Prokaryotic & Eukaryotic Cells: Structure, Metabolism & Division

Posted on Nov 20, 2024 in Biology

Prokaryotic Cell Structure

Prokaryotic cells are characterized by their smaller size compared to eukaryotic cells. Their DNA is not enclosed within a nuclear membrane but dispersed in the cytoplasm, typically as a double-stranded chain. Prokaryotes possess ribosomes, mesosomes (cell membrane folds involved in respiration), and a cell wall with a composition distinct from plants and animals, sometimes featuring multiple layers for enhanced protection.

The kingdom Monera comprises prokaryotic organisms, all of which are unicellular. Examples include bacteria and cyanobacteria (blue-green algae).

Eubacteria

Eubacteria, or true bacteria, are classified by shape and nutrition:

Shape:

  • Cocci (spherical): May occur in pairs (diplococci), chains (streptococci), or irregular clusters (staphylococci).
  • Bacilli (rod-shaped): Lactic acid bacteria are a common example.
  • Spirilla (spiral-shaped).

Nutrition:

  • Heterotrophic: Saprophytes decompose organic matter; symbiotic and commensal bacteria live within other organisms; parasites cause diseases.
  • Autotrophic: Chemosynthetic bacteria obtain energy from oxidizing inorganic materials; photosynthetic bacteria use photosynthesis.

Archaea

Archaea are divided into three groups:

  • Halophiles: Thrive in high-salt environments.
  • Methanogens: Anaerobic organisms that produce methane.
  • Thermoacidophiles: Live in hot, acidic environments.

Eukaryotic Cell Structure

Eukaryotic cells are enclosed by a plasma membrane composed of a phospholipid bilayer with cholesterol and glucids. The cytoplasm lies between the plasma membrane and the nucleus.

Nucleus: Surrounded by a double membrane, the nucleus contains DNA in the form of chromatin, a nucleolus (involved in RNA synthesis), and nucleoplasm.

Organelles:

  • Endoplasmic Reticulum (ER): A network of membranes forming flattened sacs and tubules. Includes smooth and rough ER.
  • Golgi Apparatus: Modifies and packages products from the ER.
  • Lysosomes: Vesicles containing digestive enzymes, formed by the Golgi apparatus.
  • Vacuoles: Store substances.
  • Mitochondria: Double-membrane organelles responsible for cellular respiration.
  • Plastids (in plants): Leucoplasts (store starch, proteins, or oils), chromoplasts (contain pigments), chloroplasts (contain chlorophyll).

Non-Membrane Bound Structures:

  • Ribosomes: Composed of RNA and proteins, involved in protein synthesis.
  • Cytoskeleton: Network of microfilaments and microtubules.
  • Centrosome (in animal cells): Organizes the cytoskeleton.
  • Cilia and Flagella: Elongated membrane extensions for movement.

Cellular Metabolism

Catabolism: Breakdown of molecules, releasing energy (e.g., starch to glucose).

Anabolism: Synthesis of molecules, requiring energy (e.g., amino acids to proteins).

Substance Exchange:

  • Diffusion: Movement of particles from high to low concentration.
  • Active Transport: Movement against the concentration gradient.
  • Endocytosis: Uptake of substances (phagocytosis for solids, pinocytosis for liquids).
  • Exocytosis: Release of substances.

Cellular Functions:

  • Stimulus Response: Cells respond to stimuli.
  • Encystment: Formation of a protective cyst for survival.
  • Taxis: Movement in response to stimuli (e.g., chemotaxis, phototaxis).
  • Amoeboid Movement: Movement via cytoplasmic extensions.
  • Contractile Movement: In single-celled organisms.
  • Vibratile Movement: Movement using cilia or flagella.

Cellular Division

Cell Cycle:

  • G1 Phase: Growth.
  • S Phase: DNA replication.
  • G2 Phase: Preparation for division.

Mitosis:

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
  • Cytokinesis

Meiosis

Protists and Fungi

Protists

Include algae, protozoa, and fungus-like protists.

  • Protozoa
  • Unicellular Algae: Euglenoids, dinoflagellates, diatoms, golden algae.
  • Multicellular Algae: Brown algae, green algae, red algae (various forms: filamentous, laminar, colonial).

Fungi

Characterized by hyphae (forming mycelium), which can be coenocytic or septate. Some have haustoria for nutrient absorption.

Origin and Diversity of Plants

Plants evolved from green algae, adapting to terrestrial environments with structures like:

  • Cuticle: Waterproof layer.
  • Stomata: Pores for gas exchange.
  • Gametangia: Protect embryos.
  • Internal Skeleton: Support.
  • Vascular Tissues: Transport.

Plant Classification:

  • Bryophytes (mosses, liverworts): Lack vascular tissue.
  • Pteridophytes (ferns, horsetails): Have vascular tissue.
  • Spermatophytes (seed plants): Produce seeds.

Gymnosperms:

  • Have woody tissues (xylem).
  • Possess a cuticle.
  • Reproduce using pollen grains.
  • Examples include conifers with various leaf shapes.

Angiosperms:

  • Flowering plants that produce fruits.
  • Divided into monocots and dicots based on cotyledon number and other characteristics.

Forest Types:

  • Mediterranean Evergreen Forest: Oak and cork.
  • Atlantic Deciduous Forest: Oak and beech.