6.1 Capturing the Energy in Light6.2 The Calvin Cycle

Created by Stacey Odum

Photosynthesis

Photosynthesis = process by which a cell captures the energy in sunlight and uses it to make food.

Plants, algae, and some bacteria use this process.

Like a Solar Powered Calculator with light capturing cells

Solar Cell

Water

Carbon Dioxide

Sun Light

Oxygen

Photosynthesis

Sugar

Cellular Respiration

Energy - ATP

Chloroplasts in Elodea (Lab)

Autotrophs make their own food, and most use photosynthesis to do it.

* Plants are the most common, but algae and some bacteria use it.

* All life depends on autotrophs (directly or indirectly) for food.

* Nearly all living things obtain energy either directly (like plants) or indirectly (like heterotrophs that eat plants) from the sun’s energy captured during photosynthesis.

Photosynthesis (Simply Summarized)

Leaves of plants have chloroplasts that are filled with chlorophyll, which capture the energy from the sunlight).

* Water enters through the roots.* CO2 enters through stomata openings on the underside of the leaves.* Water and CO2 move to the chloroplasts.* Chemical reactions there, produce (O2) and sugars like glucose (C6H12O6).* Cells then use the energy in the sugars (carbohydrates) to function.

Photosynthesis Equation:

CO2 + H2O + light energy → Sugar (glucose) + O2

More Detailed Equation:Using Light Energy

carbon dioxide water “yields glucose oxygen6CO2 + 6H2O → C6H12O6 + 6O2

Biochemical Pathway = Series of linked chemical reactions, in which the product of one reaction is consumed (used as the reactant) in the next reaction.

* The two stages of photosynthesis involve biochemical pathways.

* Also, the final products of plant photosynthesis (sugar and oxygen) are used by animals for another process called cellular respiration. The products of respiration are carbon dioxide and water, which plants need for photosynthesis, so both processes create a balance in the atmosphere.

The Photosynthesis Reactions:

1. Light Reactions are the first set of reactions in photosynthesis.

Background information about light:Sunlight appears white, but really many colors.

Visible Spectrum = the light waves we can see.

Wavelength = distance between wave crests(Different wavelengths reflected as different colors.)(Objects absorb and reflect certain waves.)

* Light is absorbed in the membranes of thylakoids founds in chloroplasts.- Grana = stacks of flat thylakoids.

- Stroma = the solution surrounding the thylakoids.

- Chlorophylls = pigments, which absorb violet, blue, red, yet reflect green.

- Pigment = a compound that absorbs light waves.(Chlorophyll “a” is directly involved in light reactions.

- Accessory Pigments = indirectly assist other pigments.( Chlorophyll “b” is an accessory pigment assisting “a”.)Carotenoids are other accessory pigments (yellow, orange, brown, etc.)(They absorb blue and green light.)(In the fall, plant leaves turn color because they lose

chlorophylls, which reflect green light.)

The Light Reaction in the Thylakoid Membrane

Electron Transport System of the light reactions.

* Photosystem = cluster of pigment molecules grouped in the thylakoid membrane.

(Two types: Photosystem II and Photosystem I, each with different roles.)- Note the order of II and then I in the light

reaction diagram.

* Accessory pigment molecules start the light reactions by absorbing light energy. (It is passed to other pigment molecules until it

reaches chlorophyll a molecules.)* Then a FIVE STEP process occurs . . . . . . . .

FIVE STEPS of the Electron Transport System1. Light excites electrons in the chlorophyll a molecules of

photosystem II. (Oxidation reaction, since it loses an electron.)

2. These electrons move to and are accepted by a primary electron acceptor. (This is a reduction reaction.)

3. The electrons are then transferred along a series of molecules called an electron transport chain.

4. Light excites electrons in chlorophyll a molecules of photosystem I.

5. Electrons from Photosystem I are transferred along a second chain. (The chain ends as they combine with organic NADP+

and H+ making NADPH.)

Restoring Photosystem II Electrons by splitting Water Molecules:

* Enzyme splits water to protons, electrons, & oxygen2H2O → 4H+ + 4e- + O2

So oxygen is a by-product of light reactions, and not used in the rest of photosynthesis.

(The oxygen is used by other organisms for cellular respiration.)

Chemiosmosis = a process in light reactions that synthesizes (makes) ATP. * Protons produced from the water splitting build up inside the thylakoid, and they then move down their gradient to the outside.

* ATP Synthase is a multifunctional protein serving as an enzyme and carrier protein.It uses the energy of the moving protons to add a phosphate group to ADP and make ATP (A primary chemical energy storage molecule for energy currency in the cell.)

* ATP and the NADPH provide energy for the second set of photosynthesis reactions, called the Calvin Cycle.

The Light Reaction in the Thylakoid Membrane

Primary electron acceptor Primary electron acceptor

Photosystem II

Electron Transport Chain

Photosystem I

Electron Transport Chain

Animation here: http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swf

2. The Calvin Cycle = second set of reactions (pathway), producing organic compounds from the stored energy of ATP and NADPH.

* Occurs in the stroma of the chloroplast.

* Carbon Fixation: C atoms from CO2 are bonded

(“fixed”) into organic compounds.(amino acids, lipids, and carbohydrates like glucose, fructose)

(Note: These compounds are not directly produced by photosynthesis.)

Three Steps of the Calvin Cycle:

a. CO2 combines with RuBP to form two molecules of PGA.

b. Each molecule of PGA is converted into a molecule of PGAL.

c. Most of the PGAL is converted back into RuBP, but some PGAL can be used later to make different organic compounds.

* RuBP = five-carbon carbohydrate.

* PGA and PGAL are both three-carbon molecule* C3 Plants (produce the 3-Carbon PGA),

and use only the Calvin Cycle for carbon fixation.

The Two Processes Visualized

Alternative Pathways

* First, remember that C3 Plants are those that only use

the Calvin Cycle to fix carbon.- They are called C3 plants, since they fix CO2 into a

compound with 3 carbons (PGAL).

* Others in hot, dry climates supplement the Calvin cycle with alternatives (C4 or CAM), because their

stomata openings in the leaves must close to preserve moisture, so the amount of CO2 they absorb is

reduced.

C4 Pathway – use an enzyme which fixes CO2 into

compounds with 4 carbons, which are then transported to other cells where CO2 is available to then use the

Calvin Cycle.(corn, sugar cane, are examples)

CAM Pathway – These plants open the stomata only at night to reduce water loss.They take in and fix it into compounds, which then release it during the day for use in the Calvin Cycle.(cactuses, pineapples, etc.)

Rate of Photosynthesis

* Increases as either light intensity or CO2

increase, but eventually plateaus at a maximum.

* Increases as the temperature increases, up to a certain temperature.(Beyond a certain high temperature, the rate of photosynthesis decreases.)  

Cellular Respiration