Cell Structure and Organisation

Units of Measurement

• Standard unit: metre (m).
• Cells measured in micrometres (µm): 1 \mu m = 1/1,000,000 of a metre.
• Conversion examples:
  • 0.042 x 10^{-3}m = 42 µm
  • 249,000µm = 0.249 m = 2.49 x 10^{-1}m
  • 0.001µm = 1 nm
  • 50 µm = 0.00005 m
  • 5mm = 5000,000 nm

Calculating Magnification

• Formula: Magnification = Image size / Actual size.
• Ensure consistent units.
• Example 1: Organelle length AB = 95mm, Magnification = X 20,000, Actual length = 4.75µm.
• Example 2: Alveolus distance AB = 1.5 µm, Image AB = 20mm, Magnification = X 13333.

Cell Theory

• Proposed by Robert Hooke in the 1660s.
• Principles:
  • Cells are metabolic compartments.
  • Cells perform specific functions.
  • Cells have internal structures (ultrastructure) suited to their job.
• Electron microscopes reveal ultrastructure.
• Organelles are membrane-bound (compartmentalisation).
• Cells show division of labour.
• Two types of cells:
  • Eukaryotic: true nucleus and membrane-bound organelles (e.g., plant, animal).
  • Prokaryotic: no true nucleus or membrane-bound organelles (e.g., bacteria).
• Cell Theory:
  • All living organisms are composed of one or more cells.
  • The cell is the basic unit of life.
  • Cells can only arise from pre-existing cells.

Organelles in Plant and Animal Cells

• Nucleus:
  • Largest organelle, 5-20µm.
  • Encloses chromatin (DNA).
  • Regulates cell activities.
• Nuclear Envelope:
  • Double membrane with nuclear pores (~100 nm diameter).
• Nucleolus:
  • 1-2 µm diameter.
  • Synthesizes ribosomal RNA (rRNA).
• Nucleoplasm:
  • Fluid in the nucleus.
  • Suspension medium, supports nucleus shape.
• Endoplasmic Reticulum (ER):
  • Single membrane, continuous with nuclear membrane.
  • Forms cisternae.
• Rough ER (RER):
  • Has ribosomes.
  • Synthesizes, folds, and modifies proteins.
• Smooth ER (SER):
  • No ribosomes.
  • Synthesizes, stores, and transports lipids and carbohydrates.

Other Organelles

• Ribosomes:
  • Not membrane-bound.
  • Site of protein synthesis (translation).
  • 80S in eukaryotes, 70S in prokaryotes, mitochondria, and chloroplasts.
• Golgi Apparatus:
  • Single membrane.
  • Modifies, sorts, and packages proteins and lipids.
  • Manufactures macromolecules.
  • Connected to ER.
• Mitochondria:
  • Double membrane.
  • Site of aerobic respiration and ATP synthesis.
  • Contains DNA and 70s ribosomes.
• Cell Cytoskeleton:
  • Not an organelle; network of microtubules, microfilaments, and intermediate filaments.
  • Provides support and regulates cellular activities.
  • Microtubules: 25 nm, Microfilaments: 7 nm, Intermediate filaments: 8-12 nm.
• Vacuoles and Vesicles:
  • Membrane-bound sacs; vacuoles are larger.
  • Involved in food storage, water expulsion, and storing ions and wastes.

Animal Cell Specific Features

• Lysosomes:
  • Single membrane, contain hydrolytic enzymes.
  • Intracellular digestion and recycling.
• Centrioles:
  • Part of cytoskeleton, involved in cell division.
  • Composed of nine sets of triplet microtubules.
• Cilia and Flagella:
  • Anchored to cell membrane, core of microtubules (9+2 pattern).
  • Cilia: 0.25 µm X 2-20 µm, Flagella: 0.25 µm X 10-200 µm.

Intercellular Connections (Animal Cells)

• Desmosomes: Fasten cells into strong sheets.
• Gap Junctions: Cytoplasmic channels between cells for passage of small molecules.
• Tight Junctions: Prevent leakage of extracellular fluid.

Plant Cell Specific Features

• Chloroplasts:
  • Double membrane, contain chlorophyll.
  • Site of photosynthesis.
  • Contain thylakoid membranes (grana), DNA, and 70s ribosomes.
• Cellulose Cell Wall:
  • Outside plasma membrane, provides shape and strength.
  • Made of cellulose fibres.
• Plasmodesmata:
  • Cytoplasmic strands through pores in cell walls for cell communication.
• Large Central Vacuole:
  • Regulates osmotic properties and provides support.
  • Membrane called Tonoplast.

Prokaryotes

• Lack membrane-bound organelles and a nucleus.
• Smaller (1-10µm).
• DNA:
  • Circular, naked (no histones), in nucleoid.
• Plasmids:
  • Additional circular DNA with genes like antibiotic resistance.
• Ribosomes:
  • 70S type.
• Cell Wall:
  • Peptidoglycan (murein).
• Mesosomes:
  • Infoldings of membrane for respiration.
• Capsule:
  • Some have a glycocalyx for protection.
• Flagella:
  • For locomotion.
• Fimbriae:
  • For attachment.
• Pili:
  • For conjugation.
• Photosynthetic Lamellae:
  • In some, for photosynthesis.

Endosymbiosis

• Theory explaining evolution of eukaryotic cells.
• Mitochondria and chloroplasts were once free-living prokaryotes engulfed by other cells.
• Evidence: own DNA, 70S ribosomes, division.

Viruses

• Non-cellular, consist of protein coat (capsid) and nucleic acid (DNA or RNA).
• Acellular, no cytoplasm, cell membranes, or organelles.
• Replicate inside a host cell.

Cell Differentiation

• Specialized cells for specific functions.
• Stem cells are unspecialized and can differentiate into specialized cells (totipotent).

Tissues

• Aggregation of similar cells with the same function.
• Four types:
  • Epithelial, Muscle, Connective, Nervous.

Epithelial Tissue

• Sheets of cells lining surfaces.
• Types: Squamous, Cuboidal, Columnar, Ciliated, Stratified, Glandular.

Muscle Tissue

• Allows movement.
• Types: Smooth, Skeletal, Cardiac.

Connective Tissue

• Connects, supports, and separates tissues and organs.
• Made of cells and extracellular matrix (fibres like collagen).
• Types: Supporting, Dense, Loose.

Plant Tissues

• Palisade mesophyll: Photosynthesis.
• Spongy mesophyll: Gaseous diffusion.
• Epidermis: Protection and gaseous exchange.
• Phloem: Transports organic materials.
• Xylem: Transports water and ions.

Organs and Organ Systems

• Organ: Group of tissues working together.
• Organ system: Group of organs working together.