Chapter 5Chapter 5A Closer Look At Plasma A Closer Look At Plasma

MembranesMembranes

Honors BiologyHonors Biology

It Isn’t Easy Being Single? (why not)• Concentrations of ions & other substances outside the cell may rapidly

become too high or low. A mechanism is needed to selectively permit substances to enter or leave the cell.

What is homeostasis?• Homeostasis is the name given to the dynamic processes that enable

optimum conditions to be maintained for constituent cells, in spite of continual changes taking place both internally and externally.

What mechanism is needed to selectively permit substances to enter or leave the cell?

• Phospholipid bi-layer with embedded proteins. How does a cell maintain homeostasis?

• By controlling what enters and leaves the cell. What are 2 examples of homeostasis with the human body?

Membrane Structure & Function

lipid bilayer

phospholipid molecule

water

Phospholipid molecules Phospholipid molecules form a form a bilayerbilayer..

Phosphate/glycerol head is Phosphate/glycerol head is polar & polar & hydrophilichydrophilic, two , two fatty acid tails are fatty acid tails are nonpolar & nonpolar & hydrophobichydrophobic..

Cholesterol Cholesterol (right) is another (right) is another lipid molecule lipid molecule that’s commonly that’s commonly found in the found in the plasma plasma membrane.membrane.

Note: Do hydrophilic substances have an easier time passing thru the membrane?

NO

Question: What is the arrangement of the phospholipids?

H – T - T - H

• What is the ‘Fluid Mosaic Model’ of the plasma What is the ‘Fluid Mosaic Model’ of the plasma membrane structure?membrane structure?

open channel

proteinsgated channel

proteins

transport proteinlipid

bilayer

•The lipid molecules are free to move and protein The lipid molecules are free to move and protein molecules float independently in the lipid bi-layer.molecules float independently in the lipid bi-layer.

A wide variety of protein molecules protein molecules float around in the lipid bi-layer and perform most of the functions of the

membrane.

cytoskeletal

proteins (beneath

the plasma

membrane)

ADHESION

PROTEIN

TRANSPORT PROTEINS

EXTRACELLULAR ENVIRONMENT

open

channel

protein

gated

channel

protein

(open)

gated

channel

protein

(closed)

active

transport

protein

RECEPTOR

PROTEIN

RECOGNITION

PROTEIN

LIPID

BILAYER

area of

enlargement

oligosaccharide groups

phospholipid

cholesterol

7 Types of Membrane Proteins(Note: Most of the plasma membrane functions are carried out by these proteins)

1. Cytoskeletal Proteins - composed of microtubules. Needed for internal supportinternal support and attachment.

2. Adhesion Proteins - composed of glycoproteins attached to oligosaccharides. Serve as the “glue” to hold cells togetherhold cells together.

3. Open-Channel Proteins- needed for passive transportpassive transport and facilitated diffusionfacilitated diffusion.

4. Gated Channel Proteins - help control the directional flowdirectional flow or transport of ions across the membrane.

7 types of membrane proteins..cont.

5. Carrier Proteins - carry on active active transporttransport requiring energy (ATPATP) to “actively pump” their cargo across the membrane.

6. Receptor proteins - these proteins have binding sites for hormones and enzymesbinding sites for hormones and enzymes, which allow them to do their work .

7.Recognition Proteins -these proteins are the “fingerprintsfingerprints” of the cells. These self-recognizing proteins identify their own body cells or recognize invaders.

•What will happen in the example above?

•What causes the reaction?

•What energy creates the reaction below?

•When will the reaction stop?

Concentration GradientsConcentration Gradients Molecules are in constant

motion, due to their own kinetic energykinetic energy.

If there are more molecules of a substance in one area

than another, a concentration gradientconcentration gradient

exists. Random collisions of molecules cause them to

move from an area where they’re highly concentratedhighly concentrated

to an area of low low concentrationconcentration.

Once the molecules have arranged themselves equally

throughout two adjoining regions, it’s called “dynamic dynamic

equilibriumequilibrium.”

At this point, a concentration gradient no longer exists.

DiffusionDiffusionThe net movement of like molecules down their concentration gradient is known as diffusiondiffusion.

The rate at which diffusion occurs can be affected by the following factors:

(a)(a) Molecular sizeMolecular size – smaller molecules move faster than larger ones.

(b)(b) TemperatureTemperature – heat energy causes molecules to move more rapidly & to collide more frequently.

(c)(c) Electrical gradientsElectrical gradients – the negative side of a membrane will attract positive ions & repel negative ions.

(d)(d) Pressure gradientsPressure gradients – applying pressure can speed up the rate at which molecules move.

OsmosisOsmosis

The movement of water across a selectively permeable membrane in response to concentration gradients, fluid pressure, or both is known as osmosisosmosis.

selectively permeable membrane

between two compartments

water

molecules

protein

molecules

Osmosis in a Plant Cell

TonicityTonicity The relative

concentrations of solutes in two fluids is known as tonicitytonicity.

Water tends to move to areas where solute solute concentrations are concentrations are greatergreater.

When solute concentrations in two fluids are equal, we say the two fluids are isotonicisotonic.

In an isotonic solution, there is no net no net movementmovement of water in either direction.

Water moves from a hypotonic solution to a Water moves from a hypotonic solution to a hypertonic solution.hypertonic solution.

compartment

1

compartment

2

HYPOTONICSOLUTION

HYPERTONICSOLUTION

membrane permeable to

water but not to solutesfluid volume increases

In compartment 2

TonicityTonicity

2M sucrose

solution

1 liter of

distilled water

10M sucrose

solution

2M sucrose

solution

HYPOTONIC

CONDITIONS

ISOTONIC

CONDITIONS

HYPERTONIC

CONDITIONS

A hypotonichypotonic solution has fewer solutes dissolved in

it than an adjoining solution does.

A hypertonichypertonic solution has more solutes

dissolved in it than an adjoining solution does.

As a result, water water molecules will move from molecules will move from a hypotonic solution to a a hypotonic solution to a

hypertonic solutionhypertonic solution.

Plasma membranes arePlasma membranes are selectively permeableselectively permeable.

X

O2, CO2, other small

nonpolar molecules,

as well as H2O

C6H12O6, other large, polar

water-soluble molecules,

ions (such as H+, Na+, K+,

Ca++, CI-) along with H2O

(This means that some substances are able to enter the cell and others are not.)

high

low

concentration gradient

DIFFUSION ACROSS

LIPID BILAYERS

lipid-soluble

substances as

well as water

diffuse across

PASSIVE

TRANSPORT

Water-soluble substances, and water,

diffuse through interior of transport

proteins. No energy boost required.

Also called facilitated diffusion

ACTIVE

TRANSPORT

Specific solutes are

pumped through interior

of transport proteins.

Requires energy boost

Passive Transport vs. Active TransportPassive Transport vs. Active Transport

SPECIALIZED TYPE OF VACUOLESPECIALIZED TYPE OF VACUOLE::CONTRACTILE VACUOLESCONTRACTILE VACUOLES:: PROTISTS HAVE THESE PROTISTS HAVE THESE

ORGANELLES - THEY COLLECT EXCESS WATER ORGANELLES - THEY COLLECT EXCESS WATER AND EXCRETE IT SO THE CELL DOESN’T BURST. AND EXCRETE IT SO THE CELL DOESN’T BURST. (cytolysize)(cytolysize)

contractile

vacuole (filled)

contractile

vacuole

(emptied)

PLASMOLYSISPLASMOLYSIS:: LOSS OF WATER FROM A CELL LOSS OF WATER FROM A CELL RESULTING IN A DROP IN TURGOR PRESSURERESULTING IN A DROP IN TURGOR PRESSURE..

Turgor pressure:Turgor pressure: Water pressure inside the plant cell. Water pressure inside the plant cell. (turgidity)(turgidity)

PASSIVE TRANSPORTPASSIVE TRANSPORT:: REQUIRESREQUIRES NO NO ENERGY TO MOVE SUBSTANCES ENERGY TO MOVE SUBSTANCES

ACROSS A MEMBRANE = DIFFUSION , EXAMPLE: ACROSS A MEMBRANE = DIFFUSION , EXAMPLE: WATER, NON-POLAR MOLECULES, AND LIPID-WATER, NON-POLAR MOLECULES, AND LIPID-SOLUBLE SUBSTANCES.SOLUBLE SUBSTANCES.

LIPID BILAYER KEEPS MOST SUBSTANCES OUT LIPID BILAYER KEEPS MOST SUBSTANCES OUT BECAUSE THEY’RE REPELLED OR TOO LARGE.BECAUSE THEY’RE REPELLED OR TOO LARGE.

FACILITATED DIFFUSIONFACILITATED DIFFUSION: : PROTEINS PROVIDE CONVENIENT OPENINGS FOR PROTEINS PROVIDE CONVENIENT OPENINGS FOR

PARTICLES TO PASS THROUGH - SUGARS AND PARTICLES TO PASS THROUGH - SUGARS AND AMINO ACIDS ARE MOVED THIS WAY...AMINO ACIDS ARE MOVED THIS WAY...

THIS SPEEDS UP THE DIFFUSION RATE!THIS SPEEDS UP THE DIFFUSION RATE!

Facilitated DiffusionFacilitated Diffusion

FROM FROM LOWLOW CONC. TO CONC. TO HIGHERHIGHER CONC. CONC.

In active transport, SOLUTES MOVE In active transport, SOLUTES MOVE AGAINSTAGAINST THE THE CONCENTRATION GRADIENT!CONCENTRATION GRADIENT!

Ex- Sodium-Potassium Pump-helps nerve cells transfer Ex- Sodium-Potassium Pump-helps nerve cells transfer electrical impulses by maintaining an electrical impulses by maintaining an electrical gradientelectrical gradient on on the plasma membrane.the plasma membrane.

Ex- Calcium Pump - helps keep the calcium concentration Ex- Calcium Pump - helps keep the calcium concentration high in cells.high in cells.

ACTIVE ACTIVE TRANSPORTTRANSPORT

(Requires energy)(Requires energy)

ENDOCYTOSIS:ENDOCYTOSIS: a form of active transport in which the cell a form of active transport in which the cell uses energy to bring materials into the cell by engulfing.uses energy to bring materials into the cell by engulfing.

A. A. PHAGOCYTOSISPHAGOCYTOSIS: (“CELL EATING”) Cell takes in large : (“CELL EATING”) Cell takes in large molecules by pinching in the plasma membrane.molecules by pinching in the plasma membrane.

B. B. PINOCYTOSISPINOCYTOSIS: (“CELL DRINKING”) Cell takes in fluid : (“CELL DRINKING”) Cell takes in fluid by pinching in the plasma membrane.by pinching in the plasma membrane.

ExocytosisExocytosis

Active Transport EXOCYTOSIS: a form of active transport in which

the cell excretes wastes or secretes substances needed elsewhere in the organism. These substances do not move through the plasma membrane itself.Slide 19

EXOCYTOSIS

Vesicle in cytoplasm moves to plasma membrane,

fuses with it; contents released to the outside