Lesson 3: Photosynthesis

Question 1

what is photosynthesis?

Answer 1

the process by which light energy is captured and channeled into the production of food molecules

Question 2

What is the photosynthesis equation

Answer 2

6CO2 + 6H20 + light energy → C6H12O6 +6O2

Question 3

Chemosynthetic

Answer 3

bacteria that do not need light energy at all they get their energy from other molecules

Question 4

where does photosynthesis occur?

Answer 4

• occurs in specialized organelles within plants, called chloroplasts
• Within the chloroplasts there are structures containing special light-sensitive pigments called chlorophyll

Question 5

Role of chlorophyll a

Answer 5

• to absorb energy of the red and blue wavelengths from light photons and transfer the electrons required for photosynthesis
• functional group: methyl (CH3)

Question 6

Role of chlorophyll b

Answer 6

• functional group: carbonyl
• to absorb energy of the red and blue wavelengths from light photons and pass them on through the antenna complex to chlorophyll a

Question 7

Role of other pigments

Answer 7

Xanthophylls and carotenoids absorb light wavelengths that chlorophylls a and b do not, and pass the energy they absorb on to cholorphyll a through the antenna complex

Question 8

What is an action spectrum?

Answer 8

the sum total of the absorption of light energy by all of the pigments in plants

Question 9

How does the action spectrum explain why green plants appear green?

Answer 9

None of the pigments absorb wavelengths of light in the green part of the spectrum between 500 and 600 nm. This means that those wavelengths are reflected by the leaf, while all other colours are absorbed

Question 10

Compare the roles of photosystems 680 (II) and 700 (I) in photosynthesis?

Answer 10

photosystem 680 functions to harvest photon energy and donate high-energy excited electrons to the first of the electron carriers of the ETC. Photosystem 700 accepts the electrons passed along from photosystem 680 and, using more light energy from photons, re-excites those electrons to an even higher energy level, before passing them on to the rest of the electron carriers of the ETC.

Question 11

Summarize the events occurring in the light reactions by referring to the reactants,products, and cellular components involved.

Answer 11

In light reactions:
- an electron is promoted to a higher energy level by energy absorbed by ps 680 from a photon of light. Some of this energy is also used to help split water, yielding O2 and H+ ions. An electron from water replaces the electron that was donated from chlorophyll a. The electron is then passed on, through the ETC, to photosystem 700, powering proton pumps along the way. The pumps draw even more H+ into the lumen of the thylakoid, resulting in a high concentration of H+. Upon reaching photosystem 700, another photon of energy boosts the electron to a higher energy level again, and passes it on to the rest of the ETC. The electron is finally accepted by NADP+ and H+, with the help of NADP reductase, forming NADPH. ATP synthase utilizes the high proton gradient inside the luman to drive ATP synthesis. The ATP and NADPH are then made available to power the dark reactions that fix carbon.

Question 12

List the path of an electron excited by light photons during the light reactions, identifying each cellular component it passes through.

Answer 12

An excited electron takes the following path during the light reactions: photosystem 680, chlorophyll a, pheophytin, plastoquinone (Q), chtochrome b6-f complex, plastocyanin, photosystem 700. ferrodoxin, NADP reductase, NADP/NADPH.

Question 13

Where does the energy in NADPH and ATP that feeds the Calvin cycle come from?

Answer 13

The energy in ATP and NADPH comes from the light-dependent reactions. It is transferred to the Calvin cycle during the dark reactions.

Question 14

Explain the difference between the light reactions and the dark reactions of photosynthesis?

Answer 14

• Light reactions depend on the energy provided by light protons to excite electrons and split water into oxygen and H+ ions, which are used to generate ATP and NADPH
• Dark reactions do not require light, but do require the products of the light-dependent reactions to provide the energy needed to fix carbon from the air into energy-rich molecules, which eventually becomes sugar

Question 15

What is the relationship between the light and dark reactions?

Answer 15

The light reactions use light energy and water to provide the energy (in the form of ATP and NADPH) for the dark reactions, which fix carbon from gaseous carbon dioxide into energy-rich sugar molecules

Question 16

How many ATP and NADPH molecules are required to from one glucose molecule during photosynthesis?

Answer 16

18 ATP and 12 NADPH is required to form one glucose molecule because 2 G3P molecules form one glucose molecule.

Question 17

What happens to the 3-carbon sugar produced at the end of photosynthesis?

Answer 17

G3P can be joined to form glucose, which can be added to other monosaccharides to make a dissaccharide, such as sucrose, or a polysaccharide, such as starch or cellulose

Question 18

Summarize the events of the dark reactions by referring to the reactants, products, and cellular components involved.

Answer 18

a

Question 19

What is photorespiration and why is it a problem for plants?

Answer 19

• Its the release of CO2 from an organic molecule during the process of photosynthesis. The whole process of photosynthesis is geared to the single goal of channeling energy from light into the chemical bonds of an organic compound by using CO2 as a raw material. Photorespiration undergoes what photosynthesis has just done and is therefore counterproductive and wasteful. At above 28 celcius photorespiration rates become very high and photosynthetic efficiency in C3 plants suffers greatly.

Question 20

Provide examples of plants that carry out C3, C4 and CAM photosynthesis

Answer 20

C3: trees, soybeans, wheat, barley, potatoes, sugar beets, and cotton

C4: Crabgrass, corn, sugar cane, and sorghum

CAM: Pineapples, cacti, orchids, and aloe vera

Question 21

Compare the metabolic differences between C3, C4 and CAM plants.

Answer 21

• C4 and CAM plants are better adapted for limiting photorespirative loss of CO2 than C3 plants.
• C4 plants spatially isolate high concentrations of CO2 whereas CAM plants temporally isolate this key nutrient by timing their pathways and stomatal openings.

Question 22

Stage 1: light reactions

Answer 22

1. capture of light energy
2. Transfer of light energy by intermediate energy carriers
3. Transformation of light energy into chemical potential energy (ATP and NADPH)

Question 23

Stage 2: dark reactions

Answer 23

1. Carbon fixation
2. Reduction of PGA
3. Regeneration of RuBP