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.
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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