.g. Brain Tissue. 
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Cell Size and Linear Magnification

Cell Size and Linear Magnification

Measuring specimens under a microscope can be limited by resolution. 

The resolution of the:

  • Human Eye: 100μm.
  • Light Microscope: 200nm.
  • Electron Microscope: 0.2nm.

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The Graticule and The Stage Micrometer

The Graticule and The Stage Micrometer 

  • The graticule scale is arbitrary - it represents different lengths and different magnifications, hence needs to be calibrated. 
  • The stage micrometre is a slide containing 1mm (1000μm) with 100 divisions.
  • This is placed on the microscope stage
  • To calculate the length of 1 eyepiece unit (epu):

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The Electron Microscope

The Electron Microscope

  • The light microscope has a low resolution. 
  • If magnification is above x1500, the image isn't clear.
  • The wavelength of light ranges from 400-750nm, structures closer to 200nm would appear as on object. 
  • To achieve a greater resolution, an electron microscope has to be used.
  • Electron Microscopes: Generate a beam of electrons. The beam wavelength is 0.0044nm.
  • The microscopes use magnets to focus the beam onto a specimen.
  • The image produced is projected onto a screen to make a grayscale image.

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The Electron Microscope: The need for staining in

The need for staining in EMs 

  • Staining samples with lead salts = scatter the electrons differently to give contrast.
  • Since: the final image produced is in grayscale, the image can be coloured using specialised computer software.
  • These images are labeled as 'false-colour' electron micrographs. 

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The Cytoskeleton

The Cytoskeleton

  • Refers to the network of protein fibres found within cells that give structure and shape to the cell.
    • As well as move organelles around inside cells. 
    • It provides mechanical strength to them.,
    • As well as aids transport within cells & enabling cell movement.

Some of the fibres - called Actin Filaments. Able to move against each other. These muscle fibres can move some organelles around inside cells.

-Microtubules -  used to move microorganism through a liquid or to waft a liquid past a cell.

-Microtubules are made of the tubulin (protein). 

-Other proteins present (microtubule motors) move organelles and other cell contents along the fibres. 

-This is how chromosomes are moved during mitosis and how vesicles move from the endoplasmic reticulum to the Golgi apparatus. They use ATP to drive these movements.

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Cytoskeleton: ATP Note

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Cilia and Flagella

Cilia and Flagella

  • Eukaryotes (organisms that have cells with a nucleus) have hair-like extensions that stick out from the surface of the cells.
  • Cilia often occur in large numbers in a cell.
  • The sweeping movement of these move substances such as mucus across the surfaces of cells in the ciliated epithelial tissue.
  • Flagella are longer and usually occur in ones or twos in a cell. 
  • Ones that from the tail of sperm or bacteria. E.g. able to move the whole cell, using energy from ATP. 
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Animals vs Plant Cells

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The Vesicles, Vacuoles and Cell Wall

The Vesicles, Vacuoles and Cell Wall 


  • Are membrane-bound sacs found in cells. They carry many different substances around cells.


  • The vacuole is filled with water and solutes so that it pushes the cytoplasm against the cell wall, making the cell turgid.
  • This is important as it helps support the plant. 

Cell Wall

  • The cell wall is made out of cellulose. 
  • It forms a sieve-like network of strands that makes the cell wall strong. 
  • The cell wall is held rigid by the turgor pressure, so it supports the cell & helps support the whole plant.
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The Other Organelles.

Refer to the printout of table, stating function & structure of other organelles.

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Protein Synthesis

Protein Synthesis

  • Specific instructions to make the hormone are found in a gene.
  • Genes are sections of DNA in the chromosomes in the nucleus.
  • The nucleus copies the instructions in the DNA into a molecule called mRNA.
  • The mRNA leaves the nucleus through a nuclear pore and attaches to a ribosome. 
  • The ribosome reads the instructions and uses the codes to assemble the hormone.
  • The assembled protein is pinched off in a vesicle and transported to the Golgi Apparatus.
  • The Golgi Apparatus packages the protein and may also modify it so that it is ready to release,
  • The protein now packages into a vesicle, fuses with the cell surface membrane. 
  • The membrane opens to release the hormone outside. 
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Protein Synthesis - Diagram.

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Prokaryotes and Eukaryotes

Prokaryotes and Eukaryotes 

  • Prokaryotes don't have a nucleus. They are bacteria and are much smaller than Eukaryotic cells (20-40μm). Other differences:
    • They only have one membrane.
    • They are surrounded by a cell wall made of peptidoglycan.
    • They contain much smaller ribosomes.
    • They have a single, circular loop of DNA, opposed to the linear, sperate strands of eukaryotic chromosomes.
    • Many also contain very small loops of DNA called plasmids.
    • The DNA is not surrounded by a membrane. 
    • ATP production takes place in specialised regions of the membrane called mesomes.
    • Some have Flagella. Made up of a spiral protein called flagellin. 
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Still in progress of finishing my cards.

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