Transcript Lecture 4: A Seperate Self: The Cell

BIO 10
Lecture 4
A SEPARATE SELF: THE CELL
What is a Cell?
• “The functional unit of all life forms” (Krogh)
• The smallest enclosed system whereby the
information molecule (DNA) can protect and
perpetuate itself (Selfish Gene model)
• “The smallest thing that can be said to be alive”
(Dr. Ballard)
– Direct a stream of negative entropy upon
itself
– Separate self from non-self
– Reproduce
– Evolve
Common
Characteristics of all
Cells on Earth
• Use DNA as their information molecule
• Use proteins as their “workhorses”
– Use the same 20 amino acids in their proteins
– Use only “left-handed” amino acids
• Have a phospholipid bilayer cell membrane
• Produce and use ATP as their “energy
currency”
• Except for some notable exceptions, are
between 1 micron and 100 microns in
diameter
Why are Cells
the Size
they Are?
Learn the Metric
System of
measurement
• 1 m = about 3 feet
• 1 mm = 1/1000 of a meter (= 1/10th of a cm;
Can be seen on a ruler but just barely!)
• 1 um = 1/1,000,000 of a meter (much too
small to see with the naked eye)
• 1 nm = 1/1,000,000,000 of a meter
– A DNA molecule is 2 nm wide
Prokaryotes:
1 – 10 um in diameter
Eukaryotes: 10-100 uM in diameter
1/10th of the distance
between the smallest
divisions on a metric ruler
Plant cell: 100 um diameter
Note that
this scale is
logarithmic!
• The size of cells is influenced by trade-offs
involving:
– Predation considerations
• Larger cells can engulf smaller cells more easily and are less
likely to be eaten themselves
• BUT ... larger cells must eat (or photosynthesize) more to
sustain themselves
– Surface:volume ratio
• Cells must take in nutrients and dispose of waste through
their cell membranes
• As the diameter of a cell doubles, its volume increases 8-fold
(2-fold in length, width, and height)
• Thus, as a cell’s diameter increases, the cell membrane must
become ever more efficient in order to still do its job
– Diffusion
• Molecules in the same biochemical pathways must be able to
find one another
• Prokaryotic cells have remained very
small, like the earliest cells on Earth
• Eukaryotic cells have been able to grow
100-1,000 times larger than prokaryotic
cells
– 10-fold bigger in all 3 dimensions = 1,000x
the volume
– Have a system of internal membranes
that acts to:
• Increase the total membranous surface
available for cell metabolism
• Separate the components of chemical
reactions into different internal “bags”
Prokaryotes
– “Prokaryote” means “before a nucleus”
– No internal membrane-bound organelles –
just one little bag of cytoplasm
– No nucleus
– Usually single-celled (may form simple
colonies)
– May or may not require oxygen for survival.
– Earliest types of cells on Earth
– Cell type of all bacteria and Archaea
• Much tougher than eukaryotes
– Can survive almost anywhere – and do!
• Have much greater genetic diversity than
eukaryotes
• Have a cell wall surrounding the cell
membrane (different chemistry from plant
cell wall)
Eukaryotes
– Means “true nucleus”
– The nucleus is a membrane-bound organelle
that encloses and protects the DNA
– Contains many other membrane-bound
organelles that have a variety of functions
– Many eukaryotesare multi-cellular and highly
complex
- Usually require oxygen for survival
- Cell type of all animals, plants, fungi, and
protists (e.g. amoeba, diatoms)
Common Components of
Eukaryotic Cells
• The five main components shared by all
eukaryotic cells are: a nucleus, other
membrane-bound organelles, a cytosol, a
cytoskeleton, and a plasma membrane.
Touring A Eukaryotic Cell
• Cells can be viewed as tiny factories for the
production of the proteins that sustain it (and the
precious DNA cargo it holds)
– This is simplistic, since other biomolecules are also important for
life
– But proteins do most of the work in the cell
• Therefore, following the process of protein
production is one of the best ways to tour a
eukaryotic cell for the first time
– The code for the amino acid sequence of each protein is carried
by regions of the DNA called genes
– Therefore, our tour of the cell will begin in the nucleus
• In the nucleus:
– DNA is enclosed in a double-thick membrane called the
nuclear envelope
• Humans have 46 DNA molecules per cell
• Some organisms have more, some fewer
– One strand of each gene is copied into an RNA
molecule, which exits the nucleus (through nuclear
pores in the nuclear envelope) and travels to where
proteins are made, the cytoplasm.
• Average size of a gene in bacteria: 2,000 bp DNA
• Average size of a human gene: 20,000 bp DNA
– A subset of RNAs, called messenger RNAs (mRNAs),
carry the codes for making polypeptide chains
(proteins)
– In the cytoplasm:
• mRNAs are recognized and bound by large
protein/RNA complexes called ribosomes
• Ribosomes “read” the messenger RNA
molecules to produce the correct polypeptides
• Most polypeptides are then released into the
cytoplasm, where they fold into functional
proteins
polypeptide
protein
• Some of the messenger RNAs code for proteins
that need to be secreted from the cell or be
modified in some way before they can function
properly
– These mRNAs, along with their attached
ribosomes, are transported to the endoplasmic
reticulum (ER)
• Consists of a series of flattened membrane sacs called
cisternae.
– Once at the ER membrane, the ribosomes
continue “reading” the mRNAs to produce the
encoded polypeptides
• These polypeptides are then inserted through pores in
the ER membrane into the interior of the ER for sorting,
modification, and distribution
• Some of the proteins remain in the ER at this
point
– They are “resident ER proteins” that function and
belong there
• Others continue on to the next membranebound organelle, the Golgi Apparatus
– To facilitate this transfer, the ER membrane buds
off to form balls of membranous sacs called
transport vesicles
– These sacs then travel (with their polypeptide
cargo) to the Golgi Apparatus and merge with its
membrane, dumping the polypeptides inside
• The membrane of
the Golgi Apparatus
can also bud off.
• Transport vesicles
can then merge with
the cell membrane
to release
polypeptides from
the cell in a process
called exocytosis
Other Important Organelles/Cell Structures
Animal
Cells
Include:
– Lysosomes
• The cell’s recycling organelles
• Break down large molecules from food, defective
organelles, or old proteins into their monomers for
reuse
– Mitochondria
• Extract carbon and energy from food, using oxygen
• Are themselves ancient bacteria that invaded an
early “proeukaryote”
• Contain their own bacterial DNA
• Have bacterial-type ribosomes
– The Cytoskeleton
The Lysosome
The Mitochondrion
The Cytoskeleton
Summary: The Cell as a Factory
Plant and Animal cells contain many of
the same structures and organelles
• However, plant cells also contain these
additional components:
– Cell wall: functions include: structural strength,
limiting water absorption, and protection.
Composition is cellulose and lignin.
– Central vacuole: functions include: storing
nutrients and water, retains and degrades
wastes
– Plastids: functions include: gathering and
storing nutrients, and photosynthesis
(chloroplasts)
• Chloroplasts also have a bacterial origin
The Plant Cell
The Chloroplast
Short Review of Lecture 4
• What is the smallest functional unit that can be said
to be “alive”?
• How big are cells and why?
• How are prokaryotic and eukaryotic cells the same?
In what ways are they different?
• What do plant and animal cells have in common? In
what ways do they differ and why?
• What is the purpose of DNA? For what does it encode
and by what pathway?
• How does each of the following molecules or
organelles contribute to one of the four primary
functions of life? DNA, ribosome, mitochondria,
chloroplast, cell membrane?