AQA Biology - Structure of an epithelial cell

Epithelial cells are eukaryotic cells i.e. they have a distinct nucleus and possess membrane-bound organelles. They differ from prokaryotic cells such as bacteria.

Each cell type has an internal structure that suits it for its job. This is known as the ultrastructure of the cell.

The function of epithelial cells is to absorb and secrete.

• Nuclear envelope - Double membrane that surrounds the nucleus. Its outer membrane is continuous with the endoplasmic reticulum and often has ribosomes on its surface. It controls the entry and exit of materials in and out of the nucleus.
• Nuclear Pores - Allow the passage of large molecules, e.g. RNA, out of the nucleus. There are typically around 3000 pores in each nucleus.
• 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 chromosomes take up when the cell is not dividing.
• Nucleolus - Small spherical body within the nucleoplasm. Manufactures ribosomal RNA and assembles the ribosomes.

The functions of the nucleus are to:

• Act as the control centre of the cell through the production of mRNA and hence protein synthesis
• Retain the genetic material of the cell in the form of DNA or chromosomes
• Manufacture ribosomal RNA and ribosomes

Rod-shaped organelles, made up of the following features:

• Double Membrane - Surronds the organelle. The outer membrane controls the entry and exit of material. The inner 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 involved in respiration.
• Matrix - Makes up the remainder of the mitochondrion. It is a semi-rigid material containing proteins, lipids and traces of DNA that allows the mitochondria to control the production of their own proteins. The enzymes involved in respiration are found here.

Mitochondria are the sites of certain stages of respiration. They are therefore responsible for the production of the energy-carrier molecule, ATP, from carbohydrates. As a result, in cells that have a high level of metabolic activity (and hence need a plentiful supply of ATP), the number and size of the mitochondria, and the number of their cristae, all increase.

Examples of metabolically active cells include muscle and epithelial cells.

Epithelial cells use a lot of energy when absorbing substances from the intestines by active transport.

• An elaborate, 3-D system of sheet-like membranes, spreading through the cytoplasm of the cells
• It is continuous with the outer nuclear membrane
• The membranes enclose flattened sacs called cisternae

There are two types of ER:

• Rough endoplasmic reticulum (RER) - Has ribosomes present on the outer surfaces of the membranes. Its functions are to:
  • Provide a large surface area for protein and gylcoprotein synthesis
  • Provide a pathway for the transport of materials, especially proteins, throughout the cell
• Smooth endoplasmic reticulum (SER) - Lacks ribosomes on its surface and is often more tubular in appearance. Its functions are to:
  • Synthesise, store and transport lipids and 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. the epithelial cells that line the intestines.

• Occurs in almost all eukaryotic cells and is similar to the SER in structure, except it is more compact. Consists of a stack of membranes that make up cisternae, with small rounded hollow structures called vesicles
• The proteins and lipids produced by the ER are passed through the golgi in strict sequence
• The golgi modifies these proteins often adding non-protein components, such as carbohydrate, to them. It also 'labels' them, allowing them to be 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 carbohydrate to proteins to form glycoproteins,
  • produce secretory enzymes, such as those secreted by the pancreas,
  • secrete carbohydrates, like those used in making cell walls in plants,
  • transport, modify and store lipids,
  • form lysosome
• Therefore the golgi is well developed in secretory cells.

• Formed when vesicles produced by the golgi contain enzymes such as proteases and lipases. (as many as 50 such enzymes may be contained within a single lysosome)
• Lysosomes isolate these potentially harmful enzymes from the rest of the cell before releasing them, either to the outside or into a phagocytic vesicle within the cell.
• The functions of lysosomes are:
  • 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
  • digest worn-out organelles so that the useful chemicals they are made of can be re-used
  • completely break down cells after they have died (autolysis)

Given the role that lysosomes perform, it is not surprising that they are especially abundant in secretory cells, such as epithelial cells, and in phagocytic cells.

Ribosomes:

• Small cytoplasmic granules found in all cells
• May occur in the cytoplasm or be associated with the RER
• There are two types, depending on the cells in which they are found:
  • 80S type - found in eukaryotic cells
  • 70S type - found in prokaryotic cells, it is slightly smaller
• Ribsomes have two subunits - one large and one small - each of which contains ribosomal DNA 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.

Microvilli:

• Finger-like projections of the epithelial cell
• Help to increase surface area to allow more efficient absorption