The Ultrastructure of Animal Cells

Cells are highly complex structures that contain organelles. Organelles are structures which carry out specific functions within the cell.

• Cell surface membrane: selectively permeable. Controls exchange of substances between the cell and the environment. Mostly protein and lipid.


• Cytoplasm: all the living parts of the cell without counting the nucleus. They consist of membrane-bound organelles and the cytosol (fluid part). The cytosol contains small molecules in solution and large molescules in suspention.


• Cytoskeleton: consists of miscrotubules, filaments and fibres that provide the physical support. This organelle is also involved in cell movement.


• Nucleus: enclosed by nuclear envelope (two membranes perforated by nuclear pors which control the exchange of material between the nucleus and the cytoplasm). The nucleus also contain one or more nucleoli that manufacture ribosomes.


• Chromatin: is the form of chromosomes when they are not dividing in DNA. There are two types: euchromatin (active DNA; lighter), and heterochromatin (coiled DNA, inactive; darker).


• The endoplasmic reticulum: a system of flattened membrane- bound sacs (cisternae, full of fluid) forming tubes and sheets. There are two types: the Rough Endoplasmic Reticulum (Some regions are coiled with bead-like structures, ribosomes); and the Smooth Endoplasmic Reticulum (has no ribosomes).

* The RER transports proteins made by the ribosomes.

*The SER synthesises secretes and storescarbohydrates, lipids and other non protein products.

* The SER of liver cells also contains enzymes.

• Ribosomes: tiny organelles with two subunits (one smaller) which are made of roughly equal parts of protein and RNA and they are the sites of protein synthesis.


• Mitochondria: cigar-shaped organelles. Involved in generating ATP. Each one of them is bounded by an envelope which consists of two membranes. The inner is deeply folded into shelves (cisternae). The watery matrix contains DNA, ribosomes, enzymes, calcium phosphate and granules.


• Golgi apparatus: a stack of flattened membrane-bound sacs (cisternae). A new membrane is constantly added to one end and buds off as vesicles at the other end. Processing and packing structure, enables cell materials (enzymes)to be secreted of the cell by vesicles and is involved in the formation of lysosomes. Lysosomes eliminate worn-out mitochondria and other redundant organelles. They are also involved in autolysis (breakdown of tissue).


• Centrioles: paired cylinders composed of a complex arangement of microtubules (involved in cell division).


• Flagela and celia: slender mobile extensions (involved in cell locomotion).