NOTES: CH 7 part 2 -

Transport Across the Cell

Membrane (7.3-7.5)

The Permeability of the Lipid Bilayer

● Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly

● Polar molecules, such as sugars, do not cross the membrane easily

Transport proteins:

● membrane proteins that transport specific

molecules or ions across biological

membranes:

-may provide hydrophilic tunnel thru

membrane

-may bind to a substance and physically

move it across the membrane

-are specific for the substance they move

GLUCOSE

Binding

TransportRecovery

Dissociation

Movement across the cell membrane can be:

1) PASSIVE

● cell does not have

to expend energy

2) ACTIVE

● energy-requiring

process during which a

transport protein pumps a

molecule across a

membrane, against its

conc. gradient; is

energetically “uphill”

7.3 - Passive Transport:

DIFFUSION

● net movement of a substance down a concentration gradient

-results from KE of molecules

-results from random molecular movement

-continues until equilibrium is reached (molecules continue to move but there is no net directional movement)

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

Diffusion of one solute

Net diffusion Net diffusion Equilibrium

Diffusion of two solutes

Net diffusion Net diffusion Equilibrium

7.3 - Passive Transport:

OSMOSIS

● diffusion of water across a selectively

permeable membrane; water moves down

its concentration gradient

-continues until equil. is reached

-at equil. water molecules

move in both directions

at same rate

INSIDE

THE CELL

OUTSIDE

THE CELL

Effects of Osmosis on Water Balance

● The direction of osmosis is determined

only by a difference in total solute

concentration

● Water diffuses across a membrane from the region of lower solute concentrationto the region of higher solute concentration

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

Water Balance of Cells Without Walls

● Isotonic solution: solute concentration is the

same as that inside the cell; no net water

movement across the plasma membrane

● Hypertonic solution: solute concentration is

greater than that inside the cell; cell loses

water

● Hypotonic solution: solute concentration is

less than that inside the cell; cell gains water

WATER MOVES FROM HYPO TO HYPERTONIC!!!

● Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment

● To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance

● The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

Filling vacuole50 µm

50 µmContracting vacuole

Water Balance of Cells with Walls

● Cell walls help maintain water balance

● A plant cell in a hypotonic solution swells until the

wall opposes uptake; the cell is now turgid (firm)

● If a plant cell and its surroundings are isotonic,

there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

● In a hypertonic environment, plant cells lose

water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

RECAP: In cells with cell walls:

● in a HYPERTONIC environment,

PLASMOLYSIS occurs; cells shrivel

and usually die

● in a HYPOTONIC environment, water

moves into cell, causing it to swell; cell

becomes more TURGID.

Animalcell

Lysed

H2O H2O H2O

Normal

Hypotonic solution Isotonic solution Hypertonic solution

H2O

Shriveled

H2OH2OH2OH2OPlantcell

Turgid (normal) Flaccid Plasmolyzed

7.3 - Passive Transport:

FACILITATED DIFFUSION

● diffusion of solutes across a membrane, with

the help of transport proteins;

Facilitated Diffusion:

Passive Transport Aided by Proteins

● Channel proteins provide corridors that

allow a specific molecule or ion to cross

the membrane

● Carrier proteins undergo a subtle

change in shape that translocates the

solute-binding site across the membrane

EXTRACELLULAR

FLUID

Channel protein Solute

CYTOPLASM

Carrier protein Solute

7.4 - Active transport uses energy to

move solutes against their gradients

● Facilitated diffusion is still passive because the solute moves down its concentration gradient

● Some transport proteins, however, can move solutes against their concentration gradients

The Need for Energy in Active Transport

● Active transport moves substances against

their concentration gradient

● Active transport requires energy, usually in the

form of ATP

● Active transport is

performed by specific

proteins embedded

in the membranes

Diffusion Facilitated diffusion

Passive transport

ATP

Active transport

Examples of Active Transport protein

“pumps”:

1) Sodium-Potassium Pump:

-actively pumps Na+ ions out / K+ ions in

-in every pump cycle, 3 Na+ leave and 2

K+ enter cell

-Na+ and K+ are moved against their

gradients (both concentration and electric

potential!)

Cytoplasmic Na+ bonds to

the sodium-potassium pump

CYTOPLASMNa+

[Na+] low

[K+] high

Na+

Na+

EXTRACELLULAR

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 phosphate

group.

PP

Loss of the phosphate

restores the protein’s

original conformation.

K+ is released and Na+

sites are receptive again;

the cycle repeats.

OUTSIDE

INSIDE

Maintenance of Membrane Potential

by Ion Pumps

● Membrane potential is the voltage

difference across a membrane

● Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:

-A chemical force (the ion’s concentration gradient)

-An electrical force (the effect of the membrane potential on the ion’s movement)

● Membrane Potential: voltage across

membrane; in most cells the interior is

negatively charged w/respect to outside

-favors diffusion of cations into

cell and anions out of cell

● Electrochemical Gradient: diffusion

gradient resulting from the combined

effects of membrane potential and conc.

gradient

**The Na+-K+ pump maintains the

membrane potential…HOW?**

● An electrogenic pump is a transport

protein that generates the voltage across

a membrane

● The main electrogenic pump of plants,

fungi, and bacteria is a PROTON PUMP.

ELECTROGENIC PUMPS:

2) Proton Pump: pumps protons (H+ ions) out

of the cell, creating a proton gradient (protons

are more concentrated outside the

membrane than inside)…this is an

energetically “uphill” process!

-protons then diffuse back into cell

-the force of the proton pushing back through

the membrane is used to power the

production of ATP

Examples of Active Transport protein

“pumps”:

H+

ATP

CYTOPLASM

EXTRACELLULAR

FLUID

Proton pump

H+

H+

H+

H+

H+

+

+

+

+

+

–

–

–

–

–

3) Cotransport / Coupled Channels:

process where a single ATP-powered pump

actively transports one solute and indirectly

drives the transport of other solutes against

their conc. gradients.

-Example: plants use a proton pump

coupled with sucrose-H+ transport to

load sucrose into specialized cells

H+

ATP

Proton pump

Sucrose-H+

cotransporter

Diffusion

of H+

Sucrose

H+

H+

H+

H+

H+

H+

+

+

+

+

+

+

–

–

–

–

–

–

7.5 - Bulk transport across the plasma

membrane occurs by exocytosis and

endocytosis

● Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins

● Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles

BULK TRANSPORT: EXOCYTOSIS & ENDOCYTOSIS

● transport of large molecules (e.g.

proteins and polysaccharides) across cell

membrane

Exocytosis Endocytosis

*exporting macromolecules

by fusion of vesicles w/the

plasma membrane

*vesicle buds from ER or

Golgi and migrates to plasma

membrane

*used by secretory cells to

export products (e.g. insulin in

pancreas)

Exocytosis Endocytosis

*exporting macromolecules

by fusion of vesicles w/the

plasma membrane

*vesicle buds from ER or

Golgi and migrates to plasma

membrane

*used by secretory cells to

export products (e.g. insulin in

pancreas)

*importing macromolecules

by forming vesicles derived

from plasma membrane

*vesicle forms in localized

region of plasma membrane

*used by cells to incorporate

extracellular substances (e.g.

macrophage engulfs a

bacterial cell)

EXOCYTOSIS

● In exocytosis, transport vesicles

migrate to the membrane, fuse with it,

and release their contents

ENDOCYTOSIS

● In endocytosis, the cell takes in

macromolecules by forming vesicles from

the plasma membrane

● Endocytosis is a reversal of exocytosis, involving different proteins

Three types of Endocytosis:

1) Phagocytosis: part of the cell membrane engulfs

large particles or even entire cells (“cell eating”)

Three types of Endocytosis:

2) Pinocytosis: part of the cell

membrane engulfs small

dissolved substances or fluid

droplets in vesicles (“cell

drinking”)

Three types of Endocytosis:

3) Receptor-Mediated Endocytosis: importing of specific

macromolecules by receptor proteins bind to a specific

substance which triggers the inward budding of vesicles

formed from COATED PITS (how mammalian cells take up

cholesterol)

Receptor

RECEPTOR-MEDIATED ENDOCYTOSIS

Ligand

Coated

pit

Coated

vesicle

Coat protein

Coat

protein

Plasma

membrane

0.25 µm

A coated pit

and a coated

vesicle formed

during

receptor-

mediated

endocytosis

(TEMs).