Cells and Tissues

Cell Theory and Basic Concepts

The cell theory forms the foundation of modern biology, describing the essential properties and functions of cells in living organisms.

• Cell Theory Concepts:

  • A cell is the basic structural and functional unit of living organisms.

  • The activity of an organism depends on the collective activities of its cells.

  • Principle of complementarity: Biochemical activities of cells are dictated by their structure (anatomy), which determines their function (physiology).

  • Continuity of life has a cellular basis.

• Elements Making Up Living Matter: Carbon, Hydrogen, Oxygen, Nitrogen (these four elements constitute the bulk of living matter).

• Generalized Cell: A model cell with three main regions: plasma membrane, nucleus, and cytoplasm.

• Water Content: Cells are approximately 60% water; protein is the major building material.

Structure of the Generalized Animal Cell

Cells have three main regions: plasma membrane, nucleus, and cytoplasm, each with distinct functions and structures.

• Plasma Membrane: Semipermeable boundary constructed of a double layer of phospholipids (fluid mosaic model), with cholesterol, proteins, and glycolipids interspersed. Functions include selective permeability, communication, and transport.

• Nucleus: Control center containing DNA. Composed of nuclear envelope (double membrane with pores), nucleolus (site of ribosome assembly), and chromatin (DNA wound around histones).

• Cytoplasm: Contains cytosol (fluid), inclusions (stored nutrients/cell products), and organelles (metabolic machinery).

Cell Membrane Structure and Junctions

The plasma membrane regulates entry and exit of substances and forms specialized junctions between cells.

• Phospholipid Arrangement: Hydrophilic heads face outward/inward; hydrophobic tails form the interior, making the membrane selectively permeable to water-soluble molecules.

• Cell Junctions:

  • Tight Junctions: Impermeable junctions that bind cells together, preventing leakage.

  • Desmosomes: Anchoring junctions that prevent cells from being pulled apart.

  • Gap Junctions: Allow communication between cells via connexons (protein channels).

Cell Organelles and Their Functions

Organelles are specialized structures within cells, each performing unique functions essential for cell survival.

• Mitochondria: "Powerhouses" of the cell; site of ATP production via aerobic respiration.

• Ribosomes: Sites of protein synthesis; found free in cytoplasm or attached to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; transports and modifies proteins.

  • Smooth ER: Functions in lipid metabolism and detoxification.

• Golgi Apparatus: Modifies, packages, and sorts proteins and lipids for secretion or use within the cell.

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

• Peroxisomes: Detoxify harmful substances; break down free radicals.

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

• Centrioles: Organize microtubules and direct mitotic spindle formation during cell division.

• Cell Extensions:

  • Cilia: Move materials across cell surfaces (e.g., mucus in respiratory tract).

  • Flagella: Propel cells (e.g., sperm).

  • Microvilli: Increase surface area for absorption.

Membrane Transport

Transport Processes Across the Plasma Membrane

Cells regulate the movement of substances across their membranes using passive and active transport mechanisms.

• Selective Permeability: The membrane allows certain substances to pass while restricting others.

• Passive Transport: Does not require energy (ATP); includes diffusion, osmosis, facilitated diffusion, and filtration.

• Active Transport: Requires energy (ATP); includes solute pumping and vesicular transport (endocytosis, exocytosis).

Definitions and Types of Transport

• Diffusion: Movement of molecules from high to low concentration.

  • Simple Diffusion: Direct movement through membrane.

  • Facilitated Diffusion: Uses protein channels/carriers for substances that are lipid-insoluble or too large.

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

• Filtration: Movement of water and solutes through a membrane by hydrostatic pressure.

• Active Transport (Solute Pumping): Movement against concentration gradient using protein carriers and ATP.

• Vesicular Transport:

  • Endocytosis: Uptake of substances into the cell.

  • Phagocytosis: "Cell eating"; engulfing large particles.

  • Pinocytosis: "Cell drinking"; uptake of fluids.

  • Exocytosis: Release of substances from the cell.

• Solution Types:

  • Isotonic: Equal solute and water concentrations; no net movement.

  • Hypertonic: Higher solute concentration outside; water leaves cell.

  • Hypotonic: Lower solute concentration outside; water enters cell.

Key Equations

• Diffusion Rate: Where D is diffusion rate, C1 and C2 are concentrations, d is distance.

• Osmosis: Where \Pi is osmotic pressure, i is ionization constant, M is molarity, R is gas constant, T is temperature.

Cell Division and DNA Replication

Cell Life Cycle

Cells undergo a life cycle consisting of interphase (growth and metabolic activity) and cell division (mitosis and cytokinesis).

• Interphase: Period of cell growth and DNA replication.

• DNA Replication: Duplication of genetic material before cell division.

• Mitosis: Division of the nucleus into two daughter nuclei; stages include prophase, metaphase, anaphase, telophase.

• Cytokinesis: Division of cytoplasm, resulting in two daughter cells.

Key Equation

• DNA Replication: Each daughter cell receives an identical copy of DNA.

Protein Synthesis

Roles of DNA and RNA

Protein synthesis is directed by DNA and involves three major types of RNA.

• DNA: Master blueprint for protein synthesis; contains genetic instructions.

• Messenger RNA (mRNA): Carries genetic code from DNA to ribosomes.

• Transfer RNA (tRNA): Brings amino acids to ribosomes during translation.

• Ribosomal RNA (rRNA): Forms part of the ribosome structure.

Key Equation

• Central Dogma: Describes the flow of genetic information.

Tissues of the Human Body

Major Tissue Types and Their Subcategories

The human body is composed of four major tissue types, each with distinct structural and functional characteristics.

• Epithelial Tissue: Covers and lines body surfaces; functions in protection, absorption, filtration, secretion.

  • Simple Squamous: Filtration/diffusion; lines air sacs, capillaries.

  • Simple Cuboidal: Glands, kidney tubules, ovaries.

  • Simple Columnar: Digestive tract lining; goblet cells secrete mucus.

  • Pseudostratified Columnar: Respiratory tract; absorption/secretion.

  • Stratified Squamous: Skin, mouth, esophagus; protection.

  • Transitional: Urinary organs; stretching.

  • Glandular: Endocrine (ductless, hormones), Exocrine (ducts, sweat/oil).

• Connective Tissue: Most abundant; supports, protects, insulates.

  • Bone (Osseous): Osteocytes in lacunae; hard matrix.

  • Cartilage: Chondrocytes; types include hyaline, elastic, fibrocartilage.

  • Dense Connective: Tendons, ligaments, dermis.

  • Loose Connective: Areolar (packing), adipose (fat storage), reticular (stroma).

  • Blood: Fluid matrix (plasma); transport of nutrients, wastes, gases.

• Muscle Tissue: Contractile; movement.

  • Skeletal: Voluntary, striated, multinucleate.

  • Cardiac: Involuntary, striated, one nucleus, intercalated discs.

  • Smooth: Involuntary, non-striated, spindle-shaped.

• Nervous Tissue: Conducts impulses; composed of neurons and neuroglia (support cells).

Locations of Tissue Types

Summary Table: Types of Connective Tissue

Additional info:

• Protein synthesis involves transcription (DNA to mRNA) and translation (mRNA to protein).

• Cell junctions are critical for tissue integrity and communication.

• Membrane transport is essential for maintaining homeostasis and cellular function.