Biological Membranes

• compartmentalisation: vital to a cell as metabolism involves many different and incompatible reactions.

  •  Containing reactions in separate parts of the cell allows the specific conditions required for each cellular reaction.

  •   It also allows chemical gradients to be produced.
• Phospholipids

  •       The phosphate head is hydrophilic and the

  lipid tail hydrophobic

  •       If phospholipids are completely surrounded by water, a bilayer can form.

  •       Phosphate heads on each side of the bilayer stick into the water, while the hydrophobic fatty acid tails point towards each other.

  •       This means the hydrophobic tails are held away from the water molecules.

  •       The phospholipid molecules can move freely within the plane of the membrane.
• Phospholipids 2:

  • They tend to stay on their own side but very rarely some may ‘flip flop’ from one monolayer to the other.

  •       The hydophilic head cannot easily pass through the hydrophobic region in the middle of the bilayer.

  •       This gives the bilayer some stability even though it is not actually bonded together.
• Membrane

  •       The phospholipid bilayer is the basic structural component for all biological membranes.

  •       It creates a barrier separating the cell form the outside world.

  •       Membranes cannot be seen with light microscopes but they can be seen with EMs.

  •       They appear as two dark ‘tramlines’ (the phospholipid heads) and a pale region (the fatty acid tails).

  •       Membranes are around 7 – 10 nm thick.

  •       It is Partially Permeable - Allows water and some solutes to pass across it, the rest has to move via proteins.
• Prokaryotes

  •       One outer cell surface membrane. No membrane bound organelles
• Eukaryotes

  •       Outer cell membrane and internal organelles surrounded by membranes

  •       This has allowed compartmentalisation and specialisation. Thus allowing eukaryotes to become more complex and highly successful
• Permeability of the phospholipid membrane

  •       Lipids provide a cohesive structure but they act as a barrier to the movement of polar molecules and ions.

  •       Water therefore can struggle to pass through.
• Membrane Proteins

  •       Membrane proteins are globular proteins. Globular proteins -  Spherical and water soluble.

  •       They can be

  •       Enzymes

  •       Receptors

  •       Antigens

  •       Channel proteins

  •       Carrier Proteins / Protein pumps

  •       Electron carriers
• Intrinsic

  •       Intrinsic proteins – also known as integral proteins. They are embedded through both layers of the membrane.

  •       They have amino acids with Hydrophobic R-groups that interact with the hydrophobic core of the membrane keeping them in place.

  •       Channel Proteins and Carrier Proteins are examples of intrinsic proteins.

  •       Channel Proteins – Hydrophilic channel

          that allows passive movement of polar molecules and ions

  •       Carrier Proteins – Involved in facilitated

          diffusion and active transport.
• Extrinsic

  •       They are peripheral proteins that are only present on one side of the bilayer.

  •       They have hydrophilic R-groups on the outer surface and interact with the polar heads of the phospholipids or the intrinsic proteins.

  •       They can be present on either layer and some can pass from one layer to the other.
• Carbohydrates

   

  •       Short chain polysaccharides.

  –      Glycoproteins – attached to proteins.

  –      Glycolipids – attached to lipids.

  –      Together they form the the glycocalyx its roles are

  –      Cell-cell recognition

  –      Receptors for chemical signals e.g. hormones.

  –      Assist in the binding of cells to form tissues.
• Glycoproteins

  •       Involved in cell adhesion and receptors for chemical signals.

  •       Chemical signals – When a chemical binds to the receptor it causes a direct response or cascade of events. The chemical will travel from one cell to another and cause a response in that cell.

  •       E.g. Binding of neurotransmitters triggers or prevents an impulse in the next neurone.

  •       E.g. Insulin can cause the uptake of glucose by cells.
• Glycolipids – Similar to Glycoproteins

  •       Act as cell markers or antigens
• The Fluidity of the membrane

  •       The phospholipids are a mixture. Some have saturated and some are unsaturated fatty acid tails.

  A large no of unsaturated fatty acid tails makes the membrane more fluid and many saturated fatty acid tails make it less fluid.
• Cholesterol

  •       The fluidity of the membrane is also reduced by the presence of cholesterol. This is caused by it reducing the movement of the phospholipids

  •       It is important for the membrane to remain fluid. It has been shown that some organisms change the  percentage of saturated and unsaturated fatty acids and cholesterol in their membrane depending on the temperature.
• Cholesterol 2:

  •       Cholesterol is a lipid  that has hydrophobic and hydrophilic ends. It adds stability without making the membrane too rigid.

  •       Cholesterol binds to the hydrophobic tails of phospholipids causing them to pack closer together.
• Hijacking Receptors

  •       Viruses enter cells by binding with receptors on the cell’s plasma membrane that normally bind with the host’s signalling molecules.

  •       This is how HIV can infect humans

  •       The HIV virus enters the cells of the immune system.

  •       It’s shape allows it to fit into the receptor sites of the helper T – lymphocytes.

  •       Once HIV enters this cell it enters a period of inactivity.

  •       It will then reproduce inside the cell and eventually destroy it.
• Temperature:

  ·       As the kinetic energy increases, the phospholipids move more and the gaps between them increase. This causes the membrane to become more permeable.

   

  ·       At high temperatures the protein channels and protein carriers denature. This means that any molecule can now travel through.
• Solvents (a substance that dissolves a solute resulting in a solution)

  ·       Water is a polar solvent

  o   This is very important as the non-polar tales of the phospholipids are orientated away from the water.

  o   The charge phosphate heads interact with the water.

  o   This arrangement ensures that but the membrane remains intact.

  ·       Many organic solvents are less polar than water e.g. ethanol. Or they are not polar at all eg benzene.
• Solvents 2:

  ·       The nonpolar or less polar solvent will disrupt the membrane. This causes the cell to break apart and die.

  ·       This is why alcohol is used in antiseptic wipes. It disrupts the bacterial membranes and kills them.

   

  ·       Alcoholic drinks do not have as much alcohol in them so do not dissolve membranes. However they can disrupt them.

   

  ·       Neurons need intact membranes to transmit nerve impulses, therefore a disrupted membrane can affect these impulses.
• Simple Diffusion

  •       Molecules like oxygen and carbon dioxide pass

  through the membranes unassisted. As they are very small.

  •       Some water will also pass through the membrane

  using this route. This occurs more easily where the

   fluid-mosaic contains phospholipids with unsaturated

  hydrocarbon tails as they are spaced wider apart.
• Facilitated diffusion

  •       Moves with the concentration gradient.

  So no energy required.

  •       Charged particles e.g. sodium ions or large molecules

   e.g. glucose cannot pass through the lipid bilayer.

  •       Protein carriers in cell membrane are needed to

  transport molecules across the membrane.

  •       Protein carriers are specific (they have specific binding sites).

  So they will only let through substances that match this shape.

  E.g. glucose protein carrier will only transport glucose.

  •       Specific shape is due to the tertiary structure of protein.
• Simple Diffusion:

  •       Even though water is polar (charged).

  •       The rest of the water will travel through hydrophilic channels.

  •       Membrane is made of phospholipids

  •       Fat soluble molecules can pass through the bilayer.

  •       Steroid hormones are lipid-based.
• Channel Proteins

  •       Form pores in the membrane.

  •       Shaped to allow only one type of ion through (Tertiary structure of the protein).

  •       Many are ‘gated’ so that they can be opened and closed.

  •       Gated sodium ion channels are involved in the nervous system.
• Carrier Proteins

  •       Shaped due to the tertiary structure of the protein.

  •       This only allows a specific molecule through.

  When the specific molecule fits the carrier protein changes shape to allow the molecule through
• Active Transport:

  •       Sometimes cells must move materials

   into the cell against the concentration

  gradient.

  •       E.g. Plants need magnesium ions to

  make chlorophyll. Magnesium ions are often

  in short supply in the soil so the plant

  moves them into it’s cells against

  the concentration gradient.

  •       E.g. active transport helps us absorb

   glucose from our intestines.
• Ensuring One Way Flow

  •       The molecule to be transported

  across the membrane only fits into the

   protein on one side of the membrane.

  •       ATP changes the shape of the protein.

  •       The molecule cannot travel back into

  the cell as it will no longer fit as the protein is a different shape.
• Active Transport

  •       Molecule or ion binds to receptors in the channel or carrier protein.

  •       ATP binds to carrier protein and is hydrolysed into ADP and phosphate causing the carrier protein to change shape.

  •       The ion moves into the opening and is released on the other side of the membrane.

  •       The phosphate molecule is released and the carrier protein returns to its original shape.
• Bulk Transport

  •       There are 2 types of cytosis. Endocytosis – into cell. Exocytosis – out of cell

  •       Requires energy and involves the breaking open and reforming of the phospholipid bilayer. 

  •       Energy is also needed to from vesicles and to move them around the cell.
• Osmosis

  •       Osmosis – The net movement of water molecules from an area of high water potential (dilute solution), to an area of low water potential (concentrated solution). Across a partially permeable membrane.

  •       The symbol for Water potential is psi ᴪ.

  •       Water potential is the pressure exerted by water molecules on the membrane. They are continually moving randomly and collide with the membrane. It is measured in Pa or kPa.

  •       No energy
• Osmosis 2:

  •       Passive it cannot be stopped.

  •       If you have two solutions with different water potential then osmosis will occur.

  •       During osmosis water molecules will be moving in both directions due to their kinetic energy. However, their will be a net movement of water molecules from high water potential to low water potential.

  •       Once the water potential reaches equilibrium osmosis will stop.

  •       However water will still be moving across the membrane equally in each direction.
• Low water potential

  •       E.g. in a strong sugar solution

  •       The solutes in the solution restrict the movement of water molecules. Thus reducing their kinetic energy and water potential.

  •       They do this due to the attractive forces that exist between the solute molecules and the water molecules (hydrogen bonds).
• High Water Potential

  •       There are fewer solutes in the solution so less attraction therefore it has more kinetic energy.

  •       Highest water potential is found in pure water, where there are no solutes.

  •       Hypertonic - Solution surrounding the cell has a lower water potential than the cell. Water passes out of cell into solution.

  •       Hypotonic solution- The solution surrounding the cell has a higher water potential than the cell. Water moves into the cell via osmosis

  •       Isotonic - Same strength in cell as outside.