The role of brown fat
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The role of brown fat
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[post_date] => 2024-12-23 18:21:05
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
There are two main types of adipose tissue in the body: white and brown fat. Whereas white fat is found throughout the body, brown fat, which is involved in temperature regulation, is only present in a few specific locations. In rodents, like most mammals, brown fat is located in the subscapular region and in the areas surrounding the aorta; it is ubiquitous in both newborns and adults. In humans, brown fat was originally thought to be prevalent only in newborns, but newer studies indicate that brown fat exists in the neck and upper chest of adult humans. In fact, it has also been discovered that the amount of brown adipose tissue in adult humans varies with an individual’s body mass index (BMI).
To understand the function of brown fat, scientists took adult rats under anesthesia and inserted thermal probes in the midbrain, colon, and interscapular fat. The scientists then proceeded to measure the rats’ body temperature at these various locations while the rats were exposed to external temperatures of 31°C and 20°C. The scientists also recorded the amount of oxygen the adult rats consumed during the experiment. The results are detailed in Figure 1.
Figure 1 Change in inner body temperature of rat colon, brain, and subscapular fat under 11°C external temperature change; and, the amount of consumed oxygen as a function of time
Another team of scientists discovered that the brown color of brown fat was due to the fact that the brown adipose cells were filled with mitochondria. And upon further investigation, the brown adipose mitochondria were discovered to contain a unique protein called UCP, which stands for UnCoupling Protein. UCP is a mitochondrial inner membrane protein that facilitates the transport of hydrogen ions across the membrane. The rate of diffusion of you H⁺ ions across UCP depends only on the relative concentration of H⁺ ions.
Adapted from Donhoffer SZ, Sárdy F., Szegvári GY, (1964) Brown Adipose Tissue and Thermoregulatory Heat Production in the Rat. Nature 203, 765 - 766 and from a paper by Chavin KD, Yang S, Lin HZ, Chatham J, Chacko VP, et al. (1999) Obesity induces expression of uncoupling protein-2 in hepatocytes and promotes liver ATP depletion. J Biol Chem 274
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[question] => What will happen to the production of ATP in mitochondria that contain UCP compared to a mitochondria without UCP?
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[description] => Reason for Correct Answer:
In the mitochondria, the electron transport chain is used to drive protons across the inner mitochondrial membrane and into the intermembrane space. This establishes a chemical gradient containing electrical potential energy, which ATP synthase uses to make ATP – by allowing H+ to flow down its concentration gradient and back through the inner membrane.
https://commons.wikimedia.org/wiki/File:2508_The_Electron_Transport_Chain.jpg
UCP allows H+ ions to flow passively down their concentration gradient without going through ATP synthase.
In mitochondria, this decreases the chemical potential energy used to drive the production of ATP, thus decreasing ATP production.
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[answers] => Array
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[0] => Array
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[each_answer] => A. The production of ATP will stay same.
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[each_answer] => B. The production of ATP will decrease.
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[each_answer] => C. The production of ATP will increase.
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[each_answer] => D. The production of ATP will drop to zero.
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => Microscopically, brown adipose tissue has similar features to which other type of tissue?
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[answer] => 4
[description] => Reason for Correct Answer:
The passage states that brown adipose cells are filled with mitochondria.
Both skeletal muscle and brown adipose tissue cells have more mitochondria per cell than most other cell types in the body such as epithelial, skin, or white adipose tissue. In addition, both skeletal muscle cells and brown adipose tissue participate in the regulation of body temperature. Interestingly enough, both brown adipose tissue and muscle tissue are derived from the same stem cells.
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[each_answer] => A. Epithelial cells
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[each_answer] => B. Neural cells
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[each_answer] => C. Skin cells
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[each_answer] => D. Muscle cells
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[quiz_unique_key] => 1403770772
[question] => What is mostly likely the main function of brown adipose tissue?
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[answer] => 1
[description] => Reason for Correct Answer:
Because of the diverted H+ via the UCP, energy is diverted away from ATP production.
In Figure 1, when external temperature decreases, internal temperature inside brown subscapular fat increases.
Figure 1 confirms that brown adipose tissue starts to generate heat only when the body is exposed to cool external temperatures.
The energy generated inside the mitochondria makes brown adipose tissue a location where the body generates heat. Moreover, brown fat’s location around the aorta would indicated an evolutionary adaption to maintain the blood at warm temperatures.
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[answers] => Array
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[0] => Array
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[each_answer] => A. It participates in generating heat for the body.
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[each_answer] => B. It is essential to fat storage for later use in a metabolic pathway.
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[each_answer] => C. It produces hormones that regulate body temperature.
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[each_answer] => D. It participates in cooling the body temperature.
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[question] => Which of the following conclusions can best be drawn by the data shown in Figure 1?
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[answer] => 3
[description] => Reason for Correct Answer:
Figure 1 shows changes in oxygen consumption (bottom) and temperature in various regions of rats’ bodies during exposure warmer temperature (31 celsius) followed by prolonged exposure to cooler temperature (20 celsius). You can see time on the x axis and track changes in body temperature and oxygen consumption as a function of time in the cooler condition.
The graph shows that oxygen consumption increases at T=15 minutes, after about 7 minutes of exposure to the 20 celsius condition. (Note that from 0-7 minutes, the temperature is set to 31 celsius, according to the graph.)
This change in oxygen consumption closely mirrors the change in sub-scapular temperature, because this is where brown fat is location. However, to answer this question, you have to look at the effect of this increase in oxygen consumption to the temperatures of the colon and brain.
As shown in the graph, though colon and brain temperatures decrease after exposure to the cooler temperature, their graphs level off after oxygen consumption increases. Therefore, the increase in oxygen consumption in response to the cool environment aided in temperature regulation in the colon and brain.
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[0] => Array
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[each_answer] => A. The decrease in oxygen consumption in response to the cool environment corresponds to a hibernation mode stimulated by the extreme temperature.
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[each_answer] => B. The increase in oxygen consumption in response to the cool environment aided in temperature regulation in the colon but not in the brain.
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[each_answer] => C. The increase in oxygen consumption in response to the cool environment aided in temperature regulation in the colon and brain.
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[each_answer] => D. The increase in oxygen consumption in response to the cool environment exacerbated temperature losses in both the colon and the brain.
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[question] => Which one of the following statements about brown adipose tissue is most likely true?
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[answer] => 1
[description] => Reason for Correct Answer:
If H⁺ ions are diverted away from the ATP synthase, more molecules of glucose are needed per cell to produce the same number of ATP molecules as a regular cell.
Also, from the information in Figure 1, more oxygen is required once the rats are exposed to colder temperatures.
To provide more oxygen and glucose, brown adipose fat has an increased number of capillaries surrounding it.
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[each_answer] => A. There is an increased number of capillaries surrounding brown adipose tissue.
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[each_answer] => B. In the intermembrane space of the mitochondria there are lower than normal pH levels.
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[each_answer] => C. An increase in brown adipose tissue causes the body to respond with excessive shivering.
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[each_answer] => D. Less lactic acid is stored inside brown adipose tissue than in white adipose tissue.
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[554719|4] => C
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Figure 1
