Chpt 5: Membranes and Transport Pt. 1

Transport is the controlled movement of ions and molecules across a membrane by ___.

membrane proteins

Passive transport moves substances ___ the concentration gradient.

down (high to low)

Active transport moves substances ___ the gradient using ___.

against (low to high), energy from ATP

Simple diffusion figure (a).

Passive, solute moves down its gradient, no transport protein needed

Facilitated diffusion figure (b).

Passive, down the gradient with help of a transport protein

Active transport

Against the gradient with a transport protein, and expends ATP —→ ADP + Pi (by product is ADP and inorganic phosphate)

Diffusion is the net movement from ___ to ___ concentration.

higher, lower

A concentration gradient is a form of ___ energy.

potential

Most traffic across the membrane occurs by ___.

diffusion

Dye diffusion figure: what is dynamic equilibrium?

Molecules still cross but at equal rates in both directions

Two dyes diffusing across a membrane.

Each moves down its own concentration gradient independently

Simple diffusion moves ___ gases and lipid-soluble molecules.

nonpolar (O2, N2, CO2)

The more lipid-soluble a molecule is, the ___ it diffuses.

faster

Why can't polar or charged molecules diffuse freely?

Not soluble in the hydrophobic interior and they form many hydrogen bonds with water

Osmosis is the diffusion of ___ across a ___ membrane.

water, selectively permeable

Osmosis can make cells ___ and burst or ___ and shrivel.

swell, shrink

Water moves from the side with ___ solutes to the side with ___ solutes.

less, more

Why is free water lower on the solute side?

Water molecules associate with solutes, reducing the water available to cross

Osmosis apparatus: why does the tube fluid rise?

Water flows through a cellophane membrane (permeable to water but not glucose) into the glucose solution

When does osmotic flow stop in the apparatus?

When the weight of the water column creates enough pressure to balance water entering

Define osmotic pressure.

(minimum amount of pressure required to stop the flow of water across the membrane) the pressure from the raised column that balances water movement from beaker into tube

U-tube osmosis figure.

Sugar can't cross but water can, so water moves toward the higher sugar side until concentrations are roughly equal

Tonicity is a property of a ___ with respect to a particular ___.

solution, membrane

Hypotonic solution.

Lower solute than the cell, water enters, cell swells (hypo = less)

Hypertonic solution.

Higher solute than the cell, water leaves, cell shrinks (hyper = more)

Isotonic solution.

Equal solute concentrations, water moves at the same rate in and out, no size change (iso = equal)

Animal cells keep fluids isotonic by actively transporting ___ out.

Na+

Red blood cell in hypotonic solution.

Swells and lyses (bursts)

Red blood cell in isotonic solution.

Normal shape

Red blood cell in hypertonic solution.

Shrivels (crenation)

Plant cell in hypotonic, isotonic, hypertonic.

Turgid, flaccid, plasmolyzed

Animal cells do best in ___ solutions and plant cells in ___ solutions.

isotonic, hypotonic

Turgor pressure in plants.

Osmotic pressure pushes the cell against its wall in hypotonic solution and supports soft tissues

Plasmolysis.

In hypertonic solution plant cells shrink away from the cell wall and stems and leaves wilt

Facilitated diffusion moves ___ and ___ molecules through transport proteins.

polar, charged

Facilitated diffusion is ___ and follows the ___.

specific, concentration gradient

Channel proteins are ___ membrane proteins forming hydrophilic channels for water and ions.

integral

Channel proteins that transport water are ___.

aquaporins

Ion channels transport ___ and most are ___ channels.

Na+, K+, Ca2+, Cl-, gated

Gated channels switch between ___ states.

open, closed, intermediate

Osmosis through aquaporins uses ___ metabolic energy.

no (completely passive)

Aquaporins move water in ___ and exclude ___.

single file, ions (so the cell's electrical properties are maintained)

What gives aquaporins selectivity?

Key residues in the channel filter out other molecules and small ions

How does water pass through an aquaporin (figure)?

Handed off through a succession of hydrogen-bonding sites in the channel

Aquaporins allow ___ levels of water diffusion in plant and animal cells.

massive

Ion movement across membranes matters for ___.

mitochondrial respiration, nervous system activity, leaf pores for gas exchange

All ion channels share a ___ pore through which a specific ion passes.

hydrophilic

Voltage-gated channels respond to changes in ___.

membrane potential

Ligand-gated channels open when a ___ binds.

specific ligand (neurotransmitter, drug)

Mechano-sensitive channels respond to ___.

mechanical forces on the membrane

Non-gated (leak) channels are ___.

always open

How is a gated channel anchored in the bilayer?

Outer coating of nonpolar R groups, with a pore of polar amino acids and water

How does a gated channel open?

Changes 3D shape when a ligand binds or when voltage changes

Neuron K+ channel example.

Opens in response to an electrical stimulus and K+ leaves the cell

K+ voltage-gated channel figure.

Gate closed at normal voltage, opens with voltage change so K+ moves down its gradient out of the cell

Carrier proteins bind a ___ solute and move it across (uniport).

single specific

Carrier proteins carry polar molecules like ___ and ___.

sugars, amino acids

How do carrier proteins move a solute?

Conformational change moves the binding site from one side of the membrane to the other

Carrier proteins can become ___ when there are too few of them. (Transport rate cannot go any faster)

saturated

Glucose transporter.

Carrier that lets polar glucose into cells, glucose binding causes a shape change that releases it on the other side

Carrier proteins in facilitated diffusion require ___ energy.

no (passive, down the gradient)

Carrier protein figure cycle.

Site faces high concentration, solute binds, carrier changes shape, releases solute at low concentration, returns to original shape

Active transport requires ___ and moves substances ___ the gradient.

ATP, against

Three main functions of active transport.

Nutrient uptake, waste or secretory removal, maintaining intracellular H+, Na+, K+, Ca2+

Unlike diffusion, active transport is ___.

directional (into or out of the cell as needed)

Uniporter.

Moves a single substance in one direction (like the Ca2+ pump in plasma and ER membranes)

Symporter.

Moves two substances in the same direction (like Na+ plus amino acid in intestinal cells)

Antiporter.

Moves two substances in opposite directions (like the Na+/K+ pump)

Symporters and antiporters are examples of ___ transporters.

coupled

Primary active transport.

Protein hydrolyzes ATP to power transport directly

Secondary active transport.

Uses an ion gradient built by primary transport as energy, so ATP is used indirectly

Primary active transport pumps move ___ ions such as ___.

positively charged, H+, Ca2+, Na+, K+

Ion diffusion through channels is driven by the ___ gradient.

electrochemical

The Na+/K+ pump (Na+/K+-ATPase) moves ___ Na+ out and ___ K+ in per ATP.

3, 2

The Na+/K+ pump is found in ___ animal cells.

all

Membrane potential from the Na+/K+ pump.

About -50 to -200 mV with the inside negative

Electrochemical gradient.

Difference in ion concentration and charge across the membrane, a form of potential energy

The Na+/K+ pump is important for ___.

nerve impulses (action potentials)

Na+/K+ pump steps 1-3.

Na+ binds, ATP phosphorylates the pump, shape change releases 3 Na+ outside and brings 2 K+ inside

Na+/K+ pump steps 4-6.

2 K+ bind outside, phosphate released restoring shape, 2 K+ released inside

Membrane potential is a ___ difference across the membrane caused by active transport.

voltage

Neurons and muscle cells use membrane potential for ___.

resting potential and rapid action potentials (nerve impulse transmission)

Proton pump.

Moves H+ across a membrane using ATP, creating a proton gradient

Proton pumps in prokaryotes, plants, and fungi generate ___.

membrane potential

Proton pumps in lysosomes keep pH ___ to activate enzymes.

low

Functions of proton pumps.

Organelle pH, acidifying lysosomes and endosomes, lysosome digestion and fusion, stomach acid secretion, membrane bioenergetics

Plasma membrane vs lysosome proton pumps.

Plasma membrane pumps H+ out, lysosome pumps H+ in

The calcium pump moves Ca2+ out of the cell and into the ___.

ER

Calcium is ___ in the cytosol and ___ outside cells and in the ER.

low, high

The Ca2+ gradient regulates ___.

secretion, microtubule assembly, muscle contraction

In muscle contraction Ca2+ is released from the ___.

sarcoplasmic (smooth) reticulum

Secondary active transport uses an ion gradient from a ___ pump.

primary

Seconday transporters: In symport (cotransport) the solute moves ___ the driving ion and in antiport (anti-transport) it moves ___.

with, opposite

Cotransport (secondary active transport carrier protein) moves molecules ___ a gradient alongside ions moving ___ theirs.

up, down

Na+/glucose cotransporter (SGLT) type and driver.

Example of secondary transport: symporter (moves 2 diff. moleculues) driven by the Na+/K+ pump's sodium gradient and brings glucose INTO cell

Na+/glucose symporter figure.

Na+ diffusing back into the cell powers glucose uptake against its gradient

Na+/Ca2+ exchanger.

Antiporter that pumps 1 Ca2+ out as 3 Na+ move in

Simple diffusion: energy, driving force, protein, specificity.

No energy, concentration gradient, no protein, not specific

Facilitated diffusion: energy, driving force, protein, specificity.

No energy, concentration gradient, protein required, specific

Active transport: energy, driving force, protein, specificity.

Energy required, ATP hydrolysis against gradient, protein required, specific