By Donya Pettit
Last summer I had a really strange experience. I was supposed to perform at a brand new theater. When I got to the theater site all that was there was a big squishy object (cell) floating in a pond. I was curious so I climbed up on top of the object. It was difficult because the object was made out of two layers of squishy bubbles (phospholipids).
I walked along and all of the sudden something attached to me (carrier protein) and pulled me inside (active transport).
When I looked back I saw another object larger than me get engulfed by the squishy stuff. A portion off the squishy stuff wrapped around the large object so that it was in a membrane bound sac (VESICLE). Then these little things (enzymes) came over and started eating the big molecule (phagocytosis).
I noticed that there was constant movement of particles going in and out of the cell. Even the pond water from outside was getting bumped in (osmosis). The squishy stuff was controlling all the processes. Some of the processes used energy (active transport and phagocytosis) and some didn't use energy (ATP) (passive transport, facilitated diffusion, and osmosis).
I decided to find out where the energy used by the squishy stuff was coming from. When I turned around I couldn't believe my eyes. There were different objects (organelles) every where. Each one had it's own system inside. All of the sudden I was picked up by a long skinny object (MICROTUBULE-protein rich rods; that provide pathways for organelle movement. The kinesin molecule within a cell attaches to the microtubule and to the organelle. The kinesin moves along the microtubule pulling the organelle with it) and I was pulled through the jello like surroundings (cytosol).
Microtubules Moving a Mitochnodria
I got off the skinny tube and I followed the energy to this one object that looked like a kidney bean (mitochondria). On the outside there was a six sided structure that was getting broken in half (glycolysis). When the figure was broken there seemed to be alot of energy present (formation of 2ATP). One of the two halves proceeded to go inside the bean like structure. By now I was getting even more curious, so I decided to follow the object inside, I found myself in a very thick fluid (matrix). I saw the object which now consisted of three carbons (pyruvic acid). Two of the carbons were being carried away by a carrier protein. During the breaking away process negative energy was formed (high energy electrons). The two carbons (acetic acid) were taken to meet up with four other carbons making it a six carbon structure (citric acid). Then one of the first carbons (acetic acid) broke off, forming more negative energy. Then the other carbon (acetic acid), did the same as the first. Now the four carbon molecule was the only thing left. It went back to pick up two more carbons that were from the other half of the structure that was broken apart outside the bean like structure (pyruvic acid). This process (citric acid cycle) continued. The negative energy that was released when a carbon broke off, went through another whole procedure (electron transport chain). This new process functioned in a different place (cristae) then the cycle above. The energy met with molecules that helped release energy (coenzymes). Each step of the way it lost more and more energy. By the end there was hardly any energy left. The energy that was left was used to make water. The energy that was released in this process was used to make energy (38 ADP+38P=38 ATP) that was sent out of the bean like structure.
Mitochnodria and Respiration
As I was watching these processes take place I was starting to remember what I learned in my biology class. I remembered that plants use another source to create energy, this is called Photosynthesis. The first step in Photosynthesis is the light reaction, which involves trapping light energy. When sunlight strikes a plant leaf, the energy of certain wavelengths is absorbed by special Photosynthetic pigments. Wavelengths that aren't absorbed are reflected. Chlorophyll is the major photosynthetic pigment. Chlorophyll absorbs mostly red, orange, blue and violet light, but very little yellow and green light. Because it reflects green and yellow light, chlorophyll appears green. Photosynthetic pigments are located in CHLOROPLASTS. They have an inner and outer membrane. The inner membrane folds to form stacks of disk-like structures called Thylakoids. Each thylakoid contains from 200-400 molecules of chlorophyll. Surrounding the thylakoids is a fluid called stroma. When light strikes a chlorophyll molecule, electrons absorb the energy. This energy excites the electrons and they leave the chlorophyll. A couple of things can happen to these electrons. In one pathway they are passed along a series of coenzymes very similar to the electron transport chain in cellular respiration in that ATP is produced. In the second pathway, the electrons are taken out of the thylakoid by an electron carrier molecule and are used to make carbohydrate. In this pathway the electrons cannot go back to the chlorophyll, so they are replaced by water molecules that provide the necessary electrons. The useful products of photosynthesis in the light reaction are ATP and high energy electrons.
In the Calvin cycle (dark reaction) carbon atoms are combined to form carbohydrate molecules. These reactions use the energy formed in the light reaction to function. The Calvin cycle takes place in the stroma. The stroma contains a 5 carbon sugar called Ribulose bisphosphate (RuBP). When carbon dioxide enters the chloroplast, it's single carbon atom combines with the RuBP it makes a 6-carbon molecule. The molecule is unstable and immediately splits into two molecules of phosphoglyceric acid (PGA). Each PGA has three carbons. With more energy the PGA turns into a three carbon sugar called PGAL. As the cycle continues, PGAL is converted to 4, 5, and 6-carbon sugar phosphates. With more ATP, a 5-carbon phosphate is converted back into RuBP and the cycle is completed. Three turns of the cycle results in the formation of 6 molecules of PGAL. Only one PGAL can be used by the plant to form carbohydrate.
As I was thinking about my biology class I decided to go to the nucleus of the cell and check out the chromatin material. We looked at this under a microscope in class but I thought it would be a lot better in person. I came to the center of the cell and I swam through the double membrane of the nuclear envelope. I was right the chromatin was much better in person. I could actually see the tightly wound structures. The DNA inside the chromatin contains the information used to carry out the activities of the cell. DNA is made out of repeating units called nucleotides. Each nucleotide contains one of four nitrogen bases: adenine, thymine, guanine and cytosine. The sugar of one nucleotide is bonded to the phosphate group of the next nucleotide. In order for a new cell to function properly, it must contain a complete set of genetic instructions. It is critical that every new cell receive an exact copy of the molecule. In replication, new strands of DNA are synthesized from a supply of nucleotides in the nucleus. Replication begins when the double helix of the DNA strand is unzipped by enzymes, the separation happens in many places along the strand called replication forks. Now that the bases are exposed, free floating nucleotides pair with the exposed bases. Adenine-thymine and guanine-cytosine. Separation and pairing of free nucleotides continue until the entire DNA molecule has been replicated.
There is also RNA (ribonucleic acid). The RNA carries out the genetic code for protein synthesis. First DNA is unzipped by an enzyme. This exposes the nitrogen bases. The RNA nucleotides with uracil replacing thymine are matched to their compliment along the DNA strand. The protein then covalently bonds the ribose sugar to the adjacent phosphate. This process continues until UAR is reached. Then the RNA is unzipped from the DNA and a long chain of adenine nucleotides is added. Next it leaves the nucleus and the DNA's zip back up.
During translation the m-RNA attaches to a ribosome on the ENDOPLASMIC RETICULUM-the folded membranes of the ER, are continuous with the outer portion of the nuclear envelope. Only the ribososomes that are attached to the ER are used for synthesizing proteins for secretion from the cell. Later they will be used by other cells. The beginning code on the m-RNA is AUG. Meanwhile the transfer RNA's are attaching their ends to a specific amino acid that they are coded for. T-RNA's hook up with the m-RNA. The three bases on the m-RNA are called a codon and the complimentary three bases on the t-RNA are called anticodons. Thus the codon matches with the anticodon. A second t-RNA is the anticodon to the next codon in line. Next the two amino acids form a peptide bond. The first t-RNA releases it's amino acid and leaves. The ribosome moves down the m-RNA and the process continues until UAR or the stop code is reached.
Then the protein chain is wound up and sent to the GOLGI APPARATUS-an organelle that prepares and stores proteins for secretion. There, each protein molecule under goes chemical changes, such as the addition of carbohydrates or the removal of water.
If the golgi apparatus or any other organelle is not functioning correctly then the LYSOSOMES-membrane bound sacs; site of digestion, come and break down the molecule for reuse by the cell.
Another important organelle is the VACUOLE-membrane bound, fluid filled spaces in the cytoplasm. Vacuoles are the storage rooms for the cell. They contain mostly water and they under go many chemical changes. All the proceses I have written about use the ATP made by the mitochondria to function. As you can see, all the parts of the cell help each other. The cell would not function if even one of the organelles was missing.
While I was in the cell a strange thing started happening to the cell. It started to break in to two cells. I remember Mr. Fairbank telling us that this process was called mitosis and cell division. There are five phases of mitosis. Interphase, prophase, metaphase, anaphase and telophase. The cell stays in the interphase for most of it's existence. This phase is a period of growth, when protein synthesis occurs rapidly. Towards the end of the interphase the chromosomes are spread throughout the nucleus. The entire nucleus is enclosed by the nuclear envelope. Also a daughter centriole begins to form next to each existing centriole. The next step is prophase. The first thing that happens is that the centrioles with their daughter centrioles begin to move towards opposite ends of the nucleus. Molecules of the nuclear envelope disperse and the nucleus is less defined. The replicated chromosomes condense into short, thick rods. The two replicas of a chromosome are called chromatids. They are held together by the centromere. A disk-like structure called kinetochore is located on each chromatid. As the phase progresses, the cytoskeleton breaks apart. The microtubules reposition themselves. Some form a cluster around the centrioles called an aster. Other microtubules form continuous strands that stretch between centrioles. A third group extends from the centriole to the kinetochores of the two chromatids. The entire framework of microtubules is called the mitotic spindle. By the end of the prophase, the nuclear envelope and the nucleus have disappeared. During the metaphase, a molecule of kinesine attaches itself to the pair of chromatids. The chromatids begin to move towards the equator of the cell. They are pulled along the microtubules tracks, called spindle fibers. Movement stops when all of the chromatid pairs reach the equator. Anaphase begins with the separation of chromatid pairs. Once separated each chromatid is called a chromosome. The kinetochore pulls the chromosomes towards opposite ends of the cell, called poles. By late anaphase, an equal number of chromosomes have reached each pole. Also the plasma membrane begins to change shape. The division of genetic material is called karyokinesis. During early telophase, the chromosomes begin to uncoil, becoming a mass of chromatin once again. Fragments of the original nuclear envelope reassemble around each collection of chromatin. A nucleolus reappears inside each nucleus. Outside the nucleus, the spindle fibers begin to break apart. By the end of the telophase, most of the microtubules have reassembled to form the cytoskeleton. Throughout the entire process of mitosis, daughter centrioles have been growing so that by the end of the telophase, two mature centioles appear at each pole.
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