A and B are substrate molecules or reactants; AB represents the product e.g. synthesis of lipids from fatty acids and glycerol or the formation of a polysaccharide (e.g. starch) from monosaccharide units or

Anabolic reactions are concerned with building up structures, e.g. cell walls, storage compounds (e.g. glycogen) and complex metabolites within the cell. Starch, glycogen, fats and proteins are all products of anabolic pathways.

Breakdown reactions are those in which a complex compound is split into simpler molecules:

In this case AB is the substrate while A and B are products.

e.g.

Energy is required for three main purposes:

1. To drive the cell’s anabolic reactions, i.e. synthesis of proteins, storage compounds etc;
2. For work: e.g. muscle contraction, transmission of nerve impulses, secretion by glands;
3. Maintenance: preservation of a constant internal environment and of the tissues and organs in a state of health and functional efficiency.

Energy is therefore of vital importance to an organism and much of metabolism is concerned with its production.

Note that this (and any similar equation) is a representation, a summary of the process.

[Remember the first law of thermodynamics: energy cannot be created or destroyed but may be converted from one form to another.]

The major gases in the atmosphere and their approximate concentrations are:

N₂ - 78%

O₂ - 20%

CO₂ - 0.03%

The CO₂ and O₂ ratios remain almost constant in spite of their depletion (CO₂) and enhancement (O₂) by photosynthesis because plants and animals also replenish the CO₂ and use up O₂ during respiration.

The sun (our sun, Sol) is a huge nuclear reactor liberating a lot of energy into space (in all three dimensions). Plants use v. v. very little of the sun’s energy that reaches them. The photosynthetic efficiency of plants has been estimated at about 0.2%:

plant biomass in energy terms

Following are some of the experiments and observations that led to the elucidation of the process:

Early 17^th C: Flemish doctor van Helmont grew a tree in a bucket of soil feeding the soil with rain water only. After 5 years the tree had grown significantly though the amount of soil had not diminished significantly. van Helmont concluded that the material of the tree came from water.

A century later, Stephen Hales observed that plants used mainly air as the nutrient during their growth.

Experiments of Joseph Priestly, between 1771 and 1777. Priestly burnt a candle in an enclosed volume of air and showed that the resultant air could no longer support combustion. A mouse kept in the residual air died. A green sprig of mint however, continued to live in the residual air for weeks. At the end of this time, a candle could be reactivated in the air and a mouse could breathe in it. We know now that the candle used up the oxygen which was replenished by the green plant.

There then followed the discovery that plants gave off (evolved) O₂ only in sunlight and that only the green parts of the plants carried out this process. (It was also shown that plants made ‘bad air’ in darkness and is said that physicians recommended that plants be removed from houses during the night to avoid the possibility of poisoning the occupants.)

1782 - Jean Senebier observed that plants used CO₂ as nourishment.

Early in the 19^th C it was discovered that water was consumed by plants during assimilation of CO₂.

In 1817, French chemists isolated the green substance in leaves and named it chlorophyll.

In 1845, Mayer realised that plants transformed the energy of sunlight into chemical energy.

By the middle of the last (19^th) century, the phenomenon of photosynthesis could be represented by the following relationship:

1864 - French plant physiologist found that the ratio of the volume of O₂ evolved to the volume of CO₂ consumed (the photosynthetic ratio) was almost unity (1).

1864 - Sachs (German botanist) demonstrated formation of starch grains during photosynthesis. He kept some green leaves in the dark for some hours to deplete them of starch. Then exposed one half to light and left other half in dark. Iodine test eventually showed presence of starch only in the illuminated part of the leaf.

1880 - Engelmann discovered that chlorophyll absorbed red and blue light during photosynthesis, hence chlorophyll was the photoreceptive pigment for photosynthesis.

At the beginning of this century, the knowledge of photosynthesis could be summarised:

The important features of a dicotyledonous leaf:

1. Thin and flat, collectively presenting a large surface area to the light. Thinness minimises the distance across which diffusion of CO₂ has to take place to reach site of reaction.
2. Covered by a thin layer of cuticularised epidermal cells, the (waterproof, transparent) cuticle being generally thicker on the upper than on the lower surface.
3. Inside the leaf is packed with cells containing chloroplasts. These cells are of two types. Those immediately below the upper epidermis are the palisade mesophyll cells, elongated with their long axes at right angles to the leaf surface. They are separated from each other by air spaces and are densely packed with chloroplasts. Between the palisade cells and the abaxial (lower) epidermal layer are the spongy mesophyll cells. Air spaces allow uninterrupted diffusion of gases between palisade cells and the atmosphere.
4. Numerous stomata - generally more numerous on lower surface. bordered by guard cells which can open or close the pore, these regulate the passage of CO₂, O₂ and H₂O.
5. Midrib and network of small veins.

(Note relationship between structure and function)

The leaf is the major photosynthetic organ in higher plants. Leaves contain cells that contain chloroplasts. With very few exceptions, chlorophyll is contained within chloroplasts. These are shown by electron microscopy to be approximately 4 to 10 µm in diameter and 1 µm in thickness in higher plants, but variable in size and shape. Chloroplasts arise from tiny proplastids (immature, nearly colourless plastids with few or no internal membranes) which are derived from unfertilised egg cells. These divide as the embryo develops and develop into chloroplasts as stems and leaves are formed. Young chloroplasts are also actively dividing, especially when the organ containing them is exposed to light.

Internally the chloroplast is composed of a system of lamellae or flattened thylakoids that are arranged on stacks in certain regions known as grana. Each lamella may contain two double layer membranes. The grana are embedded in a colourless matrix called the stroma and the whole chloroplast is bounded by a double membrane, the chloroplast envelope. Within a chloroplast, the grana are interconnected by a system of loosely arranged membranes called the stroma (or intergranal) lamellae. Chloroplasts also contain DNA - a circular molecule that codes for about 100 proteins - and RNA that produce 70S ribosomes (like those in bacteria).

The chlorophyll molecules are located on the membrane of the lamellae. The lamellae hold the chlorophyll molecules in a suitable position for trapping the maximum amount of light, a function that it achieves effectively. A chloroplast may contain approximately 60 grana, each consisting of about 50 stacked lamellae (economy of space). The lamellae function like shelves stacked on top each other and the chlorophyll molecules are on those shelves. This provides a large surface area in a relatively small space. Many proteins and enzymes associated with photosynthesis are also embedded in the lamellae.

The stroma contains, among other things, the enzymes responsible for the reduction of CO₂ in addition to numerous starch granules. From this, it seems that while absorption of light takes place in the lamellae, the subsequent building up of carbohydrates takes place in the stroma.