Cell Structure, Reproduction, and Development: Prokaryotic Cell Ultrastructure and Classification

Unit 2 Overview and Topic 3/4 Coverage

• Unit 2 Title: Cells, Development, Biodiversity and Conservation.
• Topic 3: Cell Structure, Reproduction and Development.
  • 3A: Cell Structure.
  • 3B: Mitosis, Meiosis and Reproduction.
  • 3C: Development of Organisms.
• Topic 4: Plant Structure and Function, Biodiversity and Conservation.
  • 4A: Plant Structure and Function.
  • 4B: Classification.
  • 4C: Biodiversity and Conservation.

Learning Objectives

• 3.1: Know that all living organisms are made of cells, sharing some common features.
• 3.2: Understand how the cells of multicellular organisms are organised into tissues, tissues into organs, and organs into organ systems.
• 3.5 (i): Know the ultrastructure of prokaryotic cells, including cell wall, capsule, plasmid, flagellum, pili, ribosomes, and circular DNA.
• 3.5 (ii): Understand the function of the structures listed above.

Success Criteria

• Draw and label a prokaryotic cell.
• Identify cell wall, capsule, plasmid, flagellum, pili, ribosomes, and circular DNA in a prokaryotic cell.
• Describe the function of cell wall, capsule, plasmid, flagellum, pili, ribosomes, and circular DNA in a prokaryotic cell.

Discovery of Cells and Cell Theory

• Historical Context:
  • Robert Hooke (1663): Known as the first scientist to solve the "great mystery of life." He observed a thin slice of cork under a microscope. He noted it looked like hundreds of tiny boxes or a honeycomb. He named these structures "cells," derived from the Latin word meaning "little rooms."
  • 19th Century Development: Extensive work by several scientists during the 19th century led to the derivation of the Cell Theory.
• Core Tenets of Cell Theory:
  • All living things are made of cells.
  • Cells are the smallest unit to perform all functions of life.
  • All cells come from pre-existing cells.

Level of Organization in Multicellular Organisms

• Organisms follow a hierarchical structural organization:
  • Chemical Level: Atoms and Molecules (e.g., DNA).
  • Cellular Level: The basic functional unit (e.g., a smooth muscle cell).
  • Tissue Level: Groups of specialised cells carrying out particular functions in the body (e.g., smooth muscle tissue).
  • Organ Level: Structures made up of several different types of tissue to carry out particular functions in the body (e.g., the stomach, which includes serous membrane, smooth muscle tissue layers, and epithelial tissue).
  • Organ System Level: A group of organs working together to carry out particular functions in the body (e.g., the Digestive System, which includes the esophagus, liver, stomach, pancreas, gallbladder, small intestine, and large intestine).
  • Organismal Level: The whole organism which is made up of all the organ systems.

Eukaryotes vs. Prokaryotes

• Eukaryotes:
  • Possess a well-defined nucleus.
  • Contain membrane-bound organelles.
• Prokaryotes:
  • No true nucleus.
  • No membrane-bound organelles.

Characteristics of Prokaryotic Cells

• General Features:
  • Regarded as the simplest type of living cells.
  • Small size range: $0.5 - 10\,\mu m$.
  • Examples: Bacteria and Cyanobacteria (blue-green algae).

Prokaryotic Cell Ultrastructure and Functions

• Always Present Structures:

  • Cell Surface Membrane: Made up of phospholipids and proteins; maintains the composition of the cytoplasm.
  • Cytoplasm: Contains dissolved proteins, lipoproteins, sugars, amino acids, fatty acids, inorganic salts, and waste products of metabolism.
  • Circular DNA: One double-stranded genetic material coiled up in the center (sometimes referred to as a chromosome or nucleoid).
  • 70S Ribosomes: The specific site of translation (protein synthesis).
  • Cell Wall: Made of peptidoglycan (a complex of protein and carbohydrate also known as murein). It provides shape and physical support and prevents the cell from swelling and bursting.

• Additional Structures (Sometimes Present):

  • Flagellum: A whip-like structure that allows mobility (locomotion). It has a very simple structure and is made up of a protein called flagellin.
  • Capsule / Slime Layer: A thick polysaccharide layer that provides additional protection. It prevents dehydration, helps cells stick together, and provides protection from phagocytes, antibodies, and antibiotics.
  • Pili: Structures used for attachment to other cells or surfaces; also involved in conjugation (sexual reproduction) to adhere to other bacterial cells.
  • Plasmid: Small circles of DNA. There may be several present. They contain non-essential genes that can provide tolerance to toxins, resistance to antibiotics, and production of toxins.
  • Mesosomes: Folded invaginations in the plasma membrane of bacteria. These can be the site of respiration.
  • Infolding of Cell Surface Membrane: May form a photosynthetic membrane or carry out nitrogen fixation.

Classification of Bacteria: Shapes

• Spherical (Cocci):
  • Streptococcus pneumoniae: Causes pneumonia.
  • Micrococcus luteus: Causes armpits to stink.
  • Streptococcus pyogenes: Causes strep throat.
  • Staphylococcus aureus: Can cause sinus infections and food poisoning.
• Rod-Shaped (Bacilli):
  • Bacillus anthracis: Causes Anthrax.
  • Salmonella enterica: Causes Typhoid.
  • Clostridium botulinum: Causes Botulism.
• Spiral-Shaped and Others:
  • Vibrio cholerae: Causes Cholera.
  • Helicobacter pylori: Can cause stomach ulcers.
  • Treponema pallidum: Causes Syphilis.

Classification of Bacteria: Gram Reaction

• Gram Staining: A staining technique developed by Christian Gram in 1884. Before staining, bacteria are colourless.
• Gram Positive ($Gram+ve$):
  • Stain Purple.
  • Possess cell walls with thick layers of peptidoglycan.
  • Contains chemical substances such as teichoic acid.
  • Produce exotoxins.
• Gram Negative ($Gram-ve$):
  • Stain Red/Pink.
  • Possess cell walls with a thin layer of peptidoglycan.
  • Contains no teichoic acid.
  • Possesses an outer membrane with lipopolysaccharide (LPS) molecules attached.
  • Produce both exotoxins and endotoxins.

Classification of Bacteria: Respiratory Requirements

• Obligate Aerobes: Specifically require oxygen for respiration.
• Obligate Anaerobes: Only respire in the absence of oxygen. These organisms are killed by the presence of oxygen.
• Facultative Anaerobes: Use oxygen if it is available but possess the ability to respire and survive without it.