Cell Membrane Function The purpose of cells membranes is to?

A. Passive transport (simple and facilitated diffusion) Large and small substances move across cell membranes in fundamentally different ways. Small molecules- A. Passive transport (simple and facilitated diffusion) B. Active transport Large molecules(endo and exocytosis)

A membrane’s molecular organization results in selective permeability Membrane permeability is influenced by size, chemical composition and charge/polarity of the molecule trying to cross the membrane a. Membranes are more permeable to small molecules than larger ones b. Membranes are more permeable to hydrophobic molecules c. Membranes are most permeable to uncharged/nonpolar molecules

Simple Diffusion Defined-the spontaneous net movement of a substance from an area of its higher concentration to an area of its lower concentration until an equilibrium is achieved Diffusion occurs because of the second law of thermodynamics

Diffusion of one solute LE 7-11a Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium Diffusion of one solute

Diffusion of two solutes LE 7-11b Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium Diffusion of two solutes

Osmosis Osmosis is a special case of diffusion It involves the diffusion of water across a differentially permeable membrane Cell and tissues can gain or lose water by osmosis depending on the type of environment they exist in

Effect of solute on cell solutions The solute concentration of the environment determines whether a cell gains or loses water The addition of solute lowers the concentration of water (makes it less than 100%) Three terms describe the tendency of one solution to gain or lose water to another solution Hypertonic (salty) solutions tend to gain water from hypotonic solutions (less salty) Isotonic solutions gain and lose water to one another at the same rate.

Lower Higher Same concentration concentration concentration of sugar LE 7-12 Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar H2O Selectively permeable mem- brane: sugar mole- cules cannot pass through pores, but water molecules can Osmosis

Environment “Cell” 0.01 M sucrose 0.03 M sucrose 0.01 M glucose LE 7-UN140 Environment “Cell” 0.01 M sucrose 0.03 M sucrose 0.01 M glucose 0.02 M glucose 0.01 M fructose

Cell survival depends on balancing water uptake and loss Plant and animal responses to being placed in A. hypertonic solution B. hypotonic solutions C. Isotonic solutions

Hypotonic solution Isotonic solution Hypertonic solution Animal cell LE 7-13 Hypotonic solution Isotonic solution Hypertonic solution Animal cell H2O H2O H2O H2O Lysed Normal Shriveled Plant cell H2O H2O H2O H2O Turgid (normal) Flaccid Plasmolyzed

LE 7-14 50 µm Filling vacuole 50 µm Contracting vacuole

Traffic across membranes A membrane’s molecular organization results in selective permeability Passive transport is diffusion across a membrane Osmosis is the passive transport of water Cell survival depends on balancing water uptake and loss The solute concentration of the environment determines whether a cell gains or loses water Specific proteins facilitate the passive transport of selected solutes (facilitated diffusion) Active transport is the pumping of solutes against their gradients Some ion pumps generate voltage across membranes In cotransport, a membrane protein couples the transport of one solute to another Exocytosis and endocytosis transport large molecules

How do small molecules move across cell membranes? Passive Transport is diffusion across a membrane A. Simple diffusion-membrane is permeable; highlow concentration; no energy required B. Facilitated diffusion-diffusion-membrane is impermeable (carrier molecule required) highlow concentration; no energy required

Facilitated diffusion LE 7-17a Passive transport Diffusion Facilitated diffusion

Facilitated Diffusion Specific proteins facilitate the passive transport of selected solutes (facilitated diffusion) Hydrophilic channels Rotating carriers (conformational changes)

LE 7-15a EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM

LE 7-15b Carrier protein Solute

Active Transport Active transport is the pumping of solutes against their gradients A. Membrane is impermeable (carrier required); movement from low concentration to high concentration; energy required B. Sodium/potassium pump (neurons) C. Plants often actively transport nutrients from soil into the root cell (advantage of doing this?)

LE 7-17b Active transport ATP

A B Both A and B Neither A nor B Solution A (.2M glucose) is separated from solution B (.4 M glucose) by a membrane which is impermeable to glucose . Which solution is hypertonic? A B Both A and B Neither A nor B

A B Both A and B Neither A nor B Solution A (.2M glucose) is separated from solution B (.4 M glucose) by a membrane which is impermeable to glucose . Which solution will have a net gain of water? A B Both A and B Neither A nor B

In the Na+/K+ pump, the ATPase enzyme is activated by Release of K+ Binding of K+ Binding of Na+ Release of Na+ phosphorylation

Figure 8.15 The sodium-potassium pump: a specific case of active transport

Cytoplasmic Na+ bonds to the sodium-potassium pump LE 7-16 EXTRACELLULAR FLUID [Na+] high [K+] low Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low [K+] high ATP P Na+ P CYTOPLASM ADP Cytoplasmic Na+ bonds to the sodium-potassium pump Na+ binding stimulates phosphorylation by ATP. Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. K+ K+ K+ K+ K+ P P K+ Extracellular K+ binds to the protein, triggering release of the phosphate group. Loss of the phosphate restores the protein’s original conformation. K+ is released and Na+ sites are receptive again; the cycle repeats.

– EXTRACELLULAR + FLUID ATP – + H+ H+ Proton pump H+ – + H+ H+ – + LE 7-18 – EXTRACELLULAR FLUID + ATP – + H+ H+ Proton pump H+ – + H+ H+ – + CYTOPLASM H+ – +

Co-transport In co-transport, a membrane protein couples the transport of one solute to another In plants, transport of sucrose into cells is coupled to the active transport of H+ ions out of the cell

Sucrose-H+ cotransporter LE 7-19 – + ATP H+ H+ – + Proton pump H+ H+ – + H+ – + H+ Diffusion of H+ Sucrose-H+ cotransporter H+ – + – + Sucrose

Movement of large molecules/cells into and out of cells Exocytosis and endocytosis transport large molecules into and out of cells Exocytosis-out Endocytosis-in Pinocytosis Phagosytosis Receptor-mediated endocytosis

PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming LE 7-20b PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). Vesicle

PHAGOCYTOSIS CYTOPLASM 1 µm Pseudopodium Pseudopodium of amoeba LE 7-20a PHAGOCYTOSIS EXTRACELLULAR FLUID CYTOPLASM 1 µm Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM)

RECEPTOR-MEDIATED ENDOCYTOSIS LE 7-20c RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs). Coat protein Plasma membrane 0.25 µm

Familial Hypercholesterolemia Symptoms/consequences Causes Genetics

Membrane Structure and Function Membrane models have evolved to fit new data (science as a process) A membrane is a fluid mosaic of lipids, proteins and carbohydrates There is a lot of experimental evidence that favors the fluid mosaic model of membrane structure.

History of Membrane Models Overton (1875) –Membranes contain lipids (like dissolve like) Langmuir(1917)-Membranes have amphipathic lipids (phospholipids) Gorter and Grendel(1925)-Phospholipid bilayer Davson and Danielli (1935)-Phospholipids and proteins (sandwich)

Figure 8.1 Artificial membranes (cross sections)

LE 7-2 WATER Hydrophilic head Hydrophobic tail WATER

Figure 8.2 Two generations of membrane models

History of Membrane Models (continued) Robertson (1950)-Electron micrographs showing “trilaminate” structure Problems with current models Singer and Nicholson (1975)-Fluid mosaic model

Figure 8.19 The three types of endocytosis in animal cells

Fluid Mosaic Model Consistent with all observations of membrane properties to date

Figure 7-01

LE 7-5 Lateral movement (~107 times per second) Flip-flop (~ once per month) Movement of phospholipids Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- carbon tails Membrane fluidity Cholesterol Cholesterol within the animal cell membrane

Facilitated diffusion ATP hydrolysis A proton pump 1 and 3 In sucrose co-transport in plants, the active transport of sucrose into plant cells is couple to Facilitated diffusion ATP hydrolysis A proton pump 1 and 3

Gorter and Grendle Davson and Danielli Singer and Nicholson Overton This model of membrane structure consisted of a phospholipid bilayer sandwiched between 2 layers of protein: Gorter and Grendle Davson and Danielli Singer and Nicholson Overton Robertson

Hypertonic environments Cooling temperatures Warming temperatures An increased synthesis of phospholipids containing unsaturated fatty acids may be an adaptation by plants to: Predators Decreasing sunlight Hypertonic environments Cooling temperatures Warming temperatures

Extracellular layer Proteins Knife Plasma membrane Cytoplasmic layer LE 7-4 Extracellular layer Proteins Knife Plasma membrane Cytoplasmic layer Extracellular layer Cytoplasmic layer

Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell LE 7-6 Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell Hybrid cell

Figure 8.9 Some functions of membrane proteins

EXTRACELLULAR SIDE N-terminus C-terminus CYTOPLASMIC SIDE a Helix LE 7-8 EXTRACELLULAR SIDE N-terminus C-terminus CYTOPLASMIC SIDE a Helix