Bio Notes

Biology Notes. Eukaryotic Cells have a distinct nucleus and possess membrane-bounded organelles. Prokaryotic Cells like bacteria are different.

The Nucleus •Most prominent feature of a eukaryotic cell. •Contains heredetary material •Control's the cell's activities. •Usually spherical - between 10 and 20 microM in diameter. Has a number of sub-parts:

•Nuclear Envelope: is a double membarane surrounding the nucleus. Its outer membrane is

continuous with the E.R and often has ribosomes on the surface. Controls entry and exit of materials in and out of the nucleus + contains reactions that take place within it. •Nuclear Pores allow large molecules such as messenger RNA out of the nucleus. There are usually around 3000 pores in each nucleus, each 40-100nm in diameter •Nucleoplasm is the granular, jelly-like material that makes up the bulk of the nucleus. •Chromatin is the DNA found within the nucleoplasm. This is the diffuse form that chromosomes take up when the cell is not dividing. •The nucleolus is a small spherical body within the nucleoplasm. It manufactures ribosomal RNA and assembles the ribosomes. •The Nucleus 1) Controls the cell through the production of mRNA and hence protein synthesis, 2) Retains the genetic material of the cell in the form of DNA or chromosomes, 3) Manufactures ribosomal RNA and ribosome

The Mitochondrion •Mitochondria are rod-shaped and 1-10microM in length. Made up of:

•Double membrane surrounds the organelle, the outer one controlling entry and exit of the

material, and the inner folded to form extensions known as cristae. •Cristae are shelf-like extensions of the inner membrane, some of which extend across the whole width of the mitochondrion. These provide a large S:A for the attachment of enzymes involved in respiration. •The matrix makes up the remainder of the organelle. It is a semi-rigid material containing protein, lipids and DNA traces that allows the mitochondria to control the production of their own proteins. The enzymes involved in respiration are found in the matrix. •Mitochondria are sites of certain stages of respiration [Krebs cycle + oxidative phosphorylation pathway]. •Therefore, they are responsible for the production of ATP from carbohydrates. •Because of this, number + size of the mitochondria and the number of their cristae all increase in cells that have a high level of metabolic activity and therefore need a high supply of ATP. These include muscle and epithelial cells. [active transport etc]

Endoplasmic Reticulum The ER is an elaborate 3D system of sheet-like membranes spreading through the cytoplasm of cells. It is continuous with the outer nuclear membrane. The membranes enclose flattened sacs called cisternae. Two types of ER:

Rough Endoplasmic Reticulum [RER] has ribosomes present on the outer surfaces of the membranes. •It provides a large S:A for synthesis of proteins and glycoproteins •It provides a pathway for the transport of materials, especially proteins, throughout the cell. Smooth Endoplasmic Reticulum [SER] does not have ribosomes on its surface and is often tubular in appearance. •It synthesises, stores and transports lipids and carbohydrates. Therefore cells that need to manufacture and store large quantities of carbohydrates, proteins, and lipids have a very extensvive ER. These include liver and secretory cells, like epithelial cells in the intestine.

Golgi Apparatus The G.A occurs in almost all eukaryotic cells and is similar to SER instructure except it's more compact. It consists of membrane stacks that make up flattened cas, or cisternae with small rounded hollow structures called vesicles. The ptroteins and lipids produced by the ER are passed through the G.A in strict sequence. The Golgi modifies these proteins often adding nonprotein components, such as carbohydrate, to them. It also 'labels' them, allowing them to be accurately sorted and sent to their correct destinations. Once sorted, these modified proteins and lipids are transported in vesicles which are regularly pinched off from the ends of the Golgi cisternae. These vesicles move to the cell surface, where they fuse with the membrane and release their contents to the outside. The GA: •Adds carbohydrate to proteins to form glycoproteins •Produces secretory enzymes, such as those secreted by the pancreas •Secretes carbohydrates, such as those used in making cell walls in plants •Transports, modifies and stores lipids •Forms lysosomes

Lysosomes These are formed when the vesicles produced by the G.A contain enzymes such as proteases and lipases. As many as 50 such enzymes may be contained in a single lysosome. Up to 1.0microM in diameter, lysosomes isolate these potentially harmful enzymes from the rest of the cell before releasing them, either to the outside or into a pagocytic vesicle within the cell. Lysosomes: •Break down material ingested by phagocytic cells such as white blood cells •Release enzymes to the outside of the cell [ this is called exocytosis ] in order to destroy material around the cell •Digest worn out organelles so that the useful chemicals they are made of can be re-used •Completely break down cells after they have died [ this is called autolysis ] Given the roles that lysosomes perform, it is not surprising that they are especially abundant in secretory cells, such as epithelial caells, and in phagocytic cells.

Ribosomes These are small cytoplasmic granules found in all cells. They may occur in the cytoplasm or be associated with the RER. There are two types, depending on the cells in which they are foudn

•80S type - found in eukaryotic cells, is around 25 nm in diameter •70S type - found in prokaryotic cells, is slightly smaller Ribosomes have 2 sub-units - one large and one small. Each of these contains ribosomal RNA and protein. Despite their small size, they occur in such vast numbers that they can account for up to 25% of the dry mass of a cell. Ribosomes are important in protein synthesis.