Photosynthesis  p. 58-64

Some historical experiments:

1. Question: Where does the material to build plants come from?

Experiment of the European scientist, van Helmont in 1600's:

CO₂ +               H₂O      +    sunlight        --->  O₂ + C₆H₁₂O₆                                    

carbon dioxide + water + sunlight---------->       oxygen + glucose

Balance the equation:   _____CO₂ +___ H₂O--->  ____O₂ + C₆H₁₂O₆ 

Conservation of matter: Matter is never gained or lost, simply rearranged.

Light is required for this reaction to occur since a low energy compound (carbon dioxide) is being converted into a high energy compound (glucose).

Energy on Earth for life comes from photosynthesis which gets its energy from the sun.

Organic molecules (molecules of life) on Earth come from photosynthesis. Animals, fungi and non-photosynthesizers are unable to make organic molecules and must consume them in their diet. 

4. Does the oxygen produced come from the water or carbon dioxide?

U.S. Scientists in the 1940’s performed an experiment with heavy oxygen.  A heavy oxygen atom has 2 more neutrons than a normal oxygen atom and an atomic weight of 18. The result is that the location of this heavy oxygen can be determined with an instrument (called a mass spectrophotometer).

H₂O was labeled with heavy oxygen and the label was found in the oxygen.    CO₂ + H₂O*-->  O₂^* + C₆H₁₂O₆ 

CO₂ was labeled with heavy oxygen and the label was found in the glucose.  CO₂^* + H₂O-->  O₂ + C₆H₁₂O₆^*

The evidence suggests that the oxygen produced in photosynthesis comes from the ________________.

5. How does CO₂ get into leaves?

a. Stomata are openings in the leaves that allow CO₂ in (fig. 4.7, p. 60).

Guard cells control opening of stomata. Stomata usual open during the day and close at night. Why?

Open stomata-->CO₂ in for photosynthesis

b. Opening stomata also lets H₂O out

In wet climates this is beneficial since as H₂O evaporates it helps cool the plant.

In the desert this is harmful since plants need water

• Some desert plants like cacti open stomata at night and close them in the day (p. 64, CAM metabolism) ! Cactus have their stomata on stems since they do not have leaves

• They take CO2 out of the air at night and store it as a 4-carbon solid compound. They store this in the vacuole. This compound is an acid so that the pads of a prickly pear would taste sour in the early morning and sweet at sunset.
• During the day this process is reversed so that, the solid carbon compound releases CO₂ internally. The plant produces its own CO₂ while keeping stomata closed!

6. How is a gas CO₂ made into a solid C₆H₁₂O₆?

a.. Chloroplasts (fig. 4.8)

Function: site of photosynthesis

b. Light capture occurs in the thylakoid membranes

1) contain chlorophyll

2) Use the captured light to:

Pull electrons from H₂O to create an electric current

O₂ is a waste product of ripping electrons off water

Use the electric current to make ATP and NADPH (high energy molecules)

c. The high energy molecules ATP and NADPH are used in the Calvin cycle to add H atoms to CO₂

1) Melvin Calvin 1950 figured out the metabolic pathway

Add ¹⁴CO₂ to algae.

Drip into tubes of boiling alcohol.

Analyze products by paper chromatography.

2) 1^st reaction in Calvin cycle (figure 4.12)

                                                                                                    Rubisco

CO₂ gas + 5-carbon compound        ---------->   2x 3-carbon

 

Rubisco is in a type of protein called an enzyme.  Rubisco is the most abundant protein in the world.

Rubisco functions to convert carbon dioxide from a gas to a solid.

7.  What are enzymes?

 Enzymes are catalysts. They help chemical reactions to occur but are not used up in the reaction

DNA: tells the cell how to build enzymes.

Faulty DNA--> Faulty enzyme--> Disease

(Example: sickle cell anemia is messed up hemoglobin protein)

             enzyme

H₂O₂          ---->                 H₂O + O₂

Cells are like tiny factories, building molecules and structures (leaves, roots, fruits) as well as breaking other structures down (chlorophyll, glucose).  Enzymes are like the machine in the factory that do the assembly or disassembly.

8.  What is light and how is light used in photosynthesis? (fig. 4.6)

a. Light travels in electromagnetic waves at the speed of light (300 million meters per second).  This was proposed by James Maxwell (1831-1879), a Scottish physicist.

All the following are electromagnetic waves (made of the same stuff as light).

• Radiowaves
• Microwaves
• Infrared
• Visible light
• Ultraviolet light
• X-rays

The wavelength (distance from the peak of one wave to the next) from radio waves to X-rays goes from longer to shorter. Shorter wavelength are damaging to life since they damage DNA.

b. Sound waves are NOT the same type of waves. They are simply vibrations of matter traveling through space at a speed of 300 meters per second (a million times slower than light). That is why if you see a flash of lightening and count until you hear the thunder, every five seconds is about a mile distance from where the lightening struck.

c. Table: Speed of light vs. Speed of sound

9. What color of light is best for plant growth?

White light= all colors

Plants reflect green light.

Engelmann did an experiment in the 1800’s.

He used a filamentous algae like Spirogyra on which he focused different colors of light by splitting up white light with a prism.

Bacteria were put on the slide and migrated towards the part of the algae producing the most oxygen. These were the blue and red light.

Green light produced the least growth of bacteria which means __________________.

a. Yellows and reds that we see in the fall are actually there all year long but covered up by the green pigment, chlorophyll. Chlorophyll is broken down in the fall and the nitrogen in the chlorophyll stored in the tree for winter so it can be used in the spring. The yellow and red pigments in leaves help to capture the other colors of light besides the red and blue.

b. Bacteriorhodopsin is a pigment found in marine bacteria that may have covered early Earth. They are purple in color and capture yellow and green light while reflecting reds and blues. Scientists believe that green algae developed below this layer of bacteriorhodopsin so that only red and blue light was available.

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