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CELL STRUCTURE AND FUNCTION
The fundamental units that make up all living things are called cells.
When cells cluster together and perform the same function, they are called tissues.
When tissues cluster together and perform the same function, they are called organs.
Organs cluster together to form organ systems, which result in the total organism.
All cells on Earth
A) Are enclosed in a membrane that maintains internal conditions different from
the surroundings.
B) Have DNA as the genetic material.
C) Can interconvert forms of energy.
D) Can interconvert chemical materials.
Every cell has three things in common:
1. Metabolic functions (using up oxygen and sugars). The sum of all of the chemical
reactions that occur in a cell collectively are called metabolism.
2. Responds to its environment
3. Capable of maintaining homeostasis within itself and within the body.
HOMEOSTASIS is maintaining a balanced internal environment, such as temperature,
pH, glucose levels, etc.
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CELL STRUCTURE AND FUNCTION
CELL ORGANIZATION
A. CYTOPLASM: Jelly-like stuff in the cell around the organelles. This is where many
of the chemical activities (cellular metabolism) of the cell occur.
Cytoplasm contains the following:
a. Mostly water
b. Things dissolved in water (sugars like glucose, and energy molecules =ATP)
c. CYTOSKELETON: made up of long protein fibers that extend throughout
cytoplasm.
Function of cytoskeleton:
1) Maintains cell shape
2) Movement
a) muscle cell contraction
b) moving organelles within the cell
c) moving the cell itself (amoeba)
B. ORGANELLES: small structures (miniature organs) that occur within the cell and
have various functions.
1. PLASMA MEMBRANE: Surrounds the entire cell.
Functions of the Plasma Membrane:
a. Plasma membranes are selectively permeable. This means that the
plasma membrane allows some substances to enter or leave a cell
more easily than others.
b. Communication with other cells and the environment
2. NUCLEUS: Usually the largest structure in a cell.
Functions of the nucleus: Stores DNA (genetic material that makes up chromosomes).
Our genes are on the chromosomes) The majority of genetic material within the
cell is contained in the nucleus
3. MITOCHONDRIA
Cells have hundreds of mitochondria.
Function of mitochondria is to make ATP (energy molecules).
NOTE: Mitochondria must have OXYGEN to convert nutrients to ATP for energy.
Therefore, the mitochondria are the organelles that carry out cellular respiration,
converting the chemical energy of foods such as sugar into the chemical energy of a
molecule called ATP.
4. ROUGH ENDOPLASMIC RETICULUM (endoplasmic = within cytoplasm;
reticulum = network; rough = surface of membrane covered with RIBOSOMES.
The function of ribosomes (and therefore, function of RER) is to make proteins.
Ribosomes make proteins out of amino acids. The messenger RNA carries the
blueprint of the DNA’s amino acid sequence into the cytoplasm; it then carries it into
the ribosomes within the RER. The ribosome has the RNA that translates the
blueprint into an exact copy of the original DNA strand, and the protein is formed
within the ribosome (like a protein factory). The protein is then taken out and carried
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CELL STRUCTURE AND FUNCTION
through the canals of the reticulum like a boat going down a canal in Venice. The
protein then floats into the Golgi complex, where it will be packaged for shipment
like a Fed-Ex center.
Some of these proteins will be used to make enzymes. Enzymes are required for the
cell to undergo various reactions. Elevated body temperatures may denature
enzymes.
5. GOLGI COMPLEX
The Golgi apparatus is a complex system of membranous channels and saccules
continuous with the plasma membrane.
Function of Golgi complex:
a. Packages the proteins made by the RER and sends them where they need
to go (like a UPS center!)
6. SMOOTH ENDOPLASMIC RETICULUM (no ribosomes)
Function of SER
a. Involved in metabolism (making use of carbohydrates (sugars) and lipids (fats).
b. Stores calcium (necessary for every cell)
c. Detoxifies harmful substances (alcohol, drugs, etc)
i. NOTE: in CSI, when they suspect poisoning, they first look at the SER
in the liver.
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CELL STRUCTURE AND FUNCTION
7. VESICLES: a sphere of membrane with something in it. Many types:
a. LYSOSOMES: contains digestive enzymes to dissolve bacteria. When
a bacterium is discovered within a cell, a lysosome bubble will fuse onto it
and release its contents in an effort to dissolve the invader. If there was a
defect in the lysosome of a white blood cell, the bacteria could
accumulate within the cell and kill a person by infection.
b. TRANSPORT VESICLES: moves material from RER to Golgi
complex, or from Golgi complex to cell membrane, etc.
c. STORAGE VESICLES
8. CILIA AND FLAGELLA
Some cells have hair-like structures that move the cell. The longer structures are
called flagella and shorter ones are cilia. Sperm cells, for instance, have flagella,
which they use to move from one place to another. Cells in our trachea have cilia
which can move mucous along the top of the cell (when you cough up mucous).
SUMMARY OF CELL FUNCTION
Each cell is near a blood vessel, which gives the cell nutrients it needs and takes products
out of the cell to distribute to the rest of the body. The nucleus has DNA which tells the
ribosomes what kind of proteins to make at the moment. The ribosomes get the amino
acids from the bloodstream to build the protein molecule. The protein is taken to the
Golgi complex, where it is packaged and sent to a storage vesicle. The cytoskeleton fibers
move the vesicle to the plasma membrane where it is released from the cell and picked up
by the bloodstream and taken to where it is needed in the body. The bloodstream drops
off sugars to the cell, which are taken to the mitochondria to be broken down into ATP,
which is the energy molecule used to fuel all the functions of the cell.
Cellular (aerobic) respiration is a process whereby mitochondria in the cells use
oxygen, produce carbon dioxide, and produce energy in the form of ATP.
Breathing involves the lungs supplying oxygen to our cells and removing carbon
dioxide. We often use the word “respiration” instead of breathing. Respiration refers to
an exchange of gases: an organism obtains oxygen and from its environment and releases
carbon dioxide as a waste product. Respiration can also apply to an individual cell when
it harvests energy from food molecules; this is called cellular respiration.
Breathing and cellular respiration are closely related. As a gymnast goes through her
routine, her lungs take up oxygen from the air and pass it to her bloodstream. The
bloodstream carries the oxygen to her muscle cells. Mitochondria in the muscle cells use
the oxygen in cellular respiration, converting energy from glucose to generate ATP,
which the cells then use to contract. Simultaneous contraction of thousands of cells,
precisely controlled by the nervous system, makes the gymnast’s body move. The
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CELL STRUCTURE AND FUNCTION
gymnast’s bloodstream and lungs also perform the vital function of disposing of the
carbon dioxide waste produced by cellular respiration.
Our body cells require a continuous supply of energy just to stay alive-- to keep the heart
pumping blood, to breathe, maintain body temperature, and digest food. These and other
life-sustaining activities use as much as 75% of the energy a person takes in as food
during a typical day. Whether you are sleeping or active, your cells are busy with cellular
respiration, producing ATP just to maintain your body.
Above and beyond the energy we need for body maintenance, the ATP made during
cellular respiration provides energy for voluntary activities. For instance, riding a bicycle
as fast as you can consumes approximately 514 calories an hour, running burns 865
calories an hour, and walking burns about 200 calories an hour. The average adult needs
to take in food that provides about 2200 calories of energy per day. This is just an
estimate of the total amount of energy a person of average weight expends, or “burns” in
both maintenance and voluntary activities.
Cell surfaces protect, support, and join cells together. To function in a coordinated
way as part of a tissue, the cells must have junctions, structures that connect them to one
another. Animal cells are embedded in a sticky layer that helps hold the cells together in
tissues and provide protection and support as well. This allows contact from one cell to
another.
Plant cell walls consist of cellulose fibers embedded in a matrix of polysaccharides
and proteins. The cell wall of one plant cell is separated from the cell wall of another
by a layer of sticky polysaccharides. Plant cell walls are multilayered structures.
Wood is primarily composed of plant cell walls. Plant cell walls protect plant cells
but they are still a layer that is permeable to selected substances.
The rate of cell division is close to the rate of cell death. For instance, 200 billion red
blood cells die every day, so 200 billion red blood cells have to be made every day. Too
few = anemia; too many is also a problem.
So, the body needs to do two things:
1. Control the rate of cell division
2. Control the rate of cell death
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