PHOTOSYNTHESIS
I.
GENERAL
A. Photosynthesis refers to the ability of plants, algae, and a few kinds of bacteria to make their
own food in the presence of sunlight
B. The organelle responsible for photosynthesis in plants and algae is the chloroplast
II. CHLOROPLAST
A. Thylakoids
1. Stacks of flattened sacs that contain the pigment chlorophyll used to capture solar
energy
2. It is the site of the light-dependent reactions in which water is split to release oxygen (O2)
while producing NADPH2 and ATP
B. Stroma
1. The fluid-filled space that contains enzymes for the light-independent reactions
2. NADPH2 and ATP produced by the light-dependent reactions are used to reduce carbon
dioxide (CO2) and form a carbohydrate
III. THE LIGHT REACTION
A. Thylakoid membranes contain 2 light-gathering units, photosystem I (PS I) and
photosystem II (PS II)
1. The pigment complex gather sunlight energy until electrons in the reaction-center
chlorophyll become so excited that they escape to a nearby primary electron acceptor
B. Noncyclic Electron Pathway
1. Pigment complex of PS II absorbs solar energy, excited electrons leave reaction-center
chlorophyll and are taken up by primary acceptor molecule, water splits to release
oxygen, electrons pass from one carrier to the next in the electron transport system as
the released energy is used for ATP production, excited electrons leave PS I to an
acceptor molecule that passes electrons on to NADP to form NADPH 2
2.
Effect of noncyclic electron pathway: water is split, oxygen and hydrogen ions released,
ATP and NADPH2 is formed
C. Chemiosmosis
1. Electron transport between PS II and PS I generates a proton gradient
a) Compared to the stroma, the large number of hydrogen ions in the thylakoid space
create an electrochemical gradient
2. When hydrogen ions flow down this gradient through channel proteins (ATP synthase
complex), ATP is formed
IV. THE CALVIN CYCLE
A. Introduction
1. The light-independent reactions (Calvin cycle) use NADPH2 and ATP from the lightdependent reactions to reduce carbon dioxide to form a carbohydrate within the stroma
of the chloroplast
V. COMPARISON OF CELLULAR RESPIRATION AND PHOTOSYNTHESIS
A. Differences
1. Overall equation for aerobic cellular respiration is the opposite of that for photosynthesis:
energy (ATP)+ 6 CO2 + 6 H2O  C6H12O6 + 6 O2 (cellular respiration)
energy (light) + 6 CO2 + 6 H2O  C6H12O6 + 6 O2 (photosynthesis)
2. Cellular respiration occurs in the mitochondria, oxidation of glucose occurs, energy and
carbon dioxide are released
a) It requires oxygen and occurs in both plants and animals, day or night
3. Photosynthesis occurs in the chloroplast, reduction occurs, energy and oxygen are
released while requiring carbon dioxide
B. Similarities
1. Both photosynthesis and cellular respiration are metabolic pathways (but occur in
opposite directions)
2. Both make use of an electron transport system located in a membrane to produce a
proton gradient and ATP