1. Homeostasis and Transport
Chapter 5
Homeostasis and Transport

2. Homeostasis
The property of a system that regulates its internal environment and tends to maintain a stable, constant condition
Cell membranes help organisms maintain homeostasis by controlling what substances enter and leave cells

3. Passive Transport
Substances crossing the cell membrane without any input of energy by the cell – move down their concentration gradient
Types:
Diffusion
Osmosis
Facilitated Diffusion
Ion Channels

4. Concentration Gradient
Concentration - how many of a substance’s molecules are sitting in a specific volume of a solution
Gradient - a measurement of how much something changes as you move from one region to another
Concentration Gradient - the difference in the concentration of molecules across a space

5. Concentration Gradient

6. Diffusion
Movement of molecules from an area of high concentration to an area of low concentration
The process by which molecules intermingle as a result of their kinetic energy of random motion.

7. Diffusion

8. Diffusion Add a sugar cube to a beaker of water It sinks to the bottom
making the concentration
of sugar higher there

9. Diffusion As the cube dissolves sugar molecules break away
and move from the bottom to
the top of the beaker

10. Diffusion Because of their kinetic energy, the molecules of
sugar are in constant motion
They keep moving until
they hit something and then
they rebound

11. Diffusion If no object blocks their movement, molecules
continue on their path
They move down their
concentration gradient from
areas of high concentration
to low concentration until
They reach equilibrium

12. Do Now What is homeostasis?
Do forms of Passive Transport require cell energy?
What is diffusion?

13. Equilibrium
Diffusion will eventually cause the concentration of molecules to be the same throughout
Equilibrium – when the concentration of the molecules of a substance is the same throughout a space
The molecules are still moving, but they are just as likely to move in one direction as the other – they balance each other

14. Equilibrium

15. Diffusion Through Cell Boundaries
Cell membrane regulates movement of dissolved molecules from the liquid on one side of the membrane to the other
Cell membrane - selectively permeable - some substances can pass across it and some cannot
Nonpolar molecules can diffuse through the lipid bilayer as well as small molecules

16. Diffusion Through Cell Boundaries
If a molecule can pass through a cell membrane, it will diffuse from an area of high concentration on one side to an area of low concentration on the other
The cell is not required to use energy for diffusion.

17. Diffusion in a Cell

18. Osmosis
Solution = solute + solvent

19. Osmosis Both solute and solvent molecules can diffuse In cells:
the solutes are organic and inorganic compounds
the solvent is water

20. Osmosis
Osmosis - diffusion of water through a selectively permeable membrane (cell membrane)
Water moves down its concentration gradient
It moves from areas of high concentration of water to low concentration of water
Osmosis does not require energy

22. Osmosis
When water is more concentrated on one side of the membrane, water will move to an area of lower concentration in order to re-establish equilibrium.
Question??
If there is more salt outside of the cell, will water move into the cell or out of the cell??

23. Osmosis
The net direction of osmosis depends on the concentration of solutes on the two sides of the semi-permeable membrane
In a cell, this can have important consequences

24. Types of Solutions
Solutions can be:
Isotonic
Hypotonic
Hypertonic

25. Isotonic Solution
The concentration of solutes is the same inside and outside cell
Water will diffuse into and out of the cell at equal rate
No net movement of water

26. Hypotonic Solution
The solution has a lower solute concentration than the cell
Water diffuses into the cell until equilibrium is established
Net movement of water – into the cell

27. Hypertonic Solution
The solution has a higher solute concentration than the cell
Water diffuses out of the cell until equilibrium is established
Net movement of water is out of the cell

29. Osmotic Pressure
Osmosis exerts a pressure (osmotic pressure) on the hypertonic side of a selectively permeable membrane. (This could cause water to rush into cells and cells could bust)
This does not happen in animal cells because they are usually in isotonic fluids
example: blood

30. Animal Blood Cells in Different Solutions
Isotonic
Hypertonic
Hypotonic

31. The Effects of Osmosis on Animal Cells

32. Osmotic Pressure
Plant and Bacteria cells are usually in hypotonic environments (water wants to diffuse into the cell:
exposed to tremendous osmotic pressure
Rigid cell wall keeps plant and bacteria cells from bursting.

33. Plant Cells in Different Solutions
Hypotonic
Isotonic
Hypertonic

35. Facilitated Diffusion
Some molecules easily pass through the cell membrane because they dissolve in lipids (alcohols) - others can not (glucose)
Specific carrier proteins allow these other molecules to pass through the cell membrane easily
This does not require energy (type of diffusion) - only occurs when concentration is higher on one side of the membrane than the other.

36. Facilitated Diffusion
Carrier protein binds to molecule and changes shape
Carrier protein shields molecule from hydrophobic lipid bilayer
Carrier protein releases molecule inside cell
Carrier protein returns to its original shape

37. Facilitated Diffusion

38. Ion Channels
Some ions are important for cell functions (Na+, K+, Ca2+ and Cl-)
Since they are charged and hydrophilic, they can’t get across the lipid bilayer
Ion channels – specific membrane proteins that help ions get across cell membrane

39. Active Transport Requires cell’s energy
Cells must move substances up their concentration gradient
Substances go from areas of lower concentration to areas of higher concentration

40. Types of Active Transport
Cell Membrane Pumps
Endocytosis
Exocytosis

41. Cell Membrane Pumps
Carrier proteins also help out in active transport and work against the concentration gradient: Low  High
Similar to facilitated diffusion, but cell’s energy is required

42. Sodium (Na+) - Potassium (K+) Pump
Many types of animal cells need to have:
a high concentration of Na+ outside the cell
a high concentration of K+ inside the cell
The sodium-potassium pump uses cell energy to maintain this concentration difference

43. Sodium (Na+) - Potassium (K+) Pump
Three Na in the cytoplasm bind to the protein pump
At the same time, the protein splits a phosphate from an ATP molecule (energy)
Protein carries the three Na across the lipid bilayer and releases them out of cell

44. Sodium (Na+) - Potassium (K+) Pump
Next the protein binds two K from outside of cell
As K bind, the phosphate is released and the protein changes shape again
Finally the protein releases the K into the cytoplasm of the cell
This creates an electrical gradient across cell membrane (nerve and muscle cells)

47. Endocytosis and Exocytosis
Some substances (macromolecules and food particles) are too large to enter cell through membrane proteins
Endocytosis and exocytosis can move these substances across cell membrane
Requires cell energy – active transport

48. Endocytosis
Taking material into the cell by means of infoldings, or pouches in the cell membrane
Pouches pinch off and form vesicles (organelles) in cytoplasm.

49. Two Types of Endocytosis
Phagocytosis: cell membrane engulfs food or whole cells (bacteria and viruses)
Pinocytosis: cell membrane engulfs liquids

50. Exocytosis
Membrane of vesicle surrounding material fuses with cell membrane, forcing contents out of the cell
Reverse of endocytosis
Release of proteins