Principles of cellular exchange+transportation mechanisms

• • The cell membrane is selectively permeable. ​
  • It lets some substances pass through rapidly and some substances pass through more slowly but prevents other substances passing through it at all.​
• Extrinsic proteins occur on the surface of the bilayer, or are partly embedded in it. They provide structural support. They also form recognition sites by identifying cells.
• Intrinsic proteins span (go right through) the phospholipid bilayer; some act as channels or carriers to facilitate the diffusion of molecules.
• • Lipid soluble non-polar substances can move through the membrane more easily than water-soluble (polar) substances (carbon dioxide, oxygen, steroid, hormones, lipids and fat-soluble vitamins). ​
  • Non-polar molecules - Small uncharged molecules freely pass through the membrane by simple diffusion. 
• Polar molecules require proteins. Polar substances such as water, glucose, amino acid or inorganic ions cannot diffuse through the hydrophobic core of the bilayer. 
• Intrinsic proteins (proteins which extend across both phospholipid layers) allow these particles to cross the membrane. Channels and carriers allow facilitated diffusion (diffusion helped by an intrinsic protein). Pumps carry out active transport.
• Diffusion = The movement of molecules from an area of high concentration to one of lower concentration until an equilibrium is reached.
• Simple  diffusion – small non-polar molecules (carbon dioxide, oxygen, steroid, hormones, lipids and fat-soluble vitamins). Does not require protein channels 
• Simple  diffusion – small non-polar molecules (carbon dioxide, oxygen, steroid, hormones, lipids and fat-soluble vitamins). Does not require protein channels 
• • Charged particles or ions and large molecules such as glucose do not readily pass through the cell membrane because they are insoluble in lipid. ​
  • In the cell membrane intrinsic protein molecules span the membrane from one side to the other and help such particles to diffuse in or out of the cells. ​
  • Facilitated diffusion – large Polar molecules (water, glucose and ions) diffusion is assisted by proteins in the membrane. There are two types of proteins which facilitate (help) diffusion – channels and carriers.
• • Channel proteins span the membrane. Channels are very selective and will accept only one type of molecule (or a few closely related molecules) for transport. ​
  • Gated channels open and close specific for particular ions such as sodium or calcium.​
  • Carrier proteins have specific sites that match the molecule to be transported e.g. a specific amino acid
• • Carrier proteins have specific sites that match the molecule to be transported e.g. a specific amino acid​
  • When the amino acid binds to the site the carrier protein changes its shape.​
  • The amino acid molecule can then pass through the protein to the other side of the membrane.​
  • Once the amino acid has left the carrier protein the protein reverts back to its original shape.
• Facilitated diffusion:

  Channel proteins consist of pores. This allows charged ions to pass through (such as Na+ ). Each channel protein is specific for one type of ion. They can also open and close depending on the needs of the cell (these are called gated channels).
• The rules of diffusion, substances move only from high concentration to low concentration.
• Carrier proteins - A particular molecule attaches to a carrier protein at its binding site and causes the carrier protein to change shape or rotate within the membrane; this action releases the molecule on the other side of the membrane.
• • In biological systems osmosis is a special form of diffusion which involves the movement of water molecules only.​
  • Osmosis is the passage of water from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
• • At high concentrations (of water molecules) water has a greater potential energy i.e. the water molecules are completely free to move about.​
  •  When a solute, such as sugar, is dissolved in water there are proportionally fewer water molecules to move about and the water potential of the solution is lowered (becomes more negative). 
• Active transport is an ATP requiring process in which ions and molecules are moved across membranes against a concentration gradient. Ions and molecules can move in the opposite direction to diffusion