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Topic 1 - cell structure

Cells:

Organ: collection of tissues with a specific function
Prokaryotic cells: do not contain any membrane-bound organelles
e
...
nucleus, mitochondria, vesicles…
• Have a plasma membrane, cytoplasm, ribosomes (but smaller),
DNA and RNA like eukaryotes
• Less developed cytoplasm than eukaryotes, no nucleus
...
All animal,
plant, fungal and protoctist cells are eukaryotic
...
g
...
5-5 µm

Average diameter – 10-20 µm

Smaller ribosomes – 20nm

Larger ribosomes – 22nm

Cell wall always present

Cell wall sometimes present

Animal cell

Plant cell
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Topic 1 - cell structure

Membrane bound organelles:

Organelles: small structures within cells, each of which has a specific function
• Enable cells to carry out many functions efficiently as organelles have specific functions
• Most of the organelles within eukaryotic cells are membrane bound, they are covered in a
membrane that is similar in structure to the cell surface membrane
• Membrane keeps each organelle separate from rest of cell - it is a discrete compartment
• Prokaryotic cells do not have membrane-bound organelles
Nucleus, nuclear envelope and nucleolus:
• The nucleus is surrounded by a double membrane – nuclear envelope
...
g
...
g
...
Here some dissolved substances
and ribosomes can pass through
• Nucleus contains genetic material (DNA and proteins) therefore controls cells activities as
provides instruction for protein synthesis
• The nucleus envelope separates the contents of the nucleus from the rest of the cell
• The nucleolus contains RNA and produces ribosomes, and does not have a membrane
Rough Endoplasmic Reticulum (RER):
• Consists of cisternae (fluid-filled cavities) and coated with ribosomes
• Provides large surface area for ribosomes to assemble amino acids into proteins, which then
actively pass through membrane into cisternae, then transported to Golgi apparatus for
modification and packaging
• RER is the intracellular transport system – forming channels for transporting substances from
one area of the cell to another
Smooth Endoplasmic Reticulum (SER):
• Consists of cisternae (fluid-filled cavities), no ribosomes on surface
• Contains enzymes that catalyse reactions involved with lipid metabolism, e
...
making of
cholesterol, lipids, steroid hormones
• Is involved with the absorption, synthesis and transport of lipids
Golgi apparatus:
• A stack of membrane-bound flattened sacs
• Secretory vesicles bring materials to and from the Golgi apparatus
• Involved in modifying proteins (e
...
adding sugar/lipid molecule; folding into 3D shape)
• Packages proteins into vesicles for storage or transportation
Mitochondria:
• Surrounded by a double membrane (inner membrane is highly folded into cristae) with a fluid
filled space between them, inner part of mitochondrion is a fluid-filled matrix
• Site of ATP production (an energy carrying molecule)
• They are self-replicating (more can be made if the cells enter need increases)
• Abundant in cells where metabolic activity takes place, eg liver cells, synapses
Chloroplasts:
• Large organelles
• Only in plants/some protoctists, lots in leaf cells especially palisade mesophyll layer
• Site of photosynthesis
• Surrounded by a double membrane, inner membrane is highly folded into thylakoids (stack –
granum) and embedded with chlorophyll
• Contain loops of DNA and starch grains
Vacuole:
• Surrounded by a membrane – tonoplast, filled with water and solutes
• Only plant cells have a large permanent vacuole
• For storage (e
...
of sap) & maintains shape of cell - pushes cytoplasm against cell wall, making
cell turgid
Lysosomes:
• Spherical sacs containing powerful hydrolytic (digestive) enzymes – used to break down old or
foreign materials
...
g
...
g
...
Gene that has coded instructions for a protein housed on chromatin in the nucleus, is transcribed
into a length of mRNA
2
...
mRNA attaches to ribosome (on RER), instructions are translated & insulin molecules assembled
4
...
Vesicles containing insulin are ‘pinched off’ from RER and transported to the Golgi apparatus, via
microtubules and motor proteins
6
...
Packaged insulin molecules travel to the plasma membrane in the vesicles
8
...
Plasma membrane opens to release insulin molecules outside

Microscopes:

Magnification: how much an image has increased in size compared to the actual image
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Topic 1 - cell structure
Resolution: clarity of an image; the ability to distinguish 2 objects, higher = clearer
Electron micrograph: photograph of an image seen using an electron microscope
Photomicrograph: photograph of an image seen using an optical microscope
Optical light microscope:
• Relatively cheap, easy to use, portable (field and laboratories), can study whole living specimens
• Rely on lenses to focus a beam of light
• Magnification – upto x 2000
• Resolution – 200nm as optical microscopes use visible light
• Ribosomes are too small to be identified
Laser scanning microscope:
• Use a laser light to scan an object point by point, can focus on different depths within a
specimen so can clearly observe whole living specimens
• High resolution and high contrast - Magnification – x 2000, Resolution – 200nm
• Have depth selection – can observe whole organism and cells, therefore used in medical
profession and biological research
Transmission electron microscope:
• Use a beam of fast travelling electrons so can be used to give clear and highly magnified images
• Specimen has to be chemically fixed by being dehydrated and stained
• The bean of electrons passes through the specimen and some electrons are focused on the
screen
...
002nm
• Large and very expensive, need a lot of skill and training to use
Scanning electron microscopes:
• Use a beam of fast travelling electrons so can be used to give clear and highly magnified images
• Electrons do not pass through the specimen, but cause secondary electrons to ‘bounce off’ the
specimen’s surface and be focused on to a screen
...
004nm
• Large and very expensive, need a lot of skill and training to use
Temporary mounts – fresh tissue samples are placed onto microscope slides
• Require the specimen to be immersed in liquid so cells do not dry out and distort
• A stain is added to provide contrast, e
...
Methylene blue
• Do not last more than a few hours
Prepared specimens – permanent specimens are prepared by dehydrating the specimen, fixing it in
wax and slicing it very thinly
Measurements:

1 millimetre = 1000 micrometres
1 micrometre = 1000 nanometres
To convert a larger unit to a smaller unit, multiply
To convert a smaller unit to a larger unit, divide

Graticules:
Eye piece graticules: placed in the eyepiece of a microscope and acts as a ruler when you view the
object under the microscope
Stage graticule: small scale that is placed on a microscope stage and used to calibrate the value of the
eyepiece divisions at different magnifications
• Eye piece graticule: a small ruler that can be inserted into the eyepiece of a microscope
• The graticule scale is superimposed on samples viewed down the microscope
• The scale on the graticule is arbitrary - one unit on the graticule will represent different lengths
depending in which objective lens is used
• The graticule must therefore be calibrated using a stage micrometer
• Ruler is 1mm long & divided into 100 divisions, each one 0
...
Superimpose the two scales
2
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Count the number of EPG divisions
4
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0 mark (eyepiece graticule)
...

The 68 mark (stage micrometer) is lined up with the
9
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So you can say that:
80 small eyepiece graticule markings = 18 stage
micrometer markings
= 18 x 0
...
18 mm
= 180 µm
So 1 small eyepiece graticule marking = 180: 80 =
2

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