photosynthesis (chemistry)

Photosynthesis (yucky chemistry details)

Hi. You are here to expand or reinforce your understanding of the chemistry behind the biology, right?
(If not, get out while the gettin' is good.)

Like most "life processes", photosynthesis can be studied at various levels ---

· the global level (how it's important to ecosystems),

· the plant part level (how leaves are organized for the process),

· and/or the cellular level (the chemical reactions occurring in the chloroplasts of the plant cells in the leaves of the plant).

I did my best to illustrate the first two levels on the "big picture" photosynthesis page which you probably have looked at already. If you haven't, take a gander now (or later) by clicking here .

OK, let's get to some detailed details.

Detail #1	Photosynthesis occurs in organelles called CHLOROPLASTS. More specifically, half of the photosynthesis chemistry stuff occurs in a liquid inside the chloroplasts called STROMA, & the other half of photosynthesis occurs in stacks of membranes inside the chloroplast called GRANA.	The inside of a chloroplast (seen through an electron microscope). I sloppily outlined the chloroplast in green.
Detail #2	Photosynthesis requires the green pigment CHLOROPHYLL, which absorbs sunlight. Turns out there are 2 types of chlorophyll, chlorophyll a & chlorophyll b. They are slightly different chemically, but similar in that they like to absorb blue, violet, orange, & red wavelengths of light, & tend to REFLECT (not absorb) yellow & green wavelengths. So what? Well, plants can't use light energy that they reflect. So, since plants reflect yellows & greens, that energy cannot be used for photosynthesis. Translation? Plants under yellow & green light will carry out photosynthesis at a lower rate than the same type of plant under other colors (blue, violet, red). (This idea makes a nifty science project.)
Detail #3	The chemical reaction of photosynthesis (as I hinted at in Detail #1) occur in two phases. Phase #1 is called "photolysis" or the "photochemical reactions" or (if you don't like those two names) the "light reactions". These reactions (whatever you call them) require light (duh). Phase #2 is called "carbon fixation" or the "dark reactions". These reactions do not require light.	Remember the parts of a chloroplast? The light reactions occur in the grana, which contain the chlorophyll for absorbing light. The dark reactions occur in the stroma. (see pic above)
Brain Refresher	Before we get into more details, please recall that the overall chemical equation for photosynthesis is the following: 6CO₂ + 12H₂O + light energy ---> C₆H₁₂O₆ + 6O₂ + 6H₂O or, in words: carbon dioxide + water + light energy ---> glucose + oxygen + water OK, onward ....
Detail #4	Detailed details about the light reactions. Of the substances in the overall equation, the light reactions use light & water, & produce the oxygen "waste product". Remember that name "photolysis"? That translates as light (photo-) splits (-lysis). Turns out that the light energy is used to split the water molecule, which produces H+ & O- ions. The oxygen is released from the plant into the air. ATP & NADPH₂ are energy molecules. They will be used up during the dark reactions, as will the H+ ions.	the light reactions
Footnote to Detail #4	Upon close inspection of the photosynthesis equation, 6CO₂ + 12H₂O + light energy ---> C₆H₁₂O₆ + 6O₂ + 6H₂O one may notice that two reactants (CO₂ & H₂O) contain oxygen. So, the skeptical folks out there would ask, "How do you know that the oxygen we breathe comes from the water molecule that is split in the light reactions & NOT from the CO₂?"	The answer: Scientists have used an isotope of oxygen called O-18 to follow oxygen atoms through the chemical maze of photosynthesis. When they tagged the "O" of the CO₂ none of the oxygen being released from the plant contained O-18. But when they tagged the "O" in the H₂O, all of the oxygen being produced contained O-18. So the O we breathe originates from water molecules used by the plant during photosynthesis.
Detail #5	Don't be afraid of the dark ... reactions, a.k.a. "carbon fixation". These reactions use the energy of the ATP & NADPH₂ (produced in the light reactions) to combine the H+ (also produced in the light reactions) with CO₂. Now the end product we're after here is glucose. To build glucose we need C, H, & O, which we have in our H+ & CO₂. Nifty, huh? So there's a bunch of reactions from which & a 3-carbon compound called PGAL is produced. Think of PGAL as 1/2 of a glucose. Then, 2 PGAL's are combined & WA-LA! A glucose is born!
Simple Summary?	The overall equation (again) & what happens to what during this fun chemical reaction ............ Simple, right?

Hints, clues & stuff to remember:

The names for the 1st phase (light reactions, photochemical reactions, photolysis) all contain clues that light is involved. So remember the names, & then ask yourself, "Self, how is light involved?" And then answer yourself, "The energy in light is use to split water".
The only things you can get by splitting water (H₂O) is hydrogen & oxygen. The oxygen exits (is released from the plant). The hydrogen is used in the dark reactions.
The dark reactions, or carbon fixation, must involve carbon (duh). The reactant with the carbon is CO₂. The CO₂ is combined with the hydrogen left over from the water split in the light reactions. First they form PGAL (a 3-C compound), & then 2 of those PGAL's are converted to our favorite 6-C compound ... glucose (C₆H₁₂O₆). Some water molecules are produced as a waste product during this step.
The dark reactions are powered by energy compounds formed during the light reactions, namely ATP & NADPH₂.
P.S. PGAL stands for phosphoglyceraldehyde.

Review & Practice Questions

1. Which equation summarizes photosynthesis?
a) water + starch ---> glucose + glucose + glucose
b) water + carbon dioxide ---> oxygen + glucose + water
c) glucose + oxygen ---> water + carbon dioxide + ATP
d) glucose + glucose ---> maltose + water

2. In what organelle does photosynthesis occur?
a) the nucleus
b) chloroplasts
c) the vacuole
d) the cell wall

3. Four identical plants are grown under different colored light bulbs. Under which color will the release of oxygen gas be slowest?
a) blue
b) green
c) orange
d) red

4. Which of the following would have the smallest effect on the rate of photosynthesis in a green plant?
a) carbon dioxide concentration
b) light intensity
c) oxygen concentration
d) water available

5. Which of the following is produced during photosynthesis?
a) carbon dioxide
b) PGAL
c) DNA
d) lactic acid

6. In what part of photosynthesis would the tracer carbon-14 be used to study the path of carbon through the chemical reactions?
a) the absorption of light by chlorophyll
b) the dark reactions
c) photolysis
d) the light reactions

7. Atmospheric oxygen that is inhaled by animals comes from:
a) carbon dioxide molecules split during the light reactions
b) carbon dioxide split during the dark reactions
c) water molecules split during the light reactions
d) water molecules split during the dark reactions

8. What change occurs during photosynthesis?
a) solar energy is converted to chemical energy
b) kinetic energy is converted to chemical energy
c) chemical energy is converted to radiant energy
d) water is converted to chemical energy

For questions #9-18 choose the best description from the list.
A - photochemical reactions only
B - carbon fixation reactions only
C - both the photochemical & carbon fixation reactions
D - neither the photochemical & carbon fixation reactions

9. Water molecules are split.
10. Carbon dioxide is used.
11. PGAL is formed.
12. Light energy is absorbed by chlorophyll.
13. The reactions occur in the stroma of the chloroplast.
14. The reactions occur in the grana of the chloroplast.
15. Glucose is formed.
16. Enzymes are present.
17. Oxygen is released.
18. Oxygen gas is used.

<ANSWER KEY>

I hope that most of the details related to this topic are a little clearer now. Remember to visit the Photosynthesis "big picture" page when you get a chance.
Stay happy & keep pluggin' away.

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<ANSWER KEY - Correct answers are in ORANGE / Explanations are in BLACK>

1. Which equation summarizes photosynthesis?
a) water + starch ---> glucose + glucose + glucose (HYDROLYSIS)
b) water + carbon dioxide ---> oxygen + glucose + water
c) glucose + oxygen ---> water + carbon dioxide + ATP (RESPIRATION)
d) glucose + glucose ---> maltose + water (DEHYDRATION SYNTHESIS)

2. In what organelle does photosynthesis occur?
a) the nucleus
b) chloroplasts
c) the vacuole
d) the cell wall

3. Four identical plants are grown under different colored light bulbs. Under which color will the release of oxygen gas be slowest?
a) blue - fastest
b) green
c) orange - fast
d) red - faster

5. Which of the following is produced during photosynthesis?
a) carbon dioxide - used
b) PGAL
c) DNA
d) lactic acid - in sore muscles do to anaerobic respiration

(note that c & d are the same thing so neither is right)

8. What change occurs during photosynthesis?
a) solar energy is converted to chemical energy - sunlight --> food energy
b) kinetic energy is converted to chemical energy
c) chemical energy is converted to radiant energy
d) water is converted to chemical energy

9. Water molecules are split. A
10. Carbon dioxide is used. B - carbon fixation, carbon dioxide
11. PGAL is formed. B
12. Light energy is absorbed by chlorophyll. A - "photo" for light
13. The reactions occur in the stroma of the chloroplast. B
14. The reactions occur in the grana of the chloroplast. A
15. Glucose is formed. B - last product, formed from PGAL
16. Enzymes are present. C - all reactions require enzymes
17. Oxygen is released. A - comes from the split water
18. Oxygen gas is used. D - oxygen is a product, not a reactant

<DONE>