1. Plants and other photosynthetic organisms undergo photosynthesis to capture energy from sunlight and store them as potential energy in covalent bonds of sugars such as in glucose.

2. Living organisms undergo cellular respiration to convert the potential energy in sugars into kinetic energy for their use.

It contains the nitrogen-containing base adenine, a ribose sugar, and 3 phosphates linked by high-energy bonds.

The high-energy bonds in the phosphates can be broken to release energy and drive cellular processes, forming ADP (adenosine diphosphate).

When energy is needed for cellular work, the ATP is broken down to ADP and P_i.

Note that this is not a closed cycle: a continuous supply of energy is needed to regenerate ATP.

• The "photo" reactions occur within the thylakoid membranes, where chlorophyll pigments absorb energy from light.

• The "synthesis" reactions occur in the semifluid stroma.

Chloroplasts contain pigment molecules such as chlorophyll a, chlorophyll b, and carotenoids that absorb light at different wavelengths.

Chlorophyll pigments absorb mainly violet-blue and red light, and reflect green.

Carotenoids absorb mostly blue and green light, and reflect orange and yellow.

• What colors are absorbed by a substance that looks white?
  • None. A white substance absorbs no colors,and reflects all colors of the visible spectrum.

The energy carried by a particular photon is measured by the length of its wave, or wavelength.

Visible light represents only a small part of the electromagnetic spectrum, between wavelengths of about 400 and 740 nanometers.

The shorter the wavelength, the greater its energy.

During spring, chlorophyll in leaves mask the presence of other pigments called carotenoids.

The chlorophyll absorb most of the visible wavelengths except those around 500 to 600 nm, and reflect these green wavelengths, so leaves appear green.

Cool temperatures in the fall cause many trees to cease producing chlorophyll, and the carotenoids reflect orange/yellow light, giving bright colors to autumn leaves.

Review:

1. "photo" reactions

2. "synthesis" reactions (Calvin cycle)

The inputs (substrates) are carbon dioxide and water, plus energy from light.

The outputs (products) are sugar and oxygen.

Note the intermediate molecules ATP and NADPH serve to store energy harvested in "photo" reactions to be used by "synthesis" reactions.

The energy is stored in ATP and NADPH molecules that are later used by the "synthesis" reactions.

These "synthesis" reactions take place in the semifluid stroma of chloroplasts in a cycle of chemical reactions called the Calvin cycle.

The inputs are oxygen and glucose, and outputs are carbon dioxide, water, and ATP.

1. Glycolysis:
   the 6-carbon glucose is split into 2 molecules of the 3-carbon pyruvate, yielding 2 ATP and NADH.

2. Acetyl-coA production:
   the pyruvate is broken down in the mitochondrion to form acetyl-coA, releasing carbon dioxide (CO₂).

3. Citric acid cycle:
   the acetyl-coA is broken down to release CO₂ and yield more ATP, as well as NADH and FADH₂.

4. Electron Transport Chain (ETC):
   NADH and FADH₂ carry high-energy electrons that are used by proton pumps to produce ATP. Oxygen is needed to accept these electrons, releasing water.
   Review:

In the presence of oxygen, high-energy electrons enter the ETC to make abundant ATP.

In anaerobic conditions (absence of oxygen), the electrons cannot enter ETC, but glycolysis is followed by fermentation, yielding either of two possible products:

• Animals produce lactic acid as an end product.

• Yeasts produce ethanol as an end product.

Review:

Many different kinds of macromolecules can be broken down to enter the Citric acid cycle at various points.