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By Levi Clancy for Student Reader on

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Photosynthesis uses light energy to produce sugar. This sugar is then processed via cellular respiration. The process of photosynthesis occurs in plants and some algae (Kingdom Protista) and takes place in the chloroplast. Photosynthesis combines CO2 and H2O to form sugar. This sugar is then used as fuel via cellular respiration. The reactions involved to forge the CO2 and H2O together are collectively called photosynthesis. Photosynthesis occurs in plants and protist algae. The overall chemical reaction involved in photosynthesis is:

6 CO2 + 6 H2O + light → C6H12O6 + 6 O2.

Photosynthesis is the source of the air (O2) we breathe and the sugar (C6H12O6) we eat.

Photosynthesis has two stages:

  • Light-Dependent Process: In grana, direct light energy is harnessed for the energy carriers in the second process. Light strikes Chlorophyll A (chlA) and excites e-'s in chlA to a higher energy state.

  • Light-Independent Process (Dark Process): In stroma, occurs using the e-'s from the light-dependent process. The Dark Process can occur even in the absence of light.

  • Light is both a particle & a wave. It comes in units called quanta, & these quanta have wavelengths. Wavelength is inversely proportional to the energy of quanta. Pigment is any light-absorbing substance. The color of a pigment comes from light reflected (not absorbed). There are three fates for quanta absorbed by a quanta. Quanta can be dissipated as heat; emitted immediately as a longer wavelength (fluorescence); their energy can trigger chemical reactions, as in photosynthesis.

    Photosynthesis takes place primarily in plant leaves, and little to none occurs in stems, etc. The parts of a typical leaf include the upper and lower epidermis, the mesophyll, the vascular bundle(s) (veins), and the stomates. The upper and lower epidermal cells do not have chloroplasts, thus photosynthesis does not occur there. They serve primarily as protection for the rest of the leaf. The stomates are holes which occur primarily in the lower epidermis and are for air exchange: they let CO2 in and O2 out. The vascular bundles or veins in a leaf are part of the plant's transportation system, moving water and nutrients around the plant as needed. The mesophyll cells have chloroplasts and this is where photosynthesis occurs.

    As you hopefully recall, the parts of a chloroplast include the outer and inner membranes, intermembrane space, stroma, and thylakoids stacked in grana. The chlorophyll is built into the membranes of the thylakoids.

    Chlorophyll looks green because it absorbs red and blue light, making these colors unavailable to be seen by our eyes. It is the green light which is NOT absorbed that finally reaches our eyes, making chlorophyll appear green. However, it is the energy from the red and blue light that are absorbed that is, thereby, able to be used to do photosynthesis. The green light we can see is not/cannot be absorbed by the plant, and thus cannot be used to do photosynthesis.

    Photosynthesis consists of two stages:

    Light reactions
  • Use solar energy to make ATP & NADPH (high energy e--carried similar to NADH)

  • Electrons packed onto NAPH come from H2O that is split.

  • Split H2O also gives rise to H+ ions and O2 (the air we breathe).

  • How can we figure out where the oxygen released from plants comes from? What are the options?

  • Dark reactions
    Calvin Cycle
  • ATP from light rxns and environmental CO2, are used to convert CO2 to organic molecules (carbon fixation).

  • Early Research on Photosynthesis

    Plant matter does not come from the soil, but rather from CO2 in the air. The CO2 in the air comes from animal respiration. If you plant an acorn in 50 pounds of soil, you might assume that you will get a 50 pound tree. However, if you measure the soil and tree after 5 years, you will find that there are approximately 49.5 pounds of soil and 215 pounds of tree. This experiment showed early scientists that plant matter does not all come from the soil. However, the origin of plant matter was still a mystery.

    If you perform the following three experiments, you will get these results:

  • A plant alone in an airtight jar. Eventually, the plant dies.

  • A cockroach alone in an airtight jar. Eventually, the cockroach dies.

  • A cockroach and a plant alone in an airtight jar. The plant and mouse both live.

  • This showed early scientists that plant and animals restore air for each other by the following mechanism:

    CO2 + H2O + light → sugar (CH2O) + O2 + H2O

    The balanced chemical reaction is:

    6CO2 + 12H2O + light → C6H12O6 + 6O2 + 6H2O