Photosynthesis

Question 1

Autotroph

Answer 1

an organism that can produce its own food using light, water, carbon dioxide, or other chemicals

Question 2

Heterotroph

Answer 2

consumers who depend on other sources for their food

Question 3

photosynthesis

Answer 3

• reverses the direction of electron flow
• water is split and electrons are transferred
• hydrogen ions from water is transferred to carbon dioxide, reducing it to sugar
• this is an endergonic process
• 6CO₂ + 6H₂O ——> C₆H₁₂O₆ + 6O₂
• takes place in chloroplasts

Question 4

photosynthesis pigments

Answer 4

• the segment most important to life is between 380nm - 750nm and is known as visible light
• the atmosphere is selective and only allows visible light to pass through

Question 5

The pigments

Answer 5

• bacteriochlorophyll a
• chlorophyll a
• chlorophyll b
• phycoerythrobilin
• β-carotene

Question 6

the action spectrum

Answer 6

• demonstrated in 1883 by Engelmann
• reveal which wavelength of light are photosynthetically important
  -b the action spectrum resembles the absorption spectrum of chlorophyll a and the accessory pigments
• showed that light in the violet - blue and red portions of the spectrum is most effective

Question 7

chlorophyll a

Answer 7

• primary photosynthetic pigment
• green pigments found in plants algae and cyanobacteria
• absorbs light that powers photosynthesis through the excitation of electrons located in the porphyrin - like rings
• make up an antenna complex that is associated to a photochemical reaction centre, forming a photosystem

Question 8

chlorophyll b

Answer 8

• accessory pigment
• has the same structure as chlorophyll a but the CH₃ is replaced by an aldehyde group (-CHO) and absorbs at 500 - 640 nm (appearing olive green)

Question 9

Carotenoids

Answer 9

• yellow, orange, red or brown pigments
• absorb strongly in the blue - violet range
• known as accessory pigments
• pass absorbed light energy to chlorophyll a
• provide photoprotection

Question 10

What are the three possible fates of energy absorbed by pigments in the light reactions of photosynthesis?

Answer 10

• Conversion to heat or a mix of heat and light
• Transfer to a neighbouring chlorophyll molecule (via resonance energy transfer)
• Transfer of a high-energy electron to a nearby molecule (an electron acceptor)

Question 11

the photosystem

Answer 11

1 - Resonance Energy Transfer – Energy from light excites electrons and is transferred between chlorophyll molecules.
2 - Charge Separation – The special pair of chlorophyll molecules loses an electron, starting electron flow.
3 - Electron Transfer – The high-energy electron is passed to an electron acceptor (quinone), forming molecule B.
4 - Photolysis – Water is split to replace the lost electron, releasing O₂ and H⁺

Question 12

Energy Coupling

Answer 12

• Exergonic reactions (e.g., catabolism) release energy by breaking down food molecules.
• This energy is captured by activated carrier molecules.
• These carriers then provide energy for endergonic reactions (e.g., anabolism), which build needed cellular molecules.
• Energy flows from food → carrier → biosynthesis.

Question 13

Role of ATP in Energy Coupling

Answer 13

• Energy from catabolic reactions converts ADP + Pi → ATP.
• ATP becomes the activated energy carrier molecule.
• ATP is then used to power anabolic (endergonic) reactions in cells.
• ATP links the two processes: catabolism fuels anabolism.

Question 14

ATP

Answer 14

• most versatile and most important of the activated carriers in cells
• is is a ribonucleotide
• the terminal group is frequently split off by hydrolysis

Question 15

How does ATP store and release energy for cellular work?

Answer 15

• ATP (Adenosine Triphosphate) stores energy in its high-energy phosphoanhydride bonds.
• Phosphorylation: Energy from sunlight or food adds a phosphate to ADP, forming ATP.
• Hydrolysis: Breaking a phosphate bond in ATP (→ ADP + Pi) releases ~11–13 kcal/mole of usable energy.
• This energy powers cellular work and chemical synthesis.

Question 16

NADP+

Answer 16

• pick up energy in the form of two-energy electrons plus a proton (H+), In other words, they can be regarded as carriers of hydride ions (H+)
• NADPH works with enzymes that catalyse anabolic reactions
• NADH is an intermediate in catabolic reactions

Question 17

the electron transport chain

Answer 17

• light energy drives the synthesis of both ATP and NADPH
• the oxygen evolving complex catalyses the splitting of two water molecules (photolysis)
• 2H₂O → 4H+ + O₂ + 4e-
• the proton gradient drives the ATP synthase to generate ATP (photophosphorylation)

Question 18

the Z-scheme

Answer 18

the coupling of photosystem II and photosystem I boosts electrons to the energy level needed to produce NADPH

Question 19

Cyclic electron flow

Answer 19

• to generate more ATP without making NADPH
• switch photosystem I into cyclic mode also known as cyclic phosphorylation
• important as ATP source in the bundle sheath chloroplasts of some C₄ plants and used in bacterial photosynthesis

Question 20

photosynthetic response to temperature

Answer 20

• temperature has a huge effect on photosynthesis which is dependent on enzymes
• increase in carboxylation rates with temperature
• decrease in affinity of Rubisco for CO₂ as temperature rises
• increased temperature also reduced CO₂ uptake

Question 21

photosynthetic response to CO₂

Answer 21

High CO₂ - more carbon is fixed by Rubisco, increased 3-phosphoglycerate (GP) and triose phosphate (TP)
Low CO₂ ribulose (RUBP) accumulates as carbon fixation is limited
- GP and TP not formed

Question 22

photosynthetic response to light

Answer 22

bright light
- increase the production of ATP, NADPH and O₂
- more power to reduce and phosphorylate GP thus increasing TP production
dim light
- increase in GP but not enough to convert to TP

Question 23

CAM Photosynthesis (Carbon Concentrating Mechanism)

Answer 23

🌙NIGHT (Stomata open):
- CO₂ enters and combines with PEP to form oxaloacetate
- Converted to malate and stored as malic acid in vacuoles
- Stomata are open → allows gas exchange but leads to some H₂O loss
☀️ DAY (Stomata closed):
- Malic acid is decarboxylated to release CO₂
- Released CO₂ enters the Calvin Cycle
- Stomata closed → prevents water loss, conserves moisture in hot/dry environments

Question 24

C₄ Photosynthesis (Carbon Concentrating Mechanism)

Answer 24

• CO₂ enters the mesophyll cells and is fixed into a 4-carbon acid (C₄ acid) using PEP carboxylase
• C₄ acid is transported to the bundle sheath cells
• In the bundle sheath, C₄ acid is decarboxylated, releasing CO₂
• The released CO₂ is used in the Calvin cycle (now isolated from oxygen, reducing photorespiration)
• This spatial separation (mesophyll vs. bundle sheath) boosts efficiency in hot, sunny environments

Question 25

How do C₄ plants concentrate CO₂?

Answer 25

- C₄ plants (like sugarcane) shuttle CO₂ from mesophyll cells to bundle sheath cells using malate or aspartate. - This creates a high CO₂ concentration near Rubisco, reducing photorespiration. - It’s an example of spatial separation of CO₂ fixation and the Calvin cycle.

Question 26

How do CAM plants minimize water loss while fixing CO₂?

Answer 26

- CAM plants (like agave) open stomata at night to take in CO₂, storing it as malic acid in vacuoles. - During the day, stomata remain closed to conserve water, and stored CO₂ is used in the Calvin cycle. - This is temporal separation of CO₂ uptake and fixation.

Question 27

How do algae concentrate CO₂ for photosynthesis?

Answer 27

- Algae use an inorganic carbon uptake system. - They rely on carbonic anhydrases and specialized structures to convert bicarbonate (HCO₃⁻) into CO₂. - This system enhances CO₂ availability around Rubisco, improving efficiency in aquatic environments.

Question 28

What is Rubisco and why is it important?

Answer 28

- Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the primary enzyme responsible for carbon fixation in the Calvin cycle. - It’s essential for life on Earth—every plant relies on it to convert CO₂ into organic compounds. - Large - Slow - It can fix O₂ instead of CO₂, leading to photorespiration, which wastes energy.

Question 29

What is photorespiration and why does it happen?

Answer 29

Photorespiration occurs when Rubisco fixes O₂ instead of CO₂, due to: - Rubisco's kinetic properties - The ratio of CO₂ to O₂ - High temperatures This triggers a salvage pathway involving the chloroplast, peroxisome, and mitochondrion. Results: - 75% of carbon is recovered and returned to the Calvin cycle - 25% of carbon is lost as CO₂ - Also consumes ATP and releases NH₃, making it energetically costly

Question 30

What is the Calvin–Benson–Bassham Cycle and what are its main phases?

Answer 30

- The CBB Cycle is the light-independent (dark) reaction of photosynthesis that fixes CO₂ into organic molecules. - Occurs in the stroma of chloroplasts. Main Phases: 1- Carbon Fixation – CO₂ is fixed by Rubisco into 3-phosphoglycerate (3-PGA) 2- Reduction – ATP and NADPH convert 3-PGA into G3P (glyceraldehyde-3-phosphate) 3 - Regeneration – Some G3P is used to regenerate RuBP, the CO₂ acceptor, using ATP - Products: G3P (used to build glucose and other carbohydrates)