Biological Membranes

• plasma membrane (cell membrane)


  a partially permeable membrane consisting of a fluid phospholipid bilayer and proteins
• what is the size of a plasma membrane

  7nm
• fluid mosaic model of membrane structure


  states that a membrane is a fluid structure with a "mosaic" of various proteins embedded in it. phospholipids are free to move and proteins embedded are randomly arranged

  
• phospholipid bilayer


  A double layer of phospholipids consisting of phosphate and lipids
• Structure of phospholipid bilayer

  their hydrophilic heads orientate towards the water. Their hydrophobic tails orientate away from the water towards the other fatty acid tails

  
• structure of cholesterol


  Cholesterol molecules are made up of four rings of hydrogen and carbon atoms. They are hydrophobic and are found among the hydrophobic tails in the lipid bilayer.
• what does the hydrophobic tail consist of in a phospholipid?


  two strings of hydrogen and carbon atoms called fatty acid chains
• function of cholesterol


  important for maintaining the level of fluidity of the membrane, and because it is hydrophobic and hydrophilic portion, it allows it to bind to the phospholipids, so preventing them to become too fluid.

  also prevents fatty acids to crystallise.
• examples of intrinsic proteins?

  channel proteins and carrier proteins
• what are peripheral proteins? examples?


  proteins attached to the surface of lipid bilayer. glycoproteins and glycolipids.
• 3 functions of proteins in general in the plasma membrane?

  enzymes to speed up chemical reactions, receptors for specific molecules, or transport materials across the cell membrane.
• function of transmembrane proteins

  to transport water soluble or charged particles through the membrane
• what is the size of a transmembrane channel?

  0.8 nm in diameter
• what is the glycolax?


  glycoprotein and glycolipid covering that surrounds the cell membranes of some bacteria, epithelia, and other cells.
• as the temperature increases, what component becomes more fluid?

  phospholipid bilayer
• what occurs in detail when the temperature increases in a phospholipid bilayer? 5

  1.the phospholipids vibrate more

  2. there is an increase in gaps in membrane

  3. bilayer becomes more fluid

  4. at 40 degrees, proteins are denatured

  5.this disrupts the membrane structure
• function of the glycolax?

  1. provides protection for plasma membrane. 2. form surface antigens by which the immune system can identify the cell
• 5 functions of cell membranes within cells?

  1. it can form vesicles for transport 2. seperates organelles from cytoplasm 3. it is the site of reactions 4.it provides surface for attachment of enzymes or ribosomes 5. controls what substances enter/leave
• what is cell signalling?


  communication between cells
• how do cell surface membranes contribute to the process of cell signalling?

  signal molecules are released by exocytosis and glycoproteins (or glycolipids) have receptors that are specific to signal as they have a complementary shape. the attachment of signal molecule causes changes inside cells. the cell surface membrane allows entry of some signal molecules
• structure of glycoproteins


  Protein with a chain of carbohydrates attached.
• function of glycoprotein?


  carbohydrate chains can form hydrogen bonds with water, stabilising the membrane structure. can act as receptors for hormones in cell signalling.
• Glycolipids


  phospholipid with a chain of carbohydrate molecules attached.
• Glycocalyx functions


  in cell adhesion, recognition and immunity.
• carrier proteins


  a protein that transports substances from a low concentration to a high concentration across a cell membrane.

  
• aquaproins


  channel protein that allows water to pass through membrane
• Enzyme


  allows reactions to take place on the membrane surface.
• what can pass through the phospholipid bilayer?

  small, non-polar molecules can pass (O2, CO2)
• what cannot pass through the phospholipid bilayer?

  large, polar molecules cannot pass (glucose,amino acids and water)
• how do receptors change cells? 3

  1. release a secondary messenger inside the cell, which can initiate a variety of effects

  2. opening or closing of a protein channel

  3.activate an enzyme
• Diffusion


  Movement of molecules from an area of higher concentration to an area of lower concentration (it is passive, it doesn't require ATP).
• facillitated diffusion


  movement of molecules from an area of high concentration to low concentration across a partially permeable membrane via protein channels or carrier proteins (it is passive, it doesn't require ATP).
• factors affecting simple diffusion

  Temperature, surface area, concentration gradient, diffusion gradient and molecule size.
• temperature


  as the temperature increases so does kinetic energy therefore increasing the rate of diffusion.
• surface area


  the larger the surface area to volume ratio the faster the rate of diffusion.
• concentration gradient


  the steeper the gradient the faster the rate of diffusion.
• diffusion distance


  the greater the distance over which diffusion must occur, the longer it takes.
• Molecule size


  the smaller the molecule the faster the rate of diffusion.
• factors affecting facilitated diffusion (in addition to simple diffusion)


  the number of channels/carriers in the membrane and whether the channel/carrier is open/closed.
• Osmosis


  passage of water molecules down their water potential gradient across a partially permeable membrane.