Membrane Structure and Transport – Lecture Review

30 question-and-answer flashcards covering membrane composition, phospholipid properties, diffusion, osmosis, tonicity, and key experimental observations from the lab.

What are biological membranes primarily composed of?

A phospholipid bilayer.

What does it mean when a molecule is described as amphipathic?

It has both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

Name the three major parts of a phospholipid.

A glycerol backbone, two fatty-acid chains, and a phosphate-containing head group.

Which region of a phospholipid is hydrophilic and which is hydrophobic?

The phosphate head is hydrophilic; the fatty-acid tails are hydrophobic.

Besides phospholipids, list four additional components commonly found in biological membranes.

Cholesterol, proteins, glycoproteins, and glycolipids.

What does "selectively permeable" mean with regard to the plasma membrane?

It allows some substances to cross while restricting or preventing others.

Which kinds of molecules cross a membrane most easily?

Small, non-polar molecules such as O₂ and CO₂.

Which kinds of molecules usually require transport proteins to cross a membrane?

Large and/or polar molecules.

Define diffusion.

The net movement of solute particles from a region of high concentration to a region of lower concentration.

What is Brownian motion?

The constant, random movement of molecules in liquids or gases due to kinetic energy.

List three factors that affect the rate of diffusion.

Molecular size (weight), steepness of the concentration gradient, and temperature.

Does passive/facilitated diffusion require cellular energy (ATP)?

No; both rely on existing concentration gradients and therefore do not require energy input.

How does molecular weight influence diffusion rate?

Smaller (lower-molecular-weight) molecules diffuse faster than larger ones.

How does the steepness of a concentration gradient affect diffusion?

A steeper gradient (larger difference) increases the rate of diffusion.

Define osmosis.

The diffusion of water across a selectively permeable membrane from a region of higher water concentration to lower water concentration.

What does isotonic mean?

The solute concentration is the same inside and outside the cell; there is no net movement of water.

What is a hypertonic solution relative to a cell, and which way does water move?

It has a higher solute concentration than the cell; water leaves the cell.

What happens to animal cells placed in hypertonic solutions?

They lose water and shrink (crenate).

What is a hypotonic solution relative to a cell, and which way does water move?

It has a lower solute concentration than the cell; water enters the cell.

Why do plant cells usually not burst in distilled (hypotonic) water?

Their rigid cell wall provides structural support, so they become turgid rather than lysing.

What term describes the shrinking of a plant cell's cytoplasm in a hypertonic solution?

Plasmolysis.

In the dialysis-tubing experiment, which substance diffused into the beaker water and why: starch-iodine (≈54,000 Da) or orange dye (≈320 Da)?

The orange dye diffused because its lower molecular weight allowed it to pass through the selectively permeable dialysis membrane.

During the agar plate experiment, which diffused farther: 0.1 M potassium permanganate (MW 158) or 0.1 M methylene blue (MW 320)? Why?

Potassium permanganate diffused farther because its smaller molecular weight allowed faster diffusion.

How did molarity affect diffusion of potassium permanganate in agar?

The 1.0 M solution spread faster/farther than the 0.1 M solution due to a steeper concentration gradient.

Describe the texture of potato slices after 30 min in distilled water, 0.2 M NaCl, and 2 M NaCl.

Distilled water: firm/turgid; 0.2 M NaCl: slightly flexible; 2 M NaCl: limp/flaccid due to water loss.

What happened to sheep red blood cells in distilled water?

They swelled and many lysed (hemolysis) because water entered the cells in a hypotonic environment.

What is the relationship between solute concentration and water concentration across a membrane?

They are inversely related: higher solute concentration means lower water concentration, and vice versa.

What structural feature prevents plant cells from bursting in hypotonic environments?

Their cellulose cell wall.

What can happen to animal cells placed in a strongly hypotonic solution?

They may swell and burst (lyse) due to excessive water intake.

Summarize the definition of a selectively permeable membrane in one sentence.

A barrier that allows certain molecules or ions to pass while blocking others based on size, polarity, or specific transport proteins.