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Membrane Structure and Function

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Presentation on theme: "Membrane Structure and Function"— Presentation transcript:

1 Membrane Structure and Function
Chs. 8 and 11

2 Cell Membrane – Introduction
Separates the living cell from its nonliving surroundings 8 nm thick Controls traffic into and out of the cell (selectively permeable) Composed of lipids (phospholipids) and proteins, but include some carbohydrates

3 Cell Membrane – Introduction
Phospholipids and most other membrane constituents are amphipathic molecules Amphipathic molecules have both hydrophobic and hydrophilic regions Described by the fluid mosaic model

4 Membrane Model Development
1895 – Charles Overton; hypothesized membranes made of lipids Observed lipid-soluble substances move across membrane easier than lipid-nonsoluble substances

5 1917 – Irving Langmuir; Dissolved phospholipids in benzene and mixed with water When benzene evaporated, phospholipid film formed on water

6 1925 – E. Gorter and F. Grendel concluded membrane must be bilayer of phospholipids
Polar phosphorous head interacts with polar water (hydrophilic) Nonpolar fatty acid tails are sheltered from the water (hydrophobic)

7 Experiments showed real membranes attract water stronger than artificial ones
Hypothesis: proteins aid in water attraction 1935 – H. Davson and J. Danielli propose sandwich model: bilayer between layers of proteins

8 Davson-Danielli model considered dominant, even after EM images
Two problems: Membranes differed in size, composition, and stained appearance Membrane proteins are amphipathic; can’t be on surface only

9 Fluid Mosaic Model 1972 – S.J. Singer and G. Nicolson present revised model; hypothesize proteins are distributed throughout and among the bilayer

10 Membranes are fluid Membrane molecules are not held together by bonds; they can slip/move past/around each other Evidence: when human and mouse cells are fused together, membrane proteins don’t stay separated.

11 Most membrane molecules can move laterally; rarely do they flip-flop
Some proteins can’t move; bound to the cytoskeleton

12 Fluidity influenced by two factors:
Temp: As temp decreases, lipids pack closer together – become more solid Saturation: unsaturated fatty acid tails make the membrane more fluid

13 Cholesterol is wedged in the plasma membrane
Warm temps: it restrains the movement of phospholipids and reduces fluidity Cool temps: it maintains fluidity by preventing tight packing

14 Membranes are mosaics

15 Membranes are mosaics Membranes each have a unique collections of proteins Membrane functions determined mostly by proteins Two types of membrane proteins: Peripheral proteins: not embedded in lipid bilayer Integral proteins: penetrate the hydrophobic core of lipid bilayer, often completely spanning the membrane (transmembrane protein)

16 Membranes are mosaics Membranes have distinctive inside and outside faces The outer surface has carbohydrates This asymmetrical orientation begins during synthesis of new membrane in the endoplasmic reticulum

17 Membranes are mosaics Membrane protein functions:

18 Cell-Cell Recognition
Ability of a cell to distinguish one type of neighboring cell from another The membrane plays the key role in cell-cell recognition Cells recognize other cells from surface molecules, often carbs, on membrane Glycolipids Glycoproteins (more common)

19 Cell-Cell Recognition
Carbs on external side of membrane vary from species to species, individual to individual, and even from cell type to cell type within the same individual Variation marks each cell type as distinct The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells It is also the basis for rejection of foreign cells by the immune system This attribute is important in cell sorting and organization as tissues and organs in development

20 Transport Membranes act as gatekeepers (selectively permeable)
Select based on size and charge Small, uncharged atoms/molecules don’t have problems Large and/or charged atoms/molecules do have problems Proteins can help transport

21 Transport Proteins Each transport protein is specific as to the substances that it will translocate Some act like a channel or tunnel through the membrane Others bind to their specific molecules and physically carry them across the membrane

22 Passive Transport No E required
Requires gradient (separation of concentrations) Movement from areas of Hi to Low (down, along, or with) concentrations Movement continues even after equilibrium is reached Rate of diffusion depends on size and charge of molecules (interaction with the membrane)

23 Passive Transport Simple Diffusion: movement of molecules from Hi to Low concentrations

24 Passive Transport Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances

25 Passive Transport Osmosis: diffusion of water across a semi-permeable membrane Osmosis continues until the solutions are isotonic When two solutions are isotonic, water molecules move at equal rates from one to the other, with no net osmosis

26 Passive Transport A solution with a higher concentration of solutes is hypertonic A solution with a lower concentration of solutes is hypotonic These are comparative terms Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water Solutions with equal solute concentrations of solute are isotonic

27 Passive Transport

28 Passive Transport Paramecia have contractile vacuoles to expel excess water

29 Passive Transport Facilitated diffusion: diffusion using “helper” molecules Those atoms and molecules that were too big or charged can still move down their concentration gradient (hi to low)

30 Passive Transport: Facilitated diffusion
Some proteins (channel) act like corridors Allow for fast, bulk flow Ex: aquiporins

31 Passive Transport: Facilitated diffusion
Some channel proteins (gated channels) open or close depending on the presence or absence of a physical or chemical stimulus The chemical stimulus is usually different from the transported molecule Ex: when neurotransmitters bind to specific gated channels on the receiving neuron, these channels open This allows sodium ions into a nerve cell When the neurotransmitters are not present, the channels are closed

32 Passive Transport: Facilitated diffusion
Some proteins change shape to physically translocate the molecules These shape changes could be triggered by the binding and release of the transported molecule

33 Transport proteins are much like enzymes
They may have specific binding sites for the solute Transport proteins can become saturated when they are translocating passengers as fast as they can Transport proteins can be inhibited by molecules that resemble the normal “substrate” When these bind to the transport proteins, they outcompete the normal substrate for transport

34 Active Transport Requires E (ATP)
Movement of molecules against or up their concentration gradients Low to Hi Performed by receptor proteins

35 Active Transport The sodium-potassium pump actively maintains the gradient of sodium (Na+) and potassium ions (K+) across the membrane Typically, an animal cell has higher concentrations of K+ and lower concentrations of Na+ inside the cell The sodium-potassium pump uses the E of one ATP to pump three Na+ out and two K+ in



38 Ions keep separate charges across a membrane
Membrane potential: voltage difference across the membrane Electrochemical gradient Gradient due to concentrations of ions Gradient due to membrane potential electrogenic pumps generate voltage gradient

39 Ions keep separate charges across a membrane
In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting H+ out of the cell Proton pumps in the cristae of mitochondria and the thylaloids of chloroplasts, concentrate H+ behind membranes These electrogenic pumps store energy that can be accessed for cellular work.

40 Cotransport A single ATP-powered pump that transports one solute can indirectly drive the active transport of several other solutes through cotransport via a different protein As the solute that has been actively transported diffuses back passively through a transport protein, its movement can be coupled with the active transport of another substance against its concentration gradient

41 Plants commonly use the gradient of H+ that is generated by proton pumps to drive the active transport of amino acids, sugars, and other nutrients into the cell The high concentration of H+ on one side of the membrane, created by the proton pump, leads to the facilitated diffusion of protons back, but only if another molecule, like sucrose, travels with the H+

42 Endo- vs. Exocytosis Both move large molecules into/out of the cell
Both use vesicles Reverse processes of each other

43 Endocytosis A small area of the plasma membrane sinks inward to form a pocket The pocket deepens, pinches in, and forms a vesicle containing the material that had been outside the cell

44 Endocytosis Two types: Receptor mediated endocytosis
Phagocytosis: cell eating Pinocytosis: cell drinking Receptor mediated endocytosis

45 Receptor mediated Endocytosis
Triggered when extracellular substances bind to special receptors, ligands, on membrane surface, especially near coated pits

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