1. Chapter 7.3
Cell Boundaries

2. Cell Membrane
All cells are surrounded by a thin, flexible barrier called the cell membrane.
The cell membrane is in charge of regulating what enters and exits a cell, it also provides protection and support.
A cell membrane is made of a lipid bilayer. This is because it is made of 2 layers of phospholipids. The phospholipid has a phosphate head and 2 fatty acid tails.
There are also proteins embedded in the phospholipid bilayer.
Carbohydrates are in the bilayer as well to help the membrane to stay flexible and to take each cell.
The cell membrane is called a “fluid mosaic model” because it is flexible (fluid) and it has many different parts stuck in it.

4. Cell Walls
Cell wall are not found in all types of cells, only in plants, algae, fungi, and many prokaryotes.
The main function is to provide support and protection.
The cell wall makes the cell very rigid.
The cell wall does NOT have any say on what enters or exits the cell.

5. Diffusion Through Cell Boundaries
Cytoplasm of a cell contains many dissolved substances.
The concentration of a solution is the mass of a solute (the thing being dissolved) in a given volume of solution (the thing doing the dissolving).
If we had 12 g of salt in 3 liter of water the concentration would be:

6. Diffusion
In solution particles are constantly moving. Particles will always move from an area of high concentration to an area of lower concentration – this is diffusion.
When a solute is the same throughout a system it has reached equilibrium.
A substance will move in or out of a cell based on concentration and diffusion. This requires NO energy.

7. Osmosis
Cell membranes are selectively permeable – this means that they don’t allow everything to move in or out of the cell.
Water can move through the membrane easily – osmosis is the diffusion of water.

8. How Osmosis Works
The concentration of water has to do with the amount of free water in a solution – if a solution has a lot of dissolved substances than it will have a low concentration of free water.
If we are looking at a cell – if there is more free water in the cell than outside the cell water will move out of the cell – from an area of high concentration of water molecules to an area of low concentration of water molecules.

9. There are 3 types of solutions when comparing the amounts of water concentration:
Isotonic – when 2 solutions have the same amount of free water.
Hypertonic – when a solution has more dissolved substances and less free water.
Hypotonic – when a solution has less dissolved substances and more free water.

10. Osmotic Pressure
If an animal cell has more solutes than the surrounding liquid, water will move into the cell and possibly cause the cell to burst. The opposite it true too, if water leaves the cell the cell will shrink.
Plant and animal cells have a cell wall – when water enters the cell it will cause the osmotic pressure to increase and cause the plant to be turgid (rigid).

11. Facilitated Diffusion
If molecules are too large they cannot simply diffuse through the membrane.
Instead there are protein channels that allow larger molecules to diffuse.
Again, this requires NO energy because it moves from an area of high concentration to low concentration.
Protein channels are very specific to a certain molecule.

12. Active Transport
If a molecule needs to move against the concentration gradient it needs energy to do this – this is active transport. Usually this requires transport proteins that are again specific to a certain molecule.

13. Endocytosis
Large molecules or even clumps of materials enter a cell through endocytosis.
In this process the cell membrane actually fold in and surrounds the material and then pinches off into the cell.
Phagocytosis – this is when the cytoplasm surrounds a particle and packages it as food. (cellular eating)
Pinocytosis – this is when liquid is surrounded and brought into the cell (cellular drinking)

14. Exocytosis
This is the process to release a large amount off material from the cell.