Chapter 7Chapter 7

Membrane Structure and Function

Overview: Life at the Edge

• The plasma membrane is the boundary that separates the living cell from its surroundings

• The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

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Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins

• Phospholipids are the most abundant lipid in the plasma membrane

• Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

• The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

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Membrane Models: Scientific Inquiry

• Membranes have been chemically analyzed and found to be made of proteins and lipids

• Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer

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Fig. 7-2

Hydrophilichead

WATER

Hydrophobictail

WATER

• In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins

• Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions

• In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-3

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

• Freeze-fracture studies of the plasma membrane supported the fluid mosaic model

• Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer

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Fig. 7-4

TECHNIQUE

Extracellularlayer

KnifeProteins Inside of extracellular layer

RESULTS

Inside of cytoplasmic layer

Cytoplasmic layerPlasma membrane

The Fluidity of Membranes

• Phospholipids in the plasma membrane can move within the bilayer

• Most of the lipids, and some proteins, drift laterally

• Rarely does a molecule flip-flop transversely across the membrane

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Fig. 7-5

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

(a) Movement of phospholipids

(b) Membrane fluidity

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-carbon tails

(c) Cholesterol within the animal cell membrane

Cholesterol

Fig. 7-5a

(a) Movement of phospholipids

Lateral movement(107 times per second)

Flip-flop( once per month)

Fig. 7-6

RESULTS

Membrane proteins

Mouse cellHuman cell

Hybrid cell

Mixed proteinsafter 1 hour

• As temperatures cool, membranes switch from a fluid state to a solid state

• The temperature at which a membrane solidifies depends on the types of lipids

• Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids

• Membranes must be fluid to work properly; they are usually about as fluid as salad oil

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-5b

(b) Membrane fluidity

Fluid

Unsaturated hydrocarbontails with kinks

Viscous

Saturated hydro-carbon tails

• The steroid cholesterol has different effects on membrane fluidity at different temperatures

• At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids

• At cool temperatures, it maintains fluidity by preventing tight packing

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-5c

Cholesterol

(c) Cholesterol within the animal cell membrane

Membrane Proteins and Their Functions

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

• Proteins determine most of the membrane’s specific functions

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-7

Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins

Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

GlycolipidEXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

• Peripheral proteins are bound to the surface of the membrane

• Integral proteins penetrate the hydrophobic core

• Integral proteins that span the membrane are called transmembrane proteins

• The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-8

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

EXTRACELLULARSIDE

• Six major functions of membrane proteins:

– Transport

– Enzymatic activity

– Signal transduction

– Cell-cell recognition

– Intercellular joining

– Attachment to the cytoskeleton and extracellular matrix (ECM)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-9

(a) Transport

ATP

(b) Enzymatic activity

Enzymes

(c) Signal transduction

Signal transduction

Signaling molecule

Receptor

(d) Cell-cell recognition

Glyco-protein

(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell Recognition

• Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane

• Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)

• Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

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Synthesis and Sidedness of Membranes

• Membranes have distinct inside and outside faces

• The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-10

ER1

Transmembraneglycoproteins

Secretoryprotein

Glycolipid

2Golgiapparatus

Vesicle

3

4

Secretedprotein

Transmembraneglycoprotein

Plasma membrane:

Cytoplasmic face

Extracellular face

Membrane glycolipid

Selectively permeable- allows certain substances to pass through

By 2 ways: active or passive transport

Passive- downhill

Active- uphill (needs energy)

Passive: downhill reaction• Simple diffusion• Osmosis• Facilitated diffusion• Filtration

Active: uphill reaction, needs ATP• Exocytosis• Endocytosis

- Pinocytosis- Phagocytosis

No Barrier:

• Substances “spread out”

• High concentration to low concentration

e.g.: Red dye placed in glass of water

• Substances diffuse

• High concentration to low concentration

• Pores in membrane must be large

• “Down the concentration gradient”

• Dynamic equilibrium, equal rates in both directions

Biological membrane:

Carrier proteins:

• Bind specific molecule & change shape

• Pass molecule through middle of protein

Osmosis- diffusion of a water through a semi-permeable membrane

• Moves down concentration gradient

e.g., Two sugar solutions of different concentrations separated by porous membrane which lets water through but not sugar

What will happen?

• More concentrated to less concentrated

• Until concentration same on both sides: isotonic

Concentration of solute less: solution is hypotonic.

Concentration of solute greater: solution is hypertonic.

Animal cells No cell walls

Isotonic environment: Influx of water equals the efflux of water No change in cell shape

Hypotonic solution: Water enters cell Bursts, or lyses

Hypertonic solution: Water leaves cell Shriveled, or crenate

Glomerular filtration

Passive transport & facilitated diffusion do NOT require

ATP

DOES require the input of

ATP

Transport proteins AGAINST concentration

gradient

outside cell

inside cell

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ATP ATP ADP + P ADP + Pii + Energy + Energy

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mucus

http://www.1lecture.com/Physiology/Endocytosis%20and%20Exocytosis/index.html

Goblet cell

Nerve Axon at Rest

Na+/K+ Pumphttp://www.1lecture.com/Biochemistry/cotransport/

H+ Pump in a Plant

• Ion channels - Voltage gated- Chemically gated- Mechanically gated

• Porins - Larger - Less specific

• Aquaporins - water

Channel Proteins

Channel Proteins:Ion Channels

Channel Proteins:Porins

• MAC• Barrel shaped protein

Channel Proteins:Aquaporins

H2O

Membrane Permeability

Cell membrane:

selectively permeable

4 factors that determine permeability

1. lipid solubility

2. molecular size

3. polarity

4.charge

Lipid solubilityMost important factor

Hydrophobic molecules

•Passively diffuse

•Hydrocarbons, carbon dioxide, & oxygen

Molecular Size and Polarity

Larger molecules, less permeable Lower kinetic energy Small pore sizes in the membrane

Polar molecules hydrophilic, less permeable Very small, polar uncharged (water) molecules can diffuse

-+

Molecular Size

Polarity

ChargeCharged molecules hydrophilic, less permeable Surrounded by coat of water (hydration shell), increases the size

You should now be able to:

1. Define the following terms: amphipathic molecules, aquaporins, diffusion

2. Explain how membrane fluidity is influenced by temperature and membrane composition

3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. Explain how transport proteins facilitate diffusion

5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps

6. Explain how large molecules are transported across a cell membrane

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings