1. Chapter 7.2 & 8.1
The Plasma Membrane

3. Review of the Plasma Membrane Structure and Function
the plasma membrane is the flexible lipid boundary of the cell and it helps the cell to maintain homeostasis through selective permeability
homeostasis is the regulation of the internal environment of the cell regardless of the external environment
remember, cells need certain chemicals and molecules in certain amounts to function correctly
selective permeability is the act of allowing certain molecules to move into and out of the cell and not others

4. Structure
Remember, the plasma membrane is a flexible lipid bilayer
It is composed of 3 types of molecules
the main type of molecule that composes the membrane is the phospholipids
these phospholipids are composed of 2 parts; the polar (they like water – hydrophilic) head and the nonpolar (they don’t like water - hydrophobic) fatty acid tails
these phospholipids line up in 2 layers to makeup the membrane; the polar heads on the outside while the nonpolar fatty acid tails point in because there is water found both inside and outside of the cell (see diagram)

5. there are also proteins found in the bilayer
there are several different types of proteins found in the membrane
some go only partway through the bilayer – these serve as enzymes and chemical “markers”
some go the entire way through the bilayer – these allow larger molecules to pass through the membrane
there is also cholesterol found in the bilayer
the cholesterol serves 2 purposes; it gives the membrane stability and helps to keep the fatty acid tails separated
We call our model of the plasma membrane the fluid mosaic model
it is made of many tiny parts that are free to move around

6. Review of Diffusion
Diffusion is the random movement of particles from an area of high concentration to low concentration
Diffusion allows particles in a solution to become evenly spread out
once the particles are evenly spread out, the random movement of particles continues but there is no overall change in the distribution of particles
this condition is called dynamic equilibrium
If nothing interferes with diffusion, it will continue until dynamic equilibrium is reached and there is no longer a difference in concentration

7. Diffusion depends on three things
concentration gradient
diffusion cannot occur unless there is a difference in the concentration of a substance
this difference in concentration in different areas called a concentration gradient
since particles randomly move from high concentration to low concentration it is said that they move with the gradient
if something makes particles move from low concentration to high concentration they are said to move against the gradient
size
it is difficult for large molecules to pass through the membrane by diffusion because there is not much room between the phospholipids to diffuse

8. charge
it is also difficult for charged particles to pass through the memebrane by diffusion because of the nonpolar fatty acid tails
the nonpolar tails do not “like” polar molecules and they will not allow them to pass through the membrane by diffusion
there are only a few molecules that can pass through the membrane by diffusion
water, oxygen, nitrogen, carbon dioxide and a few other small nonpolar molecules

9. Osmosis: Diffusion of Water - osmosis is the diffusion of water into and out of a cell

10. remember, diffusion occurs from an area of high concentration to low concentration
this means within a cell osmosis occurs when water moves into or out of a cell to even out the concentration of water
concentration refers to how many molecules are in one area relative to another area
for example, we can make salt water by adding sodium chloride to water. If we do this in 2 separate beakers, we can add more salt to one than to the other. This gives each beaker a different concentration of dissolved particles.
The concentration of dissolved particles can help us to classify the solution, but we must look at the water concentration to see in which direction the water molecules will diffuse (see diagram)
Water concentration is opposite of the dissolved substance concentration

11. What controls osmosis?
Unequal distribution of particles, called a concentration gradient, is one factor that controls osmosis.
Before
Osmosis
After
Osmosis
Water molecule
Sugar molecule
Selectively permeable membrane
The concentration of dissolved substance is opposite of the water concentration.

12. Remember, water is one of the only molecules that can diffuse across a membrane
Water will move out of the cell if the water concentration is greater inside the cell than outside
Water will move into the cell if the water concentration is greater outside of the cell than inside

13. Cells can be present or placed into three types of environments that are classified by the type of solution they are
these three types of solutions are classified by the concentration of dissolved substance outside of the cell compared to the concentration of dissolved substance inside the cell
these three types are called isotonic, hypotonic and hypertonic

14. Isotonic solutions are solutions that have the same concentration of dissolved substance that the inside of the cell has. This means that their water concentrations are equal as well.
if there is no difference in water concentration, then there will be no diffusion of water (osmosis)
water molecules will still be moving in and out, but since the concentrations are equal dynamic equilibrium is occurring
cells in isotonic solutions maintain their “normal” shape
most solutions that are injected into our bodies are isotonic so the cell is not damaged by the loss or gain of water

15. Hypotonic solutions are solutions in which the dissolved substance concentration is less outside the cell than inside the cell.
this means that the water concentration is greater outside the cell than inside, so water will move into the cell
this will cause the cell to swell, because water will move into it, the pressure inside the cell will increase

16. in plant cells; In animal cell;
this pressure is called turgor pressure.
Because plants have cell walls, they can resist turgor pressure and not be damaged
This turgor pressure helps to maintain plant cell shape and support
When turgor pressure is lost, plants wilt
In animal cell;
animal cells have no cell walls, therefore, animal cells can continue to swell until they burst (called lysis)
therefore, organisms that live in freshwater have to get rid of excess water from their cells
fish in fresh water constantly urinate
some single-celled organisms have organelles called contractile vacuoles that help them excrete excess water

17. Hypertonic solutions are solutions in which the dissolved substance has a higher concentration outside the cell than inside the cell.
this means that the water concentration is less outside the cell than inside, so water will move out of the cell.
This will cause the cell to shrink, because water is leaving the cell and the pressure decreases

18. In plant cells; In animal cells; see page 197 for diagrams
will lose water mainly from their large vacuole
this loss in pressure allows the cell membrane to pull in and away from the cell wall which is rigid and cannot move; this is called plasmolysis
this results in wilting of the plant
In animal cells;
will shrivel up, called crenation
animals that live in salt water must compensate for the constant loss of water (ex. Salt water fish drink the water and excrete the salt)
see page 197 for diagrams

20. Summary
Hypertonic – more solute, less water outside; water will move out and cell will shrink
Hypotonic – less solute, more water outside; water will move into the cell and the cell will swell
Isotonic – same concentrations inside and out; water will move in and out in equal rates, so cell will remain “normal”
Plant cells  “shrinkage” = plasmolysis, “swelling” causes turgor pressure
Animal cells  shrinkage = crenation, swelling to bursting = lysis

21. Passive Transport
passive transport is the movement of materials into and out of the cell that does not require the cell to use energy to accomplish the movement
there are 2 types of passive transport
diffusion
remember, diffusion occurs with the concentration gradient and therefore it happens randomly so the cell doesn’t have to use energy to make it happen
diffusion only occurs for a few small molecules

22. facilitated diffusion
this type of diffusion takes place through the membrane transport proteins, but it is still diffusion and therefore the particles are moving with the gradient and the movement does require the cell to expend energy
there are 2 types of transport proteins
channel proteins – these proteins are like tubes that go the entire way through the membrane and allow larger molecules to flow right in (they are like an open doorway)
carrier proteins – these proteins are like doors that open and close (by changing shape) to allow substances to pass through the membrane

23. channel proteins – these proteins are like tubes that go the entire way through the membrane and allow larger molecules to flow right in (they are like an open doorway)

24. carrier proteins – these proteins are like doors that open and close (by changing shape) to allow substances to pass through the membrane
see figure 8.5 page 198

25. Passive Transport

26. Active Transport
active transport is the movement of materials through a membrane against a concentration gradient
this means that the cell is moving materials from an area of low concentration to an area of high concentration
this can be done, it just means the cell has to use energy to move the materials

27. active transport is accomplished by the use of carrier proteins
remember, they are like doors that open and close by changing shape
generally, each carrier protein has a specific shape that fits a specific molecule or ion
when the proper molecule “binds” with the protein, cellular energy is used to change the shape of the protein allowing the material to be released on the other side of the membrane
once the material is released, the proteins original shape is restored

28. Active Transport

29. Transports of Large Molecules
some cells can move very large molecules, groups of molecules or even whole cells in and out
when large particles are moved in, it is called endocytosis
when large particles are moved out it is called exocytosis
in these processes, the materials do not pass directly through the membrane; instead they either “engulfed” by the cell during endocytosis or put in vacuoles and pushed out of the cell in exocytosis
both of these processes require cellular energy
see figure 8.7, page 200