1. Diffusion and Osmosis

2. Cells have needs too… All cells need to:
Take in food or nutrients
Take in oxygen and release carbon dioxide
Remove wastes made inside the cell
Control their water balance
All of these activities must happen across the cell membrane of every single cell

3. Methods of Transport
Passive Transport
Active Transport

4. 1. Passive Transport
Passive transport- the movement of substances across the membrane without the use of chemical energy.
Diffusion and Osmosis
Passive transport relies on the movement of substances down a concentration gradient.

5. Diffusion
Diffusion- the movement of molecules from an area of higher concentration to an area of lower concentration.
A difference in the concentrations of molecules across a space is called a concentration gradient.
Diffusion is driven by the movement of molecules
The molecules like to “spread out”

6. 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

7. Animation

8. The process of diffusion is used to deliver oxygen from
your lungs into your blood.

9. Equilibrium
Equilibrium occurs when the concentration of molecules is the same throughout a space.
Molecular movement still occurs, but in all directions  the balance is maintained.

10. Membranes
Some molecules can diffuse across a membrane (cell membrane). If a molecule can move through a membrane, that membrane is permeable to that molecule.
Ex. Water, oxygen, some ions
Not all molecules can diffuse across all membranes. Molecular movement depends on the size and type of the molecule as well as the membrane structure.
Ex. Glucose, large proteins

11. Membranes again…
Animation

12. Osmosis…
Osmosis- special case of diffusion, in which water molecules diffuse across a cell membrane from an area of high concentration of water to areas of low concentration of water.
The direction of water movement depends on the concentrations of water and solute dissolved in the solution.
Remember…
Solution- homogeneous mixture in which one or more substances are uniformly dissolved in another substance.
Solute- substance dissolved in a solution.

13. Which way will it go?
In many situations, the solute molecules can’t actually diffuse across the membrane, which creates an imbalance.
Very Simply, the concentrations of water and solutes “want” to be equal on each side of the membrane, so the water must move.

14. Osmosis is the movement of water from areas of high concentration of water to areas of low concentration of water
Animation

15. Water flows “downhill”
If the concentration of water molecules outside the cell is higher than the concentration in the cell, the solution is hypotonic to the cell. Water moves into the cell until equilibrium is reached.
Hypo means “lower”

16. Hypotonic- the cell swells
Solute
90% Water
Cell Membrane
Draw arrows
representing the
movement of water.
80% Water

17. Water flows “downhill” continued…
If the concentration of water molecules outside the cell is lower than the concentration inside the cell, the solution is hypertonic to the cell. Water moves out of the cell until equilibrium is reached.
Hyper mean “higher”

18. Hypertonic- the cell shrivels
Solute
70% Water
Cell Membrane
Draw arrows
representing the
movement of water.
90% Water

19. Water flows “downhill” continued…
When concentration of water molecules outside the cell is equal to the concentration inside the cell, the solution is isotonic to the cell. Water moves into and out of the cell at an equal rate.
“iso” means equal

20. Isotonic- at equilibrium
Solute
80% Water
Cell Membrane
Draw arrows
representing the
movement of water.
80% Water

21. Pressure
If water is moving into the cell, turgor pressure increases. If the cell takes too much water in it can burst. This is termed cytolysis.
If water is moving out of the cell, turgor pressure drops and the cell wilts. This is termed plasmolysis.

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

25. Passive Movement
Passive transport- the movement of substances across the membrane without the use of chemical energy (No ATP needed)
Diffusion and Osmosis (see previous)
Facilitated Diffusion- carrier molecules speed up the diffusion of molecules that move slowly across the membrane. No energy is required.
Larger molecules
Ex.: Glucose into and out of cells.

26. Membrane Components
Carrier molecules- transport molecules across the cell membrane from high concentration to low concentration.

27. Animation

28. 2. Active Transport
Active Transport- The movement of materials across a membrane that requires the use of energy. (Requires ATP)
Usually moves molecules Uphill, against the concentration gradient

29. It takes energy to go uphill
Cell Membrane pumps- carrier proteins that move molecules against a concentration gradient. From areas of low concentration to areas of higher concentration. This movement requires the use of chemical energy.
Animation

30. Cells eat too…
Endocytosis- the process by which cells engulf substances that are too large to pass through the cell membrane. The cell forms a vesicle around the material.
“Endo”- means “in” or “internal”

31. Types of Endocytosis
Pinocytosis- the transport of fluids or solutes into the cell. (the cell “drinks”)
Phagocytosis- movement of large molecules or entire cells/microorganisms into the cell. (the cell “eats”)

32. And get rid of wastes
Exocytosis- the passage of large molecules out of the cell.
Vacuoles merge with the cell membrane, expelling their contents.
Animation

33. Managing water balance
Isotonic
animal cell immersed in mild salt solution
Concentration of water outside=concentration inside
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

34. Managing water balance
Hypotonic
a cell in fresh water
Concentration of water outside>concentration inside
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

35. conformational change
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”