1. Unit 4 Notes: Membrane Structure IB Biology

2. Phospholipids The backbone of the membrane is a bilayer that is produced from large numbers of molecules called phospholipids. Each phospholipid is composed of glycerol (a 3 carbon compound) Two of the glycerol carbons have fatty acids attached. The third carbon is attached to a polar organic alcohol that includes a bond to a phosphate group.

3. Phospholipids Fatty acids are non-polar and therefore, not soluble in water. The organic alcohol is polar and is soluble in water. Because of this structure membranes have two distinct areas of polarity and water solubility. The polar, water soluble portion is referred to as hydrophilic (water loving). The nonpolar, water insoluble portion is referred to as hydrophobic (water fearing).

4. Phospholipids The fatty acid tails of the phospholipid molecules do not strongly attract to one another, so the membrane tends to be fluid and flexible. Animal cells have variable shapes because of this fluidity. The overall structure of the membrane is maintained by the tendency that water has to form hydrogen bonds.

5. Cholesterol In order for membranes to function properly, they must be fluid. They tend to have the consistency of olive oil. There are cholesterol molecules within the fatty acid tails in animal cells. These molecules determine membrane fluidity, which changes with temperature.

6. Cholesterol The cholesterol molecules allow the membrane to function at a wider range of temperatures than it would if they were not present. Plant cells do not have cholesterol molecules; they depend on saturated or unsaturated fatty acids to maintain membrane fluidity.

7. Proteins The proteins create diversity for membrane function. Proteins are embedded in the fluid matrix of the phospholipid bilayer. These proteins create the mosaic effect of the fluid mosaic model. There are two major types of proteins (integral proteins and peripheral proteins).

8. Proteins Integral proteins have both hydrophobic and hydrophilic regions in the same protein. – The hydrophobic region contains non-polar amino acids, is within the mid-section of the membrane, and holds the protein in place. – The hydrophilic region is exposed to the water solutions on either side of the membrane.

9. Proteins Peripheral proteins do not protrude into the middle hydrophobic region. They remain bound to the surface of the membrane. – These proteins are often anchored to an integral protein.

10. Proteins

12. Membrane protein functions There are many different proteins with many different functions within the cell membrane. The following are the general functions for proteins on membranes: – Hormone binding sites – Enzymatic action – Cell adhesion – Cell-to-cell communication – Channels for passive transport – Pumps for active transport

13. Membrane protein functions Proteins that serve as hormone binding sites have specific shapes exposed to the exterior of the cell that fit the shape of the specific hormone. – The attachment between the protein and the hormone cause a change in shape that result in a message being relayed to the interior of the cell.

14. Membrane protein functions Cells often have enzymes attached to membranes that catalyze chemical reactions. – These enzymes may be on the interior or exterior of the cell. – They tend to be grouped so that a sequence of reactions, called a metabolic pathway, may occur.

15. Membrane protein functions Cell adhesion is provided by proteins when they connect together in various ways – The connections can either be temporary or permanent. – These connections are called junctions; they may include gap junctions or tight junctions. Many cell-to-cell communication proteins are attached to carbohydrate molecules. They provide identification for other cells to recognize different types of cells or different species.

16. Membrane protein functions Some proteins contain channels that span the membrane so that substances can pass through. – The transport proteins can include passive transport, which moves materials through the channel from an area of high concentration to an area of low concentration. – In active transport, the proteins shuttle a substance from one side of the membrane to another by changing shape. This process requires energy in the form of ATP. It does not require a difference in concentration.

17. Passive and active transport Passive transport – Passive transport does not require energy in the form of ATP. It does, however, require a concentration gradient.

19. Passive and active transport Diffusion – Diffusion is the movement of particles from a region of high concentration to an area of low concentration. – Diffusion often requires a membrane in living things. – For example, oxygen gas moves outside of the cell to the inside of the cell to be used for cellular respiration. – The mitochondria use the oxygen gas when it is within the cell, thus creating a relatively lower oxygen concentration inside the cell than outside the cell. – Oxygen then diffuses into the cell. – Carbon dioxide will diffuse out of the cell because carbon dioxide is produced in cellular respiration.

20. Passive and active transport Facilitated diffusion – Facilitated diffusion is a type of diffusion that involves a membrane with carrier proteins that are capable of combining with the substance to aid in movement.

22. Passive and active transport Osmosis – Osmosis is the passive movement of water across a semi-permeable membrane. – Osmosis requires a concentration gradient. – The semi-permeable membrane only allows certain substances to pass through the membrane. – The concentration gradient allows the movement to occur based on a difference in solute concentrations on either side of the semi-permeable membrane.

24. Passive and active transport – Hyperosmotic (hypertonic) solutions have a higher concentration of solutes on the outside of the membrane than inside the membrane. – Hypo-osmotic (hypotonic) solutions have a lower concentration of solutes on the outside of the membrane than inside the membrane. – Iso-osmotic (isotonic) solutions have equal concentrations of solutes on both sides of the membrane. – Passive transport will continue until there is an equal concentration of substances on both sides of the membrane, which is called equilibrium.

26. Passive and active transport Simple diffusion Substances other than water move between phospholipid molecules or through proteins which possess channels. Facilitated diffusion Non-channel protein carriers change shape to allow movement of substances other than water. OsmosisOnly water moves through the membrane using aquaporins which are proteins with specialized channels for water movement.

28. Passive and active transport Size and charge – Substances that can move passively through the membrane are affected by two factors, size and charge. – Small substances that are non-polar move easily across the membrane. – Large substances that are polar do not move easily across the membrane.

29. Passive and active transport – Gases such as oxygen, carbon dioxide, and nitrogen are examples of small non-polar substances. – Ions such as chloride ions, potassium ions, and sodium ions have a hard time moving across the membrane passively. – Molecules such as water and glycerol are small, uncharged polar molecules that can move fairly easily across the membrane.

30. Passive and active transport Active transport and the cell – Active transport requires an input of energy in the form of ATP. – Active transport moves substances against a concentration gradient, which allows the cell to maintain interior concentrations of molecules that are different from the exterior environment.

31. Passive and active transport – For example, animal cells have a much higher concentration of potassium ions than the exterior environment, whereas sodium ions are more concentrated outside the cell. – The cell maintains a proper level of these ions by pumping potassium ions into the cell and sodium ions out of it. – An input of energy and a membrane protein must be involved for this active transport to occur.

32. Passive and active transport The sodium-potassium pump – A specific protein binds to three intracellular sodium ions. – Binding of these sodium ions causes phosphorylation by ATP. – The phosphorylation causes the protein to change its shape, thus expelling sodium ions to the exterior. – Two extracellular potassium ions bind to different regions of the protein and cause the release of the phosphate group. – Loss of the phosphate group restores the protein’s original shape thus causing the release of potassium ions into the intracellular space.

33. Passive and active transport The sodium-potassium pump illustrates the importance of proteins in active transport of particular substances and how important ATP is in active transport.

35. Endocytosis and exocytosis Endocytosis and exocytosis are necessary to move large materials into and out of the plasma membrane. Endocytosis allows macromolecules into the cell and exocytosis allows macromolecules to leave the cell. Membranes are fluid, therefore the phospholipid molecules are not closely packed together due to the loose connections between fatty acid tails. The membrane is also stable because of the hydrophilic and hydrophobic properties of the different regions of phospholipid molecules.

36. Endocytosis and exocytosis Endocytosis occurs when a portion of the plasma membrane is pinched off to enclose macromolecules. – The pinching involves a change in the shape of the membrane. – The result is the formation of a vesicle that enters the cytoplasm of the cell. – The ends of the membrane reattach because of the hydrophobic and hydrophilic properties of the phospholipids and the presence of water. – If the membrane weren’t fluid, this would not occur.

37. Endocytosis and exocytosis Exocytosis is the reverse of endocytosis. – Exocytosis usually begins in the ribosomes of the rough ER and progresses through a serious of four steps until the substance that is produced is secreted to the external environment.

38. Endocytosis and exocytosis 1.Protein produced by the ribosomes of the ER enters the lumen of the ER. 2.Protein exits the ER and enters the cis side of the Golgi apparatus; a vesicle is then involved. 3.As the protein moves through the Golgi apparatus, it is modified and exits the trans face inside a vesicle. 4.The vesicle with the modified protein moves to and fuses with the plasma membrane, which results in the secretion of the contents from the cell.

39. Endocytosis and exocytosis