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Nucleus:organelle in the cell DNA and cell’s hereditary information

Parts of the eukaryotic cell

The structures that make up a Eukaryotic cell are determined by the specific functions carried out by the cell. Thus, there is no typical Eukaryotic cell. Nevertheless, Eukaryotic cells generally have three main components: A cell membrane, a nucleus, and a variety of other organelles.


The cell membrane

  1. A cell cannot survive if it is totally isolated from its environment. The cell membrane is a complex barrier separating every cell from its external environment.
  2. This “Selectively Permeable” membrane regulates what passes into and out of the cell.
  3. The cell membrane is a fluid mosaic of proteins floating in a phospholipid bilayer.
  4. The cell membrane functions like a gate, controlling which molecules can enter and leave the cell.
  5. The cell membrane controls which substances pass into and out of the cell. Carrier proteins in or on the membrane are specific, only allowing a small group of very similar molecules through. For instance, α- glucose is able to enter; but β – glucose is not. Many molecules cannot cross at all. For this reason, the cell membrane is said to be selectively permeable.
  6. Cell membranes are made mostly of phospholipid molecules. They have only two fatty acid ‘tails’ as one has been replaced by a phosphate group (making the ‘head’).
  7. The head is charged and so polar; the tails are not charged and so are non-polar.
  8. The two ends of the phospholipid molecule have different properties in water. The phosphate head is hydrophyllic and so the head will orient itself so that it is as close as possible to water molecules.
  9. The fatty acid tails are hydrophobic and so will tend to orient themselves away from water.
  10. So, when in water, phospholipids line up on the surface with their phosphate heads sticking into the water and fatty acid tails pointing up from the surface.
  11. Cells are bathed in an aqueous environment and since the inside of a cell is also aqueous, both sides of the cell membrane are surrounded by water molecules.
  12. This causes the phospholipids of the cell membrane to form two layers, known as a phospholipid bilayer. In this, the heads face the watery fluids inside and outside the cell, whilst the fatty acid tails are sandwiched inside the bilayer.
  13. The cell membrane is constantly being formed and broken down in living cells.


Cytoplasm

  1. Everything within the cell membrane which is not the nucleus is known as the cytoplasm.
  2. Cytosol is the jelly-like mixture in which the other organelles are suspended, so cytosol + organelles = cytoplasm.
  3. Organelles carry out specific functions within the cell.
  4. In Eukaryotic cells, most organelles are surrounded by a membrane, but in Prokaryotic cells there are no membrane-bound organelles.


Fluid mosaic model of cell membranes

  1. Membranes are fluid and are rather viscous – like vegetable oil.
  2. The molecules of the cell membrane are always in motion, so the phospholipids are able to drift across the membrane, changing places with their neighbour.
  3. Proteins, both in and on the membrane, form a mosaic, floating in amongst the phospholipids.
  4. Because of this, scientists call the modern view of membrane structure the ‘Fluid Mosaic Model’.
  5. The mosaic of proteins in the cell membrane is constantly changing.


Membrane proteins

  1. A variety of protein molecules are embedded in the basic phospholipid bilayer
  2. Some proteins are attached to the surface of the cell membrane on both the internal and external surface. These may be hormone receptors, enzymes or cell recognition proteins (or antigens)
  3. Other proteins are embedded in the phospholipid bilayer itself. These are often associated with transporting molecules from one side of the membrane to the other and are referred to as carrier proteins.
  4. Some of these form channels or pores through which certain substances can pass (facilitated diffusion), whilst others bind to a substance on one side of the membrane and carry it to the other side of the membrane (active transport)
  5. Proteins exposed to the cell’s external environment often have carbohydrates attached to them which act as antigens (e.g. blood groups A & B – group AB has both; group O has neither).
  6. Some viruses may also bind here too.