RELATIVE ATOMIC MASS/RELATIVE ABUNDANCE QUESTIONS

Question 1:
Calculate the relative atomic mass of silver if it occurs naturally as 51.84% silver-107 and 48.18% silver-109.

Solution:
Relative atomic mass = 0.5184 (107) + 0.4816 (109)
Relative atomic mass = 107.96

Question 2:
Calculate the relative abundance of each isotope of copper if it consists of two isotopes, copper-63 and copper-65. Its relative atomic mass is 63.62.

Solution:
Let Abundance of copper-63 = x
Then, Abundance of copper-65 = 1-x

Relative atomic mass = 63x + 65(1-x)
                                      63x + 65(1-x) = 63.62
                                       x = 0.69
That is, 69% of copper-63 and 31% of copper-65

THE MASS SPECTROMETER

The mass spectrometer is an instrument used for analyzing samples of elements and compounds. It consists of six basic stages: Injection, Vaporization,  Ionization, Acceleration, Deflection, and  Detection.

Click to view a diagram of the mass spectrometer.

• The sample is injected into the instrument.
•  It may be vaporized by heating. 
• The vaporized sample is then passed into an ionizing beam of electrons which knock electrons off the sample to create positive ions.
• These positive ions are then accelerated by some electrostatically charged plates into a magnetic field.
• The magnetic field then deflects the particles according to their mass/charge ratio.
• The deflected ions then arrive at the detector.

You can test yourself on the atomiser by clicking this link. It is a timed quiz.

FACTORS BEHIND MEDICAL SCHOOL ADMISSION

Medical admissions committees always look at grades and MCAT test scores. But you can become a much stronger applicant by demonstrating your passion for medicine, highlighting how your goals align with the school's mission and showing your integrity and humility. Read more about this by clicking on this link.

METABOLISM AND ENZYMES

Metabolism is the building up (synthesis) or the breakdown of substances in the living organism. It consists of anabolism and catabolism.

Anabolism is the formation or synthesis of complex substances from simple ones.                           That is, A + B -----> C.
An example of anabolic reaction is photosynthesis.

Catabolism is the breakdown of complex substances into simple ones. That is, C -----> A + B.
An example of catabolic reaction is respiration.

What is photosynthesis?
This is the process whereby  plants produce their food (glucose) using carbon dioxide, water, and energy from the sun,  in the presence of chlorophyll (the green pigment in the leaves). Chlorophyll is contained in the chloroplast and it traps the energy from the sunlight. Glucose is the main product formed and oxygen is given off as waste.

This the equation that summarizes photosynthesis: 6CO₂ + 6H₂O à C₆H₁₂O₆ + 6O₂
This is carbon dioxide: CO₂ 
This is water: H₂O
This is glucose: C₆H₁₂O₆
This is oxygen: O₂

Note: You should know this equation off-hand. 

Water goes up from the roots through the plant vessel called xylem.
The glucose formed in the leaves are transported to storage sites via the plant vessel called phloem.

The glucose is stored as starch in the plants.

Respiration
Respiration involves the breakdown of glucose in the presence of oxygen to release energy in the form of ATP (ATP stands for adenosine  triphosphate, the energy currency). ATP is generated in the mitochondrium. Respiration is a combustion reaction since oxygen is needed.

There are three (3) stages in the process of respiration.
1. Glycolysis: Glucose is activated and broken down to pyruvate.
2. Kreb's cycle (also known as the Tricarboxylic Acid Cycle - TCA cycle): Some ATP, NADH, FADH_{2 are produce here.}

3. Electron Transport Chain (ETC): Most ATP's are produced here.

This equation summarizes the reaction of respiration:
 C₆H₁₂O₆ + 6O₂  à6CO₂ + 6H₂O 

Know this very well. It is simply the reverse of photosynthesis reaction.

In the absence of oxygen, pyruvate is converted into lactic acid, which  causes muscle cramps. When oxygen is available, the lactic acid is changed back to pyruvate which is then used up in the Kreb's cycle.

Fermentation
This is the process whereby glucose is changed to alcohol in the absence of oxygen.

Click to read more about enzymes and metabolism.

REPRODUCTION IN MAMMALS

Gametes refers to the sperm and the egg. During sexual reproduction, the sperm and the egg unite to form the zygote.
                           Zygote = sperm + egg

Each gamete has half the normal complement of chromosomes, that is, 23 chromosomes in each gamete. The gametes are said to be monoploids (haploid). Fusion results in the full number of chromosomes, 46 (23 pairs). The zygote is referred to as diploid.

In the male, the external reproductive organs are the penis and scrotum.
The internal organs consist of:

• The testis which contains the seminiferous tubules, where the sperms are produced.
• The interstitial cells produce testoterone, the male sex hormones.
• The sperm travels in the epididymis, the coiled tubules, into the vas deferens to the ejaculatory duct to the urethra.
• Seminal vesicles produce seminal fluid that carries the sperms.
• Prostate produces prostatic fluid that also carries the sperms.

In the female, the primary reproductive organs are the ovaries, fallopian tubes and uterus.

• Ovaries produce the eggs and the hormones, estrogen and progesterone.
• Immature egg is called oocyte.
• Matured egg is released during ovulation.
• The egg travels in the fallopian tubes, where it can be fertilized by the sperm. Note that the fertilized egg begins to divide within the tubes within few hours of conception.
• The lining of the uterus is called the endometrium.
• The fertilized egg implants in the endometrium.
• The fetus obtains nourishment via the placenta. Note that, the fetal and maternal blood do not have direct connections. Exchange of nutrients and oxygen is by diffusion.
• The fetus is connected to the placenta via the umbilical cord.
• The fetus is covered by the following membranes: amnion and chorion.
• If there is no fertilization, the uterus sheds its, endometrium. This is referred to as menstruation.

For more details on reproduction, click on this link. You will find various diagrams and details of the stages of reproduction in mammals.

MAKING INFERENCES AND CONCLUSIONS

You will be tested on your understanding of scientific experiments and your ability

 to make inferences and conclusions from various data, graphs, and scenarios. 

The science passages below can help you practice on some of these questions.

1. Reading Comprehension 1
2. Reading Comprehension 2
3. Reading Comprehension 3
4. Reading Comprehension 4
5. Reading Comprehension 5
6. Reading Comprehension 6
7. Reading Comprehension 7

ENDOCRINE SYSTEM

An endocrine gland is a gland without ducts. The secretions are released directly into the 

blood. The endocrine glands secrete hormones that regulate various metabolic activities 

in the body.

Here is a good diagram of the endocrine system. Click here. You can be given a diagram
 like this and you could be asked to identify the various glands.

Hypothalamus: The hypothalamus produces eight (8) hormones. Some of the hormones stimulate
the pituitary gland to produce other hormones.

1. Thyrotropin-releasing hormone - This acts on the pituitary to release thyroid stimulating
 hormone (TSH)
2. Growth hormone-releasing hormone-This acts on the pituitary to release growth hormone (GH)
3. Gonadotropin-releasing hormone -This acts on the pituitary to release gonadotropins, namely,
Follicle stimulating hormone (FSH) and Luteinizing hormone (LH).
4. Corticotropin-releasing hormone -This acts on the pituitary to release adrenocorticotropic
 hormone.
5. Oxytocin -This causes uterine contractions and also helps with secretion of the breast milk
 during lactation.
6. Dopamine - It is a neuroendocrine transmitter. Acts as a neuro-transmitter and also stimulates
 the release of othe hormones.
7. Vasopressin (Antidiuretic hormone -ADH) - Acts on the kidneys to prevent the lost of water
via the urine.
8. Somatostatin - It inhibits the release of growth hormone.

Pituitary gland: Has an anterior (hormone-producing glandular) portion and a
 posterior (neural) portion which is an extension of the hypothalamus.  
Two hormones ( Oxytocin and ADH) produced by the hypothalamus are stored
 in the posterior pituitary later release.

Four of the six pituitary hormones are tropic hormones. They regulate the function
of other endocrine glands.Here are the hormones produced by the anterior portion:

1.  Growth hormone (GH): It stimulates growth of all body tissues but especially
skeletal muscle and bone. GH mobilizes fats, stimulates protein synthesis, and inhibits 
glucose uptake and metabolism. Over production can lead to gigantismwhile 
underproduction can lead to dwarfism.

2. Thyroid-stimulating hormone (TSH): This promotes normal development and activity
 of the thyroid gland. 

3. Adrenocorticotropic hormone (ACTH): This stimulates the adrenal cortex to release
 corticosteroids. 

4. The gonadotropins (follicle-stimulating hormone (FSH) and luteinizing hormone (LH)):
 These regulate the functions of the gonads in both sexes. 

     (a) FSH stimulates sex cell production.  

     (b) LH stimulates gonadal hormone production. 

5. Prolactin (PRL): This promotes milk production in humans females. 

The posterior portion stores and releases two hypothalamic hormones:

1. Oxytocin: This stimulates powerful uterine contractions during labor and delivery of babies.
 It also causes milk ejection in nursing women. 

2. Antidiuretic hormone (ADH): This stimulates the kidney tubules to reabsorb and 
conserve water. This results in the production of small volumes of highly concentrated 
urine and decreased plasma osmolality. Underproduction leads to a condition 
called diabetes insipidus, where the affected person passes a lot of diluted urine.

Thyroid gland

It produces the thyroid hormone (TH), which includes thyroxine (T4) and triiodothyronine (T3).
 It increases the rate of cellular metabolism.

Calcitonin, is produced by the parafollicular cells of the thyroid gland. It decreases the blood
 calcium levels.

Parathyroid glands

It secretes parathyroid hormone (PTH), which causes an increase in blood calcium levels.

Pancreas

The pancreas is both an exocrine and an endocrine gland. Exocrine means that it
 has ducts.The endocrine portion (islets of langerhans) releases insulin and glucagon. 
It also releases smaller amounts of other hormones to the blood.

Glucagon, released by alpha (α) cells - It increases the glucose level in the blood.

Insulin is released by beta (β) cells - It reduces the glucose level in the blood. It increases
 the rate of glucose uptake and metabolism by most body cells. 

Gonads

The ovaries of the female release two main hormones - estrogens and progesterone.
 Estrogens stimulate maturation of the female reproductive system and development of 
the secondary sexual characteristics. Progesterone works with estrogens in establishing
 the menstrual cycle.

The testes of the male produce testosterone. It promotes maturation of the male reproductive
organs, development of secondary sex characteristics, and production of sperm by the testes.

Pineal gland

The pineal gland produces the hormone  melatonin, which influences daily rhythms such as
sleep and wake patterns.

Thymus 
It is an important organ of the immune system during the developmental stages of life.
It vanishes by the time of birth. The T-cells mature here.