Lecture 12 - Transport across Membranes- Review

Transport of "small" substances across the membrane is carried out by...

Diffusion

Passive transport

Does not require input of energy (ATP)

simple diffusion (passive transport)

passes directly through the membrane

Osmosis

Diffusion of water through a selectively permeable membrane

Facilitated diffusion

Passive diffusion of molecules through carrier or channel proteins

Active transport

Requires input of chemical energy (ATP)

Primary active transport

Active transport that relies directly on the hydrolysis of ATP

Secondary active transport

Form of active transport which does not use ATP as an energy source; rather, transport is coupled to ion diffusion down a concentration gradient established by primary active transport.

Diffusion

Process of random movement toward a state of equilibrium

Diffusion is net movement of water from regions of..

Greater concentration to regions of lesser concentration

At equilibrium there is...

No net movement of solutes (solute is uniformly distributed)

Rate of diffusion is affected by:

1) size/mass of the molecules or ions (heavy= slower)

2) temperature (high = increase)

3) density of solution (high = decrease)

4) concentration gradient

Diffusion is efficient at small ______

distances

Long-distance transport in humans is facilitated mainly by the

Circulatory system (each tissue cell is associated with a capillary bed)

Small nonpolar molecules that can pass through phospholipid bilayer

O2, CO2, N2

Small uncharged polar molecules that can pass through bilayer

H2O, glycerol

Large, uncharged polar molecules that can't pass right though bilayer

Glucose, sucrose

Ions that can't pass through bilayer

Cl-, K+, Na+

Simple diffusion doesn't involve

Protein carriers/ channels

- diffuse right though

The more lipid-soluble (hydrophobic) a molecule is, the more ________ diffuses through the membrane

Rapidly

Osmosis

Diffusion of water through a selectively permeable membrane

- concentration of solutes in environment determine direction of osmosis

Isotonic solutions

equal solute concentration (rate of water in and out is equal)

Hypotonic solutions

lower solute concentration outside than inside, net movement of water into cell (cell swells and burst)

Hypertonic solution

Concentrated solutes outside (cells lose water and shrivel)

Plasmolysis

Collapse of a walled cell's cytoplasm due to a lack of water

Water moves down

concentration gradient

Water diffuses from a

hypotonic solution to a hypertonic solution

Facilitated diffusion

process of diffusion in which molecules pass across the membrane through cell membrane channels or carriers

Ligand gated channel

Ligand binding causes a conformational change that allows substances to pass

Voltage gated channels

Respond to change in membrane potential (positive binds w negative to open)

Aquaporins

Specialized channels for water

Carrier proteins

Aid passive diffusion by binding substances, transport polar (larger) molecules, such as sugars and amino acids

Saturation of a Carrier Protein

The rate of transport of molecules into a cell is plotted against the concentration of those molecules outside the cell minute the concentration of those molecules inside the cell. As the concentration difference increases, the rate of transport increases and then levels off

Active transport

Moves substances against their concentration gradient, occurs by a transporter protein and requires energy

Uniporter

transporter that carries one specific ion or molecule

Symporter

transporter that carries two different ions or small molecules, both in the same direction

Antiporter

transporter that carries two ions or small molecules in different directions (sodium potassium pump)

Primary active transport

Involves direct hydrolysis of ATP

Secondary active transport

Indirectly uses ATP by ion concentration gradient established by primary active transport

- gradient forms ( molecule diffuses to push the target molecule through to its higher concentration)

Endocytosis

process by which a cell takes material into the cell by infolding of the cell membrane

- phagocytosis

- pinocytosis

- receptor mediated endocytosis (specific)

Allows large molecules in

Exocytosis

Process by which a cell releases large amounts of material

- secretion

- removal of waste

Large molecules out

Phagocytosis

Cellular eating (phagosome fuses with/ lysosome where content is digested)

Pinocytosis

"Cellular drinking" (brining fluids and dissolved substances into the cell

Receptor mediated endocytosis (specific)

Coated pits= region on the membrane containing surface receptor proteins for certain molecules; underlined in cytoplasmic side by other proteins (Clathrin)