Cell Membrane

Cell Membrane

Cell Membrane Structure: Components and Their Roles

1. Lipids: Phospholipids are the most abundant type of lipid found in a cell membrane. They are the main building block.

• Each phospholipid has 3 parts: phosphate head, glycerol backbone, and fatty acid tails

• Phosphate heads are polar and hydrophillic (water loving)
• The glycerol backbone is what holds the FA tails to the phosphate head
• Fatty acid tails are non polar and hydrophobic (water fearing) hydrocarbon chains.
• Hydrocarbon chains can be unsaturated ( having a kink in their chain) or saturated chains (do not have a kink in their chains). This will affect how closely the phospholipids sit next to one another.

• Phospholipids are amphiphatic:

• When a molecule has a hydrophillic and hydrophobic region, it is said to be amphiphatic.

• The cell membrane is a phospholipid bilayer, this forms due to their amphiphatic property.

• The hydrophillic phosphate heads will want to face water, and the hydrophobic fatty acid tails will want to stay away from water

• Remember, water can be found both inside the cell as part of the intracellular fluid and outside of the cell as part of the extracellular fluid.
• To satisfy the amphiphatic nature of phospholipids, they arrange themselves into two adjacent layers:
• The fatty acid tails of one layer will face the fatty acid tails of the other layer.
• The phosphate heads will be facing outward; with the phosphate heads of one layer facing the extracellular fluid and the phosphate heads of the other layer facing the intracellular fluid.
• This arrangement of the phopholipids causes the cell membrane to have a hydrophobic interior.
• This space will exclude any watery intracellular or extracellular fluid
• The role of phospholipids in the cell membrane:
• Contribute to the selective permeable nature of cell membranes, as they only allow the passive movement of some molecules.

1. Steroid Cholesterol: Composed of 4 fused carbon rings, making it very stable. It can be found randomly distributed throughout the cell membrane.

• The role of cholesterol in the cell membrane:
• Helps regulate what enters and exists the cell by functioning as fluidity buffers
• Key players in maintaining cell membrane fluidity and structure, which can be affected by a number of environmental factors, such as temperature.
• Low

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  temperatures stiffen the membrane; whereas

  high

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  temperature make the cell membrane too fluid.
• Cholesterol holds the phospholipids together so that they don’t separate too far when it's hot, and prevents the phospholipids from getting too close when it's cold.
• Plant cell membranes have low levels of cholesterol, as compared to animal cells. However, they do have a related steroid lipid that serves the same purpose.

1. Proteins: Most cell membranes comprise of approximately 50% (or less) protein.

• There are 3 main types of membrane proteins:

1. Integral or Transmembrane proteins

• These proteins are integrated within any part of the cell membrane

1. Peripheral proteins

• Appear on the periphery, sitting on-top of the cell membrane either on a phosphate heads or integral protein.
• Can be on either side (intracellular or extracellular).

1. Lipid anchored proteins

• Proteins that covalently attach to the cell membrane via lipid anchors.
• The role of proteins in the cell membrane:
• Act as receptors
• Help with the transport of molecules in and out of the cell

1. Carbohydrates: Carbohydrate molecules are present only on the outer surface of the cell membrane.

• Glycolipids
• Carbohydrate chain covalently bonded to lipids.
• Glycoproteins
• Carbohydrate chain covalently bonded to proteins.
• More commonly seen.
• Vary significantly from specie to specie, and even from cell to cell (within the same organism!)
• The role of carbohydrates in the cell membrane:
• Cell recognition.
• Example: ABO blood grouping
• Cell adhesion (holding cells together)

1. Fluid nature: Membranes are not static, they are dynamic! Think of the membrane to be like your skin, it's a solid but it can still move!

1. The weak hydrophobic interactions between the lipids, proteins and carbohydrates. This enables movement.
2. The degree of saturation of the fatty acid tails; the more unsaturated fatty acid tails there are, the less compact the phospholipids will be.

1. The cholesterol content since they function as fluidity buffers.

1. Mosaic appearance: What would a top view of the cell membrane look like?