Categorization Humans and biologists intrinsically categorize things Early classification e g plants vs animals was too simplistic and became insufficient with new discoveries Modern Classification Relies heavily on genetics and biochemistry rather than just physical appearance or basic functions Cell Theory Mid 1800s Pillars The cell is the fundamental unit of life All organisms are composed of one or more cells All cells arise from pre existing cells Note The formation of the first cell from primordial soup is an exception arising from molecular aggregations and self replicating molecules but subsequent cells follow the rule of arising from pre existing cells 2 1 Prokaryotic vs Eukaryotic Cells Defining Feature The fundamental difference lies in the presence or absence of a nucleus a membrane bound kernel Eukaryotes eu true karyon kernel Possess a true nucleus Prokaryotes pro before preceding karyon kernel Lack a true nucleus Feature Prokaryotes e g Bacteria Archaea Eukaryotes e g Animals Plants Fungi Nucleus Absent genetic material in nucleoid region Present DNA housed within a double membrane nucleus Cytoplasm Fluid interior nucleoid region and ribosomes visible Fluid interior cytosol containing many membrane bound organelles Organelles Generally lack membrane bound organelles minor exceptions but for MCB 150 assume none Ribosomes are present Numerous membrane bound organelles e g ER Golgi Mitochondria Lysosomes Ribosomes are present free membrane bound Genome Circular DNA typically single copy in nucleoid region Linear DNA multiple copies chromosomes in nucleus Size Typically 1 10 micrometers m Typically 10 100 micrometers m Cell Wall Most have a cell wall different composition than plants Plant cells have cell walls cellulose animal cells do not Complexity Simpler structure less compartmentalization More complex highly compartmentalized Nucleoid Region The area in a prokaryotic cell where the genome DNA is concentrated not a membrane bound organelle Contains genetic material and many ribosomes Plasma Cell Membrane Essential boundary for every living cell Key functions maintaining a constant internal environment selective permeability receiving exchanging information and cell to cell attachment Cell Wall Provides protection and shape Differs significantly in composition between prokaryotes e g peptidoglycan and plants cellulose Glycocalyx A slimy or gooey outer layer made of various molecules found on many prokaryotes A specialized subset the capsule often aids in virulence or attachment e g infectious microbes Appendages Prokaryotic Flagella Long whip like structures primarily for propulsion Pili singular pilus Shorter hair like structures usually for attachment to other cells or surfaces 2 2 The Three Domains of Life Carl Woese 1977 Revolutionized biological classification by studying and comparing small subunit ribosomal RNA ssu rRNA sequences across various organisms 1 2 ssu rRNA is a universal molecule present in all living organisms Its sequence evolves slowly making it an excellent molecular clock for determining deep evolutionary relationships Slight changes in spelling indicate relatedness Conclusion Woese s work revealed that what was previously considered one group Prokaryotes was actually two fundamentally distinct groups Domains All life on Earth is categorized into three overarching domains 1 2 3 Bacteria Ubiquitous found in every conceivable environment on Earth e g deep sea vents volcanoes soil nuclear reactors They are highly diverse Archaea Many are extremophiles thriving in harsh conditions like high pressure temperature or salinity but they are also found in temperate environments e g in this room possibly inside the human body Crucially Biochemically archaea are more similar to eukaryotes than to bacteria despite their prokaryotic cell structure Eukarya The domain that includes animals plants fungi and protists all characterized by eukaryotic cells with nuclei Evolutionary Tree All life diverged from a Last Universal Common Ancestor LUCA Bacteria represent one early branch while Archaea and Eukaryotes share a more recent common ancestor The tree of life is often described as more of a web due to horizontal gene transfer Prokaryote as a term Still acceptable to describe cells lacking a nucleus but microbiologists often prefer to distinguish between Bacteria and Archaea due to their fundamental differences and Carl Woese s impact Hint Know Carl Woese s name and contribution for Exam 1 2 3 Cell Size and Microscopy Why Cells are Small Cell size is primarily dictated by the critical surface area to volume ratio As cell size increases volume increases disproportionately faster than surface area Physiological Importance Cells are like factories they require efficient import of raw materials and export of products waste A large volume with an insufficient surface area like having too few doors for a very large factory limits metabolism and transport making the cell non functional Optimal cell size achieves a balance where the surface area is adequate for the metabolic needs of the cell s volume An elephant is multicellular not one giant cell because a single giant cell would not be able to sustain its metabolism Resolution The ability to distinguish two separate objects that are close together Human Eye Resolution limited to approximately 0 5 mm Light Microscopy Uses light and lenses to magnify images Can see entire cells down to 0 2 m but generally cannot resolve internal organelles or very small structures like viruses Electron Microscopy EM Uses a beam of electrons instead of light providing significantly higher resolution down to the atomic level 0 2 nm Transmission Electron Microscopy TEM Electrons pass through thinly sliced samples revealing internal structures Electron Micrographs Images produced by EM Key interpretation Darker areas denser material e g the two chocolate layers of an Oreo cookie in an EM represent the denser head groups of a lipid bilayer while the lighter cream middle is the less dense hydrocarbon tails Macromolecules The Building Blocks of Life Four Major Classes Proteins Nucleic Acids Carbohydrates Polysaccharides and Lipids Dry Weight After water 70 80 of cell mass is removed macromolecules constitute the majority of the cell s dry weight Proteins 60 of all macromolecules Nucleic Acids Next largest group Carbohydrates Next largest group Lipids Smallest by mass numerous but tiny Monomer Building Block Polymer Relationship Covalently