Question

Photosynthesis Homework                                                             Name:_______________________

DIRECTIONS: Use Chapter 10 to help answer the questions and fill in the table below. Bring to class completed at the beginning of lecture.

1. Briefly describe the ‘big picture’ of what photosynthesis is all about. In your description, write out the chemical formula for photosynthesis and explain how each compound in the formula is related to the ‘big picture’.

2. Describe the specific purpose of the Light Capturing Reaction and the Calvin Cycle.

3. Fill in the descriptions of the table below.

Molecule	Reaction Involved?	Purpose
Photon
H2O
CO2
ATP
NADPH

Answer 1

"The transformation of light energy in to chemical energy by green plants (and some other organisms) is termed as photosynthesis". This chemical energy is then used by heterotroph present in this planet including humans. Green plants must have pigments and pigment containing organelle (chloroplast) with specialized proteins (including enzymes) that are capable of transforming light energy in to chemical energy. Carbohydrates (sugars) are the compounds which are synthesized as chemical energy storage compounds.

The overall process of photosynthesis is completed in two phases.

Light-dependent reaction: where light energy is trapped by photosystems in the chloroplast and used for production of energy-molecules such as ATP and NADPH. Light is required for this step hence, this is called as light reaction. During light reactions, one chlorophyll molecule (pigment) absorbs one photon and one electron is lost which travels through a number of intermediate compounds such as pheophytin, quinone, cytochrome b6f complex, plastocyanin and ferredoxin and ultimately form NADPH (with the help of ferredoxin-NADP reductase). This generates a proton gradient across the chloroplast membrane and a proton motive force is created which is used by ATP synthase for ATP synthesis. The lost electron of chlorophyll is regained by photolysis of water taking place on photosystem II. This generates oxygen as a waste product and released in to the atmosphere. Light-dependent reactions for ATP and NADPH formation, can further be of two types:

• Cyclic
• Non-cyclic

Light-independent reaction: Light independent reactions involve use of energy compounds (ATP and NADPH) produced during light reactions for fixation of atmospheric carbon dioxide. As sugars have a base structure of carbon and hydrogen, carbon comes from the CO₂. The enzyme responsible for CO₂ fixation is RUBISCO (Ribulose biphosphate carboxylase / oxygenase).The set of reactions responsible for whole cycle was discovered mainly by Melvin Calvin and Andrew Benson in 1950 therefore, the name of cycle was given as " Calvin-Benson cycle" or simply "Calvin cycle".

The overall reaction of photosynthesis is as follow:

Molecule	Reaction involved	Purpose
Photon	Light reaction	Excitation of electrons from chlorophyll molecules hence, driving ATP and NADPH synthesis.
H2O	Photolysis of water (Light reaction)	It compensates for lost electron from the chlorophyll and oxygen is produced as a by product.
CO2	Dark reaction (Light-independent reaction)	Fixation of atmospheric carbon into carbohydrate. Reduction of carbon in to sugar.
ATP	Produced during Light reaction (electron transport chain) and consumed during dark reaction.	ATP provides energy for the CO2 fixation in to sugar
NADPH	Produced during Light reaction (electron transport chain) and consumed during dark reaction.	NADPH provides reducing power (Hydrogen ion) for reduction of CO2 in to the sugar.