Transport of Substances across Membranes (Exam 2)

Lipid Bilayers

Lipid Bilayers

• form when lipid molecules are aligned in paired sheets

• hydrophilic heads: interact with water

• hydrophobic tails: interact with one another

selective permeability

• small molecules cross membranes easily

• ions and large molecules diffuse across the membrane slowly or not at all

inside the membrane

• phospholipids move within membranes

• permeability is related to level of fluidity

  -higher temperature increases fluidity

  -phospholipids are in constant lateral motion

how substances move across membranes

diffusion: the spontaneous movement of molecules and ions

• solutes(dissolved substances) move randomly in all direction

• passive= no input of energy

concentration gradient

a difference in the solute concentrations across space

• if present, there will be a net movement away from regions of high concentrations of the solute

• net movement: movement of molecules from an area of greater concentration to an area of lesser concentration

process of diffusion

1. solutes are separated and each has its own concentration gradient

2. net movement of each solute from the side of high concentration to the side of low concentration

3. solutes reach equilibrium (molecules randomly distributed throughout the solution) —> movement of solute doesn’t stop

rate of diffusion

F= kA (ΔC) / d

• F= rate of diffusion

• k= diffusion constant (dependent on solute, membrane, temperature)

• A= surface area for diffusion

• ΔC= change in concentration

• d= distance for diffusion

electrochemical gradient

diffusion of charged particles depends on combination of concentration and electrical gradient

electrochemical equilibrium

when the combination of these two are balances (concentration and electrical gradient)

Consequences of Size and Shape (getting bigger)

• increased distance between surface and center

• decreased surface are to volume ratio

• center is further from the surface

• increased diffusion distance

• increased distance= reach center at a lower rate

equations for this

surface area: L x W x 6

volume: L x W x H

solutions to bigger organism diffusion

• subdivide into many smaller units

  -reduce internal volume: vacuole in plant cells

• increased surface complexity increases area for diffusion

• becoming long and thin and/or flat offsets effects of increased size

Osmosis

a special case when diffusion involves water

• occurs when solutions are separated by a semi-permeable membrane

• S-P M: allows water to cross the membrane but does not let some or all of the solute to cross

Concentration of water during osmosis

• free water moves from regions of low solute concentration to regions of high solute concentration

  -low concentration of solute = high concentration of free water

  -high concentration of solute = low concentration of free water

• causes a change in volume and a change in solute concentration on both sides

outside solutions to inside solutions (osmosis)

• hypertonic (outside): results in net flow of water out of the vesicle; vesicle shrinks

• hypotonic (outside): results in net flow of water into vesicle; vesicle swells or even bursts

• isotonic (inside and outside): no change

passive transport

when substances (ions or molecules) move across the plasma membrane in the absences of an outside an energy source

• movement occurs along the electrochemical gradient

• does not require expenditure of energy

PT- facilitated diffusion

• channel proteins:

  -selective: each protein only permits a particular type of ion or molecule to pass through it

  -forms a pore to allow movement through the membrane

• carrier proteins:

  -undergoes a change in shape to “carry” a molecule (conformational change: adjustment of a protein’s tertiary structure in response to external factors or to binding of a ligand)

Channel Proteins

• Aquaporins: allow water to move across membranes, but exclude other molecules and ions

• Gated Channels: open in response to a signal (voltage-gated K+ channel)

Carrier Proteins

1. unbound protein

2. glucose binding

3. conformational change

4. release

Active Transport

cells can move molecules in a directed manner or against the electrochemical gradient

- involves pumps (transmembrane proteins that actively move ions/ solutes against a concentration or electrochemical gradient)

Secondary Active Transport

• uses a pump to establish an electrochemical gradient

• the gradient is used to move a molecule/ion of interest

Active Transport- Pump

pumps: membrane proteins that provide active transport of molecules across the membrane

• proton pump (H+-ATPase)

  - use ATP to move protons

  - important in change pH and creating electrochemical gradients

• sodium-potassium pump (Na+/K+-ATPase)

  -uses ATP

  -transports Na+ and K+ against their concentration gradients

Active transport - Secondary Active Transport Cotransport

cotransport: simultaneous transport one substance across a membrane, coupled with the simultaneous transport of another substance across the same membrane in the same direction

• pumps move materials against their gradients to stylish different electrochemical gradients

  -ATP is not directly used to power transport

cotransporters

• symporter: transport solutes against their concentration gradients using energy released by the transport of another molecule moves in the same direction along its concentration gradient

• antiporter: transport solutes against their concentration gradient using energy released by the transport of another molecule moves in the opposite direction along its concentration gradient