Cell Membranes, Transport, and Enzyme Function: General Biology Study Notes

Cell Membranes and Cellular Transport

Functions of Plasma/Cellular Membranes

The plasma membrane is a fundamental structure in all cells, serving as a selective barrier and interface for communication and transport.

• Selective permeability: Regulates the entry and exit of substances.

• Compartmentalization: Separates internal cell environment from the external surroundings.

• Cell signaling: Contains receptors for communication with other cells.

• Structural support: Maintains cell shape and anchors cytoskeletal elements.

• Intercellular interactions: Facilitates cell adhesion and recognition.

Plasma Membrane Components

The structure and function of the plasma membrane are determined by its molecular components:

• Phospholipid bilayer: Forms the basic structural framework; amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Membrane proteins: Integral and peripheral proteins involved in transport, signaling, and structural support.

• Membrane carbohydrates: Glycolipids and glycoproteins function in cell recognition and signaling.

• Cholesterol: Modulates membrane fluidity and stability.

Functions of Plasma Membrane Proteins

Membrane proteins perform diverse roles essential for cell survival and function.

• Transport: Channels and carriers facilitate movement of molecules across the membrane.

• Enzymatic activity: Some proteins catalyze reactions at the membrane surface.

• Signal transduction: Receptors transmit signals from the external environment to the cell interior.

• Cell-cell recognition: Glycoproteins serve as identification tags.

• Intercellular joining: Proteins form junctions between adjacent cells.

• Attachment to cytoskeleton and extracellular matrix: Maintains cell shape and stabilizes membrane location.

Membrane Transport Mechanisms

Passive and Active Transport

Transport across membranes is classified as passive or active based on energy requirements.

• Passive transport: Movement of substances down their concentration gradient without energy input.

• Active transport: Movement against the concentration gradient, requiring energy (usually ATP).

Types of Passive Transport

• Simple diffusion: Direct movement of small, nonpolar molecules (e.g., O2, CO2).

• Facilitated diffusion: Movement via channel or carrier proteins (e.g., glucose, ions).

• Osmosis: Diffusion of water through a selectively permeable membrane.

Types of Active Transport

• Primary active transport: Direct use of ATP to move molecules (e.g., sodium-potassium pump).

• Secondary active transport: Uses energy from an electrochemical gradient established by primary transport.

Proteins involved: Channel proteins, carrier proteins, and pumps are involved in these transport processes.

Osmotic Balance (Osmoregulation)

Osmoregulation is the maintenance of water and solute balance in cells. In plants, the cell wall plays a key role in preventing excessive water uptake and maintaining turgor pressure.

• Animal cells: Use mechanisms like contractile vacuoles or ion pumps.

• Plant cells: Rigid cell wall prevents bursting; vacuole stores water.

Cell Signaling and Membrane Proteins

Role of Plasma Membrane Proteins in Cell Signaling

Membrane proteins are crucial for transmitting signals between cells and their environment.

• Ligand: A molecule that binds specifically to a receptor protein.

• Ligand-gated ion channel: A channel protein that opens in response to ligand binding, allowing ions to flow across the membrane. Important for rapid signaling in nerve and muscle cells.

• Intracellular receptors: Located inside the cell; respond to signals that cross the membrane (e.g., steroid hormones).

Energy and Metabolism

Concept of Energy and Heat Loss

Energy transformations in cells are governed by the laws of thermodynamics. Metabolism refers to all chemical reactions in a cell.

• Metabolism: The sum of all anabolic (building) and catabolic (breaking down) reactions.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy.

• Heat loss: Energy conversions are not 100% efficient; some energy is lost as heat.

Exergonic and Endergonic Reactions

Chemical reactions are classified by their energy changes.

• Exergonic reactions: Release free energy; spontaneous.

• Endergonic reactions: Require input of energy; non-spontaneous.

Free energy change:

$\Delta G = \Delta H - T\Delta S$

• $\Delta G$ = change in free energy

• $\Delta H$ = change in enthalpy (heat content)

• $T$ = temperature in Kelvin

• $\Delta S$ = change in entropy (disorder)

Enzymes and Their Function

Definition and Role of Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required.

• Function: Increase reaction rates without being consumed.

• Specificity: Each enzyme acts on a specific substrate.

Factors Affecting Enzyme Function

• Temperature: Affects enzyme activity; optimal range for each enzyme.

• pH: Each enzyme has an optimal pH.

• Cofactors: Non-protein molecules (e.g., metal ions, vitamins) required for activity.

• Inhibitors: Molecules that decrease enzyme activity (competitive or noncompetitive).

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions
Osmosis	No	Down gradient	Water
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump

Additional info:

• Some context and definitions were expanded for clarity and completeness.

• Table summarizes key differences between transport mechanisms.