Chapter 7: Membrane Structure & Function1. What do you know about membranes?

- Phospholipid bilayer- Selectively permeable- Fluid Mosaic Model

HydrophilicheadHydrophobictail

WATER

WATER

Chapter 7: Membrane Structure & Function1. What do you know about membranes?

- Phospholipid bilayer- Selectively permeable- Fluid Mosaic Model

2. Why fluid/what affects fluidity?-Fatty acids - Sat vs unsaturated-Temperature-Cholesterol

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-Carbon tails

(a) Movement of phospholipids

(b) Membrane fluidity

(c) Cholesterol within the animal cell membrane

Cholesterol

Chapter 7: Membrane Structure & Function1. What do you know about membranes?

- Phospholipid bilayer- Selectively permeable- Fluid Mosaic Model

2. Why fluid/what affects fluidity?- FA - Sat vs unsaturated- Temperature- Cholesterol

3. What makes a membrane mosaic?

Figure 7.7 The detailed structure of an animal cell’s plasma membrane

Glycoprotein

Carbohydrate

Microfilamentsof cytoskeleton Cholesterol Peripheral

protein Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

EXTRACELLULAR

SIDE OFMEMBRANE

Glycolipid

Fibers ofextracellularmatrix (ECM)

Chapter 7: Membrane Structure & Function1. What do you know about membranes?

- Phospholipid bilayer- Selectively permeable- Fluid Mosaic Model

2. Why fluid/what affects fluidity?- FA - Sat vs unsaturated- Temperature- Cholesterol

3. What makes a membrane mosaic?4. How are integral proteins held in the membrane?

- α-helix EXTRACELLULARSIDEN-terminus

C-terminus

HelixCYTOPLASMICSIDE

NH2

H+

COOH

Cytoplasm

Retinalchromophore

Nonpolar(hydrophobic)-helices in thecell membrane H+

Porin monomer

-pleated sheets

Bacterialoutermembrane

proton pump channel

aquaporin = water channel

Examples

H2O

H2O

H+

H+

Chapter 7: Membrane Structure & Function1. What do you know about membranes?2. Why fluid/what affects fluidity?3. What makes a membrane mosaic?4. How are integral proteins held in the membrane?5. What are the functions of integral proteins?

Figure 7.9 Some functions of membrane proteins

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 membranemay be an enzyme with its active site exposed tosubstances in the adjacent solution. In some cases,several enzymes in a membrane are organized asa team that carries out sequential steps of ametabolic pathway.

Signal transduction. A membrane protein may havea binding site with a specific shape that fits the shapeof a chemical messenger, such as a hormone. Theexternal messenger (signal) may cause aconformational change in the protein (receptor) thatrelays the message to the inside of the cell.

(a)

(b)

(c)

ATP

Enzymes

Signal

Receptor

(d)

(e)

Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells.

Intercellular joining. Membrane proteins of adjacent cellsmay hook together in various kinds of junctions, such asgap junctions or tight junctions (see Figure 6.31).

Attachment to the cytoskeleton and extracellular matrix(ECM). Microfilaments or other elements of thecytoskeleton 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 (see Figure 6.29).

(f)

Glyco-protein

Chapter 7: Membrane Structure & Function1. What do you know about membranes?2. Why fluid/what affects fluidity?3. What makes a membrane mosaic?4. How are integral proteins held in the membrane?5. What are the functions of proteins?6. Word association……

Membrane =7. What things are selected for?

- Non-polar pass easily - steroids8. What things are selected against?

- large, polar, charged molecules – glucose, amino acids, ions9. What is the difference between hypertonic & hypotonic solutions?

- Hypertonic – more total solute than another sol’n = less water- Hypotonic – less total solute than another sol’n = more water

Selectively permeable!!!!

Figure 7.11 The diffusion of solutes across a membrane(a) Diffusion of one solute. The membrane

has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions.

Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concen-tration gradient. There will be a net diffusion of the purple dye toward the left, even though the total soluteconcentration was initially greater onthe left side.

(b)

Osmosis

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

Net diffusion

Net diffusion

Net diffusion

Net diffusion Equilibrium

Equilibrium

What is osmosis?- Diffusion of water

Figure 7.12 OsmosisLowerconcentrationof solute (sugar)

Higherconcentrationof sugar

Same concentrationof sugar

Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can

More free watermolecules (higher

concentration)

Water moleculescluster around sugar molecules

Fewer free watermolecules (lowerconcentration)

Water moves from an area of higher free water concentration to an area of lower free water concentration

Osmosis

Figure 7.13 The water balance of living cells

Hypotonic solution Isotonic solution Hypertonic solutionAnimal cell. Ananimal cell fares bestin an isotonic environ-ment unless it hasspecial adaptations tooffset the osmoticuptake or loss ofwater.

Plant cell. Plant cells are turgid (firm) and generally healthiest ina hypotonic environ-ment, where theuptake of water iseventually balancedby the elastic wallpushing back on thecell.

(a)

(b)

H2O H2O H2O H2O

H2OH2OH2OH2O

Lysed Normal Shriveled

Turgid (normal) Flaccid Plasmolyzed

Aquaporins»protein channels allowing rapid

flow of water across cell membrane

» explains efficient nature of osmosis

Cell (compared to beaker) hypertonic or hypotonic

Beaker (compared to cell) hypertonic or hypotonic

Which way does the water flow? in or out of cell

.05 M .02 M

Do you understand Osmosis…

Chapter 7: Membrane Structure & Function1. What do you know about membranes?2. Why fluid/what affects fluidity?3. What makes a membrane mosaic?4. How are integral proteins held in the membrane?5. What are the functions of proteins?6. Word association……7. What things are selected for?8. What things are selected against?9. What is the difference between hypertonic & hypotonic solutions?

- Hypertonic – more total solute than another sol’n- Hypotonic – less total solute than another sol’n

10. How are substances transported across membranes?- Passive transport – no energy expended & things flow down

concentration gradient from high to low- Simple diffusion- Facilitated diffusion

- Active transport – energy required & things flow AGAINST a concentration gradient from low to high

Figure 7.17 Review: passive and active transport comparedPassive 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.

ATP

Diffusion. Hydrophobicmolecules 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.

Chapter 7: Membrane Structure & Function1. What do you know about membranes?2. Why fluid/what affects fluidity?3. What makes a membrane mosaic?4. How are integral proteins held in the membrane?5. What are the functions of proteins?6. Word association……7. What things are selected for?8. What things are selected against?9. What is the difference between hypertonic & hypotonic solutions?10. How are substances transported across membranes?

- Passive transport – no energy expended & things flow down concentration gradient- Simple diffusion- Facilitated diffusion

- Active transport – energy required & things flow AGAINST a concentration gradient

11. What are some other mechanisms of transport across membranes?

Figure 7.20 Exploring Endocytosis in Animal Cells

EXTRACELLULARFLUID

In phagocytosis, a cellengulfs a particle by wrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

PseudopodiumCYTOPLASM

“Food” or other particle

Foodvacuole

1 µm

Pseudopodiumof amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacterium viaphagocytosis (TEM).

PHAGOCYTOSIS

PINOCYTOSIS

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM).

0.5 µm

In pinocytosis, the cell “gulps” droplets of extracellular fluid into tinyvesicles. It is not the fluiditself 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 substancesit transports.

Plasmamembrane

Vesicle

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Ligand

Coat protein

Coatedpit

Coatedvesicle

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).

0.25 µm

Plasmamembrane

Coatprotein

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, but 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.