The Working Cell: Membrane Structure, Energy, and Enzyme Function

Membrane Structure and Function

Introduction

Cell membranes are essential for maintaining the internal environment of the cell and mediating interactions with the external environment. Their structure and properties enable selective transport, communication, and energy conversion.

• Plasma membrane: The boundary that separates the living cell from its surroundings, composed mainly of lipids and proteins.

• Fluid mosaic model: Describes the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

• Selective permeability: The membrane allows some substances to cross more easily than others.

Phospholipid Structure and Membrane Properties

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer in aqueous environments.

• Self-assembly: Phospholipids spontaneously form bilayers, a critical step in the origin of life.

• Membrane proteins: Serve various functions such as transport, signal transduction, and cell recognition.

Transport Across Membranes

• Diffusion: The tendency of particles to spread out evenly in an available space. $\text{Diffusion}$ does not require energy (passive transport).

• Osmosis: The diffusion of water across a selectively permeable membrane. $\text{Osmosis}$ is crucial for maintaining cell turgor and volume.

• Tonicity: The ability of a surrounding solution to cause a cell to gain or lose water.

• Types of solutions:

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

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

  • Isotonic: Equal solute concentration; no net water movement.

• Facilitated diffusion: Transport proteins help specific substances diffuse across membranes without energy input.

• Active transport: Moves substances against their concentration gradient, requiring energy (usually from ATP).

Bulk Transport: Endocytosis and Exocytosis

• Exocytosis: Export of large molecules (e.g., proteins, polysaccharides) by vesicle fusion with the plasma membrane.

• Endocytosis: Import of large molecules by engulfing them in vesicles.

  • Phagocytosis: "Cell eating"; engulfment of particles.

  • Receptor-mediated endocytosis: Uses membrane receptors for specific uptake.

Energy and the Cell

Introduction

Cells require energy to perform work, including chemical, mechanical, and transport processes. Energy transformations in cells obey the laws of thermodynamics.

• Kinetic energy: Energy of motion.

• Potential energy: Stored energy due to location or structure.

• Thermodynamics: The study of energy transformations.

  • First law: Energy cannot be created or destroyed.

  • Second law: Every energy transfer increases the entropy (disorder) of the universe.

Metabolism and ATP

• Metabolism: The sum of all chemical reactions in a cell.

• Exergonic reactions: Release energy (e.g., cellular respiration).

• Endergonic reactions: Require energy input (e.g., photosynthesis).

• ATP (adenosine triphosphate): The cell's main energy currency.

  • ATP hydrolysis releases energy for cellular work: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

  • ATP drives endergonic reactions by phosphorylation (transfer of a phosphate group).

How Enzymes Function

Introduction

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy. They are essential for regulating metabolism and enabling life processes.

• Activation energy: The initial energy needed to start a reaction.

• Enzyme-substrate complex: The substrate binds to the enzyme's active site, forming a temporary complex.

• Active site: The region on the enzyme where the substrate binds and the reaction occurs.

• Induced fit: The enzyme changes shape slightly to fit the substrate more snugly.

Enzyme Regulation

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

• Inhibitors: Molecules that decrease enzyme activity.

  • Competitive inhibitors: Bind to the active site, blocking the substrate.

  • Noncompetitive inhibitors: Bind elsewhere, changing the enzyme's shape.

• Feedback inhibition: The end product of a metabolic pathway inhibits an earlier step, regulating the pathway.

• Enzyme inhibitors in medicine: Many drugs, pesticides, and poisons act as enzyme inhibitors.

Key Terms

• activation energy

• active site

• aquaporin

• ATP

• cellular respiration

• chemical energy

• coenzyme

• cofactor

• competitive inhibitor

• concentration gradient

• diffusion

• endergonic reaction

• endocytosis

• energy

• energy coupling

• entropy

• exergonic reaction

• exocytosis

• facilitated diffusion

• feedback inhibition

• first law of thermodynamics

• fluid mosaic model

• heat

• hypotonic

• induced fit

• isotonic

• kinetic energy

• metabolic pathway

• noncompetitive inhibitor

• osmosis

• passive transport

• phagocytosis

• phosphorylation

• potential energy

• receptor-mediated endocytosis

• second law of thermodynamics

• selective permeability

• substrate

• thermal energy

• thermodynamics

• tonicity

Word Roots

• "aqua-": water (e.g., aquaporin)

• "co-": together (e.g., cofactor)

• "endo-": inner, within (e.g., endergonic reaction)

• "exo-": outside (e.g., exocytosis)

• "hyper-": exceeding (e.g., hypertonic)

• "hypo-": lower (e.g., hypotonic)

• "iso-": same (e.g., isotonic)

• "kinet-": movement (e.g., kinetic energy)

• "osmo-": pushing (e.g., osmosis)

• "phago-": eat (e.g., phagocytosis)

• "therm-": heat (e.g., thermodynamics)