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Topic 2 Summary notes (A Level Biology Edexcel B) $7.09
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Topic 2 Summary notes (A Level Biology Edexcel B)

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This document contains summarised notes for Topic 2, Cells and viruses, taken using the Pearson Edexcel B biology activate textbook. Notes with referencing to specification.

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  • Topic 2
  • May 22, 2021
  • 12
  • 2020/2021
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By: annakostova1212 • 2 year ago

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CELLS AND VIRUSES

EUKARYOTIC CELLS

OBSERVING CELLS
LIGHT MICROSCOPE/ELECTRON MICROSCOPE
Electron microscope has a higher magnification than light
Magnification – a measure of how much bigger the image is seen
Resolution – a measure of how close together two objects can be before we see them as one.

LIGHT MICROSCOPE
 Light source built in or sunlight reflected with a mirror
 Objective lens produces a magnified and inverted image, which the eyepiece lens focuses at
the eye.
 Can view living organisms, tissues and cells but most specimens will be dead, stained before
put on a slide as it makes it easier to identify types of cell.
 STAINS: methylene blue – stains nuclei blue iodine – stains starch blue-black
ADV:
 Can view living plants/animals to compare slides with living tissue
 Cheap
 Light and portable
DISADV:
 Limited powers of resolution and magnification
 Staining tissue can produce artefacts in the tissues being observed.

MAGNIFICATION OF OBJECTIVE LENS x MAGNIFICATION OF EYEPEICE LENS=TOTAL MAGNIFICATION

ELECTRON MICROSCOPE
 Uses a beam of electrons to form an image.
 Electrons behave like light waves with a small wavelength.
 Resolving power increases as wavelength decreases.
 Vacuum must be present so specimens are always dead.
 STAINS: heavy metal ions such as lead and uranium which improves scattering of the electrons
and make greater contrast image.
TWO MAIN TYPES OF ELECTRON MICROGRAPHS:
1. TRANSMISSION ELECTRON MICROGRAPHS (TEMs)
 2D images
2. SCANNING ELECTRON MICROGRAPHS (SEMs)
 Have a lower magnification, but are 3D
ADV:
 Huge powers of magnification and resolution
DISADV:
 All specimens are examined in a vacuum – air would scatter the electrons and make the image
of the tissue fuzzy – so it’s impossible to look at living material.
 Specimens undergo severe treatment that’s likely to result in artefacts.
 Extremely expensive.

,  Large, have to be kept at a constant temp and pressure and need to maintain an internal
vacuum.

CELL MEMBRANES
 Made up of lipids and proteins
 Controls what passes in and out of the cell
 Chemical processes take place on membrane surfaces
 Flexible
PHOSPHOLIPID BILAYER
 Lipids are polar
 Phospholipid molecules form a bilayer
 Water/aqueous solution on each side
 Hydrophobic tail, Hydrophilic head
MEMBRANE PROTEINS
 Proportion of phospholipids containing unsaturated fatty acids in the bilayer affects how freely
the moving proteins float about.
 Proteins help substances move across the membrane
 They can form channels or pores which can be opened or closed depending on conditions.
(gated channels)
 They may act as specific receptor molecules
 GLYCOPROTEINS – proteins with carbohydrate part, important on the surface of cells, way in
which cells recognise each other.

STRUCTURES
MEMBRANES – outer boundary to cell
CYTOSKELETON
 Web-like structure that fills the cytoplasm
 Made up of microfilaments (protein fibres, microtubules, tiny protein tubes)
 Gives cytoplasm structure, keeps organelles in place
NUCLEUS
 Largest organelle
 Spherical shape
 Surrounded by double membrane
 Chemicals can pass in and out
 Nucleic acids-DNA, RNA
NUCLEOLOUS – involved in production of ribosomes
VACUOLES
 Not permanent in animal cells
 Formed and lost when needed
CONTRACTILE VACUOLES – are in simple animals that live in fresh water as they allow water
content of the cytoplasm to be controlled
MITOCHONDRIA
 Cellular respiration takes place, produces ATP
 Cells which require less energy have few mitochondria
 Outer/inner membrane
 Contain own genetic material
 Inner membrane folds to form cristae, large SA and surrounded by fluid matrix

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