Transporting molecules across cell membranes Nutrients, wastes, hormones,

ions

Recall phospholipid bilayer • It’s a barrier! • Only a small subset of molecules may pass

through without help

Cell membrane • Sense & respond to changes in environment • Regulate exchange with environment

Cell membranes are selectively permeable

• Some compounds pass uninhibited through membrane (passive diffusion), some require assistance from membrane proteins (facilitated diffusion), and some require assistance AND energy expenditure (active transport)

1. Diffusion – Passive transport or diffusion – Carrier-mediated (facilitated) transport or diffusion

2. Active Transport – Pumps, bulk transport

PM + proteins mediate transport

Passive (Diffusion & Osmosis)…or Active

Simple diffusion; P

Osmosis; P Channel-mediated; P

Carrier- mediated; A or P

Proteins are responsible • Some are receptors that regulate bulk

transport – Induce endocytosis or exocytosis

• Cyto from “kyto” = hollow vessel

Concept Check • Membranes organize cell activities. The proteins

imbedded in the membranes are essential to their function. These membrane proteins have properties that allow them to “float” in the membrane. Which of the following describe those properties? a) The surface region of the protein in the interior

of the membrane is mostly hydrophobic. b) The surface region of the protein in the interior

of the membrane is mostly hydrophilic. c) The surface region exposed to the exterior

environment is hydrophobic. d) The surface region exposed to the interior

environment is hydrophobic.

Answer • Membranes organize cell activities.

The proteins imbedded in the membranes are essential to their function. These membrane proteins have properties that allow them to “float” in the membrane. Which of the following describe those properties? a) The surface region of the protein in the

interior of the membrane is mostly hydrophobic.

So, what is diffusion? • Solvent: The predominant liquid or gas in

a solution • Solute: The stuff that is dissolved in a

solution • Diffusion: The net movement of solute

from a higher to a lower concentration (Concentration gradient), until equilibrium is achieved. Uses intrinsic Kinetic Energy (KE).

Everything has Energy • Energy = The capacity to perform work;

to rearrange matter • 2 forms:

– Potential Energy (PE): stored energy, due to position or structure

– Kinetic Energy (KE): Energy of motion • Heat is KE associated with the movement of

molecules/atoms

Passive Diffusion

Passive Diffusion

Passive diffusion

• Kinetic energy causes particles to move

• Diffusion occurs due to random collisions between these moving particles

Osmosis • Passive diffusion of H20

down its concentration gradient – Due to solutes inside

flask, there’s more H20 outside than in

– Only H20 can pass through pores

– H20 moves down its concentration gradient and INTO flask

Osmosis + Diffusion • Both are happening all the time across

cell membranes • Osmosis (H20) occurs RAPIDLY,

diffusion (solutes) occurs SLOWLY • H20 moves into cells with high solute

concentration and out of cells with low solute concentration

What happens? • If you place a cell into a solution whose

solute concentration is much lower than that of the cell, what will happen to the cell? Will it:

1. Shrink (crenate; as H20 leaves) 2. Swell (as H20 enters) 3. Remain unchanged

Concept Check • This diagram represents osmosis

of water across a semipermeable membrane. The U-tube on the right shows the results of the osmosis. What could you do to level the solutions in the two sides of the right hand U-tube? a) Add more water to the left hand

side. b) Add more water to the right hand

side. c) Add more solute to the left hand

side. d) Add more solute to the right hand

side.

Answer • This diagram represents osmosis

of water across a semipermeable membrane. The U-tube on the right shows the results of the osmosis. What could you do to level the solutions in the two sides of the right hand U-tube?

c) Add more solute to the left hand side.

Tonicity • Tonicity: measure of the solvent concentration

of a solution and it’s osmotic pressure • Hypertonic: Solute concentration is greater

than the cytoplasm = net movement of H2O out of cell

• Hypotonic: Solute concentration of solution is less than the cytoplasm = net movement into cell

• Isotonic: Solute concentration of solution is equal to the cytoplasm = no net movement of H20

Cells in different solutions

Facilitated diffusion

• Diffusion can be simple or facilitated

• Facilitated is Passive – No energy needed to

move molecules. – Molecules move down

their concentration gradient

Active transport

1. requires ATP (energy) to move molecules.

2. Molecules move against their concentration gradient.

Maintains gradient for cotransport

Endo- & Exocytosis • Bulk transport; A • Requires ATP

– Phagocytosis: ingestion of solid particles • “phagein” = to eat

– Pinocytosis: ingestion of liquid

Endocytosis Active; Requires ATP

Exocytosis Active; Requires ATP