Chapter 3 - Structure of an Epithelial Cell

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  • Created on: 18-03-14 15:22

The Nucleus

  • Nuclear Envelope - A double membrane that surrounds the nucleaus. Its outer membrane is continuous with the endoplasmic reticulum of the cell and often has ribosomes on its surface. It controls the entry and exit of materials in and out of the nucleus and contains the reactions taking place within it
  • Nuclear Pores - Allow the passage of large molecules like messenger RNA, out of the nucleus. There are typically around 3000 pores in each nucleus, each 40-100 nm in diameter
  • Nucleoplasm - The granular jelly like material that makes up the bulk of the nucleus
  • Chromatin - The DNA found within the nucleoplasm. This is the difuse form that chromosomes take up when the cell is not dividing
  • The Nucleolus - A small spherical body within the nucleoplasm. It manufactures ribosomal RNA and assembles the robosomes

Functions of the Nucleus:

  • To act as the control centre of the cell through the production of RNA and hence protein synthesis
  • To retain the genetic material of the cell in the form of DNA or chromosomes
  • To manufactures ribosomal RNA and ribosomes
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The Mitochondrion

  • Double Membrane - Surrounds the organelle, the outer one controlling the entry and exit of material. The inner membrane is folded to form extensions known as cristae
  • Cristae - Shelf-like extensions of the inner membrane, some of which extend across thw whole width of the mitochondrion. These provide a large surface area for the attachment of enzymes involved in respiration
  • Matrix - Makes up the remainder of the mitochondrion. It is a semi-rigid material containing protein, lipids and traces of DNA that allows the mitochondria to control the production of their own proteins. The enzymes involved in respiration are found in the matrix.

Mitochondia are the sites of certain stage of respiration. They are therefore responsible for the production of the energy carrier molecule ATP from carbohydrates. Because of this the number and 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 plentiful supply of ATP. Examples pf metabolically active cells include muscle and epithelial cells. Epithelial cells use a lot of energy in the process of absorbing substances from the intestines by active transport.

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Endoplasmic Reticulum

The ER is an elaborate, 3D system of sheet-like membranes, spreading through the cytoplam of the cells. It is continuous with the outer nuclear membrane. The membranes enclose flattened sacs called cisternae. There are 2 types of ER:

  • Rough Endoplasmic Reticulum - Ribosomes present on the outer surfaces. Its functions are:

(a) To provide a large surface area for the synthesis of proteins and glycoproteins

(b) To provide a pathway for the transport of materials, especially proteins, throughout the cell

  • Smooth Endoplasmic Reticulum - Lacks ribosomes on its surface and often more tubular in apperance. Its functions are:

(a) Synthesis, store and transport lipids

(b) Sythesis, store and transport carbohydrates

Cells that need to manufacture and store large quantities of carbohydrates, proteins and lipids have a very extensive ER. Such cells include liver and secretory cells e.g. epithelial cells that line the intestines

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Golgi Apparatus

Occurs in almost all eukaryotic cells and similar to the SER in structure except it is more compact. It consists of a stack of membranes that make up flattened sacs, or cisternae, with small rounded hollow structures called vesicles. The proteins and lipids produced by the ER are passed through the golgi apparatus in strict sequence. The golgi modifies these proteins often adding nin-protein components, such as carbohydrate, to them. It also 'labels' them, allowing them to be accuratly sorted and sent to their correct destinations. Once sorted, the 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 functions of the golgi apparatus are to:

  • Add carbohydrates to proteins to form glycoproteins
  • Produce secretoy enzymes, such as those secreted by the pancreas
  • Secrete carbohydrates, such as those used in making cell walls in plants
  • Transport, modify and store lipids
  • Form lysosomes

There the golgi apparatus is especially well developed in secretory cells, such as the epithelial cells that line the intestines

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Lysosomes

These are formed when the vesicles produced the golgi apparatus contain enzymessuch as proteases and lipases. As many as 50 such enzymes may be contained in a single lysosome. Up to 10 µm in diameter, lysosomes isolate these potentially harmful enzymes from the rest of cell before releasing them, either to the outside or into a phagocytic vesicle within the cell. The function of lysosomes are to:

  • break down material ingested by phagocytic cells, such as white blood cells
  • Release enzymes to the outside of the cell (exocytosis) in order to destroy material around the cell
  • Given the roles that lysosomes perform, it is not surprising that they are especially abundant in secretory cells, such as epithelial cells, and in phagocytic cells
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Ribosomes

Ribosomes are samll cytoplasmic granule found i all cells. They may occur in the cytoplasm or be associated with the RER. There are 2 types, depending on the cells in whihc they are found:

  • 80S Type - Found in eukaryotic cells, is around 25 nm in diameter
  • 70S Type - Found in prokayotic cells, is slightly smaller

Ribosomes have 2 subunits - one large and one small, each of which 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.

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Microvilli

Microvilli are finger-like projections of the epithelial cell that increase its surface area to allow more efficient absorption.

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Relating Cell Ultrastructure to Function

Each organelle has its own function, it is possible to deduce, with reasonable accuracy, the role of a cell by looking at the number and size of the organelles it contains. For example, as mitochondria produce ATP that is used as a temporary energy store, it follows that cells with many mitochondia are likely to require a lot of ATP and therefore have a high rate of metabolism. Even within each mitochondrion, the more dense and numerous the cristae, the greater the metabolic rate of the cell possessing these mitochondria.

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