The Working Cell: Membrane Structure, Function, and Cellular Processes

The Working Cell

Introduction

This chapter explores how the plasma membrane and its proteins enable cells to survive and function. It addresses the mechanisms by which cells use membranes, water, energy, and enzymes to maintain life processes.

Membrane Structure and Function

The Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of diverse protein molecules embedded in a fluid phospholipid bilayer.

• Phospholipid bilayer: The fundamental structure of the membrane, composed of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Proteins: Scattered throughout the bilayer, these perform various functions such as transport, signaling, and structural support.

• Cholesterol: Interspersed within the bilayer, cholesterol molecules help maintain membrane fluidity.

Selective permeability is a key property of the plasma membrane, allowing some substances to cross more easily than others.

Functions of Membrane Proteins

• Transport proteins: Facilitate the movement of substances across the membrane, including channels and carriers for ions and molecules.

• Enzymatic activity: Some membrane proteins act as enzymes, catalyzing specific reactions at the membrane surface.

• Attachment proteins: Anchor the membrane to the cytoskeleton and extracellular matrix, providing structural support and facilitating communication between the cell's interior and exterior.

• Receptor proteins: Bind signaling molecules (ligands) and relay messages into the cell, initiating cellular responses.

• Junction proteins: Form intercellular junctions that connect adjacent cells, enabling communication and adhesion.

• Glycoproteins: Serve as identification tags that are recognized by other cells, important for immune response and tissue organization.

Example: Aquaporins are specialized channel proteins that facilitate rapid water transport across the membrane.

Transport Across Membranes

Passive Transport

Passive transport is the movement of substances across a membrane without the input of cellular energy.

• Diffusion: The tendency of particles to spread out evenly in available space, moving from areas of higher to lower concentration.

• Osmosis: The diffusion of water across a selectively permeable membrane.

• Facilitated diffusion: The movement of polar or charged substances across membranes with the help of transport proteins, still down their concentration gradient and without energy input.

Key Equation:

$\text{Net movement} = \text{from high concentration to low concentration}$

Water Balance and Tonicity

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

• Isotonic solution: Solute concentration is equal inside and outside the cell; animal cells are normal, plant cells are flaccid.

• Hypotonic solution: Lower solute concentration outside; cells gain water. Animal cells may lyse, plant cells become turgid (normal).

• Hypertonic solution: Higher solute concentration outside; cells lose water. Animal cells shrink (crenate), plant cells become plasmolyzed.

Solution Type	Animal Cell	Plant Cell
Isotonic	Normal	Flaccid
Hypotonic	Lysed	Turgid (normal)
Hypertonic	Shriveled	Plasmolyzed

Active Transport

Active transport requires energy (usually from ATP) to move substances against their concentration gradients.

• Transport proteins bind solutes and change shape to shuttle them across the membrane.

• ATP provides the energy for these conformational changes.

Key Equation:

$\text{Active transport:} \quad \text{Low} \rightarrow \text{High concentration} + \text{ATP input}$

Bulk Transport: Exocytosis and Endocytosis

Cells use vesicles to move large molecules across membranes.

• Exocytosis: Exports bulky molecules (e.g., proteins, polysaccharides) by fusing vesicles with the plasma membrane.

• Endocytosis: Imports large molecules by engulfing them in vesicles formed from the plasma membrane.

• Phagocytosis: "Cell eating"; the cell engulfs particles into a vacuole.

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

Energy and the Cell

Forms of Energy

• Kinetic energy: Energy of motion.

• Potential energy: Stored energy due to position or structure; includes chemical energy stored in bonds.

• Chemical energy: A form of potential energy available for release in a chemical reaction.

• Heat: Energy associated with the random movement of atoms or molecules.

Laws of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed (law of conservation of energy).

• Second Law: Every energy transfer increases the entropy (disorder) of the universe; some energy is lost as heat.

Chemical Reactions in Cells

• Exergonic reactions: Release energy; products have less potential energy than reactants.

• Endergonic reactions: Require energy input; products have more potential energy than reactants.

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

ATP and Energy Coupling

ATP (adenosine triphosphate) is the cell's energy currency, powering nearly all forms of cellular work by transferring a phosphate group to other molecules (phosphorylation).

• ATP hydrolysis releases energy:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Energy coupling links exergonic and endergonic reactions via ATP.

How Enzymes Function

Enzyme Structure and Function

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy barrier, without being consumed in the reaction.

• Each enzyme is specific to its substrate, which binds at the enzyme's active site.

• The enzyme-substrate complex undergoes a catalytic cycle, resulting in product formation and enzyme reuse.

Enzyme Inhibition and Regulation

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

• Noncompetitive inhibitors: Bind elsewhere on the enzyme, altering its shape and reducing activity.

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

Enzyme Inhibitors in Medicine and Industry

• Many drugs, pesticides, and poisons act as enzyme inhibitors, affecting metabolic processes.

• Some inhibitors are reversible, while others bind irreversibly, permanently disabling the enzyme.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose via carrier protein
Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Osmosis	No	Water: High to Low	Water via aquaporins
Exocytosis	Yes (ATP)	Out of cell	Secretion of proteins
Endocytosis	Yes (ATP)	Into cell	Phagocytosis of bacteria

Key Terms

• Plasma membrane

• Phospholipid bilayer

• Selective permeability

• Diffusion

• Osmosis

• Tonicity

• Active transport

• Exocytosis

• Endocytosis

• ATP

• Enzyme

• Activation energy

• Competitive inhibitor

• Noncompetitive inhibitor

• Feedback inhibition