Organismal Biology Topics with Explanations Thread

[QofQuimica]

This thread will function analogously to the physical science, molecular biology, and organic chemistry explanations threads. We will be adding new posts periodically on organismal biology topics that many students find difficult. Students are requested to please NOT post questions here. If you would like to ask an organismal biology question, you should post it in the Organismal Biology Question Thread.

Here is a list of all of the posts in this thread alphabetized by topic:

Table of Contents:

• Post 03: Blood pH and How It Is Controlled
• Post 04: Development of Ova (Female Gametes)
• Post 02: How to Read Biology Passages

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[Shrike]

There is no perfect answer to the question about how to attack a bio passage, but what I advise is very similar to my approach to physics passages. Possible differences:

• It may be acceptable to skim the entire passage, or skim until you get to obviously detailed information, because biology passages resemble verbal reasoning more than do physics passages.
  
  
• Because tables of experimental values are likely to be of values you understand (in contrast to physics, where that's far from guaranteed), it may make sense to look briefly at tabular data and at graphs for general trends.

These things said, they're not what I advise (nor what I personally do) -- I advise the same approach as I do for physics passages (see link above). No reading the passage; just examine the figures, graphs, and tables. At most, skim the text. Total time prior to questions should be under 45 seconds. Then, on to the questions, on which you can now spend the needed time.

 

[MollyMalone]

Important things to know about acid/base balance in the human body FOR THE MCAT:

The buffer system you need to know: HCO3-/CO2 (think HCO3- = base and CO2 = acid)

Chemical equation: H+ + HCO3- = H2CO3 = CO2 + H2O (imagine the equilibria arrows)

The lungs independently control CO2 (fast response – minutes to hours) while the kidneys independently control HCO3- (slow response – hours to days).

Physiologic pH: 7.4

When the pH goes out of whack, the body has a few different ways to deal with it: buffering, compensation, and correction. Buffering is automatic and instantaneous – it’s a function of the chemistry shown above. When the challenge to the buffering system is too great and the pH becomes materially affected, chemoreceptors pick up this signal encourage the appropriate system to take action. The response depends on whether it is a metabolic problem (HCO3- too high or low) or a respiratory problem (CO2 too high or low). The kidneys adjust to compensate for respiratory problems (can do this by a variety of mechanisms but basically you just need to know that it affects HCO3- levels) while the lungs adjust the CO2 level by hyper- or hypoventilation to compensate for metabolic disorders. These compensatory mechanisms function best as a temporary stopgap (minutes to days) until the body can manage to correct the underlying reason for the acid/base imbalance. Of course, sometimes the problem is not correctable and people wind up living in a chronically compensated state.

Here’s a handy list of the four main problems and what happens with them:

Respiratory acidosis – caused by lung problems. Levels of CO2 are too high, causing pH to drop. The kidneys respond by increasing levels of HCO3-.

Respiratory alkalosis – caused by lung problems. Levels of CO2 are too low, causing pH to rise. The kidneys respond by decreasing levels of HCO3-.

Metabolic acidosis – not caused by lungs. Levels of HCO3- are too low, causing pH to drop. The lungs respond by decreasing levels of CO2 (hyperventilation).

Metabolic alkalosis – not caused by lungs. Levels of HCO3- are too high, causing pH to rise. The lungs respond by increasing levels of CO2 (hypoventilation).

Just for fun, some causes of each (not an exhaustive list):

Respiratory acidosis: pulmonary embolism, cardiac arrest, pneumonia, COPD, airway obstruction – you get the drift. Things that impair the lungs’ ability to move air or do gas exchange.

Respiratory alkalosis: pain, fever, things that cause respiratory rate and depth to increase.

Metabolic acidosis: diarrhea (loss of base via GI tract), ketoacidosis (from diabetes), lactic acid build-up, several drugs.

Metabolic alkalosis: vomiting (loss of acid via GI tract), occasionally gain of base (saw this once in a lady who drank multiple bottles of Maalox)

 

[travelbug73]

The primary oocytes formed at birth (no more will be made, ever), and start meiosis I and are arrested in Prophase I until puberty, during which time many of the primary oocytes regress. Once puberty starts, ONE primary oocyte, every month, will complete meiosis I, resulting in the formation of a secondary oocyte and a polar body. The secondary oocyte starts meiosis II and again stops, this time at Metaphase II.
During ovulation, the secondary oocyte, which is released into the abdominal cavity before then entering the fallopian tube, will complete meiosis II and form the mature egg (ovum) and another polar body, if and only if there is a sperm available. If there is no sperm available the secondary oocyte will be discharged during menstruation without underoging meiosis II.
The first polar body also divides resulting in 2 more polar bodies. Therefore, at the end of meiosis II, there should be at least 2 and maybe 3 polar bodies (if there is fertilization).
In short, the primary oocyte is at Prophase I until puberty.