**2.7 The Calvin cycle**

The dark reactions of photosynthesis occur in the stroma of the chloroplast and are referred to as the Calvin cycle. Although the Calvin cycle does not utilize light and can happen during the daytime or at night, they employ products of the

**363**

*Microalgae: The Multifaceted Biomass of the 21st Century*

light-dependent reactions to propagate. Products of the light-dependent reaction are ATP and reduced NADP; the energized electrons from the light-dependent reactions provide the energy to produce carbohydrates from carbon dioxide molecules. **Stage 1: Carbon fixation** - Carbon-Fixing Reactions are also known as the Dark Reactions during which CO2 gas diffuses through and dissolves in the water around the walls of mesophyll cells, diffuses through the cytoplasm and chloroplast membrane into the stomata. In the stroma, CO2 undergoes a ribulose bisphosphate carboxylase (Rubisco) enzymatic catalyzed reaction with ribulose bisphosphate

The Calvin Cycle first produces phosphoglyceric acid (PGA), which is phosphorylated, using the energy carriers ATP and NADPH generated by the photosystems I and II, to produce 12 molecules of phosphoglyceraldehyde (PGAL). Two molecules of PGAL are ejected from the cycle in the form of a glucose molecule. The other ten molecules of PGAL are converted to 6 RuBP molecules, using the inherent energy in

**Stage 2: Reduction and sugar production** – The cell utilizes the high energy molecules ATP and NADPH and reduces 3-PGA to form triose phosphate, G3P.2G3P, which leaves the cycle to produce glucose, starch, cellulose, lipids, amino acids, and

**Stage 3 Regeneration** – The remaining then G3P (3-GPA) in the cycle are regenerated to RuBP, which is a 6-carbon molecule with 2 phosphates, and it requires energy to generate. This process utilizes the high energy ATP made during the light-dependent reactions. The RuBP molecule formed then interacts with more CO2 from the atmosphere and generates more PGA to keep the cycle going [20].

i i ( )

The summary of the reactions in the Calvin cycle (see Eq. (2))

+ + ++ =

3 6 69 3

CO NADPH H ATP PGAL

+ + + →−

+

2

NADP ADP H O P P inorganic phosphate

Light has properties of both waves and particles, from the quantum mechanics point of view [20]. The particulate behavior of light presents light as a stream of particles of energy, known as photons, which interact with electrons to cause the energy contained in the light to disappear and then reappear as the kinetic energy

*E hv KE photon* = = + *electron*

6 9 38 (2)

ϕ

(3)

*DOI: http://dx.doi.org/10.5772/intechopen.94090*

*The first reaction in the Calvin cycle: Carbon fixation.*

(RuBP) [19] (**Figure 7**).

**Figure 7.**

nucleotides [20].

ATP and the cycle continues [19, 20].

+

of the ejected electrons plus a work function.

2

**2.8 The inherent energy of a photon**

*Microalgae: The Multifaceted Biomass of the 21st Century DOI: http://dx.doi.org/10.5772/intechopen.94090*

#### **Figure 7.**

*Biotechnological Applications of Biomass*

**362**

**Figure 6.**

**Figure 5.**

**2.7 The Calvin cycle**

*The Calvin cycle [20].*

The dark reactions of photosynthesis occur in the stroma of the chloroplast and are referred to as the Calvin cycle. Although the Calvin cycle does not utilize light and can happen during the daytime or at night, they employ products of the

*Relative absorbance of photosynthetic pigments as a function of the wavelength of light [19].*

*The first reaction in the Calvin cycle: Carbon fixation.*

light-dependent reactions to propagate. Products of the light-dependent reaction are ATP and reduced NADP; the energized electrons from the light-dependent reactions provide the energy to produce carbohydrates from carbon dioxide molecules.

**Stage 1: Carbon fixation** - Carbon-Fixing Reactions are also known as the Dark Reactions during which CO2 gas diffuses through and dissolves in the water around the walls of mesophyll cells, diffuses through the cytoplasm and chloroplast membrane into the stomata. In the stroma, CO2 undergoes a ribulose bisphosphate carboxylase (Rubisco) enzymatic catalyzed reaction with ribulose bisphosphate (RuBP) [19] (**Figure 7**).

The Calvin Cycle first produces phosphoglyceric acid (PGA), which is phosphorylated, using the energy carriers ATP and NADPH generated by the photosystems I and II, to produce 12 molecules of phosphoglyceraldehyde (PGAL). Two molecules of PGAL are ejected from the cycle in the form of a glucose molecule. The other ten molecules of PGAL are converted to 6 RuBP molecules, using the inherent energy in ATP and the cycle continues [19, 20].

**Stage 2: Reduction and sugar production** – The cell utilizes the high energy molecules ATP and NADPH and reduces 3-PGA to form triose phosphate, G3P.2G3P, which leaves the cycle to produce glucose, starch, cellulose, lipids, amino acids, and nucleotides [20].

**Stage 3 Regeneration** – The remaining then G3P (3-GPA) in the cycle are regenerated to RuBP, which is a 6-carbon molecule with 2 phosphates, and it requires energy to generate. This process utilizes the high energy ATP made during the light-dependent reactions. The RuBP molecule formed then interacts with more CO2 from the atmosphere and generates more PGA to keep the cycle going [20].

The summary of the reactions in the Calvin cycle (see Eq. (2))

$$\begin{aligned} &\mathbf{3CO}\_2 + \mathbf{6NADPH} + \mathbf{6H}^\* + \mathbf{9ATP} \rightarrow \mathbf{3-PGAL} \\ &+ \mathbf{6NADP}^\* + \mathbf{9ADP} + \mathbf{3H}\_2\mathbf{O} + \mathbf{8P\_i} \left(\mathbf{P\_i} = \text{inorganic phosphate}\right) \end{aligned} \tag{2}$$

#### **2.8 The inherent energy of a photon**

Light has properties of both waves and particles, from the quantum mechanics point of view [20]. The particulate behavior of light presents light as a stream of particles of energy, known as photons, which interact with electrons to cause the energy contained in the light to disappear and then reappear as the kinetic energy of the ejected electrons plus a work function.

$$E\_{photon} = h\nu = KE\_{electron} + \varphi \tag{3}$$

where *Ephoton* is the energy of a photon, *h* is Planck's constant (6.626 × 10−34 J·s),

and *v* is the frequency of the light wave, *KEelectron* is the kinetic energy of electron andϕ is the work function, which defines the minimum amount of energy that is necessary to induce photoemission of electrons from the surface of a metal, and the value ofϕ depends on the metal. We are dealing with biotic materials in the context of this chapter, so we may assume that ϕ= 0 .

By definition, *<sup>c</sup> v* λ <sup>=</sup> , where *c* is the velocity of light (3 × 108 m/s in a vacuum), and λ is the wavelength of light. It is important to note that the energy content of light of shorter wavelength is higher than that of longer wavelengths; and for one mole of photons, the energy is the total of the energies of all the particles in one mole, which is given in Eq. (4).

$$E = \mathsf{NE}\_{electron} \tag{4}$$

**365**

**Figure 8.**

*Photo-isomerization of all-trans to 13-cis retinal in bR [21].*

*Microalgae: The Multifaceted Biomass of the 21st Century*

λ

chromophore covalently bonded in the central region via a protonated Schiff base

c1, and chlorophyll c2. Each pigment registers a maximum signal at a particular

λ

*B m k B <sup>P</sup> <sup>I</sup>* α

maximum value of the photosynthetic rate of the system <sup>2</sup>

indicate the half-saturation constant when the photosynthetic rate is half the

measure of photosynthetic efficiency of solar energy conversion into chemical energy, and it takes into account that the light absorbed by the algal cell is

ing of light, microalgae are seen in their defined color. Consequently, in the blue

max = 675 *nm* (see **Figure 5**) [22]. As the microalgae cells are irradiated with light from a source, the photosynthetic process is initiated and propagated. The photosynthetic rate increases as the intensity of the irradiance are increased; and the level of irradiance is reached where the rate of photosynthesis attains a maximum and begins to retard (see **Figure 9**). At this stage, the cells are said to be in photoinhibition. Photoinhibition is thus a phenomenon that describes the inability of the photosynthetic organism to support photosynthesis in the presence of excess

The most common chlorophylls are chlorophyll a, chlorophyll b, and chlorophyll

max = 440 *nm* and in the red region of the spectrum,

<sup>=</sup> ) and photo-inhibition (β) parameters

*<sup>B</sup> Pm <sup>P</sup>* <sup>=</sup> 

. α<sup>B</sup>

is the

max ), this coupled with the selective scatter-

*DOI: http://dx.doi.org/10.5772/intechopen.94090*

to a lysine residue (see **Figure 8**).

region of the spectrum,

λ

wavelength of maximum absorption (

illumination from a light source [19].

The saturation irradiance (

where *N* is the Avogadro's number (6.02 × 1023 molecules or photons/mol). Thus

$$E = NE\_{electron} = h\nu = \text{Nh}\frac{c}{\lambda} \tag{5}$$

Thus for sunlight with a wavelength of 650 nm (650 × 10−9 m), the energy is computed in Eq. (6).

$$E = \left(6.02 \text{x10}^{\text{>3}} \text{photons} / mol\right) \left(6.626 \text{x10}^{-\text{>4}} \text{J}\text{s}\right) \left(\frac{\text{3x10}^{\text{8}} \text{m} / \text{s}}{650 \text{x10}^{-\text{9}} \text{m}}\right)$$

$$\begin{array}{l} = 184100.86 \text{ J/mol} \\ \equiv 184.1 \text{ kJ/mol} \end{array} \tag{6}$$

If all this were to be used for synthesizing ATP from ADP and **P**i it would be enough to synthesize several moles [20].

Chlorophylls b, c, d, and e are accessory pigments with xanthophylls, and carotenoids in algae and protistans, Pigments that are not accessory to chlorophyll absorb light energy at wavelengths that do not stimulate chlorophyll. Light energy absorbed by accessory pigments is channeled to the reaction site and is converted into chemical energy. The ability to absorb some energy from the longer, more penetrating wavelengths probably conferred an advantage to the benthic photosynthetic algae. Depending upon turbidity of the water, the shorter, high energy wavelengths penetrate very little in the euphotic zone (below 5 meters) in seawater [7, 8]. Chlorophyll molecules being the main producers of pigments are bound to proteins of the photosynthetic membranes and capture the sunlight in oxygenic plants, and convert light energy into chemical energy. This is facilitated by pigment-protein complexes known as Photosystem I (PSI) and Photosystem II (PSII) reaction sites [9]. In PS II water is *split* and the electrons are used to replenish excited electrons that are lost from the photosystem. The loss of electrons during the oxidation of water results in the formation of O2 gas. In PS I the electron acceptor is first in an electron transport system in the thylakoid membrane. Electrons pass through the chain via a series of redox reactions until it hit the final electron acceptor. The final electron acceptor is NADP<sup>+</sup> which is reduced to NADPH. ATP is produced throughout the whole process via chemical osmosis, meaning using an H+ gradient during electron transport (photophosphorylation). It has been shown that [6] the protein is composed of seven transmembrane helices with a retinal

*Biotechnological Applications of Biomass*

of this chapter, so we may assume that

*v* λ

= 184100.86

enough to synthesize several moles [20].

acceptor. The final electron acceptor is NADP<sup>+</sup>

≅

184.1 kJ/mo

J/mol

*E mol* <sup>−</sup>

l

andϕ

and λ

value of

ϕ

By definition, *<sup>c</sup>*

mole, which is given in Eq. (4).

computed in Eq. (6).

where *Ephoton* is the energy of a photon, *h* is Planck's constant (6.626 × 10−34 J·s),

 is the work function, which defines the minimum amount of energy that is necessary to induce photoemission of electrons from the surface of a metal, and the

depends on the metal. We are dealing with biotic materials in the context

m/s in a vacuum),

9

−

0 m

(6)

50

x1

which is reduced to NADPH. ATP

*E NE* = *electron* (4)

λ= = = (5)

and *v* is the frequency of the light wave, *KEelectron* is the kinetic energy of electron

ϕ= 0 .

<sup>=</sup> , where *c* is the velocity of light (3 × 108

 is the wavelength of light. It is important to note that the energy content of light of shorter wavelength is higher than that of longer wavelengths; and for one mole of photons, the energy is the total of the energies of all the particles in one

where *N* is the Avogadro's number (6.02 × 1023 molecules or photons/mol). Thus

*electron <sup>c</sup> E NE hv Nh*

Thus for sunlight with a wavelength of 650 nm (650 × 10−9 m), the energy is

( )( ) <sup>8</sup> 23 34

If all this were to be used for synthesizing ATP from ADP and **P**i it would be

Chlorophylls b, c, d, and e are accessory pigments with xanthophylls, and carotenoids in algae and protistans, Pigments that are not accessory to chlorophyll absorb light energy at wavelengths that do not stimulate chlorophyll. Light energy absorbed by accessory pigments is channeled to the reaction site and is converted into chemical energy. The ability to absorb some energy from the longer, more penetrating wavelengths probably conferred an advantage to the benthic photosynthetic algae. Depending upon turbidity of the water, the shorter, high energy wavelengths penetrate very little in the euphotic zone (below 5 meters) in seawater [7, 8]. Chlorophyll molecules being the main producers of pigments are bound to proteins of the photosynthetic membranes and capture the sunlight in oxygenic plants, and convert light energy into chemical energy. This is facilitated by pigment-protein complexes known as Photosystem I (PSI) and Photosystem II (PSII) reaction sites [9]. In PS II water is *split* and the electrons are used to replenish excited electrons that are lost from the photosystem. The loss of electrons during the oxidation of water results in the formation of O2 gas. In PS I the electron acceptor is first in an electron transport system in the thylakoid membrane. Electrons pass through the chain via a series of redox reactions until it hit the final electron

is produced throughout the whole process via chemical osmosis, meaning using an

 gradient during electron transport (photophosphorylation). It has been shown that [6] the protein is composed of seven transmembrane helices with a retinal

3x10 m / s 6.02x10 photons / 6.626x10 J.s <sup>6</sup>

<sup>=</sup>

**364**

H+

chromophore covalently bonded in the central region via a protonated Schiff base to a lysine residue (see **Figure 8**).

The most common chlorophylls are chlorophyll a, chlorophyll b, and chlorophyll c1, and chlorophyll c2. Each pigment registers a maximum signal at a particular wavelength of maximum absorption ( λmax ), this coupled with the selective scattering of light, microalgae are seen in their defined color. Consequently, in the blue region of the spectrum, λmax = 440 *nm* and in the red region of the spectrum, λmax = 675 *nm* (see **Figure 5**) [22]. As the microalgae cells are irradiated with light from a source, the photosynthetic process is initiated and propagated. The photosynthetic rate increases as the intensity of the irradiance are increased; and the level of irradiance is reached where the rate of photosynthesis attains a maximum and begins to retard (see **Figure 9**). At this stage, the cells are said to be in photoinhibition. Photoinhibition is thus a phenomenon that describes the inability of the photosynthetic organism to support photosynthesis in the presence of excess illumination from a light source [19].

The saturation irradiance ( *B m k B <sup>P</sup> <sup>I</sup>* α <sup>=</sup> ) and photo-inhibition (β) parameters indicate the half-saturation constant when the photosynthetic rate is half the maximum value of the photosynthetic rate of the system <sup>2</sup> *<sup>B</sup> Pm <sup>P</sup>* <sup>=</sup> . α<sup>B</sup> is the measure of photosynthetic efficiency of solar energy conversion into chemical energy, and it takes into account that the light absorbed by the algal cell is

**Figure 8.** *Photo-isomerization of all-trans to 13-cis retinal in bR [21].*

**Figure 9.** *Photosynthesis – Irradiance curve.*

proportional to the functional absorption cross-section of the effective area that PS II presents to an incoming photon. *<sup>B</sup> Pm* is the assimilation number which is the maximum photosynthetic rate [22].
