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 Recovery Transport 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)

Diffusion of one solute Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium Diffusion of one solute

Diffusion of two solutes Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium Diffusion of two solutes

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

OUTSIDE THE CELL INSIDE 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

Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar H2O Selectively permeable mem- brane: sugar mole- cules cannot pass through pores, but water molecules can 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

50 µm Filling vacuole 50 µm Contracting 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.

Hypotonic solution Isotonic solution Hypertonic solution Animal cell H2O H2O H2O H2O Lysed Normal Shriveled Plant cell H2O H2O H2O H2O 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

Passive transport Active transport ATP Diffusion Facilitated diffusion

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 EXTRACELLULAR FLUID [Na+] high [K+] low Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low [K+] high ATP P Na+ P CYTOPLASM ADP Cytoplasmic Na+ bonds to the sodium-potassium pump Na+ binding stimulates phosphorylation by ATP. Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. K+ K+ K+ K+ K+ P P K+ Extracellular K+ binds to the protein, triggering release of the phosphate group. 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

Electrochemical Gradient

**The Na+-K+ pump maintains the membrane potential…HOW?**

ELECTROGENIC PUMPS: ● 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.

Examples of Active Transport protein “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

Proton Pump!

Producing ATP!

– EXTRACELLULAR FLUID + ATP – + H+ H+ Proton pump H+ – + H+ H+ – + CYTOPLASM 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

Sucrose-H+ cotransporter – + ATP H+ H+ – + Proton pump H+ H+ – + H+ – + H+ Diffusion of H+ Sucrose-H+ cotransporter H+ – + – + Sucrose

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-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs). Coat protein Plasma membrane 0.25 µm