Unit 2: Topic 5 - Membrane Permeability Flashcards
Why are cell membranes selectively permeable?
Phospholipids, the main component of the cell membrane, are amphipathic: a molecule that contains both hydrophobic and hydrophilic parts. Phospholipids are amphipathic because each phospholipid has a hydrophilic phosphate group head and two hydrophobic fatty acid hydrocarbon tails. Phospholipids arrange themselves into a tightly packed bilayer, with the hydrophobic tails facing inward. As a result, charged and large polar molecules cannot cross the hydrophobic center.
In relation to selective permeability, what is the main function of cell membranes?
Due to its selective permeability, the cell membrane regulates the transport of specific molecules in and out of the cell. The cell membrane prevents the cell from entering harmful molecules and interacting with the outside environment. Cell membranes are a barrier between the internal environment of the cell and the external environment.
What is the fluid mosaic model?
A model first proposed in the 1970s, the fluid mosaic model displays a cell membrane primarily composed of phospholipids. However, within the phospholipids are embedded proteins and carbohydrates (resembling a mosaic). These components are “fluid” and able to move around laterally freely.
How can saturated and unsaturated fatty acid tails of phospholipids cause the permeability of the membrane to change?
Unsaturated fatty acid tails have double bonds, resulting in “kinks” in the shape of the phospholipids. On the other hand, saturated fatty acids do not have double bonds, resulting in relatively straight phospholipids that can pack tightly together. Thus, having more saturated fatty acid phospholipids decreases the membrane’s fluidity. A more fluid membrane tends to let more substances through and thus, be more permeable.
How can cholesterol cause the permeability of the membrane to change?
Cholesterol is a small lipid steroid found in the membrane that prevents abrupt changes in membrane fluidity. It bidirectionally influences the membrane’s fluidity by preventing lower temperatures from inhibiting the membrane’s fluidity and preventing higher temperatures from increasing fluidity. In low temperatures, the nonpolar tail of cholesterol interferes with the tight packing of nearby phospholipids and increases fluidity. In high temperatures, cholesterol can form strong interactions with nearby phospholipids, decreasing fluidity. A more fluid membrane tends to let more substances through and thus, be more permeable.
How and why does the transport of small, nonpolar molecules like CO2 differ from that of charged ions like Na+?
Small, nonpolar molecules like CO2 can easily diffuse through the cellular membrane because they are not repelled by the hydrophobic tails of the phospholipid bilayer. In contrast, charged ions like Na+ are repelled by the hydrophobic region of the phospholipid bilayer and must travel through the membrane by channel/carrier proteins.
Unlike large polar molecules, why can small amounts of small polar molecules like water pass through the cell membrane?
A polar molecule must break its interactions with water to cross the hydrophobic region of the phospholipid bilayer. Breaking these interactions requires an input of energy. For smaller polar molecules, the energy cost is not large enough to prevent diffusion from completely occurring on a relevant timescale.
What are cell walls and what is their purpose?
Cell walls are protective and rigid layers around cells. The function of cell walls are to provide structure, support, and protection. In plant cells, cell walls prevent cells from bursting after an influx of water by helping maintain turgor pressure (the pressure of the internal fluid against the cell wall).
What are similarities and differences of the components of the cell walls of plants, prokaryotes, and fungi?
The cell walls of plants, bacteria, and fungi are all composed of complex carbohydrates. However, they vary in the type of complex carbohydrates. Cell walls in plants are mainly composed of cellulose, while in bacteria, they are mainly composed of peptidoglycan, and in fungi, they are mainly composed of chitin.
