Membrane Transport and Cell Signaling – Study Notes (Campbell Biology in Focus, Chapter 5)

Membrane Structure and Function

Overview of Cellular Membranes

Cellular membranes are dynamic structures essential for maintaining the internal environment of the cell and mediating communication with the external environment. The plasma membrane, in particular, regulates the movement of substances into and out of the cell and plays a key role in cell signaling.

• Plasma Membrane: The boundary that separates the living cell from its surroundings.

• Selective Permeability: The property that allows some substances to cross more easily than others.

• Key Components: Lipids (mainly phospholipids), proteins, and carbohydrates.

Phospholipids and the Fluid Mosaic Model

The structure of the plasma membrane is described by the fluid mosaic model, which depicts the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

• Phospholipids: Amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Bilayer Formation: Phospholipids arrange themselves into a bilayer, creating a stable boundary between two aqueous compartments.

• Fluid Mosaic Model: Membrane proteins are dispersed and individually inserted into the lipid bilayer, creating a mosaic pattern.

Membrane Fluidity

Membrane fluidity is crucial for proper membrane function and is influenced by lipid composition and temperature.

• Hydrophobic Interactions: Hold the membrane together but are weaker than covalent bonds, allowing lateral movement of lipids and proteins.

• Unsaturated Hydrocarbon Tails: Increase fluidity by preventing tight packing of phospholipids.

• Cholesterol: Acts as a fluidity buffer, restraining movement at high temperatures and preventing tight packing at low temperatures.

Membrane Proteins and Their Functions

Proteins embedded in the membrane perform most of its functions. They can be classified as integral or peripheral proteins.

• Integral Proteins: Penetrate the hydrophobic core; many are transmembrane proteins that span the membrane.

• Peripheral Proteins: Loosely bound to the membrane surface.

• Major Functions:

  • Transport

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to the cytoskeleton and extracellular matrix (ECM)

Membrane Carbohydrates and Cell-Cell Recognition

Carbohydrates attached to lipids (glycolipids) or proteins (glycoproteins) on the extracellular surface of the membrane are involved in cell-cell recognition.

• Glycoproteins and Glycolipids: Serve as identification tags recognized by other cells.

• Variation: The composition of membrane carbohydrates varies among species, individuals, and cell types.

Synthesis and Sidedness of Membranes

Membranes have distinct inside and outside faces, established during their synthesis in the endoplasmic reticulum (ER) and Golgi apparatus.

• Asymmetry: The arrangement of proteins, lipids, and carbohydrates is determined during membrane assembly and modification.

Membrane Transport

Selective Permeability

The plasma membrane allows some substances to cross more easily than others, maintaining the cell's internal environment.

• Hydrophobic (Nonpolar) Molecules: Such as hydrocarbons, can dissolve in the lipid bilayer and pass through rapidly.

• Polar Molecules: Such as sugars and ions, do not cross the membrane easily.

Transport Proteins

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

• Channel Proteins: Provide hydrophilic channels for specific molecules or ions (e.g., aquaporins for water).

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

• Specificity: Each transport protein is specific for the substance it transports.

Passive Transport: Diffusion and Osmosis

Passive transport is the movement of substances across the membrane without energy investment.

• Diffusion: The tendency of molecules to spread out evenly; substances move down their concentration gradient.

• Osmosis: The diffusion of free water across a selectively permeable membrane from lower to higher solute concentration.

Tonicity and Water Balance

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

Solution Type	Solute Concentration	Effect on Animal Cell	Effect on Plant Cell
Isotonic	Same as inside cell	Normal	Flaccid
Hypertonic	Greater than inside cell	Cell shrivels	Plasmolyzed
Hypotonic	Less than inside cell	Cell lyses (bursts)	Turgid (normal)

• Osmoregulation: The control of solute concentrations and water balance, necessary for life in varying environments.

• Example: Paramecium caudatum uses a contractile vacuole to pump excess water out.

Facilitated Diffusion

Facilitated diffusion is a type of passive transport aided by proteins, allowing specific molecules to cross the membrane more efficiently.

• Channel Proteins: Provide corridors for specific molecules or ions (e.g., aquaporins, ion channels).

• Gated Channels: Open or close in response to stimuli.

• Carrier Proteins: Undergo shape changes to move solutes across the membrane.

• No energy input is required.

Active Transport

Active transport moves substances against their concentration gradients and requires energy, usually from ATP.

• Allows cells to maintain internal concentrations of small molecules that differ from those in their environment.

• Sodium-Potassium Pump: Exchanges Na+ for K+ across the plasma membrane in animal cells.

Equation for Active Transport (Sodium-Potassium Pump):

$3\,\mathrm{Na}^+_{\text{(in)}} + 2\,\mathrm{K}^+_{\text{(out)}} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na}^+_{\text{(out)}} + 2\,\mathrm{K}^+_{\text{(in)}} + \mathrm{ADP} + \mathrm{P}_i$

Bulk Transport: Exocytosis and Endocytosis

Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles, a process that requires energy.

• Exocytosis: Transport vesicles fuse with the membrane, releasing their contents outside the cell.

• Endocytosis: The cell takes in molecules by forming vesicles from the plasma membrane. Types include:

  • Phagocytosis: "Cellular eating"; cell engulfs large particles.

  • Pinocytosis: "Cellular drinking"; cell engulfs extracellular fluid.

  • Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors.

• Example: Human cells use receptor-mediated endocytosis to take in cholesterol via low-density lipoproteins (LDLs).

Cell Signaling

Overview of Cell Communication

Cell-to-cell communication is essential for the coordination of activities in multicellular organisms. The plasma membrane plays a central role in this process.

• Local Signaling: Includes direct contact (gap junctions in animals, plasmodesmata in plants) and local regulators (paracrine and synaptic signaling).

• Long-Distance Signaling: Involves hormones (endocrine signaling) that travel through the circulatory system.

Types of Cell Signaling

Type	Description	Example
Paracrine	Local regulators affect nearby cells	Growth factors
Synaptic	Neurotransmitters cross synapses	Nerve cell signaling
Endocrine	Hormones travel via bloodstream	Insulin signaling

Three Stages of Cell Signaling

Cell signaling typically involves three main stages:

1. Reception: The target cell detects a signaling molecule (ligand) that binds to a receptor protein on the cell surface.

2. Transduction: The binding of the ligand changes the receptor protein in some way, initiating a signal transduction pathway.

3. Response: The transduced signal triggers a specific cellular response.

• Ligand: A molecule that specifically binds to another molecule, often a receptor.

• Signal Transduction Pathway: A series of steps by which a signal on a cell's surface is converted into a specific cellular response.

Reception: Binding of a Signal Molecule

The interaction between a ligand and its receptor is highly specific and often causes a conformational change in the receptor, activating it. Most signal receptors are plasma membrane proteins.