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Cell structure - A-Level Biology notes
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Cell structure - A-Level Biology notes
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Chapter 3: Cell structure3.1 Methods of studying cells
Microscopy:
- Produces a magnified image
- Convex glass lens
- Light microscopes can only distinguish between objects which are 0.2um apart
- Electron microscopes can distinguish objects that are 0.1um apart.
Magnification The size of the image is compared to real life
Resolution The smallest distance where 2 discrete objects will be seen as separate
Cell fractionation Process where cells are broken up and the cells are separated
Conditions necessary:
1. Cold – to reduce enzyme activity
2. Equal water potential as tissue – to prevent organelles from bursting and shrinking
3. Buffered – so pH doesn’t fluctuate
Stage 1: Cell are broken up by a homogeniser
Homogenation This releases organelles from the cell = homogenate
The homogenate is filtered
Stage 2: A process where the homogenate is separated in a centrifuge.
Ultracentrifuge Tube of filtrate is placed in centrifuge and spun at low speed
Heaviest organelle (nuclei) is forced to the bottom which forms a sediment/pellet
The supernatant (fluid on top) is removed, leaving the sediment
The centrifuge is spun at a faster speed to form a second sediment consisting of the next heaviest
organelle (mitochondria)
The process continues
3.2 The electron microscope
- Beam has a short wavelength meaning it has a high resolution power
- As electrons are negatively charged the beam can be formed by electromagnets
Transmission Electron Microscope (TEM) Scanning Electron Microscope (SEM)
Electron gun produces beam Electron gun produces beam
Beam focused onto specimen by a condenser electromagnet Beam focused onto specimen by a condenser
Beam passes through the specimen. If beam falls onto the electromagnet
specimen, it’s absorbed and that part appear dark. If beam Beam bounces off the surface of the specimen to
doesn’t fall onto the specimen, it appears white produce a 3D image
- Photomicrograph (it can be photographed) - Photomicrograph (it can be photographed)
Limitation: - Specimen doesn’t need to be thin
- Must be in a vacuum, therefore no living specimen - Lower resolution than TEM
- Complex staining process therefore may contain artefacts
- Image is in black and white and in 2D
- Specimen must be very thin
3.3 Measurements and calculations
You can measure cells by using an eyepiece graticule (a glass disk). Depending on the magnification it will need to be
calibrated. Calibrating the microscope:
1. Place of stage micrometre onto the stage of the light microscope
2. Focus the microscope so that both the stage micrometre and eyepiece graticule are in clear view
, Chapter 3: Cell structure
3. Count the number of divisions of the eyepiece graticule that represents the total length of the stage
micrometre (x)
4. Calculate the actual length (y) represented by one division of the eyepiece graticule
5. The actual length represented by the whole length of the eyepiece graticule (z):
z = y times by the number of divisions on the graphical
3.4 Eukaryotic cell structure
Ultrastructure – each cell has an
internal structure for its function
Nucleus
Nuclear Double membrane; outer layer is
envelope continuous with the endoplasmic
reticulum of the cell and often has
ribosomes on the surface
Nucleoplasm Granular jelly like substance that
makes up the bulk of the of the
nuclei
Nucleolus a small spherical region
within the nucleoplasm which
manufactures ribosomal RNA and
assembles ribosomes there may
be several in one nucleus
Chromosomes Carry genetic material
Nuclear pores Allow passage of big molecules ex.
mRNA
Nucleolus A small spherical region within the
nucleoplasm which manufactures Functions:
ribosomal RNA and assembles - Acts as a control center
ribosomes there may be several in - Retain genetic material
one nucleus - Manufacture ribosomal RNA and ribosomes
Mitochondrion
Double The inner layer is folded to form cristae
membrane (extensions) to give a large surface area
Matrix Makes up the remainder of the cell
Functions:
- Aerobic respiration
- Responsible for ATP
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