1. Chapter 8. Movement across the Cell Membrane

2. Diffusion
Diffusion
movement from high  low concentration of any substance
Can be movement across a membrane or not (think about perfume spreading throughout an entire room)
Movement from high concentration of that substance to low concentration of that substance.

3. Diffusion of 2 solutes
Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances
Things tend to get mixed up evenly.

4. Diffusion Move for HIGH to LOW concentration diffusion osmosis
“passive transport”
no energy needed
diffusion
osmosis

5. Cell (plasma) membrane
Cells need an inside & an outside…
separate cell from its environment
cell membrane is the boundary
Can it be an impenetrable boundary?
NO!
OUT
waste
ammonia
salts
CO2
H2O
products IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
OUT IN
cell needs materials in & products or waste out

6. Simple diffusion across membrane
Which way will lipid move?
lipid
lipid
lipid
inside cell
lipid
lipid
lipid
low
high 
lipid
outside cell
lipid
lipid
lipid
lipid
lipid
lipid
lipid

7. Permeable cell membrane
Need to allow more material through
membrane needs to be permeable to…
all materials a cell needs to bring in
all waste a cell needs excrete out
all products a cell needs to export out
inside cell aa
H2O
sugar
lipid
“holes”, or channels, in cell membrane allow material in & out
salt
NH3
outside cell

8. Semi-permeable cell membrane
But the cell still needs control
membrane needs to be semi-permeable
specific channels allow specific material in & out
inside cell
H2O aa
sugar
salt
outside cell
NH3

9. Getting through cell membrane
Passive transport
diffusion of hydrophobic (lipids) molecules
high  low concentration gradient
Facilitated transport
diffusion of hydrophilic molecules
through a protein channel
Active transport
diffusion against concentration gradient
low  high
uses a protein pump
requires ATP

10. Facilitated Diffusion
Globular proteins act as doors in membrane
channels to move specific molecules through cell membrane
open channel = fast transport
high
Donuts!
Each transport protein is specific as to the substances that it will translocate (move).
For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer.
Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane.
These channel proteins allow fast transport.
For example, water channel proteins, aquaprorins, facilitate massive amounts of diffusion.
low
“The Bouncer”

11. Facilitated diffusion
Move from HIGH to LOW concentration through a protein channel
passive transport
no energy needed
facilitated = with help

12. Gated channels
Some channel proteins open only in presence of stimulus (signal)
stimulus usually different from transported molecule
ex: ion-gated channels when neurotransmitters bind to a specific gated channels on a neuron, these channels open = allows Na+ ions to enter nerve cell
ex: voltage-gated channels change in electrical charge across nerve cell membrane opens Na+ & K+ channels
When the neurotransmitters are not present, the channels are closed.

13. conformational change
Active Transport
Globular proteins act as ferry for specific molecules
shape change transports solute from one side of membrane to other  protein “pump”
“costs” energy
conformational change
low
Cellular Donuts!
Each transport protein is specific as to the substances that it will translocate (move).
For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer.
Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane.These channel proteins allow fast transport.
For example, water channel proteins, aquaprorins, facilitate massive amounts of osmosis.
Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport.
high
“The Doorman”

14. Na+/K+ pump in nerve cell membranes
Active transport
Cells may need molecules to move against concentration situation
need to pump against concentration
protein pump
requires energy
ATP
Plants have nitrate & phosphate pumps in their roots.
Why?
Nitrate for amino acids
Phosphate for DNA & membranes
Not coincidentally these are the main constituents of fertilizer.
Na+/K+ pump in nerve cell membranes

15. Active transport Many models & mechanisms using ATP using ATP
Plants: nitrate & phosphate pumps in roots.
Why?
Nitrate for amino acids
Phosphate for DNA & membranes
Not coincidentally these are the main constituents of fertilizer
Supplying these nutrients to plants
Replenishing the soil since plants are depleting it

16. How about large molecules?
Moving large molecules into & out of cell also requires energy
through vesicles & vacuoles
endocytosis
phagocytosis = “cellular eating”
pinocytosis = “cellular drinking”
receptor-mediated endocytosis
exocytosis
exocytosis

17. receptor-mediated endocytosis
fuse with lysosome for digestion
phagocytosis
non-specific process
pinocytosis
triggered by ligand signal
receptor-mediated endocytosis

18. Transport summary

19. The Special Case of Water Movement of water across the cell membrane

20. Osmosis is diffusion of water
Water is very important, so we talk about water separately
Diffusion of water from high concentration of water to low concentration of water
across a semi-permeable membrane

21. Concentration of water
Direction of osmosis is determined by comparing total solute concentrations
Hypertonic - more solute, less water
Hypotonic - less solute, more water
Isotonic - equal solute, equal water
hypotonic
hypertonic
water
net movement of water

22. Managing water balance
Cell survival depends on balancing water uptake & loss
freshwater
balanced
saltwater

23. Managing water balance
Isotonic
animal cell immersed in isotonic solution
blood cells in blood
no net movement of water across plasma membrane
water flows across membrane, at same rate in both directions
volume of cell is stable

24. Managing water balance
Hypotonic
animal cell in hypotonic solution will gain water, swell & burst
Paramecium vs. pond water
Pond water is hypotonic
H2O continually enters cell
to solve problem, specialized organelle, contractile vacuole
pumps H2O out of cell = ATP
plant cell
turgid

25. Water regulation
Contractile vacuole in Paramecium

26. Managing water balance
Hypertonic
animal cell in hypertonic solution will loose water, shrivel & probably die
Why a salt water fish cannot be added to a freshwater aquarium – the water would be hypertonic compared to the fish cells causing the cells to leach water out into the aquarium
plant cells
plasmolysis = wilt

27. Osmosis…
.05 M
.03 M
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell