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 concentration to the region of higher solute concentration

Lowerconcentrationof solute (sugar)

Higherconcentrationof sugar

Same concentrationof sugar

Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater 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

EXTRACELLULARFLUID

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 tothe sodium-potassium pump

CYTOPLASMNa+

[Na+] low[K+] high

Na+

Na+

EXTRACELLULARFLUID

[Na+] high[K+] low

Na+

Na+

Na+

ATP

ADP

P

Na+ binding stimulatesphosphorylation by ATP.

Na+

Na+

Na+

K+

Phosphorylation causesthe protein to change itsconformation, expelling Na+

to the outside.

P

Extracellular K+ bindsto the protein, triggeringrelease of the phosphategroup.

PP

Loss of the phosphaterestores the protein’soriginal conformation.

K+ is released and Na+

sites are receptive again;the cycle repeats.

K+

K+

K+

K+

K+

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

EXTRACELLULARFLUID

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

Diffusionof 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

Coatedpit

Coatedvesicle

Coat protein

Coat protein

Plasmamembrane

0.25 µm

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).