Thursday November 20

• Objectives You will be able to:– Explain the concept of diffusion and how it relates to cells– Describe the different ways that molecules can be passively

transported across the cell membrane• Agenda

– Reading Quiz– Start Notes on Passive Transport (Chapter 5-1)

• Homework Due today– Read section 5-1– Outline of section

Passive Transport

Section 5-1

Passive Transport

• Doesn’t require energy• Concentration is the amount of solute

dissolved in a fixed amount of solvent• Concentration gradient is the difference in the

concentration across space

Diffusion

• Diffusion - the movement of molecules from high concentrations to low concentrations– Molecules are in constant motion (kinetic energy)– This movement drives diffusion

Equilibrium

• Concentration is the same throughout– Solutions move to equilibrium

• Even at equilibrium, molecules are still moving– Movement in many directions balances

concentration

Diffusion Across Membranes

• Some molecules can move from high concentration to low across the cell membrane– Depends on size, shape, and nature

• Molecules that dissolve in lipids can move across the membrane– Molecules that don’t must move through protein

pores

Osmosis• Water moving across a membrane• Hypotonic is when the solute concentration outside is lower

(water moves in- cell expands) • Hypertonic is when the solute concentration outside is higher

(water moves out- cell shrinks)• Isotonic is when concentrations are equal

HypOtonic

Inside

Outside

HypERtponic

Inside

Outside

Osmosis in Cells• Hypotonic environment– Contractile vacuole- collects water and pumps out– Plants use cell wall- holds up against turgor

pressure (the pressure water exerts)– Some pump solutes out of cytosol– Cytolysis (bursting) (red blood cells)

• Hypertonic environment– Plant cell membrane shrinks against cell wall

(plasmolysis), plant wilts– Red blood cells shrink and shrivel

PlantsHypotonic environment

Hypertonic environment

Red Blood Cells

Facilitated Diffusion

• Carrier proteins help some molecules that cant diffuse rapidly through the membrane

• Not soluble in lipids, or too large– Still diffusion because moving from high to low

• Molecule binds to protein, shields it from the hydrophobic cell membrane, and releases to other side of the cell– Carrier molecule is specific to molecule• Example: glucose

Ion Channels

• Provide passage for ions that can’t diffuse on their own (not soluble)

• Specific to each ion (Na+ K+ Ca2+ Cl-)• Some are always open but some have gates

that respond to:– Stretching of cell membrane– Electrical signals– Chemicals in cytosol or environment

Active Transport

Section 5-2

Sodium Potassium Pump• Same as passive transport but from low to high• NA+ binds to a carrier protein from inside• Phosphate from ATP changes shape of the protein• New shape allows K+ to bind to protein (outside)

and phosphate leaves• Loss of phosphate changes shape again and K+ is

released into the cell• Inside becomes negative while outside becomes

positive-– conducts electrical impulses

Endocytosis

• Cells ingest external fluid, macromolecules, and large cells

• Materials enclosed by cell and pinched off into a vesicle– Can fuse with organelles and digested

• Pinocytosis involves fluid• Phagocytosis involves large particles– Phagocytes ingest bacteria and viruses • destroyed by lysozymes

Exocytosis

• The reverse of endocytosis• Vesicles fuse with cell membrane and contents

are released into the environment• Used to export large molecules such as

proteins