Cell Memb Trans L2

8/14/2019 Cell Memb Trans L2

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AP BiologyAP Biology

The Cell MembraneCell Physiology

[email protected]


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AP Biology

Overview

Cell membrane separates living cell fromnonliving surroundingsu thin barrier = 8nm thick

Controls traffic in & out of the cellu selectively permeableu allows some substances to cross more

easily than others hydrophobic vs hydrophilic

Made ofphospholipids, proteins & othermacromolecules


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Phospholipids

Fatty acid

Phosphate

Fatty acid tailsu hydrophobic

Phosphate group headu

hydrophilic Arranged as a bilayer


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Phospholipid bilayer

polarhydrophilic

heads

nonpolarhydrophobic

tails

polarhydrophilic

heads


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More than lipids

In 1972, S.J. Singer & G. Nicolsonproposed that membrane proteins areinserted into the phospholipid bilayer


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Membrane fat composition varies

Fat composition affects flexibilityu membrane must be fluid & flexible

about as fluid as thick salad oil

u % unsaturated fatty acids inphospholipids keep membrane less viscous

u cholesterol in membrane


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Membrane Proteins

Proteins determine membranes specific functionsu cell membrane & organelle membranes each

have unique collections of proteins

Membrane proteins:u peripheral proteins

loosely bound to surface of membrane

cell surface identity marker (antigens)

u integral proteins

penetrate lipid bilayer, usually across wholemembrane

transmembrane protein

transport proteins

w channels, permeases (pumps)


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Proteins domains anchor molecule

Within membraneu nonpolaramino

acids hydrophobic

anchors proteinintomembrane

On outer surfaces of

membraneu polaramino acids

hydrophilic

extend into

extracellular

Polar areasof protein

Nonpolar areas of protein


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Many Functions of Membrane Proteins

Outside

Plasmamembrane

Inside Transporter Cell surfacereceptor

Enzymeactivity

Cell surfaceidentity marker Attachment to thecytoskeletonCell adhesion


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Membrane carbohydrates

Play a key role in cell-cell recognitionu ability of a cell to distinguish one cell

from another antigens

u important in organ &tissue development

u basis for rejection of

foreign cells byimmune system


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Movement across the

Cell Membrane


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Diffusion

2nd Law of Thermodynamicsgoverns biological systemsu universe tends towards disorder

(entropy)

Diffusionu

movement from high

low concentration

Diffusionu

movement from high

low concentration


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Diffusion

Move from HIGH to LOW concentrationu passive transport

u no energy needed

diffusion osmosis

movement of water


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Diffusion across cell membrane

Cell membrane is the boundary betweeninside & outsideu separates cell from its environment

INfood

carbohydratessugars, proteinsamino acidslipidssalts, O2,H2O

OUTwaste

ammoniasaltsCO2H2O

products

cell needs materials in & products or waste out

IN

OUT

Can it be an impenetrable boundary? NO!


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Diffusion through phospholipid bilayer

What molecules can get through directly?u fats & other lipids

inside cell

outside cell

lip

id

salt

aa

H

2

O

sugar

What molecules can

NOT get throughdirectly?

upolar molecules

H2O

uionssalts, ammonia

ularge moleculesstarches,

proteins


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Channels through cell membrane

Membrane becomes semi-permeablewith protein channelsu specific channels allow specific

material across cell membrane

inside cell

outside cell

suga

r

aa

H

2

O

sal


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Facilitated Diffusion

Diffusion through protein channelsu channels move specific molecules across

cell membrane

u no energy needed

The BouncerThe Bouncer

open channel = fast transport

facilitated = with help

high

low


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Active Transport

The DoormanThe Doorman

conformationalchange

Cells may need to move molecules againstconcentration gradientu shape change transports solute from

one side of membrane to otheru protein pumpu costs energy = ATP

ATP

low

high


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Active transport

Many models & mechanisms

ATP

ATP


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Getting through cell membrane

Passive Transportu Simple diffusion

diffusion of nonpolar, hydrophobic moleculesw lipids

w high low concentration gradient

u Facilitated transport diffusion of polar, hydrophilic molecules

through a protein channelw high low concentration gradient

Active transportu diffusion againstconcentration gradient

low high

u uses a protein pumpu requires ATP

ATP


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Transport summary

simplediffusion

facilitateddiffusion

activetransport

ATP


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How about large molecules?

Moving large molecules into & out of cellu through vesicles & vacuoles

u endocytosis phagocytosis = cellular eating

pinocytosis = cellular drinking

u exocytosis

exocytosis


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Endocytosis

phagocytosis

pinocytosis

receptor-mediatedendocytosis

fuse withlysosome fordigestion

non-specificprocess

triggered bymolecularsignal


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The Special Case of Water

Movement of water acrossthe cell membrane


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Osmosis is diffusion of water

Water is very important to life,so we talk about water separately

Diffusion of water fromhigh concentration of water tolow concentration of wateru across a

semi-permeable

membrane


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Concentration of water

Direction of osmosis is determined bycomparing total solute concentrations

u Hypertonic - more solute, less water

u Hypotonic - less solute, more wateru Isotonic - equal solute, equal water

hypotonic hypertonic

water

net movement of water


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Managing water balance

Isotonicu animal cell immersed in

mild salt solution

example:

blood cells in bloodplasma

problem: none

wno net movement

of watern flows across

membraneequally, inbothdirections

wvolume of cell is balanced


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Hypotonicu a cell in fresh water

example: Paramecium

problem: gains water,swells & can burst

wwater continually enters

Paramecium cell

solution: contractile vacuole

wpumps water out of cell

w ATP


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Hypertonicua cell in salt water

example: shellfish

problem: lose water & diesolution: take up water or

pump out salt


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Aquaporins

Water moves rapidly into & out of cellsu evidence that there were water

channels

1991 | 2003

Peter AgreJohn Hopkins

Roderick MacKinnonRockefeller


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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 .03 M

Osmosis

Cell Memb Trans L2

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