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Chapter 10

The photosynthetic process

10.1 Introduction Photosynthesis is the making (synthesis) of organic structures and chemical energy stores by the action of solar radiation (photo). It is by far the most important renewable energy process, because living organisms are made from material fixed by photosynthesis, and our activities rely on oxygen in which the solar energy is mostly stored. For instance, human metabolism continuously releases about 150 W per person from food. Thus, both the materials and the energy for all life are made available in gases circulating in the Earth's atmosphere, namely carbon dioxide and oxygen. Sadly, despite photosynthesis being a physically induced process and the driving function of natural engineering, the subject is missing from most physics and engineering texts. This chapter tries to rectify this omission by describing a cheap process that provides abundant stored energy ? an engineer's dream, but a natural phenomenon.

The continuous photosynthetic output flux on the Earth is about 0 9 ? 1014 W (i.e. about 15 kW per person; the power output of 100 000 large nuclear power stations). This chapter discovers how the process occurs within molecules and cells, and how eventually it may be utilised at these levels. Energy supply from plant and animal materials, biomass, is discussed in Chapter 11. Solar radiation incident on green plants and other photosynthetic organisms relates to two main effects: (1) temperature control for chemical reactions to proceed, especially in leaves, and (2) photo excitation of electrons for the production of oxygen and carbon structural material. It is so important to maintain leaf temperature in the correct range that some solar radiation is reflected or transmitted, rather than absorbed (this is why leaves are seldom black). The energy processes in photosynthesis depend on the photons (energy packets) of the solar radiation, labelled `h ', where h is Planck's constant and is the frequency of the radiation. The organic material produced is mainly carbohydrate, with carbon in a medium position of oxidation and reduction (e.g. glucose, C6H12O6). If this (dry) material is burnt in oxygen, the heat released is about 16 MJ kg-1 (4.8 eV per carbon atom, 460 kJ per mole of carbon). The fixation of one carbon atom from

10.1 Introduction 325

atmospheric CO2 to carbohydrate proceeds by a series of stages in green plants, including algae:

1 Reactions in light, in which photons produce protons from H2O, with O2 as an important by-product, and electrons are excited in two stages to produce strong reducing chemicals.

2 Reactions not requiring light (called dark reactions), in which these reducing chemicals reduce CO2 to carbohydrates, proteins and fats.

Combining both the light and the dark reactions gives an overall reaction, neglecting many intermediate steps:

CO2 + 2H2O --l-igh- t O 2 + CH2O + H2O

(10.1)

where the products have about 4.8 eV per carbon atom more enthalpy (energy production potential) than the initial material because of the absorption of at least eight photons. Here CH2O represents a basic unit of carbohydrate, so the reaction for sucrose production is

12CO2 + 24H2O --l-igh- t 12O 2 + C12H22O11 + 13H2O

(10.2)

In these equations, the oxygen atoms initially in CO2 and H2O are distinguished, the latter being shown with a dots over the O.

Most studies of photosynthesis depend on biochemical analysis considering the many complex chemical processes involved. This chapter, however, will emphasise the physical processes, and will relate to the branch of spectroscopy called photophysics. There will also be interesting similarities and comparisons with photovoltaic devices (Chapter 7). We shall proceed in three stages:

1 The trophic level (Figure 10.1) 2 The plant level (Figure 10.2) 3 The molecular level (Figure 10.3): this is a complex system, which will

be studied in Section 10.6.

There is extensive variety in all aspects of photosynthesis, from the scale of plants down to molecular level. It must not be assumed that any one system is as straightforward as described in this chapter, which concentrates on the general physical principles. However, the end result is that energy from the Sun is stored in stable chemicals for later use ? a principle goal of renewable energy technology, yet happening all around us.

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Figure 10.1 Trophic level global photosynthesis, also requiring water. Fluxes: energy, 1014 W; carbon, 1011 t/y; CO2 4 ? 1011 t/y; oxygen, 3 ? 1011 t/y; water (as reactant) 3 ? 1011 t/y. Atmospheric concentrations: oxygen, 21%;

CO2, 0.030% by volume (pre-industrial, in 1850) but increasing due to anthropogenic activity (was 0.037% by year 2000, now increasing at a proportional rate of 0 4%/y).

Solar radiation

. O2

Leaf

Light Dark reaction reaction

CO2

Chemical exchange

. H2O

O2

Roots

CO2

Nutrients

. H2O

Figure 10.2 Plant level photosynthesis.

10.2 Trophic level photosynthesis

Animals exist by obtaining energy and materials directly or indirectly from plants. This is called the trophic (feeding) system. Figure 10.1 is an extremely simplified diagram to emphasise the essential processes of natural ecology. We should remember, however, that the box labelled `animals' might also include the human fossil fuel?based activities of industry, transport, heating, etc.

10.2 Trophic level photosynthesis 327

(a)

Solar radiation photons

Pigment molecules

Photosystem 1 Electron acceptor

A1

NADPH Reducing agent NADP

Electrons e?

Photosystem 1 Electron donor D1

Photosystem 2 Electron acceptor

A2

e?

ATP Energy store ADP

Electrons e?

Pigment molecules

Photosystem 2 Electron donor D2

.

.

H2O

O2

Environment

Protons H+

(b) NADPH

Reducing agent NADP

Calvin cycle

C6H12O6 Carbon fixation

CO2

Figure 10.3 Molecular level photosynthesis. Vertical scale indicates the excitation energy of the electron. (a) Light reaction, indicating the flow of energy and materials in the two interacting photosystems of green plants. (b) Dark reaction, using the reducing agent produced from the light reaction of photosystem 1.

During photosynthesis CO2 and H2O are absorbed to form carbohydrates, proteins and fats. The generalised symbol CH2O is used to indicate the basic building block for these products. CO2 is released during respiration of both plants and animals, and by the combustion of biological

material. This simplified explanation is satisfactory for energy studies, but

neglects the essential roles of nitrogen, nutrients and environmental param-

eters in the processes.

328 The photosynthetic process

The net energy absorbed from solar radiation during photosynthesis can be measured from combustion, since

H + CO2 + 2H2O --p--ho--to-s-y-n-th-e-s-is- CH2O + O2 + H2O combustion

H = 460 kJ per mole C = 4 8 eV per atom C 16 MJ kg-1of dry carbohydrate material

(10.3)

Here H is the enthalpy change of the combustion process, equal to the

energy absorbed from the photons of solar radiation in photosynthesis,

less the energy of respiration during growth and losses during precursor

reactions. H may be considered as the heat of combustion. Note that combustion requires temperatures of 400 C, whereas respiration proceeds by catalytic enzyme reactions at 20 C. The uptake of CO2 by a plant leaf is a function of many factors, especially temperature, CO2 concentration and the intensity and wavelength distributions of light (Figure 10.4).

Photosynthesis can occur by reducing CO2 in reactions with compounds other than water. In general these reactions are of the form

CO2 + 2H2X CH2O + X2 + H2O

(10.4)

For example, X = S, relating to certain photosynthetic bacteria that grow in the absence of oxygen by such mechanisms, as was the dominant process on Earth before the present `oxygen-rich' atmosphere was formed.

The efficiency of photosynthesis is defined for a wide range of circumstances. It is the ratio of the net enthalpy gain of the biomass per unit area H/A to the incident solar energy per unit area E/A , during the particular biomass growth over some specified period:

= H/A E/A

(10.5)

Here A may range from the surface area of the Earth (including deserts) to the land area of a forest, the area of a field of grain, and the exposed or total surface area of a leaf. Periods range from several years to minutes, and conditions may be natural or laboratory controlled. It is particularly important with crops to determine whether quoted growth refers to just the growing season or a whole year. Table 10.1 gives values of for different conditions.

The quantities involved in a trophic level description of photosynthesis can be appreciated from the following example. Healthy green leaves in sunlight produce about 3 litres of O2 per hour per kg of leaf (wet basis). This is an energy flow of 16 W, and would be obtained from an exposed

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