Sunday, November 25, 2012

Roles of Hormone in Menstrual Cycle




Menstrual cycle consists of 4 phases:
a) Follicular phase
b) Ovulation
c) Luteal phase
d) Menstruation


  • Hypothalamus secretes GnRH(gonadotrophin releasing hormone).
  • GnRH stimulates anterior pitituary gland to secrete FSH(follicle-stimulating hormone) & low levels of LH(luteinising hormone).
  • FSH stimulates development of follicles in ovary. Only one follicle will mature to form Graafian follicle.
  • Follicles secrete oestrogen.
  • Oestrogen causes healing & repair of uterine lining. At low levels, secretion of FSH & LH are inhibited. At high levels, secretion of LH is stimulated, causing ovulation. Secondary oocyte is released  into fallopian tube.
  • LH stimulates convertion of empty follicle into corpus luteum & stimulate corpus luteum to produce oestrogen & progesterone.
  • Both oestrogen & progesterone promotes thickening of endometrium/uterine lining to prepare for the implantation of blastocyst.
  • Oestrogen & progesterone also inhibits secretion of FSH & LH. Development of new follicles is prevented.
  • If fertilisation does not occur, corpus luteum degenerates. Oestrogen & progesterone decreases. Endometrium breaks down. Menstruation occurs.


Mechanism of Hormone Action

Mechanism of Hormone Action : Gene Activation


  • Lipid soluble hormones such as steroids pass through plasma membrane easily.
  • In the cytoplasm or nucleus, the hormone binds to its receptor to form hormone-receptor complex.
  • The hormone-receptor complex binds to specific part of DNA.
  • Transcription occurs to produce mRNA.
  • mRNA diffuse out of nucleus & binds to ribosome. Translation occurs. Specific proteins are synthesised.
  • Complete proteins carry out specific functions.

Mechanism of Hormone Action : Second Messenger(cAMP) using adrenaline hormone.

  • Hormones such as adrenaline are first messengers.
  • In the plasma membrane, hormone binds to its receptor to form hormone-receptor complex.
  • The hormone-receptor complex binds to G-protein(guanine nucleotide-binding protein) & activates it.
  • G-protein binds to enzyme adenylyl cyclase & activates it.
  • Adenylyl cyclase catalyses conversion of ATP to cAMP(cyclic adenine monophosphate). cAMP is second messenger.
  • cAMP activates enzyme protein kinase which activates enzyme phosphorylase kinase. This stimulates another enzymatic reaction that in turn produces the 'cascade effect'.

Mechanism of Muscular Contraction : Sliding Filament Theory

Mechanism of Muscular Contraction : Sliding Filament Theory


  1. Action potential spreads through T tubules. This stimulates release of Ca2+ ions from sarcoplasmis reticulum.
  2. Ca2+ ions binds to troponin & rearrange the troponin-tropomyosin complexes. Tropomyosin movesaway to expose binding sites on actin filaments.
  3. ATP attaches to myosin head & is hydrolysed into ADP.
  4. High energy myosin head binds to exposed binding site on actin filament, forming cross-bridges (actomysosin bridges).
  5. Myosin head bends 45 degrees & pull actin filament towards centre of sarcomere. (ADP is released)
  6. ATP attaches to myosin head again. Myosin head detaches from binding site, swings back & reattaches again. This is 'ratchet mechanism'. Actin filaments are pulled closer towards centre of sarcomere.
  7. Muscle contracts.

Ornithine Cycle

Chapter Homeostasis : Ornithine Cycle




  • NH3 combine with CO2 & H2O to form carbomoyl phosphate.
  • Carbomoyl phosphate react with ornithine to form citrulline. This process occurs in matrix of mitochondria in liver cells.
  • Citrulline react with aspartate to form argininosuccinate.
  • Argininosuccinate breaks down to form fumarate & arginine. Fumarate enters Krebs Cycle.
  • Arginine is hydrolysed by H2O to form urea & ornithine.
  • Ornithine is released & reused.
  • Urea is carried to the kidneys to be excreted in urine.

Munch's Pressure (Mass Flow) Hypothesis


  • Leaves are 'sugar source'. Sugar diffuse from mesophyll cells into the sieve tubes of the phloem through symplast route/cytoplasm.
  • Concentration of sugar in sieve tube is high, causing water potential in leaves to decrease/low.
  • H2O enters sieve tube from the xylem, creating high hydrostatic pressure.
  • Growing roots, shoot tips, stems & other parts of the plants are 'sugar sink'.
  • Sugar diffuse out of sieve tube/phloem into 'sink tissues'. Water potential in 'sink tissues' decreases.
  • As a result, H2O moves into other cells by osmosis, creating low hydrostatic pressure in sieve tubes.
  • The hydrostatic pressure gradient formed between 'sugar source' & 'sugar sink' drives mass flow of H2O.

Photosynthesis: Light Reaction & Dark Reaction

Light Reaction

Non-cyclic photophosphorylation : produce ATP & NADPH






  • Reaction centre for photosystem II is P680.
  • H2O is broken down to provide 2 electrons and O2.
  • 2 electrons excited by light energy and achieve high energy potential/energy level, accepted by phaeophytin(primary acceptor).
  • 2 electrons flow to/received by plastoquinone, received by cytochrome complex, received by plastocyanin, and received by photosystem I(ADP is used & ATP is produced).
  • The flow of electrons provide the energy to pump protons across the thylakoid membrane. This activates ATP synthase to produce ATP.
  • From photosystem I, the reaction centre is P700.
  • The 2 electron excited by light energy and achieve high energy level, and accepted by FeS.
  • 2 electron flow to/received by Ferredoxin and received by NADP reductase.
  • Enzyme NADP reductase converts NADP+ to NADPH and H+ ion.

Cyclic photophosphorylation : produce ATP only

  • Reaction centre for photosystem I is P700.
  • 2 electrons excited by light energy, achieve high energy level, accepted by FeS(primary acceptor).
  • The 2 electrons flow to/received by Ferredoxin, received by cytochrome complex, then received by plastocyanin, then received back by photosystem I.(ADP is used & ATP is produced).
  • The flow of electrons provide energy to pump protons across thylakoid membrane. This activated ATP synthase to produce ATP.

Dark Reaction

Calvin Cycle




  • CO2 combines with 5 carbon compound, RuBP with the help of enzyme rubisco to form unstable 6 carbon compound.------>(CARBON DIOXIDE FIXATION phase)
  • 6 carbon compound  split into 2 molecules of 3-phosphoglycerate/phospho-3-glycerate.
  • 3-phosphoglycerate undergoes a series of reduction processes to form glycealdehyde-3-phosphate/triose phosphate. ATP & NADPH is used.------>(REDUCTION phase)
  • Glyceraldehyde-3-phosphate undergoes regeneration to regenerate RuBP.------>(REGENERATION OF CO2 ACCEPTOR phase)
  • Some glyceraldehyde-3-phosphate assimilate to form carbohydrates, lipids, & proteins.------>(PRODUCT SYNTHESIS phase)

DNA Replication Essay

DNA Replication


Using the image above as reference, this is a simple way to memorise the process.

  • Enzyme helicase unwind the parental double helix.
  • The single strand binding proteins stablise the parental DNA
  • The leading strand is synthesised continously in the 5' to 3' direction by DNA polymerase III
  • The lagging strand is synthesised discontinuously. Primase synthesises a short RNA primer to form Okazaki fragment
  • RNA primer is replaced by DNA polymerase I, then DNA ligase joins the Okazaki fragment to the growing strand.

Thursday, November 8, 2012

Argumentative Writing: 'People are becoming more materialistic. They are concerned with making more money and what money can buy. What is your opinion on being materialistic?'

People are becoming more materialistic. They are concerned with making more money and what money can buy. What is your opinion on being materialistic?

          "Life shouldn't be printed on dollar bills". This quote from Clifford Odets tells us that life does not necessarily revolve around money and material wealth. As time proceeds into a modern era, money and materialism have been one of the most influential forces in the world. Some may think that money and wealth is the way of life. They believe that everything luxurious in this world has a price and only by accumulating money and wealth can one enjoy them. In my opinion, I think that people really are becoming more materialistic. This essay will discuss how materialism has affected people, the modern society, and the country's economy.
          Materialism has geared individuals towards the path of making more money. This is so that they can afford life's indulgences like a spacious home and expensive delicacies. They are driven by the want to enjoy a carefree and luxurious lifestyle that seems so popular nowadays. In addition, people work hard to earn more income for the purpose of affording expensive and branded items such as Chanel handbags, IPhones, and tablet personal computers. As people become more focused on wealth and improving living standards, they tend to forget matters that are substantial in life, namely health, family, and friends.
         A materialistic mind-set can drive people to overspend or buy things that they cannot afford yet. This will give a domino effect on the country's economy. Of course, the spending power of the society can help to boost the economy, as they increase sales and business opportunities. However, when people overspend to extreme ends using plastics money or credit cards and take loans that they cannot afford to reimburse them, the country's debt margin will surely rise. For instance, in America, banks are giving loans to individuals who aspire to own big houses that they cannot pay for. Their inability to pay for it became one of the factors that caused the collapse of America's economy.
          Materialism has also affected people's spiritual and moral beliefs. Some people are fortunate enough to have a proper occupation that they can work hard for, but some people who are jobless or have low income can also develop avaricious characteristics. Since they cannot afford the treats in life yet desire it, they can turn to a life of crime for the resolution to satisfy their materialistic needs. This can be observed through the increasing crime rates for theft, break-ins, snatch thefts, frauds, blackmail, kidnapping for ransom, and bank account hacking. Those who commit these crimes are materialistic criminals that lost sight of moral and spiritual values as they are blinded by greed and wealth. Besides that, parents may neglect their children's emotional and spiritual needs because they are fully engrossed in money-making activities. Despite all the comforts and extravagance, the children can become lonely and crave for parental affection, and thus indulge in bad habits like smoking or vandalizing public property as an act to eradicate the loneliness or seek attention.
          In conclusion, materialism is becoming a common quality in the 21st century. People are compelled to work harder to afford indulgences beyond their reach. Even though materialism may spur individuals to strive in life to improve their way of living, the act of overspending can lead to the downfall of the one's well-being and the economy. For me, I believe that money is not everything in life. It is undeniable that without money, one cannot survive in this harsh world, but having money in abundance in meaningless when one is unhappy. As quoted from Virgil, "The greatest wealth is health." I think that there are more significant things in our lives that seem to be overshadowed by our busy daily lives and worldly delights, and they are family, friends, health, and true happiness.

(626 words.)

Band 4