Membrane Structure & Function (Learning Objectives)

• Review the basic function and biochemical composition of the plasma membrane. p p

• Learn the fluid state of membranes and the movement of its lipids and proteins.

• Explain the importance of cholesterol in maintaining fluidity of animal cells membranes.

• Review the mosaics of membrane structures that dictate its function.

• Summarize the functions of membrane proteins.

• Compare and contrast movement of small and large molecules across the plasma membrane.

• Explain the basis of selective membrane permeability and the distinguishing features of substances that can move across freely and those that require protein transporters. 

• Compare and contrast passive transport (diffusion and facilitated diffusion) with active transport. 

• Explain the driving force behind diffusion and osmosis. Anticipate the movement of water into & out of the cells as a function of the concentration of water soluble substances.

E l i th l f t i t t i i ifi l l th b• Explain the role of protein transporters in moving specific molecules across the membrane down their concentration gradient and uphill against that gradient.

• Describe the role of ATP and phosphorylation in active transport. 

• Compare and contrast exocytosis with endocytosis and the three types of endocytosis• Compare and contrast exocytosis with endocytosis and the three types of endocytosis.

MEMBRANE STRUCTURE AND FUNCTION

Membranes divide the cell into compartments where different chemical cellular activities canwhere different chemical cellular activities can take place.

The plasma membrane is the outer boundary ofThe plasma membrane is the outer boundary of the cell and is selectively permeable

It t l th fl f b t i t t fIt controls the flow of substances into or out of the cell

Outside of cell

Cytoplasm

M 200

,000

 

Figure 5.10

TEM

Membranes are made of 

h h li id bilCH2

CH

CH3

CHCH3N

+Hydrophilic head

phospholipids bilayerOne hydrophilic head 

CH2CH3

OO O–P

OCH2CHCH2

Phosphategroup

Two hydrophobic tailsCH2

CH2

CH

CH2

CH2

CH

22

C O C OO O

CH2

CH2

CH2

CH2

CHCH2

CH2

CH2

CH2

CH2 SymbolCH2

CH2

CH2

CH2

CH2

CH2

CHCH

CH2CH

CH2

Symbol

CH2

CH2

CH2

CHCH2

CH2CH2CH2CH2CH2

Figure 5.11A

CH2

CH3

CH3

Hydrophobic tails

– The heads of the phospholipid bilayer face outward– The heads of the phospholipid bilayer, face outward and the tails face inward

WaterHydrophilicheads

Hydrophobictails

W

Figure 5.11B

Water

In addition to phospholipids membranesIn addition to phospholipids, membranes contain proteins that determine the function of the membraneof the membrane

Hydrophilic regionof proteinof protein

Phospholipidbilayerbilayer

Hydrophobic region of protein

Cholesterol is a steroid lipid with a carbon skeleton consisting of four fused carbon rings present in plasma membranes of ganimal cells

• Cholesterol is wedged between phospholipid molecules in the plasma membrane of animalsmolecules in the plasma membrane of animals cells.

• At warm temperatures, it restrains the movementAt warm temperatures, it restrains the movement of phospholipids and reduces fluidity.

• At cool temperatures, it maintains fluidity by p , y ypreventing tight packing.

Ch l t lCholesterolCholesterol within the animal cell membrane

A fluid mosaic of phospholipids and proteins

Membrane fluidity affects activity of membrane boundMembrane fluidity affects activity of membrane‐bound enzymes. http://www.insidecancer.org/

Fibers of the extracellular matrix

Carbohydrate(of glycoprotein)

Glycoprotein

Plasmamembrane

Glycolipid

Phospholipid

Figure 5.12Microfilamentsof cytoskeleton Cholesterol

Proteins

Cytoplasm

Functions of plasma membrane proteinsFunctions of plasma membrane proteins

SignalEnzymes

Signal

R tReceptorATP

Transport Enzymatic activity Signal transduction

Enzymes catalyze chemical reactionsEnzymes catalyze chemical reactions

Figure 5.13A

Receptors receive chemical messages from other cells

Messenger molecule

RReceptor

Activatedl l

Figure 5.13B

molecule

Transporters move substances across the bmembrane

ATP

Figure 5.13C

ATP

Traffic Across Membranes

A. Ions and monomers (small molecules)Move physically through the membrane

B Macromolecules and large particlesB. Macromolecules and large particlesmove across inside vacuoles & vesicles

Traffic of ions and monomers

Selective permeability depends on interaction of that molecule with the hydrophobic core and presence of specific proteins

• non‐polar molecules can pass: 

• polar and ionic molecules and inorganic ions are assisted by membrane proteins

Traffic of ions and monomers

Selective permeability depends on interaction of that molecule with the hydrophobic core and presence of specific proteins

• non‐polar molecules can pass: 

hydrocarbons, CO2, and O2

• polar and ionic molecules and inorganic ions are assisted by membrane proteins– nutrients (monomers of sugars and amino acids) and metabolic 

waste products

N + K+ C 2+ d Cl– Na+, K+, Ca2+, and Cl‐

Fig 8 16 Both diffusion and facilitated diffusion are forms of passive transport of moleculesFig. 8.16 Both diffusion and facilitated diffusion are forms of passive transport of molecules down their concentration gradient, while active transport requires an investment of energy to move molecules against their concentration gradient.

Movement across membranes

I Passive transportI. Passive transportFrom an area of high concentration to one with lower concentration down awith lower concentration, down a concentration gradient, no ATP required 

II. Active transportFrom an area of low concentration to oneFrom an area of low concentration to one  with higher concentration, requires ATP 

I. Passive transport is diffusion across a membrane‐ without work by the cellwithout work by the cell‐ Spreading from areas of high concentration to areas of low concentrationareas of low concentration

EquilibriumMembraneMolecules of dye EquilibriumMembraneMolecules of dye

EquilibriumFigure 5.14A

Figure 5.14B

I. Passive TransportI. Passive Transport

1. Simple diffusion (gases & hydrocarbons)

2. Osmosis diffusion of solvent (H2O)2. Osmosis diffusion of solvent (H2O)

3. Facilitated diffusion (via protein transporters)

Solvation of ionic compounds in waterSolvation of ionic compounds in water

http://programs.northlandcollege.edu/biology/Biology1111/animations/dissolve htmlBiology1111/animations/dissolve.html

Osmosis

http://physioweb.med.uvm.edu/bodyfluids/osmp //p y / y /osis.htm

Osmosis is the diffusion of water across a membraneWater travels from a solution of lower soluteWater travels from a solution of lower solute concentration to one of higher solute concentration

L Hi hEqual

Lowerconcentration

of solute

Higherconcentration

of solute

concentrationof solute

H2OSolute molecule

Selectively permeabley pmembrane

Watermolecule

Solute molecule with

Figure 5.16

cluster of water molecules

Net flow of water

Water balance between cells and their surroundings is crucial to organisms

Osmosis causes cells to shrink in hypertonic solutionsswell in hypotonic solutionsswell in hypotonic solutionsnot change in isotonic solutions

In isotonic solutions

Animal cells are normal, but plant cells are limp

Isotonic solution Hypotonic solution Hypertonic solution

H2O H2OH2O H2O

Isotonic solution Hypotonic solution Hypertonic solution

l(1) Normal (2) Lysed (3) Shriveled

Animalcell

H2O H2O H2O H2OPlasma

membrane

Plantcell

(4) Flaccid (5) Turgid(6) Shriveled

(plasmol ed)

Figure 5.17

( ) ( ) g(plasmolyzed)

Transport proteins may facilitate diffusion across b b idi h lmembranes by providing passages or channels

Solutemoleculemolecule

Transport

Figure 5.15

Transportprotein

II Active transport uses cellular energyII. Active transport uses cellular energy – Transport proteins can move solutes against a concentration gradientconcentration gradient

– ATP provides a phosphate group to change the shape of the transporter protein

Transportprotein

the transporter protein 

PP PProtein

changes shapePhosphatedetaches

ATPADPSolute

Solute binding1 Phosphorylation2 Transport3 Protein reversion4Figure 5.18

B. Transport of large macromolecules across l b (E t i d E d t i )plasma membrane (Exocytosis and Endocytosis) 

• Uptake of macromolecules is known asUptake of macromolecules is known as endocytosis

• Secretion or excretion of macromolecules is known as exocytosis e.g. insulin (protein hormone) by pancreatic cells.

Endocytosis and exocytosishttp://highered mcgrawhttp://highered.mcgraw-

hill.com/sites/0072437316/student_view0/chapter6/animations.html#r6/animations.html#

In exocytosis a vesicle fuses with the membraneIn exocytosis,  a vesicle fuses with the membrane and expels its contents 

Fluid outside cell

ProteinVesicle

CytoplasmFigure 5.19A

In endocytosis a membrane vesicle folds inwardIn endocytosis,  a membrane vesicle folds inward enclosing material from the outside

Vesicle forming

Figure 5.19B

Endocytosis can occur in three waysEndocytosis can occur in three ways

• Phagocytosis

• Pinocytosis• Pinocytosis

• Receptor‐mediated endocytosis

Plasma membrane

Pseudopodium of amoebaFood being ingested Material bound to receptor proteins

PIT

00

,500

 

Phagocytosis Pinocytosis Receptor‐mediated endocytosis

CytoplasmTEM 54,00

TEM 96,

LM 230

Figure 5.19C

Faulty membranes can overload the blood with ycholesterol– Harmful levels of cholesterol can accumulate in theHarmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors

Ph h li id t lLDL particle Phospholipid outer layer

Vesicle

Cholesterol

Protein

ReceptorPlasma

b CytoplasmReceptorprotein

membrane

Figure 5.20