The difference between magnification and resolution.

Electron microscopes can magnify objects many times to a good resolution.

Magnification is how many times the image has been enlarged.

Resolution is how clear the picture is. If resolution is 1nm it means that if two objects are one nanometer or more apart the microscope will show them as two different objects; if they are any closer than that they will look like one object.

Waves are only affected by objects that are bigger than half of their length: so big wavelength will only get diffracted by a relatively big object in the way- think of the sea with massive waves- if you put a twig in front of it the water would not be disturbed, you would have to put a tree in front of it to make any difference.

A small wavelength will be diffracted by tiny objects in the way: so the ripples from a raindrop would be affected by a twig. Electrons have a very small wavelength and so are effected by tiny objects- this means that more detail shows up in an electron microscope where the waves have been disturbed than it does on a light microscope (with a bigger wavelength.)

Candidates should be able to apply their knowledge of organelles in explaining adaptations of other eukaryotic cells.

A cell will have organelles to suit its job. The ultrastructure of a cell is just the detail inside it- so its types and amounts of organelles.

Here are some examples of where having more of an organelle fits a cells function:

• Mitochondria produce energy (ATP) so are beneficial in muscle cells which need energy for movement and epithelial cells which need energy for active transport.
• Endoplasmic reticulum produces protein (among other things) and so is needed in large amounts for cells that secrete enzymes like the epithelial cells and liver cells
• The same is true for ribosomes which will increase with ER as it covers the surface
• Golgi apparatus enables a cell to egest products like enzymes so would again be helpful in secretory cells; it also produces lysosomes which are needed a lot in phagocytic cells like white blood cells
• Microvilli are helpful in cells that need to absorb things as they increase the surface area, hence why there are so many in the epithelial cells

There can also be adaptations in the organelles- for example in a cell that needs a lot of energy the mitochondria may have more cistae so there is more respiration can happen.

The appearance, ultrastructure and function of the nucleus.

Nucleus:

• Nuclear envelope
• Double membrane
• Controls the movement of substances in and out of the nucleus
• Can have ribosomes on the surface
• Can be continuous with (joined/next to) endoplasmic reticulum
• Contains chromatin
• DNA for the cell
• Form chromosomes take when the cell is not dividing
• Nucleolus
• Makes rRNA (coding for ribosomes)
• Assembles ribosomes
• Makes mRNA (coding for proteins) and controls protein synthesis
• Nucleoplasam
• Viscous liquid
• Molecules and organelles of the nucleus suspended in it
• Holds the structure of the nucleus

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The appearance, ultrastructure and function of microvilli.

Microvilli are tiny protrusions that help to increase surface area. Epithelial cells in the small intestine need have these in order to increase the rate of diffusion: there is a bigger surface area so more things can diffuse at the same time.

They are also thin walled to decrease the diffusion distance and so increase the rate of diffusion. This is key in the small intestine so that all the useful molecules from food eaten can be absorbed by the cells before the food moves on.

Under a light microscope the microvilli in the intestines are so tightly packed they look like a bush and are referred to as a 'bush border'.

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The appearance, ultrastructure and function of Golgi apparatus.

The Golgi apparatus receives proteins in vesicles (packages) from the endoplasmic reticulum: it modifies them (like adding carbohydrate) and packages them in a new vesicle which is labelled (so it can travel to the right place). The Golgi apparatus can also transport, modify and store lipids.

Mostly the vesicles are going to the cell surface membrane to release their content outside the cell: an example of this in practice is enzymes being secrete in the pancreas to digest food.

The Golgi apparatus is made up of flattened sacs called cisternae. To make a vesicle a bit of the cisternae breaks off into a circular shaped package- this will contain either proteins, carbohydrates or lipids.

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The appearance, ultrastructure and function of endoplasmic reticulum.

Endoplasmic reticulum (ER) provides a surface for the synthesis of different molecules needed in a cell. It can also store and transport these materials.

The molecules involved depend on the type of ER:

• Rough endoplasmic reticulum (RER) makes and transport proteins and glycoproteins; it has ribosomes on its surface because they are involved in protein synthesis.
• Smooth endoplasmic reticulum (SER) stores, transports and synthesises lipids and carbohydrates.

ER is sometimes continuous with (attached to) the nucleus of a cell.

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ER is made up of 'flattened sacs' called cisternae surrounded by a membrane. Both types are complex structures but SER looks more tubular compared to the 'liney' RER.

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The appearance, ultrastructure and function of ribosomes.

Riosomes play a role in protein synthesis. mRNA (coding for proteins) is read by them and used to assemble amino acids.

They can be found in the cytoplasm or on the surface of rough endoplasmic reticulum.

They consist of two sub-units (a large and a small) that are not bound by a membrane.

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