Unit 2 Biology Notes

Light or optical microscope has been the main tool for observing cells, with it magnifying up to 1500 times, whereas electron microscopes provide the highest levels of magnification.

Light microscope:

·         Living matter can be observed directly.

·         Cheap – readily available.

·         Preparation methods may contribute to what is seen under the microscope.

·         Limited powers of resolution and magnification.

Electron microscope:

·         High powers of magnification and resolution.

·         Specimens are examined in a vacuum so not able to view living matter

·         Preparation includes severe treatment – artefacts

·         Expensive, large, inaccessible and have to be kept in specific conditions.

Eukaryotes

Animals, plants, protoctists and many fungi have membrane-bound organelles (nucleus, mitochondria and chloroplasts).

Prokaryotes

Bacteria and blue-green algae lack the structure and organisation of eukaryotic cells, with their genetic material coiled up in the centre – nucleoid, and sometimes contain additional bits of genetic material – plasmids.

The cytoplasm contains enzymes, ribosomes and food-storage granules and respiration occurs in the mesosome.

Animal cell

Surrounded by a cell surface membrane which surrounds the cytoplasm and nucleus – protoplasm. The structure of each part of the cell is related to its function.

Membranes

Important as an outer boundary to the cell and in the multitude of internal membranes. Cell membranes control the movement of substances as well as having other functions.

The protoplasm

The cytoplasm contains many organelles – this detailed organisation is known as the ultrastructure of the cell.

Nucleus

The largest organelle in the cell – spherical in shape and surrounded by a double nuclear membrane containing pores in which chemicals can pass in and out of so events can be controlled. Inside the envelope there are nucleic acids – DNA and RNA, and proteins. When the cell is not actively dividing the DNA is bonded to a protein to form chromatin. In the nucleus there is at least one nucleolus containing pure DNA and protein. The nucleolus is involved in the production of ribosomes.

Mitochondria

Tiny rod like structures in the cytoplasm act as ‘powerhouses’ of the cell. Energy is released from food by respiration using oxygen. This energy is in the form of ATP which can be used to drive the other functions of the cell and the organism. Cells that require little energy have few mitochondria. Any cell with an energy-demanding function or that carry out a lot of active transport will contain large numbers of mitochondria.

They are surrounded by an outer and inner membrane and contain their own genetic material – the inner membrane is folded to form cristae surrounded by a fluid matrix. They originated as eubacteria living inside early cells.

Centrioles

There is a pair of centrioles near the nucleus, each of which is made up of a bunch of 9 tubules. These are involved in cell division, where these centrioles pull apart to produce a spindle of microtubules.

The cytoskeleton

A feature of all eukaryotic cells, this web like…