Concepts of Biology · Cell Structure and Function
Passive Transport
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Cells must constantly exchange materials with their surroundings — bringing in oxygen and nutrients, letting out carbon dioxide and wastes. Much of that exchange costs the cell nothing, because it rides on Passive transport Movement of molecules across a membrane down their gradient, with no energy input Full entry →: the movement of molecules across the membrane down their Concentration gradient Difference in concentration of a substance between two regions Full entry → (from where they are more concentrated to where they are less concentrated) without any energy input from the cell. This topic covers the three forms — Simple diffusion Diffusion directly through the lipid bilayer Full entry →, Facilitated diffusion Diffusion through channel or carrier proteins (through channel and carrier proteins), and Osmosis Diffusion of water across a membrane toward higher solute concentration Full entry → (the Diffusion Net movement of molecules from high to low concentration of water) — along with tonicity, which describes what happens to a cell placed in a solution. The central idea is simple but powerful: concentration gradients are stored energy, and passive transport spends that gradient energy instead of ATP. Master this, and you understand how oxygen reaches your tissues, how glucose enters your cells, and why red blood cells swell or shrink in different fluids.
Why this matters
Passive transport is how your body does a huge share of its daily business. Every breath relies on oxygen diffusing down its gradient from the air in the lungs into the blood, and carbon dioxide diffusing back out. Glucose enters many cells by facilitated diffusion through carrier proteins. The movement of water across membranes (osmosis) governs fluid balance in every tissue, which is why "Isotonic Solution with the same solute concentration as the cell Full entry →," "Hypotonic Solution more dilute than the cell Full entry →," and "Hypertonic Solution more concentrated than the cell Full entry →" are essential vocabulary for anyone studying medicine or nursing: the wrong IV fluid can swell or shrink red blood cells. Expect exam questions that ask you to predict which way molecules move, whether a process needs energy, and what happens to a cell in a given solution. Getting the energy logic right — passive = with the gradient = no ATP — is the single most reliable exam strategy in this chapter.
The college version
Core Concepts
The concentration gradient: the driving force
A concentration gradient exists when a substance is more concentrated in one region than in another. Molecules are in constant random motion, and when a gradient exists, net movement runs from high concentration to low concentration — not because molecules "want" to, but because random motion sends more particles from the crowded side to the empty side than the reverse. This continues until equilibrium: concentrations are equal, and movement continues but with no net change. The gradient itself is a form of potential energy — passive transport releases it; active transport (Topic 6) spends ATP to fight it.
Simple diffusion: straight through the membrane
Simple diffusion is the direct movement of molecules through the phospholipid bilayer, down their concentration gradient, with no protein help and no energy cost. What can diffuse this way?
- Small, nonpolar molecules: oxygen (O₂) and carbon dioxide (CO₂) diffuse freely — this is how gas exchange works in lungs and tissues.
- Small uncharged polar molecules like water cross the bilayer itself, though slowly.
- Lipid-soluble molecules (steroid hormones, many drugs) dissolve into the lipid core easily.
The diffusion rate is affected by the steepness of the gradient, temperature, molecular size, and the membrane's surface area. Charged ions and larger polar molecules cannot take this route — the hydrophobic core blocks them.
Facilitated diffusion: help from proteins, still no energy
Facilitated diffusion moves substances that cannot cross the lipid core — ions and larger polar molecules like glucose — down their concentration gradient through transport proteins. It is still passive: no ATP is spent; the gradient provides the energy. Two kinds of transport proteins:
- Channel proteins — water-filled pores that let specific ions or water pass. Many are gated: they open or close in response to a signal (a neurotransmitter binding, a voltage change). Aquaporins are the water channels that let water cross membranes far faster than diffusion through the bilayer.
- Carrier proteins — bind the molecule on one side, change shape, and release it on the other. The classic example is the glucose transporter (GLUT), which carries glucose into many cells down its gradient. Carriers are specific and can become saturated when all carriers are busy — so facilitated diffusion has a maximum rate, unlike simple diffusion.
Osmosis: the diffusion of water
Osmosis is the diffusion of water across a selectively permeable membrane, from the side where water is more concentrated to the side where it is less concentrated. Because dissolved solutes lower water concentration, osmosis is often described as water moving toward the region of higher solute concentration. The membrane lets water pass but blocks the solute, so water flows until concentrations balance (or until pressure stops it). Key point: in osmosis, the solutes do not move — only the water does. Aquaporins speed water movement in cells that need rapid water flux, such as kidney tubule cells.
Tonicity: what a solution does to a cell
Tonicity describes how a solution affects the volume of a cell placed in it — it depends on the concentration of non-penetrating solutes outside the cell compared with inside:
- Isotonic — same solute concentration as the cell interior; no net water movement; the cell keeps its shape.
- Hypotonic — more dilute than the cell; water enters; animal cells swell and may burst (lysis).
- Hypertonic — more concentrated than the cell; water leaves; animal cells shrink (crenation in red blood cells).
Plant cells respond differently because of the cell wall: in a hypotonic solution, water enters and the central vacuole fills, creating Turgor pressure Pressure of the central vacuole against the plant cell wall Full entry → that stiffens the plant (the normal, healthy state); in a hypertonic solution, water leaves, the membrane pulls away from the wall, and the cell undergoes Plasmolysis Shrinking of plant cell contents away from the wall in a hypertonic solution Full entry → — the wilting you see in a plant watered with salt water.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Facilitated diffusion | Active transport | Both use carrier proteins, but facilitated diffusion moves with the gradient and needs no energy; active transport moves against the gradient and requires ATP |
| Osmosis | Diffusion of solutes | Osmosis is specifically the movement of water; diffusion moves any molecule. Water moves toward the side with more solute |
| Hypotonic | Hypertonic | Hypotonic = more dilute outside → water enters, cell swells; hypertonic = more concentrated outside → water leaves, cell shrinks |
| Channel protein | Carrier protein | A channel is an open (often gated) pore; a carrier binds the molecule and changes shape to shuttle it |
| Simple diffusion | Facilitated diffusion | Simple diffusion goes through the lipid bilayer (small nonpolar molecules only); facilitated diffusion goes through proteins (ions, polar molecules) |
| Osmosis | Active water pumping | Osmosis is passive — driven by concentration differences, not by pumps or ATP |
| Equilibrium | No movement | At equilibrium molecules still move, but there is no net movement — rates are equal in both directions |
| Isotonic | "Normal" for every cell | Isotonic means no net water movement for a given cell type; the matching concentration depends on that cell's internal solute levels |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a hallway full of kids on one side and an empty hallway on the other, with a door between. The kids naturally push through the door until both hallways are equally full — that's diffusion, and it costs no energy. Some doors are open to everyone (simple diffusion), and some are special doors that only let in kids wearing a certain shirt (channel and carrier proteins — facilitated diffusion). Water has its own trick: if one side has lots of salt and the other doesn't, water sneaks through the walls toward the salty side to balance things out — that's osmosis. And it all happens by itself, with zero energy from the cell.
Worked example
A hospital patient needs fluids, and the nurse selects the IV solution with the cell in mind. Consider what happens to a red blood cell depending on the fluid around it — an educational illustration of tonicity (actual clinical decisions follow current guidelines and patient assessment). Isotonic fluid (same solute concentration as the blood cells): water moves in and out equally, the cells keep their biconcave shape, and the blood works normally. If a hypotonic fluid were infused (more dilute than the cells), water would rush into the red blood cells by osmosis; they would swell and, in the extreme, burst (hemolysis), releasing hemoglobin — dangerous. If a hypertonic fluid were infused (more concentrated), water would leave the cells; they would shrivel (crenate) and lose their ability to squeeze through capillaries. Meanwhile, in the same blood, oxygen from the lungs is diffusing simply across membranes into tissues, and glucose is entering cells through carrier proteins — both passive, both free. Tonicity vocabulary — iso/hypo/hypertonic — is why the fluid choice matters and why these terms are tested so heavily.
Key takeaways
- Passive transport = down the gradient = no ATP. Three forms: simple diffusion, facilitated diffusion, osmosis.
- Simple diffusion: small nonpolar molecules (O₂, CO₂) straight through the bilayer.
- Facilitated diffusion: ions and polar molecules (glucose) through channel or carrier proteins — still no energy.
- Carriers saturate (maximum rate); channels are often gated. Aquaporins speed water movement.
- Osmosis = water moves toward higher solute concentration. Solutes stay put; water does the moving.
- Tonicity: isotonic = no net movement; hypotonic = water in, cells swell (lysis risk); hypertonic = water out, cells shrink (crenation).
- Plant cells: hypotonic → turgor (firm); hypertonic → plasmolysis (wilting).
- Diffusion rate factors: gradient steepness, temperature, molecular size, membrane surface area.
- Exam trap: facilitated diffusion uses proteins but is NOT active transport — no energy required.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What is the single feature that defines all passive transport?
Show answer
Passive transport moves substances down their concentration gradient without energy input from the cell (no ATP). It relies on the gradient itself as the energy source.
Why can oxygen use simple diffusion but glucose cannot?
Show answer
Oxygen is small and nonpolar, so it dissolves through the hydrophobic core of the phospholipid bilayer. Glucose is a large polar molecule that cannot pass through the lipid core; it needs a carrier protein (facilitated diffusion).
How does facilitated diffusion differ from simple diffusion?
Show answer
Simple diffusion moves small nonpolar molecules directly through the lipid bilayer. Facilitated diffusion moves ions and polar molecules through channel or carrier proteins — still down the gradient with no energy, but requiring protein help.
A cell is placed in a hypertonic solution. Which way does water move, and what happens to an animal cell?
Show answer
The solution outside is more concentrated, so water moves out of the cell by osmosis. An animal cell shrinks (crenates in red blood cells).
What is the difference between a Channel protein Water-filled pore (often gated) for specific ions/water Full entry → and a carrier protein?
Show answer
A channel protein is a water-filled pore (often gated) through which specific ions or water pass. A carrier protein binds the molecule, changes shape, and releases it on the other side — and can become saturated.
Why do plant cells not burst in a hypotonic solution, while animal cells can?
Show answer
Plant cells have a rigid cellulose cell wall. In a hypotonic solution, water enters and the central vacuole fills, pressing against the wall (turgor pressure), which resists further expansion. Animal cells have no wall, so they keep swelling until the membrane may burst (lysis).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Passive transport
- Movement of molecules across a membrane down their gradient, with no energy input
- Concentration gradient
- Difference in concentration of a substance between two regions
- Diffusion
- Net movement of molecules from high to low concentration
- Simple diffusion
- Diffusion directly through the lipid bilayer
- Facilitated diffusion
- Diffusion through channel or carrier proteins
- Channel protein
- Water-filled pore (often gated) for specific ions/water
- Carrier protein
- Protein that binds a molecule and changes shape to move it
- Aquaporin
- Water channel protein
- Osmosis
- Diffusion of water across a membrane toward higher solute concentration
- Isotonic
- Solution with the same solute concentration as the cell
- Hypotonic
- Solution more dilute than the cell
- Hypertonic
- Solution more concentrated than the cell
- Turgor pressure
- Pressure of the central vacuole against the plant cell wall
- Plasmolysis
- Shrinking of plant cell contents away from the wall in a hypertonic solution
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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