BIO110 - Plasma Membranes and Transport Processes

Reason why a particular organism would evolve to use a particular membrane lipid composition (saturated, unsaturated, cholesterol), based on the environment.

Organisms adapt their membrane lipid composition to maintain membrane fluidity. For example, in cold environments, membranes contain more unsaturated fatty acids to prevent rigid packing, while in hot environments, saturated fatty acids provide stability. Cholesterol acts as a buffer, preventing the extracellular membrane from becoming too rigid in the cold or too fluid in the heat.

Explain how the amino acid sequence of a protein can be used to identify membrane proteins. What makes membrane protein sequences different from soluble proteins?

The amino acid sequence of integral membrane proteins often contains long stretches of hydrophobic residues, which embed into the lipid leaflet. In contrast, soluble proteins lack these hydrophobic regions and are found in the cytoplasm. Peripheral membrane proteins may also be identified because they interact with glycolipids or glycoproteins on the extracellular membrane without spanning the bilayer.

Use ABO blood groups to explain the concept of self/non-self recognition.

ABO blood type is determined by glycolipids and glycoproteins present on the extracellular membrane of red blood cells. The immune system uses these as markers of self vs. non-self. For example, type A blood has A antigens, so exposure to type B antigens is recognized as non-self, triggering an immune response.

Describe the fluid mosaic model of the membrane.

(LARGE PROTEIN ICEBURGS IN SEA OF LIPIDS) The fluid mosaic model describes the cell membrane as a selectively-permeable (AMPHIPATHIC) bilayer where lipids, proteins, and carbohydrates move laterally like a fluid. Integral membrane proteins and peripheral membrane proteins form the “mosaic,” while glycolipids and glycoproteins on the extracellular membrane play roles in recognition and communication.

List and explain factors that affect the diffusion rate of molecules.

Diffusion rate depends on the concentration gradient, temperature, molecule size, and membrane permeability. A steeper concentration gradient and higher temperature increase diffusion, while larger molecules diffuse more slowly. Selectively-permeable membranes also regulate diffusion.

Explain how osmosis is just a special case of diffusion.

Osmosis is the diffusion of water molecules across a selectively-permeable membrane, moving from regions of low osmolarity (low solute concentration) to regions of high osmolarity (high solute concentration). It follows the same principles as diffusion but specifically applies to water.

Calculate osmolarity from the molarity of a solution (very basic only).

Osmolarity = Molarity × number of particles per solute. For example, 1 M NaCl has an osmolarity of 2 Osm/L because it dissociates into Na⁺ and Cl⁻.

Identify if cells are present in hypertonic, isotonic, or hypotonic solution.

In a hypertonic solution, cells lose water and shrink; in an isotonic solution, water movement is balanced and cells stay the same size; in a hypotonic solution, cells gain water and may swell or burst. Tonicity describes this effect relative to the cell’s osmolarity.

Identify molecules that can pass the membrane using unassisted transport and explain why that is possible.

Small, nonpolar molecules like O₂, CO₂, and steroid hormones can cross the lipid leaflet via passive, unassisted transport because they dissolve in the hydrophobic membrane core. In contrast, ions and polar molecules usually require proteins for assisted transport.

Differentiate between carriers and channels. Which can do assisted or passive transport? Which only do passive?

Channels form hydrophilic pores allowing passive, assisted transport along the concentration gradient. Carriers undergo conformational changes to move molecules; they can mediate passive, assisted transport (facilitated diffusion) or active, assisted transport if coupled to energy input.

Explain why some types of transport take the input of energy—i.e. they are active. (Hint: they run against the concentration gradient.)

Active, assisted transport requires energy because molecules move against their concentration gradient. For example, the Na⁺/K⁺ pump uses ATP to move Na⁺ out and K⁺ in, maintaining gradients essential for cell function.

Differentiate and explain primary and secondary active transport.

Primary active transport uses direct ATP hydrolysis (e.g., Na⁺/K⁺ pump). Secondary active transport uses the energy of an ion gradient created by primary active transport. For instance, the SGLT (Na⁺/glucose symporter) uses Na⁺ moving down its concentration gradient to drive glucose uptake against its gradient.

Explain the differences and similarities between the three key types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Endocytosis is the process of taking material into the cell via vesicles. In phagocytosis, the cell engulfs large particles or pathogens. In pinocytosis, the cell takes in extracellular fluid and dissolved solutes. In receptor-mediated endocytosis, specific molecules bind to receptors (often glycoproteins) on the extracellular membrane before being internalized. All three involve membrane invagination and vesicle formation but differ in selectivity and cargo.