Topic 11 - Photosynthesis

Question 1

Photosynthesis

Answer 1

The process by which plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water; oxygen is generated as a byproduct

Question 2

What is the chemical equation for photosynthesis?

Answer 2

6CO2 + 12H2O + sunlight → C6H12O6 + 6H2O + 6O2

Question 3

Where does photosynthesis take place?

Answer 3

Chloroplasts

Question 4

Thylakoid

Answer 4

Disc-shaped, membrane-bound structure inside a chloroplast where the light-dependent reactions of photosynthesis take place; stacks of thylakoid are called grana (granum singular)

Question 5

Stroma

Answer 5

Fluid-filled space surrounding the grana inside a chloroplast where the light-independent (dark) reactions of photosynthesis take place

Question 6

Pigment

Answer 6

Molecule that is capable of absorbing certain wavelengths and reflecting others (which accounts for its color); embedded in thylakoid membranes and transfers energy to electrons
- Chlorophyll a: absorbs violet-blue and red light; has a bluish-green color
- Chlorophyll b: secondary pigment that absorbs blue and red-orange light; has a yellowish-green color

Question 7

What are the two stages of photosynthesis?

Answer 7

1. Light-dependent reactions
   - Energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy
2. Light-independent (dark) reactions
   - Chemical energy harvested during light-dependent reactions drives the assembly of sugar molecules from CO2

Question 8

What is the function of light-dependent reactions?

Answer 8

To convert solar energy into chemical energy (in the form of ATP and NADPH)
- In light-dependent reactions, electrons are removed from water and passed through photosystem II and photosystem I before ending up in NADPH. An electrochemical gradient produces ATP via chemiosmosis
- This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules

Question 9

Where do the light-dependent reactions take place?

Answer 9

Tylakoid

Question 10

Photosystem I

Answer 10

Integral pigment and protein complex in thylakoid membranes that uses light energy to transport electrons from plastocyanin to NADP+ (which becomes reduced to NADPH)

Question 11

Photosystem II

Answer 11

Integral pigment and protein complex in thylakoid membranes that uses light energy to transport electrons from water to the electron transport chain; oxygen is a product of PSII

Question 12

Describe the steps of the light reactions.

Answer 12

1. Chlorophyll in PSII absorbs light (photons) and energizes an electron, which is transferred to a primary electron acceptor
2. Carrier molecule transports the electron down the ETC
3. Water molecule is split, releasing a new electron into the system and oxygen into the atmosphere (as a waste gas)
4. Chlorophyll in PSI absorbs light, energizing an electron
5. Carrier removes the electron and the electron from PSII replaces it
6. The electron is accepted by NADP+ at the end of the ETC (NADP+ reduced to NADPH)
7. Proton gradient pushes H+ through ATP synthase into stroma, producing ATP
8. NADPH and ATP go into Calvin cycle

Question 13

What are the reactants of the light reactions?

Answer 13

Sunlight (photons) and water

Question 14

What are the products of the light reactions?

Answer 14

O2, NADPH, and ATP

Question 15

ATP is made from…

Answer 15

Photosystem II

Question 16

NADPH is made from…

Answer 16

Photosystem I

Question 17

Calvin cycle

Answer 17

Light-independent reactions of photosynthesis that convert CO2 into carbohydrates using the energy and reducing power of ATP and NADPH

Question 18

Where does the Calvin cycle (light-independent reactions) take place?

Answer 18

Stroma

Question 19

What are the three stages of the Calvin cycle?

Answer 19

1. Fixation
2. Reduction
3. Regeneration

Question 20

Describe what happens in each stage of the Calvin Cycle.

Answer 20

1. Fixation
   - CO2 is “fixed” from an inorganic form into organic molecules
   - CO2 binds to ribulose biphosphate (RuBP) with the help of the enzyme RuBisCO, creating a 6-carbon compound that splits into two molecules of a 3-carbon compound called 3-PGA
2. Reduction
   - ATP and NADPH are used to convert 3-PGA molecules into molecules of a 3-carbon sugar, G3P (used to create glucose)
   - NADPH donates electrons to (reduces) a 3-carbon intermediate to make G3P
3. Regeneration
   - Only one G3P molecule leaves the Calvin cycle to be converted into glucose and other organic compounds
   - Remaining G3P molecules are recycled to regenerate the RuBP acceptor

Question 21

Compare and contrast the functions of photosynthesis and cellular respiration.

Answer 21

• Both processes use ETCs to capture energy necessary to drive other reactions.
• In photosynthesis, light energy is converted into chemical energy and stored in sugars. Initially, the light energy is converted into chemical energy during ATP synthesis in a process that gives off oxygen. The energy in ATP is then used to reduce CO2 to simple sugars.
• In contrast, cellular respiration is the process in which the chemical energy stored in sugars is converted to ATP, a source of chemical energy that can be used by the rest of the cell. In the process of converting energy stored in sugars to ATP, CO2 is released and oxygen is consumed.