1. Transport of Substances Across a Cell Membrane

2. Diffusion 2nd Law of Thermodynamics governs biological systems
universe tends towards disorder (entropy)
Movement from high concentration of that substance to low concentration of that substance.
Diffusion
movement from high  low concentration

3. Diffusion Move from HIGH to LOW concentration movement of water
“passive transport”
no energy needed
movement of water
diffusion
osmosis

4. Diffusion across cell membrane
Cell membrane is the boundary between inside & outside…
separates cell from its environment
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

5. Diffusion through phospholipid bilayer
What molecules can get through directly?
fats & other lipids
What molecules can NOT get through directly?
polar molecules
H2O
ions
salts, ammonia
large molecules
starches, proteins
lipid
inside cell
outside cell
salt
NH3
sugar aa
H2O

6. Small nonpolar molecules such as fats, O2 and CO2
Diffuse easily across the phospholipid bilayer of a membrane (nonpolar molecules)

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

8. 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

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

10. Managing water balance
Isotonic
animal cell immersed in mild salt solution
example: blood cells in blood plasma
problem: none
no net movement of water
flows across membrane equally, in both directions
volume of cell is stable
balanced

11. Managing water balance
Hypotonic
a cell in fresh water
example: Paramecium
problem: gains water, swells & can burst
water continually enters Paramecium cell
solution: contractile vacuole
pumps water out of cell
ATP
plant cells
turgid
ATP
freshwater

12. Water regulation
Contractile vacuole in Paramecium
ATP

13. Managing water balance
Hypertonic
a cell in salt water
example: shellfish
problem: lose water & die
solution: take up water or pump out salt
plant cells
plasmolysis = wilt
saltwater

14. 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

15. Transport proteins may facilitate diffusion across membranes
Many kinds of molecules do not diffuse freely across membranes (size, charge, polarity)
For these molecules, transport proteins
Provide passage across membranes through a process called facilitated diffusion
Solute molecule
Transport protein
Figure 5.15

16. Channels through cell membrane
Membrane becomes semi-permeable with protein channels
specific channels allow specific material across cell membrane
inside cell
H2O aa
sugar
salt
outside cell
NH3

17. Facilitated Diffusion
Diffusion through protein channels
channels move specific molecules across cell membrane
no energy needed
facilitated = with help
open channel = fast transport
high
low
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, aquaporins, facilitate massive amounts of diffusion.
“The Bouncer”

18. Facilitated Diffusion

19. Ion Channels allow specific ions to pass through the protein channel.
regulated by the cell and are either open or closed to control the passage of substances into the cell

20. Carrier Proteins
bind to specific molecules, change shape and then deposit the molecules across the membrane.
Once the transaction is complete the proteins return to their original position.

21. Active Transport
Cells may need to move molecules against concentration gradient
shape change transports solute from one side of membrane to other
protein “pump”
“costs” energy = ATP
conformational change
low
high
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.
ATP
“The Doorman”

22. Active transport Many models & mechanisms antiport symport ATP 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
antiport
symport

23. Sodium Potassium Pump

24. Cells expend energy for active transport
Transport proteins can move solutes against a concentration gradient
Through active transport, which requires ATP P
Protein changes shape
Phosphate detaches
ATP
ADP
Solute
Transport protein
Solute binding 1
Phosphorylation 2
Transport 3
Protein reversion 4
Figure 5.18

25. Getting through cell membrane
Passive Transport
Simple diffusion
diffusion of nonpolar, hydrophobic molecules
lipids
high  low concentration gradient
Facilitated transport
diffusion of polar, hydrophilic molecules
through a protein channel
Active transport
diffusion against concentration gradient
low  high
uses a protein pump
requires ATP
ATP

26. Transport summary simple diffusion facilitated diffusion
ATP
active transport

27. How about large molecules?
Moving large molecules into & out of cell
through vesicles & vacuoles
endocytosis
phagocytosis = “cellular eating”
pinocytosis = “cellular drinking”
exocytosis
exocytosis

28. Exocytosis and endocytosis transport large molecules
To move large molecules or particles through a membrane
A vesicle may fuse with the membrane and expel its contents (exocytosis)
Fluid outside cell
Cytoplasm
Protein
Vesicle
Insulin, Crying
Figure 5.19A

29. Membranes may fold inward
Enclosing material from the outside (endocytosis)
Vesicle forming
Figure 5.19B

30. Endocytosis fuse with lysosome for digestion phagocytosis
non-specific process
pinocytosis
triggered by molecular signal
receptor-mediated endocytosis