Cell Membranes and Membrane Transport

two cell membranes and intracellular space (rat)

• CONTROL what goes in and out of the cell

• But they must also allow for cell FREEDOM for movement, communication, growth, repair, transport...

Membranes Must

Disruption of the membrane during mucus secretion (rat goblet cell)

• Form spontaneously

• Re-seal after a puncture

• (really?)

Membranes Must

Endocytosis (rat)

Lipids (mainly phospholipids)

Proteins

Carbohydrates

Membrane Composition:

• Transport

• Structural

• Enzymatic

• Antigens

Membrane Protein Functions

How do you ‘anchor’ proteins in a cell membrane?

Membrane Transport Processes

There are two ways to classify them…

1. Passive or ActiveTransport WITH or AGAINST the gradient

What’s a Gradient?

Active vs. Passive Transport

Particle moves WITH it’s gradient (all by itself - no energy needed)

Particle moves AGAINST it’s gradient (requires energy)

2. Carrier-Mediated or

Non-Carrier-Mediated

How do we know if there’s a carrier?

• Carrier-mediated transport is characterized by:

• Specificity

• Competition

• Saturation

How do we know if there’s a carrier?

Specificity: The Ca++ ATPase transports ONLY Ca++ into the SR in muscle cells

SPECIFICITY

COMPETITION

SATURATION

Non-Carrier Mediated Transport Processes

The movement of particles from areas of high concentration to areas of low concentration until equilibrium is reached

Diffusion

Do all things diffuse at the same rate?

• Osmosis is the net diffusion of water through a semi-permeable membrane

• If solutes cannot cross the membrane, water will move to follow the solute to try to diffuse a gradient

Osmosis

Water moves to follow the solute!

Carrier-Mediated Transport

Facilitated Diffusion: Passive, Carrier-Mediated

• Particles are pumped against their gradient

Active Transport: Active, Carrier-Mediated

• Solute pumped against its gradient and DIRECT ENERGY is used (usually ATP)

• Ex: Na+/K+ pumps

Primary Active Transport

• One substance is moved against it’s gradient, powered by another substance moving WITH it’s gradient

• Goal: get the O pumped inside the cell

• Method: use the power of the X gradient

Secondary Active Transport = Cotransport

X XX

X

X

XX

XX

X

X

X

XX

OOO

OO

O

OO

O

O

O

• One substance is moved against it’s gradient, powered by another substance moving WITH it’s gradient

• You can go to the movies with your date, but you have to bring your little sister

Secondary Active Transport Analogy

• Symport = same direction

• Ex: Na+ / glucose symporter

• Antiport = opposite direction

• Ex: Na+ / Ca++ antiporter

Secondary Active Transport = Cotransport

Other Transport Processes

Endocytosis and

Exocytosis

• Endocytosis (cell-gulping) may be general or specific

• Receptor-mediated endocytosis

• Exocytosis (cell-barfing) is how many secretions are released from cells

Endocytosis (phagocytosis)

exocytosis of neurotransmitters between 2 neurons

Transcytosis