Unit 1 Biology – Plasma Membrane & Transport

A comprehensive set of Q&A flashcards covering plasma membrane structure, passive and active transport mechanisms, diffusion, osmosis, and related cellular concepts.

What are the three major structural components of the plasma membrane?

Phospholipids, proteins, and cholesterol.

Why do cells need membranes?

To separate the intracellular environment from the extracellular environment and regulate the passage of materials.

Describe the polarity of a phospholipid.

It has a polar hydrophilic head and two non-polar hydrophobic tails.

What is meant by the phrase “like dissolves like” in the context of polarity?

Polar substances dissolve in polar solvents and non-polar substances dissolve in non-polar solvents; polar and non-polar substances generally do not mix.

What structure do phospholipids form in water and why?

A bilayer with hydrophobic tails inward and hydrophilic heads outward, minimizing contact between water and the non-polar tails.

Explain the ‘fluid’ part of the Fluid Mosaic Model.

Phospholipids and some proteins move laterally within the layer, allowing membrane flexibility, self-repair, and shape change.

Explain the ‘mosaic’ part of the Fluid Mosaic Model.

Proteins are embedded like tiles in a mosaic, giving the membrane a patchwork appearance.

Differentiate between integral, peripheral, and transmembrane proteins.

Integral proteins are permanently embedded; peripheral proteins are temporarily attached; transmembrane proteins span the entire bilayer.

List four functions of membrane proteins.

Transport, enzymatic activity, cell communication (receptors), adhesion/structural support, and identity markers.

What are glycoproteins and glycolipids and what do they do?

Proteins or lipids with attached carbohydrate chains; they function in cell recognition and signaling.

How does cholesterol affect the plasma membrane?

It increases membrane fluidity in cold temperatures and stabilizes the membrane in warm temperatures.

Define passive transport.

Movement of substances across a membrane without energy input, down their concentration gradient.

Define active transport.

Movement of substances against their concentration gradient using energy (ATP) and transport proteins.

What is a concentration gradient?

A difference in the concentration of a substance between two regions.

State the direction of net movement in simple diffusion.

From an area of higher concentration to an area of lower concentration.

Which kinds of molecules can cross the phospholipid bilayer by simple diffusion?

Small non-polar molecules (e.g., O₂, CO₂), lipids, and small weakly polar molecules like H₂O.

Name two types of molecules that cannot cross the bilayer unaided.

Charged ions (e.g., K⁺, Cl⁻) and large polar molecules (e.g., glucose, proteins).

Describe facilitated diffusion.

Passive transport of large or charged molecules through protein channels or carrier proteins.

How do channel proteins differ from carrier proteins?

Channel proteins provide open or gated pores, whereas carrier proteins change shape to shuttle molecules across.

List two factors that increase the rate of diffusion.

Steeper concentration gradient and higher temperature.

Define osmosis.

Passive movement of free water molecules across a semi-permeable membrane from high free-water (low solute) to low free-water (high solute) concentration.

What is meant by ‘free’ water molecules?

Water molecules not bound to solute particles and thus able to move freely.

Compare hypotonic, hypertonic, and isotonic solutions relative to a cell.

Hypotonic: higher free water (lower solute) than the cell; hypertonic: lower free water (higher solute); isotonic: equal solute concentration.

What happens to an animal cell in a hypotonic solution?

It swells and may burst (lysis) due to water influx.

What happens to an animal cell in a hypertonic solution?

It shrivels (crenation) as water leaves the cell.

Describe plasmolysis in plant cells.

In a hypertonic solution, the cell membrane pulls away from the cell wall as the vacuole loses water.

Why don’t plant cells burst in hypotonic solutions?

The rigid cell wall resists excessive expansion, leading to turgor pressure instead.

What is protein-mediated active transport?

Transport proteins use ATP to move substances against their concentration gradient.

Explain the general mechanism of endocytosis.

The membrane folds inward, encloses material, pinches off as a vesicle, and carries the content into the cell.

Explain exocytosis.

Vesicles fuse with the plasma membrane, releasing their contents outside the cell and adding their phospholipids to the membrane.

Give two cellular examples that rely on exocytosis.

Secretion of neurotransmitters from nerve cells and release of hormones from endocrine cells.

Which form of transport requires ATP: facilitated diffusion or active transport?

Active transport.

What molecule serves as the primary energy currency for active transport?

ATP (adenosine triphosphate).

After ATP is used in active transport, what does it become?

ADP (adenosine diphosphate), analogous to a ‘flat battery.’

How does temperature affect molecular motion?

Higher temperatures provide more kinetic energy, increasing molecular speed and diffusion rate.

Why do charged ions require protein channels to cross membranes?

The hydrophobic core of the phospholipid bilayer repels charged particles.