Homeostasis & Cell Transport

• Passive Transport– Diffusion aka Simple Diffusion– Osmosis– Facilitate Diffusion

• *No energy required

• Active Transport– Cell Membrane Pumps– Endocytosis and Exocytosis

• *Requires energy

Passive transport is diffusion across a membrane

• What is diffusion or simple diffusion?– Movement of molecules from an area of HIGH concentration to

LOW concentration.– Driven entirely by kinetic energy the molecules possess. Molecules

are in constant motion.

• Think about when you open a bottle of perfume…The bottle has a high concentration of perfume the air has a _____ concentration. So the molecules in the perfume want to go???

• A simple rule of diffusion: a substance will diffuse from where it is more concentrated to where it is less concentrated. To put it another way, any substance will diffuse down its concentration gradient.

More Diffusion…

• When does diffusion stop?– When the concentration of the molecules of a

substance is the same throughout a space called?

• What types of molecules can move into a cell by diffusion?– Oxygen and carbon dioxide because they

dissolve in lipids.

More Diffusion…

• Is diffusion regulated?– Is the cell membrane selective?– What kind of molecules are going to diffuse

across a cell membrane? • Size and type of the molecule…the smaller the

better!

• Chemical nature of the membrane…if they dissolve in lipids!

Osmosis is the passive transport of water

• What is osmosis?– The process by which water molecules diffuse across a

cell membrane from an area of higher concentration to an area of lower concentration.

• How does osmosis affect a cell?– Under HYPERTONIC conditions? – H2O out

– Under HYPOTONIC conditions? – H2O in

– Under ISOTONIC conditions? – H2O same

• Remember…where salt goes water will follow!!!

More osmosis…

• How do cells deal with osmosis?– Vertebrate animals living on land and most living

in the sea…cells function in an isotonic external environment – NO PROBLEM when balancing movement of water.

– Unicellular freshwater organisms…cells function in hypotonic environments – Water CONSTANTLY diffuses into these organisms.

• How do they get rid of this excess water that enters by osmosis?

– Contractile Vacuole

More osmosis…

– How do plant cells deal with osmosis? Remember they live in hypotonic environments…

• Turgor pressure – if pressure against the cell walls is so great…water will be pushed out of the cell.

– In a hypertonic environment…water leaves plant cells by osmosis.

• The cells shrink away from the cell walls and turgor pressure is lost…this is called plasmolysis.

– Changes to a cell’s environment can create problems…Human Red Bloods Cells

• Cytolysis – hypotonic environment…cells burst!!!

Facilitated Diffusion

• What is facilitated diffusion?• How is this different from simple diffusion?

– How does the selectivity compare?• MORE selective than simple diffusion…molecules are

assisted by specific proteins in the membrane.• What are carrier proteins? • Example…How does glucose, a molecule that many

cells need for their energy, get into the cell?

• Must diffuse down its concentration gradient!!!

What did we learn?

• Diffusion is SLOOOOOOWWWW!!!

• Osmosis is the diffusion of water.

• Hypertonic, hypotonic, isotonic (remember where salt goes water will follow).

• Facilitated diffusion uses carrier proteins to help large molecules across the cell membrane.

Active Transport

• What is active transport?• How is it different than passive transport?

– Where does the energy come from to drive active transport?

• What are cell membrane pumps?• What kind of proteins do they use?• An example of a cell membrane pump…

– Sodium-Potassium pump…the mother of all pumps– How does the Sodium-Potassium Pump work?

• Exchanges sodium (Na+) for potassium (K+) across the cell membrane of animal cells.

Other types of active transport…

• What is Endocytosis?– What are the two types of endocytosis?– What does pinocytosis involve?– What does phagocytosis involve?

• What is Exocytosis?

What did we learn today?

• Active transport uses ATP (it’s energy currency) to move molecules from low to high concentration.

• Cell membrane pumps ex. Sodium-Potassium Pump.

• Endocytosis and Exocytosis