Photosynthesis

Photosynthesis

Chapter 8

Autotrophs vs. Heterotrophs

• All living things depend upon energy to carry out life’s processes.

• Plants and some other types of organisms are able to use light energy from the sun to produce food- autotrophs.

• Other organisms cannot make their own food/use the sun’s direct energy and must consume either other heterotrophs or autotrophs in order to get energy- heterotrophs.

Chemical Energy and ATP• ATP: Adenosine Triphosphate• ATP is energy stored in a chemical compound.• ATP has three phosphate groups attached to

adenine and ribose (5-C sugar).• As the bonds between the phosphate groups

are broken, energy is released that can be used by the organism.

History of Photosynthesis

• Van Helmont- after measuring a plant’s water intake and mass increase, he concludes that plants gain most of their mass from water.

• Priestly- Using a jar, candle and a plant, he determines that plants release oxygen.

• Ingenhousz- finds that aquatic plants produce oxygen in the light but not in the dark; concludes that plants need sunlight to grow and release oxygen.

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Photosynthesis

• Equation is:6CO2 + 6H2O -----> C6H12O6 + 6O2

• Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high-energy sugars and oxygen.

• Plants then use those sugars to make complex carbohydrates and starches.

(Light)

Aquatic Plants.wmv Review of Plants.wmv

Light and Pigments• In addition to water and carbon dioxide, photosynthesis requires light and

chlorophyll, a molecule in chloroplasts.• Light travels from the sun to earth in wavelengths; we perceive that as color. • Pigments attract the wavelengths of color from the light spectrum, and reflect

the color that is not absorbed. For example, if my shirt is blue, it is blue because the blue pigment is absorbing all the other colors of the spectrum and reflecting blue; that’s why we see the shirt as blue.

• Plants gather the sun’s energy with light-absorbing pigments.• Chlorophyll absorbs light well in blue/violet/red regions of light spectrum;

reflects green/yellow.• Chlorophyll absorbs light and then that energy is transferred to electrons in the

chlorophyll molecule, raising the energy of these molecules which make photosynthesis work.

Chlorophyll A dark green Heaviest

Chlorophyll B light green Lighter than A

Xanthophyll yellow Lighter than B

Carotene orange Lightest

Inside the Chloroplast

• Photosynthesis takes place inside the chloroplasts (an organelle) in the plant cells.

• Contains membranes called thylakoids.• Thylakoids arranged in stacks called grana.• Proteins in the thylakoid membrane organize

chlorophyll and other substances into clusters called photosystems.

• The photosystems are light-collecting units of the chloroplast.

Inside a Chloroplast

Photosynthesis Overview

• The reactions of the photosystems take place in two parts.1. Light-dependent reactions

- take place within the thylakoid membranes2. Light-independent reactions (Calvin Cycle)

- takes place in the stroma (the region outside the thylakoid membranes.

Electron Carriers• Chlorophyll contains electrons.• Exposure to sunlight charges up these electrons.• These high-energy electrons need a carrier because

they are so full of energy.• NADP+ is an electron carrier.• Once it accepts the high-energy electrons, it then

becomes NADPH, trapping the energy from the sun in a chemical form.

• The high-energy electrons continue to be carried along the electron transport chain, with minimal energy lost from the electrons.

example: a pan can be used to carry hot coals.

Light-Dependent Reactions

• First set of photosynthetic reactions.• They require light.• This set of reactions uses light energy and

produces oxygen; they convert ADP and NADP+ into the energy carriers ATP and NADPH.

Light Dependent Reactions

A. Photosystem II absorbs light in electrons. Water (H2O) breaks up into 2H+ ions and 1 oxygen atom.

B. High-energy electrons and the H+ ions move through the electron transport chain from photosystem II to photosystem I.

C. The H+ ions are added to NADP+ to make NADPH.D. H+ ions also continue to build up, so the inner

thylakoid membrane becomes more positively charged. This makes energy that turns ADP to ATP.

Light-Independent Reactions(Calvin Cycle)

• The ATP and NADPH from the light-dependent reactions have a lot of chemical energy, but are not stable enough to store the energy long-term.

• The Calvin Cycle uses ATP and NADPH to produce high-energy sugars that are longer-lasting forms of energy that can be stored.

• Reactions do not require light.

Light-Independent Reactions(Calvin Cycle)

A. 6 CO2 molecules enter the cycle from the atmosphere and combine with 6 five-carbon molecules = 12 three-carbon molecules.

CO2 CO2 CO2 CO2 CO2 CO2

+ 5-C 5-C 5-C 5-C 5-C 5-C_____________________________________________________________________________________________________

3-C 3-C 3-C 3-C 3-C 3-C3-C 3-C 3-C 3-C 3-C 3-C

Light-Independent Reactions(Calvin Cycle)

B. The 12 three-carbon molecules gain more energy from ATP and NADPH.

C. 2 of the 12 are taken from the cycle to make glucose/ other compounds for the plant.

D. The remaining 10 are converted back into 6 five-carbon molecules. They will combine with 6 new CO2 molecules to begin the cycle again.

Factors Affecting Photosynthesis

• Water- needs H+ ions from the H2O• Temperature- extreme temps can denature

enzymes needed to make the photosynthesis reactions occur.

• Light intensity- varies from plant to plant.• Presence of carbon in the atmosphere