Lecture 3
Cells: The Working Units of Life
Cell Theory
 All living organisms are comprised of one or more cells
 All cells come from preexisting cells
 Cells are the basic units of life
Cells: The Working Units of Life
 Viruses blur the boundary between living and non-living
 Possess some of the features of life
 Lack some features of life
Notably, they are not cellular
Cells: The Working Units of Life
 The earliest cells arose on Earth over 3.5 billion years ago
 All other cells can be traced back to these earliest cells
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Cells: The Working Units of Life
 Multicellular organisms consist of cells and materials outside of cells
 Many of these materials are themselves produced by cells
 e.g., Hormones, calcified tissue of bones, etc.
 Cells are able to specialize
Cells: The Working Units of Life
 Muscle cells  contraction
 Nerve cells  signal transmission
 Despite their diverse specializations, all cells are fundamentally similar—similar parts, basic
functions
Prokaryotic & Eukaryotic Cells
 There are two fundamentally different types of cells
 All cells are either:
 Prokaryotic
 Eukaryotic
 Prokaryotic Cells
Prokaryotic & Eukaryotic Cells
 Bacteria and Archaea
 Eukaryotic Cells
 All other cells
 e.g., Plants, Fungi, Animals (including humans), etc.
 Prokaryotic Cells
Prokaryotic & Eukaryotic Cells
 “Before nucleus”
 DNA is not enclosed within a nucleus
 Eukaryotic Cells
 “True nucleus”
 DNA is enclosed within a nucleus
Prokaryotic & Eukaryotic Cells
Both prokaryotes and eukaryotes display fantastic diversity and success
Procaryotes: vital for life on the plant, in spite of their small size
Procaryotes: representative of first life forms on planet
Discussion later in Diversity
Eukaryotic Cells
 Eukaryotic cells have five major components
 The nucleus
 Other organelles
 The cytosol
 The cytoskeleton
 The plasma membrane
 These five structures have smaller structures within them
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The Protein Pathway
 Cells produce lots of proteins, regardless of the cell type or the organism in which the cell is
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found
This process involves several structures within the cell
“Interconnectedness”
Protein pathway: model for cell activities
The Protein Pathway
The nucleus
 Contains DNA
 DNA contains information for protein production
 Surrounded by a double membrane
 “Nuclear envelope”
The Protein Pathway
The nucleus
 The nuclear envelope is studded with nuclear pores
 Allow transport of molecules to and from the nucleus
The Protein Pathway
The nucleus
 DNA’s information is copied into mRNA
 “Messenger RNA”
 mRNA is transported to the cytoplasm
 Exits nucleus through nuclear pores
The Protein Pathway
Ribosomes
 Present in thousands of copies
 Organelles serving as sites of protein synthesis
 Binds to an mRNA molecule
 Reads information on the mRNA molecule
 Assembles amino acids to form a protein
The Protein Pathway
Ribosomes
 Some proteins are destined for export
 After a short portion of such a protein is made, the ribosome attaches to the rough
endoplasmic reticulum
The Protein Pathway
Rough Endoplasmic Reticulum (RER)
 Folded up continuation of nuclear envelope
 First component of endomembrane system
 Aids in protein processing
 Appears rough due to ribosomes studding surface
The Protein Pathway
Rough Endoplasmic Reticulum (RER)
 Ribosome docks on surface of RER
 Chain of amino acids is threaded into the chamber of the RER
 “Cisternal space”
 Protein folds into appropriate shape
 Sugar groups may be added to the protein
 “Protein processing”
The Protein Pathway
Transport Vesicles
 Second component of endomembrane system
 A piece of the RER membrane can “bud off” to form a transport vesicle
 The newly made protein is enclosed within this vesicle
 Transports protein to Golgi complex
The Protein Pathway
Golgi Complex
 Series of connected membrane sacs
 Third component of endomembrane system
 Transport vesicle fuses with Golgi complex
 Protein now present in the cisternal space of the Golgi complex
The Protein Pathway
Golgi Complex
 Protein modification
 e.g., Sugar groups trimmed
 e.g., Phosphate groups added
The Protein Pathway
Golgi Complex
 Sorting and shipping of proteins
 Adds chemical tags to proteins
 Tags function as zip codes
 90210  Beverly Hills, CA (plasma membrane)
 55113  Roseville, MN (lysosome)
The Protein Pathway
Golgi Complex
 Transport vesicles containing processed and tagged proteins bud from outer face of Golgi
complex
The Protein Pathway
After the Golgi Complex
 Some transport vesicles fuse with the plasma membrane
 Protein contents are ejected from cell
 “Exocytosis”
 Some transport vesicles reach other destinations
 e.g., Other organelles
Other Cell Structures
 Cells are involved in many processes in addition to protein synthesis and shipment
 Other organelles are involved in these processes
Other Cell Structures
Smooth Endoplasmic Reticulum (SER)
 Network of folded membranes continuous with the RER
 Surface NOT studded with ribosomes
 Surface appears smooth
 NOT involved in protein synthesis
Other Cell Structures
Smooth Endoplasmic Reticulum (SER)
 Site of synthesis of various lipids
 Fats synthesized in SER of liver cells
 Steroid hormones (estrogen and testosterone) synthesized in SER of ovaries and testes
 Detoxification of harmful substances
 e.g., Alcohol detoxified largely in SER of liver cells
Other Cell Structures
Lysosomes
 Present in hundreds of copies
 Membrane-bound sac
 Acidic interior
 Contains various hydrolytic enzymes
Other Cell Structures
Lysosomes
 Digests worn-out cellular materials
 Digests foreign materials entering cell
 Useful molecules reused
 Waste molecules expelled from cell
Other Cell Structures
Mitochondria
 Descended from bacteria living inside cells
 “Endosymbiosis”
 Present in multiple copies
 Oxidize (burn) food to release energy
 Oxygen is required for this process
 This energy is used to fuel various cellular processes
 Literally, the cell’s skeleton
 Network of protein filaments
 Functions
Cytoskeleton
 Cell structure
 Movement of cells
 Movement of materials within cells
 Three types of fibers
Cytoskeleton
 Microfilaments
 Intermediate filaments
 Microtubules
Cytoskeleton
Microfilaments
 Structural role
 Movement of cell
 e.g., Formation of pseudopodia caused by extension of microfilaments
Intermediate filaments
 Structural role
 Form fairly permanent network
Cytoskeleton
Cytoskeleton
Microtubules
 Largest cytoskeletal elements
 Structural role
 Railroad tracks (freeways) within cell
 e.g., Transport vesicles travel along microtubules from RER to Golgi complex
Cytoskeleton
Microtubules
 Components cilia and flagella
 Hairlike extensions of cell
 Movement of microtubules within these structures moves these structures
Cytoskeleton
Cilia
 Present in large numbers on ciliated cells
 Beat in unison
 Beating moves the cell
 e.g., Some microorganisms
 Moves material across the cell
 e.g., Cells in human respiratory system and oviducts
Flagella
 Present in one or few copies on flagellated cells
 Beating moves the cell
 e.g., Human sperm cell
The Plant Cell
 Many of the structures present in animal cells are also present in other types of cells
 e.g., Fungi, plant cells, etc.
 Other types of cells have some structures absent in animal cells
 We will now study plant cells in more detail
The Plant Cell
 Plant cells also possess certain structures absent in animal cells
 Cell wall
 Chloroplasts
 Central vacuole
Central Vacuole
 Very prominent in appearance
 May comprise 90% of cell volume
 Stores nutrients
 Degrades waste products
 Balances cell pH
The Plant Cell
The Plant Cell
Cell Wall
 Protective covering external to membrane
 Present in virtually all plants
 Contain cellulose
 Different materials comprise cell walls of many different organisms
 Limits water uptake
 Limits flexibility
The Plant Cell
Plastids
 Possessed only by plants and algae
 Various functions
 Gather and store nutrients
 Contain pigments
 One important type of plastid is the chloroplast
The Plant Cell
Chloroplasts
 Site of photosynthesis
 Use sunlight to convert CO2 into food
 Ultimately supports virtually all organisms
 Produces O2 as a byproduct
 Also important to many organisms
The Size of Cells
 Cells are small
 Small can encompass many orders of magnitude
The Size of Cells
Why are cells so small?
 Cells are chemical factories
 Factories require shipments in and out
 Cells need enough surface area to allow these shipments
 As volume increases
The Size of Cells
 Surface area increases, but not as much
 Surface area:volume ratio decreases
 The cells requirement for materials increases, but the ability to ship these materials does not
increase enough
Endosymbiosis
 Mitochondria are the descendents of free-living bacteria
 So are chloroplasts
Bacteria invaded early eukaryotic cells
Took up permanent residence
Both became dependant upon the other
How do we know that this happened?
Endosymbiosis
Evidence for an endosymbiotic origin
 Mitochondria and chloroplasts
 Possess bacterial ribosomes
 Possess bacterial DNA
 Divide like bacteria
 Extra-nuclear nucleic acids used in study of evolution—trace maternal sources
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Endosymbiosis
Why did endosymbiosis happen?
 Eukaryotic cells invaded were rather intolerant of oxygen
 These bacteria were tolerant of oxygen
 Both components of this symbiotic relationship derived benefit
 Inner workings of a cell
 Unicellular organism: Amoeba
Cell Video