1. Membranes and Transport
Chapters 7.3

2. Plasma Membrane Fluid Mosaic Model Phospholipids Bilayers
Membrane Proteins
Cholesterol

3. Transport across Membranes
Gradient
Passive Transport
diffusion
osmosis
facilitated diffusion
Active Transport
Cell Surfaces

4. Fluid Mosaic Model
describes both the structure and function of biological membranes
states that the membrane is a “fluid” (not static) structure, membrane molecules “float” or “slide” past one another
also is a “mosaic” - art term describing a large picture made of many small parts

5. 1st Major Part: Phospholipids
are amphipathic
Contain two regions:
head
phosphate
polar, hydrophilic, interacts with water
tails (2)
fatty acids (saturated or unsaturated)
nonpolar, hydrophobic, repelled by water

6. Major Part 2-Membrane Proteins
are either inserted through the membrane = integral proteins
transport proteins - channels and carriers
enzymes
receptor proteins
or are associated w/ the membrane surface = peripheral proteins
act as sites for attachment to the cytoskeleton or extracellular matrix

7. Major Part 3 - Cholesterol:
is the basis of all steroid molecules
acts to stabilize the structure of membranes
affects the “fluidity” of the membrane

8. Bilayers:
are arranged from two layers of phospholipids; arranged tail-to-tail
are the basic structure for all membranes
have a hydrophobic region (lipid tails) that acts as a selectively permeable barrier
give membranes a condition known as selective permeability

9. SELECTIVE PERMEABILITY
Some things can cross the membrane; other things cannot...
This means the membrane is important for HOMEOSTASIS-keeps the internal part of the cell stable.
The Membrane is the Cell’s BOUNCER

10. Cross Easily small molecules nonpolar molecules water oxygen
carbon dioxide
nonpolar molecules
lipids
steroids

11. Doesn’t Cross Easily Large molecules Polar and ionic compounds
DNA, Proteins, etc.
Polar and ionic compounds
sugar
salts

12. Gradient
definition: difference in concentrations, ions, etc. over a distance
helps describe how substances move from one place to another
Due to the random movement of molecules
leads to movement from areas of high concentration to areas of low concentration

13. Passive Transport any movement of molecules “down” gradient
does not require energy
diffusion: movement of a substance from high concentrations to low concentrations
osmosis: diffusion of water across a selectively permeable membrane
facilitated diffusion: diffusion of a molecule across a membrane through a membrane protein

14. Osmosis: diffusion of water across a selectively permeable membrane
hypertonic: solution with a higher concentration of solutes than its surroundings
hypotonic: solution with a lower concentration of solutes than its surroundings
isotonic: solution with concentrations equal to the surroundings

15. Active Transport
movement of molecules against or “up” their concentration gradient
REQUIRES ENERGY = ATP
compare:both are types of transport
contrast:
active requires energy; moves substances up the gradient and from low to high concentration
passive doesn’t require energy; moves down the gradient and from high to low concentration

16. Explain the movement…
of water across a selectively permeable membrane in terms of hyper-, iso-, and hypotonic solutions
water moves “down” its concentration gradient
water’s concentration gradient is opposite that of its solutes, SO water moves from regions of low solute concentration to regions of high solute concentration
water moves from hypotonic to hypertonic

17. Water Balance w/out walls
hypertonic-lose water and shrivel (crenation)
hypotonic-gain water, swell, and possibly burst (lyse)
must live in isotonic environments or have adaptations for osmoregulation

18. Water Balance - cont’d. with walls
hypotonic-water moves into the cell and presses against the wall, causing a turgid cell
isotonic solutions-flaccid cells
hypertonic-plasmolysis - membrane pulls away from the cell wall as it shrivels

19. Exocytosis movement of large molecules out of the cell
requires vesicles for transport

20. Endocyotsis movement of large molecules into the cell
phagocytosis: cell eating-one cell engulfs another
pinocytosis: cell drinking-cells bring in fluid from the surroundings

21. Transport Terms to Know
Diffusion
Osmosis
Facilitated Diffusion
Active transport
Exocytosis
Endocytosis