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.