Campbell Biology Chapter 7: Membrane Structure and Function

Flashcards generated from Campbell Biology Chapter 7 lecture notes covering membrane structure, fluidity, transport proteins, passive and active transport, osmosis, cotransport, and bulk transport processes.

How do small and large molecules cross the plasma membrane based on energy requirements?

Small molecules cross via passive transport (which requires no energy input and may use transport proteins) or active transport (which requires energy and a transport protein); large molecules cross in bulk via exocytosis or endocytosis.

What does it mean for a phospholipid to be an amphipathic molecule?

It contains both hydrophobic ("water-fearing") regions, consisting of nonpolar fatty acid tails, and hydrophilic ("water-loving") regions, consisting of a phosphate head.

How does the fluid mosaic model describe the structure of the cellular membrane?

It depicts the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

What did the 1970 experiment by L. D. Frye and M. Edidin demonstrate regarding membrane proteins?

It demonstrated that membrane proteins can move laterally within the lipid bilayer, as evidenced by the rapid intermixing of cell surface antigens after forming mouse-human heterokaryons (hybrid cells) after $1\text{ hour}$.

How do unsaturated and saturated hydrocarbon tails affect membrane fluidity?

Unsaturated hydrocarbon tails have kinks that prevent packing, keeping the membrane fluid at lower temperatures, whereas saturated hydrocarbon tails pack tightly together, making the membrane viscous.

How does cholesterol regulate membrane fluidity in animal cells at different temperatures?

At relatively high temperatures (such as $37\,^\circ\text{C}$), cholesterol restrains phospholipid movement to reduce fluidity; at low temperatures, it hinders solidification by disrupting the tight packing of phospholipids.

How does winter wheat adapt its membrane lipid composition to survive cold seasonal temperatures?

The percentage of unsaturated phospholipids increases in autumn to prevent membrane solidification during winter.

What is the key difference between peripheral proteins and integral proteins?

Peripheral proteins are bound to the surface of the membrane, while integral proteins penetrate the hydrophobic core of the lipid bilayer.

What structural feature characterizes transmembrane proteins?

They are integral proteins that span the entire membrane, with hydrophobic regions composed of nonpolar amino acids often coiled into $\alpha$ helices.

What six major functional roles do cell-surface membrane proteins perform?

Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix (ECM).

Why are individuals who lack the co-receptor CCR5 resistant to HIV infection?

HIV must bind to both the CD4 receptor and the CCR5 co-receptor to infect immune cells; without CCR5 on the cell surface, HIV cannot enter the cell.

What is the difference between glycolipids and glycoproteins?

Glycolipids are membrane carbohydrates bonded to lipids, whereas glycoproteins are membrane carbohydrates bonded to proteins.

Which types of molecules cross the hydrophobic core of the lipid bilayer rapidly without assistance?

Hydrophobic (nonpolar) molecules, such as hydrocarbons, $\text{CO}_2$, and $\text{O}_2$.

What are aquaporins, and how many water molecules can pass through them per second?

Aquaporins are channel proteins made of four polypeptide subunits that facilitate water passage, allowing up to $3\text{ billion}$ water molecules to pass through per second.

How do channel proteins differ from carrier proteins in facilitated diffusion?

Channel proteins provide a hydrophilic channel that acts as a tunnel for specific molecules or ions, while carrier proteins bind to molecules and undergo a subtle change in shape to shuttle them across.

What occurs when a system of diffusing particles reaches dynamic equilibrium?

As many molecules cross the membrane in one direction as cross in the opposite direction, resulting in no net change in concentration.

Define osmosis.

Osmosis is the diffusion of free water molecules across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.

According to the provided diagram, what happens to animal and plant cells in hypotonic, isotonic, and hypertonic solutions?

In a hypotonic solution, animal cells lyse and plant cells become turgid (normal); in an isotonic solution, animal cells are normal and plant cells become flaccid; in a hypertonic solution, animal cells shrivel and plant cells undergo plasmolysis.

How does Paramecium manage osmoregulation in its hypotonic freshwater environment?

It uses a contractile vacuole to continuously pump excess water out of the cell.

What is plasmolysis in plant cells, and under what environmental condition does it occur?

Plasmolysis is the shrinking of a plant cell causing its plasma membrane to pull away from the cell wall, occurring when the cell is placed in a hypertonic environment.

What are gated channels, and what causes them to open or close?

Gated channels are ion channels that open or close in response to a specific stimulus, such as an electrical stimulus (in nerve cells) or a chemical stimulus (binding of a ligand).

How does active transport differ from passive transport in terms of concentration gradients and energy requirement?

Active transport moves solutes against their concentration gradients and requires energy (usually ATP hydrolysis), whereas passive transport moves solutes down their concentration gradients without energy expenditure.

How does the sodium-potassium pump function in animal cells?

It uses energy from ATP hydrolysis (transfer of a phosphate group) to actively pump sodium ions ($\text{Na}^+$) out of the cell and potassium ions ($\text{K}^+$) into the cell against their concentration gradients.

What two combined forces constitute an ion's electrochemical gradient?

A chemical force (the ion's concentration gradient) and an electrical force (the effect of the membrane potential on the ion's movement).

What is the primary electrogenic pump in animal cells versus plant, fungal, and bacterial cells?

In animal cells, the primary electrogenic pump is the sodium-potassium pump; in plants, fungi, and bacteria, it is the proton pump, which actively transports hydrogen ions ($\text{H}^+$) out of the cell.

What is cotransport, and how do plant cells utilize it to load sucrose?

Cotransport occurs when active transport of a solute indirectly drives the transport of other substances; plant cells use a proton pump to create an $\text{H}^+$ gradient, and an $\text{H}^+$/sucrose cotransporter couples the downhill diffusion of $\text{H}^+$ to the active transport of sucrose into the cell.

Why is drinking a concentrated salt (NaCl) and glucose solution used to treat severe diarrhea?

It enables solute and water reabsorption in the intestine through $\text{Na}^+$/glucose cotransporters, rapidly restoring sodium levels in the body.

Identify the three types of endocytosis shown in this diagram and briefly state how they differ.

1. Phagocytosis ("cellular eating"): cell engulfs particles using pseudopodia into a food vacuole.
2. Pinocytosis ("cellular drinking"): cell nonspecifically gulps extracellular fluid into coated vesicles.
3. Receptor-mediated endocytosis: solute binding to specific receptors in coated pits triggers coated vesicle formation.

What causes cholesterol accumulation in the blood of individuals with familial hypercholesterolemia?

They have missing or defective low-density lipoprotein (LDL) receptor proteins, preventing receptor-mediated endocytosis of cholesterol into cells.