1. Cell Membrane and the Movement across it!

2. 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

3. Cell Membrane…Fluid Mosaic Model!
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.

4. Lipids of cell membrane
Membrane is made of amphipathic phospholipids
phospholipid bilayer
phosphate
hydrophilic
inside cell
outside cell
lipid
hydrophobic

5. Phospholipid bilayer What molecules can get through directly?
fats & other lipids
can slip directly through the phospholipid cell membrane, but…
what about other stuff?
lipid
inside cell
outside cell
salt
NH3
sugar aa
H2O

6. Membrane Proteins
Proteins determine most of membrane’s specific functions
cell membrane & organelle membranes each have unique collections of proteins
Membrane proteins:
peripheral proteins = loosely bound to surface of membrane
integral proteins = penetrate into lipid bilayer, often completely spanning the membrane =
“transmembrane proteins”
Integral – enzyme activity, intercellular joining,
Peripheral – protein hormones

7. Membrane Protein Types
Channel proteins – wide open passage
Ion channels – gated
Aquaporins – water
Carrier proteins – change shape
Transport proteins – require ATP
Recognition proteins - glycoproteins
Adhesion proteins – anchors
Receptor proteins - hormones
Aquaporin- kidney & plant root

8. A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.

9. Membrane Carbohydrates
Play a key role in cell-cell recognition
ability of a cell to distinguish neighboring cells from another
important in organ & tissue development
basis for rejection of foreign cells by immune system
The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

10. Cholesterol
Provides stability in animal cells
Sterols in plants

11. Synthesis of Membrane components
1. Starts in the ER- production of proteins and lipids
2. Inside the golgi- glycoproteins are modified
3. Transported in vesicles- to plasma membrane
4. Vesicles fuse- release secretory proteins

12. Getting through cell membrane
Passive transport
No energy needed
Movement down concentration gradient
Active transport
Movement against concentration gradient
low  high
requires ATP

13. Diffusion
(2nd Law of Thermodynamics) - Universe tends towards disorder
Organization tends to disorder. Molecules move to disarray.
Diffusion
movement from high  low concentration

14. 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

15. 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.

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

17. 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.

18. 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.

19. Sodium-Potassium Pump
Na+/K+ pump in nerve cell membranes

20. Cotransport
A single ATP powered pump that transports a specific solute can indirectly drive the active transport of other solutes.

21. Transport summary

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

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

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

25. Osmosis is diffusion of water
Diffusion of water from high concentration of water to low concentration of water
across a semi-permeable membrane

26. 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

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

28. Hypotonicity
animal cell in hypotonic solution will gain water, swell & possibly burst (cytolysis)
Paramecium vs. pond water
Paramecium is hypertonic
H2O continually enters cell
to solve problem, specialized organelle, contractile vacuole
pumps H2O out of cell = ATP
plant cell
Turgid (turgor pressure)
Cell wall

29. Hypertonicity
animal cell in hypertonic solution will loose water, shrivel & probably die
salt water organisms are hypotonic compared to their environment
they have to take up water & pump out salt
plant cells
plasmolysis = wilt

30. 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

31. Water Potential
Water moves from a place of greater water potential to a place of lesser water potential (net).
As the concentration of a solute increases in a solution, the water potential will decrease accordingly.
Which has the greater water potential:
.2M or .8M?
20% or 80% water?
.2 greater
80% greater