IB Biology SL: Theme B - Form & Function

B2.1 - Membranes & Membrane Transport

The Gateway of the Cell: Structure, Function, and Transport

🧪 Introduction to Cell Membranes

The cell membrane (also called the plasma membrane) is one of the most fundamental structures in biology. It forms the boundary of every cell, separating the internal cellular environment from the external surroundings. This remarkable structure is not just a simple barrier—it's a dynamic, selective gateway that controls what enters and leaves the cell.

Cell membranes are selectively permeable (also called semi-permeable), meaning they allow some substances to pass through while blocking others. This selective permeability is essential for maintaining the cell's internal environment, acquiring nutrients, eliminating waste, and responding to signals.

Understanding membrane structure and transport mechanisms is crucial for comprehending how cells maintain homeostasis, communicate, and respond to their environment.

🏗️ Structure of Cell Membranes

💧 The Phospholipid Bilayer: Foundation of Membranes

What is the Phospholipid Bilayer?

The phospholipid bilayer is the primary structural component of all cell membranes. It consists of two layers (hence "bilayer") of phospholipid molecules arranged with their hydrophobic tails facing inward and their hydrophilic heads facing outward toward the aqueous environments inside and outside the cell.

This arrangement forms a stable barrier that separates the cell's interior from its external environment while maintaining fluidity.

Structure of Phospholipids

Phospholipids are amphipathic molecules (or amphiphilic) with two distinct regions:

Hydrophilic Head

Polar (water-loving)
Contains phosphate group
Attracted to water
Faces aqueous environments
Points outward/inward in bilayer

Hydrophobic Tails

Non-polar (water-fearing)
Consist of fatty acid chains
Repelled by water
Hide from aqueous environments
Face inward in bilayer

🔑 Key Point:

The amphipathic nature of phospholipids causes them to spontaneously arrange into a bilayer in aqueous environments. The hydrophobic tails cluster together in the interior, away from water, while the hydrophilic heads face the water on both sides.

Functions of the Phospholipid Bilayer

1. Barrier Function

Creates a hydrophobic barrier that prevents free passage of polar molecules and ions, maintaining different concentrations of substances inside and outside the cell.

2. Selective Permeability

Allows small, non-polar molecules (O₂, CO₂, N₂) and small uncharged polar molecules (water, ethanol) to pass through while blocking larger or charged molecules.

3. Fluidity

Phospholipids can move laterally within their layer, giving the membrane flexibility and allowing it to self-repair small tears.

4. Compartmentalization

Enables formation of distinct cellular compartments and organelles, each with specialized functions.

🧩 The Fluid Mosaic Model

What is the Fluid Mosaic Model?

The Fluid Mosaic Model, proposed by Singer and Nicolson in 1972, is the currently accepted model for describing the structure of cell membranes. It describes the membrane as a dynamic, flexible structure where components can move.

The name comes from two key features: Fluid (components can move) and Mosaic (diverse components create a pattern).

Two Key Features:

1. FLUID

The phospholipid bilayer is viscous and flexible
Individual phospholipids can move laterally within their layer
Proteins can also drift within the membrane (unless anchored)
Membrane has liquid-like properties at physiological temperatures
This fluidity allows membranes to be self-sealing and flexible

2. MOSAIC

The membrane contains a variety of components: phospholipids, proteins, cholesterol, carbohydrates
Proteins are embedded in or attached to the phospholipid bilayer
These proteins create a mosaic pattern when viewed from above
Different membranes have different protein compositions depending on function
Creates a heterogeneous structure with diverse functional regions

Additional Membrane Components

Cholesterol (in animal cells)

Interspersed between phospholipids; regulates membrane fluidity (prevents it from becoming too fluid or too rigid) and adds stability.

Glycoproteins & Glycolipids

Carbohydrate chains attached to proteins or lipids; always face the extracellular side; involved in cell recognition and adhesion.

Membrane Proteins

Embedded in or attached to the bilayer; perform various functions (transport, signaling, recognition, enzymatic activity).

🔬 Membrane Proteins

Proteins are essential components of cell membranes, accounting for approximately 50% of membrane mass. They perform most of the membrane's specialized functions, including transport, cell signaling, recognition, and enzymatic activity.

Membrane proteins are classified into two main types based on how they associate with the membrane: integral proteins and peripheral proteins.

🔷 Integral Proteins (Intrinsic Proteins)

Characteristics

Permanently embedded in the phospholipid bilayer
Contain hydrophobic amino acids that interact with the hydrophobic core of the membrane
Many are transmembrane proteins—they span the entire membrane, with regions exposed on both sides
Cannot be easily removed without disrupting the membrane (requires detergents or organic solvents)
Amphipathic structure: hydrophobic regions within the bilayer, hydrophilic regions exposed to aqueous environments

Examples of Integral Proteins

Channel proteins: Form pores for ion transport
Carrier proteins: Bind and transport specific molecules
Protein pumps: Actively transport substances using ATP
Receptor proteins: Bind signaling molecules (hormones, neurotransmitters)
Glycoproteins: Have carbohydrate chains attached; involved in cell recognition

🔶 Peripheral Proteins (Extrinsic Proteins)

Characteristics

Temporarily attached to the membrane surface
Located on the inner (cytoplasmic) or outer (extracellular) surface of the membrane
Do not penetrate the hydrophobic core of the bilayer
Attached through weak bonds (hydrogen bonds, ionic bonds) with integral proteins or phospholipid heads
Can be easily removed by changes in pH or ionic strength without disrupting the membrane

Examples of Peripheral Proteins

Cytoskeletal proteins: Connect membrane to internal cell structure
Enzymes: Catalyze reactions near the membrane
G proteins: Involved in signal transduction pathways
Regulatory proteins: Control membrane protein activity

⚙️ Functions of Membrane Proteins: JET RAT

Mnemonic: JET RAT

Junctions · Enzymes · Transport · Recognition · Anchorage · Transduction

Junctions

Connect and join two cells together (e.g., tight junctions, gap junctions)

Enzymes

Catalyze reactions; fixing to membranes localizes metabolic pathways

Transport

Move substances across membrane (channels, carriers, pumps); responsible for facilitated diffusion and active transport

Recognition

Function as markers for cellular identification (e.g., immune system recognition, blood type antigens)

Anchorage

Attachment points for cytoskeleton and extracellular matrix; maintain cell shape

Transduction

Function as receptors for signaling molecules (hormones, growth factors); transmit signals into cell

🚪 Membrane Permeability

What Determines Permeability?

Membrane permeability refers to the ability of substances to pass through a cell membrane. The phospholipid bilayer is selectively permeable—it allows some substances to pass through easily while restricting or blocking others.

Permeability depends primarily on two factors: size of the molecule and its polarity/charge.

What Can Cross the Membrane?

Permeability	Type of Molecules	Examples	Reason
HIGH	Small, non-polar molecules	O₂, CO₂, N₂	Can dissolve in and diffuse through lipid bilayer
MODERATE	Small, uncharged polar molecules	H₂O, ethanol, urea	Small enough to slip through, despite being polar
LOW	Large, uncharged polar molecules	Glucose, amino acids	Too large and polar; need transport proteins
VERY LOW/NONE	Ions (charged particles)	Na⁺, K⁺, Ca²⁺, Cl^-	Charged; repelled by hydrophobic core; need channel proteins

🔑 Key Principles of Permeability

1. Size Matters

Smaller molecules cross more easily than larger ones. Very large molecules (proteins, polysaccharides) cannot cross the membrane without specialized transport mechanisms.

2. Polarity/Charge Matters

Non-polar (hydrophobic) molecules cross easily because they dissolve in the lipid bilayer. Polar and charged molecules are repelled by the hydrophobic core and require protein channels or carriers.

3. Selective Permeability Enables Control

The membrane's selective permeability allows cells to maintain different concentrations of substances inside vs. outside, regulate their internal environment, and respond to external signals.

➡️ Simple Diffusion

What is Diffusion?

Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration down a concentration gradient.

Diffusion is a passive process—it requires NO energy input (no ATP). It occurs due to the random kinetic motion of molecules.

🔬 How Diffusion Works

The Concentration Gradient

A concentration gradient exists when there is a difference in the concentration of a substance between two regions. Molecules naturally move down the concentration gradient (from high to low) until equilibrium is reached.

Process of Diffusion:

Molecules are in constant random motion due to their kinetic energy
In a region of high concentration, molecules collide frequently and spread out
They move randomly in all directions, but net movement is toward lower concentration
Eventually, molecules are evenly distributed—this is called equilibrium
At equilibrium, molecules still move randomly, but there is no net movement in any direction

Characteristics of Simple Diffusion

Passive process: No energy (ATP) required
Down concentration gradient: Always from high to low concentration
Random molecular motion: Driven by kinetic energy
Continues until equilibrium: Stops when concentrations are equal
No selectivity: Any molecule that can cross the membrane will diffuse (based only on size and polarity)
Directly through membrane: Molecules dissolve in and pass through the lipid bilayer

⚡ Factors Affecting Rate of Diffusion

1. Concentration Gradient

Greater concentration difference = faster diffusion. The steeper the gradient, the faster the net movement of molecules.

2. Temperature

Higher temperature = faster diffusion. Molecules have more kinetic energy and move faster at higher temperatures.

3. Surface Area

Larger surface area = faster diffusion. More membrane area allows more molecules to cross simultaneously.

4. Distance

Shorter distance = faster diffusion. Molecules reach equilibrium faster over short distances.

5. Molecular Size

Smaller molecules = faster diffusion. Smaller molecules move more quickly and can pass through the membrane more easily.

📝 Fick's Law of Diffusion:

Rate of Diffusion ∝ (Surface Area × Concentration Difference) / Distance

💧 Osmosis: Diffusion of Water

What is Osmosis?

Osmosis is the diffusion of water molecules across a selectively permeable membrane from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration).

Osmosis is a special case of diffusion—it applies specifically to the movement of water. Like diffusion, it is a passive process requiring no ATP.

🔬 Understanding Osmosis

Key Concepts

Water Concentration vs. Solute Concentration:

High solute concentration = Low water concentration
Low solute concentration = High water concentration
Water moves from areas of lower solute concentration to areas of higher solute concentration
This is equivalent to water moving from higher water concentration to lower water concentration

🔑 Key Point:

Osmosis is driven by the water concentration gradient, not the solute gradient. Water always moves toward the area where it is less concentrated (which is where solutes are more concentrated).

Osmotic Solutions: Tonicity

Tonicity describes the relative solute concentration of two solutions separated by a selectively permeable membrane. It determines the direction of water movement.

1. Isotonic Solution

Equal solute concentration inside and outside the cell
No net water movement—equilibrium
Cell maintains normal shape and volume
Example: 0.9% saline solution for red blood cells

2. Hypotonic Solution

Lower solute concentration outside the cell (higher water concentration outside)
Water moves INTO the cell by osmosis
Cell swells and may burst (lysis)
Animal cells: cytolysis (bursting)
Plant cells: Become turgid (firm) due to cell wall preventing bursting

3. Hypertonic Solution

Higher solute concentration outside the cell (lower water concentration outside)
Water moves OUT OF the cell by osmosis
Cell shrinks and shrivels
Animal cells: Crenation (shriveling)
Plant cells: Plasmolysis (cell membrane pulls away from cell wall)

📊 Effects of Osmosis on Cells

Solution Type	Water Movement	Animal Cell Effect	Plant Cell Effect
Isotonic	No net movement	Normal shape	Normal, slightly flaccid
Hypotonic	Into cell	Swells, may lyse (burst)	Turgid (firm), cell wall prevents bursting
Hypertonic	Out of cell	Shrivels (crenation)	Plasmolyzed (membrane pulls away from wall)

🌱 Why Plant Cells Don't Burst:

Plant cells have a rigid cell wall outside the plasma membrane. This wall provides structural support and prevents the cell from bursting when water enters. The pressure of water against the cell wall creates turgor pressure, which helps plants maintain their upright structure.

🚪 Facilitated Diffusion

What is Facilitated Diffusion?

Facilitated diffusion is the passive movement of molecules across a cell membrane with the help of transport proteins. It is used by molecules that cannot freely cross the phospholipid bilayer—typically large polar molecules and ions.

Like simple diffusion, facilitated diffusion is passive (no ATP required) and moves substances down their concentration gradient. The difference is that it requires protein assistance.

🔬 Transport Proteins in Facilitated Diffusion

Two types of transport proteins mediate facilitated diffusion:

1. Channel Proteins

Structure & Function:

Form hydrophilic pores (tunnels) through the membrane
Allow specific ions to pass through when open
Lined with polar amino acids to allow passage of charged particles
Most are gated—can open or close in response to stimuli
Very fast transport: Thousands to millions of ions per second
Highly specific: Each channel only allows certain ions (e.g., Na⁺ channels, K⁺ channels, Ca²⁺ channels)

Types of Gating:

Voltage-gated: Open/close in response to changes in electrical potential across membrane
Ligand-gated: Open/close when specific molecule binds to the channel
Mechanically-gated: Open/close in response to physical pressure or stretch

Example: Sodium (Na⁺) channels in nerve cells open in response to voltage changes, allowing rapid influx of Na⁺ ions during action potentials.

2. Carrier Proteins (Transporters)

Structure & Function:

Undergo conformational changes (shape changes) to transport molecules
Bind specific molecules on one side of the membrane
Change shape to move the molecule to the other side
Release the molecule on the opposite side
Slower than channels: ~1,000 molecules per second
Highly specific: Each carrier has a specific binding site for its substrate

How Carrier Proteins Work:

Binding: Specific molecule binds to carrier protein's binding site
Conformational change: Protein changes shape (like an enzyme-substrate interaction)
Translocation: Molecule is moved across the membrane
Release: Molecule is released on the other side
Return: Protein returns to original shape, ready for another cycle

🔍 Example: Glucose transporters (GLUT proteins) in cell membranes bind glucose on one side, change shape, and release glucose on the other side. This allows cells to take up glucose from the bloodstream without using ATP.

⚖️ Channel vs. Carrier Proteins

Feature	Channel Proteins	Carrier Proteins
Structure	Form pores/channels through membrane	No pore; change shape to transport
What They Transport	Ions (Na⁺, K⁺, Ca²⁺, Cl^-)	Large polar molecules (glucose, amino acids)
Speed	Very fast (millions per second)	Slower (~1,000 per second)
Mechanism	Molecules pass through open channel	Binding and conformational change
Regulation	Can be gated (open/close)	Always "active" when substrate present
Specificity	High (ion-specific)	Very high (molecule-specific)

⚡ Active Transport

What is Active Transport?

Active transport is the movement of molecules across a cell membrane against their concentration gradient (from low concentration to high concentration) using energy from ATP.

Unlike diffusion and facilitated diffusion, active transport is NOT passive—it requires direct energy input in the form of ATP hydrolysis.

🔬 How Active Transport Works

Pump Proteins (Protein Pumps)

Active transport is carried out by special carrier proteins called pump proteins or simply pumps. These proteins use energy from ATP hydrolysis to change shape and move substances against their concentration gradient.

General Mechanism:

Binding: Specific ions/molecules bind to the pump protein
ATP hydrolysis: ATP is broken down to ADP + Pi (inorganic phosphate)
Phosphorylation: Phosphate group attaches to the pump protein
Conformational change: Protein changes shape due to phosphorylation
Transport: Ions/molecules are moved across the membrane against their gradient
Release: Ions/molecules are released on the other side
Dephosphorylation: Phosphate is removed; protein returns to original shape
Cycle repeats: Pump is ready for another transport cycle

Key Features of Active Transport

Requires energy: Uses ATP (adenosine triphosphate)
Against concentration gradient: Moves substances from low to high concentration
Uses pump proteins: Specialized carrier proteins with ATP-binding sites
Highly selective: Each pump is specific for certain ions/molecules
Creates concentration gradients: Maintains different concentrations inside vs. outside cell
Essential for cell function: Required for maintaining ion balance, nutrient uptake, waste removal

⚡ The Sodium-Potassium Pump (Na⁺/K⁺-ATPase)

The sodium-potassium pump is the most important example of active transport in animal cells. It is found in the plasma membrane of virtually all animal cells and is critical for maintaining cell function.

What It Does:

Pumps 3 Na⁺ ions OUT + Pumps 2 K⁺ ions IN

For each ATP molecule hydrolyzed

Detailed Mechanism:

Step 1: Binding of Na⁺

3 sodium ions (Na⁺) from the cytoplasm bind to specific sites on the pump protein

Step 2: ATP Hydrolysis & Phosphorylation

ATP binds to the pump and is hydrolyzed to ADP + Pi. The phosphate group attaches to the pump protein (phosphorylation)

Step 3: Conformational Change

Phosphorylation causes the pump to change shape, opening toward the extracellular side

Step 4: Release of Na⁺

3 Na⁺ ions are released to the extracellular space (outside the cell)

Step 5: Binding of K⁺

2 potassium ions (K⁺) from outside the cell bind to the pump

Step 6: Dephosphorylation

K⁺ binding triggers release of the phosphate group (dephosphorylation)

Step 7: Return to Original Shape

Pump returns to original conformation, opening toward the cytoplasm

Step 8: Release of K⁺

2 K⁺ ions are released into the cytoplasm (inside the cell). Cycle repeats.

Functions of the Na⁺/K⁺ Pump

1. Maintains Concentration Gradients

Keeps Na⁺ low inside, high outside; K⁺ high inside, low outside. These gradients are essential for many cellular processes.

2. Maintains Cell Volume (Osmoregulation)

By controlling ion concentrations, the pump helps regulate water balance and prevents cells from swelling or shrinking excessively.

3. Establishes Membrane Potential

Creates an electrical gradient (more negative inside). Essential for nerve impulse transmission and muscle contraction.

4. Drives Secondary Active Transport

The Na⁺ gradient created by the pump is used to transport other substances (glucose, amino acids) via cotransport.

5. Critical for Nerve and Muscle Function

The gradients established by the pump are essential for action potentials in neurons and contraction in muscle cells.

⚡ Energy Cost:

The Na⁺/K⁺ pump consumes approximately 30% of a cell's ATP at rest, and up to 70% in nerve cells. This shows how crucial it is for cell function!

📊 Comparison of Membrane Transport Methods

Feature	Simple Diffusion	Facilitated Diffusion	Active Transport
Direction	Down gradient (high → low)	Down gradient (high → low)	Against gradient (low → high)
Energy Required	No (passive)	No (passive)	Yes (requires ATP)
Proteins Involved	None	Channel or carrier proteins	Pump proteins
Selectivity	Not selective (based on size/polarity only)	Highly selective (specific proteins)	Highly selective (specific pumps)
What Crosses	Small non-polar molecules (O₂, CO₂)	Large polar molecules, ions	Ions, molecules against gradient
Examples	O₂ entering cells, CO₂ leaving cells	Glucose uptake, ion channels	Na⁺/K⁺ pump, Ca²⁺ pump
Saturation	No (rate increases linearly)	Yes (limited by protein number)	Yes (limited by pump capacity & ATP)

🎯 Key Concepts Summary

✓ Fluid Mosaic Model

Cell membranes consist of a phospholipid bilayer with embedded proteins. The membrane is fluid (components can move) and mosaic (diverse components create patterns).

✓ Selective Permeability

Membranes allow small non-polar molecules to pass easily, but restrict large/polar molecules and ions, which require transport proteins. This enables cells to control their internal environment.

✓ Membrane Proteins: JET RAT

Integral and peripheral proteins perform six main functions: Junctions, Enzymes, Transport, Recognition, Anchorage, and Transduction (signal reception).

✓ Passive Transport

Simple diffusion and facilitated diffusion (via channels/carriers) move substances down concentration gradients without ATP. Osmosis is the diffusion of water across selectively permeable membranes.

✓ Active Transport

Pump proteins use ATP to move substances against concentration gradients (low → high). The Na⁺/K⁺ pump exchanges 3 Na⁺ out for 2 K⁺ in, maintaining vital concentration gradients.

✓ Osmotic Solutions

Isotonic (no net movement), Hypotonic (water enters cell, swelling/lysis), Hypertonic (water leaves cell, shrinking/crenation). Plant cells have cell walls preventing bursting in hypotonic solutions.

📚 About the Author

Adam

Co-Founder @RevisionTown

Adam is a dedicated mathematics and science educator with extensive experience in international curricula. As Co-Founder of RevisionTown, he is committed to creating comprehensive, high-quality educational resources that help students excel in their studies.

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