Cell Structure and Function

Introduction

Cells are the fundamental units of life, forming the basis of structure and function in all living organisms. Understanding cell structure and function is essential for the study of Anatomy & Physiology, as it underpins all physiological processes in the body.

3.1 The Cell Theory and Basic Cell Structure

The Cell Theory

• Definition: The cell is the basic structural and functional unit of life.

• All organisms are composed of one or more cells.

• Cells arise only from pre-existing cells by division.

Major Regions of a Generalized Cell

• Plasma Membrane: The selectively permeable outer barrier of the cell.

• Cytoplasm: The intracellular fluid containing organelles.

• Nucleus: The organelle that controls cellular activity and contains genetic material.

Extracellular Materials

• Extracellular fluid: Includes interstitial fluid, blood plasma, and cerebrospinal fluid, containing nutrients, regulatory substances, and wastes.

• Cellular secretions: Digestive secretions, lubricating fluids, etc.

• Extracellular matrix: A network of proteins and polysaccharides that help bind cells together and provide structural support.

3.2 The Plasma Membrane

Structure and Composition

• The plasma membrane is a double layer of phospholipids with embedded proteins, forming a fluid mosaic model.

• Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Cholesterol: Interspersed within the bilayer, providing stability and fluidity.

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, enzymes, and anchors.

Membrane Carbohydrates and the Glycocalyx

• Glycocalyx: A fuzzy, sticky, carbohydrate-rich area at the cell surface, consisting of glycoproteins and glycolipids. It acts as a biological marker for cell recognition.

3.3 Intercellular Junctions

Types of Cell Junctions

• Tight Junctions: Integral proteins fuse adjacent cells, preventing passage of molecules between them.

• Desmosomes: Mechanical couplings that anchor cells together, providing strength and reducing the chance of tearing.

• Gap Junctions: Hollow cylinders (connexons) that allow ions and small molecules to pass between cells, facilitating communication.

3.4 Passive Membrane Transport

Overview

Passive transport involves the movement of substances across the membrane without energy input, down their concentration gradient.

Types of Passive Transport

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., gases, steroids) directly through the lipid bilayer.

• Facilitated Diffusion: Movement of larger or polar molecules (e.g., glucose, ions) via carrier proteins or channels.

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

Key Concepts

• Diffusion: Movement from high to low concentration, driven by kinetic energy.

• Osmolarity: The total concentration of solute particles in a solution.

• Tonicity: The ability of a solution to change the shape or tone of cells by altering their internal water volume.

• Isotonic: Same solute concentration as the cell; no net water movement.

• Hypertonic: Higher solute concentration than the cell; water moves out, cell shrinks.

• Hypotonic: Lower solute concentration than the cell; water moves in, cell swells.

Summary Table: Types of Passive Membrane Transport

3.5 Active Membrane Transport

Overview

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

Types of Active Transport

• Primary Active Transport: Direct use of ATP to pump substances (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses energy stored in gradients created by primary active transport (e.g., symporters and antiporters).

Vesicular Transport

• Endocytosis: Uptake of large particles or fluids via vesicles (includes phagocytosis and pinocytosis).

• Exocytosis: Release of substances from the cell via vesicles.

• Receptor-mediated Endocytosis: Selective uptake of specific molecules via receptor binding.

3.6 Membrane Potentials and Electrochemical Gradients

Membrane Potential

• A voltage across the cell membrane due to separation of oppositely charged particles (ions).

• Resting membrane potential is typically -50 to -90 mV, with the inside of the cell more negative than the outside.

Key Equation

The Nernst equation describes the equilibrium potential for a particular ion:

Role of Potassium (K+)

• K+ diffuses out of the cell, creating a negative charge inside.

• Active transport (Na+/K+ pump) maintains the gradient and prevents equilibrium.

3.7 Cell Adhesion Molecules and Membrane Receptors

Cell Adhesion Molecules (CAMs)

• Glycoproteins that anchor cells to extracellular matrix and to each other.

• Assist in cell movement, signaling, and immune responses.

Plasma Membrane Receptors

• Integral proteins that bind ligands (chemical signals) and initiate cellular responses.

• G protein-coupled receptors: Activate second messengers (often via ATP) to propagate signals inside the cell.

PART 2: The Cytoplasm

Composition of the Cytosol

• The cytosol is the fluid portion of the cytoplasm, containing water, solutes, and suspended particles.

Organelles and Their Functions

• Mitochondria: Produce most of the cell's ATP via aerobic respiration.

• Ribosomes: Sites of protein synthesis; can be free in cytosol or bound to rough ER.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER synthesizes proteins, smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes to break down waste and cellular debris.

• Peroxisomes: Contain enzymes for detoxification and lipid metabolism.

• Cytoskeleton: Network of protein filaments (microfilaments, intermediate filaments, microtubules) providing structural support and facilitating movement.

Specialized Cell Structures

• Cilia and Flagella: Motile extensions for movement or moving substances along cell surfaces.

• Microvilli: Increase surface area for absorption.

Autophagy and Proteasomes

• Autophagy: The process by which cells degrade and recycle their own components via lysosomes.

• Proteasomes: Protein complexes that degrade unneeded or damaged proteins by proteolysis.

Apoptosis

• Programmed cell death involving caspases and cell shrinkage, essential for development and homeostasis.

Additional info: These notes are based on a detailed outline of cell structure and function, including key learning outcomes, figures, and tables referenced in a typical Anatomy & Physiology textbook chapter.