Apply Transport - IB Biology

SECTION 1

Historical Context & Motivation

Every living cell is surrounded by a membrane that controls what enters and exits. For centuries, scientists puzzled over how certain substances seemed to pass freely through animal and plant tissues while others were blocked entirely. The discovery of membrane transport mechanisms — from simple diffusion to energy-driven pumps — transformed biology from a purely descriptive science into one that could explain how organisms regulate their internal environments at the molecular level.

1748 • Osmosis Observed

Jean-Antoine Nollet placed a pig bladder over a flask of alcohol submerged in water and observed the bladder swell, providing the first recorded demonstration of osmosis.

1855 • Fick's Laws of Diffusion

Adolf Fick formalized diffusion mathematically, showing that the rate of movement depends on concentration gradients. This gave biologists a quantitative framework for passive transport.

1925 • Lipid Bilayer Proposed

Gorter and Grendel extracted lipids from red blood cells and calculated that there was enough lipid to form a double layer, establishing the bilayer model of cell membranes.

1957 • Na⁺/K⁺ Pump Identified

Jens Christian Skou discovered the sodium-potassium ATPase, the first identified active transport protein. He later received the Nobel Prize for this work.

1972 • Fluid Mosaic Model

Singer and Nicolson proposed that the membrane is a fluid mosaic of phospholipids and proteins, providing the structural context for understanding all forms of transport.

These discoveries raised a central question that still drives cell biology today: how does a membrane only 7–8 nm thick manage to selectively allow some molecules through while blocking others, and how can cells move substances against their natural concentration gradients? Answering that question requires understanding both passive transport and active transport.

SECTION 2

Core Principles of Membrane Transport

Transport across biological membranes can be organized around a few key ideas. Every form of transport depends on the structure of the phospholipid bilayer, which is selectively permeable — small, nonpolar molecules pass through easily, while large or charged molecules generally cannot cross without help. The direction and type of transport depend on the concentration gradient (the difference in solute concentration between two regions) and whether or not cellular energy is required.

Simple Diffusion

Small, nonpolar molecules (O₂, CO₂) move directly through the bilayer from high to low concentration. No proteins or energy required.

Facilitated Diffusion

Polar or charged substances (glucose, ions) pass through channel or carrier proteins down their concentration gradient. No ATP is used.

Osmosis

Water moves through aquaporins or directly through the bilayer from a region of lower solute concentration to higher solute concentration.

Active Transport

Carrier proteins use ATP to pump substances against their concentration gradient — from low to high concentration — maintaining essential cellular conditions.

Vesicle Transport

Large molecules or bulk quantities are moved by endocytosis (into the cell) or exocytosis (out of the cell) using membrane-bound vesicles. This also requires energy.

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KEY TAKEAWAY

Think of a cell membrane like the security system at a concert venue. Some people (small nonpolar molecules) can walk right through the open gates — that's simple diffusion. Others need to show their ticket to a specific turnstile — that's facilitated diffusion. And if someone needs to be escorted backstage against the flow of the crowd, a security guard has to spend energy to make it happen — that's active transport.

SECTION 3

Visualizing Transport Across the Membrane

This diagram shows the four major transport mechanisms across the phospholipid bilayer. On the left, simple diffusion allows small nonpolar molecules like O₂ to pass directly through. Facilitated diffusion uses channel proteins for ions. Osmosis moves water through aquaporins. On the right, active transport uses ATP to pump ions against their gradient.

Notice how the diagram separates passive processes (left side) from active ones (right side). In every passive mechanism, molecules move down their concentration gradient — from a region where they are more concentrated to where they are less concentrated. This movement is spontaneous and requires no cellular energy. Active transport, by contrast, moves substances against their concentration gradient, which is why the cell must spend ATP. The Na⁺/K⁺ pump shown in the diagram is the classic IB example: it pumps 3 Na⁺ ions out and 2 K⁺ ions in for every molecule of ATP hydrolyzed.

SECTION 4

How Transport Mechanisms Work

Passive Transport: Driven by Gradients

All passive transport relies on the kinetic energy that molecules naturally possess. In a solution, solute particles are in constant random motion, colliding with each other and with the surrounding solvent. This random motion leads to a net movement from regions of higher concentration to regions of lower concentration — a process described by Fick's first law of diffusion.

FICK'S FIRST LAW (SIMPLIFIED)

Rate of diffusion ∝ (Surface area × Concentration difference) ÷ Membrane thickness

A larger surface area and a steeper concentration gradient increase the rate. A thicker membrane slows diffusion. In IB Biology, you should know these factors qualitatively rather than calculating exact values.

Osmosis and Tonicity

Osmosis is essentially the diffusion of water. Water moves through the membrane toward the side with a higher solute concentration because the water molecules themselves are more concentrated on the other side. The concept of tonicity describes the relative solute concentration of a solution compared to the cell's interior. A hypertonic solution has more solute than the cell, causing water to leave and the cell to shrink. A hypotonic solution has less solute, causing water to enter and the cell to swell. An isotonic solution has equal solute concentration, so there is no net water movement.

Active Transport: Against the Gradient

When cells need to accumulate substances at a higher concentration inside than outside (or vice versa), they use active transport proteins powered by ATP. The protein undergoes a conformational change — literally changing its 3D shape — to shuttle the molecule across. Primary active transport uses ATP directly. Secondary active transport (cotransport) uses the gradient established by a primary pump to drive another molecule across — for example, the sodium gradient drives glucose uptake in the small intestine.

Bulk Transport: Vesicles

Some molecules are simply too large or too numerous to cross through individual protein channels. In endocytosis, the membrane folds inward to engulf material, forming a vesicle that enters the cytoplasm. Exocytosis is the reverse: a vesicle fuses with the plasma membrane and releases its contents outside the cell. Both processes require energy and involve the dynamic remodeling of the membrane itself.

SECTION 5

Classifying Transport: A Detailed Breakdown

This flowchart organizes all forms of membrane transport. Follow the branches to see how passive and active mechanisms differ in energy requirements, direction of movement, and the molecules they transport.

Feature	Passive Transport	Active Transport
Energy (ATP)	Not required	Required
Direction	Down the concentration gradient (high → low)	Against the concentration gradient (low → high)
Proteins needed?	Sometimes (channel/carrier for facilitated; aquaporins for osmosis)	Always (pumps or vesicle machinery)
Specificity	Varies — simple diffusion is non-specific; facilitated is specific	Highly specific — each pump transports particular ions or molecules
Examples	O₂ into red blood cells; glucose into cells via GLUT transporters	Na⁺/K⁺ pump in neurons; phagocytosis of bacteria by white blood cells
Saturation	Simple diffusion: no; facilitated: yes (limited carrier/channel number)	Yes — limited by number of pump proteins and ATP availability

Comparison of passive and active transport features

An important IB application is understanding how osmosis affects cells differently depending on whether they have a cell wall. Animal cells placed in a hypotonic solution will swell and may lyse (burst). Plant cells in the same solution become turgid — the rigid cell wall prevents them from bursting and instead provides structural support. In a hypertonic solution, animal cells crenate (shrivel), while plant cells undergo plasmolysis — the cell membrane pulls away from the cell wall as the cytoplasm shrinks.

SECTION 6

Worked Example: Predicting Osmotic Effects

Red Blood Cells in Different Solutions

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Step 1 — Identify the Scenario

A red blood cell (RBC) with a cytoplasmic solute concentration of 0.9% NaCl is placed into three different solutions: (A) 0.9% NaCl, (B) 0.2% NaCl, and (C) 3.0% NaCl. Predict what happens to the cell in each solution.
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Step 2 — Determine Tonicity for Solution A

Solution A has the same solute concentration (0.9%) as the cell. This makes it isotonic. Water moves in and out at equal rates, so the cell maintains its normal biconcave shape.

No net water movement → cell stays normal.
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Step 3 — Determine Tonicity for Solution B

Solution B (0.2% NaCl) has a lower solute concentration than the cell. The solution is hypotonic relative to the cell. Water will move into the cell by osmosis because the water potential is higher outside the cell (fewer solutes = more free water molecules).

Net water movement into the cell → cell swells → may lyse (hemolysis).
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Step 4 — Determine Tonicity for Solution C

Solution C (3.0% NaCl) has a higher solute concentration than the cell. The solution is hypertonic relative to the cell. Water will leave the cell by osmosis, moving toward the region of higher solute concentration.

Net water movement out of the cell → cell shrinks (crenation).
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Step 5 — Summarize & Generalize

The key principle is that water always moves from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). For animal cells without a cell wall, extreme osmotic conditions can be fatal. This is why intravenous fluids given to patients are always isotonic (0.9% saline).

Isotonic → normal | Hypotonic → lysis | Hypertonic → crenation

SECTION 7

Strengths and Limitations of Each Transport Type

Transport Type	Strengths / Advantages	Limitations
Simple diffusion	Fast, requires no energy or proteins, works continuously for small nonpolar molecules	Cannot transport polar or large molecules; rate limited by membrane thickness and gradient
Facilitated diffusion	Allows selective transport of polar molecules and ions; highly specific proteins prevent unwanted molecules from entering	Can become saturated when all proteins are occupied; still depends on a favorable gradient
Active transport	Can move substances against their gradient; essential for maintaining ion gradients (e.g., nerve impulses)	Requires ATP — metabolically expensive; stops if cellular respiration is inhibited
Vesicle transport	Can move very large molecules, particles, or even whole cells; allows bulk quantities to be transported at once	Energy-intensive; slower than protein-mediated transport; requires complex membrane remodeling

Advantages and limitations of each transport mechanism

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KEY TAKEAWAY

No single transport mechanism can handle everything a cell needs. Think of it like a delivery system: small letters (simple diffusion) slide under the door easily, packages (facilitated diffusion) need a mail slot of the right size, and heavy furniture (vesicle transport) requires a crew with energy to carry it in. Cells use all of these methods simultaneously to maintain homeostasis.

SECTION 8

Connections to Advanced Topics

The transport concepts you've learned here are foundational for many higher-level topics in IB Biology and beyond. Understanding how ions are moved across membranes leads directly into how neurons generate action potentials — rapid electrical signals that depend on the Na⁺/K⁺ pump maintaining resting membrane potential. Similarly, understanding osmosis is critical for grasping how the kidneys filter blood and reabsorb water.

This Lesson	Advanced Application
Na⁺/K⁺ pump (active transport)	Establishes resting membrane potential in neurons (−70 mV); essential for nerve impulse transmission
Osmosis and tonicity	Kidney nephrons use osmotic gradients to reabsorb water; loop of Henle creates a concentration gradient in the medulla
Facilitated diffusion (glucose transporters)	Insulin increases GLUT4 transporter insertion into cell membranes; failure leads to Type 2 diabetes
Endocytosis / exocytosis	Neurotransmitter release at synapses (exocytosis of vesicles); receptor-mediated endocytosis of cholesterol (LDL)
Cotransport (secondary active)	Glucose absorption in the small intestine via Na⁺-glucose symporter; proton gradients in chemiosmosis (ATP synthesis)

How transport concepts connect to advanced IB Biology topics

Perhaps the most elegant connection is to chemiosmosis — the process by which mitochondria and chloroplasts produce ATP. In both organelles, the electron transport chain uses active transport to pump protons (H⁺) across a membrane, creating a concentration gradient. Those protons then flow back through ATP synthase by facilitated diffusion, and the energy of that flow drives ATP production. The very same principles of passive and active transport you studied in this lesson are what power life at the cellular level.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Explain why oxygen (O₂) can cross the cell membrane by simple diffusion, but sodium ions (Na⁺) cannot. In your answer, refer to the properties of both the molecule/ion and the membrane.

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Oxygen is a small, nonpolar molecule, which means it can dissolve in and pass through the hydrophobic core of the phospholipid bilayer without assistance. Sodium ions, however, carry a positive charge. Charged particles are repelled by the nonpolar fatty acid tails in the interior of the bilayer, making it thermodynamically unfavorable for them to cross unaided. Therefore, Na⁺ ions require specific channel proteins or carrier proteins (facilitated diffusion) or pumps (active transport) to move across the membrane.

Question 2: BASIC CALCULATION

A plant cell is placed in a 10% sucrose solution. The cell's cytoplasm has a sucrose concentration of 5%. (a) Is the external solution hypertonic, hypotonic, or isotonic relative to the cell? (b) In which direction will water move? (c) What will happen to the cell?

Show answer

(a) The external solution (10% sucrose) has a higher solute concentration than the cell's cytoplasm (5%), so it is hypertonic relative to the cell. (b) Water will move out of the cell by osmosis — from the region of lower solute concentration (inside) to higher solute concentration (outside). (c) The plant cell will undergo plasmolysis: the cytoplasm will shrink and the cell membrane will pull away from the rigid cell wall. The cell becomes flaccid.

Question 3: INTERMEDIATE

A scientist observes that glucose uptake into intestinal epithelial cells stops when a metabolic poison (cyanide) is added, even though there is a higher concentration of glucose in the intestinal lumen than inside the cells. Explain this observation, given that glucose normally enters these cells via a sodium-glucose cotransporter.

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Cyanide inhibits the electron transport chain in mitochondria, halting ATP production. Although glucose moves into the cell via the sodium-glucose cotransporter (which itself does not directly consume ATP), the process depends on the sodium gradient maintained by the Na⁺/K⁺ ATPase pump. Without ATP, the Na⁺/K⁺ pump stops, sodium accumulates inside the cell, and the sodium gradient dissipates. Once there is no sodium gradient to drive cotransport, glucose can no longer be pulled into the cell via the symporter, even though glucose itself is at a higher concentration outside. This demonstrates that cotransport (secondary active transport) is indirectly dependent on ATP.

Question 4: APPLIED

A farmer accidentally over-fertilizes a field, dramatically increasing the solute concentration in the soil water. Within hours, the crop plants begin to wilt badly, even though there is plenty of water in the soil. Use your knowledge of osmosis and transport to explain why the plants wilt, and suggest what the farmer could do to save the crops.

Show answer

The excess fertilizer raised the solute concentration in the soil water, making it hypertonic relative to the root hair cells. By osmosis, water moved out of the root cells into the soil, rather than being absorbed. Without water uptake, the plant cannot maintain turgor pressure in its cells, causing the stems and leaves to become flaccid — this is the visible wilting. The phenomenon is sometimes called 'fertilizer burn.' To save the crops, the farmer should flood the field with large volumes of pure water to dilute the soil solution, lowering its solute concentration and restoring the osmotic gradient that allows water to flow into the roots.

Question 5: CRITICAL THINKING

Some cells, such as those lining the kidney collecting duct, can regulate the number of aquaporin channels in their plasma membrane in response to the hormone ADH (antidiuretic hormone). Explain how inserting more aquaporins into the membrane would affect the rate of osmosis, and discuss why this regulation is important for whole-body water balance. Consider what would happen if the cell could not regulate its aquaporin number.

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Inserting additional aquaporins increases the membrane's permeability to water, effectively increasing the surface area available for osmotic water flow. According to the factors affecting diffusion rate, a greater number of channels means water can cross the membrane faster for any given osmotic gradient. When ADH is released (e.g., when the body is dehydrated), more aquaporins are inserted, allowing the kidney collecting duct to reabsorb more water back into the blood, producing concentrated urine. When ADH levels drop (the body is well-hydrated), aquaporins are removed via endocytosis, reducing water reabsorption and producing dilute urine. If the cell could not regulate aquaporin number, the kidneys would not be able to adjust urine concentration. The body would either lose too much water (leading to dehydration) or retain too much (leading to water intoxication and dangerously low blood solute concentrations). This highlights how active membrane regulation — not just passive diffusion — is essential for homeostasis.

SUMMARY

Lesson Summary

Summary

Membrane transport is the foundation of how cells interact with their environment. Simple diffusion moves small nonpolar molecules directly through the phospholipid bilayer down the concentration gradient. Facilitated diffusion uses channel or carrier proteins to transport polar molecules and ions without energy. Osmosis is the net movement of water from a region of lower solute concentration to a region of higher solute concentration, and its effects on cells depend on whether the external solution is hypertonic, hypotonic, or isotonic.

Active transport uses ATP to move substances against their gradient — the Na⁺/K⁺ ATPase is the classic example, pumping 3 Na⁺ out and 2 K⁺ in per ATP. Vesicle transport (endocytosis and exocytosis) handles bulk materials too large for individual protein channels. These mechanisms work together to maintain homeostasis and connect to advanced topics including nerve impulse transmission, kidney function, and chemiosmosis in cellular respiration and photosynthesis.