Homeostasis: Movement Across the Cell Membrane

Students know cells are enclosed within semipermeable membranes that regulate their interaction with their surroundings.

A. Cell Membrane

Cell Membranes surround ALL cells! Controls what molecules enter and leave the

cell- food & oxygen molecules must enter the cell- waste products need to be released

SEMIPERMEABLE – it allows some things through but not others

Structure of Cell Membrane Phospholipid bilayer

Polar heads “love” water (hydrophilic) Nonpolar tails “fear” water (hydrophobic)

C. Fluid Mosaic Model Describes the various molecules found in

the cell membrane Example:

-Proteins: forms channels and pumps to move materials across membranes- Carbohydrates: act like identification cards so cells can identify one another

D. Transport Across a Membrane Molecules can enter/leave the cell in 2

ways:1. Passive Transport

* Simple Diffusion* Carrier-Facilitated (“helped”) Diffusion* Osmosis

2. Active transport* Protein Pump* Endocytosis* Exocytosis

Passive Transport

Energy is not required for movement across the membrane to occur

Molecules move from area of high concentration to area of low concentration

Example: Like riding a bike downhill

Two factors determine if passive transport takes place, equilibrium and permeability- Equilibrium: when the concentrations on both sides of the membrane are equal

membrane

- Permeability: ability of a molecule to diffuse (move) across a membrane

* Impermeable: molecules that cannot pass across a membrane* semi-permeable/selective permeability: Some molecules can pass across the membrane while other molecules

cannot

higher conc. lower conc.

Cell membrane

A. A. Diffusion Molecules moving from area of high

concentration to area of low concentration No energy is required Occurs because molecules constantly move and

collide with each otherClick link

B. Carrier-Facilitated Diffusion No energy required Molecules “helped” across by carrier

proteins from high to low concentration

Video (click here)

C. Osmosis Osmosis is a special type of diffusion

The diffusion of water molecules through a selectively permeable membrane

*Click pic

ISOTONIC Concentration of

solutes (dissolved substance) is the same in and out of the cell

Equal amount of water leaving and entering cell

HYPERTONIC Solution has a

higher solute concentration than the inside of cell

More water leaves cell, causing cell to shrink

HYPOTONIC Solution has a

lower solute concentration than the inside of cell

More water enters cell, causing cell to expand (get big)

*Click pic

Active TransPort Energy required for molecules to pass

across◦ ATP – the “battery” of the cell◦ Breaking a bond in ATP releases energy◦ Can “pump” molecules from low to high

concentration (against concentration gradient) Example: Like riding a bike uphill

ATP ATP

A. Protein Pump◦Protein Pumps: pulls molecule through

◦ATP Energy is required

*Click Pic

B. EndocytosisB. Endocytosis Endocytosis: cell membrane engulfs the larger molecule bring it into the cell

Two types:◦Phagocytosis: cytoplasm surrounds a molecule and packages it in a food vacuole

◦Pinocytosis: cell membrane forms pockets filled with liquid and pinch off to form vacuoles in a cell

C. Exocytosis Exocytosis: The membrane of the vacuole

surrounding the material fuses with the cell membrane, releasing the material outside the cell

*Click Pic