AS Biology - Cell Structure - Cells & Organelles Part 2

Cell Surface (Plasma) Membrane 

Definition: The membrane found on the surface of animal cells and just inside the cell wall of plant cells and prokaryotic cells. It's made mainly of lipids and proteins.

Function: Regulates the movement of substances into and out of the cell. It also has receptor molecules on it, which allow it to respond to chemicals like hormones.

Microvilli

Definition: These are folds in the cell membrane.

Function: They're found on cells involved in processes like absorption, such as epithelial cells in the small intestine. They increase the surface area of the membrane.

Nucleus

Definition: A large organelle surrounded by a nuclear envelope (double membrane) which contians many pores. The nucleus contains chromatin and often a structure called the nucleolus.

Function: Chromatin is made from proteins and DNa (DNA controls the cell's activities). The pores allow substances (e.g. RNA) to move between the nucleus and the cytoplasm. The nucleolus makes ribosomes.

Lysosome

Definition: A round organelle surrounded by a membrane, with no clear internal structure.

Function: Contains digestive enzymes. These are kept separate from the cytoplasm by the surrounding membrane, and can be used to digest invading cells or to break down worn out components of the cell. 

Ribosome

Definition: Very small organelle that floats free in the cytoplasm or is attached to the rough endoplasmic reticulum.

Function: It's the site where proteins are made.

Endoplasmic Reticulum

Definition: There are two types of endoplasmic reticulum: (1) The smooth endoplasmic reticulum is a system of membranes enclosing a fluid-filled space.
(2) The rough endoplasmic reticulum is similar but covered in ribosomes.

Function: (1) Smooth endoplasmic reticulum synthesises and processes lipids.
(2) Rough reticulum folds and processes proteins made in the ribosomes.

Golgi apparatus

Definiition: A group of fluid-filled flattened sacs 

 Function: Processes and packages new lipids and proteins and also makes lysosomes.

Mitochondrion

Definition: Usually oval-shaped. Have a double membrane - inner one folded to form structures called cristae. Inside is the matrix which contains enzymes involved in respiration.

Function: Site of aerobic respiration, found in large numbers in cells that are vert active and require lots of energy.

Vesicle

Definition: Small fluid-filled sac in the cytoplasm surrounded by a membrane.

Function: Transports substances in and out of the cell (via the cell surface membrane) and between organelles. Some are formed by the Golgi apparatus or the endoplasmic reticulum, while others are formed at the cell surface. 

Cell wall

Definition: Rigid structure that surrounds plant cells. Made mainly of carbohydrate cellulose.

Function: Supports plant cells.

Chloroplasts

Definition: Small, flattened structure found in plant cells, surrounded by a double membrane and also has membranes inside called thylakoid membranes. These are stacked up in some parts of the chloroplast to form grana. Grana are linked together by lamellae - thin flat pieces of thylakoid membrane.

Function: Site where photosynthesis happens - some parts of photosynthesis happen in the grana, and other parts in the storma (a thick fluid found in chloroplasts).

Centriole

Definition: Small hollow cylinders containing a ring of microtubules (tiny protein cylinders).

Function: Involved with separation of chromosomes during cell division.

Cilia

Definition: Small hair-like structure found on the surface membrane of some animal cells. In cross-section they have an outer membrane and a ring of 9 pairs of protein microtubules inside, with a single pair of microtubules in the middle.

Function: Microtubules allow the cilia to move. This movement is used by the cell to move substances along the cell surface.

Flagellum

Definition: Flagella on eukaryotic cells are like cilia but longer, they stick out from the cell surface and are surrounded by the cell membrane. Inside they're like cilia too - 2 microtubules in the centre and 9 pairs around the edge.

Function: Microtubules contract to make the flagellum move. Flagella are used like outboard motors to propel cell forward e.g. when a sperm swims.