CELL THEORY
Cells are the basic unit of life – all living things are composed of one or more cells.
Even though cells were observed as early as 17th century, it was not until 1838,
when Matthias Schleiden stated that all plants are simply aggregates of these cells.
He considered cells to be individual, independent beings. In 1939, German
physiologist Theodor Schwann added that all animal tissues also are composed of
cells. And so, Cell theory was born:
1. all organisms are composed of one or more cells, and the life processes of
metabolism and heredity occur within these cells.
2. Cells are the smallest living things, the basic units of organization of all
organisms.
3. Cells arise only by division of a previously existing cell.
Cell size is limited
Most cells are small so that diffusion can occur efficiently. Rate of diffusion can vary
due to many factors:
1. surface area
2. temperature
3. concentration gradient of diffusing substance
4. distance over which diffusion must occur
Larger cells
1. produce more waste
2. have higher energy requirements
3. need to synthesize more macromolecules
4. takes longer for waste to be removed from molecule
Organisms with many small cells have an advantage, because they have a relatively
smaller surface area to volume ratio. As radius increases, volume increases, but
surface area to volume ratio decreases:
SA = 4πr2
V=(4/3)πr3
Therefore, SA:V=3/r
V is directly proportional to r, whereas SA is inversely proportionate. As radius
increases, volume increases, but SA:V becomes increasingly small. Example: If
r=1μm, SA:V=3; if r increases to 10, then SA:V=0.3; if r increases to 100, then
SA:V=0.03. Steep decrease in SA as radius increases.
What happens to the rate of diffusion when there is a small surface area?
There are some large cells, e.g., skeletal muscle and neuron cells. These have
overcome the SA:V problem. Skeletal muscle cells have many nuclei to spread
genetic information around. Neuron cells are long and skinny, so any point within
the cell is close to a membrane and does not have to travel too far to diffuse out of
the cell.
STRUCTURAL SIMILARITIES OF CELLS
1. Centrally located genetic material:
a. Microbes (prokaryotes) contain a nucleoid – genetic material forms a
circle near the center of the cell, but is not separated from the rest of
the cell by membranes
b. Eukaryotes contain a nucleus – genetic material is mostly contained
within the nucleus; nucleus enclosed by a double membrane called
the nuclear envelope.
2. Cytoplasm: semifluid matrix that fills the inside of a cell. Contains all of the
sugars, amino acids, and proteins the cell uses to function.
Cytoplasm is composed of two components:
a. Organelles are discrete structures within the cytoplasm that are
specialized for specific functions.
b. Cytosol the fluid part of the cytoplasm that contains organic
molecules and ions in solution.
3. Plasma membrane: encloses a cell and separates its contents from its
surroundings. This is a phospholipid bilayer with proteins embedded in it.
The proteins allow the cell to interact with its environment. Transport
proteins help molecules and ions pass from outside the cell to the inside, or
vice versa. Receptor proteins induce changes within the cell when they
come in contact with specific molecules, such as hormones, or other
molecules on neighboring cells. These cells are important in multicellular
organisms that need to recognize each other to form tissue.
PROKARYOTIC CELLS
Main difference between prokaryotic and eukaryotic cells is that prokaryotic cells
lack membrane-bound nucleus and organelles.
Relatively Simple Organization:
- small
- cytoplasm surrounded by plasma membrane
- encased within a rigid cell wall
- no distinct interior compartments
QUESTION: if you wanted to maximize a prokaryotic cell’s size, how would you
modify it?
Bacteria
Bacteria do not contain membrane-bound organelles, therefore the plasma
membrane carries out some of the functions of the organelles. For example, a
photosynthetic bacterium will have a highly folded cell membrane that contains
photosynthetic pigments that carry out photosynthesis, like a plant.
Most bacteria are encased in a strong cell wall. Cell wall consists mostly of
peptidoglycan – carbohydrate matrix cross linked by short polypeptide units (how
does this differ from eukaryotic cells?) – eukaryotes have microtubules that
make up the cytoskeleton; plants, fungi, and most protists have cell walls, but theirs
is different in structure than peptidoglycan.
What are the advantages of having a cell wall if you are a single-celled
organism?
- helps maintain shape
- prevents excessive uptake or loss of water
Archaea
Archaea lack peptidoglycan, but still have cell walls.
Cell walls composed of various compounds, including polysaccharides, proteins, and
maybe even inorganic components. Not much is known about archaea beyond
genetic structure, because they are DIFFICULT TO CULTURE.
Another difference between archaea and bacteria is the makeup of their
membrane lipids.
single layer of saturated hydrocarbons attached covalently to glycerol at
both ends
- advantage: great thermal stability
- disadvantage: cannot adapt to surrounding temperatures, b/c they cannot
alter their degree of saturation of hydrocarbons.
Archaea are more closely related to eukaryotes than prokaryotes.
-
Rotating flagella
Long, thread-like structures protruding from the surface of a cell – used for
locomotion.
Protein fibers extending from the cell.
May be one or more, or none.
EUKARYOTIC CELLS
The main difference between prokaryotic and eukaryotic cells is that eukaryotes
contain a nucleus and membrane bound organelles.
The nucleus contains the genetic material in eukaryotic cells. Where is the genetic
material in prokaryotes?
Many nuclei also contain a nucleolus. This is where ribosomal RNA (rRNA) is
synthesized.
The surface of the nucleus is bound by two phospholipid bilayer membranes.
The cytoplasm contains an internal membrane system called the endoplasmic
reticulum. The nuclear envelope is continuous with this membrane system.
Nuclear pores allow some ions and small molecules to pass freely between
cytoplasm and nucleoplasm. Pores control the passage of proteins and proteinbound RNA. Protein formed in the cytoplasm can pass into the nucleoplasm and
protein bound RNA can pass from the nucleoplasm into the cytoplasm.
Chromatin
DNA is packaged into linear chromosomes organized with proteins into a complex
structure called chromatin. Chromatin affects all aspects of DNA function. Changes
in gene expression can happen, even if there is no change in the DNA sequence are
called epigenetic changes. These involve changes in chromatin.
**What is epigenetics?** Discuss examples
Nucleolus
“little nucleus” Manufactures ribosomes inside the nucleus.
Ribosomes
Synthesize proteins. Although they are assembled in the nucleolus, they are
transported into the cytoplasm where they make proteins.
Made up of two subunits – large and small. Each is made up of ribosomal RNA
(rRNA) and proteins.
Subunits join during synthesis of proteins.
During protein synthesis, two other forms of RNA are required: messenger RNA
(mRNA) and transfer RNA (tRNA).
Endoplasmic Reticulum
“Within cytoplasm” and “little net”
Phospholipid bilayer with proteins embedded.
Rough (RER)
Is associated with many ribosomes. Proteins produced by these ribosomes
are exported from the cell, sent to lysosomes or vacuoles, or embedded in the
plasma membrane.
Proteins synthesized here first enter the cisternal space (lumen), then get
sorted to be transported to their final destination.
**What determines where the ribosome’s ultimate destination (i.e., whether it
will become a cytoplasmic ribosome, or stay with the RER?**
Newly synthesized proteins can be modified by the addition of short-chain
carbohydrates to form glycoproteins. These glycoproteins can be used for
secretion and are often packaged into vesicles that move to the Golgi, then
are further modified and packaged. They are then transported to other
locations within the cell.
Smooth (SER)
Contains few ribosomes compared to RER. Synthesizes many carbohydrates
and lipids and steroid hormones using enzymes.
Stores intracellular Ca2+ to maintain low concentration within the cytoplasm.
When Ca2+ is released, it is used as a signaling in many pathways, such as
muscle function, neurotransmitter release, and transcription.
Calcium is moved into areas in the endoplasmic reticulum via calcium pumps
against a concentration gradient.
**How does this differ from diffusion?**
The ratio of RER to SER depends on the organ and tissue type. A high
demand for lipids, such as in the brain, will yield higher SER, whereas cells
that secrete proteins, such as snot in your nose, would contain more RER.
SER also modifies foreign substances to make them less toxic. SER in liver
cells would perform this function.
Golgi Apparatus
Individual stacks of membrane called cisternae.
Protists only contain a few (1 or more)
Animals contain 20 or more
Plants contain hundreds.
In animals, they are extensive in glandular cells, which manufacture and secrete
substances.
Proteins and lipids made in the ER are transported to Golgi apparatus. When they
pass through, they are modified, usually into glycoproteins or glycolipids. These are
packaged, pinched off, and transported to their final destination in the cell.
Synthesize cell wall components. These do not contain cellulose - they contain other
polysaccharides that are integral to plant cell walls (e.g., hemicellulose and pectin).
These pollysaccharides are transported to the cell wall of the plant cell where they
bind to the cellulose of the cell wall.
**Why do plants contain so many golgi bodies compared to animals?**
Lysosomes
Manufactured in the Golgi apparatus.
Contain digestive enzymes that break down proteins, lipids, carbohydrares, and
nucleic acids. The component molecules of these macromolecules get recycled to
make room for new organelles.
Lysosome enzymes function best at a low pH. These organelles are activated by
fusing with either “food vesicles” that are produced by phagocytosis, or with oher
organelles. Low pH results from proton pump activation in the lysosome membrane
once the lysosome fuses with another body. Protons stream into the lysosome,
decreasing the internal pH. This ultimately caused the destruction of molecules in
the food vasicle, or the destruction of the old organelle.
**what role does the lysosome play in eliminating pathogens in the cell?**
Peroxisomes
Important for fatty acid oxidation. Produces hydrogen peroxide as biproduct, which
is highly reactive and, therefore, dangerous. Peroxisomes can break down hydrogen
peroxide into water and oxygen, because they also contain catalase.
Vacuoles
Storage and water balance. Are the largest organelles in a plant cell. Its membrane
is called a tonoplast. This name refers to the channels of water that help maintain
the cell’s tonicity, or osmotic balance.
Different types of vacuoles have different roles, depending on the cell type.
The central vacuole is has a number of different roles in plant cells:
1. maintain the tonicity of the cell along with the water channels within the
tonoplast
2. cell growth; plant cells grow by expanding their vacuole, rather than
increasing cytoplasmic volume
Vacuoles also found in some fungi and protists. One type is a contractile vacuole
employed by some protists. This type of vacuole contracts and pumps water out
into the cytoplasm to maintain water balance.
**How does pumping water out of the vacuole maintain water balance?**
Other vacuoles are used for storage, or to remove toxins from the cell.
Mitochondria
Found in all eukaryotic cells. Bound by two membranes: a smooth outer and a
folded inner membrane. The folded layers of the inner membrane are called
cristae. The area enclosed by the inner membrane is called the matrix. The area
between the two membranes is called the intermembrane space. Embedded
within the inner membrane, as well as on its surface, are proteins that carry out
oxidative metabolism - the process that requires oxygen to produce ATP.
Mitochondria contain their own DNA. This is DNA codes for genes that are essential
to the mitochondrion’s oxidative metabolism. So, the mitochondrion is like a cell
that codes proteins for its own function. The mitochondrion is not, however
completely self-sustaining. Many of the genes responsible for much of oxidative
metabolism are made in the nucleus of the cell.
Mitochondria divide when cells divide and are distributed between the two new
cells. The genes necessary for most of the components used in mitochondrial
division are produced in the nucleus. There are then translated by cytoplasmic
ribosomes.
Chloroplasts
Give plants their green color.
Use light to generate glucose - their food.
Bound by a double membrane, like mitochondria, but are more complex. Contain
grana – stacked membranes that reside inside the inner membrane. One granum
consists of several disk-shaped structures called thylekoids. The surface of the
thylekoid contains light-harvesting photosynthetic pigments. The fluid surrounding
the thylekoids is called the stroma. This is where the enzymes used to synthesize
glucose during photosynthesis are found.
Like mitochondria, chloroplasts have their own DNA, but most of the genes that
code for chloroplast components are found in the nucleus. Some of the elements
used in photosynthesis, including proteins necessary to carry out the reaction, are
produced entirely in the chloroplast.
Other DNA-containing organelles in plants are called leukoplasts. One example is
starch (amylose) storage cells, or amyloplasts. Collectively all the “plasts” are
called plastids.
Endosymbiosis
The theory that eukaryotic cells evolved by the close relationship between two or
more free-living cells. Mitochondria and chloroplasts are thought to be descendants
of these symbiotic bacteria.
Cytoskeleton
Made up of protein fibers that criss-cross to support the cell structure and anchor
organelles in place. Constantly assembling and disassembling. Fibers consist of
polymers of identical protein subunits. These attract each other, forming long
chains that are also broken apart unit by unit.
Actin filaments (microfilaments)
Long fibers that look like two strands of pearls loosely intertwined. Each
“pearl” is an actin protein.
Actin exhibits polarity - they have +ve and -ve ends. These direct the growth
of the filament. These filaments are formed spontaneously. Polymerization
is turned on and off by the cell via other proteins that act as switches when
appropriate.
Microtubules
Largest cytoskeletal elements. Hollow tubes, each composed of a ring of 13
protein protofilaments made of a and b-tubulin subunits that polymerize to
form these protofilaments. These are arranged in a tube, giving the
mirotubule its shape.
Microtubules organize the cytoplasm and are responsible for moving
materials within the cell. They are constantly assembling and disassembling.
Intermediate filaments
Tough fibrous protein molecules twined together in an overlapping
arrangement.
Most durable element of the cytoskeleton in animal cells. Do not disassemble
once formed. Consist of a mixed group of cytoskeletal fibers, or protein
subunits.
The most common type of cytoskeletal fibers, which are composed of protein
subunits called vimentin, provides structural stability for many types of
cells. Another is keratin, which provides support for epithelial cells, hair,
and fingernails. The intermediate filaments of nerve cells are called
neurofilaments.
Centrosomes
Plant Cell walls
Animal cells – extracellular matrix
Cell to Cell interactions