Ultrastructure of cells

• Nucleoid


  • Central feature of prokaryotic cells
  • Region within the cell where the DNA is located
  • DNA is not segregated from the rest of the cell's contents, but instead floats freely within the cytoplasm
  • Prokaryotic DNA is circular and exists as a single, large, coiled chromosome
  • Location is thought to result from the cell's need to distribute its genetic material evenly during cell division, ensuring genetic material is transferred accurately to offspring cells
• Plasmids


  • Small, circular DNA molecules that exist separately from the main DNA in the nucleoid
  • Can replicate independently
  • Often carry genes that may offer an evolutionary advantage to the organism, such as antibiotic resistance genes or genes necessary for metabolising uncommon substrates
  • Can be transferred between prokaryotic cells in a process known as horizontal gene transfer, allowing for the rapid spread of beneficial traits through a population
• Cell Wall


  • Vital structure in prokaryotes
  • Provides physical protection and rigidity to the cell and prevents the cell from bursting under different osmotic pressures
  • Composition varies between bacteria and archaea
  • Bacteria possess a peptidoglycan cell wall, a complex lattice structure that encloses the entire bacterium and provides structural strength
  • Archaea do not contain peptidoglycan in their cell walls, instead their cell walls can contain a variety of other polysaccharides, proteins or pseudopeptidoglycan, providing a unique chemical signature that separates archaea from bacteria
• Ribosomes


  • Sites of protein synthesis in all living cells, translating the genetic code from messenger RNA into amino acid sequences to create proteins
  • Prokaryotic ribosomes are 70S (Svedberg units), smaller than the 80S ribosomes found in eukaryotes
  • Consist of a large (50S) and a small (30S) subunit
  • Can be found scattered throughout the cytoplasm or attached to the plasma membrane, allowing for simultaneous transcription and translation, a feature not seen in eukaryotes due to the separation of the nucleus and cytoplasm
• Bacteria


  • Most abundant group of prokaryotes and are among the earliest forms of life on Earth
  • Metabolically diverse, capable of exploiting a wide range of organic and inorganic substances to obtain energy
  • Can thrive in a variety of environments, including extreme conditions such as hot springs and the deep sea, to more familiar settings like soil, water, and the human body
  • Cells are typically a few micrometres in length and come in several shapes, including spheres (cocci), rods (bacilli), and spirals (spirilla)
  • Can have additional structures like pili (used for attachment), a capsule (for protection), and flagella (for movement)
• Archaea


  • Single-celled prokaryotic organisms similar to bacteria in form and function, but their genetic makeup and biochemical characteristics distinguish them significantly
  • Some of their genes and metabolic pathways more closely resemble those in eukaryotes, leading some scientists to propose archaea as a stepping stone in the evolution of eukaryotic life
  • Known for their ability to survive in extreme environments, such as high salt concentrations (halophiles), high temperatures (thermophiles), and anaerobic conditions like swamps and animal guts (methanogens)
  • Their unique properties and survival mechanisms have led to intense scientific research, investigating their potential applications in biotechnology and their implications for life in extreme environments
• Microscopy

  (Nature of Science)

  • progress in biology have been due to advances in the design of the microscope
  • Light Microscopy ( late 19th century)
  • Allowed for discovery of bacteria, other unicellular organisms
  • Chromosomes, process of mitosis, meiosis and gamete formation
  • complexity of organs, some organelles
  • Electron Microscopy ( Germany, 1930s)
  • revealed the ultrastructure of cells
  • examples: ribosomes, lysosomes and rER
  • Electron tomography (producing 3D images by electron microscopy)
  • Table of Resolution
• Prokaryotic Cell Structure

  • first organisms to evolve on earth
  • simplest cell structure / no compartments
  • cell wall, made of peptidoglycan, maintains shape and prevents bursting
  • biochemically still complex as cytoplasm is single uninterrupted chamber
  • Ribosomes 70s
• • complex cell structure
  • cells compartmentalised
  • enzymes and substrates can be more concentrated
  • substances that could damage cell are contained
  • condition for processes like pH or temperature can be kept at ideal levels easily
  • organelles with contents can move around easily
  • Ribosomes 80s
• Organelles of Eukaryotic Cells

  • Nucleus: double nuclear membrane, with pores; contains chromosomes, DNA associated with histone proteins; uncoiled chromosomes called chromatin; where DNA is replicated and transcribed to form mRNA, exported via pores to cytoplasm
• • rER (Rough Endoplasmic Reticulum) : flattened membrane stacks (linear) called cisternae; attached to them are ribosomes (80s); main function is to synthesise protein for secretion outside cell; protein when synthesised carried by vesicles, that bud off and move to Golgi apparatus
• • Golgi Apparatus: flattened membrane stacks (not long, curved); no attached ribosomes; have many vesicles in close proximity; processes proteins brought from rER; then carried to plasma membrane for secretion
• Lysosome: approximately spherical; formed from Golgi apparatus; high concentration of digestive enzymes used to breakdown ingested food, organelles or even whole cell
• • Mitochondrion: double membrane, inner membrane invaginated to form cristae; fluid inside called matrix; produce ATP for aerobic cell respiration; fat is digested here if used as energy source
• • Free Ribosomes: appear as dark granules; not surrounded by membrane; synthesise protein, releasing it to work in cytoplasm as enzymes etc. constructed in nucleolus (labelled above)
• • Chloroplast: double membrane; inner stacks called thylakoids (flattened stacks of membrane); produce glucose and other organic compounds by photosynthesis; cytoplasm called stroma;
• • Vacuoles & Vesicles: single membrane with fluid inside; plant cells have large permanent vacuoles; animals absorb foods from outside and digest in vesicles; unicellular organisms use vacuoles to expel excess water (contractile vacuole); used to transport materials inside cell
• • Microtubules and Centrioles: small cylindrical fibres; move chromosomes during cell division; animal cells have centrioles ( two groups of nine triple microtubules); they form anchor during cell division.
• • Cilia and Flagella: whip like structures projecting from cell surface; ring if nine double microtubules ( + 2 central ones); flagella is larger and only one present; cilia are smaller and present in multitude; locomotion; cilia used to create current in fluid next to cell