Photosynthesis

Question 1

What is photosynthesis

Answer 1

The process of making glucose from carbon dioxide, water and light energy

Question 2

What does it mean if an organism is chemosynthetic?

Answer 2

An organism that is chemosynthetic is one which doesn’t require the use of light energy to form carbohydrates

Question 3

Where in plants does photosynthesis occur?

Answer 3

Photosynthesis occurs in plant structures known as chloroplast which contain chlorophyll (light-sensitive pigments)

Question 4

Where is chlorophyll found in green leaves?

Answer 4

In green leaves, chlorophyll is found right under the epidermis within the palisade mesophyll cells and the spongy mesophyll cells (less frequent).

Question 5

Describe the structural makeup of green leaves

Answer 5

• The outer layers of the leaves are called cuticles (waxy layer)
• under the cuticle is another layer (plant skin) known as the upper epidermis
• The inner layer as a whole is called the mesophyll which consists of palisade mesophyll (cells are tightly packed) and spongy mesophyll (pores)
• Chloroplasts are found within mostly the palisade mesophyll and less frequently within the sponge mesophyll
  ( ROUND PORES)
• The lower epidermis (plant skin)
• In the lower epidermis are guard cells and stroma which regulate the entrance and exit of different molecules in and out of the leaf
• another cuticle under the cuticle

Question 6

What are thylakoids?

Answer 6

Thylakoids are small stacks of disk-like structures found within chloroplasts

Question 7

What is the difference between stroma and lumen?

Answer 7

Lumer is the internal space of thylakoids and stroma is the aqueous medium in which all plant structures are suspended within

Question 8

Where is chlorophyll found?

Answer 8

Chlorophyll is found bound to thylakoid membranes

Question 9

What is the main significance of chlorophyll (function)?

Answer 9

The main significance of chlorophyll in plants is to give them their green colour and also give them their ability to absorb light energy

Question 10

What are the two main forms of chlorophyll?

Answer 10

1) Chlorophyll a
2) Chlorophyll b

Question 11

What are pigments?

Answer 11

Pigments define the electromagnetic wavelength at which a plant may absorb light energy. Plant pigments absorb light mostly within the visible spectrum.

Question 12

What are the different stages of the series of reactions which take place during photosynthesis?

Answer 12

The two stages which photosynthetic reactions are split into include:

1) “light-dependent reactions”: reactions which require the use of light

• Energy harvesting stage

2) Dark reactions: those reactions which take place with or without the presence of light (Calvin cycle)

• Where carbon is “fixed” or combined using the harvested energy to form glucose molecules

Question 13

Break down the two major stages of photosynthetic reactions into 3 steps each

Answer 13

Light-dependent reactions:
1)Capturing light
2)Transfering light to energy carriers
3)Converting into ATP and NADH

Dark reactions: (Calvin cycle)
1) Carbon fixation
2) Reduction of PGA
3) Regeneration RuBP

Question 14

What happens during the first step of the light-dependent reactions?

Answer 14

First, a photon of light hits a chlorophyll molecule in the reaction center of the antenna system. From the electron center, the primary electron acceptor oxidizes the reaction center to gain the electron.

Question 15

What is a photon?

Answer 15

A photon refers to a wave of light energy which strikes chlorophyll molecules bound to thylakoid membranes.

Question 16

What is the antenna system?

Answer 16

The antenna system consists of a bunch of pigments including chlorophyll a, b and accessory pigments referred to the reaction center as a whole. The pigment which makes up of the reaction center converts the light energy which strikes the chloroplast into electrons. Along with this, a primary electron acceptor.

Question 17

What are the two main proteins directly associated with the antenna system?

Answer 17

the two main proteins include Photosystem II and Photosystem I

Question 18

What happens during the second step of light-dependent reactions?

Answer 18

The second step of the light-dependent reaction being the transfer of electrons by electron carriers starts with photosystem II. This complex is used to perform photolysis which splits 2 water molecules removing electrons, and releasing H+ ions into the thylakoid lumen. This creates a chemiosmotic gradient of protons in the lumen compared to protons in the stroma. Electrons removed during photolysis are transferred from photosystem I to photosystem II and so on throughout a chain of proteins and metal ion energy carriers. This is known as the elctron transport chain. While moving through the electron transport chain, energy is consistently used at each complex to pump H+ ions into the lumen or lost to heat. The chemiosomotic gradient result in the production of ATP molecules at end of the electron transport chain. At photosystem I, electrons are recharged by more light energy, similar to photosystem II. From photosystem I, an energy carrier called ferredoxin carries the excited electron the NADP reductase in which reduction takes place and 2 NADPH molecules are produced.

Question 19

What is photolysis?

Answer 19

Photolysis in cellular respiration refers to the use of light energy to split water molecules removing electrons, releasing protons (H+) and creating the bi-product oxygen.

Question 20

Describe what happens in the last step of light dependent reactions during photosynthesis

Answer 20

In the final step which consists of transforming the photon energy into chemical energy, ATP is produced through phosphorylation at the end of the ETC when protons come down the ATP synthase. After electrons are recharged in photosystem I, some of the energy is used to reduce NADP in the NADP reductase which attaches H+ ions from the stroma.

Question 21

State the order in which each structure involved in the light-dependent reactions is found embedded in the thylakoid membrane

Answer 21

1) photosystem I
2) ETC
3) photosystem II
4) NADP reductase
5) ATP synthase

Question 22

Where are the ATP and NADPH produced during the light-dependent reactions?

Answer 22

In the stroma

Question 23

Where are protons pumped to using energy from electrons passing through the ETC?

Answer 23

The Lumen (inner thylakoid space)

Question 24

What is the proton motive force which drives chemiosmosis and the production of ATP?

Answer 24

The proton motive force which drives the chemiosmosis of protons from the stroma to the lumen in photosystem II is the chemiosmotic gradient created by the proton pump increasing the concentration of hydrogen atoms in the lumen.

Question 25

What is important to remember about phosphorylation in green plants compared to normal phosphorylation?

Answer 25

In green plants, it is non-cyclic, meaning that the pigment never returns or reuses the electrons extracted from chlorophyll a after a photon strikes the antennae system and passes through the ETC. All electrons extracted at the start of light-dependent reactions are transferred to NADP at the end of the process for NADPH. Electrons are replenished by the supply of electrons released through the splitting of water

Question 26

What is cyclic phosphorylation?

Answer 26

Cyclic phosphorylation is when the electrons transferred from the chlorophyll a are returned at the end of the light-dependent reactions. Therefore, NADP is not reduced and there is no production of NADPH+

Question 27

what are the three steps in which the dark reactions of photosynthesis may be split into?

Answer 27

The three steps in which the dark reactions (The Calvin Cycle) is split into include:
1) carbon fixation
2) the reduction of PGA
3) the regenerations of RuBP

Question 28

Describe the first phase of the dark reactions

Answer 28

The dark reactions start with the fixation of carbon. a CO2 molecule is added to a RuBP (rubisco) molecule and through a series of exergonic reactions, catalyzed by RuBP carboxylase resulting in the formation of a G3P molecule after a total of 3 carbon molecules pass through the cycle

Question 29

Describe the second phase of the dark reactions

Answer 29

With the help of PGA kinase, PGA produced through carbon fixation becomes PGAP (gains energy) with the addition of a phosphate group from the phosphorylation of 6 ATP molecules before the production of PGAP. The PGAP then undergoes a series of reduction reactions in which the use of NADPH produced during the reactions comes into use resulting in the final production of a glucose molecule. The entire cycle must be repeated twice to produce 1 glucose molecule. After the PGA is completely reduced into 6G3P molecules, 1 of the 6 leaves the cycle to produce a sugar while the other 5 remain within the cycle to replenish the supply of RuBP (rubisco).

Question 30

What happens in the final stage of the dark reactions?

Answer 30

The remaining 5 G3P molecules within the cycle move on to combine with 3 ATP molecules and 3 phosphates to create 3 Rubisco molecules

Question 31

How do plants store extra glucose molecules synthesized through photosynthesis?

Answer 31

Extra glucose molecules in plants are stored as starches

Question 32

What are the different variations of photosynthesis and what creates this variation

Answer 32

The different types of photosynthesis have to do with the number of carbons formed during carbon fixation.

The different types include C3, C4 and another division of C4 photosynthesis as for the way the CO2 is processed, this is called CAM

Question 33

Define photorespiration

Answer 33

Photorespiration is the process in which plants reduce the efficiency of photosynthesis in unfavourable temperatures

Question 34

What are C3 plants?

Answer 34

• C3 plants produce a 3-carbon molecule during fixation
• Stomata are usually open during the day and closed at night allowing for efficient transportation
• More susceptible to photorespiration in warm and dry climates
• More water loss through transpiration
• rice (95% of biomass)
• carbon fixation in mesophyll

Question 34

What are C4 plants?

Answer 34

• starts with the fixation of a 4-carbon molecule
• efficient in hot temperatures
• susceptible to photorespiration cooler temperatures
• less water loss due to transpiration
• stomata are normally closed
• separation between carbon fixation (mesophyll) and Calvin cycle (Bundle sheath cell)
• corn

Question 34

CAM plants

Answer 34

• Similar to C4 pathway though carbon fixation and Calvin cycle are not spatially separated
• stomata remain closed during the day, respiration occurs at night
• reduced water loss
• carbon fixation takes place during the night, Calvin cycle takes place during the day

Question 35

What are stomata?

Answer 35

Stomata are pores found on the surface of leaves which allow the exit and entrance of CO2 and Oxygen in and out of the cell