Biological Membranes¶

Part of Module 2: Foundations in biology.

Membranes are the boundary systems of biology. They connect cell structure, transport, signalling, and practical investigation. A strong understanding of this topic explains not just what membranes are made of, but why that composition produces selective permeability, controlled movement, and intercellular communication.

Learning Objectives¶

The Fluid Mosaic Model¶

In 1972, the fluid mosaic model was proposed to describe the structure of cell membranes. The two words capture the essential features:

Fluid refers to the fact that phospholipid molecules are constantly moving laterally within the bilayer, so the membrane is not a rigid, fixed structure.

Mosaic refers to the varied mixture of proteins of different sizes and shapes embedded within the phospholipid bilayer — like tiles in a mosaic — alongside glycoproteins, glycolipids, and cholesterol.

This model replaced earlier static descriptions of membranes and better accounts for how membrane components can move, aggregate, and change over time.

Components of Cell Membranes¶

Phospholipid Bilayer¶

Each phospholipid molecule consists of a hydrophilic "head" (containing a phosphate group) and two hydrophobic fatty acid "tails." This dual nature causes phospholipids to spontaneously arrange into a bilayer in aqueous environments: the hydrophilic heads face outward toward the watery cytoplasm and extracellular fluid, while the hydrophobic tails face inward and are shielded from water.

This arrangement creates a hydrophobic core in the centre of the bilayer. As a result: - Water-soluble (polar) substances cannot pass directly through the hydrophobic centre. - Lipid-soluble (non-polar) substances can dissolve in the bilayer and pass directly through. Small, non-polar molecules such as oxygen and carbon dioxide cross by simple diffusion for this reason.

Cholesterol¶

Cholesterol molecules are interspersed between phospholipids in the bilayer. Each cholesterol molecule has a hydrophilic and a hydrophobic region: the hydrophobic regions bind to the fatty acid tails of phospholipids, causing them to pack more closely together. This reduces the fluidity of the cell membrane and increases its stability, making it less permeable to water-soluble substances. At physiological temperatures, cholesterol is essential for preventing the membrane from becoming either too fluid (and fragile) or too rigid.

Proteins: Intrinsic and Extrinsic¶

Proteins are described in two categories depending on their position in the bilayer:

Intrinsic (integral) proteins are embedded through both sides of the phospholipid bilayer (transmembrane proteins). These include: - Channel proteins, which form hydrophilic pores through which specific ions can move passively. - Carrier proteins, which transport specific larger molecules or ions across the membrane — either passively (facilitated diffusion) or actively (active transport using ATP).

Extrinsic (peripheral) proteins are present on only one side of the bilayer and are not embedded through it. They provide structural support to the membrane or are involved in cell signalling.

Glycoproteins and Glycolipids¶

Glycoproteins consist of intrinsic proteins with attached carbohydrate chains; glycolipids consist of lipids with attached carbohydrates. Both project from the outer surface of the membrane and are involved in three related functions:

• Cell adhesion: the attachment of cells to one another or to a substrate.
• Cell recognition: allowing cells to identify and distinguish themselves from other cells (including recognition of "self" versus "non-self" in the immune system).
• Cell signalling: communication between cells, including the binding of hormones, drugs, and neurotransmitters to receptor proteins.

Roles of Cell Membranes¶

There are two main types of cell membrane:

Cell-surface (plasma) membranes surround the cell, acting as a selectively permeable barrier between the cell contents and the external environment. They control which substances enter and leave.

Organelle membranes surround internal organelles such as mitochondria, chloroplasts, the nucleus, and the endoplasmic reticulum. They separate organelle contents from the cytoplasm, creating distinct compartments (compartmentalisation) in which specific biochemical reactions can occur under controlled conditions — such as the inner mitochondrial membrane being the site of the electron transport chain.

Both types are partially permeable: they allow some molecules to pass but exclude others.

Factors Affecting Membrane Structure and Permeability¶

Changes to the structure of cell membranes alter their permeability, allowing more (or fewer) substances to pass through.

Temperature¶

Temperature has predictable effects at different ranges:

Below 0 °C: Phospholipids have little kinetic energy and pack closely together, forming a rigid, almost impermeable membrane. The membrane may also rupture as ice crystals form inside the cell.

0–40 °C: As temperature increases, phospholipids gain kinetic energy and move faster, packing less closely. The membrane becomes progressively more fluid and permeable in a controlled, gradual way. This is the functional range for most membrane processes.

Above 40 °C: The phospholipid bilayer begins to break down. Channel and carrier proteins denature, meaning they can no longer control what enters or leaves the cell. Permeability increases markedly and uncontrollably — the membrane loses its selective function.

A practical investigation frequently associated with this topic involves beetroot discs: the pigment anthocyanin is normally retained inside the vacuole, but increasing temperature causes membranes to become more permeable, and pigment leaks out. The amount of pigment in the surrounding water (measured by colorimetry or absorbance) is a proxy for membrane permeability.

Solvents¶

Organic solvents such as ethanol dissolve the phospholipid component of membranes. When cells are placed in solvent, the phospholipids dissolve and the membrane becomes more fluid and disorganised, increasing permeability. Higher solvent concentration produces greater disruption. This is why alcohol-based solvents denature cell membranes and can kill cells at sufficient concentrations.

Movement Across Membranes¶

Simple Diffusion¶

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration — down a concentration gradient. It is a passive process requiring no energy. Particles continue to diffuse until equilibrium is reached (equal distribution), though individual molecules continue to move.

Simple diffusion across membranes is possible for molecules that are: - Small — can pass through gaps between phospholipids. - Non-polar — can dissolve in the hydrophobic core.

Oxygen and carbon dioxide are the key examples: both are small and non-polar and cross membranes by simple diffusion.

Factors affecting the rate of simple diffusion: - Temperature: higher temperatures give particles more kinetic energy, increasing diffusion rate. - Concentration gradient: steeper gradients produce faster diffusion. - Thickness of membrane: thinner exchange surfaces reduce the distance particles must travel. - Surface area: larger surface areas allow more particles to cross simultaneously.

Facilitated Diffusion¶

Large or polar molecules cannot pass through the hydrophobic bilayer core directly. They cross by facilitated diffusion — still passive, still down a concentration gradient, but via specific membrane proteins.

Channel proteins form hydrophilic pores through the membrane. They are highly specific, typically transporting one or two types of ion. Ions travel through the pore when the concentration gradient favours their movement.

Carrier proteins transport larger molecules. The process is: 1. A large molecule binds to the carrier protein on one side of the membrane. 2. This causes the carrier protein to change shape. 3. The molecule is released on the opposite side of the membrane. 4. The carrier protein returns to its original shape, ready for the next molecule.

Each carrier protein is highly specific, recognising only its particular substrate. The rate of facilitated diffusion can be increased by a greater number of channel or carrier proteins, as well as the same factors that apply to simple diffusion (temperature, concentration gradient, membrane thickness, surface area).

Active Transport¶

Active transport moves particles from an area of lower concentration to an area of higher concentration — against the concentration gradient. It requires energy from respiration in the form of ATP, and uses carrier proteins.

The mechanism: 1. The molecule or ion binds to the carrier protein. 2. ATP binds to the carrier protein. 3. Hydrolysis of ATP to ADP and inorganic phosphate (Pi) causes the carrier protein to change shape, moving the molecule against the gradient and releasing it on the other side. 4. The phosphate group is released from the protein, allowing it to return to its original shape and be used again.

Factors affecting the rate of active transport: - Temperature: higher temperatures increase kinetic energy and respiratory rate, providing more ATP; very high temperatures denature carrier proteins. - Thickness of membrane: thinner membranes reduce distance travelled. - Number of carrier proteins: more carriers allow faster transport. - Rate of respiration: more respiration produces more ATP, enabling more active transport.

Bulk Transport: Endocytosis and Exocytosis¶

Large molecules — such as proteins, hormones, or polysaccharides — are too large to pass through even carrier proteins. Cells move them in bulk via vesicle-based mechanisms, both of which require energy (ATP).

Endocytosis transports materials into cells. The cell-surface membrane engulfs the material, folding inward to form a vesicle that moves into the cytoplasm. There are two forms: - Phagocytosis: uptake of solid materials (e.g. bacteria engulfed by macrophages). - Pinocytosis: uptake of liquid materials.

Exocytosis transports materials out of cells. Vesicles (mostly produced by the Golgi apparatus, where proteins are packaged) move toward and fuse with the cell-surface membrane, releasing their contents outside the cell. This is how secretory cells release hormones, digestive enzymes, and neurotransmitters.

Osmosis and Water Potential¶

Water Potential¶

Water potential (symbol Ψ, pronounced "psi") is the pressure exerted by water molecules on the membrane or container surrounding a solution, measured in kilopascals (kPa). It reflects the tendency of water to move from one region to another.

Key points about water potential: - Pure water has a water potential of 0 kPa — this is the maximum possible value. - Adding solute lowers water potential, making it more negative. - High water potential = high water concentration = dilute solution (little solute). - Low water potential = low water concentration = concentrated solution (lots of solute).

The Process of Osmosis¶

Osmosis is the diffusion of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential. Water molecules are small enough to pass directly through the cell membrane. Net movement continues down the water potential gradient until equilibrium is reached (equal water potential on both sides).

The key distinction from general diffusion: osmosis refers specifically to water, and specifically across a partially permeable membrane.

Osmosis in Animal Cells¶

Animal cells lack a rigid cell wall, so their response to osmosis depends entirely on the water potential of the surrounding solution:

• Hypotonic solution (higher water potential than the cell): water enters the cell by osmosis. The cell swells and may burst (lyse or crenate in reverse — lysis).
• Isotonic solution (same water potential as the cell): no net movement of water. The cell remains unchanged in size.
• Hypertonic solution (lower water potential than the cell): water leaves the cell by osmosis. The cell shrinks (crenates).

Osmosis in Plant Cells¶

Plant cells have a rigid cell wall that prevents the cell from bursting, so their responses differ:

• Hypotonic solution: water enters the cell by osmosis. The vacuole expands and the membrane presses outward against the cell wall. The cell wall exerts a back-pressure (wall pressure). The cell becomes turgid — firm and swollen, but not burst.
• Isotonic solution: no net movement of water; the cell remains the same.
• Hypertonic solution: water leaves the cell by osmosis. The cell contents shrink away from the cell wall. The membrane pulls away from the wall — a state called plasmolysis. The cell is described as plasmolysed.

Factors affecting the rate of osmosis: - Temperature: higher temperatures increase kinetic energy of water molecules. - Water potential gradient: steeper gradients drive faster net movement. - Membrane thickness: thinner membranes reduce diffusion distance. - Surface area: larger surface areas allow more water to cross simultaneously.

Common Confusions¶

Facilitated diffusion does not require ATP. It uses proteins as a structural aid, but the movement itself is driven by the concentration gradient — no energy input is needed.

Active transport is not simply "faster diffusion." The critical distinction is direction: diffusion moves particles down a gradient; active transport moves particles against a gradient, which requires energy expenditure.

Osmosis is only about water. It is not a general term for substances moving through a membrane. The specific definition — net movement of water down a water potential gradient across a partially permeable membrane — must be used precisely.

Solute molecules do not move through the partially permeable membrane during osmosis; water moves toward the solute, not solute toward more dilute areas.

Key Terms¶

• Fluid mosaic model: the description of membranes as a dynamic bilayer of phospholipids with embedded proteins and other components, in which molecules can move laterally.
• Phospholipid bilayer: the two-layered core structure of a membrane, with hydrophilic heads facing outward and hydrophobic tails facing inward.
• Hydrophilic: water-attracting; describes the phosphate head of a phospholipid.
• Hydrophobic: water-repelling; describes the fatty acid tails of a phospholipid.
• Intrinsic (integral) protein: a membrane protein that spans the entire bilayer, including channel and carrier proteins.
• Extrinsic (peripheral) protein: a membrane protein present on only one side of the bilayer; involved in support or signalling.
• Channel protein: an intrinsic protein forming a hydrophilic pore through which specific ions pass by facilitated diffusion.
• Carrier protein: an intrinsic protein that changes shape to transport specific molecules across the membrane, used in both facilitated diffusion and active transport.
• Cholesterol: a lipid molecule interspersed in the bilayer that reduces fluidity and increases membrane stability.
• Glycoprotein: an intrinsic protein with attached carbohydrate chains, involved in cell adhesion, recognition, and signalling.
• Glycolipid: a lipid with attached carbohydrate, also involved in cell recognition and signalling.
• Partially permeable: allowing some substances to pass through but not others; a property of all biological membranes.
• Compartmentalisation: the division of the cell into distinct regions by organelle membranes, allowing different reactions to occur simultaneously.
• Simple diffusion: passive movement of small, non-polar molecules directly through the phospholipid bilayer, down a concentration gradient.
• Facilitated diffusion: passive movement of large or polar molecules through specific channel or carrier proteins, down a concentration gradient.
• Active transport: movement of particles against a concentration gradient using carrier proteins and ATP.
• Endocytosis: bulk uptake of material into a cell by the cell-surface membrane engulfing it to form a vesicle; includes phagocytosis (solids) and pinocytosis (liquids).
• Exocytosis: bulk release of material from a cell when vesicles fuse with the cell-surface membrane.
• Phagocytosis: endocytosis of solid material.
• Pinocytosis: endocytosis of liquid material.
• Water potential (Ψ): the pressure exerted by water molecules on their surrounding membrane or container; measured in kPa; pure water = 0 kPa, decreasing as solute concentration increases.
• Osmosis: the diffusion of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.
• Hypotonic solution: a solution with a higher water potential than the cell; water enters the cell.
• Isotonic solution: a solution with the same water potential as the cell; no net water movement.
• Hypertonic solution: a solution with a lower water potential than the cell; water leaves the cell.
• Turgid: the state of a plant cell that has taken up water by osmosis; the vacuole is full and the membrane presses against the cell wall.
• Plasmolysis: the state of a plant cell that has lost water by osmosis; the membrane has pulled away from the cell wall.
• Crenation: shrinkage of an animal cell due to water loss by osmosis in a hypertonic solution.

Connected Pages¶

• 2.1.1 Cell structure
• 2.1.4 Enzymes
• 2.1.6 Cell division, cell diversity and cellular organisation
• 3.1.1 Exchange surfaces