Biology notes that will help you get an A*
Notes have been made to cater to the AQA Biology Mark Scheme
Contains everything on the spec for Cells:
- Microscopes
- Eukaryotic Cells
- Prokaryotes & Viruses
- Cell Division
- Cell Membrane
- Diffusion
- Osmosis
- Active Transport
- Phago...
Studying Cells
Studying Cells
Microscopes are instruments that produce a magnified image of an object
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Magnification = How much bigger the image is than the object.
·
Resolution / Resolving power = The ability to distinguish between two Magnification = Size of image
points that are close together. Size of real object
Optical Microscope
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Uses light to form an image. = disadvantage
·
Maximum resolution of 0.2 micrometers - can't see ribosomes, lysosomes or RER.
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Low resolution due to the wavelength of light being too long. Any 2 objects which are closer than 0.2
micrometers will appear as a single item.
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Maximum useful magnification is x1500 - low Preparing slides:
Light is focused onto sample with diaphragm 1. Add drop of water onto slide.
Cheap
A live specimen can be used
Am Eyepiece
2. Use tweezers to get thin slice of
specimen and place on slide
·
Arm
3. Add stain - iodine in potassium iodide
External view of specimen Objective lens
Stage solution is used to stain starch grains and
Image can be viewed directly Diaphragm
Coarse focus eosin makes the cytoplasm show up
Fine focus
Staining of the specimen is required Light 4. Add coverslip carefully: stand it
Image in 2D and colour ↑
Base upright then carefully tilt and cover it.
Electron Microscope
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Uses electrons to form an image.
High resolution - can see smaller organelles that can't be seen with optical.
Maximum resolution of 0.0002 micrometers si units:
Maximum useful magnification of x 1500000 - high Milli 2x1000 (mm)
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Image can not be viewed directly Micro
Nano I
x00
Expensive
Live specimen can not be used because samples are placed in a vacuum PicO (pm) (400
⑮
Complex staining process required
Image in black and white
The image produced on the screen can be photographed to give a photomicrograph
, Scanning electron microscope Transmission electron microscope
condenser Electromagnet used to focus a beam of
A beam of electrons scanned across specimen. This knocks
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off electrons from the specimen which are gathered in aelectrons which is transmitted through the specimen
cathode ray tube to form an image Denser parts of the specimen absorb more electrons
External view of the specimen -shows the surface which make them look darker in the image. Other parts
3d allow electrons to pass through so appear bright
Thick specimens can be used Section view of the specimen- internal structures of
X200000 maximum effective magnification organelles can be viewed
20nm maximum resolution 2d
X2000000 maximum effective Magnification
Artefacts= things such as air bubbles that obstruct the 0.1nm maximum resolution
view of the specimen and are not meant to be there. Thin specimens can be used only
The full resolving power of electron microscopes can not always be achieved because:
Difficulties preparing the specimen limit the resolution that can be achieved.
A higher energy electron beam is required and this may destroy the specimen
Cell fractionation
Cell fractionation separates organelles from the rest of the cell so that they can be seen under an
electron microscope
1. Homogensation:
Breaking down the cell - Cells are blended in a homogeniser forming the resultant fluid called the homogenate. This can also be done by vibrating,
grinding or crushing. This breaks up the plasma membrane and releases the organelles into solution. The solution must be ice-cold to slow down
enzyme activity. The solution should be isotonic to prevent damage to the organelles through osmosis. A buffer solution should be used so that
enzymes are not denatured.
2. Filtration
The homogenate is filtered through a gauze to separate any useless parts and large cell debris
3. Ultracentrifugation
The solution you are left with contains a mixture of organelles. It is poured into a tube which is placed in a centrifuge and spun at a
low speed. The heaviest organelles, such as the nuclei, are forced to the bottom of the tube and they form a thin sediment /pellet
The fluid above is called the supernatant and it is transferred to another tube and spun again faster The next heaviest, the mitochondria,
forms the pellet.. The process is repeated at higher speeds until all the organelles are separated out.
, Eukaryotic Cells
Nucleus
Eukaryotic cells are complex and include animal, plant, fungal and protist Mitochondria
Rough Endoplasmic Reticulum
cells. They have a distinct nucleus and membrane-bounded organelles. Nucleoplasm
Cytoplasm
·
Lysosome
Cell-surface membrane: Nucleolus
Cell membrane
Regulates the movement of substances into and out of the cell.
:
Covered in receptors to respond to hormones. Nuclear Envelope -
Golgi vesicle
Made mainly of lipids and protein.
Ribosomes
Nucleus: Nuclear pore
Golgi apparatus
Chromatin Smooth
Stores chromosomes to make polypeptides. Where transcription occurs Endoplasmic
Reticulum
Controls cell's activities
The nuclear envelope is a double membrane that surrounds the nucleus. Controls entry and exit of materials
in and out of nucleus and contains the reactions taking place within.
Nuclear pores allow substances to move between cytoplasm and nucleus.
Nucleolus makes ribosomes. There can be more than one.
Mitochondria:
Has a double membrane - the inner one is folded to form structures called cristae. Cristae provide a large
surface area for the attachment of enzymes and other proteins involved in respiration.
Inside is the matrix which contains enzymes for respiration.
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↑
It is the site of aerobic respiration, where ATP is produced mu
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Rough Endoplasmic Reticulum: matrix
Filled with fluid which is enclosed by membranes
It provides a large surface area for protein synthesis
The surface is covered with ribosomes.
Folds and processes proteins that have been made at the ribosomes
Smooth Endoplasmic Reticulum:
Synthesises and processes lipids
System of membrane bound sacs
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