Friday, April 29, 2011

Negative Feedback and Blood Glucose Regulation

     Our ability to maintain relatively constant internal conditions is due to homeostasis. It may sound pretty boring, but without homeostasis we would have to change the way we do a lot of things. For example, without the ability to regulate our body temperature we'd have to rely on external sources of heat to warm us (think of a lizard sitting on a rock in the sun...not very productive, right?). Homeostasis is also important for regulation blood glucose levels, which I'll explain in a little more detail.
      Glucose is important for healthy functioning because it is required for the formation of ATP, the molecule of energy transfer in our bodies. Two important hormones for blood glucose regulation are insulin and glucagon. Insulin is secreted by beta cells while alpha cells secrete glucagon. As the food we eat is broken down, our blood sugar rises due to the carbohydrates. In order to avoid serious problems such as kidney and cardiovascular damage, blood sugar levels have to be decreased. 

     So, how does our body regulate our blood glucose levels to maintain homeostasis? The internal mechanism for blood glucose regulation is negative feedback. As you can see from the figure, depending on whether glucose levels are rising or falling, the body has a different response. When levels increase, the beta cells secrete insulin which then converts glucose to glycogen so that extra glucose can be stored restoring glucose levels to a normal level. When levels fall, the alpha cells secrete glucagon which converts stored glycogen to glucose increasing levels back to normal. 

Thursday, April 28, 2011

Don't Be Soooo Negative!!

What does an air conditioner have to do with a negative feedback loop you ask? Actually, more than you would think! Generally, an air conditioner has a thermostat that you set to a specific temperature. Throughout the day and especially during the summer, your home heats up to the point where it gets warmer than the temperature set on your air conditioner thermostat. As the temperature gets to hot, the air conditioner kicks on and cools the house to the set temperature. When it reaches that set temperature, the air conditioner turns off and the process continues again. This system ensures that your home is never too hot or too cold!

A negative feedback loop in our bodies occurs much like an air conditioner. One definition states that a negative feedback loop is the process of shutting bodily systems off once a set level is achieved, exactly like an air conditioner!! In the body, a negative feedback loop occurs in response to a physiological change which triggers a hormone release. The hormone rise triggers the endocrine system to STOP producing the hormone once it reaches a certain level. This negative feedback prevents the overproduction of hormones which could ultimately lead to disease if the hormone levels stay high for an extended period of time.

An example of a negative feedback loop in our bodies is after you eat a HUGE piece of birthday cake. After eating the high amount of sugar, our blood sugar rises. After it rises, the endocrine system signals the pancreas to release insulin. The insulin is release from the pancreas which travels through the bloodstream to cells. This insulin helps the body cells to take in glucose which helps to lower the amount of sugar in the bloodstream. After the blood sugar lowers and the glucose levels fall, the insulin release is inhibited. Now you can see how an air conditioner and our bodies are similar! Who would have thought...

One of the questions I got wrong on the assessment test was one about the drug Taxol and its role in mitosis. Although I had heard about this drug before, I got the question wrong because I thought it would inhibit anaphase and not necessarily the mitotic spindle. Because I didn't really understand the mechanism that makes Taxol work, I wanted to research it a little more.

Taxol, an anti-cancer drug, is isolated from the Pacific Yew Tree. It was originally isolated from the bark but was found in small amounts in the needles and cones as well. The relative amount able to be isolated from these trees was fairly minute and therefore many trees were used to get enough for one patient's treatment.

A couple of the cancers that taxol can be used to treat include breast cancer and lung cancer. The way Taxol works is to inhibit the cell division and growth of the cancer cells. It affects the microtubules involved in mitosis and can therefore inhibit mitosis of the cells.

Taxol is fairly expensive to extract and purify from the Pacific Yew Tree. Many different researchers have been trying to figure out new ways to synthesize this important drug and to make it more available to the general population.

In metaphase of mitosis, there are spindle fibers that attach to the centromeres of the chromosomes so that the chromatids can be moved to opposite poles during anaphase. When taxol is applied to the cells the spindle fibers are interrupted and the cell remains in metaphase. After the cell is in metaphase for a while, apoptosis occurs and the cell dies. This is why taxol is such a good cancer treatment because it can inhibit cell division and eventually kills the cancer cells. The end. :)

Friday, April 1, 2011

Endosymbiotic Origins

Eukaryotic Cell
Prokaryotic Cell
      There are two basic types of cells found in organisms, prokaryotic cells and eukaryotic cells. Prokaryotic cells are those found in bacteria and archaea, while protists, fungi, animals, and plants are composed of eukaryotic cells. One of the main differences between these two types of cells is that chromosomes are located in the membrane-bound nucleus in eukaryotes and in a non-enclosed area known as the nucleoid in prokaryotes. Eukaryotes also contain other membrane-bound organelles while prokaryotes do not. Endosymbiotic theory posits that these membrane-bound organelles were originally small prokaryotes that were engulfed by larger predatorial cells.
     There is a large amount of evidence in support of this theory. For example, both mitochondria and chloroplasts have their own DNA which is found in one circular molecule not associated with histones. These organelles also have their own transfer RNAs, ribosomes, and other molecules needed for transcription and translate. Also, mitochondria and chloroplasts both divide in a process more similar to that of binary fission which occurs in prokaryotes. Mitochondria and chloroplasts have ribosomes which are more similar to prokaryotic ribosomes in terms of nucleotide sequence, size, and antibiotic sensitivities.
     You might be wondering how and why this would happen. As far as how, the smaller prokaryotic cells most likely entered the eukaryotic cells as internal parasites or undigested prey. From there a mutual relationship developed, benefiting both cells. The smaller cells gained protection in an increasingly oxygen enriched environment from the eukaryotic host, while the host was able to use nutrients from the photosynthetic prokaryote. Both cells became increasingly dependent on the benefits gained from the relationship until they were no longer separate organisms.