Biology 11 (home) and Biology 12 (home)

june 2002 exam

Cell Transport Notes

1. How material moves in and out of the cell
a. Diffusion - Diffusion is the movement of molecules, other than water, from an area of high concentration to an area of low concentration. No ATP energy is used. eg. Perfume filling a whole room.
-Molecules move like this naturally, so no chemical or cellular energy is needed.

How does the size of molecules, temperataure and concentration gradient affect diffusion
- Size of the molecules, temperature, and the size of the concentration gradient affects the speed of diffusion. eg. large molecules diffuse slower that small ones, the greater the concentration difference the faster the rate of diffusion and increased temperature speeds up diffusion. The reason diffusion takes place faster is because of the increased number of collisions of the particles. When you heat up something the particles vibrate faster, collide more often and spread out faster. When the concentration is high, there is a greater number of particles in the solution. With a greater number of particles, there is a greater chance of collision and thus spreading out. Large molecules need more energy to begin moving and thus diffuse slower.
-Lipid-soluble molecules can diffuse through the membrane easily. Oxygen and carbon dioxide pass through easily. Water passes through easily even though it is lipid insoluble. (see protein lined pores)

b. Osmosis - Flow of water from a high concentration to a low concentration across a selectively permeable membrane. eg. water moving from the large intestine into the blood.

c. Facilitated transport - is also the movement of molecules from a high concentration to a low concentration. Lipid insoluble substances such as glucose and amino acids are taken across by "carrier proteins". These carrier proteins are embedded in the plasma membrane and will pick up, carry and regulate the rate that specific molecules move into the cell.No chemical energy is required in this process 
.eg. amino acids, glucose and other breakdown products of food are absorbed by the small intestine.

d. Active Transport - It is the movement of molecules across a living membrane from an area of low concentration to an area of high concentration with the aid of a carrier protein and using energy or ATP.

The diagram below represents the sodium/potassium pump a kind of active transport. Read the diagram from top to bottom. 

Two kinds of active transport:
i. Endocytosis - Surrounding a substance with the cell membrane and the subsequent formation of a vesicle to bring these substances into the cell. Energy is used. The diagrams below shows endocytosis of a large particle. Be able recognize the large particle, the cell membrane and the vesicle.

The diagram below is read from left to right

                      
-There are two kinds of Endocytosis:
a. Phagocytosis - involves the ingestion of large food particles or cellular debris 
b. Pinocytosis - involves the ingestion of fluids or dissolved particles.

ii. Exocytosis - is the opposite of endocytosis. Materials are surrounded by a vesicle in the cytoplasm of the cell and released from the cell as the vesicle merges with the plasma membrane. Materials such as waste, useless cellular debris, or useful hormones for other cells are released in this manner. Energy is used.

2. What is a Solution?
A solution is a combination of solute (a solid) that has been dissolved in a solvent ( a liquid like water)
-There are three kinds of solutions:
a. isotonic solution - is a solution where the solute concentration of the solution that the cell is in is the same as the solute concentration of the cell's cytoplasm.
b. hypotonic solution - is a solution where the solute concentration of the solution that the cell is in is lower than the solute concentration of a cell's cytoplasm
c. hypertonic solution - is a solution where the solute concentration of the solution that the cell is in is higher than the solute concentration of a cell's cytoplasm

3. What happens when cells are place in different kinds of solutions
a. Hypertonic solutions

The cells shrink or shrivel due to water leaving the cell. If a cell is placed in a hypertonic solution (higher solute concentration outside the cell) water will leave the cell, the net movement of water is from the inside to the outside of the cell. If blood cells are placed in a salt solution, the cell will shrink or "crenate". When this occurs in a plant cell it is said to plasmolyze. The blood cells below have shrunken (lost water) because they were placed in a salt solution.

b. Hypotonic solutions

The cells will swell due to water entering the cell. If a cell is placed in a hypotonic solution (lower solute concentration outside the cell) water will enter the cell, the net movement of water is from the outside to the inside of the cell. If blood cells are placed in a distilled water solution, the will swell or burst. This is called  hemolysis in blood cells and lysis in non blood cells. In plant cells it is called turgor pressure because the plant cell wall prevents the cell from bursting. 

The diagram below shows the effect of placing red blood cells into a hypotonic solution. After two minutes they have swelled.

c. Isotonic solutions

When a cell is placed in a solution where the solute concentration is the same on both sides of the cell membrane, the cell will neither shrink nor swell. 0.9% sodium chloride (salt) is isotonic to blood cells.

4. Fluid-Mosaic Model of the Cell Membrane - The cell membrane is made up a phospholipid bilayer (double layer) with proteins embedded in it.  The phospholipid bilayer has a fluid consistency, comparable to light oil. Proteins are scattered throughout the membrane, they form the mosaic. (eg. stained glass windows are mosaics. Small individual pieces of glass make up a whole picture.)

a. Function of the parts:
i. Phospholipid bilayer (labelled X) - The phospolipid bilayer is the structural element that forms the physical boundary of the cell membrane. Materials which can dissolve in fat, like alcohol, can move across phospholipid bilayer with ease. Water soluble substance are unable to cross through the bilayer and must enter the cell through channel proteins.

ii. Proteins - are involved in the passage of molecules through the membrane.

A. Channel proteins - a protein that allows a particular molecule or ion to freely cross the membrane as it enters or leaves the cell.
B. Carrier proteins - a protein that selectively interacts with a specific molecule or ion so that it can cross the cell membrane to enter or exit the cell. 
C. Receptor proteins - A protein that has a specific shape so that specific molecules can bind to them. The binding of a molecule, such as a hormone, can influence the metabolism of the cell. 
D. Enzyme proteins - An enzyme that catalyzes  a specific reaction.
E. Cell-recognition proteins - glycoproteins that identify the cell. They make up the cellular fingerprint by which cells can recognize each other.

5. Surface Area and Volume

A cell with a large volume has a smaller surface area to volume ratio than a cell with a small volume. As the volume increases the surface area/ volume ration decreases. The table below shows this

Linear dimension of the cell                  surface (mm²)                        Volume (mm²)              Ratio

1 mm                                                               6                                                1                              6 to 1

2 mm                                                             24                                                8                             3 to 1

4 mm                                                             96                                              64                             1.5 to 1  

If all the raw materials for metabolism and all the wastes produced as a result of metabolism must diffuse through the cell membrane, then a cell with a small volume (higher surface area/volume ratio) can have a higher metabolic rate. This is because the surface area allows the cell to obtain the necessary raw materials and get rid of the wastes that build up. Conversely a larger cell will not be able to supply the necessary raw materials to fuel a high metabolic rate or get rid of the resulting wastes, therefore it has a slower metabolic rate.

EG.

an elephant has a lower surface area to volume ratio than a mouse. If heat is a byproduct of metabolic rate, explain why the elephant has a much lower metabolic rate than the mouse.

-heat is a waste product that an animal has to get rid of or it will overheat, the enzyme will denature and the animal will die

-Since the elephant has a very high volume, it has a lower volume/surface area ratio than the mouse.

-The mouse must metabolize at a high rate because it loses so much heat (because of it high Surface area/volume ratio.) The elephant must metabolize at a lower rate because it cannot get rid of the heat it would produce at a higher metabolic rate

1. What is the definition of diffusion

2. What is the definition of osmosis

Endocytosis, Exocytosis