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Membrane Structure and

Function

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Membrane Composition Membrane fluidity

Fatty acids, cholesterol Membrane Proteins Membrane Permeability

Diffusion, osmosis, tonicity, osmoregulation Active transport, membrane potential

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Boundary that separates living cell from itsenvironment

Selective permeability- allowing some substances to

cross it more easily than others

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Composed of lipids,proteins, carbohydrates

Phospholipids- most abundantlipid in the plasma membrane

Amphipathicmolecules-

containing hydrophobic &hydrophilic regions

Hydrophilic

Hydrophilic

Hydrophobic

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Membrane is a fluid structure with a mosaic ofvarious proteins embedded in it

Hydrophilic region

of protein

Hydrophobic region of protein

Phospholipidbilayer

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Phospholipids in the plasma membrane can move within

the bilayer (fluid as salad oil)

Held in place by hydrophobic interactions (weak)

Lipids, (some proteins), drift laterally Rarely- molecule flip-flop across the membrane

Lateral movement

(~107times per second)

Flip-flop

(~ once per month)

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As temperatures cool, membranes switch from afluid state to a solid state

Temperature where membrane solidifies dependson types of lipids (fatty acids)

Membranes rich in unsaturated fatty acids aremore fluid at lower temperature than saturatedfatty acids

*IMPORTANT CONCEPT!

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ViscousFluid

Unsaturated hydrocarbontails with kinks

Saturated hydro-carbon tails

Cannot pack together

due to double bondsbetween carbons in the

hydrocarbon chain

No double bonds

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Cholesterol (steroid) - effects membrane fluiditydepending on the temperature

Warm temperatures- restrains movement ofphospholipids, making them less fluid

Cool temperatures- maintains fluidity bypreventing tight packing Lowers the temperature required for a membrane to

solidify

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Cholesterol

Cholesterol within the animal cell membrane

Fluidity must be maintained to work properly- balance

-too fluid or not fluid enough will not function

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Membrane Proteins

Integral or peripheral

Functions

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Proteins in the plasma membrane can move Different types of cells have different types of

proteins in their plasma membrane

Integral protein- penetrate the hydrophobic core of thelipid bilayer

Transmembrane protein- spans the entire membrane

Peripheral protein- not embedded in the membrane,bound to it

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(type of integral protein)

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EXTRACELLULARSIDEN-terminus

C-terminusCYTOPLASMIC

SIDEHelix

Integral protein- hydrophobic regions consist ofnonpolar AAs, often coiled into a-helices

Transmembrane Integral Protein

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Transport

Enzymatic activity

Signal transduction Cell-cell recognition

Intercellular joining

Attachment to the cytoskeleton and ECM

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ATP

Enzymes

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Signal

Receptor

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Cells recognize each other by binding to surfacemolecules

Recognize self vs non-self (pathogens, transplant tissue)

Surface molecules usually carbohydrates Covalently bonded to lipids- glycolipids

Covalently bonded to proteins- glycoproteins

Markers on your blood cells

Type A, B, AB or O

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Permeability of lipid bilayer

Polar vs Nonpolar

Transport proteins

Types of transport (sec 4)

Diffusion Facilitated diffusion

Active transport

Bulk transport

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Hydrophobic (nonpolar) molecules- dissolve in thelipid bilayer & pass through the membrane rapidly

O2, CO2, hydrocarbons

Hydrophilic (polar) molecules- do not cross the

membrane easily Sugars (glucose), ions, water

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Allow passage of hydrophilic substances- veryspecific

Channel proteins- hydrophilic channel that specificmolecules or ions can use as a tunnel

Aquaporins- facilitate the passage of water

Carrier proteins- bind to molecules and changeshape to shuttle them across the membrane

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ATP

Channel

Protein CarrierProtein

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Types of transport

Passive (diffusion): high to low

Channel proteins Aquaporin or ion channel

Carrier proteins

Active: low to high ONLY carrier proteins

Hydrophobic

substances diffuse

right across

membrane. Dontneed transport

proteins.

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Thermal motion (heat)- movement of molecules

Diffusion- molecules spread out evenly into the

available space

Each molecule moves randomly (Brownian Motion)

All the of molecules- movement in one direction

Molecules move HIGH concentration to LOW concentration

Equally spread throughout

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Heat: increase heat, increase molecular collisions,increase diffusion rate

Direct relationship

Size: smaller molecules move faster than largermolecules, smaller molecule will diffuse faster

Inverse relationship

Concentration gradient: steeper gradient, fasterdiffusion

Direct relationship

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Molecules of dye Membrane

WATER

Net diffusion Net diffusion Equilibrium

Diffusion

Dye is diffusing down its own concentration gradient.

NO work- no energy is added to the system.

Spontaneous process.

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Concentration Gradient- the difference in concentration of

a substance from one area to another

Moves HIGH to LOW

Passive transport- requires no energy from the cell tomake it happen

Only happens in a cell if the substance can readily cross

the plasma membrane

Water can diffuse across the cell membrane through

aquaporins (hydrophilic channels)

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Diffusionrefers to the movement of

SOLUTE (molecules) down its OWN

concentration gradient

Do NOT confuse this with osmosis- thediffusion of WATER (solution)

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Osmosis-diffusion of water across a selectivelypermeable membrane

Water is diffusing on its OWN concentrationgradient From a high to low concentration

Water diffuses across a membrane- from an area oflow solute concentration (high water) to an area ofhigh solute concentration (low water)

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Lower

concentration

of solute (sugar)

Higher

concentration

of sugar

Same concentration

of sugar

Selectively

permeable mem-

brane: sugar mole-

cules cannot pass

through pores, but

water molecules can

H2O

Osmosis- diffusion

of water (not

solute- sugar)

High

H2O Low

H2O

Solutes Suck

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Ability of a solutionto cause a cell to gain or lose water

Isotonic solution: solute concentration is the same as that

inside the cell; no net water movement across the plasmamembrane

Hypertonic solution: solute concentration is greater than

that inside the cell; cell loses water (cell shrivel up)

Hypotonic solution: solute concentration is less than that

inside the cell; cell gains water (cell burst)

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To maintain their internal environment-osmoregulate

Control the balance of water within themselvesand the outside environment

Paramecium lives in a hypotonic environment(pond water)

Contractile vacuole that acts as a pump

50

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Filling vacuole50 m

50 mContracting vacuole

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Hypotonicenvironment- plantcell swells, cell wallexerts pressure, opposes uptake- turgid (firm)

Isotonic environment- no net movement of water;flaccid (wilt)

Hypertonicenvironment- plant cells lose water Plasmolysis- plasma membrane pulls away from the wall

***Plants, bacteria, fungi***

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Tonicity- ability of a solutionto cause a cell togain or lose water

Osmoregulation- animals/plants control thebalance of water within themselves and theoutside environment

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Animalcell

Lysed

H2O H2O H2O

Normal

Hypotonic solution Isotonic solution Hypertonic solution

H2O

Shriveled

H2OH2OH2OH2OPlantcell

Turgid (normal) Flaccid Plasmolyzed

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Transport proteins speed movement of polar moleculesacross the plasma membrane Very specific, water or small ions

Channel proteins- provide corridors that allow a specificmolecule or ion to cross the membrane

Ion channels- gated channels, stimulus causes them to

open/close Stimulus- chemical or electrical

Chemical- not the substance that will be transported

Neurotransmitter can open Na+ ion channels

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Carrier proteins undergo change in shape thattranslocates the solute-binding site across themembrane EXTRACELLULAR

FLUID

Channel protein SoluteCYTOPLASM

Carrier protein Solute

Passive diffusion-

Moving down its

concentration gradient

(high to low)

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Moving solutes from LOWconcentration to aHIGHconcentration

Active transport- requires energy, usually ATP

Performed by carrier proteins only Channels are just open passage ways

Sodium-potassium pump- higher concentration ofK+ and lower concentration of Na+ inside the cellcompared to environment

EXTRACELLULAR [Na+] high Na+

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Cytoplasmic Na+bonds to

the sodium-potassium pump

CYTOPLASMNa+

[Na+] low

[K+] high

Na+

Na+

FLUID[Na ] high

[K+] low

Na+

Na+

Na+

ATP

ADP

P

Na+binding stimulates

phosphorylation by ATP.

Na

Na+

Na+

Phosphorylation causes

the protein to change its

conformation, expelling Na+

to the outside.

P

Extracellular K+binds

to the protein, triggering

release of the phosphategroup.

P

P

Loss of the phosphate

restores the proteins

original conformation.

K+is released and Na+

sites are receptive again;

the cycle repeats.

add a phosphate group

Passive transport Active transport

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Diffusion Facilitated diffusion

Passive transport

ATP

Active transport

Nonpolar PolarPolar

hydrophobic hydrophilic hydrophilic

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All cells have voltage across their plasma membranes

Voltage-electrical potential energy, separation of opposite

charges

Cytoplasm has a negativecharge compared to

extracellular fluid

Unequal distribution of anions (- ions) & cations (+ ions)

Membrane potential- voltage difference across a

membrane

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Range -50 to -200 mV Minus sign indicated negative inside the cell compared to

outside

Acts like a battery- energy source that affects thetrafficking of all charged substances across themembrane

Inside cell is negative- membrane potential favorsthe passive transport of cations (+) in to the cell &anions (-)outof the cell

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Two combined forces drive the diffusion of ionsacross a membrane: Ions concentration gradient- chemical force

Effect of the membrane potential on the ionsmovement-electrical force

Passive diffusion- ion moves down itselectrochemical gradient Includes concentration, as well as charge across

membrane

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Resting nerve cell: [Na+] lower inside cellthan out

Stimulated- gated Na+ channels open up Na+ ions fall down electrochemical gradient

1. Driven by concentrationgradient of Na+

2. Driven by the attraction of cations to thenegative interior of the cell (charge)

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Membrane proteins contribute to membranepotential (difference of voltage across the membrane)

Sodium-potassium pump- pumps 3 Na+ ions outfor every 2 K+ ions pumps in

Net transfer of +1 charge to extracellular environment

Stores energy in form of voltage

Electrogenic pump - transport protein thatgenerates the voltage across a membrane

El t i P t

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H+

ATP

CYTOPLASM

EXTRACELLULAR

FLUID

Proton pump

H+

H+

H+

H+

H+

+

+

+

+

+

Electrogenic Pump- proton pump

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H+

ATP

Proton pump

Sucrose-H+

cotransporter

Diffusion

of H+

Sucrose

H+

H+

H+

H+

H+H+

+

+

+

+

+

+

Downhilldiffusion of

H+ coupled

to uphill

transport ofsucrose

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Large molecules cross the membrane via vesicles

Exocytosis-transport vesicles migrate to

membrane, fuse with it, & release their contents Many secretory cells use exocytosis to export their

products

Endocytosis-cell takes in macromolecules byforming vesicles from the plasma membrane

Reversal of exocytosis, involving different proteins

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Phagocytosis (eating)- cell engulfs particles in avacuole

Pinocytosis (drinking)- cell creates vesicle aroundfluid

Receptor-mediated endocytosis- binding ofligands to receptors triggers vesicle formation

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Receptor

RECEPTOR-MEDIATED ENDOCYTOSIS

Ligand

Coated

pit

Coated

vesicle

Coat protein

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1. Passive- NO energy High to LowA. Diffusion across membrane

Small or hydrophobic (nonpolar) molecules only

B. Facilitated diffusion (through a transportprotein)

1. Channel protein- specific, some gated2. Carrier protein- specific

2. Active- ENERGY (ATP!) Low to HighA. Molecule must go through transport protein

CARRIER proteins only!

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Pump- Active ATP- Active

Diffusion- Passive

Explain the concept of voltage, electrochemical

gradient, membrane potential in your ownwords.

A. Plasma membrane content

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1. Phospholipids

2. Proteins

B. Membrane Fluidity

1. Unsaturated/saturated fats- cold temp

2. Cholesterol- cold/hot temp

C. Membrane Proteins

1. Types

A. Integral (transmembrane)

B. Peripheral

2. Functions

D. Types of Transport

Passive (diffusion): high to low Channel proteins- aquaporin or ion channel

Carrier proteins

Active: low to high

ONLY carrier proteins

A. Tonicity

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1. Plants & animal cells

2. Isotonic, hypertonic, hypotonic

B. Na/K+ Pump (electrogenic pump)

1. How does it function?

2. What are the consequences on the cell?

C. Membrane Potential

1. Electrochemical gradient- gradient of ions based on concentration &

charge

2. Voltage

3. Electrogenic pump- Na/K+ pump and proton pump (establish

volatage)

4. Co-transportD. Bulk Transport

1. Endocytosis: pinocytosis, phagocytosis, receptor-mediated

2. Exocytosis