PHOTOSYNTHESIS

How plants use the sun’s energy to make sugar

Occurs in the chloroplasts of plant cells

I. The process is broken into 3 sets of reactions

A. “capture light” – collect sunlight

B. “light reaction” – use solar energy to make chemical energy

C. “dark reaction” – aka…Calvin cycle – use chemical energy to make glucose

6CO2 + 6H2O C6H12O6 + 6O2

II. The process

A. Capture light

1. Light travels in waves – each color is a different wavelength

a. violet – short wavelength

b. red – long wavelength

2. Three things can happen when light hits an object

a. absorbed

b. reflected

c. transmitted

3. What absorbs light in autotrophs? (organisms that make their own food)

a. pigments (molecules that absorb particles of light)

b. some pigments absorb all wavelengths

c. some pigments only absorb certain wavelengths

d. chlorophyll – pigment found in most plants – two types

chlorophyll a – major one… absorbs violet and red

chlorophyll b – absorbs blue and orange (must be a gator)

e. carotenoids – protect plants from UV rays (like sunscreen)

f. accessory pigments – pigments that absorb wavelengths that are reflected by chlorophyll a and b (what you see in the fall)

4. Where are these pigments found in a plant? Chloroplasts

a. stroma – made of protein rich solution (liquid) -- where the dark reaction occurs

b. thylakoid – where the pigment is, therefore where light is absorbed…acts like a solar panel

-- where the light reaction occurs

c. grana – stacks of thylakoids

Thylakoid Stroma

Grana

5. How does a chloroplast capture light energy?

a. light waves are made of photons (particles)

b. a photon of a certain wavelength hits a pigment and is absorbed by that molecule

c. energy from the photon excites an electron in the pigment molecule

d. the excited electron moves to a higher energy level (outer orbit) and is unstable

e. at the higher level, the electron is passed to an “electron carrier”, which is the first molecule in an electron transport chain

f. the lost electron is replaced by an electron from the splitting of water. An enzyme in the chloroplast is able to split water into two hydrogen ions and one oxygen atom, which immediately combines with another to make O2

B. Light Reaction – reaction where light energy is used to make chemical energy

1. There are two electron transport chains involved.

a. the first makes ATP via chemiosmosis

b. the second makes NADPH

2. Once the electron gets “excited” and bound to the primary electron carrier it gets passed down the 1st chain.

3. Chemiosmosis occurs so that ATP is made

a. The energy lost as the electron gets passed is used to pump H+ ions across the thylakoid membrane. The H+ ions flow with the concentration gradient back into the stroma through ATP synthase, causing ATP to be made.

b. The final electron acceptor is a pigment molecule.

4. The electron gets excited again by the sun and is passed down a 2nd ETC where NADP+ is the final electron acceptor to make NADPH.

C. Dark reaction -- "Calvin Cycle" -- The energy stored in the bonds of ATP and NADPH are used to make glucose.1. Carbon fixation a. CO2 diffuses into the stroma from the environment b. 3 molecules of CO2 bond with 3, 5 carbon molecules called RuBP

= 3 (6 carbon molecules)= 18 carbons total

2. Reduction a. The six carbon molecules split because they are unstable

= 6, 3 carbon molecules of PGA= 18 carbons total

b. PGA receives phosphate group from ATP and H from NADPH to make G3P

= 6 G3P molecules= 18 carbons total

3. Regeneration a. One G3P is used to make glucose. The other five use ATP to regenerate 3 molecules of RuBP

5 molecules of G3P (15 carbons total)

3 molecules of RuBP (15 carbons total)

II. Chemosynthesis

A. some organisms are able to make food using energy from inorganic compounds instead of the sun

1. Bacteria called “methanogens”

a. live where there is no oxygen (they are poisoned by it)

b. they combine CO2 and H2 to produce methane (CH4)

2. Other bacteria may use Hydrogen Sulfide (H2S), ferrous ions (Fe2+)