Metabolic reactions: Biochemical reactions carried out to maintain homeostasis.

The importance of ATP: a denotive triphosphate → energy currency of cells

• ATP → ADP + Pi diphosphate

The role of NADPH:

• NADPH → NADP^+ + H + 2e (which can be donated to another molecule)

How much do you really know about this bulk reaction equation?

6 H₂O + CO₂ → 6O₂ + C₆H₁₂O₆

6 H₂O → 6O₂ is from light

CO₂ → C₆H₁₂O₆ is from chloroplasts

C₆H₁₂O₆ = glucose. This NOT the first product of photosynthesis!

A. LIGHT AND PLANT PIGMENTS

Sunlight-What is it?

• electromagnetic energy → radiation that travels through space in waves

Artificial light

• portion of spectrum → visible (mostly)

• grow lamps → optimized for photosynthesis and far red light

Plant pigments:

Chlorophyll a → primary pigment

• Absorption spectra: blue/violet and red visible

• Functions: convert light energy into chemical energy via loss of electrons

Chlorophyll b →

• Absorption spectra: blue/green and orange

• Functions: expands on spectrum of chl. a and can transfer energy to chl. a

Accessory pigments: multiple/variable

• Absorption spectra: most absorb in UV spectrum of some green

• Functions: carotenoids

• Transfer light energy to chlorophyll → protection

• Dissipate light energy excess short wave

How did we first find out that photosynthesis is most active in the blue- and red-light spectra?

• Red light is absorbed effectively by chlorophyll which ranges within this wavelength. It is utilized in the maximum amount. This red light is followed by blue light which falls next to red in the visible spectrum, thus it is also absorbed in a very effective way for photosynthesis.

Photosynthesis reactions can be divided into 2 general “sets” of reactions:

1. Light-dependent reactions:

   1. in and across thyaloid membranes of chloroplasts

   2. H2O split

   3. O2 byproduct

   4. ATP and NADPH produced

2. Light-independent reactions

   1. or “carbon reactions” or “dark reactions”

   2. in chloroplast stroma

   3. CO2 is input → ATP and NADPH used

   4. 3-carbon compounds produced

B. LIGHT-DEPENDENT REACTIONS

Chloroplast pigments are anchored to and occur throughout the thylakoid membranes.

These pigments (primarily chlorophyll a and b) cluster together forming light-harvesting complexes and a reaction center.

Together, these complexes (proteins of 200-300 pigment molecules) and centers are called photosystems (PS). There are 2 of them. They work in order and simultaneously.

The first photosystem (PS II, P680):

• H2O split

• excited chl. a → 2 electrons transported to ETC (electron transport chain)

The second photosystem (PS I, P700): 2 electrons transferred to NADPH

The electron transport chain connecting them provides the energy that is required to synthesize ATP.

So the primary products of light reactions are O2, ATP, and NADPH.

Some factors (internal and external) influencing the light-dependent reactions:

1. H2O availability: < 1% of H2O used in photosynthesis

2. light quantity

3. light quality

4. temperature

C. LIGHT-INDEPENDENT REACTIONS (CARBON-FIXING REACTIONS)

Inputs: CO2, ATP, NADPH

Products: 3-carbon compounds (more complex structures) → used to produce glucose

The Calvin-Benson cycle occurs in three general phases or steps:

1. Carbon fixation

The enzyme required to carry out this phase is called Rubisco (ribulose 1,5-bisphosphate carboxylase oxygenase). Rubisco “fixes” inorganic CO2 by incorporating it into organic compounds. Rubisco is likely the most abundant protein on Earth.

2. Reduction of 3-carbon compounds.

3. Regeneration of 5-carbon compounds.

Some factors (internal and external) influencing the light-independent reactions:

1. CO2 quantity

2. temperature

3. energy from light reactions (ATP + NADPH)

4. type of plant (C3, C4, CAM)

D. PHOTORESPIRATION

Photorespiration is a metabolic phenomenon that can occur when environmental conditions are hot and/or dry.

Stomata close to conserve water, thereby not allowing CO2 to enter for the Calvin cycle. How do most plants respond to a decline in internal CO2 concentrations?

CO2 < 50 ppm

Rucisco reacts with O2 instead of CO2

• yields [2] 3 C compounds instead of 6

• Drawbacks: requires time, energy input (ATP), other organelles

  • NOT energy efficient

E. VARIATIONS IN CARBON FIXATION STRATEGIES

1. C3 plants (account for 94% of all planetary biomass)

   1. CO2 from air diffuses into plant cells and fixed direction into 3C compounds in Calvin cycle

Examples: tree species, bamboo, rice, soy, cotton, sweet potato, wheat, peanut

Climates in which C3 plants are prevalent: temperate to subtropical

Potential for photorespiration in these plants under hot or dry conditions: High! easily stressed!

2. C4 plants (4-Carbon Pathway, 5% of all biomass)

3. Adapted to carry out photosynthesis more efficiently under moderate-to-hot, drier (but not extremely hot) conditions. Adaptation comes in the form of a separation in space for initial carbon fixation and the Calvin-Benson Cycle.

4. Examples: 2/3 of all grasses including corn, sugarcane, sorgum

   1. “warm/subtropical”

The “4” designation:

Potential for photorespiration in these plants under hot or dry conditions:

3. CAM plants (Crassulacean Acid Metabolism, 1% of biomass)

Adapted to carry about photosynthesis more efficiently under extremely hot and tropical conditions. Adaptation comes in the form of a separation in time for carbon fixated and cycling.

Example plants: jade, pineapple, orchids, cacti, “air plants” including Spanish moss

What’s with this name?

• crassalaceae → succulent family

• native to southern Africa of southern north America

The mechanism occurs in the same cell:

• stomata open only at night → potential for water loss is lowest

• At night, CO2 is fixed into 4C compounds and stand in vacuoles

• during day, CO2 released from 4C compounds and transported to chloroplasts for use in Calvin cycle

Potential for photorespiration in these plants under hot or dry conditions:

• low to zero

• rarely occurs