BIOB 160 Principles of Living Systems · Unit 2: Energy and the cell · on Exam 2
2.5 Membrane structure and transport
Study notes for Membrane structure and transport in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in StudyCat.
Big ideas
- The fluid mosaic model: a phospholipid bilayer with embedded proteins that drift laterally. Unsaturated tails keep it fluid; cholesterol buffers fluidity against temperature changes.
- Membrane proteins are integral (span the bilayer) or peripheral; they transport, act as enzymes and receptors, and help cells recognize and attach to each other (glycoproteins).
- Selective permeability: small nonpolar molecules (O₂, CO₂) cross freely; ions and polar molecules need transport proteins: channels (including aquaporins for water) and carriers.
- Passive transport goes down a concentration gradient with no energy: diffusion, osmosis (water) and facilitated diffusion. Tonicity: in a hypotonic solution water enters (animal cells may burst; plant cells become turgid), in a hypertonic one water leaves (animal cells shrivel; plant cells plasmolyze), and isotonic means no net movement.
- Active transport uses energy to move solutes against their gradient: the sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in per ATP, building the membrane potential; cotransport uses one gradient to drive another. Bulk transport: exocytosis and endocytosis (phagocytosis, pinocytosis, receptor-mediated).
Worked example
A red blood cell (about 0.9% solutes) is placed in 3% salt water. Which way does water move, and what happens to the cell? What about a plant cell in pure water?
Show the solution
The 3% solution is hypertonic to the cell, so water leaves and the red blood cell shrivels (crenates). A plant cell in pure water (hypotonic) gains water and becomes turgid; its wall prevents bursting.
Common mistakes
- Describing osmosis as solute movement. It is the movement of water toward higher solute concentration.
- Calling facilitated diffusion active transport. It uses proteins but no energy.
- Getting the pump backwards: 3 Na⁺ out, 2 K⁺ in.
Exam tipReading: Campbell ch. 7 (pp. 126–142). For tonicity questions, compare solute concentrations first, then say which way water goes.
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