Biology: Chapter 4: Cell surface membrane: Components of cell membranes

Biology: Chapter 4: Cell surface membrane: Components of cell membranes

Proteins

Two types of proteins: 

• Intrinsic (integral) proteins: Embedded in the membrane. Found in inner and outer layer, or most commonly spanning the entire membrane, which are called trans-membrane proteins.
  The hydrophobic regions in trans-membrane proteins is made up of one of more alpha-helical chains.
• Extrinsic (peripheral) proteins: Entirely outside the membrane, found on the inner and outer surface. Bound by weak bonds (dipole bonds, ionic bonds) or to intrinsic proteins. 

Intrinsic (integral) proteins

• Have hydrophobic and hydrophilic regions 
• Stay in the membrane because the hydrophobic regions, made from non-polar amino acids are next to the hydrophobic phospholipid tails and get repelled by the watery environment on either side of the membrane
  The hydrophilic regions, made up of polar amino acids, are repelled by the non-polar interior of the membrane and therefore face the aqueous environment inside and outside the cell, or line the hydrophilic pores which pass through the membrane.
•  Float like mobile icebergs in the phospholipid layers, though some are fixed to structures inside/outside the cell and do not move.
• Many have short carbohydrate chains attached to the outer side that protrude into the aqueous environment, called glycoproteins.

Phospholipids

• Form the bi-layer - basic structure of the membrane
• Tails are non-polar - difficult for polar molecules (ions) to pass through the membrane
• Act as a barrier to most water-soluble substances; molecules such as sugars, amino acids and proteins cannot leak out of the cell, while unwanted water-soluble molecules can't enter.
• Some phospholipid molecules can be chemically modified to act as signalling molecules; these may move around the bi-layer activating other molecules eg. enzymes.
• Some may be hydrolysed into smaller, water-soluble molecules(vesicles) which diffuse through the cytoplasm and bind to special receptors -eg. Release of calcium ions from storage in the ER, resulting in exocytosis of digestive enzymes.

Cholesterol

• Small
• Unsaturated - More bent
• Also have hydrophilic heads and hydrophobic tails
• Fit neatly between phospholipids in cell membrane
• Animal cell surface membranes contain almost as much cholesterol as phospholipids
• Plant cells have less and prokaryotes have none - similar compounds do the same function.
• Cholesterol increases fluidity of membrane at low temperatures
  Done by preventing it from becoming too rigid by prohibiting close packing of the phospholipid tails.
  This allows cells to survive in colder temperatures
• Cholesterol also maintains stability at higher temperatures- prevent it from becoming too fluid
  Without this cell membranes would quickly break and burst open
• Hydrophobic regions prevent ions or polar molecules from passing through membrane
  Important in myelin sheath around nerve cells, where ion leakage would slow down nerve impulses.

Glycolipids and Glycoproteins

• Many lipid molecules on the outer surface and probably all protein molecules have short carbohydrate chains attached to them.
• Glycolipid: Attached to a lipid molecule
• Glycoprotein: Attached to a protein molecule
• Chains project into watery exterior surrounding cell
• Form hydrogen bonds with water and stabilize membrane structure
• Glycocalyx: Sugary coating formed by carbohydrate chains
• The glococalyx in animal cells is formed mainly from proteins, while in plants it is mainly from glycolipids.
• Act as receptor molecules; bind with certain molecules at cell surface.
• Different cells have different receptors depending on their function.

Three major groups of receptors:

1. Signalling receptors: Part of the signalling system that coordinates the activities of cells.
   Recognize messenger molecules like hormones and neurotransmitters.
   When messenger molecules bind to the receptor, it triggers a series of chemical reactions inside the cell.
2. Receptors involved in Endocytosis: Bind to molecules that are parts of structures to be engulfed by the cell surface membrane.
   Endocytosis: Form of active transport where a cell transport molecules (eg. proteins) into the cell by engulfing them.
3. Cell adhesion: Binding cells to other cells

• Some glycolipids and glycoproteins act as cell markers or antigens, allowing cell to cell recognition.
  Each type of cell has its own type of antigen eg. ABO blood group proteins all have small differences in their carbohydrate chains.

Proteins (functions)

Transport proteins

• Provide hydrophilic channels or passageways for ions and polar molecules to pass through membrane.
  Two types of transport protein: Channel proteins and carrier proteins.
• Each transport protein is specific for a particular type of ion or molecule. Therefore types of substances that leave or enter the cell can be controlled.

Enzymes

• Some proteins on the inside of the cell surface are attached to the cytoskeleton (system of protein filaments inside the cell). 
• Decide and maintain shape of cell.
• Involved in changes of shape when cells move.