1 Membrane Structure and Function

2 Plasma Membrane boundary Is the boundary that separates the living cell from its nonliving surroundings Selectively Permeable Selectively Permeable (it chooses what may cross the membrane) bilayer Lipid bilayer (2 layers of phospholipids) Contains embedded proteins

3Phospholipids most abundant Are the most abundant lipid in the plasma membrane amphipathic Are amphipathic, containing both hydrophilic (head) and hydrophobic regions (tails) Head hydrophilic Head composed of phosphate group attached to one carbon of glycerol is hydrophilic tailshydrophobic Two fatty acid tails are hydrophobic

4 Hydrophilic head Hydrophobic tail WATER Phospholipid Bilayer

5 Phospholipid bilayer Hydrophilic region of protein Hydrophobic region of protein

6 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 laterally making the membrane fluid ure=fvwp&v=Rl5EmUQdkuI

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8 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)

9 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

10 The Fluidity of Membranes steroid cholesterol The steroid cholesterol has different effects on membrane fluidity Figure 7.5 Cholesterol 5bQ

11 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)

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

14 Through the Cell Membrane cell must exchange materials with its surroundings A cell must exchange materials with its surroundings, a process controlled by the plasma membrane

15 Permeability of the Lipid Bilayer Hydrophobic molecules rapidly can pass through the membrane rapidly Hydrophillic molecules rapidly Do NOT cross the membrane rapidly and therefore need help from transport proteins Phospholipid bilayer Hydrophilic region of protein Hydrophobic region of protein

16 Transport Proteins Transport proteins hydrophilic substances Allow passage of hydrophilic substances across the membrane

Passive transport. Substances move 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

18 1. 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

19 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 QEc

20 Effects of Osmosis on Water BalanceOsmosis Is the movement of water across a semipermeable membrane affected by the concentration gradient of dissolved substances Is affected by the concentration gradient of dissolved substances

21 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

22 Hypertonic Solution hypertonic If a solution is hypertonic concentration of solutesgreater outside the cell The concentration of solutes is greater outside the cell than it is inside the cell lose water The cell will lose water

23 Hypotonic Solutions hypotonic If a solution is hypotonic concentration of solutes outside the cell isles The concentration of solutes outside the cell is less than it is inside the cell gain water The cell will gain water

24 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.

25 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

26 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

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

28 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

29 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. gXOhY

30 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.

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

32 2. Active Transport Active transport Uses energyagainst Uses energy to move solutes against their concentration gradients ATP (Adenosine Triphosphate) Requires energy, usually in the form of ATP (Adenosine Triphosphate)

33 Proton Pump EXTRACELLULAR FLUID + H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Proton pump ATP CYTOPLASM – – – – – + qzzL4

34 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

35 Bulk Transport exocytosis and endocytosis Bulk transport across the plasma membrane occurs by exocytosis and endocytosis of large molecules (e.g. proteins and polysaccharides)

36 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 a4Y8

37 Endocytosis

38 Exocytosis