Unit 1: Transport Across Membranes

3 sources of energy for active transport pumps

1. The coupled pump (electrochemical gradient) 2. The ATP-driven pump (ATP as energy) 3. The light-driven pump (light as energy)

Antiport

A membrane transport process that carries two substances in opposite directions. One substance goes with its concentration gradient, and the other in the opposite direction against its concentration gradient.

Symport

A membrane transport process that carries two substances in the SAME direction across the membrane. One molecule with its concentration gradient, the other against its concentration gradient.

Resting potential

Between -20 and -200 mV for most animal cells.

Channel

Can be "open" or "closed"

Membrane potential

Difference in charge across the membrane, aka voltage gradient.

Active transport

Energy-requiring process that moves material across a cell membrane against concentration gradient. Can only happen through transporters, not channels. Also called pumps.

Coupled pumps

Exploit Na/K gradient to move other molecules. Two different molecules are moving across the membrane. Usually one is moving DOWN its gradient (supplying th energy for the conformational change), and the other molecule is USUALLY moving UP its gradient.

Electrochemical gradient

For a given ion, it is the sum of ion gradient and cell voltage gradient. Provides stored energy for the cell.

Na+K+ pump

Forms the most important electrochemical gradient in animal cells. Uses 30% of the cell's ATP. 3 Na+ are pumped out for every 2 K+ pumped in. Sodium is in charge of electrochemical gradient since leaky K+ neutralizes gradient.

Most channels facilitate ion passage and are ____

Gated.

Example of coupled pumps

Glucose-Na+ symport: Glucose movement through various pumps in intestine. Couples glucose uptake with sodium, leading to a high level of glucose in cells, and low level of glucose outside the cells. Passive glucose uniport releases glucose outside the cell for use by other tissues (goes into the blood stream).

Transporter

Has a specific binding site. Undergoes a conformational change to move each molecule. Like a revolving door, while a channel is like a sliding door.

Levels of Cl- outside of cell

High outside

Levels of Na+ inside and outside of cell

High outside, low inside

For most cells, ___ is more negative than ____

Inside is more negative than outside (just like people!)

3 things critical for cell function

Ion gradients, electrochemical gradients, and membrane potential.

Ion channels show...

Ion selectivity (open in response to voltage, ligand, or mechanotransduction).

K+ leak channel

K+ "leaks" out of the cell until there is a balance between the strength of K+ gradient and voltage gradient (resting membrane potential).

What sets up the resting membrane potential?

K+ leak channels and resting membrane potential

What do Ca2+ pumps do?

Keep cytosolic Ca2+ low so that a cell can detect a small spike in its concentration to carry out the signal transduction.

Levels of K+ inside and outside of cell

Low outside, high inside

What are ion gradients formed by?

Membrane protein transporters

Ion channels

Most channels facilitate ion passage and are gated. Ion channels show ion selectivity to move ions across the membrane. They open in response to voltage, ligand, or mechanostranduction.

Facilitated transport

Most molecules need help to get across the lipid bilayer. Proteins use facilitated transport to help large and/or polar molecules across the plasma membrane. Like entryways in a building.

Voltage-gated channels

Open in response to change in electrical potential

Channels

Open/closed "doors" through which molecules can pass through. Example of allosteric regulation.

Transport can be ___ or ____

Passive or active

Mechanically-gated channels

Regulated by a mechanical force applied to a protein to result in a change in the protein. Ex: membrane is stretched.

Ligand-gated channels

Regulated by the attachment of a stimuli

Simple diffusion

Small and uncharged molecules can move across the lipid bilayer slowly or through water channels (aquaporins) through simple diffusion.

Passive transport

Transportation of materials across a plasma membrane without using energy; moves with the concentration gradient (down the concentration gradient).

Facilitated transport is by ___ or ____

Transporter or channel