Chapter 7: Cell Membrane and Transport

the movement of molecules from an area of high concentration of the molecules to an area with a lower concentration.

selectively permeable

Allows some substances to cross more easily than others

membrane held together by weak interactions

phospholipids, proteins, carbs

Bilayer

Amphipathic = hydrophilic head, hydrophobic tail

Hydrophobic barrier: keeps hydrophilic molecules out

hydrophilic head, hydrophobic tail

Embedded in membrane

Determined by freeze fracture

Transmembrane with hydrophilic heads/tails and hydrophobic middles

Extracellular or cytoplasmic sides of membrane

NOT embedded

Held in place by the cytoskeleton or ECM

Provides stronger framework

Function: cell-cell recognition; developing organisms

Glycolipids, glycoproteins

(eg. blood transfusions are type-specific)

keeps membranes fluid and stable

Transport

Enzymatic activity

Signal transduction

Intercellular joining

Cell-cell recognition

Attachment to the cytoskeleton & extracellular matrix

(just for a visual)

refers to the TOTAL dissolved solutes in a solution...relative to another solution across a nearby membrane

Because the molecules tend to move so that the solution, not the amount of water, reaches equillibrium.

solution with a HIGHER solute concentration.

solution with a LOWER solute concentration.

solution has the same dissolved solute concentration.

Lysed → bursts because water rushes in

“Normal“ → there is an equal flow of water in and out of the cell

Shriveled → Water rushes out, causing it to shrink

Turgid (normal) → plants like to keep themselves full of water

water rushes in

Flaccid → there is an equal flow of water in and out of the cell

Plasmolyzed → plant is dehydrated/dying, water rushes out, causing only the membrane to shrink

The cell wall does not shrink

No. Only the membrane shrinks

diffusion of H2O/water across a semi-permeable membrane

the gradual change in the concentration of solutes in a solution as a function of distance through a solution.

hypotonic, isotonic or hypertonic

cross easily: hydrocarbons, hydrophobic molecules, CO2, O2, N2

Polar molecules, including H2O – pass in small amounts through aquaporin proteins

Hydrophobic core prevents passage of ions, large polar molecules – movement through embedded channel and transport proteins

NO ENERGY (ATP) needed!

Diffusiondownconcentration gradient(high to low concentration)

Molecules diffuse right across the phospholipid bilayer. (passive)

(eg: CO2, O2, N2)

(passive)

Transport proteins (channel or carrier proteins) help hydrophilic substances cross

Two ways:

• Provide hydrophilic channel

• Loosely bind/carry molecule across

Eg. ions, polar molecules (H2O, glucose) water: uses aquaporins

(channel or carrier proteins) help hydrophilic substances cross

Requires ENERGY (ATP)

Proteins transport substances against concentration gradient (low to high conc.)

Eg. Na⁺/K⁺ pump, proton pump

generate voltage across membrane

Pump Na⁺ out, K⁺ into cell

Nerve transmission

1. The binding of cytoplasmic/intracellular Na+ to the protein stimulates phosphorylation by ATP.

2. Phosphorylation causes the protein to change its conformation.

3. The conformational change expels Na+ to the outside and the extracellular K+ binds.

4. K+ binding triggers the release of the phosphate group.

5. Loss of phosphate restores original conformation.

6. K+ is released and Na+ sites are receptive again; the cycle repeats.

Push protons (H⁺) across membrane

Eg. mitochondria (ATP production)

membrane protein enables “downhill” diffusion of one solute to drive “uphill” transport of other

Eg. sucrose-H+ cotransporter (sugar-loading in plants)

Passive:

Little or no Energy

High to low concentrations

DOWN the concentration gradient

eg. diffusion, osmosis, facilitated diffusion (w/transport protein)

Active:

Requires Energy (ATP)

Low to high concentrations

AGAINST the concentration gradient

eg. pumps, exo/endocytosis

Control solute & water balance

Contractile vacuole: “bilge pump” forces out fresh water as it enters by osmosis. video

Eg. paramecium caudatum – freshwater protist

“bilge pump” forces out fresh water as it enters by osmosis. video

Transport of proteins, polysaccharides, large molecules

exocytosis & endocytosis

take in macromolecules and particulate matter, form new vesicles from plasma membrane

vesicles fuse with plasma membrane, secrete contents out of cell

Phagocytosis & Pinocytosis

“cellular eating” - solids

“cellular drinking” - fluids

Ligands bind to specific receptors on cell surface

Ex: Growth hormones, LDL’s attached to cholesterols

H2O moves from high ψ to low ψ potential

ψ = ψs + ψp

free energy of water/a measure of water's potential to do work.

solute concentration (osmotic potential) Solute potential is ALWAYS negative.

physical pressure on solution; turgor pressure (plants)

ψP = 0 MPa

ψP = 1 MPa

ψS = -iCRT

i = ionization constant (# particles made in water)

C = molar concentration

R = pressure constant (0.0831 liter bars/mole-K)

T = temperature in K (273 + °C)

lowers the solute potential (more negative) and therefore decreases the water potential.

From an area of:

higher ψ to lower ψ (more negative ψ)

low solute conc. to high solute conc.

high pressure to low pressure

Moving from the roots of a tree up to the air, where transpiration occurs, water potential DECREASES

This will help allow water movement up a tree.