1. 1 Membrane Structure and Function

2. 2 Plasma Membrane boundary The plasma membrane is the boundary that separates the living cell from its nonliving surroundings Selectively Permeable Selectively Permeable Fluid mosaic Fluid mosaic of lipids and proteins bilayer Phospholipid bilayer Contains embedded proteins Contains embedded proteins

3. 3Phospholipids amphipathic Phospholipids are amphipathic, containing both hydrophilic (head) and hydrophobic regions (tails)

4. 4 Hydrophilic head Hydrophobic tail WATER Phospholipid Bilayer

5. 5 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 laterally Membrane proteins also move side to side or laterally making the membrane fluid

6. 6 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)

7. 7 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

8. 8 steroid cholesterol The steroid cholesterol Has different effects on membrane fluidity at different temperatures Figure 7.5 Cholesterol

9. 9 Membrane Proteins and Their Functions collage of different proteins embedded A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Fibers of extracellular matrix (ECM)

10. 10 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

11. 11 Types of Membrane Proteins Peripheral proteins Are appendages loosely bound to the surface of the membrane

12. 12 Six Major Functions of Membrane Proteins Figure 7.9 Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane. Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. (a) (b) (c) ATP Enzymes Signal Receptor

13. 13 Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (d) (e) (f) Glyco- protein Six Major Functions of Membrane Proteins

14. Types of intercellular junctions in animals Tight junctions prevent fluid from moving across a layer of cells Tight junction 0.5 µm 1 µm Space between cells Plasma membranes of adjacent cells Extracellular matrix Gap junction Tight junctions 0.1 µm Intermediate filaments Desmosome Gap junctions At tight junctions, the membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins (purple). Forming continu- ous seals around the cells, tight junctions prevent leakage of extracellular fluid across A layer of epithelial cells. Desmosomes (also called anchoring junctions) function like rivets, fastening cells Together into strong sheets. Intermediate Filaments made of sturdy keratin proteins Anchor desmosomes in the cytoplasm. Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for commu- nication between cells in many types of tissues, including heart muscle and animal embryos. TIGHT JUNCTIONS DESMOSOMES GAP JUNCTIONS Figure 6.31

15. 15 The Role of Membrane Carbohydrates in Cell-Cell Recognition Cell-cell recognition I Is a cell’s ability to distinguish one type of neighboring cell from another Membrane carbohydrates Interact with the surface molecules of other cells, facilitating cell-cell recognition

16. 16 Synthesis and Sidedness of Membranes distinct inside and outside faces Membranes have distinct inside and outside faces affects the movement proteins synthesized endomembrane system (Golgi and ER) This affects the movement of proteins synthesized in the endomembrane system (Golgi and ER)

17. 17 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

18. 18 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

19. What needs to cross the membrane? Oxygen Carbon Dioxide Sucrose Bacteria Waste Sodium + Potassium + Glucose

20. What can diffuse across the membrane? Oxygen Carbon Dioxide Sucrose Bacteria Waste Sodium + Potassium + Glucose

21. 21 Permeability of the Lipid Bilayer Hydrophobic molecules lipid soluble rapidly Are lipid soluble and can pass through the membrane rapidly Polar molecules rapidly Do NOT cross the membrane rapidly

22. 22 Transport Proteins Transport proteins hydrophilic substances Allow passage of hydrophilic substances across the membrane

23. 23 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

24. 24 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

25. 25 Effects of Osmosis on Water BalanceOsmosis Is the movement of water across a semipermeable membrane affected by the concentration gradient of dissolved substances called the solution’s tonicity Is affected by the concentration gradient of dissolved substances called the solution’s tonicity

26. 26 Water Balance of Cells Without Walls 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

27. 27 Three States of Tonicity

28. 28 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

29. 29 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)

30. 30 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

31. 31 Water Balance in Cells Without Walls isotonic Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.

32. 32 Water Balance of Cells with Walls Cell Walls Help maintain water balance Turgor pressure Is the pressure of water inside a plant cell pushing outward against the cell membrane turgid If a plant cell is turgid hypotonic It is in a hypotonic environment firm, a healthy state in most plants It is very firm, a healthy state in most plants flaccid If a plant cell is flaccid isotonic or hypertonic It is in an isotonic or hypertonic environment

33. 33 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

34. 34 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.

35. 35 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.

36. 36 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

37. 37 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

38. 38 Comparison of Passive & Active Transport Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. 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

39. 39 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

40. 40 Proton Pump EXTRACELLULAR FLUID + H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Proton pump ATP CYTOPLASM + + + + – – – – – +

41. 41 CotransportCotransport active transport Occurs when active transport of a specific solute indirectly drives the active transport of another solute membrane protein Involves transport by a membrane protein concentration gradient Driven by a concentration gradient

42. 42 Example of Cotransport driven by a concentration gradient Cotransport: active transport driven by a concentration gradient

43. 43 Bulk Transport exocytosis and endocytosis Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Large proteins Cross the membrane by different mechanisms

44. 44 Exocytosis & Endocytosis exocytosis In exocytosis Transport vesicles Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents endocytosis In endocytosis forming new vesicles from the plasma membrane The cell takes in macromolecules by forming new vesicles from the plasma membrane

45. 45 Endocytosis

46. 46 Exocytosis

47. 47 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.

48. 48 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 coat 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.