Membrane Transport, Osmosis, and Tonicity Study Pack

Kibin's free study pack on Membrane Transport, Osmosis, and Tonicity includes a 3-section study guide, 8 quiz questions, 10 flashcards, and 1 open-ended Explain review question. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.

Last updated May 21, 2026

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Membrane Transport, Osmosis, and Tonicity Study Guide

Master the mechanics of how substances cross the plasma membrane — from simple and facilitated diffusion to active transport via the sodium-potassium pump. This pack breaks down osmosis, water potential gradients, and how hypotonic, isotonic, and hypertonic solutions affect cell volume, plus bulk transport through endocytosis and exocytosis.

Key Takeaways

  • •The plasma membrane is selectively permeable, allowing small nonpolar molecules to cross freely while restricting ions and large polar molecules that require protein-mediated transport.
  • •Passive transport moves substances down their concentration gradient without energy input, encompassing simple diffusion, facilitated diffusion through channel and carrier proteins, and osmosis.
  • •Osmosis is the net movement of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration, driven by the water potential gradient.
  • •Tonicity describes how a solution's solute concentration affects the volume of a cell by determining the direction of osmotic water movement; solutions are classified as hypotonic, isotonic, or hypertonic relative to the cell's interior.
  • •Active transport uses ATP or electrochemical gradients to move substances against their concentration gradient, with the sodium-potassium pump (Na⁺/K⁺-ATPase) being the most studied example in animal cells.
  • •Bulk transport mechanisms — endocytosis and exocytosis — move large molecules or particles across the membrane by forming or fusing vesicles, and both require energy.

The Selectively Permeable Plasma Membrane

The plasma membrane does not function as a simple barrier that blocks everything; it discriminates between substances based on their chemical properties, allowing some to cross freely while requiring others to use specific protein machinery.

Phospholipid Bilayer and Its Permeability Rules

  • •The hydrophobic core of the bilayer — formed by the fatty acid tails of phospholipids — creates a zone that repels charged and polar molecules.
  • •Small, nonpolar molecules such as O₂, CO₂, and lipid-soluble steroid hormones dissolve into and cross the bilayer directly without assistance.
  • •Small uncharged polar molecules like water (H₂O) and ethanol cross slowly on their own, while larger polar molecules such as glucose cannot cross without protein help.
  • •Ions such as Na⁺, K⁺, Cl⁻, and Ca²⁺ cannot cross the hydrophobic core unaided regardless of their size, making membrane proteins essential for ion movement.

Membrane Proteins That Enable Transport

  • •Channel proteins form hydrophilic pores through the bilayer; aquaporins are a specific class of channel protein that greatly accelerate water transport.
  • •Carrier proteins bind a specific solute and undergo a conformational change — a physical shape shift — to shuttle that solute across, then reset to accept another molecule.
  • •The combination of the lipid bilayer and these transport proteins gives the membrane its selective permeability.

Passive Transport: Movement Without Energy Input

Passive transport encompasses all mechanisms by which substances cross the membrane along their concentration or electrochemical gradient, releasing free energy rather than consuming ATP.

Simple Diffusion

  • •Simple diffusion is the unassisted net movement of a substance from a region of higher concentration to a region of lower concentration, driven by the kinetic energy of molecules.
  • •The difference in concentration between two regions is the concentration gradient; diffusion continues until equilibrium is reached and no net movement occurs.
  • •Rate of simple diffusion depends on the steepness of the concentration gradient, the temperature, and the molecular size and polarity of the substance.

Facilitated Diffusion via Channel Proteins

  • •Facilitated diffusion uses integral membrane proteins to move polar or charged substances down their gradient without ATP expenditure.
  • •Ion channels are gated — they open or close in response to voltage changes, ligand binding, or mechanical stimuli — which allows cells to regulate ion flow precisely.
  • •The rate of facilitated diffusion can be saturated when all available protein transporters are occupied, unlike simple diffusion which has no such ceiling.

Facilitated Diffusion via Carrier Proteins

  • •Carrier proteins such as the GLUT glucose transporters bind glucose on the high-concentration side, shift conformation, and release glucose on the low-concentration side.
  • •Carrier-mediated transport is highly specific; a carrier for glucose will not transport galactose, even though the two molecules differ only in the orientation of one hydroxyl group.

About this Study Pack

Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.

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