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Chapter 5

Membrane Structure and Function

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5.1 Membrane Models

The fluid-mosaic model consists

of a fluid phospholipid bilayer

with embedded proteins

scientists first noticed that lipid-soluble molecules entered cells

more rapidly than water-soluble

moleculesin 1925, Gorter and Grendel

suggested the phospholipid bilayer

based on lipid content of RBCs

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5.1 Membrane Models

in 1940s, Danielli and Davson

suggested the presence of proteins

coating the inside and outside of the

bilayer

by late 1950s, electron microscopy

allowed viewing of the membrane;

Robertson suggested protein with

the hydrophilic heads ofphospholipids

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Fig. 5.1a RBC plasma membrane

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5.1 Membrane Models

Fluid-Mosaic Modelintroduced in 1972 by Singer and

Nicolson

proteins are partially or wholly

embedded in a fluid phospholipid

bilayer

proteins are scattered

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Fig. 5.1b Two possible models

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Fig. 5.1c Freeze-fracture

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Fig. 5.1d Freeze-fractured membrane

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Fig. 5.2 Fluid-mosaic model

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5.2 Membrane Structure & Function

The plasma membrane consists

of a phospholipid bilayer and

associated proteins

phospholipid bilayer means 2 layersof phospholipids

phospholipid structure

2 long, hydrophobic fatty acids(tails)

hydrophilic glycerol/phosphate

area (head)

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Membrane Structure

Glycerol/Phosphate Head

Fatty Acid Tails

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Membrane Structure

HydrophobicRegion

Hydrophilic

Hydrophilic

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HydrophobicRegion

Hydrophilic

Hydrophilic

Outside Cell

Inside Cell

M

E

M

B

R

A

NE

Membrane Structure

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Cholesterol stiffens and

strengthens the membrane, helping

regulate its fluidity

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Peripheral proteins

on one side of membrane

often have a structural role

Integral proteins

embedded in membrane

called transmembrane proteins

when they span the membrane

diverse in their functions

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Transmembrane protein

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Glycolipids

phospholipids with carbohydrate

chains attached

only on outside of membrane

Glycoproteins

proteins with carbohydrate chains

attached

only on outside of membranetherefore, the inside of the membrane

is not identical to the outside

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Carbohydrate ChainsGlycocalyx

a sugar coat of glycoproteins in

animal cells

protects cell; facilitates adhesion to

other cells, reception of signal

molecules, and cell-to-cell

recognition

basis for cell identification (like A, B,

O blood groups) due to variety

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Fluidity of the Plasma Membraneconsistency of olive oil at body

temperature

more unsaturated fatty acids = more

fluid

more saturated fatty acids = more

solid

phospholipids and proteins both drift

through the membrane (demonstrated

by fusing mouse and human cells)

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Fig. 5.3 Lateral drifting of proteins

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Functions of the Proteins

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Membrane protein diversity

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Structural Summary

Fluid Mosaic ModelConstantly shifting mosaic of

proteins moving in a semi-liquid

lipid

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5.3 Permeability of the Membrane

The plasma membrane is

selectively permeablehydrophilic edges allow membrane to

exist in aqueous environment

hydrophobic core is major barrier to

charged substances

this makes the membrane selectively

permeable, allowing some things to

pass but not others

critical for sustaining the cells life

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What can cross passively (without

using energy):

water

small, uncharged molecules lipid-soluble molecules, carbon

dioxide, oxygen, glycerol, alcohol

sugars and amino acids slowly

usually assisted by channel proteinsor carrier proteins

involves movement down concen-

tration gradient (from high to low)

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Diffusion

O2O

2

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Diffusion

O2O2

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Fig. 5.5 Membrane permeability

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What cannot cross passively (requires

energy input)

charged molecules and ionsCa2+, Cl-

macromoleculesanything against its concentration

gradient

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How molecules cross the membrane

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Diffusion and OsmosisDiffusion: movement of molecules

from a higher to lower concentration

(down a concentration gradient) until

equilibrium is reached

done by H2O, O2, CO2increases entropy (high potential

energy to low potential energy)temperature, pressure, electrical

currents, and molecular size all

affect diffusion rate

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Fig. 5.6 Diffusion

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Diffusion of a gas

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Fig. 5.7 Gas exchange in lungs

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Diffusion and Osmosis, cont.Osmosis: movement of water

molecules across a selectively

permeable membrane due to a

concentration gradient

water generally moves to dilute the

more concentrated solution (from

higher water concentration to lower)osmotic pressure is pressure that

develops due to osmosis

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Osmosis

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Diffusion and Osmosis, cont.Isotonic solutions have the same

solute concentrations as one another

Hypotonic solutions have a lower

solute concentration than other

solutions

plant cells in hypotonic solutions

swell (turgor pressure)Hypertonic solutions have a higher

solute concentration than other

solutions

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Osmosis demonstration

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Osmosis

2% NaCl

98% H2O

10% NaCl

90% H2O

Hypotonic Solution: Less Solute

Hypertonic Solution: More Solute

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Osmosis

6% NaCl

94% H2O

6% NaCl

94% H2O

Isotonic Solutions: Equal Solute Concentration

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Osmosis in cells

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RBC osmosis simulation

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RBCs in different solutions

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Osmosis summary

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Transport by Carrier Proteinsuses integral membrane proteins to

transport material across membrane

that couldnt normally cross

carrier protein: permease or gatecarries large or charged particles

each permease is specific for a

particular substance

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Transport by Carrier Proteins, cont.Facilitated Transport

movement from high concentration

to low through a carrier protein

no cellular energy requiredreaches equilibrium

example: glucose (polar)

3 P bili f h M b

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Transport by Carrier Proteins, cont.Active Transport

movement from low concentration

to high (against the concentration

gradient) through a proteinrequires cellular energy (ATP)

example: Na+/K+ pump (sodium-

potassium pump)

Fi 5 11 S di t i

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Fig. 5.11 Sodium-potassium pump

5 3 P bilit f th M b

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Vesicle Formationused to obtain or release large

particles or quantities

Exocytosis

vesicle fuses with cell membraneand releases contents

can be used to release digestive

enzymes or hormones

Fi 5 12 E t i

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Fig. 5.12 Exocytosis

E t i i ti

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Exocytosis animation

5 3 P bilit f th M b

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Vesicle Formation, cont.Endocytosis

cell surrounds and engulfs material,

forms a vesicle

Phagocytosis: endocytosis of largematerial, such as a food particle or

another cellseen in amoeba and white blood cells

Pinocytosis: endocytosis of a liquid

or very small particlesseen in RBCs, plant root cells

Fi 5 13 Ph t i d i

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Fig. 5.13a Phagocytosis drawing

Fi 5 13 Ph t i h t h

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Fig. 5.13a Phagocytosis photograph

Ph t i i ti

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Phagocytosis animation

WBC tt ki E li

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WBC attacking E. coli

Amoeba phagoc tosis

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Amoeba phagocytosis

Fig 5 13b Pinocytosis drawing

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Fig. 5.13b Pinocytosis drawing

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Pinocytosis animation

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Pinocytosis animation

5 3 Permeability of the Membrane

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Vesicle Formation, cont.Endocytosis, cont.

Receptor-Mediated Endocytosis:

uses receptor proteins bound to

signal molecules (e.g., vitamins,hormones, or lipoproteins) to initiate

endocytosis

selective and more efficient

Fig 5 13c Receptor mediated

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Fig. 5.13c Receptor-mediated

Fig 5 13c Receptor mediated

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Fig. 5.13c Receptor-mediated

Transport review

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Transport review

DiffusionOsmosis

Carrier Assisted TransportFacilitated Transport

Active Transport

Vesicle Mediated TransportExocytosis

Endocytosis

Phagocytosis

PinocytosisRece tor-mediated endoc tosis

Transport review

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Transport review

Fig 5 14a Adhesion junction

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Fig. 5.14a Adhesion junction

Fig 5 14b Tight junction

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Fig. 5.14b Tight junction

Fig 5 14c Gap junction

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Fig. 5.14c Gap junction

Fig 5 15 Extracellular matrix

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Fig. 5.15 Extracellular matrix

5 4 Modification of Cell Surfaces

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5.4 Modification of Cell Surfaces

Plant Cell Wallssurround the plasma membraneporous

Primary cell wall

all plant cells have itcontains cellulose fibrils

contains pectins to allow the cell

wall to stretch during growthnoncellulose polysaccharides

harden the wall when the cell is

mature


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Plant Cell WallsMiddle lamella

pectin-rich layer that serves as an

adhesive between cells

Secondary cell wallcan form inside primary cell wall

contains more cellulose

cellulose fibrils at right angleslignin for strength

Plant cell wall structure

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Plant cell wall structure


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Plant Cell WallsPlasmodesmata

connect the cytoplasm of plant cells

only allow water and small solutes

to pass freely

Fig 5 16 Plasmodesmata

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Fig. 5.16 Plasmodesmata

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