1

Chapter 7Plasma membrane

“the edge of life”

Fig. 6.2a (TEArt)

Fatty acidPhosphorylated

alcohol

Fatty acidGLYCEROL

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

membranes = lipids, proteins & carbohydrates

• MOSTLY phospholipids

HYDROPHILIC - polar! Usually(Ionic – polar – usually)

HYDROPHOBIC

Polar(hydrophilic) region

Nonpolar (hydrophobic) region

2

Lipid monolayer Ben Franklin

Cells – amphiphileamphipathic

H2O inside and outside lipid bilayer

6-8 nm thick – keep out water soluble

SELF ASSEMBLE

•relatively weakhydrophobicinteractions

WHAT IS HAPPENING WITH THE WATER MOLECULES HERE?

WHAT IS HAPPENING WITH THE WATER MOLECULES HERE?

Fig. 3.22(TE Art)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Saturated fat

Unsaturated fat

OC

HCH

HCH

HCH

HCH CH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

O

OC

HCH

HCH

HCH

HCH CH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

O

OC

HCH

HCH

HCH

HCH CH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

H

HCH

HCH

H

HCH

HCH

HO

H HCH

HCH

HOC

HCH CH

HCH

HCH

HCH

HCH

HCH

HCH

CH

HC

HC C

H H H

HC

HC C

HO

HC

HCH

HCH

HOC

HCH CH

HCH

HCH

HCH

HCH

HCH

HCH

CH

HC

H HCH

CH

HC

H HCHH

CH H

O

HHC

HCH

HCH

HOC

HCH CH

HCH

HCH

HCH

HCH

HCH

HCH

CH

HC

HCH

CH

HC

HCH

CH H

O

H

H

H

H

kinks prevent tight packingunsaturated hydrocarbon tails-fluid when cold

Is solid bad?Lose permeability Enzymes inactivated

3

steroid cholesterol -wedged between phospholipidswhen cold -prevents tight packing of phosoholipids.

•Cholesterol influenced by temperaturephospholipids alone are usually closely packed in cold

How could you make plantsthat will grow in cold

temperatures?

membranes are fluid

•lateral movements •rapid

•2 microns per second

kinks in tail preventtight packing

Singer & Nicholson - 1972

4

• drift–directed (motor proteins)

• anchored

1 hour

What happened to the “presidential” stem cell lines?

Micron = micrometer = 1 millionth of a meterNanometer = 1 billionth of a meter

• 100,000 nanometers (nm) = 1 hair

• 100 microns (um)= 1 hair

• Membrane = 6-8 nanometers

• Bacteria, mitochondria, chloroplasts = ½ -1 X 5 um

Freeze-fracture and freeze-etch

5

bumps are proteins

Sidedness of the plasma membrane

Membrane models have evolved

SINGER AND NICOLSONFluid mosaic model (1972)A membrane is a fluid structure with proteins embedded or attached to a double layer of phospholipids.

6

• Cell membrane components– phospholipid bilayer– transmembrane proteins– interior protein network– cell surface markers

transmembrane protein

Proteins:

•integral

•transmembrane

•peripheral

•Also amphipathic

• Anchoring proteins in the bilayer• Transmembrane proteins

• single-pass anchors• multiple-pass channels and carriers• pores

7

Carbohydrates – oligosaccharides

some attached to lipids – glycolipid (glyco=carbohydrate)

cell-cell recognition

recognition

8

Glycoproteins— combination of proteinsand carbohydrates often used as cell-surface receptors

(some hormones)

– NOTE: Lipoproteins—proteins and lipids transport fat throughaqueous bloodbstream.

• High-Density Lipoproteins (HDL)—greater proportion of dense protein;also known as “good cholesterol.”

• Low-Density Lipoproteins (LDL)—“bad cholesterol” because they carrycholesterol to outlying tissues (arteries) where they accumulate inblockages called plaques.

• VIRUS host range:• “lock-and-key” fit –membrane proteins

• rabies virus - several species• most- single species

(except pigs!)

• Most specific tissues– Human cold viruses - upper respiratory tract– AIDS virus - certain white blood cells

Membrane Proteins• transporters• enzymes• cell surface identity markers• cell adhesion proteins• attachments to cytoskeleton

9

Some functions of membrane proteins

proteins have to change shape – membranes must be fluid

Passive Transport• Diffusion

– Random motion causes a net movement ofsubstances from regions of high concentrationto regions of low concentration.

The diffusion of solutes across membranes

passive transportbased on thermal motion (heat) – intrinsic kinetic energy

10

Facilitated Diffusion• Carriers transport ions and other solutes across

the plasma membrane.– Facilitate movement by physically binding molecules

on one side of the membrane, and releasing them onthe other.

• essential characteristics– specific– passive

–saturates

Osmosis –passive diffusion of water

Rate = change time

Rate = ΔV ΔT

HIGH SOLUTESLOW WATER

11

Osmosis

• Osmotic concentration - concentration of allsolutes in solution– Hyperosmotic - solution with the higher solute

concentration– Hypoosmotic - solution with the lower solute

concentration– Isosmotic - solute concentrations are equal

• Hypertonic – higher osmotic pressure• Higher solute concentration

• Hypotonic – lower osmotic pressure• Low solutes

• aquaporins

Maintaining Osmotic Balance

lyse

12

The water balance of living cells

hypo and hyper – SOLUTE CONCENTRATION in solution

Fig. 6.15c (TEArt)

Cell body shrinksfrom cell wall

Flaccid cell Normal turgid cell

Plant cells

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The contractile vacuole of Paramecium: an evolutionary adaptation forosmoregulation WATER BALANCE

•Solutions to beinghypoosmotic to theenvironment

–extrusion

–isosmotic solutions

–turgor

13

Bulk Transport• Endocytosis - enveloping food

– phagocytosis - material taken in is in particulateform

– pinocytosis - material taken in is in liquid form– receptor-mediated - transport of specific

molecules

• Exocytosis - discharge of material fromvesicles at the cell surface

The three types of endocytosis in animal cells

•phagocytosis

•pinocytosis

•receptor-mediated

14

Secretoryvesicle

Plasmamembrane

Cytoplasm

Secretoryproduct

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

15

• no membranes-

– dead!

• Disequilibrium –membranes defining feature of life