2.1.5 Biological Membranes

What are the roles of membranes

• Separates the components of the cell from their environments

• Separates the different organelles within cells from each other and the cytosol

• Acts as the interface for communication

• Uses diffusion, osmosis and active transport to move substances across it

What is the structure of phospholipids

• Only 2 fatty acids (the third has been replaced by a phosphate ion)

• Contains a phosphate group, glycerol and 2 fatty acid tails

What is the structure of the plasma membrane

• Formed by a phospholipid bilayer

  • The hydrophilic phosphate heads of the phospholipids form both the inner and outer layer of the membrane

  • The hydrophobic fatty acid tails form the hydrophilic centre

• Fluid - phospholipids and proteins are free to move within the layer (mostly sideways) via diffusion

• Mosaic - The proteins embedded in the layer vary in shape, size and position (in the very same as the tiles of the mosaic

What is a micelle

• If a phospholipid is mixed/shake with water they form spheres called a micelle

• Hydrophilic head faces outwards

• Hydrophobic fatty acid tails face inwards

Name the parts of the phospholipid bi-layer protein

• A = Glycolipid

• B = Extrinsic/peripheral protein

• C = Glycoprotein

• D = Carbohydrate part of the glycoprotein

• E = Phospholipid

• F = Cholesterol

• G = Transport/Intrinsic/Integral Protein

What is cholesterols role in the phospholipid bi layer

• Regulates the fluidity of the membrane by being between the phospholipids

• They prevent phospholipids from packing to closely together and crystallising

• Also allows membrane to be impermeable to ions, to increase strength & stability

What is the structure of cholesterol

Lipid with a hydrophilic end and a hydrophobic end

What makes membranes less fluid

• More saturated fatty acid chains, pack together tightly, high number of intermolecular forces between the chains

  • This is due to the double bonds of the saturated fatty acid chains having a double bond, meaning there’s less space and therefore less movement

• Lower temperatures, molecules have less energy, not moving as freely, structure becomes more closely packed

What makes membranes more fluid

• More unsaturated fatty acid chains, pack tether less tightly, less molecular forces between the chains

• Higher temperatures, molecules have more energy, move freely, structure becomes more fluid

What are the roles of glycolipds and glycoproteins in the membrane

• Can act as receptor molecules by binding to surfaces at the cells surface

• Cn be involved in:

  • Signalling receptors for hormones and neurotransmitters

  • Receptors involved in endocytosis

  • Receptors involved inn cell adhesion and stabilisation

  • Cell markers/antigens for cell-to-cell communication (glycolipids)

  • Some drugs work by binding to cell receptors. E.g. Beta Blockers

What is the structure of a glycoprotein

• Intrinsic proteins embedded in the cell surface membrane

• Carbohydrate chains of varying lengths and shapes are attached

What is the structure of a glycolipid

Lipids with attached carbohydrate chains

What are the roles of intrinsic proteins

• Channel (pore) proteins: Create hydrophilic channels which allow ions and polar molecules to travel through membranes

• Carrier proteins - the protein changes shape to allow this to happen

• Each transport protein is specific to what it is transporting

• The more intrinsic proteins inside a membrane, the faster the rate of diffusion

• They are embedded through both layers of a membrane

How do membranes act as a site of chemical reactions

• The enzymes of photosynthesis are found on membrane stacks within the chloroplasts

• The proteins must be in particular positions for this to work

• For example, the electron carriers and the enzyme ATP must be in the correct positions within the cristae for the production of ATP in respiration