Essay on the Structure and Function of Cell Organelles
Cell wallA rigid cell wall surrounding the cell, consisting of cellulose microfibrils running through a matrix of other complex polysaccharides, namely hemicelluloses and pectic substances. May be secondarily thickened in some cells.Provides mechanical support and protection. It prevents osmotic bursting of the cell. It is totally permeable and hence pathway for movement of water and mineral salts. Various modifications, such as Lignification, for specialised functions.
Middle lamellaThin layer of pectic substances (calcium and magnesium pectates).Cements neighbouring cell together.
PlasmadesmataA fine cytoplasmic thread linking the cytoplasm of two neighbouring cells through a fine pores in the cell walls.Enables a continuous system of protoplasm, to be formed between neighbouring cells for transport of substances between cells.
Plasma membraneTwo layers of lipid (bilayer) sandwiched between two protein layers.A differentially permeable barrier controlling exchange of water and minerals between the cell and its outside.
Endoplasmic reticulumA system of flattened, membrane- bounded sacs called cisternae, forming tubes and sheets. It is continuous with the outer membrane of the nuclear envelope.If ribosomes are found on its surface it is called rough ER and transports proteins made by the ribosomes through the cisternae. Smooth ER, (no ribosomes) is a site of lipid and steroid synthesis.
ChloroplastLarge plastid containing chlorophyll and carrying out photosynthesis. It is surrounded by an envelope of two membranes and contains a gel-like stroma through which runs a system of membranes that are stacked in places to form grana. It may store starch. The storma also contains ribosomes, a circular DNA molecule and lipid droplets.It is the organelle in which photosynthesis takes place, producing sugars and other substances from carbon dioxide and water using light energy trapped by chlorophyll. Light energy is converted to chemical energy.
MitochondriaSurrounded by two membranes, the inner being folded to form cristae. Contains a matrix with a few ribosomes and circular DNA molecule.In aerobic respiration cristae are the sites of oxidative phosphorylation and electron transport, and the matrix is the site of Krebs cycle enzymes and fatty acid oxidation.
Golgi apparatusA stack of flattened, membrane bound sacs, called cisternae, continuously being formed at one end of the stack and budded off as vesicles at the other. Stacks may form discrete dictyosomes as in plant cells, or an extensive network as in many animals cells.Processing in cisternae and transport in vesicles of many cell materials, such as enzymes from the ER. Often involved in secretion and lysosome formation.
RibosomesSmallest cell organelle with cell membrane. It consists of a large and a small subunit. They are made î roughly equal parts of protein and RNA. Ribosomes are found ir mitochondria and chloroplasts in plants.Sites of protein synthesis. They are either bound to the ER or lie free in the cytoplasm. They may form polysomes (polyribosomes), chain of ribosomes—along messenger RNA.
LysosomeA simple spherical sac bounded b) a single membrane and containing digestive (hydrolytic) enzymes Contents appear homogeneous.Many functions, all concerned with break down of structures or molecules like autophagy, autolysis, endocytosis and exocytosis.
They are called as suicidal bags of the cell, they are made up of powerful digestive enzymes. So if there is any problem in cellular metabolism. Then cells may be burst and eat themselves.
MicrobodiesA roughly spherical organelle bounded by a single membrane. Its contents appear finely granular except for occasional crystalloid or filamentous deposits.All contain catalase, an enzyme that breaks down hydrogen peroxide. All are associated with oxidation reactions, in plants, are the site of the glyoxylate cycle.
CentrioleCylindrical structure without limiting membrane. It consists of 9 triplet fibres, each consisting of three secondary fibres or tubules. These are enclosed in amorphous matrix. Centrosome separates into a pair of centrioles. Mostly found in animals and a few plants.Centrioles take part in formation of basal bodies and mitotic spindle. These also facilitate the movement of chromosomes during cell division.