Organelles- structure and function

Structure

• the nucleus is the largest organelle
• when stained, it shows dark patches known as chromatin
• it is surrounded by a nuclear envelope, a structure made of two membranes with fluid between them
• holes called nuclear pores go right throught the envelope, these holes are large enough for relatively large molecules to pass through
• inside the nucleus is a dense, spherical structure called the nucleolus

Function

• the nucleus houses nearly all the cell's genetic material, the chromatin consists of DNA and proteins, and the nucleus has the instructions for making proteins, some regulate cell's activities
• when cells divide, chromatin condenses into visible chromosomes
• the nucleolus makes RNA and ribosomes
• these pass into the cytoplasm and proteins are assembled at them

Structure

• ER consists of a series of flattened, membrane-bound sacs called cisternae
• they are continuous with the outer nuclear membrane
• rough endoplasmic reticulum is studded with ribosomes
• smooth endoplasmic reticulum does not have ribosomes

Function

• rough ER transports proteins that were made in the attached ribosomes
• some of these proteins may be secreted from the cell
• some will be placed on the cell surface membrane
• smooth ER is involved in making the lipids that the cell needs

Structure

• a stack of membrane-bound flattened sacs

Function

• golgi apparatus recieves proteins from the ER and modifies them
• it may add sugar molecules to them
• the golgi apparatus then packages the modified protiens into vesicles that can be transported
• some modified proteins may go to the surface of the cell so that they may be secreted

Structure

• these may be spherical or sausage-shaped
• they have two membranes seperated by a fluid-filled space
• the inner membrane is highly folded to form cristae
• the central part of the mitochondrian is called the matrix

Function

• mitochondria are the site where ATP is produced during respiration
• ATP is sometimes called the universal energy carrier because almost all activities that need energy in the cell are driven by the energy released from ATP

Structure

• these are found only in plant cells and the cells of some protoctists
• chloroplasts also have two membranes seperated by a fluid-filled space
• the inner membrane is continuous, with an elborate network of flattened membrane sacs called thylakoids, which look like piles of plates
• a stack of thylakoids is called a granum
• chlorophyll molecules are present on the thylakoid membranes and in the intergranal membranes

Function

• chloroplasts are the site of photosynthesis in plant cells
• light energy is used to drive the reactions of photosynthesis, in which carbohydrate molecules are made from carbon dioxide and water

Structure

• spherical sacs surrounded by a single membrane

Function

• lysosomes contain powerful digestive enzymes
• their role is to break down materials
• for example: white blood cell lysosomes help to break down invading microorganisms; the specialised lysosome (acrosome) in the head of a sperm cell helps to penetrate the egg by breaking down the material surrounding the egg

Structure

• tiny organelles, some in the cytoplasm, some are bound to ER
• each ribosome consists of two subunits

Function

• ribosomes are the site of protein synthesis in the cell (where new proteins are made)
• they act as an assembly line where coded information (mRNA) from the nucleus is used to assemble proteins from amino acids

Structure

• these are small tubes of protein fibres (microtubules)
• there is a pair of them next to the nucleus in animal cells and in the cells of some protocists

Function

• centrioles take part in cell division
• they form fibres, known as the spindle, which move chromosomes during nuclear division