Cells: The building blocks of life.
Much of this information will be a review….some will be new. Read casually and
find the NEW stuff.
Vocab review…
Chromatin- DNA and histone proteins in loose molecular form, not visible under a
microscope. (interphase)
Polypeptide- a large chemical formed from a dehydration reaction forming
peptide bonds, usually refers to proteins, which are sometimes called enzymes.
Vesicle- a membrane “bubble” that moves substances from one organelle to
another or into/out of the cell (endo/exocytosis)
Metabolic or biochemical pathway- a series of chemical reactions in which the
products of one reaction (usually) are the reactants in the next.
In 1665, Dutch microscope maker Anton van Leeuwenhoek wrote about the tiny
objects he saw in rainwater, fabric, sperm, & feces. Another samples he called
them “animalcules” he believed that their movement defined life, but he was
incorrect. Later, Robert Hook magnified pieces thinly sliced cork and noted the
compartments. He called them cells. This term stuck. In the 1820s botanist Robert
Brown was the 1st to identify a cell nucleus Mathias Schleiden hypothesized that
plants were also made out of individual units or cells. Theodore Schwann
concluded that the tissue of animals was also made out of subunits of cells.
Rudolph Virchow, a physiologist realized that all cells descended from other living
cells. The cell theory was born. Though originally stated in 1839, it was not widely
accepted until around 1900.
The cell theory states:
1. Every living organism consists of one or more cells.
2. The cell is the structural and functional unit of all organisms. A cell is the
smallest unit of life individually a lie, even as part of a multicelled organism.
3. All living cells arise by division of pre-existing cells.
4. Cells contain hereditary material which they pass to their offspring during
division.
Cells always consist of at least 3 main parts, the plasma membrane, cytoplasm,
and genetic material. Cells are limited in size by a surface to volume ratio. They
must maintain a balance of moving materials in and moving waste product out to
maintain homeostasis. Surface to volume ratio’s also affect the body plans of
multicelled species. Some cells are shaped long and cylindrically to maintain a
proper surface to volume ratio, example skeletal muscle cells, or filamentous
algae.
Cells are considered to have 2 main varieties:
PROKARYOTES or EUKARYOTES.
Prokaryotic cells are generally smaller than eukaryotic cells, usually 1-10 microns.
They do not bind their DNA in a nuclear envelope, which is a type of lipid bilayer.
The majority of their DNA typically occurs in a large circular molecule within an
irregularly shaped region of cytoplasm called the nucleoid. They also contain
plasmids, which are small circles of DNA composed of a few genes. Some
nucleotides are enclosed by membrane. The term prokaryotic is now considered
an informal designation due to the fact that prokaryotes contain 2 domains which
are quite different from one another in metabolic activity and DNA composition.
The domain Eubacteria contains the bacteria we refer to as germs or bacteria.
The domain Archae contain the ancient Archaebacteria, which tend to live in
unusual and often hostile environments. Both type of prokaryotic cells contain
many ribosomes and often use their membranes to do many cellular functions.
They also are surrounded by a cell wall, which can be made out of a variety of
materials, many have a flagella to aid in movement. They may also contain a pilus,
which is a straw like structure through which plasmids may be transferred from
one cell to another. There are 3 main shapes, cocci, Bacillus, & spirilla, which
makes us in singles, doubles, clumps or strands. But we will get into that a little
later.
Bacteria often live so close together that entire communities share a layer of
secreted polysaccharides and proteins. This arrangement provides protection
from being swept away from the surface. This layer of slime is called a biofilm and
consists of many species usually bacteria, algae, fungus, protists, and Archae.
Biofilms can be particularly troublesome as dental plaque. Biofilms are also
problematic on heart valves and in transplant surgeries.
Eukaryotic cells are much more familiar to you. They vary greatly in size and
structure, and form multicellular organisms. They are from 10-100 microns
generally, however ova of different species may be really large. Eukaryotes are
specialized due to their membranes, which protect and perform several functions
for the organism. They have a phospholipid bilayer we call the plasma membrane,
and most, other than animals have a cell wall also. They all enclose their genetic
material, which is made up of several molecules of DNA, in a selectively
permeable nuclear envelope. A variety of organelles may be present according to
the function of the cell, and many have a membrane system.
The Endomembrane System is a complex network of membranes, many of which
are attached. The Nuclear envelope is a type of lipid bilayer which contains many
pores to aid in specific transport of materials into and out of the nucleus. The
pores are anchored by the nuclear lamina, a mesh of proteins that support the
inner surface of the envelope. Pores help RNA and proteins which cannot cross on
their own. Remember both are crucial in protein synthesis. The envelope encloses
the nucleoplasm in which chromatin(DNA w/histone protein) is suspended. At
least one nucleolus (a region of proteins and nucleic acids responsible for the
assembly of ribosome subunits) is present. The Endoplasmic Reticulum is an
extension of the nuclear envelope. The inner part studded with ribosomes is
called rough due to its appearance. It secretes proteins directly into the interior of
the ER where they will take on their tertiary structure. Some may become part of
the membrane. Rough ER secretes a lot of protein and gland cells contain quite a
bit of it. Smooth ER does not secrete proteins, but helps assemble lipids that
form the cell membranes and break down carbs, fatty acids, some drugs and
poisons. Some of the proteins made in the rough ER become part of the smooth
ER and act as enzymes. *There is a special type of Smooth ER that stores calcium
in muscle cells and plays a role in cell signaling for muscle contraction. The
smooth ER also produces special vesicles called peroxisomes that move around
the cell and break down fatty acids, amino acids and toxins like alcohol. Vesicles
also move materials everywhere including proteins and lipids from ERs to Golgi
body for modification. The Golgi attaches phosphate groups, oligosaccharides and
break certain polypeptides. The Golgi also produces Lysosomes which digest
unwanted materials.
Other organelles include the Mitochondria and Plastids. (Remember the
endosymbiont hypothesis?) Mitochondria are found in all Aerobic eukaryotes.
They are 1-4 microns long, have a double membrane, contain their own DNA, can
self-replicate, or merge into one, may be branched or change shape. They vary in
number based upon the energy needs of the cell. Mitochondria specialize in
making ATP during aerobic respiration, which yields more energy from organic
compounds than any other metabolic pathway. Mitochondria build up H+ ions in
the membrane space to create a gradient and potential energy to drive the
metabolic pathway. Anaerobic eukaryotes have a special organelle called
Hydrogenosomes, which produce ATP. Plastids are double membraned, and are
used for photosynthesis or storage in plants and algal cells. The three most
common are Chloroplasts, chromoplasts, and amyloplasts. Chloroplasts are
specialized for photosynthesis in protists and plants. The chloroplast contains its
own DNA and has a double membrane surrounding a liquid stroma and a very
folded thylakoid membranes which contain pigments to collect the suns energy.
The chloroplast creates high energy molecules such as ATP to drive the
production of glucose and other carbohydrates. Chromoplasts make and store
pigments such as carotenoids, which are oranges and reds. As a fruit ripens it may
convert plastids from chloroplasts to chromoplasts. Amyloplasts are
unpigmented plastids. They tend to store starch in stems and tubers. They are
very dense and often function as gravity sensing organelles.
I always have downplayed the cytoskeleton in regular biology but it is an amazing
and crazy structure.
The Cytoskeleton is made up of 3 types of protein filaments and some accessory
proteins. Microtubules, made of tubulin protein rapidly assemble when needed
and disassemble when they are not. (spindle fibers in replication) Microfilaments,
made of actin protein, aid in cell strength and shape. They are often arranged in
bundles or Rays, compose the cell cortex which is a reinforcing mesh under the
plasma membrane. Actin is one of the two protein filaments that allow for
skeletal muscle contraction. Intermediate filaments, made of lamin protein are
the most stable and the network under membranes help in support (nuclear
lamina). Motor Proteins made of dynein, are contractile proteins which pull
vesicles along microfilaments and microtubules for transport reasons. They also
allow for movement of cilia and flagella. See below.
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Cilia and flagella have a specialized arrangement of microtubules known as a 9 +2
array which interact with a dynein motor protein. The microtubules grow from a
centriole which remains below the finished array as a basal body. Alternating the
contraction of these tubules allows for movement back and forth. Amoeba and
other eukaryotes have pseudopods which allow for movement. In this form of
movement the elongating microfilaments allow the cell membrane and cytoplasm
to stream in a particular direction forming a lobe.
Cell membranes have many specializations depending on the type of cell. Some
eukaryotes have additional walls around their plasma membranes as well as a
matrix of cytoskeleton underneath. The extracellular matrix (ECM) is secreted by
the cell and is composed of a variety of fibrous proteins and polysaccharides. Cell
walls are in example of an ECM. A plant cell wall forms as a young cell secretes
pectin and other polysaccharides on its outer surface, the sticky coating, shared
between adjacent cells binds them together. Each cell then forms a primary cell
wall by secreting strands of cellulose into the coating. Some of the coding remains
as the middle lamellae. The primary cell wall is porous and flexible allow for
growth. At maturity deposits formed on the inner wall forming a secondary wall
made of lignin, which is stronger, more waterproof in less susceptible to injury by
pests. A cuticle is a type of ECM secreted at the body surface of a leaf or stem
consisting of waxes and proteins, helps retain water and fend off insects. Bone is
a type of ECM that is secreted by an osteocyte, and composed mostly of
hardened calcium and phosphorus deposits. Cell junctions are important
structures that connect cells to other cells, allowing for more rapid
communication. In plants channels called plasmodesmata allow for
communication diffusion and osmosis. Tight junctions between plasma
membranes, prevent body fluids from seeping between adjacent cells in tissues
that line body surfaces and internal cavities. Example, stomach acid remains in
the stomach. Gap junctions (look like gauges) form channels that connect
cytoplasm of adjoining cells permitting very fast communication. This is very
important heart muscles. Adhering junctions (desmosomes in animals) have
fibrous anchors that hold the cells together especially if tissue is subject to
abrasion.
http://www.cengage.com/biology/book_content/9781111425692_starr_udl13e/
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What does it mean to be alive?
We say that life is a property that emerges from cellular components, but a
collection of those components in the right amounts and proportions is not
necessarily alive.
1. They make and use organic molecules of life.
2. They consist of one or more cells.
3. They engage in self-sustaining biological processes such as metabolism and
homeostasis.
4. They change over their lifetime. For example, by growing maturing and
aging.
5. They use DNA as they are hereditary material.
6. They have the collective capacity to change over successive generations.
For example, by adapting to environmental pressures. They evolve.
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