Source: Campbell Biology, Chapter 7 (Sections 7.3–7.4)
Tags: passive transport, diffusion, concentration gradient, osmosis, tonicity, isotonic, hypertonic, hypotonic, facilitated diffusion, active transport, sodium-potassium pump, membrane potential, electrochemical gradient, cotransport
Difficulty: Intermediate | Prerequisites: Sections 7.1 and 7.2 (membrane structure, lipid bilayer properties, transport proteins). You should also be comfortable with the concept of chemical energy and ATP from Chapter 8.
Now that you know what the membrane is made of, these sections explain how substances actually get across it. Section 7.3 covers passive transport: movement driven by concentration gradients, requiring no energy from the cell. Section 7.4 introduces active transport: movement against the gradient, powered by ATP. Together they explain how cells maintain their internal environment, regulate ion concentrations, and generate the electrical potential across their membranes. This is foundational for understanding nerve impulses, muscle contraction, and nutrient uptake later in the course.
Substances cross membranes either passively (down their concentration gradient, no energy needed) or actively (against their gradient, using ATP). Osmosis is a special case of passive transport involving water. Active transport by pumps like the sodium-potassium pump creates and maintains the electrical and chemical gradients that cells depend on for signalling, nutrient uptake, and volume control.
Diffusion
The movement of particles of any substance so that they spread out into the available space. Substances move from regions of higher concentration to regions of lower concentration. Think of it as molecules naturally spreading out until they are evenly distributed.
Concentration gradient
The region along which the density of a substance increases or decreases. Each substance diffuses down its own gradient independently of other substances.
Passive transport
The diffusion of a substance across a biological membrane with no energy investment by the cell. The concentration gradient itself provides the driving force.
Thermal energy
The energy associated with the random movement of molecules. It is what keeps molecules in constant motion and makes diffusion possible.
Dynamic equilibrium
The state in which roughly as many molecules of a substance cross the membrane in one direction as in the other per unit time. The molecules are still moving, but there is no net directional flow.
Osmosis
The diffusion of free water across a selectively permeable membrane, from a region of higher free water concentration (lower solute concentration) to a region of lower free water concentration (higher solute concentration).
Tonicity
The ability of a surrounding solution to cause a cell to gain or lose water. It depends on the concentration of non-penetrating solutes outside the cell relative to inside.
Isotonic
A solution in which the concentration of non-penetrating solutes is equal to that inside the cell. No net water movement occurs. In simple terms, the water flows in and out at the same rate.
Hypertonic
A solution with a higher concentration of non-penetrating solutes than the cell interior. Water leaves the cell, causing it to shrivel.
Hypotonic
A solution with a lower concentration of non-penetrating solutes than the cell interior. Water enters the cell, which may swell and burst (lyse).
Osmoregulation
The control of solute concentrations and water balance within a cell or organism.
Flaccid
The limp state of a plant cell in an isotonic environment, where there is no net water intake and the cell lacks turgor pressure. The plant wilts.
Plasmolysis
The shrinking of a plant cell's cytoplasm as the plasma membrane pulls away from the cell wall in a hypertonic environment.
Turgor pressure
The pressure exerted by water inside a plant cell pushing outward against the cell wall. A turgid (firm) cell is the healthy state for plants.
Facilitated diffusion
Passive transport in which polar molecules and ions, impeded by the lipid bilayer, cross the membrane with the help of transport proteins. No ATP is required because movement is still down the concentration gradient.
Ion channel
A channel protein that specifically transports ions. Many are gated, opening or closing in response to a stimulus.
Active transport
The pumping of a solute across a membrane against its concentration gradient, requiring energy (usually from ATP hydrolysis).
Sodium-potassium pump (Na+/K+ pump)
A carrier protein that actively transports three sodium ions out of the cell and two potassium ions into the cell per cycle, using one ATP. This creates a net movement of one positive charge out of the cell.
Membrane potential
The voltage (electrical potential difference) across a cell's plasma membrane, resulting from the unequal distribution of ions on either side. The cytoplasmic side is typically negative relative to the extracellular side.
Electrochemical gradient
The combined effect of a concentration gradient (chemical force) and a membrane potential (electrical force) on the movement of an ion across a membrane.
Electrogenic pump
A transport protein that generates voltage across a membrane. The sodium-potassium pump is the main electrogenic pump in animal cells. The proton pump (H+ pump) is the main one in plants, fungi, and bacteria.
Cotransport (coupled transport)
A mechanism in which a transport protein couples the "downhill" diffusion of one solute to the "uphill" transport of another against its concentration gradient. The energy stored in one gradient drives movement of the second substance.
Molecules are in constant random motion because of their thermal energy. Diffusion is the net result of that motion: particles spread from where they are more concentrated to where they are less concentrated, down their concentration gradient.
Key principles:
Each substance diffuses down its own concentration gradient independently of other gradients.
Diffusion is spontaneous and requires no energy input from the cell.
At dynamic equilibrium, molecules still cross the membrane in both directions, but there is no net movement.
Passive transport is simply diffusion across a biological membrane. The concentration gradient itself represents stored potential energy that drives the movement.
Example: oxygen uptake during cellular respiration. Dissolved O2 diffuses into the cell because the cell continually consumes O2 internally, maintaining a concentration gradient that favours inward movement.
How osmosis works
Consider a U-shaped tube with a selectively permeable membrane separating two sugar solutions. The pores allow water through but not sugar. Because sugar is hydrophilic, water molecules cluster around sugar molecules, reducing the number of free water molecules available to cross. Water diffuses from the side with more free water (lower solute concentration) to the side with less free water (higher solute concentration), until solute concentrations are roughly equal.
Osmosis is the diffusion of free water across a selectively permeable membrane.
Tonicity and cells without cell walls (animal cells)
Tonicity describes the effect of a surrounding solution on cell water balance, based on the concentration of non-penetrating solutes:
Isotonic: solute concentration outside equals that inside. No net water movement. The cell maintains its normal shape.
Hypertonic: solute concentration outside is higher. Water leaves the cell. The cell shrivels and may die.
Hypotonic: solute concentration outside is lower. Water enters the cell. The cell swells and may lyse (burst).
Animal cells cannot tolerate too much or too little water. They require osmoregulation to control solute concentrations and water balance. Some single-celled organisms (e.g. Paramecium) use a contractile vacuole that pumps water out as fast as it enters.
Tonicity and cells with cell walls (plant cells)
Isotonic: the plant cell becomes flaccid (limp). The plant wilts.
Hypertonic: the cell loses water and shrinks. The plasma membrane pulls away from the cell wall. This is plasmolysis.
Hypotonic: water enters by osmosis, but the cell does not burst. The rigid cell wall pushes back, creating turgor pressure. A turgid (firm) cell is the healthy state for plants.
Facilitated diffusion is passive transport aided by proteins. Polar molecules and ions that cannot cross the hydrophobic bilayer on their own diffuse through transport proteins, still moving down their concentration gradient with no ATP required.
Channel proteins in facilitated diffusion
Provide corridors for specific molecules to cross.
Aquaporins are channel proteins dedicated to water transport.
Ion channels are channel proteins that transport ions. Many are gated (they open or close in response to a stimulus).
Carrier proteins in facilitated diffusion
Undergo a subtle change in shape that moves the solute across.
Example: the glucose transporter binds glucose on one side, changes conformation, and releases it on the other side.
Both channel and carrier proteins in facilitated diffusion require no ATP. The concentration gradient provides all the energy.
Active transport
Active transport moves a solute against its concentration gradient, from lower to higher concentration. This requires energy, almost always from ATP hydrolysis.
The transport proteins involved are all carrier proteins (not channel proteins).
ATP transfers its terminal phosphate group directly to the carrier protein, inducing a conformational change that moves the bound solute across the membrane.
The sodium-potassium pump
The Na+/K+ pump is the best-known example of active transport:
Per cycle, it pumps 3 Na+ out of the cell and 2 K+ into the cell.
This creates a net export of one positive charge per cycle.
It uses one ATP per cycle.
Membrane potential
All cells maintain a voltage (membrane potential) across their plasma membrane. The cytoplasmic side is negative relative to the extracellular side, because of the unequal distribution of ions.
Two forces drive ion movement across a membrane:
A chemical force: the concentration gradient.
An electrical force: the effect of the membrane potential on the ion.
The combination of these two forces is called the electrochemical gradient. It determines the net direction of ion movement.
Electrogenic pumps
An electrogenic pump is a transport protein that generates voltage across a membrane:
In animal cells, the sodium-potassium pump is the major electrogenic pump.
In plants, fungi, and bacteria, the proton pump (H+ pump) is the main one. It actively transports protons out of the cell, storing energy that can be tapped for cellular work.
Proton gradients are also essential for ATP synthesis during cellular respiration.
Cotransport
Cotransport couples the "downhill" diffusion of one solute to the "uphill" transport of another substance against its own gradient. The energy comes from the gradient of the first solute, not directly from ATP.
Example: in plant cells, ATP-powered proton pumps create an H+ gradient outside the cell. An H+/sucrose cotransporter then uses the downhill flow of H+ back into the cell to drive the uptake of sucrose against its own gradient. ATP hydrolysis indirectly provides the energy.
Intravenous (IV) fluids in hospitals are isotonic saline precisely because a hypertonic or hypotonic solution would damage red blood cells. Kidney dialysis machines rely on controlled osmosis to remove waste from the blood. Turgor pressure is why fresh vegetables are crisp (cells are turgid) and wilted ones are limp (cells are flaccid, having lost water).
Students often think osmosis moves solutes. It does not. Osmosis is the movement of water, not solute.
A common error is assuming that water moves toward the side with "more water." Water moves toward the side with more solute (less free water). The direction is always from low solute concentration to high solute concentration.
Students sometimes think facilitated diffusion is active transport because proteins are involved. Facilitated diffusion is passive: no ATP is used. The involvement of a protein does not make it active.
The sodium-potassium pump is frequently confused with a channel. It is a carrier protein that uses ATP. Channels are passive; the pump is active.
⚠️ Be able to distinguish passive from active transport clearly: direction relative to gradient and energy source.
⚠️ Osmosis questions are extremely common. Know which way water moves relative to solute concentration, and be ready to predict what happens to animal cells and plant cells in isotonic, hypertonic, and hypotonic solutions.
⚠️ Understand the sodium-potassium pump mechanism step by step: 3 Na+ out, 2 K+ in, one ATP per cycle. Know that it is a carrier protein, not a channel.
⚠️ The electrochemical gradient concept is tested frequently. Be able to explain why ion movement depends on both the concentration gradient and the membrane potential.
⚠️ Cotransport is a favourite short-answer topic. Be ready to explain the H+/sucrose cotransporter example and why ATP is needed indirectly (to run the proton pump that builds the gradient).
True or False: Diffusion requires ATP. (False, diffusion is spontaneous.)
Fill in the blank: Water moves by osmosis from a ______ solute concentration to a ______ solute concentration. (Low, high.)
True or False: A plant cell in a hypotonic solution will burst. (False, the cell wall prevents bursting; the cell becomes turgid.)
Fill in the blank: The sodium-potassium pump moves __ Na+ out and __ K+ in per cycle. (3, 2.)
True or False: Facilitated diffusion uses ATP. (False, it is passive transport aided by proteins.)
Q: A red blood cell is placed in a hypertonic solution. What happens and why?
A: Water leaves the cell by osmosis because the surrounding solution has a higher concentration of non-penetrating solutes, meaning lower free water concentration outside the cell. The red blood cell shrivels (crenation).
Q: Explain why a plant in a hypotonic solution does not burst, while an animal cell in the same solution would.
A: The plant cell has a rigid cell wall that resists expansion. As water enters by osmosis, the cell wall pushes back (turgor pressure), preventing the cell from bursting. An animal cell lacks a cell wall and has no structural resistance to swelling, so it lyses.
Q: Describe how the sodium-potassium pump contributes to membrane potential.
A: The pump moves 3 Na+ out and 2 K+ in per cycle, creating a net export of one positive charge. Over time, this contributes to the inside of the cell being negative relative to the outside. The pump is an electrogenic pump because it generates voltage across the membrane.
Q: What is the difference between passive transport and active transport? Give one example of each.
A: Passive transport moves substances down their concentration gradient without energy input (e.g. oxygen diffusing across the membrane, or glucose moving through a carrier protein by facilitated diffusion). Active transport moves substances against their concentration gradient, requiring ATP (e.g. the sodium-potassium pump).
Q: Explain how cotransport works, using the H+/sucrose cotransporter as an example.
A: An ATP-powered proton pump first builds an H+ gradient by pumping protons out of the cell. The H+/sucrose cotransporter then allows H+ to flow back into the cell down its electrochemical gradient, and couples that energy to the simultaneous uptake of sucrose against its own concentration gradient. ATP is not used directly by the cotransporter, but indirectly powers the process by maintaining the H+ gradient.
Passive and active transport connect directly to Chapter 9 (cellular respiration), where the proton gradient across the inner mitochondrial membrane drives ATP synthesis via chemiosmosis. They also lay the groundwork for Chapter 48 (neurons and nerve impulses), which depends on the sodium-potassium pump to maintain resting membrane potential. Osmosis and tonicity are revisited in Chapter 44 (osmoregulation and excretion in animals).
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