Explain how the reactions of photosynthesis transform light energy into chemical bond
energy. Include in your discussion the relationship between chloroplast structure and
the light and dark reactions.
Light first hits an antenna complex in the thylakoid membrane of the chloroplast.
This light antenna consists of several hundred molecules of chlorophyll A, B, and
carotenoid. The energy from the photon that has struck the antenna molecules is passed
from chlorophyll molecule to chlorophyll molecule until it reaches the reaction site. This
reaction site is a specific chlorophyll molecule where the initial chemical reaction that
occurs in photosynthesis is driven by light energy. The reaction site consists of a
specialized molecule known as the primary electron acceptor, which takes on the electron
from the chlorophyll molecule. This sharing of electrons from chlorophyll to the
acceptor is the beginning of the light reaction. The energy that drives the light reactions
knocks a series of electrons from the chlorophyll molecules. This electron cascade is
what splits a water molecule, and what also generates ATP. The hydrogen from the split
water molecule goes to reduce NADP+, and the oxygen from the water molecule is
released into the atmosphere.
The ATP and NADPH from the light reaction go into the dark reaction (the
Calvin cycle) by being pushed from the thylakoid to the stroma. The carbon dioxide is
taken in and bonded with a five-carbon sugar biphosphate, catalyzed by rubisco. The sixcarbon sugar immediately breaks down two molecules of three phosphoglyocerates.
Each molecule from the phosphogylocerates gains an additional phosphate from ATP.
The NADPH generated from the light cycle reduces the biphosphoglycerate to a threecarbon sugar that stores more potential energy, known as G3P. Then the remaining fivecarbon molecule is changed into RuBP ready to accept another CO2 molecule.
The chloroplast structure is very important in this process. Chlorophyll exists in
the membrane of the thylakoid. Thylakoids are often stacked in structures called grana.
The chloroplast itself contains the grana and a dense fluid called stroma, incased in a
double membrane. As mentioned above, the light reaction takes place in the thylakoid
membrane, then the products of that, ATP and NADPH go to the stroma for the dark
reaction.
With love, from Thanjina, Emily, John BizzyBohman, Arexa Cray, Jolie Bolie, and Chris