Photosynthesis

Question 1

What is the overall chemical reaction for photosynthesis?

Answer 1

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2. Carbon dioxide and water combine using light energy to produce glucose and oxygen.

Question 2

Define autotroph.

Answer 2

An autotroph is an organism that makes its own food from inorganic sources, such as CO2 and water, using light or chemical energy.

Question 3

Define heterotroph.

Answer 3

A heterotroph is an organism that must obtain energy by consuming other organisms or organic molecules.

Question 4

Define producer and consumer.

Answer 4

A producer (autotroph) creates its own food using sunlight or chemicals, while a consumer (heterotroph) must eat producers or other consumers for energy.

Question 5

What is the function of stomata?

Answer 5

Stomata are pores on the underside of leaves that allow for gas exchange—CO2 enters, and O2 and water vapor exit.

Question 6

Where does photosynthesis take place in the cell?

Answer 6

Photosynthesis occurs in the chloroplasts of plant cells, mainly within the mesophyll tissue of leaves.

Question 7

What are the two stages of photosynthesis and where do they occur?

Answer 7

1) Light-dependent reactions – occur in the thylakoid membranes; convert light energy to ATP and NADPH. 2) Calvin cycle (light-independent reactions) – occurs in the stroma; uses ATP and NADPH to fix CO2 into sugar.

Question 8

What colors of light are most effective for photosynthesis?

Answer 8

Red and blue light are most effective for photosynthesis because they are absorbed by chlorophyll pigments, while green light is least effective because it is reflected.

Question 9

What is the wavelength range of visible light?

Answer 9

Visible light ranges from about 380 to 750 nanometers (nm).

Question 10

What are the main photosynthetic pigments in plants?

Answer 10

The main pigments are chlorophyll a, chlorophyll b, and carotenoids. Chlorophyll a is the primary pigment; chlorophyll b and carotenoids are accessory pigments.

Question 11

What is the structural difference between chlorophyll a and chlorophyll b?

Answer 11

Chlorophyll a has a CH3 group, while chlorophyll b has a CHO group on its porphyrin ring.

Question 12

What role do carotenoids play in photosynthesis?

Answer 12

Carotenoids absorb wavelengths not absorbed by chlorophyll and protect the plant from excess light by acting as photoprotective pigments.

Question 13

What happens when a photon hits a pigment molecule?

Answer 13

The photon excites an electron to a higher energy level; this energy is transferred through pigment molecules until it reaches the reaction center.

Question 14

What is a photosystem?

Answer 14

A photosystem is a complex of pigments and proteins that captures light energy. It contains a reaction center with a special pair of chlorophyll molecules and a primary electron acceptor.

Question 15

What are the two types of photosystems in photosynthesis?

Answer 15

Photosystem II (PSII) with a reaction center called P680, and Photosystem I (PSI) with a reaction center called P700.

Question 16

What is the function of Photosystem II?

Answer 16

PSII captures light energy to split water molecules (photolysis), releasing O2, protons (H⁺), and electrons that enter the electron transport chain.

Question 17

How many photons are required to split two water molecules?

Answer 17

It takes four photons to split two H2O molecules into O2, 4 H⁺, and 4 e⁻.

Question 18

What is the electron transport chain in photosynthesis?

Answer 18

A series of carriers including plastoquinone (Pq), cytochrome complex, and plastocyanin (Pc) that move electrons from PSII to PSI, creating a proton gradient used for ATP synthesis.

Question 19

What is the function of Photosystem I?

Answer 19

PSI receives electrons from the ETC and re-energizes them with light energy to reduce NADP⁺ to NADPH.

Question 20

What are the two main energy products of the light reactions?

Answer 20

ATP and NADPH, which are used in the Calvin cycle to power carbon fixation.

Question 21

How is ATP produced during the light reactions?

Answer 21

ATP is produced by chemiosmosis when ATP synthase uses the proton gradient created across the thylakoid membrane to phosphorylate ADP.

Question 22

Where are the light reactions located?

Answer 22

In the thylakoid membranes of the chloroplast.

Question 23

What is the role of NADPH in photosynthesis?

Answer 23

NADPH provides high-energy electrons and reducing power for the Calvin cycle to synthesize carbohydrates.

Question 24

What is the Calvin cycle?

Answer 24

The Calvin cycle (light-independent reactions) uses CO2, ATP, and NADPH to produce G3P (glyceraldehyde-3-phosphate), a precursor to glucose.

Question 25

What are the three main phases of the Calvin cycle?

Answer 25

1) **Carbon fixation** by **RuBisCO** enzyme attaching CO2 to RuBP. 2) **Reduction** phase using ATP and NADPH to form G3P. 3) **Regeneration** of RuBP from G3P using ATP.

Question 26

What enzyme catalyzes carbon fixation in the Calvin cycle?

Answer 26

**RuBisCO** (ribulose bisphosphate carboxylase/oxygenase) catalyzes the reaction of CO2 with **RuBP**.

Question 27

How many CO2 molecules are needed to produce one G3P?

Answer 27

It takes **3 CO2 molecules** to produce **one G3P** (three turns of the Calvin cycle).

Question 28

How many CO2 molecules are needed to produce one glucose molecule?

Answer 28

It takes **6 CO2 molecules** (six turns of the Calvin cycle) to produce one molecule of glucose.

Question 29

What are the inputs and outputs of the Calvin cycle?

Answer 29

**Inputs:** CO2, ATP, NADPH. **Outputs:** G3P (sugar), ADP, NADP⁺, and inorganic phosphate (Pi).

Question 30

What is the relationship between the light reactions and the Calvin cycle?

Answer 30

The **light reactions** provide **ATP and NADPH** to power the **Calvin cycle**, which returns **ADP, Pi, and NADP⁺** back to the light reactions.

Question 31

What is C3 photosynthesis?

Answer 31

**C3 photosynthesis** is the normal Calvin cycle pathway where CO2 is fixed directly by RuBisCO to form a 3-carbon compound (3-phosphoglycerate).

Question 32

What problem can occur in C3 plants?

Answer 32

In hot, dry conditions, **photorespiration** occurs when RuBisCO binds O2 instead of CO2, wasting energy and reducing photosynthetic efficiency.

Question 33

What is C4 photosynthesis?

Answer 33

**C4 plants** (e.g., corn, sugarcane) fix CO2 into a 4-carbon compound using **PEP carboxylase** in mesophyll cells, then transport CO2 to bundle-sheath cells for the Calvin cycle.

Question 34

Why is C4 photosynthesis advantageous?

Answer 34

It **minimizes photorespiration** and increases CO2 concentration for RuBisCO, improving efficiency in **hot, sunny climates**.

Question 35

What is CAM photosynthesis?

Answer 35

**CAM (Crassulacean Acid Metabolism)** plants (e.g., cacti, pineapple) open **stomata at night** to fix CO2 into organic acids, then close stomata during the day to conserve water.

Question 36

Why is CAM photosynthesis advantageous?

Answer 36

It conserves water in **arid environments** by separating CO2 uptake (night) and Calvin cycle (day) by time.

Question 37

What is the relationship between photosynthesis and cellular respiration?

Answer 37

Photosynthesis **stores energy** in glucose molecules by using CO2 and H2O, while cellular respiration **releases energy** by breaking down glucose into CO2 and H2O. They are **complementary processes** in the carbon and energy cycles.