Transcript Why Are Organisms Made of Cells

Why Are Organisms
Made of Cells
Chapter 4
Antonie van Leeuwenhoek, a Dutch
merchant made glass lenses by
polishing bits of glass and mounting
lenses between gold plates to examine
and magnify things. He was first to
see red blood cells, little animalcules,
and that insects hatched from eggs.
He sent these reports to the Royal
Society of London (over 375 reports).
In 1683 he was the first person to see
bacteria. Robert Hooke was curator
of instruments for the Royal Society.
In 1665 he published a book,
Micrographia. He also coined the
term cellular from looking at cork.
Cells resembled the little rooms in a
monastery.
Neither man’s work was taken
seriously because they were
commoners. Also there was a great
deal of bias left from the Middle Ages
where science was left to magic and
superstition.
Leeuwenhoek’s lenses magnified up
to 300x, sophisticated them but
primitive now. In the 1820’s better
microscopes led to the discovery of
nuclei and that “juice” described by
Hooke was protoplasm.
Lenses of electron microscopes are
electromagnets that bend the path of
electrons. Two kinds of them the
TEM (transmission electron
microscope) and SEM (scanning
electron microscope) can produce an
image on a screen. In the TEM light
passes through the specimen
revealing internal structure. The
SEM shows surface detail.
In 1838 Schleiden theorized that all
plants were made of cells. In 1839
Schwann said the same about animals.
Noncellular organisms do not exist.
They proposed cells crystallized out of
shapeless material but eventually in
1858 Virchow, a physician who saw
cells divide, formalized the phrase”all
cells from cells”.
This concluded that cells could not
come from non-living matter and that
diseases are caused by changes in
cells. Their work makes up the “cell
theory”.
1.All organisms come from cells.
2.Cells are units of structure and
function of organisms.
3.Cells come only from other cells.
Every cell consists of a boundary, cell
body, and set of genes. The plasma
membrane is the boundary, highly
organized and responsive – defines the
limits and regulates the internal
environment.
The genes are DNA, in eukaryotes
contained in a nucleus. Prokaryotes
are the eubacteria and archaebacteria.
DNA is in nucleid with no membrane.
Organelles do specialized tasks.
Cytoplasm not contained in organelles
is cytosol. Most of the cell’s
biochemical work takes place there.
Protein fibers run through it forming a
cytoskeleton, giving the cell shape and
helps in cell movement.
A cell is alive because it is made of
organized parts, performs chemical
reactions, responds to the
environment, changes over time,
reproduces, and shares evolutionary
history.
Organelles work together to maintain
homeostasis. Most capture energy
from glucose, oxidize it to CO2 and
H2O which takes place in organelles
or in the cytosol.
Cells change over time chemically
and mechanically: muscle cells
shorten, some change size and shape.
To do this eukaryotes have a
cytoskeleton.
Cells can copy genes for reproduction
( read and duplicate DNA).
Different cells make different proteins.
Wastes, CO2, and ammonia are
excreted. Most cells are in a similar
size range 10-100 micrometers.
(eukaryotes) Prokaryotes are 0.4-0.5
micrometers. Ostrich eggs are huge.
Plant fibers can be meter- long cells,
and over meter long cells in a giraffe’s
leg.
Size is limited by the cell’s need to
regulate its internal environment.
Cells need to maintain homeostasis,
same internal pH, concentration of
salts, take in useful molecules, and get
rid of wastes. Size is limited by the
plasma membrane’s ability to do this.
Protists are able to do these functions
(Didinium and Paramecium).
Cells are limited by the surface – to –
volume ratio. Larger cells have smaller
surface to volume ratios to regulate the
internal environment. Large cells have a
hard time getting nutrients, getting rid of
wastes, and regulating internal
concentrations of ions and molecules.
How can eukaryotic cells be larger than
prokaryotic?
Eukaryotes have special adaptations to
increase their surface areas, convoluted
membranes and elaborate internal
membrane systems.
RBC’s carry O2, muscle cells contract,
cells of plants absorb nutrients.
Cells live and die independently of the
whole organism. Skin, blood, and
intestines replace themselves. The life
of a multicellular organism can extend
beyond the life of a cell. Many different
organelles can be compared to a walled
city- there are power stations, a library,
warehouses to package proteins.
The development of the TEM in the
1940’s-50’s surprised scientists with
the complexity of internal
membranes, and number of
compartments. Vesicles (empty sacs
can be 100 nm., and can be 95% of a
plant cell.
The Nucleus is 5-19% of the cell. Inside
are chromosomes – complexes of DNA
and protein. The nucleus is the library
and contains instructions for forming
new cells. The boundary is a double –
membrane, a nuclear envelope with
nuclear pores. The pores are channels
between the inside and the cytoplasm to
control movement of materials in and out
of the nucleus.
The DNA is the code for building all
polypeptides the body will ever need.
The cytosol is about ½ of the cell
volume. It can be separated from the
cell by breaking open cells and
spinning the solution in a centrifuge at
100,000x gravity (2,000 – 80,000
r.p.m.’s)
Heavier larger cell fragments
concentrate at the bottom. There are
thousands of enzymes in the cytosol
that produce building blocks, degrade
small molecules, and synthesize
proteins.
Cytosol is aqueous but about 20%
protein giving it a viscosity like jello.
Granules of energy rich droplets of fat
called ribosomes (15-30 nanometers)
and smaller proteosomes contain RNA
and are where proteins are put
together with peptide bonds. They are
bound to thr rough E.R. or float freely.
Proteosomes brek up old proteins and
recycle amino acids.
Peroxisomes contain enzymes that
transfer hydrogen from substrates to
toxygen forming H2O2. They can
break down fatty acids and detoxify
substances. Glyoxysomes are found in
fat storing seeds of plants.
The endoplasmic reticulum makes
proteins and lipids. It forms a
convoluted network throughout the
cell. The ER membrane encloses a
network of cavities and channels
called the lumen that make up 15% of
the cell’s volume. The E.R. consists of
the rough area dotted with ribosomes
on the cytosol side to make proteins to
be exported from the cell.
The smooth E.R. has no ribosomes; it
synthesizes lipids and breaks down
toxins. Both rough and smooth E.R.
are in eukaryote cells. Specialized cells
may have more of one type than the
other.
The pancreas has a lot of rough E.R.
(makes digestive enzymes and
insulin). Cells that produce lipid and
steroid hormones (adrenal glands and
liver) have a lot of smooth E.R.
The Golgi Complex is a packaging center.
In cell reproduction and maintenance it
forms structures that stay in the cell like
lysosomes and prepares materials for
export. It is made of sets of flattened
discs (in 6’s) with small vesicles at the
ends. Cells that make glycoproteins have
more.
Glycoproteins are proteins with attached
sugars. (albumin in egg white for ex.).
Palade and Farquhar labeled new
glycoproteins with radioactive tracers.
Proteins to be exported appeared first in the
rough E.R., then the Golgi complex. The
Golgi complex modifies the glycoproteins
and packages them in secretory vesicles.
Vesicles fuse with the plasma membrane and
discharge their contents.
The Golgi complex manages the flow of
proteins to different destinations by modifying
the carbohydrates on glycoproteins- labels
them with tags that direct them to specific
locations, lysosomes, or outside the cell.
Lysosomes are in all eukaryotic cells. They
contain enzymes that break down proteins,
nucleic acids, sugars, and lipids.
Vacuoles of plant cells are like large
lysosomes. Lysosomes are numerous in
phagocytic cells that consume and digest
food. (amebas, other protists, and our
white blood cells).
The membrane of a lysosome keeps
enzymes from digesting the cell’s cytosol.
If the membrane breaks down the cell
digests itself. Lysosomes are formed by
the Golgi complex. Its enzymes are made
by ribosomes of the rough E.R.
Mitochondria obtain energy from
nutrients. They make most of the ATP
for the chemical reactions of the cell.
Mitochondria convert sugar to ATP.
Under the TEM they are the most
numerous organelles of a eukaryotic cell.
Different kinds of cells have different
numbers of them. Liver and heart cells
may contain thousands (1/4 the cell’s
volume. A TEM shows 2 membranes.
The double membrane and having their
own DNA suggests mitochondria evolved
from eukaryotes that captured bacteria
which evolved into mitochondria.
Plastids also have a double membrane.
Chloroplasts make and store sugar for
food. Most organisms depend on them
for food. They are generally large, round,
and green.
Chloroplasts have internal folded
membranes, thylakoids, piled in stacks of
10 called grana. They have DNA and
make protein so are thought to have
evolved as free-living organisms captured
by early eukaryotic cells that became
chloroplasts.
Chromoplasts contain yellow, orange, or
red pigments and form from chloroplasts
that reshape the membrane and break
down chlorophyll. (tomatoes ripen)
Amyloplasts store starches in roots of
potatoes and seeds like wheat and rice.
The cytoskeleton is a network of protein
filaments that are visible with the TEM.
3 types of filaments are: microtublues,
actin, and intermediate filaments.
The cytoskleton gives support and force
for cell movement, changes in shape and
transport of materials through the cell.
Some proteins are specilaized for muscle
movement (actin). Microtubules are
cylinders functioning in cell division and
dividing materials to daughter cells.
Microtubules originate from microtubule
organizng centers (MTOC’s) near the
nucleus in a zone called the centrosome
that contains a centriole.
Microtubules consist of 2 globular protein
molecules called tubulins. 50 proteins can
form microtubules. Actin are finer,
anchored to the cell surface in muscle
fibers, necessary for contraction.
Intermediate filaments, are fibrous like
keratin that forms hair. They are in parts
of cells that are subject to stress.
Cilia (short and numerous) and flagella
(longer for propulsion) have the same
arrangement in cross section. Around the
periphery are 9 pairs and 2 in the center.
Membranes limit cell size because the
membrane increases in size more slowly than
the volume. It must supply nutrients and O2
and remove wastes. Its functions are:
1.Forms a boundary
2.Regulates contents.
3.Place for chemical reactions and secretion
of enzymes.
4.Participates in energy conversion.
Membranes of cells have lipid bilayers.
RBC’s have been studied. Biochemists
break them open, extract the hemoglobin,
leaving RBC “ghosts”. The most
important lipids are phospholipids which
are amphipathic (hydrophilic heads and
hydrophobic tails).
Proteins occupy the 2 layers of the
plasma membrane (peripheral) and also
the space between (transmembrane).
Membrane proteins are amphipathic.
Enzymes can break down lipids in the
outer or inner layer but not the other.
Some carbohydrates appear only on the
outer surface attached only to proteins
(glycoproteins) or to lipids (glycolipids)
The inner and outer layers differ in how
they interact with membrane proteins .
Proteins that span the entire thickness
orient themselves in a certain direction.
The orientation of the protein is crucial
to allowing molecules to pass through.
Diffusion and Osmosis
 Water
and small molecules can diffuse
through the membrane. Other molecules
must be selectively pumped. Some lipids
move freely through the membrane.
About half of the proteins move freely
within each layer, half remain tightly bound.
Lipids and proteins move easily because
they are in layers of fluid. In 1972 Singer
and Garth proposed it was a fluid mosaic
model and a lipid bilayer.
Proteins are peripheral or transmembrane.
The membrane is a hydrophobic barrier.
Some membrane proteins transport
molecules across the membrane.
Membranes are selectively permeable.
Proteins and ions cannot pass through.
Homeostasis is dependent on the
membrane molecules that actively pull
small molecules and ions through..
Water diffuses. If you add solute like salt to
water its concentration is highest where it
was added but it diffuses until it is
distributed evenly. Still the salt particles
continue to move randomly. A difference
in concentration between 2 areas is a
concentration gradient..
Diffusion and mixing eliminate the
gradient. Rate of diffusion is affected by
temperature, size of molecules, steepness
of the gradient (greater with greater
difference in concentrations).
Osmosis is diffusion of water toward
where concentration of water molecules is
less. Water will move into a cell with a
high solute concentration.
The movement of water down the
concentration gradient to try to equalize
concentration of water is osmosis. As
water flows into a cell it creates force
(osmotic pressure) against the inner
surface of the membrane. It stretches the
surface until its resistance is equal to the
water within it unless it breaks.
When a cell is dropped in water (hypotonic)
it swells. If the cell is in a solution equally
sugary to it, it is isotonic. A red blood cell in
an isotonic solution like plasma remains
intact. In a hypertonic salty solutions it
shrivels (plasmolysis). In a hypotonic solution
like plain water it may take in water until it
bursts (cytolysis).
In plant cells with a cell wall in hypotonic
solution the water moves in and is
contained in a vacuole which may push
against the cell wall until the pressure of
the wall is equal to the pressure of
osmosis, called turgor pressure.
In hypertonic solution water escapes the
vacuole , the cell shrinks away from the
cell wall.. Turgor pressure provides
support to non-woody plants
Passive transport occurs without cell
energy, resulting in equal concentration of
molecules on each side of a membrane.
Active transport moves molecules against
the concentration gradient using cell
energy (ATP).
Some molecules move passively faster
than they diffuse, a process called
facilitated diffusion – the increased rate
of transport depends on membrane
molecules to move faster.
Glucose is facilitated by a
transmembrane protein. The glucose
transporter only binds to glucose.
Active transport can require a special pump
as in the case of the Na-K pump. 3 Na+
are pumped out of cells and 2 K+ pumped
in so that the inside of a cell is negatively
charged with respect to the outside. This
creates resting membrane potential and is
important in conduction of nerve impulses.
There are many such pumps.
Non-animal cells surround themselves
with a rigid carbohydrate cell wall which
can continue after cell death. Animal cells
make their own environment, often a
diffuse network of carbohydrates and
proteins called extracellular matrix.
Membranes help consume and excrete
materials by membrane fusion.
Membranes avoid water , are fluid and
rapidly change shape. In cell division the
membrane of a parent cell reseals itself
around each daughter cell.
In phagocytosis cells engulf large particles
such as microorganisms. Part of the
membrane surrounds particles and forms a
vesicle which takes the particle into the cell,
fuses with a lysosome exposing it to
enzymes.
In endocytosis particles are excreted in
vesicles by fusing with the membrane and
releasing the particles outside the cell.
Pinocytosis takes in bits of liquid. In
receptor-mediated endocytosis the cell
takes up specific substances which it
recognizes by special proteins on the cells
surface called receptors.
Ex: Cholesterol passes through the blood
with an amphipathic protein, LDL, which
enters the cell by receptor mediated
endocytosis.
In exocytosis a cell reverses
pinocytosis. Substances excreted this
way are digestive enzymes, hormones,
and neurotransmitters.
Regulation of exocytosis is important
in cell function. Rapid increase in
intracellular calcium ions stimulate
exocytosis.
Sometimes the vesicles fuse with those of
other cells. Communication occurs
chemically between cells by
neurotransmitters and hormones.
Plant cells have rigid walls with fine
channels (plasmodesmata) through which
thin strands of cytoplasm stream,
allowing molecules to flow from cell to
cell.
Animal cells lack walls. Ions and molecules
pass through gap junctions in membranes of
adjacent cells. Heart cells have gap junctions
to coordinate contraction. Animals also have
adhering junctions (desmosomes) that allow
passage of molecules from cell to cell and
help connect internal cytoskeletons.
Tight junctions fuse membranes in a sheet
so fluid cannot leak between cells, EX.
Distant cells communicate with nerves
chemically. Plant cells turn toward the sun
by a chemical signal.
Release of signaling molecules
bind to other cells at receptors
(ligands) which are specialized
proteins on the surface of a cell or
within a cell that only recognize
that molecule. They then trigger a
chain of events.