OCR AS Biology: Module 1, Unit 1

Revision notes on the following topics:

  • light microscopes
  • electron microscopes
  • cells and organelles
  • the phospholipid bilayer
  • the fluid mosaic model
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  • Created by: Jessica
  • Created on: 16-12-11 10:36
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Biology Revision Notes - Unit 1, Module 1
1.1) Microscopes
Magnification: the degree to which the size of an image is larger than the object itself. It is the
image size divided by the actual size of the object.
Resolution: the degree to which it is possible to distinguish between two objects that are very close
together. The higher the resolution, the greater the amount of detail that can be seen.
The light microscope:
capable of a magnification of up to x1500
have a maximum resolving power of 200nm
can be used to view a large range of specimens ­ including some living organisms
it's relatively low resolution means it can't give detailed information about the
internal cell structure
Ways of preparing specimens for the light microscope:
Staining (allows the sample to be seen/increases contrast)
Sectioning (specimens are embedded in wax to prevent being distorted when cut)
The electron microscope:
(1) Transmission electron microscope
use electromagnets to focus a beam of electrons, which is transmitted
through the specimen
denser parts absorb more electrons, producing a contrast
can only be used on thin specimens
resolution of 0.1nm
produce a 2D image
magnification of x500 000
(2) Scanning electron microscope
scan a beam of electrons across the specimen
produces a 3D image
magnification of x100 000
resolution of 50nm
Limitations of the electron microscope:
electron microscopes are extremely expensive
preparing samples and using an electron microscope both require a high degree of
skill and training
samples must be placed in a vacuum because electron beams are deflected by
molecules in the air
1.2) Cells and Organelles

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Division of labour: refers to each organelle within the cell having a specific role which contributes to
the cell's survival.
The cytoskeleton: regards the network of protein fibres found within cells that gives structure and
shape to the cell, and also moves organelles around inside.
Plant Cell: Animal Cell:
Functions of organelles:
The nucleus houses all of the cells genetic material. The chromatin consists of DNA
and proteins. It has the instructions for making proteins. Some of these proteins
regulate the cell's activities.…read more

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Cell Membranes
The phospholipid bilayer: is the basic structural component of plasma membranes (cell surface
membranes). It consists of two layers of phospholipid molecules. Proteins are embedded in this
layer.…read more

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The fluid mosaic model: refers to the model of cell membrane structure. The lipid molecules give
fluidity and proteins in the membrane give it a mosaic (patchwork) appearance.…read more


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