Biological membranes (2.2, 2.3, 2.4)
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Membranes are partially permeable barriers with the same basic structure, regardless of their location.
At the cell surface, the cell surface membrane separates the cell and its environment and regulates the exchange of ions, nutrients, and waste, maintaining homeostasis.
Between membrane–bound organelles and cytoplasm, the membrane separates organelle contents from the cytoplasm, ensuring compartmentalisation and facilitating specialised functions.
Within organelles, the membrane creates distinct compartments, such as the inner mitochondrial membrane, which is essential for processes such as cellular respiration.
Partially permeable membranes allow certain substances to pass through, but restrict or prevent the movement of others.
Cell surface membranes are partially permeable membranes.
Often, small molecules, such as and can diffuse across cell membranes, whereas larger or charged molecules require transport proteins.
Membranes can function as sites of chemical reactions.
Membranes provide a surface for enzymes to bind to and interact with substrates. Membrane organisation supports efficient, sequential enzyme activity in metabolic processes.
Examples:
Respiration: ATP-producing enzymes are embedded in the inner mitochondrial membrane.
Photosynthesis: In chloroplasts, thylakoid membranes house enzymes for light-dependent reactions.
Membranes can function as sites of cell communication and cell recognition.
Receptor proteins bind to signalling molecules such as hormones (e.g. insulin) and neurotransmitters to trigger intracellular responses via signal transduction pathways.
The carbohydrate chains attached to glycoproteins and glycolipids facilitate cell recognition and aid in immune responses and tissue formation.
In 1935, Davson and Danielli, developed the Davson–Danielli model to describe the cell membrane as a phospholipid bilayer sandwiched between two continuous protein layers (giving a ‘protein–lipid–protein’ structure).
The protein layers in the model coated both membrane surfaces, providing stability and resulting in partial permeability by acting as barriers or channels for molecule movement.
Cell membranes consists of 4 main components:
- A phospholipid bilayer, with hydrophilic phosphate heads facing outward and hydrophobic fatty acid tails inward, creating a partially permeable barrier
- Embedded intrinsic (channel and transport) proteins spanning both layers of the membrane and extrinsic proteins in one side of the membrane, facilitating transport, cell signalling, and structural support
- Glycoproteins and glycolipids on the extracellular surface, involved in cell recognition and communication
- Cholesterol interspersed within the bilayer, regulating membrane fluidity and stability, particularly in response to temperature fluctuations.
In 1935, Davson and Danielli, developed the Davson–Danielli model to describe the cell membrane as a phospholipid bilayer sandwiched between two continuous protein layers (giving a ‘protein–lipid–protein’ structure).
The protein layers in the model coated both membrane surfaces, providing stability and resulting in partial permeability by acting as barriers or channels for molecule movement.
The Davson–Danielli model has limitations:
- Electron microscopy showed that proteins were scattered, and did not form continuous layers.
- The model failed to explain the dynamic nature of membranes, which show fluidity and lateral movement of molecules.
- The discovery of intrinsic proteins (sometimes called integral proteins) spanning the bilayer contradicted the idea of surface protein layers.
Following advancements in electron microscopy and membrane studies, Singer and Nicolson proposed the fluid mosaic model of cell membrane structure in 1972. This model replaced the Davson–Danielli model.
The model describes the cell membrane as having a dynamic structure formed from a mosaic of proteins, lipids, and carbohydrates.
This structure allows fluid movement of molecules, such as phospholipids and proteins, within the lipid bilayer, and also enables self-repair.
In the fluid mosaic model, cell membranes are made up of multiple components, each with specific functions.
Phospholipids are the main component of membranes. They are amphipathic, consisting of a hydrophilic head and two hydrophobic tails.
In an aqueous environment, phospholipids arrange themselves into a bilayer with heads facing the extracellular and intracellular fluids and tails facing inwards, away from water.
The phospholipid bilayer acts as a partially permeable barrier, allowing only certain small and non-polar molecules to pass through freely, while larger and polar molecules require assistance from membrane proteins.
In the fluid mosaic model, cell membranes are made up of multiple components, each with specific functions.
Intrinsic membrane proteins (transmembrane proteins) span both layers of the phospholipid bilayer.
These proteins can be receptors (e.g., for hormones or neurotransmitters), enzymes (to catalyse reactions at the membrane surface), structural (e.g., involved in cell adhesion), electron carriers (e.g., in mitochondrial or thylakoid membranes), or involved in transport across membranes.
Intrinsic proteins involved in transport:
- Carrier proteins bind specific molecules and change shape to move them across a membrane.
- Channel proteins form pores that allow ions or water-soluble molecules to cross a membrane.
In the fluid mosaic model, cell membranes are made up of multiple components, each with specific functions.
Extrinsic membrane proteins are attached to one side of a membrane but can sometimes move between layers.
They are involved in signalling and maintaining cell shape and structure (e.g., connecting the membrane to the cytoskeleton).
In the fluid mosaic model, cell membranes are made up of multiple components, each with specific functions.
Cholesterol molecules are interspersed within the phospholipid bilayer and stabilise membrane fluidity and rigidity by:
- Making the membrane less permeable to small, water-soluble molecules that could otherwise pass through freely, thus preventing leakage from the cell
- Preventing the membrane from becoming too rigid at cold temperatures and too fluid at high temperatures
- Restricting the movement of other cell membrane components.
In the fluid mosaic model, cell membranes are made up of multiple components, each with specific functions.
Glycolipids are lipids (embedded in the membrane) with a carbohydrate chain attached (extending outwards into the extracellular space).
These molecules are involved in cell recognition (e.g., antigens), signalling, maintaining membrane stability, cell attachment, and the formation of tissues.
In the fluid mosaic model, cell membranes are made up of multiple components, each with specific functions.
Glycoproteins are intrinsic proteins with carbohydrate chains attached. They are crucial for cell adhesion, recognition, and immune responses.
Glycoproteins on the cell surface are involved in cell signalling as receptors for molecules, such as hormones, neurotransmitters, and drugs, which bind to trigger intracellular events (e.g., receptor for acetylcholine at nerve cell synapses).
Cell surface membranes control the movement of molecules into and out of cells, maintaining the internal environment and allowing communication with the external environment. There are two main categories of transport: passive transport, which does not require energy, and active transport, which does.
Diffusion and facilitated diffusion are the two main types of passive transport.
Diffusion is the movement of molecules directly through a phospholipid bilayer down their concentration gradient (high to low concentration) without requiring ATP. The process continues until there is equilibrium in molecule concentration between both sides of the membrane.
Molecules that can freely diffuse across membranes are typically small, non-polar, and hydrophobic (lipid soluble). Examples include gases such as and ), steroid hormones such as testosterone, and vitamins such as vitamin D.
Diffusion and facilitated diffusion are the two main types of passive transport.
Facilitated diffusion is the process by which molecules and ions cross membranes using transport proteins. Molecules move down their concentration gradient (high to low concentration) without using ATP.
Channel proteins form hydrophilic pores through which specific molecules/ions can diffuse across the membrane.
Carrier proteins undergo conformational changes once specific molecules/ions bind to them, which enables those molecules/ions to be transported from one side of a membrane to the other.
Examples of molecules transported by facilitated diffusion include large, polar molecules, such as glucose, and charged molecules/ions, such as and .
Active transport is the movement of ions/molecules against their concentration gradient (low to high concentration) using ATP as a source of energy and carrier proteins (which act as pumps).
- The ion/molecule to be transported binds to a receptor on the carrier protein.
- On the other side of the membrane, ATP also binds to the carrier protein.
- The ATP is hydrolysed to ADP + .
- The carrier protein undergoes a conformation change and opens, allowing the molecule/ion in.
- The molecule is released (and recombines with ADP to form ATP) and the protein returns to its original shape.
Example: -ATPase pumping out of and into neurons to restore and maintain the resting membrane potential.
Bulk transport is a special kind of active transport that moves material (large molecules or whole cells) that is too large to pass through the membrane via proteins.
In endocytosis, cells engulf external substances by invagination of the membrane moving them into the cell in vesicles. This requires ATP.
- Phagocytosis is where large, solid particles, such as bacteria, are engulfed.
- Pinocytosis is where the cell ingests extracellular fluid and its solutes.
Bulk transport is a special kind of active transport that moves material (large molecules or whole cells) that is too large to pass through the membrane via proteins.
In exocytosis, cells expel materials in vesicles that fuse with the plasma membrane and release their contents outside the cell. This requires ATP. The secretion of neurotransmitters, hormones, and digestive enzymes are examples.
Osmosis is the diffusion of water molecules across a partially permeable membrane. This process continues until equilibrium is reached. It does not require ATP.