Membrane Transport, Osmosis, and Tonicity Study Pack
Kibin's free study pack on Membrane Transport, Osmosis, and Tonicity includes a 6-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. 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 27, 2026
Membrane Transport, Osmosis, and Tonicity Study Guide
Master the mechanisms behind how cells control what enters and exits — from passive diffusion and facilitated transport to active pumps like Na⁺/K⁺ ATPase. This pack clarifies osmosis, tonicity, and why cells shrink, swell, or stay stable in hypertonic, hypotonic, and isotonic solutions.
Key Takeaways
- •The plasma membrane is selectively permeable, allowing some molecules to cross freely while others require specific transport proteins or energy input.
- •Passive transport moves substances down their concentration gradient without ATP; active transport moves substances against their gradient and requires ATP.
- •Osmosis is the passive diffusion of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.
- •Tonicity describes how the solute concentration of an external solution affects the direction and magnitude of osmotic water movement into or out of a cell.
- •Cells placed in hypertonic solutions lose water and shrink (crenation in animal cells; plasmolysis in plant cells), while cells in hypotonic solutions gain water and may lyse or become turgid.
- •Facilitated diffusion uses channel proteins or carrier proteins to move polar molecules and ions down their gradient without ATP, while active transport uses pumps such as the sodium-potassium ATPase to move solutes against their gradient.
The Selectively Permeable Plasma Membrane
The plasma membrane acts as a regulated boundary between a cell's interior and its environment, and its structure determines which substances can cross it and how.
Phospholipid Bilayer as a Selective Barrier
- •The membrane consists of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails forming the interior, creating a barrier that repels charged and polar molecules.
- •Small nonpolar molecules such as oxygen (O₂), carbon dioxide (CO₂), and lipids dissolve into and pass through the hydrophobic core rapidly without assistance.
- •Water molecules, despite being polar, cross the membrane slowly on their own and much faster through specialized channel proteins called aquaporins.
Why Ions and Large Molecules Cannot Freely Cross
- •Charged ions such as Na⁺, K⁺, and Cl⁻ are repelled by the hydrophobic interior and cannot diffuse through the lipid bilayer without protein assistance.
- •Large polar molecules such as glucose and amino acids are also excluded from free diffusion, requiring dedicated transport proteins to enter or exit the cell.
Passive Transport: Moving with the Gradient
Passive transport encompasses all mechanisms that move substances across the membrane along their concentration gradient, requiring no energy expenditure from the cell.
Simple Diffusion
- •Simple diffusion is the net movement of a substance from an area of higher concentration to an area of lower concentration due to the random kinetic motion of particles.
- •The driving force is the concentration gradient; diffusion continues until equilibrium is reached, at which point movement continues in both directions at equal rates.
- •Small nonpolar molecules (O₂, CO₂, ethanol) and some small uncharged polar molecules cross the membrane by simple diffusion.
Facilitated Diffusion via Channel Proteins
- •Channel proteins form hydrophilic pores through the membrane that allow specific ions or water molecules to pass; transport is passive and driven by the electrochemical gradient.
- •Ion channels are often gated, opening or closing in response to voltage changes, ligand binding, or mechanical stimuli, giving cells precise control over ion movement.
- •Aquaporins are a specialized class of channel proteins dedicated to rapid water transport; their abundance in kidney tubule cells and red blood cells reflects the high demand for fast osmotic adjustment.
Facilitated Diffusion via Carrier Proteins
- •Carrier proteins bind a specific solute on one side of the membrane, undergo a conformational change, and release the solute on the other side — all without ATP.
- •Glucose transporters (GLUTs) are classic examples; GLUT1, for instance, moves glucose into red blood cells down its concentration gradient.
- •Unlike channel proteins, carrier proteins have a maximum transport rate (saturation point) because each protein must physically change shape for every molecule transported.
Osmosis and Water Potential
Osmosis is a specific case of passive transport involving only water, and understanding it requires understanding how dissolved solutes affect the tendency of water to move.
Definition and Mechanism of Osmosis
- •Osmosis is the diffusion of water across a selectively permeable membrane from the side with a lower solute concentration (higher free water concentration) to the side with a higher solute concentration (lower free water concentration).
- •Water moves to dilute the more concentrated solution, a process driven by the difference in water potential across the membrane.
Osmotic Pressure
- •Osmotic pressure is the pressure that would need to be applied to a solution to prevent water from entering it by osmosis; it is directly proportional to solute concentration.
- •A solution with more dissolved solutes has a higher osmotic pressure and a stronger tendency to draw water toward it.
- •In plant cells, the rigid cell wall generates turgor pressure that eventually opposes further water entry, creating an equilibrium without cell lysis.
Osmolarity as a Measure of Solute Concentration
- •Osmolarity measures the total number of dissolved solute particles per liter of solution and determines the direction of osmotic water flow.
- •When comparing two solutions separated by a membrane, water moves from the solution with lower osmolarity to the solution with higher osmolarity.
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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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Question 1 of 25
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Which molecules can cross the plasma membrane by simple diffusion without any protein assistance?
Card 1 of 30
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Concept 1 of 5
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Selective Permeability of the Plasma Membrane
Explain selective permeability in your own words. Why can some molecules cross the plasma membrane freely while others cannot, and what determines which category a molecule falls into?
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