Cell Theory and Generalized Cell Structure

Cell Theory

The cell theory is a fundamental concept in biology stating that all living organisms are composed of cells, which are the basic units of structure and function in life.

• All living things are made of cells.

• Cells are the basic unit of life.

• All cells arise from pre-existing cells.

Structure of the Generalized Cell

A typical eukaryotic cell consists of several key components, each with specialized functions.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Cytoplasm: Gel-like substance where organelles are suspended.

• Organelles: Specialized structures performing distinct cellular functions (e.g., mitochondria, ribosomes).

• Plasma Membrane: Semi-permeable barrier regulating entry and exit of substances.

Plasma Membrane

Structure

The plasma membrane is a dynamic structure that separates the cell from its environment and controls molecular traffic.

• Fluid Mosaic Model: Describes the membrane as a phospholipid bilayer with embedded proteins.

• Phospholipid Bilayer: Composed of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Membrane Proteins:

  • Integral Proteins: Span the membrane; include transmembrane proteins.

  • Peripheral Proteins: Attached to the membrane surface.

  • Functions:

    • Transport

    • Enzymatic activity

    • Receptors for signaling

    • Cell adhesion

    • Cytoskeletal attachment

• Glycocalyx: Carbohydrate-rich area on the cell surface for cell recognition.

Specializations of the Plasma Membrane

• Microvilli: Increase surface area for absorption.

• Membrane Junctions:

  • Tight Junctions: Prevent leakage between cells.

  • Desmosomes: Anchor cells together.

  • Gap Junctions: Allow communication between cells.

Membrane Transport

Passive Transport

Passive transport involves movement of substances across the membrane without energy input.

• Simple Diffusion: Movement down the concentration gradient.

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

  • Osmolarity: Total concentration of solute particles in water.

  • Tonicity: Effect of solution on cell volume (hypertonic, hypotonic, isotonic).

• Facilitated Diffusion: Uses membrane proteins to transport substances.

• Filtration: Movement of water and solutes due to hydrostatic pressure.

Active Transport

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

• Solute Pumps: Use ATP to transport ions and molecules.

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

• Secondary Active Transport: Uses energy from gradients established by primary transport.

• Bulk Transport: Movement of large particles via vesicles.

  • Exocytosis: Expulsion of materials from the cell.

  • Endocytosis: Uptake of materials into the cell (phagocytosis, pinocytosis, receptor-mediated endocytosis).

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the plasma membrane.

• Created by separation of charges (positive and negative) on either side of the membrane.

• Determined mainly by potassium (K+) and sodium (Na+) ions.

• Cells are polarized, with the inside negative relative to the outside.

Cell-Environment Interactions

• Membrane Receptors: Proteins that bind signaling molecules (ligands).

• Contact Signaling: For cell recognition.

• Chemical Signaling: Ligands bind to receptors to trigger cellular responses.

Cytoplasm

Components

The cytoplasm consists of cytosol, organelles, and inclusions.

• Cytosol: Fluid portion containing dissolved substances.

• Organelles: Specialized structures with specific functions.

• Inclusions: Non-living substances (e.g., glycogen, lipid droplets).

Organelles

• Mitochondria: Powerhouse of the cell; site of ATP production.

  • Double membrane, own DNA and RNA, self-replicating.

• Ribosomes: Sites of protein synthesis.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; protein synthesis and modification.

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

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

• Lysosomes: Contain digestive enzymes (acid hydrolases) for breakdown of cellular debris.

• Peroxisomes: Contain enzymes for detoxification and breakdown of hydrogen peroxide ().

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

  • Microtubules: Hollow tubes for cell shape and transport.

  • Microfilaments: Actin filaments for movement.

  • Intermediate Filaments: Structural support.

• Centrosome & Centrioles: Organize microtubules during cell division.

• Cilia & Flagella: Motile structures for movement.

  • Cilia: Move substances across cell surface.

  • Flagella: Propel the cell (e.g., sperm).

Nucleus

• Nuclear Envelope: Double membrane with nuclear pores.

• Nucleolus: Site of ribosome synthesis.

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

Extracellular Materials

• Body Fluids: Interstitial fluid, blood plasma, cerebrospinal fluid.

• Cellular Secretions: Substances produced and released by cells.

• Extracellular Matrix: Network of proteins and polysaccharides providing structural support; known as the "universal cell glue."

Protein Synthesis

General Principles

Protein synthesis is the process by which cells build proteins based on genetic instructions.

• DNA: Dictates the structure of proteins.

• Gene: Segment of DNA carrying instructions for one polypeptide chain.

• Each triplet of DNA bases specifies an amino acid.

• Exons: Coding sequences; Introns: Non-coding sequences.

RNA Types and Functions

• rRNA: Forms ribosomes.

• tRNA: Transfers amino acids to ribosome.

• mRNA: Carries genetic code from DNA to ribosome.

Transcription and Translation

• Transcription: DNA is used as a template to synthesize mRNA.

• Translation: mRNA is decoded by ribosomes to build a protein.

• Each set of three nucleotides on mRNA (codon) specifies an amino acid.

• Start Codon: AUG (codes for methionine).

• Stop Codons: Signal end of translation.

Example Equation:

Cell Life Cycle

Interphase

Interphase is the period of cell growth and DNA replication preceding cell division.

• G1 (Gap 1): Cell growth and metabolic activities.

• S (Synthesis): DNA replication.

• G2 (Gap 2): Preparation for cell division.

Cell Division

• Mitosis: Division of the nucleus.

  • Prophase: Chromatin condenses, spindle forms.

  • Metaphase: Chromosomes align at cell equator.

  • Anaphase: Chromatids separate and move to opposite poles.

  • Telophase: Nuclear envelopes reform, chromosomes decondense.

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

Developmental Aspects of Cells

• Cell Specialization: Cells differentiate to perform specific functions.

• Cell Differentiation: Process by which cells become specialized.

• Hypertrophy: Increase in cell size.

• Hyperplasia: Increase in cell number.

Aging and Cell Death

Wear-and-Tear Theory

Suggests that cells and tissues are damaged by accumulated insults over time.

Immunological Theory

• Autoimmune responses increase with age.

• Immune system progressively weakens.

Genetic Theory

• Telomere Clock: Telomeres shorten with each cell division, eventually leading to cell aging and death.

Additional info: Expanded explanations and definitions have been added for clarity and completeness.