1. Passive Transport across Plasma Membrane

2. Plasma Membrane Functions
Membrane Transport
Cells bathed in interstitial fluid – rich in nutrients (amino acids, sugars, fatty acids, vitamins, hormones & neurotransmitters, salts, wastes)
Selectively Permeable – allows some things into cell, keeps others out, also keeps substances in the cell but wastes can exit.
Damaged cells (burns) – ruin permeability – fluids, proteins, ions exit dead or damaged cell.

3. 2 types of membrane transport
Passive transport – move across membrane without energy input
Diffusion – transport for every cell.
Filtration – across capillary walls
Active Transport – cell provides ATP to move substances across membrane.

4. Diffusion
Molecules & ions scatter evenly throughout - due to kinetic energy.
Movement from high concentration to low concentration (down or along the concentration gradient)
Faster diffusion with greater difference in gradient
Molecules diffuse through physical barrier if…
Lipid soluble
Small enough to pass through channels
Assisted by carrier molecule

5. Passive Membrane Transport: Diffusion
Simple diffusion – nonpolar and lipid-soluble substances
Diffuse directly through the lipid bilayer
Substances Diffuse through channel proteins
Oxygen, carbon dioxide, fat-soluble vitamins.
PLAY
Diffusion

6. Passive Membrane Transport: Diffusion
Facilitated diffusion
Transport of glucose, amino acids, and ions
Not transported through lipid bilayer
Transported substances bind carrier proteins or pass through protein channels
Follows concentration gradient

7. Carrier Proteins – Part of Facilitated Diffusion
Are integral transmembrane integral proteins
Show specificity for certain polar molecules too large to pass through channels (including sugars and amino acids )
Oxygen, water, glucose, ions = passive transport to save ATP.

8. Diffusion Through the Plasma Membrane
Extracellular fluid
Small lipid-
insoluble
solutes
Lipid-
soluble
solutes
Lipid-insoluble
solutes
Water
molecules
Lipid
bilayer
Cytoplasm
(a) Simple diffusion
directly through the
phospholipid bilayer
(b) Carrier-mediated facilitated
diffusion via protein carrier
specific for one chemical; binding
of substrate causes shape change
in transport protein
(c) Channel-mediated
facilitated diffusion
through a channel
protein; mostly ions
selected on basis of
size and charge
(d) Osmosis, diffusion
through a specific
channel protein
(aquaporin) or
through the lipid
bilayer
Figure 3.7

9. Diffusion Through the Plasma Membrane
Extracellular fluid
Lipid-
soluble
solutes
Cytoplasm
(a) Simple diffusion
directly through the
phospholipid bilayer
Figure 3.7

10. Diffusion Through the Plasma Membrane
Lipid-insoluble
solutes
(b) Carrier-mediated facilitated
diffusion via protein carrier
specific for one chemical; binding
of substrate causes shape change
in transport protein
Figure 3.7

11. Diffusion Through the Plasma Membrane
Small lipid-
insoluble
solutes
(c) Channel-mediated facilitated
diffusion through a channel
protein; mostly ions selected
on basis of size and charge
Figure 3.7

12. Diffusion Through the Plasma Membrane
Water
molecules
Lipid
bilayer
(d) Osmosis, diffusion through
a specific channel protein
(aquaporin) or through the
lipid bilayer
Figure 3.7

13. Diffusion Through the Plasma Membrane
Extracellular fluid
Small lipid-
insoluble
solutes
Lipid-
soluble
solutes
Lipid-insoluble
solutes
Water
molecules
Lipid
bilayer
Cytoplasm
(a) Simple diffusion
directly through the
phospholipid bilayer
(b) Carrier-mediated facilitated
diffusion via protein carrier
specific for one chemical; binding
of substrate causes shape change
in transport protein
(c) Channel-mediated
facilitated diffusion
through a channel
protein; mostly ions
selected on basis of
size and charge
(d) Osmosis, diffusion
through a specific
channel protein
(aquaporin) or
through the lipid
bilayer
Figure 3.7

14. Passive Membrane Transport: Osmosis
Occurs when the concentration of a solvent is different on opposite sides of a membrane
Diffusion of water across a semipermeable membrane
Osmolarity – total concentration of solute particles in a solution
Tonicity – how a solution affects cell volume
PLAY
Osmosis

15. Effect of Membrane Permeability on Diffusion and Osmosis
Figure 3.8a

16. Effect of Membrane Permeability on Diffusion and Osmosis
Figure 3.8b

17. Passive Membrane Transport: Filtration
The passage of water and solutes through a membrane by hydrostatic pressure (still passive but through pressure gradient)
Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area
Application – water forced out of capillaries, urine forced out of glomerulus. (Concentration gradient may bring molecules back into capillaries)

18. Effects of Solutions of Varying Tonicity
Isotonic – solutions with the same solute concentration as that of the cytosol
Not net gain of loss of water in or out of cell
Hypertonic – solutions having greater solute concentration than that of the cytosol
Cell in strong saline solution, cell loses water, shrinks
Hypotonic – solutions having lesser solute concentration than that of the cytosol
Cell in Distilled Water, cell fills up, can burst