Lecture 16: Transport across cell membranes

Non-mediated Transport Def

Movement of substances across cell membrane that doesn’t directly involve a transport protein

Mediated Transport Def

Movement of substances across cell membrane involving a transport protein

Passive Transport Def

Moves substances down their electrochemical gradient using only their kinetic energy (can be mediated or non-mediated)

Active Transport Def

Primary Active transport Def

Secondary Active Transport Def

Uses energy to move substances against their concentration/electrochemical gradients

Primary - Energy is directly derived from the hydrolysis of ATP

Secondary - uses energy stored in ion gradients created by primary active transporters to move substances against their own concentration gradient. Indirectly use the energy obtained by hydrolysis of ATP. Examples =

• Na+ symporters: Glucose or AA’s plus Na+ ions rush inwards

• Na+ Antiporters: Na+ rush inwards while Ca2+ or H+ are pushed out

Vesicular Transport Def

Moves material via vesicles

Non-mediated Transport: Diffusion through lipid bilayer Function

Involves nonpolar, hydrophobic molecules like oxygen, carbon dioxide, fatty acids, vitamins. Important for nutrient absorption and waste excretion

Mediated Transport: Diffusion through ion channels Function

Channels form a water-filled pore, shielding ions from the hydrophobic core of the lipid bilayer. Specific AA’s lining the pore determine the channel’s selectivity to ions which allows the channel to harness energy stored in different ion gradients. Ions don’t bind to channel pore allowing rapid transport.

Properties of Channels: Gating Function

Channels contain gates that control the opening and closing of the pore. Different stimuli can gate channel opening and closing like voltage, pH, ligand binding, cell volume, phosphorylation

Carrier Mediated Transport Def and Function

Substrate interacts directly with transporter protein, transport is slow compared to channels as transporter undergoes conformational change once substance binds to it (can be passive or active). (Transport proteins mediate transport at a faster than normal rate but don’t catalyze chemical reactions)

Facilitated Diffusion of Glucose (Passive transport) 3 steps

1. Glucose binds to transport protein (GLUT)

2. Transport protein changes shape, allowing glucose to move across cell membrane down it’s concentration gradient

3. Kinase enzyme reduces glucose concentration inside the cell by transforming glucose into glucose-6-phosphate

   -Conversion of glucose maintains the concentration gradient for glucose entry

   - Glucose transport occurs until all binding sites are saturated and max transport is is reached

Primary active Transport: Na/K ATPase 4 steps and Function

1. Na+ binds

2. ATP hydrolysis and NA+ pushed out

3. K+ binds and phosphate release

4. K+ is pushed in

-3 Na+ ions are removed from the cell and 2K+ ions are brought into the cell. Because Na+ and K+ continually leak back into the cell down their gradients the pump works continuously.