4 Organelles & Cell Structure

• Viruses challenge the cell theory and concepts of what is considered living
• Overview of Topic


  • This topic discusses the typical model of prokaryotes and eukaryotes, the structure of various organelles in relation to their functions, and the endomembrane system in eukaryotic cells
• References: Reece, K., Jane, B. Taylor, Martha, R., Simon, Eric, J., Dickey, Jean, L., Hogan (2020) Campbell Biology (12th Edition), Raven, P., Johnson, G., Mason, K., Losos, J. and Duncan, T. (2022) Biology (13th Edition), Alberts, B., Heald, R., Johnson, A., Morgan, D., Raff, M., Roberts, K. and Walter, P. (2022) Molecular Biology of the Cell (7th Edition)
• Viral reproductive cycles will be discussed in further details in the lectures for the topic on Viruses
• Endomembrane system in eukaryotic cells

  Organelles and structures inside and around a eukaryotic cell are part of the endomembrane system that work together to serve important cellular functions
• Contents

  • The Cell Theory
  • Prokaryotic Cell
  • Peptidoglycan Cell Wall
  • Glycocalyx
  • Cell Surface Membrane
  • Appendages
  • Genetic Material
  • Ribosomes
  • Storage Granules
  • Eukaryotic Cell
  • Non-membranous Organelles
  • Ribosomes
  • Centrioles
• The Endomembrane System


  • Secretion of Substances
  • Digestion of Food
• Robert Hooke discovered cells in 1665 using one of the first microscopes ever invented
• Membranous Organelles

  • Nucleus
  • Endoplasmic Reticulum
  • Golgi Apparatus
  • Lysosome
• According to the cell theory, cells are the smallest unit of life, all cells come from pre-existing cells, and living organisms are composed of cells
• Knowledge on viruses will be covered in more detail under the topic of Viruses
• Double-Membrane Organelles

  • Mitochondrion
  • Chloroplast
• Non-membranous Organelles

  • Ribosomes
  • Centrioles
• Cell Fractionation

  1. Homogenisation

  2. Centrifugation
• There are two broad categories of cells: prokaryotic cells and eukaryotic cells, grouped within the domains Bacteria, Archaea, and Eukarya
• Viruses do not completely fulfil the definition of cells and challenge the cell theory as they are acellular, lack cellular organelles, do not carry out metabolism, lack the ability to reproduce independently, do not grow or undergo developmental changes, and do not respond to stimuli outside the host cell
• Matthias Schleiden and Theodor Schwann concluded that all living organisms consist of cells, forming the foundation for understanding the reproduction and growth of all organisms
• Eukaryotic organisms are grouped in the domain Eukarya
• Archaea share certain traits with bacteria and others with eukaryotes. However, archaea also have many unique characteristics of their own. For our syllabus, we will focus on domain Bacteria as prokaryotic cells
• Prokaryotes, such as bacteria, are very small (~ 0.2 μm in diameter and 1-8 μm in length) and are unicellular. They lack membrane-bound organelles but can still survive and reproduce
• Outline the structure of a typical bacterial cell (small and unicellular, peptidoglycan cell wall, circular DNA, 70S ribosomes and lack of membrane-bound organelles
• Appendages


  Appendages may be present in some bacteria. Fimbriae and pili are hollow, hair-like structures composed of protein. Fimbriae are short, bristle-like fibres involved in attachment to surfaces or other bacteria/organisms. Pili are longer and involved in making contact with a surface and retracting to pull the bacteria forward. Flagellum is a long appendage for motility
• Glycocalyx

  Some bacteria have a layer of polysaccharides known as glycocalyx (sugar coat) surrounding the peptidoglycan cell wall. The glycocalyx can take the form of a slime layer (a diffused mass) or capsule (a distinct and thicker layer)
• Ways bacteria may be classified

  • Morphology (coccus, bacillus, spiral, filamentous)
  • Amount of peptidoglycan in the cell wall
  • Phylogeny
• Prokaryotic organisms are grouped in the domain Bacteria or domain Archaea
• Phylogeny refers to the evolutionary history of a species or a group of related species
• Some differences between prokaryotic and eukaryotic cells include:
• Peptidoglycan Cell Wall

  The bacterial cell wall surrounds the plasma membrane of the bacterial cell. It consists of a polymer called peptidoglycan, which are long polysaccharide chains cross-linked by short chains of amino acids. The cell wall protects the bacterial cell from osmotic lysis
• Cell Surface Membrane

  It is made up of a phospholipid bilayer. In addition to the roles of a cell surface membrane, the membrane of a bacteria is also where the proteins and enzymes used during photosynthesis and/or respiration of eukaryotic cells are found. The plasma membrane folds inward to form the mesosome, the site of respiration and functionally equivalent to the mitochondria
• Genetic Material


  It consists of a single circular DNA (bacterial chromosome), which is double-stranded and associated with histone-like proteins. It is not located within a membrane-bound nucleus but in the nucleoid
• Storage Granules

  • Nutrients and chemical reserves may be stored in the cytoplasm in the form of granules such as glycogen, lipids, and ions like phosphorous and magnesium
• Animal and plant cells are examples of eukaryotic cells. A typical eukaryotic cell consists of the following: Cell Surface Membrane, Nucleus, Cytoplasm
• The nucleoid region of a bacterial cell is stained less densely than the surrounding cytoplasm. Condensation of the bacterial DNA is made possible through its association with histone-like proteins
• Nucleus


  • Contains the hereditary material, DNA, which directs cellular activities
• Subcellular contents of cells are seen in greater detail under an electron microscope compared to a light microscope. Cell ultrastructure refers to a cell’s anatomy revealed under an electron microscope
• Structure of Ribosomes will be covered in greater detail under the topic of Bacteria
• Organelles

  • Discrete subcellular structures with specific functions. Some are enclosed by membranes while others are not. Some form part of the endomembrane system
• Ribosomes

  • 70S ribosomes are present for protein synthesis. Each 70S ribosome consists of a small (30S) subunit and a large (50S) subunit. They give the cytoplasm of bacteria a granular appearance in electron micrographs
• Genetic Material

  • Consists of a single circular DNA (known as the bacterial chromosome), which is double-stranded and associated with histone-like proteins
  • Not located within a membrane-bound nucleus but in the nucleoid (a region of cytoplasm that appears lighter than the surrounding cytoplasm in electron micrographs)
  • Plasmids are usually present in bacterial cells and are much smaller, double-stranded, circular, extra-chromosomal DNA (outside of the chromosome)
  • Plasmids are autonomously replicating DNA (replicate independently of the bacterial cell cycle)
  • Plasmids mostly carry only a few non-essential genes that may confer advantages on bacteria living in stressful environments (e.g. antibiotic resistance)
  • Plasmids occur as multiple copies per cell
• Cell Surface Membrane


  • Defines the boundary of a cell and retains its contents. Acts as a barrier controlling the movement of substances in and out of the cell due to its selective permeability