CHAPTER 12: Photosynthesis: Makin’ Food in the Kitchen of Life summary

Introduction

• Photosynthesis is the process by which light energy (from the sun) is transferred into chemical energy that can be utilized by living things.
• Photosynthesis can be represented by the chemical equation below:
  • 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

Getting what plants need

• Plants are autotrophs hence they can make their food.
• Plants need water, carbon dioxide, and sunlight to make food through photosynthesis.
• Plant structures are special to absorb materials they require:
  • Roots absorb water from the soil.
  • Xylem helps in the movement of water from roots to all plant parts.
  • Phloem move sugars from leaves.
  • Stomates allow carbon dioxide into leaves.
  • Chlorophyll absorbs sunlight.

Capturing suns energy with pigments

• All photosynthetic organisms have chlorophyll as well as other photosynthetic pigments.
• Chlorophyll increases the wavelength of light energy photosynthetic organisms can absorb for use in the photosynthesis process.

The light reactions and Calvin cycle

• Photosynthesis takes place through two reactions: light reactions and the Calvin cycle.
• In light reactions,  light energy is captured from sunlight by plants. 
• The light energy is transformed into chemical energy in the form of ATP.
• Plants transfer electrons from water to NADPH.
• In the Calvin cycle, plants absorb carbon dioxide and convert it to carbohydrates.
• For conversion to take place, they need NADPH and ATP from the light stage reaction.

The light reaction

• Plants spread their leaves for the absorption of light energy.
• Plants then transform the light energy into ATP.
• Cells use ATP as a chemical energy source.

Transferring light energy to chemical energy

• Photosynthetic pigments absorb light energy and convert it to chemical energy in the form of ATP.
• The electron transport chain makes this energy transfer possible.
• When plant cells use an electron transport chain and sunlight to make ATP, it is referred to as photophosphorylation.
• In photophosphorylation, plant cells add phosphate to ADP to form ATP.
• The electron transport chains for the process of photosynthesis are embedded in the thylakoid membranes in chloroplasts.
• The chlorophyll molecules form reaction centers.
• When the chlorophyll receives light energy, electrons are excited to a high energy state.
• The electrons move to outer orbitals and jump to electron transport chain proteins.
• Chemical energy in the form of ATP is then generated.

Steps of photophosphorylation

There two photophosphorylation types in cells:

• Noncyclic photophosphorylation (Z-scheme) involves electron transfer from chlorophyll through the electron transfer chain to reduce NADP+ to NADPH.
• Cyclic phosphorylation involves the transfer of excited electrons from the chlorophyll and returns the electrons to the chlorophyll after their energy is transferred to ATP.

Steps of complete photophosphorylation

• Light energy is absorbed by chlorophyll and the energy is transferred to reaction centers.
• Electrons from chlorophyll reaction centers are excited and move to outer orbitals.
• The electrons are transferred to proteins in the electron transport chain.
• Chemiosmosis process is used to generate ATP.
• The electrons are accepted by NADP+ which is reduced to NADPH +H.
• Some of the light energy absorbed from sunlight is used to split water by the process of photolysis.

The Calvin cycle

• This process transforms inorganic carbon in the form of carbon dioxide to organic carbon in the form of carbohydrates.
• This Calvin cycle is light-independent.

Steps of the Calvin cycle

• The enzyme Rubisco joins carbon dioxide molecules to ribulose biphosphate(Rubp), a 5-carbon sugar.
• Unstable 6-carbon molecules are formed which immediately dissociate into two 3-carbon molecules called 3-phosphoglycerate.
• 3-phosphoglycerate molecules are phosphorylated to form glyceraldehyde-3-phosphate.
• This is formed by transferring a phosphate group from ATP to 3-phosphoglycerate.
• Some molecules of glyceraldehyde-3-phosphate are used to make glucose and other sugars.
• Some ATP and glyceraldehyde-3-phosphate are used to make ribulose-1,5-bisphosphate, the 5-carbon sugar used by the plant in the first step.

Photosynthesis in the real world

• The carbohydrates generated by plants, algae, and bacteria serve as a source of food and energy for all living things on the earth.
• Food chains help to show food flow.
• A simple food chain starts with autotrophs that produce food followed by heterotrophs that feed on autotrophs.
• An example of a  simple food chain looks the one below:
  • Grass → Cow → You (human)

Revision

What is energy and how does it relate to work? Based on that, why do living things need energy?
Energy is the ability to do work. All living things need energy to survive. Energy is needed to metabolize, reproduce, develop, move, perform functions.

Why might photosynthesis be the most important biochemical process in the world? Give evidence to support your answer.
Photosynthesis cleans the air by removing the CO2 from the atmosphere. Photosynthesis makes it possible for living things to breathe because it produces oxygen. Photosynthesis breaks a water molecule and releases oxygen as a byproduct of the Light-dependent reaction. Plants are producers, so photosynthesis makes it possible for them to produce their own food, which makes it possible for us to have food since we eat them and we eat other organisms that eat them.

Define what is meant by chemical potential energy. How does the chemical potential energy of ATP compare to that of ADP? Why is this important?
The energy stored in chemical bonds between atoms. It is used to hold molecules together.
ATP: there is a tremendous amount of chemical potential energy stored in the terminal phosphate bond.
ADP: the terminal phosphate bond is broken in ADP, so the energy is no longer potential, it is kinetic.
It is important because it is two different types of energy.

Describe how photosynthesis happens overall.
-The chlorophyll captures the light energy. The roots of the plants absorb the water. The leaves capture the CO2.
-The LDR captures the solar energy to make ATP and NADPH. Oxygen is produced as a byproduct of this reaction when the LDR breaks the water molecules.
-The CO2 and water are combined together to produce glucose — this is powered by the energy from the ADP and NADPH (this is also known as the Calvin Cycle).

How do the light-dependent reactions go about making ATP and NADPH? Describe the story in detail.

1. Light hits photosystem II and charges an electron
2. The electron goes onto the ETC (electron transport chain)
3. The water molecule breaks to replenish the electrons that are lost to the ETC.
4. As the electron moves through the ETC,
   -electron carriers absorb some of the electron’s energy
   -electron carriers use this energy to pull H+ ions from outside the thylakoid membrane to the inside.
   -This causes the inside to become more positive and the outside more negative (imbalance of H+).
5. The electron reaches Photosystem I
   -The electron is slightly depleted of energy.
   -Light hits Photosystem I which recharges the electron.
6. The energized electron exits the ETC.
   -NADP+ uses the energy from this electron to bond to a H+ ion
   -NADP+ H+ —-> NADPH = Battery #1
7. Due to the increasing imbalance caused by the inbound H+ ions…
   -H+ ions exit to the outside of the membrane through ATP synthase
   -This movement causes the ATP synthase to spin
   -Spinning motion provides ADP the energy it needs to pick up a third phosphate.
   -ADP + P —-> ATP = Battery #2

How does the Calvin cycle use ATP and NADPH to produce glucose?
6 CO2 molecules combine with 6 5-carbon molecules, the result is twelve 3-carbon molecules, which are then converted into higher-energy forms. The 12 charged ATP molecules are used and decharged into ADPs. The 12 NADPHs are decharged into NADP+. Two of the 12 3-carbon molecules are removed from the cycle. The molecules are used to produce sugars, lipids, amino acids, and other compounds.

1. Be able to explain how plants get the energy they need to produce food (where does this energy come from?).
   The plants get energy from the sun. Their leaves absorb CO2, their roots absorb water, and the chlorophyll takes in the sunlight.
2. Describe the difference between heterotrophs and autotrophs.
   Autotrophs are Organisms that can capture energy from sunlight/chemicals and use that energy to produce food. Heterotrophs are Organisms that rely on other organisms for their energy and food supply.
3. What are the 3 components of an ATP molecule?

• Adenine
• Ribose (5-carbon sugar)
• 3 phosphate groups

1. What does the abbreviation ATP stand for?
   Adenosine Triphosphate
2. Be able to explain the difference between an ADP and an ATP molecule.
   An ATP molecule has 3 phosphate groups and an ADP molecule only has 2. ADP is the
3. What is the purpose of ATP and what role does it play in cellular activities?
   ATP stores energy in a way that cells can use it to do work.
4. Be able to define photosynthesis and write out the balanced equation.
   The process in which green plants use the energy of sunlight to convert water and carbon dioxide into high-energy carbohydrates (sugars) and oxygen.
   6CO2+6H2O–Light→ 6O2 + C6H12O6 Glucose
   [Reactant] [_Products__]
5. Why is photosynthesis perhaps the most important biochemical process? Defend your answer.
   photosynthesis makes it possible for living things to breathe because it removes CO2 from the air. We would not be able to survive without it because the plants convert the CO2 into the oxygen we rely on. It makes it so plants can make food without using other resources; it uses the sun to make its food instead of taking from the Earth.
6. Where does photosynthesis occur? (…in which organelle?)
   Photosynthesis occurs in the chloroplast
7. Be able to list and describe all of the parts of a chloroplast.
   -Thylakoid: saclike photosynthetic membranes. It is where that Light-dependent reaction happens.
   -Grana: Stacks of thylakoids.
   -Stroma: The liquid portion of the chloroplast.
8. Explain how photosynthesis works overall (i.e. how do the LDR and Calvin cycle work together).
   The LDR converts the sunlight into the energy that the plant can use, and then the Calvin cycle uses that energy to convert CO2 and H2O to glucose.
9. Describe in detail what occurs during the Light Dependent Reaction
10. Light hits photosystem II and charges an electron
11. The electron goes onto the ETC (electron transport chain)
12. The water molecule breaks to replenish the electrons that are lost to the ETC.
13. As the electron moves through the ETC,
    -electron carriers absorb some of the electron’s energy
    -electron carriers use this energy to pull H+ ions from outside the thylakoid membrane to the inside.
    -This causes the inside to become more positive and the outside more negative (imbalance of H+).
14. The electron reaches Photosystem I
    -The electron is slightly depleted of energy.
    -Light hits Photosystem I which recharges the electron.
15. The energized electron exits the ETC.
    -NADP+ uses the energy from this electron to bond to a H+ ion
    -NADP+ H+ —-> NADPH = Battery #1
16. Due to the increasing imbalance caused by the inbound H+ ions…
    -H+ ions exit to the outside of the membrane through ATP synthase
    -This movement causes the ATP synthase to spin
    -Spinning motion provides ADP the energy it needs to pick up a third phosphate.
    -ADP + P —-> ATP = Battery #2
17. What is the purpose of the Calvin cycle and why is it important?
    The Calvin cycle is the process that uses energy from the LDR to convert CO2 into sugar. It is important because glucose is used by the plant as an energy source.
18. Be able to list the various factors that may affect the rate of photosynthesis.
    Amount of sunlight
    Amount of Water
    Amount of CO2
    Amount of Oxygen and Nutrients
    Temperature
19. Be able to define and describe an atom. (What are the 3 parts?)
    An atom is the basic unit of matter. They are indestructible. The consist of:
    -Protons: positive charge
    -Neutrons: no charge (neutral)
    -Electrons: negative charge