Membrane Structure and Function Study Guide

Membrane Structure and Function

Membrane Composition

Phospholipids: The primary component of animal cell membranes, forming a bilayer.
- Polar End: Contains a phosphate group, which is hydrophilic (water-loving). Water molecules are polar due to their oxygen-hydrogen bonds, allowing them to form hydrogen bonds with the phosphate group, enabling interaction with the aqueous cellular environment.
- Nonpolar End: Called the "tail" and is hydrophobic (water-hating).
Cholesterol: Acts as a "buffer" to maintain optimal membrane fluidity.
- Function: Prevents excessive fluidity at high temperatures and rigidity at low temperatures. It is embedded within the phospholipid bilayer, regulating fluidity by preventing phospholipid tails from packing too tightly, maintaining membrane stability across different temperatures.
Identity Markers: Often composed of carbohydrate chains attached to proteins (glycoproteins) or lipids (glycolipids).
- Function: Crucial for cell recognition and play a vital role in the immune response by allowing the body to distinguish between "self" cells and foreign invaders. They act as molecular ID badges for cells.
Membrane Proteins: Can be integral (embedded in the membrane) or peripheral (on the surface), each serving distinct roles.
- Transmembrane Proteins: Play roles in transport and signaling. They are embedded within the lipid bilayer and are normally not soluble in water, as they contain long stretches of nonpolar amino acids that are hydrophobic.
  - Example: GABA receptors function as gated ion channels, allowing ions to flow in response to signals.

Membrane Components and Their Functions

Phospholipid Bilayer
- Composition: Phospholipid molecules.
- Function: Provides a permeability barrier and matrix for proteins.
- How it works: Excludes water-soluble molecules from the nonpolar interior of the bilayer and cell. The bilayer is impermeable to large water-soluble molecules.
- Example: Impermeable to glucose.
Transmembrane Proteins
- Carriers
  - Function: Actively or passively transport molecules across the membrane.
  - How it works: Move specific molecules through the membrane via a series of conformational changes.
  - Examples: Glycophorin carrier for sugar transport; sodium–potassium pump.
- Channels
  - Function: Passively transport molecules across the membrane.
  - How it works: Create a selective tunnel that acts as a passage through the membrane.
  - Examples: Sodium and potassium channels in nerve, heart, and muscle cells.
- Receptors
  - Function: Transmit information into the cell.
  - How it works: Signal molecules bind to the cell-surface portion of the receptor protein, altering the portion of the receptor protein within the cell and inducing activity.
  - Examples: Specific receptors bind peptide hormones and neurotransmitters.
Interior Protein Network
- Spectrins
  - Function: Determine the shape of the cell.
  - How it works: Form a supporting scaffold beneath the membrane, anchored to both the membrane and the cytoskeleton.
  - Example: Red Blood Cells (RBCs).
- Clathrins
  - Function: Anchor certain proteins to specific sites, especially on the exterior plasma membrane in receptor-mediated endocytosis.
  - How it works: Proteins line coated pits and facilitate binding to specific molecules.
  - Example: Localization of low-density lipoprotein receptor within coated pits.
Cell-Surface Markers
- Glycoproteins
  - Function: "Self" recognition.
  - How it works: Create a protein/carbohydrate chain shape characteristic of an individual.
  - Example: Major histocompatibility complex protein recognized by the immune system.
- Glycolipid
  - Function: Tissue recognition.
  - How it works: Create a lipid/carbohydrate chain shape characteristic of a tissue.
  - Example: A, B, O blood group markers.

Amino Acid Properties in Membrane Domains

Polar (hydrophilic) and Nonpolar (hydrophobic) Amino Acids: Play crucial roles in membrane protein structure.
- Polar Amino Acids: Have hydrophilic side chains and tend to be located on the surface of proteins exposed to the aqueous environment (e.g., cytoplasm).
- Nonpolar Amino Acids: Have hydrophobic side chains and are typically found within the membrane, where they interact with the lipid bilayer.

Membrane Dynamics

Fluid Mosaic Model: Proposed by Singer and Nicolson in 1972. Described the cell membrane as a phospholipid bilayer with proteins embedded. Emphasizes the fluidity of the membrane, allowing lateral movement of phospholipids.
Membrane Fluidity Influences: Influenced by temperature, cholesterol content, and fatty acid composition.
- Case Study: Cyclodextrins: Can disrupt membrane cholesterol, affecting protein transport and membrane fluidity.
- Temperature: Higher temperatures increase fluidity.
- Fatty Acids: Unsaturated fatty acids enhance membrane flexibility (contain C=C double bonds causing kinking in the fatty acid tails).
- Low Temperatures: Incubation at low temperatures can create rigidity within the Golgi membrane and block protein release from the Golgi apparatus, affecting secretion via exocytosis.

Membrane Proteins and Their General Functions

Transport Proteins: Facilitate the movement of substances across the membrane.
Receptor Proteins: Receive signals from the environment, triggering cellular responses.
Membrane Proteins: Involved in transport, communication, and structural support.
Enzymatic Activity: They can act as enzymes, catalyzing reactions at the membrane surface.

Passive Transport

Definition: Includes diffusion and facilitated diffusion, which do not require energy.
Simple Diffusion
- Mechanism: Movement of substances down their concentration gradients without energy input, from areas of high concentration to low concentration until equilibrium is reached.
- Rate Influences: The rate of diffusion is influenced by the size of the concentration difference, membrane permeability, and temperature (higher temperatures generally lead to faster diffusion rates).
Facilitated Diffusion
- Mechanism: Involves carrier proteins and is specific but does not require energy. Allows molecules to move across membranes via specific protein channels or carriers. Movement is always along (or down) their concentration gradient, from high to low concentration.
- Carrier Specificity: Carrier molecules must be specific to the transported molecule, ensuring selective permeability.
- Saturation Point: A saturation point can be reached if there are not enough carrier proteins to bring in solute.
- Example: Glucose Transport: Glucose transport into cells via GLUT transporters, which are specific for glucose.
  - Insulin's Role: Insulin increases the number of glucose transporter proteins (GLUT4) on cell membranes, enhancing glucose uptake, especially in muscle and fat cells.
  - Low Insulin: Low insulin levels can decrease the number of available transporters, affecting glucose homeostasis.
  - Diabetes: In diabetes, GLUT transporters can become saturated, leading to glucose excretion in urine (in untreated diabetes, high blood glucose levels cause glucose in urine due to transporter saturation).
Osmosis (Water Diffusion)
- Mechanism: The diffusion of water across a semipermeable membrane, influenced by solute concentration.
- Aquaporins: Water can also move through aquaporins, specialized protein channels in the membrane.

Active Transport

Definition: Requires energy (ATP) to move substances against (or up) their concentration gradient.
Carrier Proteins: Essential for this process, utilizing ATP to function.
Sodium-Potassium Pump (Na^+/K^+ pump)
- Mechanism: A protein embedded in the cell membrane that actively transports sodium ions out of the cell and potassium ions into the cell, utilizing energy from ATP.
- Function: Crucial for maintaining the proper ion balance necessary for nerve impulse transmission (action potentials) in neurons by establishing a sodium concentration gradient across the membrane.
- Conformational Change: When sodium ions bind to the pump on the intracellular side, it triggers a conformational change, allowing the pump to release 3 sodium ions (3 Na^+) outside the cell and take in 2 potassium ions (2 K^+) from the extracellular fluid into the cell.

Bulk Transport

Definition: Processes for the bulk transport of materials into and out of cells (endocytosis and exocytosis).
Energy Requirement: Both endocytosis and exocytosis require energy as they are active transport mechanisms.
Phagocytosis ("Cell Eating")
- Mechanism: The uptake of large particles.
- Performed by: Primarily by specialized immune cells like macrophages and neutrophils.
Pinocytosis ("Cell Drinking")
- Mechanism: Involves the ingestion of fluids and small molecules.
Receptor-Mediated Endocytosis
- Definition: An active process that requires energy and allows cells to take in specific molecules bound to receptors.
- Mechanism: Involves clathrin-coated vesicles.
- Substances Brought In: Nutrients (e.g., LDL and transferrin), hormones (e.g., insulin), cholesterol uptake (helps create cell membranes and steroid hormones), certain viruses, and toxins.

Osmosis and Concentration Gradients (Tonicity)

Water Movement Direction: Determined by the concentration of solutes on either side of the semi-permeable membrane. Water moves to dilute, meaning it will move to areas where the solute concentration is higher until equilibrium is reached.
Tonicity: Describes the relative concentration of solutes in solutions, affecting cell volume.
- Isotonic Solutions: Equal solute concentration on both sides of the membrane; maintain cell volume and are ideal for cell health.
- Hypertonic (Hyperosmotic) Solutions: Higher solute concentration outside the cell; lead to cell shrinkage.
- Hypotonic (Hypoosmotic) Solutions: Lower solute concentration outside the cell; cause cells to swell.
  - Example: Red blood cells lyse in hypoosmotic solutions due to water influx.
  - Plant Cells: When a plant cell is in a hypotonic solution, water moves into the cell via osmosis, causing it to swell and exert pressure against the cell wall, known as turgor pressure. The cell wall in bacteria, fungi, and plants helps counter the high osmotic pressure generated inside the cell.

Summary of Membrane Transport Processes

Passive Processes

Diffusion (Direct or Simple)
- How it works: Random molecular motion produces net migration of nonpolar molecules toward a region of lower concentration.
- Example: Movement of oxygen into cells.
Facilitated Diffusion
- Protein Channel
  - How it works: Polar molecules or ions move through a protein channel; net movement is toward a region of lower concentration.
  - Example: Movement of ions in or out of cell.
- Protein Carrier
  - How it works: Molecule binds to a carrier protein in the membrane and is transported across; net movement is toward a region of lower concentration.
  - Example: Movement of glucose into cells.
Osmosis (Aquaporin)
- How it works: Diffusion of water across the membrane via osmosis; requires an osmotic gradient.
- Example: Movement of water into cells placed in a hypotonic solution.

Active Processes

Active Transport
- Protein Carrier (Na^+/K^+ pump)
  - How it works: Carrier uses energy to move a substance across a membrane against its concentration gradient.
  - Example: Na^+ and K^+ against their concentration gradients.
- Coupled Transport
  - How it works: Molecules are transported across a membrane against their concentration gradients by the cotransport of sodium ions or protons down their concentration gradients.
  - Example: Coupled uptake of glucose into cells against its concentration gradient using a Na^+ gradient.
Endocytosis
- Membrane Vesicle (Phagocytosis)
  - How it works: Particle is engulfed by the membrane, which folds around it and forms a vesicle.
  - Example: Ingestion of bacteria by white blood cell ("cell eating").
- Membrane Vesicle (Pinocytosis)
  - How it works: Fluid droplets are engulfed by the membrane, which forms vesicles around them.
  - Example: "Nursing" of human egg cells ("cell drinking").
- Receptor-Mediated Endocytosis
  - How it works: Endocytosis triggered by a specific receptor, forming clathrin-coated vesicles.
  - Example: Cholesterol uptake.
Exocytosis
- Membrane Vesicle
  - How it works: Vesicles fuse with the plasma membrane and eject contents.
  - Examples: Secretion of mucus; release of neurotransmitters.