Module 2: Cell Membrane
Learning Objectives
Describe the structure of cell membranes.
List components and describe their basic functions.
Describe the movement of molecules across a membrane.
Describe passive transport and provide examples.
The Plasma Membrane: A Fluid Mosaic
The plasma membrane is a fundamental, dynamic structure common to ALL cells, both prokaryotic and eukaryotic.
Key Functions of the Plasma Membrane:
Boundary Definition: Separates the internal cytoplasm (intracellular environment) from the external environment (extracellular environment) of the cell, maintaining cellular integrity.
Compartmentalization: Allows incompatible chemical reactions to occur simultaneously within different regions of the cell (via internal membranes in eukaryotes) or within the cell vs. its surroundings.
Selective Permeability: Regulates the passage of substances into and out of the cell, allowing necessary nutrients to enter and waste products to exit, while preventing entry of harmful substances.
Cell Communication: Contains receptors that bind to signaling molecules, enabling the cell to receive and respond to information from its environment.
Cell Adhesion: Mediates cell-to-cell contact and attachment to the extracellular matrix, crucial for tissue formation and stability.
The plasma membrane is described as a fluid mosaic model, a concept developed by S.J. Singer and G.L. Nicolson in 1972. This model likens its dynamic structure to a mosaic made of various components (lipids, proteins, carbohydrates) that are able to move and shift within the membrane. The term "fluid" refers to the constant movement and flexibility of the lipid bilayer and its embedded components, while "mosaic" describes the patchwork arrangement of proteins and other molecules.
Components of the Plasma Membrane
Lipid Component (Phospholipid Bilayer):
Phospholipids are the primary constituents, forming the fundamental bilayer structure. Each phospholipid molecule is amphipathic, possessing both hydrophobic and hydrophilic regions.
Hydrophilic heads (water-loving) contain a phosphate group and face the aqueous environments at both surfaces of the membrane: the internal cytoplasm and the external extracellular fluid.
Hydrophobic tails (water-fearing) consist of two fatty acid chains and make up the nonpolar interior of the membrane, creating a significant barrier to the passage of water-soluble (polar) substances and ions.
The phospholipid bilayer exhibits fluidity, allowing phospholipids to move laterally, rotate, and flex their tails. However, "flip-flop" (transverse diffusion) between layers is rare due to the energy barrier.
Membrane Proteins: These diverse proteins perform most of the membrane's specific functions. They can be classified based on their association with the lipid bilayer:
Integral proteins: Firmly embedded in the membrane.
Transmembrane proteins: Span the entire lipid bilayer, with portions exposed on both the extracellular and cytoplasmic sides. Many function as channels, carriers, receptors, or enzymes.
Lipid-anchored proteins: Covalently attached to a lipid molecule (e.g., a fatty acid chain) that inserts into the lipid bilayer, holding the protein to the membrane surface.
Peripheral proteins: Loosely associated with the membrane surface, often attached to integral proteins or the polar heads of phospholipids via non-covalent interactions. They are usually found on the inner (cytoplasmic) membrane surface and can be easily removed without disrupting the membrane.
Cholesterol: Primarily found in animal cell membranes (absent in most plant and bacterial cells).
Affects the fluidity and stability of the membrane.
Acts as a fluidity buffer:
At moderate (body) temperatures, cholesterol reduces membrane fluidity by restricting the movement of phospholipids, making the membrane less permeable.
At low temperatures, it hinders solidification by disrupting the tight packing of phospholipid tails, preventing the membrane from becoming too rigid.
Carbohydrate Chains: Always found on the exterior surface of the plasma membrane, extending into the extracellular space.
Form glycolipids (carbohydrate attached to a lipid) and glycoproteins (carbohydrate attached to a protein).
These carbohydrate components collectively form the glycocalyx, a sugar coat on the cell surface.
Play crucial roles in:
Cell recognition: Essential for distinguishing "self" from "non-self" cells (e.g., in immune responses, tissue typing, and blood groups).
Cell adhesion: Helping cells to stick together to form tissues.
Protective barrier: Protecting the cell from mechanical damage.
Types of Membrane Proteins and Their Functions
Channel Proteins:
Function: Provide a hydrophilic channel or pore through the membrane, selectively allowing a particular molecule or ion to cross the plasma membrane freely and rapidly. Many are gated, meaning they can open or close in response to specific signals (e.g., voltage changes, ligand binding).
Examples:
In cystic fibrosis, an inherited disorder, a faulty chloride ( \text{Cl}^- ) channel (CFTR protein) causes thick, sticky mucus to collect in airways and in pancreatic and liver ducts, severely impairing organ function.
Aquaporins are channel proteins specific for water molecules.
Carrier Proteins:
Function: Selectively bind to a specific molecule or ion on one side of the membrane, undergo a conformational change, and then release the molecule on the other side. This process can facilitate both passive transport (down a concentration gradient) and active transport (against a concentration gradient, requiring energy).
Example: The family of GLUT carriers (Glucose Transporters) facilitates the diffusion of glucose into and out of various cell types. An inability to use energy efficiently for the sodium-potassium ( \text{Na}^+ -- \text{K}^+ ) pump (which is also a carrier protein) has been suggested as a cause of obesity in some individuals due to its role in maintaining membrane potential and cellular volume.
Cell Recognition Proteins:
Function: Glycoproteins (proteins with attached carbohydrate chains) that serve as identification tags. They help the body recognize "self" cells and distinguish them from foreign cells or cancerous cells. This is critical for immune responses and tissue compatibility. For example, the major histocompatibility complex (MHC) proteins on cell surfaces are a type of cell recognition protein.
Receptor Proteins:
Function: Bind to specific signaling molecules (ligands) from outside the cell (e.g., hormones, neurotransmitters). This binding triggers a specific response or chain of events inside the cell, allowing cells to communicate and coordinate activities.
Example: Insulin receptors on muscle and liver cells bind insulin, signaling the cells to take up glucose from the blood.
Enzymatic Proteins:
Function: Carry out metabolic reactions directly on the plasma membrane or act as catalysts for specific biochemical reactions.
Example: Adenylate cyclase, an enzyme embedded in the plasma membrane, converts ATP to cyclic AMP (cAMP), an important secondary messenger in cell signaling.
Junction Proteins:
Function: Involved in forming various types of intercellular junctions (e.g., tight junctions, gap junctions, desmosomes) that physically connect adjacent cells. These connections allow for cell adhesion and direct cell-to-cell communication.
Example: Cadherins are a class of transmembrane proteins that play a vital role in cell adhesion, particularly in adherens junctions and desmosomes.
Attachment Proteins:
Function: Help anchor the plasma membrane to the cytoskeleton within the cell (maintaining cell shape) and to the extracellular matrix (providing structural stability to tissues).
Example: Integrins are transmembrane proteins that link the cell's cytoskeleton to elements of the extracellular matrix, playing roles in cell signaling, migration, and adhesion.