Structure of an epithelial cell

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Epithelial cell

Are eukaryotic cells - they have true nucleus           

   function is to absorb and secrete

contain these organelles

  • the nucleus
  • the mitochondriom
  • endoplasmic reticulum
  • golgi apparatus
  • lysosomes
  • ribosomes
  • cell membrane
  • cytoplasm
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The nucleus

  • contains genetic material and controlls the cells activities.
  • Its usually spherical and between 10 and 20 micrometers in diameter.
  • Nuclear Envelope - double membrane, its outer memebrane is continuous with the endoplasmic reticulum, this often has ribosomes (Rough endoplasmic reticulum). It controls the entry and exit of materials and contains the reactions taking place within it.
  • Nuclear Pores - allows the passage of large molecules, such as RNA, out of the nucleus. There are usuallt 3000 pores in each nucleus, each 40-100nm in diameter.
  • Nucleoplasm - granular, jelly - like material that makes up the bulk of the nucleus.
  • Chromatin - the DNA found within the nucleoplasm. This is the diffuse form that chomosomes take up when the cell is not dividing.
  • The nucleolus - small spherical body within the nucleoplasm. It manufactures robosomal RNA and assembles the ribosomes.

FUCTIONS: control center of the cell through the production of mRNA and hence protein synthesis. Retain genetic material of the cell in the form of DNA or chromosomes. Manufacture ribosomal RNA and ribosomes.

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The Mitochondrion

1-10 micrometer in length.

  • double membrane - surrounds the organelle, outer one controls the entry and exit of materials. The inner membrane is folded to form extensions known as cristae.
  • Cristae - shelf like extensions of the inner membrane. These provide a large surface area for the attachment of enzymes involves in respiration.
  • Matrix - the remainder of the mitochondria, it is a semi-rigid material containing proteins, lipids and traces of DNA that allows the mitochondria to control the productions of their oen proteins . The enzymes envolved in resiration are found in the matrix.

Functions - responsible for the production of ATP from carbohydrates. This is why in cells where there is a high level of metabolic activity, the number and size of mitochondria , and the number of their cristae all increase. This is because they need a plentiful supply of ATP. E.g muscle cells.

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Endoplasmic reticulum

  • elaborate, three-dimensional system of sheet like membranes, spreading through the cytoplasm of the cells.
  • It is continous with the outer nuclear membrane . The membranes enclose flattened sacs called cisternae. There are two types of ER:

Rough endoplamic reticulum (RER) - ribosomes present on the outer surface of the membrane.

  • provide large surface area for the synthesis of proteins and glycoproteins
  • provide a pathway for the transport of materials , especially proteins throughout the cell.

Smooth endoplasmic reticulum (SER) - lacks ribosomes on the surface. More tubular in appearance.

  • synthesise store and transport lipids and carbohydrates

Cells that need to manufacture large quantities of crabohydrates, proteins and lipids have a very extensive ER. E.g liver and epithelial cells that line the small intestine.

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Golgi Apparatus

In ALL eukaryotic cells. Simillar to SER in structure, apart from its more compact. Consists of a stack of membranes that make up flatterened stacks called cristernae, with small rounded hollow structures called vesicles. The proteins and lipids made by the ER pass through the golgi aparatus in strict sequence. The golgi modifies them by adding non-protein components, such as crabohydrate. It also accurately sorts them and sends them to their correct desitnations. They are then transported in vesicles, they then move to the cell surface where they fuse with the membrane and release their contents to the outside.

Functions

  • add carbohydrate to proteins to form glycoproteins.
  • produce secretory enzymes, such as those secreted by the pancreas (insulin.
  • Secrete carbohydrates e.g those used to make cell walls in plants
  • transport, modify adn store lipids
  • form lysosomes.
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Lysosomes

- formed in the vesicles produced by the golgi apparatus (contain enzymes such as proteases and lipases. As masny as 50 such enzymes may be contained in a single lysosome. Up to 1.0 micrometer in diameter, lysosomes isolate potentially harmfull enzymes from the rest of the cell before releasing them, either to the outside on into a phagocytic vesicle within the cell.

  • break down material ingested by phagosytic cells, such as white blood cells
  • release enzymes to the outside of the cell (exocytosis) in order to destroy material around the cell
  • digest, worn out organelles so that the usefull chemicals they are made of can be re-used.
  • completely break down cells after they have died  (autolysis)

Abundant in secretory cells e.g epithelial and phagocytic cells

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Ribosomes

small cytoplasmic granules found in all cells. They may be found in the cytoplasm or be assosiated with the RER.

There are two types:

  • 80s type - eukaryotic cells - is around 25nm in diameter.
  • 70s type - found in paryotic cells, is slightly smaller.

They have two subunits , one large and one small, each of which contains ribosomal RNA and protein. Despite their small siz, they occur in such vast numbers that they can account for up to 25% of the dry mass of the cell.

Protein synthesis.

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