Chapter 7 (Membranes) Concepts
1.) Explain why phospholipids are amphipathic molecules and how this quality contributes to their role in membranes.
Amphipathic molecules have parts that are hydrophobic and parts that are hydrophilic. The 2 hydrocarbon tails of phospholipids are composed of chains of carbon atoms noncovalently bonded to hydrogen atoms making the tails hydrophobic. The phosphate group on the phospholipid head is charged making it hydrophilic. This amphipathic nature allows for the bi- layer to form with the hydrophobic tails turning inwards away from the aqueous environment of the inside and outside of the cell with the hydrophilic phosphate head being in contact with the water.
2.) Describe the components of a cell membrane in terms of the Fluid-Mosaic model.
The “fluid” part refers to the phospholipids that can move laterally in the membrane. The “mosaic” part is the proteins that are embedded in the phospholipid bi-layer.
3.) Explain how membrane fluidity is influenced by temperature and membrane composition.
As it becomes colder, the phospholipid bi-layer becomes less fluid and more viscous. If the hydrocarbon tails have more unsaturated bonds, it will be more fluid at lower temperatures compared to tails that are more saturated. More cholesterol molecules interspersed through the cell membrane will keep the membrane more fluid at lower temperatures as well.
4.) Distinguish between peripheral and integral membrane proteins. What roles do they play in cell membranes.
Peripheral proteins are composed of only hydrophilic regions and are present just on the outside or inside of the membrane. Integral membranes are amphipathic and extend through the cell membrane.
5.) Explain the role of membrane carbohydrates in cell-cell recognition.
Short carbohydrate chains are attached to the proteins on the outside of the cell membrane. Different cell types and different individuals have unique populations of these carbohydrates which act as ID tags.
6.) Compare the following various types of transport across the cell membrane and describe what types of molecules would use each type: Diffusion: Passive movement of molecules from areas of high to low concentration
Osmosis: Diffusion of water across a selectively permeable membrane, special form of facilitated diffusion
Facilitated diffusion: Passive movement of molecules from areas of high to low concentration but requires the help of transport proteins to provide a path for polar or charged molecules to move across the hydrophobic core of the cell membrane
Active transport: Movement of molecules from areas of low to high concentration requiring the use of cellular energy in the form of ATP
Phagocytosis: Cellular “eating”; the engulfing of large food particles by the rearrangement of the cell membrane around the particles which bring them inside making food vesicles
Pinocytosis: Cellular “drinking”; the engulfing of extracellular fluid which brings it inside the cell
Receptor mediated endocytosis: More specific form of pinocytosis in which specific receptors on the outside of the cell membrane bind with extracellular ligands to bring them inside the cell
7.) Compare and contrast passive and active transport.
Passive transport typically moves molecules from areas of high to low concentration and requires no energy. Active transport moves molecules from areas of low to high concentration, against the concentration gradient and requires energy in the form of ATP.
8.) Distinguish between hypertonic, hypotonic and isotonic solutions.
In relative comparisons, a hypertonic solution has a higher concentration of solutes to a hypotonic solution. A hypotonic solution would have a lower concentration of solutes compared to a hypertonic solution. Isotonic solutions have equal concentration of solutes to each other.
9.) Predict the direction of water movement based on differences in solute concentrations across a membrane.
Water will always move down its concentration gradient from areas of high to low concentration. A hypotonic solution has more water than solute compared to a hypertonic solution so water will move from the hypotonic solution into the hypertonic solution.
10.) Describe what would happen to an animal and plant cell if placed in solutions of various solute concentrations.
An animal cell placed in an isotonic solution will remain “normal” and water will move back and forth across the cell membrane. If placed in a hypotonic solution (compared to its internal solute concentration), it will burst or lyse as water moves into the cell. If placed in a hypertonic solution, water will leave the cell and the cell will shrivel or crenate.
A plant cell in an isotonic solution will be called “flaccid” as water enters and leaves the cell at the same rate. If placed in hypotonic solution, it will be called “turgid” as the cell wall pushes back against the water pressure and prevents the cell from bursting. In a hypertonic solution, the water will leave the cell, causing it to shrivel and undergo plasmolysis.