1. What macromolecule is the major component of cell membranes? This type of molecule has a hydrophilic and hydrophobic part. Diagram the molecule (in "cartoon" form), and label the hydrophilic and hydrophobic parts. How do the chemical properties of this molecule enable it to function well as a component of cell membranes?

Membranes are composed primarily of phospholipids. A cartoon of a phospholipid is shown on the right. The hydrophilic head is at the top and the hydrophobic tails are at the bottom. The hydrophobic center of the lipid bilayer is what allows selective permeability.

2. The cell membrane is sometimes referred to as a "lipid bilayer." Explain the rationale for this term.

 A membrane consists of two layers of phopholipids with the tails facing each other. Therefore, there are essentially two layers of lipid, or a lipid bilayer.

3. The cell membrane is selectively permeable. What does this term mean? How is it important to the functioning of the cell?

 Selectively permeable means that the membrane lets only some substances through. This selectivity to solutes is essential for the cell to control its internal environment and make it different from its external environment (and to vary the concentrations of solutes within intracellular compartments.

4. Cell membranes are fluid mosaics. How do membranes remain fluid? What macromolecules are embedded in membranes to form the "mosaic" and what functions do they serve?

 Membranes remain fluid in two ways. They can vary the saturation of the fatty acid tails (more saturation increases the stability of the membrane in hot temperatures, less saturation increases the fluidity of the membrane at cold temperatures). Membranes can also contain cholesterol, which interferes with the movement of the phospholipids and keeps them together better at high temperatures, but also interferes with dense packing of phospholipids and keeps the membrane from solidifying at cold temperatures.

Proteins are the main macromolecules embedded in membranes. They serve a myriad of functions including transport of solutes across the membrane, enzymes to catalyze reactions, cell-cell recognition, signal reception, cell adhesion, and attachment sites for the cytoskeleton.

5. What is diffusion? What is osmosis? How are the two different?

 Diffusion is the movement of any molecule from an area of high concentration to an area of low concentration. Osmosis is the special case of diffusion involving the movement of water from its own high concentration to its low concentration across a selectively permeable membrane. These two differ in that diffusion refers to any molecule, and does not have to be across a membrane, whereas osmosis refers to only water and must be across a membrane.

6. What is osmolarity?

 Osmolarity is the concentration, in moles/liter, of solute particles in a solution. The solute particles could be all of one kind or could be a mixture of solutes. The osmolarity of a solution includes all solutes to get total solute concentration.

7. Three important mechanisms for transporting substances across cell membranes are simple diffusion, facilitated diffusion, and active transport. Which of these transport mechanisms requires a protein channel to transport the substance? Which requires a concentration gradient to move the substance? Which requires energy? Which can move a substance against a concentration gradient? What properties determine whether a substance can cross the membrane unaided or whether it requires a protein carrier?

 

Mechanism	protein req'd?	moves [high] to [low]?	energy req'd?	moves [low] to [high]?
Simple diffusion	NO	YES	NO	NO
Facilitated diffusion	YES	YES	NO	NO
Active transport	YES	CAN	YES	YES

Only small, nonpolar, uncharged molecules can pass through a membrane by simple diffusion. Until a couple of years ago, we thought water was an exception. But protein channels specific for water (aquaporins) have been found in almost every membrane examined. This is why water moves as if it can freely pass through membranes.

8. Suppose we construct an artificial cell surrounded by a strong semi-permeable "membrane" through which only water can pass. On the inside of the cell we place a 5% glucose solution. What would happen to the cell when placed in each of the following solutions?

a. distilled water - Water would move into the "cell" expanding it until either the membrane ruptured or the force exerted by the stretching membrane was equal to the force of the water trying to come in.

b. 5% Na⁺ solution - No net change in the cell. Water could pass into and out of the "cell," but would do so at equal rates and not change the cell size. Neither solute could move.

c. 10% Na⁺ solution - Water would move out of the "cell" until the solution inside the cell was 10% glucose. Remember that as water moves out, the concentration of the solute inside increases.

 9. Assume we start with the same artificial cell with 5% glucose solution inside. This time, however, presume that we have built into the semi-permeable membrane many Na⁺ channels so that sodium ions can pass through the membrane. Describe how the cell would differ between its starting state and its state AT EQUILIBRIUM in each of the solutions above.

 a. distilled water - No change from the first situation here because there are no sodium ions in the system. Water would move into the "cell" expanding it until either the membrane ruptured or the force exerted by the stretching membrane was equal to the force of the water trying to come in.

b. 5% Na⁺ solution - In this case, Na⁺ would move into the cell, down its own concentration gradient until the Na⁺ inside and outside of the cell were at equilibrium (presumably at ~5% if there is a lot more solution outside than inside). Since Na⁺ was moving in, that would increase the solute concentration inside the cell and cause water to move in as well, expanding the cell over its initial size.

c. 10% Na⁺ solution - Just as in the 5% case above, Na would move into the cell until the inside had ~10% Na⁺ as well as the 5% glucose. This would cause water to move into the cell and expand it even more than in the case above...until the inside of the cell had a total solute concentration of 10% to match the outside, or until the cell ruptured, or until the force of the membrane stopped more water from coming in.

10. How do exocytosis and endocytosis differ from the transport mechanisms in the above question?

 Endo- and exo-cytosis differ in that the transported substances never actually cross a membrane. In these mechanisms, the substances remain inside membrane-bound compartments inside the cell and the membrane fuses to capture or release the substances.

11. As science officer aboard the USS Enterprise, you beam down to the planet Krisko to explore its life forms. Organisms on this planet, like those on earth, are composed of carbohydrates, lipids, proteins, and nucleic acids and cells are surrounded by membranes composed of phospholipids However, on Krisko, lipids are the solvent in which all biological reactions occur (on earth the solvent is water).

Given these conditions, how will the phospholipids in the membranes be arranged so as to form a selectively permeable membrane? DRAW a diagram and EXPLAIN your reasoning. (Don't worry about the transport proteins.)

The membranes on planet Krisko would have to have the hydrophobic lipid tails on the outside and the hydrophilic heads on the inside because the solvent for all solutions is hydrophobic lipid.

12. Identify and label the designated structures in the diagrams. Be familiar with the appearance and function of each of the organelles. Which are surrounded by membranes? Why is it important for organelles to have membranes?

All internal organelles except ribosomes are surrounded by membranes. The membranes allow the organelle to regulate its internal environment such that it can be different from the cytosol.

nucleus - site of chromosomes (DNA+proteins) and their regulatory proteins	rough endoplasmic reticulum (ER) - site of protein manufacture for exported proteins
chloroplast - manufactures glucose from sunlight and CO2	smooth ER - site of steroid manufacture and detoxification of poisons in the liver
Golgi body - processes and sorts proteins, makes some macromolecules	lysosome - sacs of digestive enzymes to break down macromolecules
central vacuole - contains water and water-soluble substances. Functions to enlarge the cell during growth and to maintain turgor pressure in a mature plant cell	vesicle (for transport) - any membrane-bound compartment that in not clearly some other organelle can be called a vesicle. Most commonly, these are used to transport materials into or out of a cell, or include contents that must be isolated from the rest of the cell (e.g., lysosome)
mitochondrion - processes energy for a cell. Site of conversion of pyruvate to ATP	ribosomes - sites of protein synthesis. Ribosomes can be free in the cytoplasm (where the manufacture proteins for intracellular use) or associated with ER (where they manufacture proteins for export out of the cell)

 

13. What are the functions of the cytoskeleton? What are the three main types of fibers that make up the cytoskeleton?

The cytoskeleton funtions to maintain the shape and structure of the cell. It is also invovled movement of cells and in moving cell contents, such as chromosomes during cell divisions. Microfibers, microfilaments and intermediate filaments.

14. Describe the function of each of the following intercellular junctions:

tight junctions - fusion of adjacent animal cell membranes to prevent leakage of intercellular fluids between cells.

gap junctions - membrane-lined channels between adjacent animal cells to facilitate chemical communication.

desmosomes - "rivet-like" connections that fasten animal cells together but are not water-tight.

plasmodesmata - membrane-lined channels between adjacent plant cells that allow cytoplasm to be continuous from one cell to the next.

 

15. How do animal and plant cells differ? How are these differences important to the functions each cell type must perform?

 Animal and plant cells differ in 2 fundamental ways: 1) plant cells have a cell wall and animal cells don't, and 2) plant cells have chloroplasts and animals cells don't. Both of these differences are related to the differences in the fundamental way of life of plants and animals. Plants use their cell wall to allow their cells to use turgor pressure for structural support rather than build a skeleton. But the cell walls restrict movement and plants are stationary. Animals spend a lot of energy on a skeleton, but the articulations in the skeleton allow movement. Plants use light as their energy source, and therefore have chloroplasts, whereas animals use other organisms as their energy sources, and therefore do not have chloroplasts.

16. How do eukaryotic and prokaryotic cells differ? Which of the structures listed in question #12 are found in each kind of cell?

 Eukaryotic cells have many membrane-bound (surrounded by membrane) organelles including a nucleus, mitochondria, chloroplasts (in plants), ER, Golgi, vesicles. They also have ribosomes, which are not membrane-bound. Prokaryotes have no membrane-bound organelles. They accomplish the functions that require internal membranes using infoldings of the cell membrane. Prokaryotic cells do have ribosomes (which are not membrane bound).

17. How does the abundance of certain organelles and structures specialize a cell for a particular function?

 Any cell that is specialized for a particular function will reflect that function in an abundance of the organelles that specialize in that function. For example, a cell that is specialized for protein secretion has lots of rough ER, lots of Golgi bodies, and lots of transport vesicles.

18. Assume you find a cell with an abundance of mitochondria. What might you suspect about the function of that cell? What about a cell with a highly folded surface?

 The abundance of mitochondria suggest that this cell needs a lot of energy. It is likely a cell that moves in some way...a muscle cell or maybe a swimming cell. A cell with a highly folded surface must have a function that requires a lot of surface area. High surface area is usually associated with a cell that does absorption from the environment (e.g., an intestinal epithelial cell).

19. Consider the following cell. Label the indicated organelles. What organelles are in abundance? What process is the cell carrying out at the top? What do you think is the function of this cell?

This cell has lots of rough ER, lots of mitochondria, and lots of transport vesicles. The vesicles are undergoing exocytosis at the top of the cell. Given the abundance of rough ER and transport vesicles, as well as the exocytosis, this cell looks like a cell secreting proteins.