transport/ cell membrane

• The plasma membrane (also known as the cell membrane) is a biological membrane that separates the interior of a cell from its outside environment.​
• The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components —including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character.
• • N.B. Cell membranes are fluid, meaning they are not fixed in position. Structures in the cell membrane are moving.​
  • The plasma membrane that surrounds these cells has two layers (a bilayer) of phospholipids​
• label

  A) glycoprotein

  B) carbohydrate

  C) hydrophilic head

  D) phospholipid bilayer

  E) phospholipid molecule

  F) hydrophobic tail

  G) alpha helix protein (integral protein)

  H) surface protein

  I) globular protein

  J) peripheral protein

  K) glycolipid

  L) cholesterol

  M) globular protein

  N) protein channel

  
• The phosopholipids allows the cell membrane to be selectively permeable.​

  ​
• • Only small uncharged molecules can diffuse freely through phospholipid bilayers​
  • Small molecules, such as O2 and CO2, are soluble in the lipid bilayer and therefore can readily cross cell membranes. ​
  • but larger uncharged molecules, such as glucose, cannot. ​
  • Charged molecules, such as ions, are unable to diffuse through a phospholipid bilayer regardless of size.​
• integral proteins:​

  As their name suggests, integrated (embedded)  into the membrane. span from one side of the phospholipid bilayer to the other.​
• Many integral proteins are carrier molecules or channels.​ These help transport substances, such as ions, sugars and amino acids, that cannot diffuse across the membrane because they are too large or too Hydrophilic. Other integral proteins are receptors for hormones and neurotransmitters, or enzymes for catalyzing reactions.​
• Peripheral membrane proteins, also called extrinsic proteins, are only temporarily associated with the membrane. ​

  Most peripheral membrane proteins are hydrophilic (water loving) so usually they are either attached to integral membrane proteins, or they can directly bound to a polar head group of the bilayer.​

  • they tend to be more loosely attached.​
• peripheral protein:​

  • They function as receptors for hormones or neurotransmitters​
  • Extrinsic proteins functions as recognition centers, where they act ‘markers’ to identify the cell as ‘friendly’.​
• cholesterol​- Like phospholipids, cholesterol molecules have hydrophilic heads and hydrophobic tails, so they fit neatly between the phospholipid molecules with their heads at the membrane surface​
• • Hydrophobic and interact with the fatty acid tails. ​
  • Gives Stability to the membrane and makes the membrane less fluid at high temperatures. ​
  • Helps prevent the leakage of water and ions from the cell.​
  • The cholesterol holds the phospholipids together so that they don’t separate too far,, or compact too tightly, restricting movement across the membrane.​
  • It makes it more fluid in very cold temperatures, by not allowing the membrane to come in too close.​
  •  In too warm temperatures it decreases fluidity.​
• Carbohydrates are the third major component of plasma membranes. ​

  In general, they are found on the outside surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids).​
• • These are lipids and proteins with carbohydrate chain attached.​
  • They act as cell recognition molecules- They recognize and bind to carbohydrate receptors  on adjacent cells, leading to cell-cell attachment as well as intracellular responses in the interacting cells.​
  • Help maintain the stability of the membrane.​
  • Help cells in tissues attach to each other.​
• • The surface area refers to the total area of the organism that is exposed to the external environment. ​​
  • The volume refers to the total internal volume of the organism (total amount of space inside the organism)​
• Diffusion is the net movement of particles down a concentration gradient: from a region of high concentration to a region of low concentration.​ No metabolic energy is expended during diffusion so it is an example of passive transport.
• • Small organisms like the amoeba do not need a circulatory system but rely on diffusion to get the materials they need.​
  • These small organisms obtain oxygen through their cell membrane, they have a very large surface area to volume ratio.​
  • Therefore, If an organism is small and has a large surface area to volume ratio, all the nutrients and respiratory gases can be taken in by diffusion across the body surface.​
• As the surface area of the membrane increases, the rate of diffusion also increases, as there is more space for molecules to diffuse across the membrane.

  Having a larger SA : V ratio maximizes the amount of area exposed for an organism.With more area exposed, there is a greater possibility for diffusion of gases and nutrients to occur.§ A greater surface area and smaller volume also enables diffusion to happen more efficiency.
• §As the surface area of the membrane increases, the rate of diffusion also increases, as there is more space for molecules to diffuse across the membrane.

  §Having a larger SA : V ratio maximizes the amount of area exposed for an organism.

  § With more area exposed, there is a greater possibility for diffusion of gases and nutrients to occur.

  § A greater surface area and smaller volume also enables diffusion to happen more efficiency.
• All cells are surrounded by a partially-permeable membrane that controls what substances can enter and exit the cell.A cell needs to be able to import the substances it needs to survive, and to export waste materials and substances that are needed outside the cell
• FACILITATED DIFFUSION-Large polar (charged) molecules, such as glucose and amino acids, cannot diffuse through the phospholipid bilayer. Nor can ions such as sodium (Na+) or chloride (Cl−). These can only cross the membrane with the help of certain protein molecules.
• Facilitated diffusion is the diffusion of a substance through transport proteins in a cell membrane without the use of energy.
• Facilitated diffusion is just like ordinary diffusion, except that molecules or ions only get through the membrane if they bump into a channel.

  There are two kinds of transport protein:

  .Channel proteins

  .Carrier proteins
• Channel proteins Channel proteins are water-filled pores. They allow charged substances, usually ions, to diffuse through the membrane
• Most channel proteins are ‘gated’. This means that part of the protein molecule on the inside surface of the membrane can move to close or open the pore, like a gate.This allows control of ion exchange.
• There are two types of carrier proteins:

  .The first type, involve in facilitated diffusion, does not require energy to move the substance across the cell membrane.

  .The second type, active transport, does use energy.
• Carrier proteins can change their shape to move a target molecule from one side of the membrane to the other.

  The substance will bind on the side where it at a high concentration and be released where it is at a low concentration.

  As a result, the binding site is alternately open to one side of the membrane, then the other

   
• In general, channel proteins transport molecules much more quickly than do carrier proteins.

  This is because channel proteins are simple tunnels; unlike carrier proteins, they don’t need to change shape and “reset” each time they move a molecule.

  A typical channel protein might facilitate diffusion at a rate of tens of millions of molecules per second, whereas a carrier protein might work at a rate of a thousand or so molecules per second
• What is simple diffusion?

   Diffusion is the movement of molecules from an area of high concentration of the molecules to an area with a lower concentration until evenly distributed.
• Osmosis-water always moves down a water potential gradient. This will happen until the water potential is the same throughout the system, at which point we can say that equilibrium has been reached.

  A solution containing a lot of water has a high concentration of water and is a dilute solution.

  A solution containing a lot of solutes and little water has a low concentration of water and is a concentrated solution.
• Active transport-Active transport is the movement of molecules or ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP.

  It is achieved by carrier proteins, each of which is specific for a particular type of molecule or ion.

  However, unlike facilitated diffusion, active transport requires energy, because movement occurs up a concentration gradient.
• Active transport-The energy is supplied by the molecule ATP (adenosine triphosphate) which is produced during respiration inside the cell.

  The energy is used to make the carrier protein change its shape, transferring the molecules or ions across the membrane in the process
• The coupled transport of two distinct molecules is called co-transport.

  •Co-transport is the coupled movement of substances across a cell membrane via a carrier protein

  •It involves a combination of facilitated diffusion and active transport

  •A well-known example of a co-transporter protein can be found on the cell surface membrane of the epithelial cells lining the mammalian ileum