A Closer Look at Cell Membranes

CH. 5 CELL MEMBRANE AND TRANSPORT

BOZEMAN VIDEO—CELL MEMBRANEShttp://www.youtube.com/watch?v=y31DlJ6uGgE

http://www.youtube.com/watch?v=S7CJ7xZOjm0

http://www.youtube.com/watch?v=y31DlJ6uGgE

http://www.youtube.com/watch?v=S7CJ7xZOjm0

Lipid BilayerCell membranes consist of a lipid bilayer

containing different proteinsMembrane is a continuous boundary layer

that selectively controls the flow of substances across it

hydrophilicparts

hydrophobicparts

b

a

fluid

fluid

one layer of lipidsone layer of lipids

cross-sectionthrough lipid bilayer

Fig. 5.3, pg. 76

Fluid Mosaic ModelEvery cell membrane has a mixed

composition of phospholipids, glycolipids, sterols, and proteins

KNOW THE CAMPBELL DIAGRAM “CELL MEMBRANE MOSAIC”

Fluid Mosaic Model

adhesionprotein

cytoskeletal proteins just beneath the plasma

membrane

active transporter(calcium pump)

passivetransporter

active transporter(ATPase pump)

recognitionprotein

receptor

phospholipid

Cytoplasm

PlasmaMembrane

Lipid bilayer

cholesterol

Fig. 5.4, pg. 77

Studying Membranes

Fig. 5.5a, pg. 77

Stepped Art

Overview of Membrane Proteins

AdhesionProteins

CommunicationProteins

Fig. 5.6, p.78

Overview of Membrane Proteins

ReceptorProteins

RecognitionProteins

PassiveTransporters

ActiveTransporters

Fig. 5.6, p.79

Transport ProteinsSpan the lipid bilayerInterior is able to open to both sidesChange shape when they interact with solute Play roles in active and passive transport

Concentration Gradient Means the number of molecules or ions in

one region is different than the number in another region

In the absence of other forces, a substance moves from a region where it is more concentrated to one one where it’s less concentrated - “down” gradient

DiffusionThe net movement of like molecules or ions

down a concentration gradient

Although molecules collide randomly, the net movement is away from the place with the most collisions (down gradient)

Diffusion

Fig. 5.7a, p.80Stepped Art

Factors Affecting Diffusion Rate

Steepness of concentration gradientSteeper gradient, faster diffusion

Molecular sizeSmaller molecules, faster diffusion

TemperatureHigher temperature, faster diffusion

Electrical or pressure gradients

Membrane Crossing MechanismsDiffusion across lipid bilayer

Passive transport

Active transport

Endocytosis

Exocytosis

Cell Membranes Show Selective Permeability

oxygen, carbon dioxide, and other small, nonpolar molecules; some water molecules

glucose and other large, polar, water-soluble molecules; ions (e.g.,H+, Na+, K+, Ca++, Cl–); water molecules

Fig. 5-8, p.80

Fig. 5-9, p.81

Membrane Crossing: Overview IHigh

Concentrationgradient acrosscell membrane

Low

Diffusion oflipid-solubleSubstancesacross bilayer

Passive transport of water-soluble substancesthrough channel protein;no energy input needed

Active transportthrough ATPase;requires energyinput from ATP

ATP

Passive Transport Flow of solutes through the interior of

passive transport proteins down their concentration gradients

Passive transport proteins allow solutes to move both ways

Does not require any energy input

TYPES OF PASSIVE TRANSPORTDIFFUSION

FACILITATED DIFFUSION—diffusion that is “helped out” by membrane transport proteins (such as helping larger molecules pass)

Passive Transport

Stepped Art

glucose transportersolute (glucose)

high

low

Fig. 5.10, p.80

Active TransportNet diffusion of solute is against

concentration gradientTransport protein must be activated (requires

an integral membrane protein)ATP gives up phosphate to activate proteinBinding of ATP changes protein shape and

affinity for solute

TYPES OF ACTIVE TRANSPORTSODIUM-POTASSIUM PUMP—3 Na move out and

allow 2 K to enter; needs net 1 ATP molecule to dothis

PROTON PUMP– pushes H ions out of the membrane allowing them to eventually diffuse back in; generates ATP for the cell; used in cell respiration

This uses ATP synthase enzymeCOTRANSPORT– when molecules travel together

across the membrane

Types of active transport ContinuedEndocytosis—”engulfing “things into cell

membrane; difficult for plants to do Ex: white blood cells engulfing bacteria—to fight

infectionsphagocytosis (large food)

pinocytosis (small things) Receptor-mediated endocytosis—use of receptor

proteins to help pull things into cell

Exocytosis—”spitting” things out of cell

Osmosis Diffusion of water molecules across a

selectively permeable membranewater molecules protein molecules

semipermeable membranebetween two compartments

• Direction of net flow is determined by water concentration gradient

• Side with the most solute molecules has the lowest water concentration

Osmosis

p.84

Tonicity Refers to relative solute concentration of two fluids

Hypotonic - having fewer solutes

Hypertonic - having more solutes

Isotonic - having same amount

2% sucrose solutio

n

1 liter of distilled

water

1 liter of 10% sucrose

solution

1 liter of 2%

sucrose solution

HypotonicConditions

HypertonicConditions

IsotonicConditions

Fig. 5-13, p.85

Tonicity and

Osmosis

DIALYSIS BAG % MASS CHANGE

Pressure and OsmosisHydrostatic pressure

Pressure exerted by fluid on the walls that contain it

The greater the solute concentration of the fluid, the greater the hydrostatic pressure

Osmotic pressureAmount of pressure necessary to prevent

further increase of a solution’s volume

Increase in Fluid Volume

hypotonic solution

membrane permeable towater but not to solutes

hypertonicsolution

fluid volume rises in second compartment

second compartment

first compartme

nt

Fig. 5.14, p.85

ENDOCYTOSIS AND EXOCYTOSIS VIDEOhttp://www.youtube.com/watch?v=DuDmvlbpjHQ

Endocytosis and ExocytosisExocytosis: A cytoplasmic vesicle fuses with the

plasma membrane and contents are released outside the cell

3 types: phagocytosis—engulfs large molecules; difficult for plant cells to do (WHY??)

pinocytosis—engulfs small molecules receptor-mediated—uses receptor membrane proteins to pull things in

Endocytosis: A small patch of plasma membrane sinks inward and seals back on itself, forming a vesicle inside the cytoplasm – membrane receptors often mediate this process

Fig 5.17, p.87

Macrophage engulfing Leishmania mexicana

parasite macrophage

bacterium phagocytic vesicleFig. 5-17b, p.87

Phagocytosis

Contractile Vacuole

contractile vacuole filled

contractile vacuole emptied

Fig. 5.21, pg. 89

Plasmolysis

Plasmolysis

cytoskeletal

proteins

adhesion protein

lipid bilayer

recognition

protein

receptor protein

cytoplasm

active transporter

(calcium pump)

active transporter

(ATPase pump)

passive transporter

Fig. 5-19, p.88

BOZEMAN VIDEO—CELL TRANSPORT ACROSS MEMBRANES AND ANIMATIONhttp://www.youtube.com/watch?v=RPAZvs4hvGA

http://www.youtube.com/watch?v=RPAZvs4hvGA