Membrane Transport and Facilitated Diffusion

Selective permeability

Some substances can cross the membrane more easily than others

Easy passage

small, nonpolar hydrophobic molecules (hydrocarbons, CO₂, O₂, N₂)

Difficult passage or protein assisted passage

Hydrophilic, polar, large molecules, ions (sugars, water)

Passive transport

Transport of a molecule that does not require energy from the cell because a solute is moving with its concentration

Involved in the import of materials and export of waste

Examples of passive transport

diffusion, osmosis, facilitated diffusion

Diffusion

spontaneous process resulting from the constant motion of molecules (high to low)

Osmosis

The diffusion of water down its concentration gradient across a selectively permeable membrane OR

diffusion of water from areas of low solute concentration to areas of high solute concentration

Facilitated Diffusion

diffusion of molecules through the membrane via transport proteins

Facilitated diffusion on rate of diffusion

increases rate of diffusion for small ions, water, and carbohydrates

Channel proteins

provide a channel for molecules and ions to pass 

Only allow passage with stimulus

Aquaporins

Specific channel protein for water

Carrier proteins

Undergo conformational changes for substance to pass

Active transport

Transport of a molecule that requires energy because it moves a solute against its concentration gradient

Examples of active transport

Pumps, cotransport, exocytosis, endocytosis

Pumps

Maintain membrane potential

Membrane potential

Unequal concentrations of ions across the membrane which results in an electrical charge (electrochemical gradient)

Cytoplasm charge

Negative

Electrogenic pumps

Proteins that generate voltage across membranes which can be used later as an energy source for cellular processes

Example of electrogenic pump

Sodium potassium pump

Sodium Potassium pump

Animal cells will regulate their relative concentrations of Na+ and K+

1+ net charge to extracellular fluid

Sodium Potassium Pump process

1. Protein has affinity towards Na+

2. ATP phosphorylates the protein, which causes a conformational change

3. Protein has affinity towards K+

4. When K+ binds to protein, phosphate group will be released

5. Causes protein to return to original shape

Proton pump

Integral membrane protein that builds up a proton gradient across the membrane

pumps H+ out of cell

Organisms with proton pumps

Plants, fungi, and bacteria

Cotransport

the coupling of a favorable movement of one substance with an unfavorable movement of another substance

How does cotransport work?

uses energy stored in electrochemical gradients (generated by pumps) to move substance against their concentration gradient

Unfavorable and favorable movement in cotransport

Favorable = downhill

Unfavorable = uphill

Transport of large molecules

exocytosis and endocytosis

Exocytosis

the secretion of molecules via vesicles that fuse to the plasma membrane

Process of exocytosis

• Vesicles can fuse to the membrane by forming a bilayer

• Contents of vesicle are released to the extracellular fluid

Endocytosis

The uptake of molecules from vesicles fused from the plasma membrane

Examples of endocytosis

Phagocytosis, pinocytosis, receptor mediated

Phagocytosis

When a cell engulfs particles to be later digest by lysosomes

Process of Phagocytosis

1. Cell surrounds particle with pseudopodia

2. Cells packages particles into a food vacuole

3. Food vacuole fuses with lysosome to be digested

Pinocytosis

Nonspecific uptake of extracellular fluid containing dissolved molecules

Process of pinocytosis

1. Cell takes in dissolved molecules in a protein coated vesicle

2. Protein coat helps to mediate transport of proteins

Receptor mediated endocytosis

Specific uptake of molecules via binding to receptors on the plasma membrane

When solutes bind to the receptors they cluster in a coated vesicle to be taken into the cell