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
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: The Working Units of Life
Cells are able to specialize
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 & Eukaryotic Cells
Prokaryotic Cells
Bacteria and Archaea
Eukaryotic Cells
All other cells
e.g., Plants, Fungi, Animals (including humans), etc.
Prokaryotic & Eukaryotic Cells
Prokaryotic 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
The Protein Pathway
Cells produce lots of proteins, regardless of the cell type or the organism in which the cell is 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
Cytoskeleton
Literally, the cell’s skeleton
Network of protein filaments
Functions
Cell structure
Movement of cells
Movement of materials within cells
Cytoskeleton
Three types of fibers
Microfilaments
Intermediate filaments
Microtubules
Cytoskeleton
Microfilaments
Structural role
Movement of cell
e.g., Formation of pseudopodia caused by extension of microfilaments
Cytoskeleton
Intermediate filaments
Structural role
Form fairly permanent network
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
The Plant Cell
Central Vacuole
Very prominent in appearance
May comprise 90% of cell volume
Stores nutrients
Degrades waste products
Balances cell pH
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
The Size of Cells
As volume increases
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
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
Cell Video
Inner workings of a cell
Unicellular organism: Amoeba