Chapter 7 Membrane Transport

Lecture 19

Passive transport

(no energy required) molecules more down their concentration gradient

3 types of passive transport

Simple diffusion, facilitated diffusion, osmosis

Active transport

(requires energy) Moves molecules against (up) their concentration gradient (low to high)

Permeability of the Lipid Bilayer 

Molecules can cross a phospholipid bilayer (without a transporter) and molecules that can not cross

Molecules that CAN cross a phospholipid bilayer (without a transporter)

Small polar uncharged

Water

Hydrophobic (nonpolar) molecules

Molecules that CANNOT cross a phospholipid bilayer (without a transporter)

Large polar molecules (like sugars, amino acids)

Charged molecules (ions)

Passive transport has 2 diffusions across a membrane

Simple diffusion and facilitated diffusion

Simple diffusion

Tendency for molecules to spread out evenly into the available space

Molecules diffuse down their concentration gradient from high to low concentration

Diffusion requires

no energy

Equilibrium means no net movement

Movement in both in and out at an equal rate

Facilitated diffusion

Molecules move down their concentration gradient from high to low concentration

Transport proteins

Membrane proteins that assist ions and large polar molecules across the membrane

Each transporter is very specific for the molecules it moves 

Types of transport proteins 

Channel protein (tiny particles)

Carrier Proteins (large particles)

Channel proteins 

Provide a corridor (pore) for a molecule or ion to cross the membrane 

Ex; Aquaporins, ion channels 

Aquaporins (2003 nobel prize Peter Agre)

Water channel, a transport protein

Found only in some cells and allows rapid water movement

Ion channels

In neurons, heart, and muscle cells

They move down their electrochemical gradients

Carrier proteins 

Change shape into move substances across the membrane For large polar amino acids and sugars 

Osmosis (Memorize) 

Movement of water across a semipermeable membrane 

Water moves from a region of low osmolarity to higher osmolarity

Calculating osmolarity

Molarity x (number of solute particles) 

Units= Osmolar (OSM)

Calculating osmolarity for molecules (have covalent bonds)

Molarity and osmolarity are the same= 1 particle

Calculating osmolarity for ionic compounds

Depends on the number of particles in the salt

Which way will water move? 1M glucose II 2M glucose

1×1=1 —> 2×1=2

Tonicity

Ability of a solution to cause a cell to gain or lose water

What does tonicity refer to

The osmolarity of a solution relative to the osmolarity inside a cell and nonpenetrating particles

Hypertonic solutions

Has a higher osmolarity than inside the cell

Causes cell to lose water, shrivel

Hypotonic solutions

Has a lower osmolarity than inside the cell

Causes cells to gain water, swell or get bigger

Isotonic solutions

Has the same osmolarity as inside the cell

No net movement of water, looks the same

Tonicity: flaccid 

Active Transport

Move substances against (up) their concentration gradient (from low to high concentration)

Requires energy and a membrane protein

Electrogenic pumps 

Create a voltage across a membrane  (sodium-potassium ATPase)

Contransport

Passive transport of 1 solute derives the active transport of another solute

Proton pump

The main electrogenic pump in plants, fungi, bacteria

Membrane potential

The charge difference (voltage) across a membrane

Created by the separation of opposite charges across a membrane

Electrochemical Gradient

Created by a unequal concentration of ions across a membrane

Has 2 forces: A chemical force (the concentration gradient)

An electrical force (the membrane potential)

Proton motive force (memorize)

The potential energy stored in a gradient of protons across a membrane 

-Key force for a cellular respiration

-H+ are trapped bc ions cant cross membranes

1. If a red blood cell is placed in a hypotonic solution, which way will water move? What will happen to the cell and why? 

Water will move into the cell, gaining the water. The cell will swell as it is absorbed with water

Why don’t plant cells burst like animal cells do when they are put in water?

Because they have a rigid cell wall surrounding the plasma membrane