Apply Membranes & Membrane Transport — Apply Membranes and...

SECTION 1

Historical Context & Motivation

Understanding how cells control what enters and exits has been one of biology's most important pursuits. Every living cell is surrounded by a plasma membrane — a thin, flexible boundary that separates the cell's internal chemistry from the unpredictable outside environment. Without this barrier, the carefully balanced reactions inside a cell would quickly fall apart. The study of membranes has evolved over more than a century, with each breakthrough revealing deeper complexity in how cells manage the transport of molecules.

1895

Overton's Lipid Hypothesis

Charles Overton observed that lipid-soluble molecules entered cells far more easily than water-soluble ones, suggesting the membrane contained a lipid layer.

1925

Gorter & Grendel's Bilayer

Evert Gorter and François Grendel extracted lipids from red blood cells and showed they could form a layer twice the surface area of the cells, establishing the concept of a phospholipid bilayer.

1972

Fluid Mosaic Model

Singer and Nicolson proposed the fluid mosaic model, describing the membrane as a dynamic structure with proteins floating in a sea of phospholipids. This model remains the foundation of membrane biology today.

2003

Aquaporin Nobel Prize

Peter Agre received the Nobel Prize for discovering aquaporins — channel proteins that allow rapid water transport across membranes, solving the mystery of how cells move water so efficiently.

These discoveries leave us with an essential question for IB Biology: how do we apply our knowledge of membrane structure and transport to solve problems, interpret experimental data, and construct clear explanations? This lesson focuses on building exactly those skills.

SECTION 2

Core Principles of Membrane Transport

Before tackling application questions, you need a firm grip on the foundational principles that govern how substances cross membranes. Every IB question on this topic draws from one or more of these core ideas, so understanding them deeply is essential for consistent success.

Selective Permeability

The phospholipid bilayer allows small, nonpolar molecules (O₂, CO₂) to pass freely, but blocks large or charged molecules (glucose, ions). This selective permeability is the reason cells need transport proteins.

Passive Transport

Movement down the concentration gradient — from high to low concentration — requires no energy input. This includes simple diffusion, facilitated diffusion (via channel or carrier proteins), and osmosis (water movement through a selectively permeable membrane).

Active Transport

Movement against the concentration gradient — from low to high concentration — requires energy, usually in the form of ATP. Protein pumps, such as the sodium-potassium pump, carry out this work.

Vesicle Transport

Very large molecules or bulk quantities are moved via endocytosis (into the cell) and exocytosis (out of the cell), both of which involve membrane-bound vesicles and require energy.

Concentration Gradient

The difference in concentration of a substance between two regions is the concentration gradient. The steeper the gradient, the faster the rate of passive transport — a relationship that appears frequently in IB data-based questions.

✦ KEY TAKEAWAY

Think of the cell membrane like the security system at a concert venue. Small items (like phones) pass through easily — that's simple diffusion. Ticket holders go through specific gates — that's facilitated diffusion. VIP guests who need a staff escort against the crowd flow are like molecules in active transport — energy is spent to move them in the 'wrong' direction. And the delivery trucks that bring in entire pallets of supplies? That's vesicle transport.

SECTION 3

Visual Explanation — The Membrane in Action

The diagram below illustrates the fluid mosaic model of the plasma membrane, showing the major components and the different transport mechanisms side by side. Study how each molecule crosses the membrane through a specific pathway — this is exactly how IB examiners expect you to annotate diagrams and explain transport processes.

The diagram shows four transport mechanisms across the phospholipid bilayer. From left to right: simple diffusion of small nonpolar molecules directly through the bilayer; facilitated diffusion through channel and carrier proteins; active transport via protein pumps using ATP; and endocytosis involving vesicle formation.

When answering IB questions, you should be able to identify each type of transport from a diagram like this. Notice that passive processes (simple diffusion, facilitated diffusion, osmosis) always move substances down the concentration gradient, while active transport moves substances against the concentration gradient. If an IB question tells you that a metabolic poison stops ATP production and transport of a substance halts, you can confidently conclude that the substance was being moved by active transport.

SECTION 4

Mechanisms & Quantitative Relationships

While IB Biology does not require extensive mathematical derivations for membrane transport, understanding a few key quantitative relationships helps you interpret data-based questions confidently. The most important involve osmosis and the rate of diffusion.

Osmosis and Tonicity

Osmosis is the net movement of water molecules from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential) across a selectively permeable membrane. When describing the environment around a cell, you should use three key terms: a hypertonic solution has more solute than the cell's interior, causing the cell to lose water; a hypotonic solution has less solute, causing water to enter the cell; and an isotonic solution has equal solute concentration, resulting in no net water movement.

PERCENTAGE CHANGE IN MASS (OSMOSIS LAB)

% change in mass = ((final mass − initial mass) ÷ initial mass) × 100

This formula is used to calculate mass changes of tissue samples placed in solutions of different concentrations. A positive value indicates net water gain (hypotonic solution), while a negative value indicates net water loss (hypertonic solution).

Factors Affecting Rate of Diffusion

FICK'S LAW (SIMPLIFIED)

Rate of diffusion ∝ (surface area × concentration gradient) ÷ membrane thickness

Although you won't be asked to perform full Fick's Law calculations, IB examiners expect you to explain how each variable affects transport rate. A larger surface area or steeper concentration gradient increases the rate, while a thicker membrane decreases it.

💡 IB EXAM TIP

When a graph shows the rate of transport reaching a plateau as substrate concentration increases, this indicates saturation of transport proteins — a hallmark of facilitated diffusion or active transport. Simple diffusion, by contrast, shows a linear increase because it does not rely on proteins.

SECTION 5

Interpreting Data-Based Questions

IB Biology assessments frequently present data in tables and graphs related to membrane transport experiments. Developing a systematic approach to reading and interpreting this data is just as important as knowing the content itself. The diagram below shows a typical osmosis experiment result — the kind you will encounter in Paper 2 or Paper 3 questions.

This graph shows data from a typical osmosis experiment where potato tissue was placed in sucrose solutions of varying concentrations. The isotonic point (approximately 0.4 M) is where the line crosses 0% change — the solute concentration of the surrounding solution equals that of the potato cells. To the left, solutions are hypotonic (cells gain mass). To the right, solutions are hypertonic (cells lose mass).

How to Approach Data Questions

Step 1 — Identify variables. What is the independent variable (x-axis)? What is the dependent variable (y-axis)? In the graph above, sucrose concentration is independent and % change in mass is dependent.
Step 2 — Describe the trend. Use data points in your answer: 'As sucrose concentration increases from 0.0 to 1.0 M, the percentage change in mass decreases from +23% to −22%.'
Step 3 — Explain using biology. Connect the trend to osmosis: at low sucrose concentrations, water moves into cells by osmosis because the external solution is hypotonic. At high concentrations, water moves out because the solution is hypertonic.
Step 4 — Identify key features. Find the isotonic point (where the line crosses zero) and state its value. Examiners award marks for reading values accurately from graphs.

SECTION 6

Worked Example — Osmosis Data Analysis

Let's work through a typical IB-style question step by step. This example mirrors the kind of data-based question you will encounter on Paper 2.

📝 QUESTION

A student places cylinders of potato tissue (each initially 3.2 g) into five different sucrose solutions. After 24 hours, she records the final masses. The results are: 0.0 M → 3.9 g; 0.2 M → 3.6 g; 0.4 M → 3.2 g; 0.6 M → 2.8 g; 0.8 M → 2.5 g. (a) Calculate the percentage change in mass for the 0.0 M and 0.8 M solutions. (b) Estimate the solute concentration of the potato cells. (c) Explain the result at 0.6 M using the concept of osmosis.

Solution

Step 1 — Calculate % change in mass for 0.0 MApply the formula: % change = ((final − initial) ÷ initial) × 100. Substituting: ((3.9 − 3.2) ÷ 3.2) × 100 = (0.7 ÷ 3.2) × 100.

% change at 0.0 M = +21.9%

Step 2 — Calculate % change in mass for 0.8 MUsing the same formula: ((2.5 − 3.2) ÷ 3.2) × 100 = (−0.7 ÷ 3.2) × 100.

% change at 0.8 M = −21.9%

Step 3 — Estimate solute concentration of potato cellsThe isotonic point is the concentration at which there is no net movement of water (0% change in mass). At 0.4 M, the final mass equals the initial mass (3.2 g), so % change = 0%. This means the solute concentration inside the potato cells is approximately equal to the external solution.

Estimated solute concentration of potato cells ≈ 0.4 mol dm⁻³

Step 4 — Explain the result at 0.6 MAt 0.6 M sucrose, the external solution has a higher solute concentration than the potato cells (which are approximately 0.4 M). This means the external solution is hypertonic relative to the cell. Water moves out of the potato cells by osmosis, down the water potential gradient (from higher water potential inside the cell to lower water potential outside). This causes the cells to lose water and the tissue to decrease in mass from 3.2 g to 2.8 g, representing a −12.5% change.

The potato tissue loses mass because water exits the cells by osmosis into the hypertonic solution.

SECTION 7

Comparing Types of Membrane Transport

IB examiners love comparison questions. Being able to clearly distinguish between passive and active transport, and among the subtypes within each category, is essential for earning full marks. The table below summarizes the key features that you should memorize and be ready to apply.

Comparison of the three main types of membrane transport

Feature	Simple Diffusion	Facilitated Diffusion	Active Transport
Direction	Down concentration gradient	Down concentration gradient	Against concentration gradient
Energy (ATP)	Not required	Not required	Required
Proteins needed?	No	Yes (channels or carriers)	Yes (pump proteins)
Specificity	Low — depends on size and polarity	High — protein shape is specific	High — pump is specific
Saturation?	No — rate increases linearly	Yes — rate plateaus	Yes — rate plateaus
Examples	O₂, CO₂, ethanol	Glucose (via GLUT), ions (via channels)	Na⁺/K⁺ pump, H⁺ pump in roots
Effect of metabolic poison	No effect	No effect	Transport stops

✦ KEY TAKEAWAY

The single most powerful IB question-answering tool for membrane transport is the metabolic poison test. If adding cyanide or another poison to cells stops the transport of a substance, then that substance was being moved by active transport (because the poison blocks ATP production). If transport continues, it must be passive. This logic appears in IB exams repeatedly.

SECTION 8

Connections to HL Topics & Advanced Theory

The principles covered in this lesson form the foundation for several more advanced topics in IB Biology, especially at Higher Level. Understanding membrane transport deeply will help you tackle topics like nerve impulse transmission, kidney function, and cellular signaling with greater confidence.

How SL membrane transport concepts connect to HL content

SL Foundation	HL / Advanced Extension
Active transport moves ions against gradients	The Na⁺/K⁺ pump maintains the resting membrane potential in neurons (−70 mV), enabling action potentials
Osmosis moves water across membranes	The loop of Henle in the kidney uses osmotic gradients to concentrate urine via countercurrent multiplier
Facilitated diffusion uses channel proteins	Voltage-gated Na⁺ and K⁺ channels open and close during depolarization and repolarization of neurons
Endocytosis brings large molecules into cells	Receptor-mediated endocytosis allows cells to selectively import specific molecules (e.g., cholesterol via LDL receptors)

If you are studying at Higher Level, keep in mind that the same logic you use to solve SL membrane questions — identifying gradient direction, energy requirements, and protein involvement — scales directly to these more complex scenarios. The analytical framework stays the same; only the biological context becomes more detailed.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

A cell is placed in a solution and is observed to shrink over time. State whether the external solution is hypertonic, hypotonic, or isotonic, and explain why the cell changes size.

PROBLEM 2 — BASIC CALCULATION

A piece of potato tissue has an initial mass of 2.5 g. After being immersed in a 0.1 M sucrose solution for 24 hours, its mass is 2.9 g. Calculate the percentage change in mass and state what this tells you about the solution's tonicity.

PROBLEM 3 — INTERMEDIATE

An experiment measures the rate of glucose uptake by intestinal cells at different external glucose concentrations. The rate increases steeply at low concentrations but reaches a plateau at high concentrations. Explain this pattern and identify the type of transport involved.

PROBLEM 4 — APPLIED

A researcher treats a batch of kidney cells with cyanide (a metabolic poison that blocks ATP production). She then tests the cells' ability to absorb potassium ions (K⁺) and oxygen (O₂). Predict and explain the results for each substance.

PROBLEM 5 — CRITICAL THINKING

A student conducts an osmosis experiment and plots percentage change in mass against sucrose concentration. She finds the isotonic point is at 0.35 M. However, her teacher notes that the actual solute concentration of the cells should be higher than 0.35 M because sucrose molecules are too large to enter the cells. Evaluate the teacher's reasoning and suggest one limitation of using percentage change in mass to determine internal solute concentration.

SUMMARY

Lesson Summary

Cell membranes are built on the fluid mosaic model — a phospholipid bilayer with embedded proteins that gives cells selective permeability. Substances cross membranes by simple diffusion (small, nonpolar molecules moving down their gradient), facilitated diffusion (using channel or carrier proteins, still down the gradient), active transport (against the gradient, requiring ATP), or vesicle transport (endocytosis and exocytosis for bulk materials). Osmosis is the passive movement of water from lower solute concentration to higher solute concentration.

When solving IB data-based questions, use the percentage change in mass formula for osmosis experiments and identify the isotonic point where the graph crosses zero. Apply Fick's Law to explain how surface area, gradient steepness, and membrane thickness affect diffusion rates. Use the metabolic poison test to distinguish active from passive transport: if a poison stops transport, it was active; if transport continues, it was passive. Always cite data values from tables and graphs, describe trends precisely, and connect your observations to biological mechanisms for full marks.