Organelles- structure and function

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  • Created by: eikceb
  • Created on: 23-03-15 17:07

Nucleus

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
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Endoplasmic reticulum

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
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Golgi apparatus

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
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Mitochondria

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
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Chloroplasts

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
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Lysosomes

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
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Ribosomes (no surrounding membrane)

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
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Centrioles (no surrounding membrane)

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
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