Membrane Structure and Function

The Plasma Membrane

The plasma membrane is a fundamental structure that separates the living cell from its external environment. It is selectively permeable, allowing certain substances to cross more easily than others.

• Phospholipids are the most abundant lipids in membranes and are amphipathic, containing both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions.

• The phospholipid bilayer forms a stable boundary between two aqueous compartments.

• Most membrane proteins are also amphipathic and reside within the bilayer, with hydrophilic portions exposed.

• The fluid mosaic model describes the membrane as a mosaic of proteins floating in a fluid bilayer of phospholipids.

• Specialized patches of proteins or lipids may form within the membrane.

Membrane Fluidity

Membrane fluidity is crucial for proper function and is influenced by temperature, lipid composition, and cholesterol content.

• Lipids and some proteins move laterally within the membrane; phospholipids move rapidly, proteins more slowly.

• As temperature decreases, membranes become less fluid and may solidify.

• The solidification temperature depends on lipid types; unsaturated lipids (with double bonds) increase fluidity by disrupting Van der Waals forces.

• Cholesterol modulates fluidity: at warm temperatures, it restrains phospholipid movement; at cool temperatures, it prevents tight packing.

Van der Waals Forces and Lipid Composition

Van der Waals forces are weak interactions that help stabilize membrane structure. Unsaturated lipids disrupt these forces, increasing membrane fluidity.

• Membrane lipid composition varies among species, reflecting adaptations to environmental conditions.

Membrane Proteins

Proteins embedded in the membrane determine its specific functions.

• Integral proteins penetrate the hydrophobic interior; most are transmembrane proteins.

• Hydrophobic regions of integral proteins are composed of nonpolar amino acids, often forming alpha helices.

• Peripheral proteins are loosely attached to the membrane surface.

Membrane Carbohydrates and Cell Recognition

Membrane carbohydrates play a key role in cell-cell recognition.

• Carbohydrates may be covalently bonded to lipids (glycolipids) or proteins (glycoproteins).

• Membranes have distinct inside and outside faces due to asymmetrical arrangement of proteins, lipids, and carbohydrates.

Membrane Transport

Selective Permeability

The plasma membrane regulates the transport of substances, allowing hydrophobic molecules to cross easily while restricting polar molecules.

• Hydrophobic molecules (e.g., hydrocarbons) dissolve in the lipid bilayer and cross easily.

• Polar molecules (e.g., sugars) and even water cross less easily.

Transport Proteins

Transport proteins facilitate the movement of hydrophilic substances across the membrane.

• Channel proteins provide hydrophilic tunnels for molecules or ions; aquaporins specifically facilitate water transport.

• Carrier proteins bind to molecules and change shape to shuttle them across; each is specific for its substrate.

Diffusion and Osmosis

Diffusion is the movement of molecules from areas of high concentration to low concentration, down a concentration gradient. Osmosis is the diffusion of water across a selectively permeable membrane.

• At dynamic equilibrium, molecules cross the membrane equally in both directions.

• Water moves from regions of lower solute concentration to higher solute concentration until equilibrium is reached.

Water Balance in Cells

Cells must maintain water balance to survive, especially in environments with varying solute concentrations.

• Tonicity describes the ability of a solution to cause a cell to gain or lose water.

• Isotonic solution: Solute concentration equal inside and outside; no net water movement.

• Hypertonic solution: Higher solute concentration outside; cell loses water.

• Hypotonic solution: Lower solute concentration outside; cell gains water.

Cell Signaling

Local Signaling

Cells communicate locally through direct contact or by secreting signaling molecules.

• Gap junctions (animal cells) and plasmodesmata (plant cells) allow direct cytoplasmic exchange.

• Local regulators, such as growth factors, stimulate nearby cells (paracrine signaling).

• Synaptic signaling in nerves involves neurotransmitter release across synapses.

Long-Distance Signaling

Hormones are used for long-distance communication in plants and animals.

• Endocrine signaling involves hormone release into the circulatory system.

• Hormones vary in size and shape.

Stages of Cell Signaling

Cell signaling involves three main stages: reception, transduction, and response.

• Reception: Detection of a signal by binding to a receptor protein.

• Transduction: Conversion of the signal to a cellular response, often via a signal transduction pathway.

• Response: Cellular activity triggered by the signal.

Reception: Membrane and Intracellular Receptors

Signal molecules (ligands) bind to highly specific receptor proteins, causing shape changes and activation.

• Most signal receptors are plasma membrane proteins.

• Two main types of membrane receptors: G protein-coupled receptors (GPCRs) and ligand-gated ion channels.

• GPCRs interact with G proteins that bind GTP; these pathways are diverse.

• Ligand-gated ion channels open to allow specific ions (e.g., Na+, Ca2+) through when a ligand binds.

• Intracellular receptors are found in the cytoplasm or nucleus; small or hydrophobic messengers (e.g., steroid hormones, nitric oxide) can activate them.

Transduction: Signal Cascades

Signal transduction often involves multiple steps, amplifying the signal and allowing regulation.

• Protein phosphorylation (by kinases) and dephosphorylation (by phosphatases) regulate protein activity.

• Kinases transfer phosphate groups from ATP to proteins.

• Phosphatases remove phosphate groups.

Second Messengers

Second messengers are small, nonprotein molecules or ions that relay signals inside the cell.

• cAMP (cyclic AMP) is a widely used second messenger.

• Adenylyl cyclase converts ATP to cAMP in response to extracellular signals.

Response: Cellular Activities

Signal transduction pathways ultimately regulate cellular activities, either in the cytoplasm or nucleus.

• Responses may include regulation of enzyme activity, opening of ion channels, or changes in gene expression.

Summary Table: Types of Membrane Proteins

Key Equations

• Diffusion: Movement down concentration gradient

• Osmosis: Water moves from low to high solute concentration

• cAMP Formation:

Takeaways

• Membranes are dynamic structures with selective permeability.

• Transport proteins facilitate movement of substances across membranes.

• Cell signaling involves local and long-distance communication, with three main stages: reception, transduction, and response.

• Second messengers and phosphorylation cascades amplify and regulate cellular responses.