Chapter 03: Cells – The Living Unit

Introduction

This chapter explores the fundamental unit of life—the cell. It covers cell theory, the diversity of cell types, extracellular materials, the plasma membrane, and essential cellular processes. Understanding these concepts is crucial for studying human anatomy and physiology.

Cell Theory and Cell Diversity

Cell Theory

• Definition: The cell is the smallest unit of life.

• Principles:

  • All organisms are composed of one or more cells.

  • Cells arise only from pre-existing cells.

• Human Body: Contains 50 to 100 trillion cells.

Cell Diversity

• There are over 250 different types of human cells, each specialized for specific functions.

Extracellular Materials

Types of Extracellular Materials

• Extracellular fluids (ECFs):

  • Interstitial fluid: Bathes and surrounds cells.

  • Blood plasma: Fluid component of blood.

  • Cerebrospinal fluid: Surrounds nervous system organs.

• Cellular secretions: Includes saliva, mucus, and gastric fluids.

• Extracellular matrix: A network of proteins and polysaccharides that acts as a glue to hold cells together.

Plasma Membrane Structure and Function

Overview

• The plasma membrane acts as a selective barrier, separating intracellular fluid (ICF) from extracellular fluid (ECF).

• Controls what enters and exits the cell.

Fluid Mosaic Model

• The plasma membrane is a dynamic structure composed of:

  • Phospholipids: Form a bilayer, providing fluidity and barrier properties.

  • Cholesterol: Interspersed within the bilayer, increases membrane stiffness and stability.

  • Membrane proteins: Integral and peripheral proteins perform various functions, including transport, signaling, and cell recognition.

  • Glycocalyx: Surface sugars attached to lipids (glycolipids) and proteins (glycoproteins); functions as biological markers for cell recognition and immune response.

Cell Junctions

Types of Animal Cell Junctions

• Tight junctions: Prevent leakage of extracellular fluid between cells (e.g., in intestinal lining).

• Desmosomes: Anchor cells together, providing mechanical strength (e.g., in skin and heart muscle).

• Gap junctions: Allow direct communication between cells via channels (e.g., in cardiac muscle).

Membrane Transport Processes

Passive Transport

• Simple diffusion: Movement of molecules from high to low concentration without energy input.

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

• Facilitated diffusion: Movement of molecules via membrane proteins (channels or carriers).

• Factors affecting diffusion rate:

  • Concentration gradient

  • Molecular size

  • Temperature

Active Transport

• Primary active transport: Direct use of ATP to move substances against their concentration gradient (e.g., Na+-K+ pump).

• Secondary active transport: Uses energy stored in ion gradients created by primary active transport; involves cotransport proteins.

Vesicular Transport

• Endocytosis: Transport into the cell. Types include:

  • Phagocytosis (cell eating)

  • Pinocytosis (cell drinking)

  • Receptor-mediated endocytosis

• Exocytosis: Transport out of the cell; substances are ejected in secretory vesicles.

• Transcytosis: Movement into, across, and out of the cell.

• Vesicular trafficking: Movement of substances within the cell.

Osmolarity and Tonicity

Definitions

• Osmolarity: Total concentration of all solute particles in a solution, expressed in osmoles per liter (osmol/L).

• Tonicity: The ability of a solution to change the shape or tension of the plasma membrane by altering the cell's internal water volume.

Effects on Cells

• Isotonic solution: No net movement of water; cell shape remains unchanged.

• Hypertonic solution: Water moves out of the cell; cell shrinks (crenation).

• Hypotonic solution: Water moves into the cell; cell swells and may burst (lysis).

Membrane Potential and Cell Signaling

Resting Membrane Potential (RMP)

• Voltage across the plasma membrane due to separation of charged particles; typically -50 to -70 mV.

Cell Adhesion Molecules (CAMs) and Receptors

• CAMs: Anchor cells, assist movement, attract immune cells, and transmit signals.

• Membrane receptors: Involved in chemical and contact signaling.

Cytoplasm and Organelles

Cytoplasm Components

• Cytosol: Gel-like solution containing water, proteins, salts, and sugars.

• Inclusions: Insoluble molecules (e.g., glycogen granules, pigments, lipid droplets).

• Organelles: Specialized structures performing metabolic tasks.

Major Organelles

• Mitochondria: Powerhouse of the cell; site of ATP production via aerobic respiration. Contains its own DNA and ribosomes.

• Ribosomes: Site of protein synthesis; can be free or membrane-bound.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion and membrane incorporation.

  • Smooth ER: Lipid metabolism, detoxification, glycogen breakdown, calcium storage.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

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

• Peroxisomes: Detoxify harmful substances; neutralize free radicals.

• Cytoskeleton: Network of protein rods for cell shape, movement, and division.

• Centrosome and Centrioles: Organize microtubules and mitotic spindle; form bases of cilia and flagella.

• Cilia and Flagella: Cell surface extensions for movement.

• Microvilli: Increase cell surface area for absorption.

Nucleus and Genetic Material

Nucleus Structure

• Nuclear envelope: Double membrane with pores for transport.

• Nucleoli: Sites of ribosomal RNA synthesis and ribosome assembly.

• Chromatin: DNA wrapped around histone proteins; condenses to form chromosomes during cell division.

Cell Cycle and Division

Phases of the Cell Cycle

• Interphase: Cell growth and DNA replication; includes G1, S, and G2 phases.

• Mitotic phase: Cell division (mitosis and cytokinesis).

Mitosis Stages

• Prophase: Chromatin condenses, spindle forms.

• Metaphase: Chromosomes align at cell equator.

• Anaphase: Sister chromatids separate.

• Telophase: Nuclear envelopes reform.

• Cytokinesis: Cytoplasm divides, forming two daughter cells.

DNA Replication

• Occurs during S phase of interphase.

• Semiconservative replication: Each new DNA molecule consists of one old strand and one new strand.

Protein Synthesis

Transcription and Translation

• Transcription: DNA information is coded into messenger RNA (mRNA).

• Translation: mRNA is decoded to assemble polypeptides (proteins).

• Genetic code: Three-base DNA sequence (triplet) corresponds to a codon on mRNA.

mRNA Processing

• Pre-mRNA is edited; introns are removed, leaving only exon coding regions.

Cell Cycle Control and Cell Death

Control Systems

• Cell division is regulated by surface area-to-volume ratio, chemical signals, and contact inhibition.

• Checkpoints (e.g., G1 checkpoint) ensure proper division; faulty cells are stopped for repair or enter a non-dividing state.

Mechanisms of Cell Death

• Autophagy: Disposal of nonfunctional organelles and cytoplasmic bits.

• Ubiquitin-Proteasome Pathway: Degrades misfolded or damaged proteins.

• Apoptosis: Programmed cell death; involves caspase activation, DNA and cytoskeleton degradation, and phagocytosis of dead cells.

Developmental Aspects and Aging

Cell Differentiation and Growth

• Cell differentiation: Development of specific features and functions.

• Hyperplasia: Increased cell numbers due to accelerated growth.

• Atrophy: Decrease in cell size due to loss of stimulation or use.

Aging Theories

• Wear and tear theory: Accumulated damage from chemicals and free radicals.

• Mitochondrial theory: Free radicals diminish energy production.

• Immune theory: Autoimmune responses and weakened immunity.

• Genetic theory: Programmed cell aging; telomere shortening limits cell division.

• Telomerase: Enzyme that lengthens telomeres; present in germ cells and cancer cells.

Summary Table: Types of Membrane Transport

Key Equations

• Osmolarity:

• Resting Membrane Potential: (Typically -50 to -70 mV for most cells)

Example Application

• Hypertonic IV solution: Used to draw water out of swollen cells in cases of edema.

• Na+-K+ pump: Maintains cell membrane potential and regulates cell volume.

Additional info: Some details, such as the specific roles of organelles and the mechanisms of cell death, have been expanded for clarity and completeness.