Passive Transport, Active Transport, and Vesicles

Core Concepts

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, driven by random thermal motion. No energy input is required — diffusion is a passive process. Diffusion continues until the substance is evenly distributed (dynamic equilibrium), at which point molecules still move but with no net change in concentration.

Key points:

• Diffusion is fastest over short distances.

• Diffusion rate depends on temperature (faster at higher temperatures), concentration gradient (faster with steeper gradients), and the size and polarity of the diffusing substance.

• In the context of cell membranes, small, nonpolar molecules (O2, CO2) diffuse directly through the phospholipid bilayer. This is simple diffusion.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from a region of lower solute concentration (higher water concentration) to a region of higher solute concentration (lower water concentration). You can think of it as water moving to dilute the more concentrated solution.

In biological membranes, water crosses through the bilayer (slowly) and through specialized channel proteins called aquaporins (rapidly).

Tonicity

Tonicity describes the ability of an external solution to cause a cell to gain or lose water. Tonicity depends on the concentration of solutes that cannot cross the membrane (nonpenetrating solutes).

• Isotonic: The solute concentration is the same inside and outside the cell. No net water movement; the cell maintains its normal volume. (Animal cells are healthiest in isotonic environments.)

• Hypotonic: The external solute concentration is lower than inside the cell (the solution is more dilute). Water enters the cell by osmosis. Animal cells swell and may burst (lyse). Plant cells become turgid — the plasma membrane presses against the cell wall, which is the normal, healthy state for most plant cells.

• Hypertonic: The external solute concentration is higher than inside the cell. Water leaves the cell by osmosis. Animal cells shrivel (crenate). Plant cells undergo plasmolysis — the plasma membrane pulls away from the cell wall as the central vacuole loses water, causing the plant to wilt.

Facilitated diffusion

Facilitated diffusion is passive transport — solutes move down their concentration gradient — but the solutes cannot cross the bilayer on their own and require the assistance of transport proteins. No energy (ATP) is consumed.

Two types of transport proteins mediate facilitated diffusion:

• Channel proteins: Form hydrophilic tunnels through the membrane. Many channels are gated — they open or close in response to stimuli (e.g., voltage changes, ligand binding, mechanical force). Ion channels are specific for particular ions (Na+, K+, Ca2+, Cl−). Aquaporins are water-specific channels.

• Carrier proteins: Bind the solute on one side of the membrane, undergo a conformational change, and release the solute on the other side. Carriers are more selective than channels but generally slower because each transport event requires a shape change. The glucose transporter (GLUT) is a well-known carrier.

Active transport

Active transport moves solutes against their concentration (or electrochemical) gradient — from lower to higher concentration. This requires energy, typically from ATP hydrolysis. Active transport is carried out by pumps, which are integral membrane proteins.

Key features:

• Active transport is directional — it pumps a specific solute in a specific direction.

• Active transport creates and maintains concentration gradients that are essential for cell function (e.g., the Na+ and K+ gradients in animal cells).

• Active transport is saturable — the rate reaches a maximum when all pumps are engaged.

The sodium-potassium pump (Na+/K+ ATPase)

The sodium-potassium pump is the most important active transporter in animal cells. It pumps 3 Na+ out of the cell and 2 K+ into the cell for each ATP hydrolyzed. This pump:

• Maintains the Na+ gradient (high Na+ outside, low Na+ inside) and K+ gradient (high K+ inside, low K+ outside).

• Contributes to the negative membrane potential (more positive charges are pumped out than in — 3 Na+ out vs. 2 K+ in — creating a net export of positive charge).

• Drives secondary active transport (cotransport) by maintaining the Na+ gradient that other transporters use.

Cotransport (secondary active transport)

In cotransport, a solute is transported against its concentration gradient by coupling its movement to the movement of another solute down its gradient (which was established by a pump using ATP). The energy for the uphill transport comes indirectly from ATP, through the ion gradient. An example is the Na+/glucose cotransporter in intestinal cells: Na+ moves down its gradient (into the cell), and glucose is pulled in against its gradient.

Endocytosis

Endocytosis is the process by which the cell takes in large particles or quantities of extracellular material by engulfing them in vesicles formed from the plasma membrane. It requires energy. There are three major types:

• Phagocytosis ("cell eating"): The cell engulfs large particles (e.g., bacteria, cell debris) by extending pseudopodia around them, forming a phagocytic vesicle (phagosome) that fuses with a lysosome for digestion. This is performed primarily by specialized cells of the immune system.

• Pinocytosis ("cell drinking"): The cell takes in extracellular fluid and dissolved solutes in small vesicles. This is nonselective and occurs in many cell types.

• Receptor-mediated endocytosis: Specific molecules (ligands) bind to receptors on the cell surface. The receptors cluster in coated pits, which invaginate and form vesicles. This is highly selective and efficient — it concentrates specific ligands even when they are present at low concentrations. Example: cells take up cholesterol via LDL (low-density lipoprotein) receptors.

Exocytosis

Exocytosis is the process by which the cell secretes large molecules or quantities of material by fusing vesicles with the plasma membrane. The vesicle membrane becomes part of the plasma membrane, and the vesicle contents are released to the extracellular space. Exocytosis is used for:

• Secretion of proteins (digestive enzymes, hormones, antibodies)

• Secretion of neurotransmitters (at synapses)

• Export of waste products

• Delivery of new membrane components (lipids and proteins) to the plasma membrane

Transport summary: Simple diffusion (no energy, down gradient, no protein). Osmosis (water, down gradient). Facilitated diffusion (protein, down gradient). Active transport (ATP, against gradient). Cotransport (indirect ATP, one solute against). Endo/exocytosis (bulk, vesicle-based).

ELI Example

Imagine a water balloon filled with salty water, placed in a tub of pure water. The balloon's rubber is like a selectively permeable membrane — water can pass through, but salt cannot. Water molecules move through the rubber from the tub (where water is more concentrated — it is pure water) into the balloon (where water is less concentrated — it is mixed with salt). The balloon swells. That is osmosis. If instead you put the salty-water balloon into a tub of very salty water, water would leave the balloon, and it would shrink. That is hypertonic. If the tub water has the same saltiness as the balloon's interior, water moves in both directions equally — no net change. That is isotonic.

Do Not Confuse

Lab Link

Diffusion and osmosis are frequently investigated in introductory biology laboratories. Common investigations include: (1) placing dialysis tubing containing various solutions in water or other solutions and measuring mass changes (osmosis); (2) observing red blood cells or Elodea (aquatic plant) cells in solutions of different tonicities; (3) measuring the rate of diffusion of a dye (such as potassium permanganate) through agar; (4) using beet root to measure membrane damage (pigment leakage) under different conditions. Understanding the principles in this chapter allows you to predict and explain the observations in these investigations.

High-Yield Memory Anchors

• Passive = down gradient, no ATP. Active = against gradient, needs ATP.

• Osmosis = water moving toward the side with more dissolved stuff.

• Hypo-tonic = cell swells. Hyper-tonic = cell shrinks. Iso-tonic = no change.

• Plant cells like hypotonic (turgid). Animal cells like isotonic.

• Sodium-potassium pump: 3 Na+ out, 2 K+ in, 1 ATP.

Quick Check

Q1 (Foundational): A red blood cell is placed in a beaker of distilled water. Describe what will happen to the cell and explain why, using the terms osmosis, hypotonic, and selectively permeable membrane.

Q2 (Application): A marine fish lives in seawater, which has a higher solute concentration than its body fluids. The fish must constantly drink seawater and actively pump salt out through its gills. Explain why the fish would lose water to its environment and why active transport is necessary for its survival.

Q3 (Comparison/Reasoning): Compare the transport of glucose into a cell by the GLUT carrier protein (facilitated diffusion) versus the transport of glucose by the Na+/glucose cotransporter (secondary active transport). Under what circumstances would each mechanism be most important?

Quick Check Answers

A1: Distilled water is hypotonic relative to the red blood cell's interior — it has a much lower solute concentration. Because the plasma membrane is selectively permeable (water can cross, but most solutes cannot), water enters the cell by osmosis, moving from the region of higher water concentration (outside) to the region of lower water concentration (inside, where solutes are present). The cell swells and, without a cell wall to restrain it, will eventually burst (lyse).

A2: Seawater is hypertonic relative to the fish's body fluids. Water moves by osmosis from the fish's body (lower solute, higher water concentration) into the surrounding seawater (higher solute, lower water concentration). The fish loses water to its environment. To compensate, the fish drinks seawater, which brings in both water and excess salt. The excess salt must be actively pumped out (active transport) through the gills against its concentration gradient — salt concentrations are higher in seawater than in the fish's cells. Active transport is necessary because passive mechanisms cannot move salt from a lower concentration (inside) to a higher concentration (outside).

A3: GLUT carrier (facilitated diffusion): Glucose moves down its concentration gradient from higher glucose (outside the cell, after a meal) to lower glucose (inside the cell, where it is being metabolized). No ATP is used. This mechanism is important when blood glucose is higher than intracellular glucose — the typical fed state. Na+/glucose cotransporter (secondary active transport): Glucose is transported against its concentration gradient, coupled to the movement of Na+ down its electrochemical gradient. The Na+ gradient was established by the Na+/K+ pump (which uses ATP). This mechanism is important in the intestinal epithelium and kidney tubules, where glucose must be absorbed even when its concentration in the lumen is lower than inside the cells.

Chapter Summary

Passive transport (diffusion, osmosis, facilitated diffusion) moves solutes down gradients without energy. Active transport (sodium-potassium pump) uses ATP to move solutes against gradients. Cotransport couples uphill to downhill movement. Bulk transport (endo/exocytosis) moves large cargo via vesicles.

Common Mistakes

Mistake: "Water moves from high solute concentration to low solute concentration."

Reality: Water moves from high WATER concentration (low solute) to low WATER concentration (high solute). Put another way: water moves toward the side with more dissolved particles — it moves to dilute the concentrated side.

Mistake: "A cell placed in pure water will not change size because it has a cell membrane."

Reality: A cell (especially an animal cell lacking a cell wall) in pure (hypotonic) water will swell as water enters by osmosis. If the gradient is steep enough, the cell may burst (lyse). Red blood cells, for example, lyse in distilled water.

Mistake: "Plant cells burst in hypotonic solutions."

Reality: Plant cells become turgid (firm) in hypotonic solutions, which is their normal, healthy state. The cell wall prevents bursting. Plant cells actually wilt in isotonic solutions and plasmolyze (the membrane pulls away from the cell wall) in hypertonic solutions.

Mistake: "Active transport directly uses the energy from ATP to move all solutes."

Reality: In secondary active transport (cotransport), the energy comes indirectly from ATP — the ion gradient created by a pump (which used ATP) provides the driving force. The cotransporter protein itself does not hydrolyze ATP.