The Working Cell

Introduction

The cell is the fundamental unit of life, and its ability to function depends on the structure and properties of its membrane. This chapter explores the composition, structure, and function of the cell membrane, as well as the mechanisms by which substances move into and out of cells.

The Cell Membrane

Structure and Function

• Cell Membrane: Also known as the plasma membrane, it separates the inside of the cell from the external environment and from specialized organelles within the cell.

• Phospholipid Bilayer: The membrane is primarily composed of a double layer of phospholipids, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Selective Permeability: The membrane allows certain substances to pass while restricting others, maintaining the internal environment of the cell.

Example: The cell membrane prevents large or charged molecules from freely entering or leaving the cell, while allowing gases like O2 and CO2 to diffuse through.

The Fluid Mosaic Model

Membrane Composition

• Fluid Mosaic Model: Describes the cell membrane as a dynamic structure with proteins floating in or on the fluid lipid bilayer.

• Membrane Proteins: Integral and peripheral proteins serve various functions, including transport, signaling, and structural support.

• Carbohydrates: Often attached to proteins or lipids on the extracellular surface, playing roles in cell recognition.

Additional info: Cholesterol molecules are interspersed within the bilayer, contributing to membrane fluidity and stability.

Membrane Proteins

Types and Functions

• Transport Proteins: Facilitate the movement of substances across the membrane.

• Enzymatic Proteins: Catalyze specific reactions at the membrane surface.

• Receptor Proteins: Receive and transmit signals from the environment.

• Attachment Proteins: Anchor the membrane to the cytoskeleton and extracellular matrix.

Membrane Transport

Overview

Substances move across the cell membrane by various mechanisms, which can be classified as passive or active transport.

Passive Transport

• Diffusion: The movement of molecules from an area of higher concentration to an area of lower concentration, driven by the concentration gradient.

• Facilitated Diffusion: The movement of molecules across the membrane via specific transport proteins, still down their concentration gradient.

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

Equation for Diffusion Rate:

where is the flux, is the diffusion coefficient, and is the concentration gradient.

Tonicity

• Isotonic Solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypotonic Solution: Lower solute concentration outside the cell; water enters the cell, which may swell or burst.

• Hypertonic Solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.

Active Transport

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually from ATP).

• Carrier Proteins: Undergo conformational changes to transport molecules across the membrane.

Example: The sodium-potassium pump (-ATPase) moves sodium ions out of and potassium ions into the cell against their concentration gradients.

Bulk Transport

• Endocytosis: The process by which cells engulf large particles or fluids by forming vesicles from the plasma membrane.

• Exocytosis: The process by which cells expel materials in vesicles that fuse with the plasma membrane.

• Types of Endocytosis: Includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.

Energy and the Cell

Forms of Energy

• Kinetic Energy: Energy of motion, including thermal energy (heat).

• Potential Energy: Stored energy, such as chemical energy in bonds.

Thermodynamics in Biology

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

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

Energy and Chemical Reactions

• Exergonic Reactions: Release energy; products have less free energy than reactants.

• Endergonic Reactions: Require input of energy; products have more free energy than reactants.

Gibbs Free Energy Equation:

where is the change in free energy, is the change in enthalpy, is temperature, and is the change in entropy.

ATP and Energy Transfer

ATP as the Energy Currency

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• ATP Hydrolysis: Releases energy by breaking a phosphate bond:

• Energy Coupling: The use of exergonic processes to drive endergonic ones via ATP.

Enzymes and Metabolism

Enzyme Structure and Function

• Enzymes: Biological catalysts that speed up chemical reactions by lowering activation energy.

• Active Site: The region on the enzyme where the substrate binds.

• Substrate: The reactant molecule upon which an enzyme acts.

Activation Energy Diagram:

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature for activity.

• pH: Each enzyme has an optimal pH range.

• Substrate Concentration: Increasing substrate increases reaction rate up to a point (saturation).

Enzyme Inhibition

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

• Noncompetitive Inhibitors: Bind elsewhere on the enzyme, changing its shape and reducing activity.

Negative Feedback and Regulation

• Feedback Inhibition: The end product of a metabolic pathway inhibits an earlier step, preventing overproduction.

Key Terms

• Fluid mosaic model

• Membrane proteins

• Diffusion

• Facilitated diffusion

• Osmosis

• Tonicity

• Passive transport

• Active transport

• Endocytosis

• Phagocytosis

• Pinocytosis

• Exocytosis

• Energy

• Heat

• Chemical energy

• Endergonic reactions

• Exergonic reactions

• Activation energy

• Enzyme

• Coenzyme

• Competitive inhibitor

• Noncompetitive inhibitor

• Negative feedback