CH 7:Membrane Structure

AP Biology

CH 7:Membrane Structure

and Function

AP Biology

Overview



 thin barrier = 8nm thick



 selectively permeable

 allows some substances to cross more easily than others

 hydrophobic vs hydrophilic



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Phospholipids

Fatty acid Phosphate

 Fatty acid tails

 hydrophobic

 Phosphate group head

 hydrophilic

 Arranged as a bilayer

Aaaah,

one of those

structure–function examples

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Phospholipid bilayer

polar

hydrophilic heads

nonpolar hydrophobic tails

polar

hydrophilic heads

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Fluid Mosaic Model

 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 is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembrane proteins

Filaments of cytoskeleton Peripheral

protein Glycoprotein

Phospholipids

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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Figure 7.6

Lateral movement occurs

10⁷ times per second.

Flip-flopping across the membrane is rare ( once per month).

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Membrane fat composition varies

 Fat composition affects flexibility

 membrane must be fluid to work properly

 about as fluid as thick salad oil

 higher unsaturated fatty acids in

phospholipids keep membrane less viscous

 cold-adapted organisms, like winter wheat

 increase % of unsaturated fatty acids in autumn

• 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

© 2011 Pearson Education, Inc.

Figure 7.8

Fluid

Unsaturated hydrocarbon tails

Viscous

Saturated hydrocarbon tails

(a) Unsaturated versus saturated hydrocarbon tails

(b) Cholesterol within the animal

cell membrane prevent tight packing

Cholesterol

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Membrane Proteins



 cell membrane & organelle membranes each have unique collections of proteins



 peripheral proteins

 loosely bound to surface of membrane

 cell surface identity marker (antigens)

 integral proteins

 penetrate lipid bilayer, usually across whole membrane

 transmembrane protein

 transport proteins

 channels, permeases (pumps)

AP Biology 2007-2008

Why are proteins the perfect molecule to build structures

in the cell membrane?

They’re Bi-polar

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Classes of amino acids

What do these amino acids have in common?

nonpolar & hydrophobic

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Classes of amino acids

What do these amino acids have in common?

polar & hydrophilic

I like the polar ones the best!

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Proteins domains anchor molecule

 Within membrane

 nonpolar amino acids

 hydrophobic

 anchors protein into membrane

 On outer surfaces of membrane

 polar amino acids

 hydrophilic

 extend into

extracellular fluid &

into cytosol

Polar areas of protein

Nonpolar areas of protein

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NH₂

H⁺

COOH

Cytoplasm Retinal

chromophore

Nonpolar (hydrophobic) a-helices in the

cell membrane _H⁺ Porin monomer

b-pleated sheets Bacterial outer

membrane

proton pump channel

in photosynthetic bacteria water channel

in bacteria

function through

conformational change = shape change

Examples

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Many Functions of Membrane Proteins

Outside

Plasma membrane

Inside

Transporter Cell surface

receptor Enzyme

activity

Cell surface identity marker

Attachment to the cytoskeleton

Cell adhesion

1. Transport Proteins

Transport proteins allow passage of hydrophilic

substances across the membrane

Some transport proteins, called channel proteins, have a hydrophilic

channel that certain

molecules or ions can use as a tunnel

Channel proteins called aquaporins facilitate the passage of water

Channel proteins Aquaporins

Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane

A transport protein is

specific for the substance it moves

© 2011 Pearson Education, Inc.

• An electrogenic pump is a transport protein that generates voltage across a membrane

• The sodium-potassium pump is the major electrogenic pump of animal cells

© 2011 Pearson Education, Inc.

Figure 7.20

CYTOPLASM

ATP EXTRACELLULAR

FLUID Proton pump

H^

H^

H^ H^

H^ H^

















The main electrogenic pump of plants, fungi, and bacteria is a proton pump

Electrogenic pumps help store energy that can be used for cellular work

Cotransport: Coupled Transport by a Membrane Protein

• Cotransport occurs when active transport of a solute indirectly drives transport of other

solutes

• Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive

active transport of nutrients into the cell

© 2011 Pearson Education, Inc.

Figure 7.21

ATP

H^

H^

H^ H^

H^

H^

H^ H^

Proton pump

Sucrose-H^ cotransporter

Sucrose

Sucrose Diffusion of H^

















2. Membrane Protein as Enzymes

Embedded enzymes have active site for attachment of substrate

Embedded Enzymes in inner mitochondrial membrane required for cellular respiration

3. Cell Surface Receptor Protein

An example of Cell surface receptor protein is Tyrosine kinases. They cause Signal transduction.

Signal Transduction is

conversion of signal from a signaling molecule to a form that can bring about a

specific cellular response.

1.Signal molecules attach to membrane embedded receptor tyrosine kinase proteins.

2. Two monomers of tyrosine kinase form dimer after attachment.

3. Phosphorylation by ATP molecules activate them.

4. On activation, tyrosine kinases activate relay protein which causes signal transduction and appropriate cellular response.

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4. Cell Surface Identity Marker

 Membrane Carbohydrate

Play a key role in cell-cell recognition

 ability of a cell to distinguish one cell from another

 antigens

 important in organ &

tissue development

 basis for rejection of foreign cells by

immune system

Figure 7.11

Receptor (CD4)

Co-receptor (CCR5)

HIV

Receptor (CD4)

but no CCR5 Plasma

membrane HIV can infect a cell that

has CCR5 on its surface, as in most people.

HIV cannot infect a cell lacking CCR5 on its surface, as in

resistant individuals.

Membrane proteins attached to

Extracellular Matrix cytoskeleton

Examples are selectin and integrin

They are anchoring molecules

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The Membrane Regulates !!!!

 2nd Law of Thermodynamics governs biological systems

 universe tends towards disorder (entropy)

 ^Diffusion

 movement from high  low concentration

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Diffusion

 Move from HIGH to LOW concentration

 “passive transport”

 no energy needed

diffusion osmosis

movement of water

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Diffusion across cell membrane

 Cell membrane is the boundary between inside & outside…

 separates cell from its environment

IN

food

carbohydrates sugars, proteins amino acids

lipids

salts, O₂, H₂O

OUT

waste

ammonia salts

CO₂ H₂O

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?

 fats & other lipids

inside cell

outside cell

lipid

salt

aa ^H₂^O sugar

NH₃



directly?

 polar molecules

 H₂O

 ions

 salts, ammonia

 large molecules

 starches, proteins

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Channels through cell membrane

 Membrane becomes semi-permeable with protein channels

 specific channels allow specific material across cell membrane

inside cell

outside cell sugar aa

H₂O

salt NH₃

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Facilitated Diffusion



 channels move specific molecules across cell membrane

 no energy needed

“The Bouncer”

open channel = fast transport facilitated = with help

high

low

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Active Transport

“The Doorman”

conformational change



 shape change transports solute from one side of membrane to other

 protein “pump”

 “costs” energy = ATP

ATP

low

high

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Getting through cell membrane



 Simple diffusion

 diffusion of nonpolar, hydrophobic molecules

 lipids

 high  low concentration gradient

 Facilitated transport

 diffusion of polar, hydrophilic molecules

 through a protein channel

 high  low concentration gradient



 diffusion against concentration gradient

 low  high

 uses a protein pump

 requires ATP

ATP

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Transport summary

simple diffusion

facilitated diffusion

active

transport

ATP

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How about large molecules?

 Moving large molecules into & out of cell

 through vesicles & vacuoles

 endocytosis

 phagocytosis = “cellular eating”

 pinocytosis = “cellular drinking”

 exocytosis

exocytosis

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Endocytosis

phagocytosis

pinocytosis

receptor-mediated endocytosis

fuse with

lysosome for digestion

non-specific process

triggered by molecular signal

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The Special Case for Water

 Osmosis is diffusion of water!!!

 Water is very important to life,

so we talk about water separately

 Diffusion of water from high concentration of water to low concentration of water

 across a

semi-permeable membrane

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Concentration of water

 Direction of osmosis is determined by comparing total solute concentrations

 Hypertonic - more solute, less water

 Hypotonic - less solute, more water

 Isotonic - equal solute, equal water

hypotonic hypertonic water

net movement of water

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Cell survival depends on balancing water

uptake & loss

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Turgid Cell

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Flaccid Cell

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Plasmolyzed Cell

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Managing water balance

 ^Isotonic

 animal cell immersed in mild salt solution

 example:

blood cells in blood plasma

 problem: none

no net movement of water

 flows across membrane equally, in both directions

volume of cell is stable

balanced

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Managing water balance

 ^Hypotonic

 a cell in fresh water

 example: Paramecium

 problem: gains water, swells & can burst

 water continually enters Paramecium cell

 solution: contractile vacuole

 pumps water out of cell

 ATP

 plant cells

 turgid

freshwater

ATP

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Water regulation

 Contractile vacuole in Paramecium

ATP

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Managing water balance

 Hypertonic

 a cell in salt water

 example: shellfish

 problem: lose water & die

 solution: take up water and pump out salt. Salt is removed by chloride secretory cells in the gills, which

actively transport salts from the blood into the surrounding water.

 plant cells

 plasmolysis = wilt

saltwater

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Aquaporins

 Water moves rapidly into & out of cells

 evidence that there were water channels

1991 | 2003

Peter Agre John Hopkins

Roderick MacKinnon Rockefeller

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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…

Water Potential

Osmosis & Plant cells

Water Potential



Water potential is the potential energy of water per unit volume. It is also called free energy per mole of water.



Water potential measures the tendency of water to diffuse from one compartment to another compartment.



Water moves from an area of higher water

potential or higher free energy to an area of

lower water potential or lower free energy.

Hypotonic Solution Hypertonic Solution More Free Water Less Free Water

Higher Water Potential Lower Water Potential

Water potential



Water Potential symbol is 

  is called



Water potential is calculated from two major components:

1.) solute concentration

2.) physical pressure (cell wall)

Calculating Water Potential



 = 

+ 

Or

Water = pressure + solute

Potential potential potential

Solute Potential 

 Measure of the tendency of water

to move into a region due to the

presence of solutes.

Solute Potential 



Solutes bind water molecules reducing the number of free water molecules  lowers waters

ability to do work.

Solute Potential

Solute potential is affected by

concentration of solute in a solution



of any solution at atmospheric pressure is always negative – why?

Answer = less free water molecules to do work

Solute potential = - osmotic pressure

Pressure Potential 

 

is measure of physical (hydrostatic) pressure on a solution exerted by cell walls.



Pressure potential in living plant system is

positive except for xylem tissues.

Turgor pressure is the outward pressure that occurs in a plant cell

when the cytoplasm and vacuoles fill up with

water and the cell membrane pushes against the cell wall.

Pressure potential = Turgor pressure

Plant cells are surrounded by rigid cellulose walls, (unlike animal cells).

Plant cells do not burst when placed in

pure water because their cellulose cell

walls limit how much water can move

in.

Plant Cell when placed in hypotonic solution becomes turgid and attains equilibrium

Turgid cell is normal plant cell

Adhesion

Two types of water-conducting

cells

Cohesion

300 m Direction

of water movement

Adhesion cause water transport in plants.

• As water evaporates from a leaf, hydrogen

bonds cause water molecules leaving the veins to tug on molecules farther down, and the

upward pull is transmitted through the water conducting cells all the way to the roots.

• Adhesion of water to cell walls by hydrogen bonds helps counter the downward pull of gravity.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 3.2

Plants transport water by

maintaining positive potential in gradient form by changing concentration of solutes in their cytoplasm. The cells mostly have low water potential compared to

surrounding

solution causing

water to move in to the cell.

The transport vessels in plants, xylem and phloem, are typically long and slender like pipes (form fits function).

Xylem transports and

stores water and water-soluble nutrients in vascular plants.

Phloem is responsible for transporting sugars, proteins, and other organic molecules in plants.

The sucrose is actively transported against its concentration gradient (a process

requiring ATP) into the phloem cells using cotransport pump (refer to slide 25).

Water conduction

1. Transpiration pulls water up xylem vessels

2. Source cells (leaves) of sucrose load it into

phloem reducing their water potential.

3. Water is taken up from xylem vessels by

osmosis (higher to lower water potential).

4. Internal pressure

differences drive the sap down the phloem to sink cells (roots).

5. Sucrose is unloaded into sink cells.

6. Water moves back to xylem vessels by osmosis.

Stomata



Transpiration is regulated through the opening and closing of stomata on the leaf surface.

Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as

light intensity and quality, leaf water status,

and carbon dioxide concentrations.

Stomata regulates transpiration



Stomata must open to allow air containing

carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is

lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss. Plants

growing in low water environment have lower

number of stomata.

Solve for water potential



Pressure potential for cells at atmospheric pressure is 0.



Solute potential for pure water is 0 (highest value)



Adding solutes to water lowers it water

potential. Solute potential for a solution is

always negative

Calculating Solute potential



Need solute concentration



Use the equation 

^{= - iCRT}

i = # particles molecule makes in water C = Molar concentration

R =

0.0831 liter bar mole

K T = temperature in degrees Kelvin

= 273 +

C

Solve for water potential (literal equation)



Knowing solute potential, water

potential can be calculated by inserting values into the water potential

equation.

 = 

+ 

In an open container, 

^{= 0}