1. 1 Membrane Structure and Function

2. 2 Plasma Membrane Overview Structure= Fluid mosaic Structure= Fluid mosaic of lipids and proteins bilayer - Lipid bilayer - Contains embedded proteins boundary Function= the boundary that separates the living cell from its nonliving surroundings - Selectively Permeable - Selectively Permeable (chooses what may cross the membrane)

3. 3 Fluid Mosaic Model fluid structure A membrane is a fluid structure with a “mosaic” of various proteins embedded in it when viewed from the top Phospholipidslaterally Phospholipids can move laterally a small amount and can “flex” their tails Membrane proteins laterall Membrane proteins also move side to side or laterally making the membrane fluid

4. 4 The Fluidity of Membranes Phospholipids in the plasma membrane Can move within the bilayer two ways Lateral movement (~10 7 times per second) Flip-flop (~ once per month)

5. 5 type of hydrocarbon tails The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane FluidViscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails The Fluidity of Membranes

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7. 7Phospholipids most abundant - Are the most abundant lipid in the plasma membrane amphipathic - Are amphipathic, containing both hydrophilic (head) and hydrophobic regions (tails) 1. Head- hydrophilic 1. Head- composed of phosphate group attached to one carbon of glycerol is hydrophilic tails hydrophobic 2. Tails- two fatty acid tails are hydrophobic

8. 8 Hydrophilic head Hydrophobic tail WATER Phospholipid Bilayer

9. Steroid Cholesterol steroid cholesterol The steroid cholesterol moderates membrane fluidity (different effects at different temperatures) Cholesterol http://telstar.ote.cmu.edu/biology/MembranePage/index2.html

10. 10 Membrane Carbohydrates - Interact with the surface molecules of other cells, facilitating cell-cell recognition Cell-cell recognition- I Cell-cell recognition- Is a cell’s ability to distinguish one type of neighboring cell from another

11. 11 Membrane Proteins 1972 membrane proteins inserted into the phospholipid bilayer of the plasma membrane In 1972, Singer and Nicolson, Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer of the plasma membrane Phospholipid bilayer Hydrophilic region of protein Hydrophobic region of protein

12. 12 Types of Membrane Proteins Integral proteins Penetrate the hydrophobic core of the lipid bilayer transmembrane proteins Are often transmembrane proteins, completely spanning the membrane EXTRACELLULAR SIDE

13. 13 Types of Membrane Proteins Peripheral proteins Are appendages loosely bound to the surface of the membrane

14. 14 Functions of Membrane Proteins Figure 7.9 1.Transport 2.Enzymatic activity 3.Signal transduction 4.Cell to cell recognition 5.Intercellular joining 6.Attachment to the cytoskeleton and extracellular matrix (ECM). ATP Enzymes Signal Receptor Glyco- protein

15. 15 Fibers of extracellular matrix (ECM)

16. 16 Synthesis and Sidedness of Membranes Membrane proteins and lipids ER and Golgi apparatus Membrane proteins and lipids are made in the ER and Golgi apparatus ER

17. 17 Membrane Permeability structure selective permeability Membrane structure results in selective permeability cell must exchange materials with its surroundings A cell must exchange materials with its surroundings, a process controlled by the plasma membrane

18. 18 Permeability of the Lipid Bilayer Polar molecules rapidly - Do NOT cross the membrane rapidly - Often assisted by transport proteins Hydrophobic molecules lipid soluble rapidly Are lipid soluble and can pass through the membrane rapidly

19. 19 Passive Transport Passive transport no energy Passive transport is diffusion of a substance across a membrane with no energy investment CO 2, H 2 O, and O 2 CO 2, H 2 O, and O 2 easily diffuse across plasma membranes Osmosis Diffusion of water is known as Osmosis

20. 20 Simple DiffusionDiffusion spread out evenly Is the tendency for molecules of any substance to spread out evenly into the available space high to low concentration Move from high to low concentration Down Down the concentration gradient

21. 21 Osmosis Osmosis - Is the movement of water across a semi-permeable membrane - Is affected by the concentration gradient of dissolved substances called the solution’s tonicity Tonicity- Is the ability of a solution to cause a cell to gain or lose water impact on cells without walls - Has a great impact on cells without walls

22. 22 Three States of Tonicity

23. 23 Hypertonic Solution hypertonic If a solution is hypertonic concentration of solutesgreater The concentration of solutes is greater than it is inside the cell lose water (PLASMOLYSIS) The cell will lose water (PLASMOLYSIS)

24. 24 Isotonic Solutions isotonic If a solution is isotonic concentration of solutes same The concentration of solutes is the same as it is inside the cell NO NET There will be NO NET movement of WATER

25. 25 Hypotonic Solutions hypotonic If a solution is hypotonic concentration of solutesles The concentration of solutes is less than it is inside the cell gain water The cell will gain water

26. 26 Water Balance in Cells Without Walls

27. 27 Water Balance in Cells with Walls turgid (firm hypotonic environment Plant cell- Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.

28. 28 How Will Water Move Across Semi-Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 100 molecules of fructose per ml How will the water molecules move? no net movement of water There will be no net movement of water since the concentration of solute in each solution is equal ANSWER:

29. 29 How Will Water Move Across Semi-Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 75 molecules of fructose per ml How will the water molecules move? There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute ANSWER:

30. 30 How Will Water Move Across Semi-Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 100 molecules of NaCl per ml How will the water molecules move? Each molecule of NaCl will dissociate to form a Na+ ion and a Cl- ion, making the final concentration of solutes 200 molecules per ml. Therefore, there will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute ANSWER:

31. 31 Facilitated Diffusion Facilitated diffusion Passive Proteins Is a type of Passive Transport Aided by Proteins In facilitated diffusion Transport proteins Transport proteins speed the movement of molecules across the plasma membrane

32. 32 Facilitated Diffusion & Proteins Channel proteins Provide corridors that allow a specific molecule or ion to cross the membrane EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM A channel protein (purple) has a channel through which water molecules or a specific solute can pass.

33. 33 Facilitated Diffusion & Proteins Carrier proteins Undergo a subtle change in shape that translocates the solute-binding site across the membrane carrier proteinalternates between two conformations can transport the solute in either direction down the concentration gradient A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.

34. 34 Active Transport Active transport Uses energy against Uses energy to move solutes against their concentration gradients ATP Requires energy, usually in the form of ATP

35. 35 sodium-potassium pump The sodium-potassium pump Is one type of active transport system Active Transport P P i EXTRACELLULAR FLUID Na+ binding stimulates phosphorylation by ATP. 2 Na + Cytoplasmic Na + binds to the sodium-potassium pump. 1 K + is released and Na + sites are receptive again; the cycle repeats. 3 Phosphorylation causes the protein to change its conformation, expelling Na + to the outside. 4 Extracellular K + binds to the protein, triggering release of the Phosphate group. 6 Loss of the phosphate restores the protein’s original conformation. 5 CYTOPLASM [Na + ] low [K + ] high Na + P ATP Na + P ADP K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ [Na + ] high [K + ] low

36. 36 Passive vs. Active Transport Passive transport. Active transport. Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins. ATP

37. 37 Maintenance of Membrane Potential by Ion Pumps Membrane potential Is the voltage difference across a membrane electrochemical gradient An electrochemical gradient Is caused by the concentration electrical gradient of ions across a membrane electrogenic pump An electrogenic pump Is a transport protein that generates the voltage across a membrane

38. 38 Proton Pump EXTRACELLULAR FLUID + H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Proton pump ATP CYTOPLASM + + + + – – – – – +

39. 39 Example of Cotransport active transport driven by a concentration gradient) Cotransport-Occurs when active transport of a specific solute indirectly drives the active transport of another solute (driven by a concentration gradient)

40. 40 Bulk Transport Exocytosis- transport vesicles Exocytosis- transport vesicles migrate to the plasma membrane, fuse with it, and release their contents

41. 41 Bulk Transport ndocytosis- t forming new vesicles from the plasma membrane Endocytosis- the cell takes in macromolecules by forming new vesicles from the plasma membrane

42. 42 phagocytosis In phagocytosis, a cell engulfs a particle by Wrapping pseudopodia around it and packaging it within a membrane- enclosed sac large enough to be classified vacuole as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. Three Types of Endocytosis PHAGOCYTOSIS pinocytosis In pinocytosis, the cell “gulps” droplets of extracellular fluid extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports.

43. 43 0.25 µm RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Ligand Coat protein Coated pit Coated vesicle A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs). Plasma membrane Coat protein Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coa proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.

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