OCR F211 Cell Structure

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  • Created on: 29-04-16 17:52
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Module 1: Cells
1.1.1 Cell Structure
(a) state the resolution and magnification that can be achieved by a light microscope, a transmission electron
microscope and a scanning electron microscope
Light Microscope Transmission Electron Scanning Electron
Microscope (TEM) Microscope (SEM)
Radiation Source Light Electrons Electrons
Maximum Resolution 200nm 0.1nm 0.1nm
Maximum Magnification x1500 x500,000 x100,000
Image Focusing Lenses Magnets Magnets
Observe Live Specimens? Yes No No
Specimen in vacuum? No Yes Yes
Type of Image Produced 2D Colour 2D Greyscale 3D Greyscale
Ease of Use Easy Not Easy Not Easy
Ease of Portability Easy Not Easy Not Easy
Cost Cheap Expensive Expensive
Light Microscope uses a number of lenses to produce an image that can be viewed directly at the eyepiece.
Light passes from a bulb under the stage, through a condenser lens and then through the specimen. This beam of
light is passed through an objective lens (x4, x10, x40) and then the eyepiece lens (x10).
Overall Magnification = Objective Lens Magnification x Eyepiece Lens Magnification
Transmission Electron Microscope (TEM) uses electromagnets to focus a beam of electrons, which is
transmitted through the specimen. Denser parts of the specimen absorb more electrons , which makes them look
darker on the image you end up with creating a contrast .
Scanning Electron Microscope (SEM) an electron beam is scanned across the specimen. The electrons don't
pass through the specimen , they bounce off and are detected at multiple detectors.
Advantages of Electron Microscopes Disadvantages of Electron Microscopes
The resolution is 0.1nm (2000x more than the light Electron beams are deflected by air molecules, so the
microscope) sample has to be placed in a vacuum specimen is not
Can produce detailed images of the structures inside Electron microscopes are extremely expensive
SEM produces a 3D image Preparing samples and using the electron microscope
both require a high degree of skill and training
(b) explain the difference between magnification and resolution
Magnification the number of times greater an image is than the object itself
Resolution the ability to distinguish two separate points as distinct from each other
(c) explain the need for staining sample for use in light microscopy and electron microscopy
Material needs to be stained to allow the specimen to be seen under the microscope . Images are produced
because some parts of the object absorb more light/electrons than others to create contrast, but sometimes the

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Module 1: Cells
object being viewed is completely transparent this makes the whole thing look white because the light
rays/electrons pass straight through. This problem is solved by staining the specimen. Different types of stains
bind to different specific cell structures :
Acetic Orcein stains DNA dark red
Genetian Violet stains bacterial cell walls
Some specimens are embedded in wax , to prevent distortion of the structure when being cut.…read more

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Module 1: Cells
(f) outline the functions of the structures listed in (e)
Organelle Description Function
Nucleus Largest organelle 1020µm Contains chromatin mixture of DNA and
Roughly spherical associated proteins
Double membrane bound Controls protein synthesis makes mRNA
Nucleolus Area of dense chromatin within the nucleus Makes ribosomal RNA (rRNA) and makes
Granular structure not membrane bound ribosomes
Rough Flattened sacs cisternae Ribosomes make proteins which is then
Endoplasmic Continuous outer nuclear membrane transported within the ER's flattened sacs
Reticulum Studded with ribosomes Some proteins…read more

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Module 1: Cells
Contains chlorophyll: a photosynthetic
Cellulose On the outside of plant cells' plasma Supports the cell
Cell Wall membrane Helps maintain cell shape
Vacuoles A large membrane bound sac containing Storage of water and solutes
water and solutes Maintains cell turgidity
(g) outline the interrelationship between the organelles involved in the production and secretion of proteins
1. In the nucleus is the DNA needed to make proteins. The DNA from the nucleus is copied into a molecule called
2.…read more

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Module 1: Cells
DNA Location In nucleus nucleus present Free in cytoplasm
no nucleus
Size of ribosomes Large 22nm (80S) Small 18nm (70S)
(j) compare and contrast, with the aid of diagrams and electron micrographs, the structure and ultrastructure of
plant cells and animal cells
Plant Cells Animal Cells
Shape Rectangular fixed Round irregular
shape shape
Exterior of cell includes Cell wall and cell Cell membrane
Chloroplasts present? Yes No
Vacuoles Large single vacuole One or more
Excess glucose storage Stored as starch…read more


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