Plant structure and photosynthesis

Two factors that make plants as living organisms different from animals are their structure and their method of obtaining the energy to “power” their life processes. Structurally most plants are based largely on cellulose, a natural polymer made up of atoms of carbon, hydrogen, and oxygen (cellulose is a complex carbohydrate). Plants derive their energy from sunlight, which they use in the process called photosynthesis to bring about the chemical combination of carbon dioxide (from the air) and water (from the soil) to make simple sugars. In addition, for healthy growth plants need various other elements—particularly nitrogen and phosphorus—which they must also obtain from the soil. Transport of the various elements and compounds through the tissues of the plant is carried out by water, and so water plays a key role both in photosynthesis and in running the biochemical factory of which every plant consists. Water is the chief constituent of the fluid within the plant cells that make up the various tissues and structures of a plant.

All green plants, from the mosses on the fallen log to the lush jungle vegetation and the huge buttress-rooted tree, build their structures from chemical substances derived ultimately from the products of photosynthesis. Thus sunlight is the source of energy for the growth of all green plants.

Cell structure

In 1665, when the British scientist Robert Hooke first turned his simple microscope onto a thin slice of cork (the outermost layer of a woody plant stem), he described what he saw as resembling a collection of “little boxes.” Later he coined the word “cell” for the units of plant tissue (later to be applied to animal tissue as well).

Plant cells are “packets” of living protoplasm surrounded by a dead cell wall composed mostly of cellulose, a carbohydrate manufactured by a plant cell’s cytoplasm. Cellulose fibers give strength to plant stems, roots, and leaves. It is the substance that makes plant stems stiff.

The cell wall surrounds a central mass of living cytoplasm, which in turn encloses a large vacuole containing cell sap, a mixture of various chemicals in a watery solution. The cytoplasm, which is bounded by a thin cell membrane, is not uniform in appearance or texture but contains various structures called organelles. These include mitochondria, the cell’s energy-producers; chloroplasts, which conduct the process of photosynthesis; and lyso-somes, which contain digestive enzymes to break down any substances that enter the cell. Also in the cytoplasm is the many-folded endoplasmic reticulum, which produces fats and proteins, and the nucleus, the center for nucleic acids and the genetic material of the cell.

The cells of a plant begin as small units produced at growing points, called meristems. Those at the growing tips of shoots or roots are known as apical meristems; those in the body of a plant include cambium, which produces vascular tissue, and cork cambium, which produces the outer tissue called cork.

Plant tissues

As cells grow they differentiate into the various tissues of the plant. For instance the outermost covering of cells known as the epidermis forms a continuous layer around the plant. The characteristics of epidermal cells depend on their position on the plant, but most of them are flat, platelike cells. Modified versions include the guard cells of stomata (the “breathing holes” in a leaf) and the fine hairs on roots and rootlets. Epidermal cells secrete a waxy cuticle onto their surface and have cell walls containing a great deal of a fatty water-repellent substance called cutin, both of which help to protect the plant.

In older stems and roots the epidermis is frequently replaced by the cork, which is better at withstanding damage. It contains three different kinds of cells: the cork cambium (phellogen), a typical meristematic tissue that divides to produce cork cells (phellem) to the outside and cortical cells (phelloderm) to the inside. The cork cells include a layer of water-repellent suberin.

The water-conducting tissues (the xylem) are long tubes running the length of the plant and form part of the vascular bundle in angio-sperms. There are two types—the tracheids and the vessels—with walls that characteristically show spiral, annular, scalariform (ladderlike), or reticulate (netlike) thickening. Tracheids are long narrow cells about .5 to 3 inches (1 to 7.5 centimeters) long and 0.04 to 0.06 millimeter across, which join at their ends with a very slanting wall. Vessel cells are small but some of the walls between them break down to form a long tube; those walls that remain are perforated by pores. The vessels may be as narrow as only 0.006 millimeter across or up to 0.7 millimeter wide, but in trees they may be as long as 16 feet (5 meters).

The conducting tissues—the xylem and phloem—are produced by a layer of meristematic tissue called the cambium. This vascular tissue is formed primarily in discrete bundles, but the cambium may extend to form a complete ring of tissue within the plant stem. Cells produced by the cambium differentiate into xylem on the inner side and phloem on the outer side. Both occur throughout the plant.

Xylem’s main function concerns the movement of water around the plant (by the tracheids and vessels), although it is also a supporting tissue (by means of sclereid and fiber cells) and has some storage functions (using its parenchyma cells). Phloem is concerned with the movement and storage of food, and it also acts as a supporting tissue. Food movement is achieved by sieve tubes. The surrounding tissue, the parenchyma, is again the food store, and phloem fibers are the supporting elements.

Parenchyma consists of living cells capable of growth and division. They have various shapes from stellate (star-shaped in cross section) to the more common polyhedral conformation. Some have thickened walls, and some may be specialized for secretion or excretion. Their general functions are in photosynthesis, storage, and repair. The type specialized as a supporting tissue—with unevenly thickened walls—is called collenchyma. It occurs as strands or bundles in stems, leafstalks, and leaf veins.

Sclereids and fibers occurring as mechanical tissue in both xylem and phloem are examples of sclerenchyma. To fulfill their function as support and strengthening cells they have thickened and often lignified walls. The two types vary in shape: fibers are long and thin, whereas sclereids may vary between polyhedral and elongated and are sometimes branched.

Water conduction

Most herbaceous plants contain from 70 to 95 per cent water, and some algae may contain as much as 98 per cent. The water has a number of functions. One is to act as a hydrostatic skeleton that helps to support the plant (most plants deprived of water soon wilt). Another role of water is as a medium in which biochemical processes take place. But one of its most important functions is to transport various materials—dissolved gases, minerals, and nutrients—around the plant’s tissues.

The water-conducting vessels, consisting of joined-up long narrow cells, can be clearly seen in the scanning electron micrograph of a piece of wood. Xylem vessels may have various structures, with the walls thickened in a characteristic way. Common forms, shown here in longitudinal section, are spiral (A), annular (B), and the ladderlike scalariform (C). Like the blood vessels in an animal, the vessels become increasingly finer toward a plant’s extremities, until eventually they form the threadlike veins of the leaves.

The continual movement of water through a plant is known as the transpiration stream. Water from the soil enters root hairs by osmosis (that is, it crosses the semipermeable membranes surrounding the cells) and thus dilutes the cell contents. This causes water to flow through the membranes into the next cell, and an osmotic chain is set up through the cells to the water-conducting tissues. A similar chain is set up in the leaves. Cells surrounding the air spaces lose water by evaporation, which concentrates the cell sap. Water from surrounding cells passes in, establishing a chain back to the water-conducting tissues.

The transpiration stream therefore comprises the movement of water into and through the roots, along the xylem vessels, and through the leaf cells to be evaporated or transpired through the stomata of the leaves.

The water in the conducting tissues passes up the plant partly because of the osmotic “root pressure/’ partly because of the capillary effect of the narrow tubes, and partly because of the osmotic “suction pressure” from the leaves.

The rate of movement in the conducting tissues varies, but rates of 10 inches and 13 feet (25 and 400 centimeters) per hour have been recorded. The amount of water loss per plant also varies. A corn plant (Zea mays), for example, may lose nearly a gallon (3.8 liters) of water per day, or approximately 325,000 gallons of water per acre (3,040,000 liters per hectare) in a growing season.

Photosynthesis

Apart from a few bacteria, plants are the only living organisms that are capable of manufacturing their own food. It is the energy of sunlight that powers this process, which is appropriately called photosynthesis. Animals cannot make their own food but rely—-directly or indirectly—on plants to produce it.

Photosynthesis takes place in two stages. The overall outcome of the light reaction is the splitting of water molecules—using the energy of sunlight absorbed by chlorophyll—to release oxygen and generate energy-rich compounds such as ATP. A cycle of reactions (the Calvin cycle) in the dark reaction combines carbon dioxide with RUBP, hydrogen from NADPH2, to form the sugar glucose.

This is the fundamental difference between plants and animals. All other differences follow on. Since an animal cannot make its own food it must go out and find it—hence it must be mobile. Animals must be able to recognize food when they meet it—hence the sensory and nervous systems that plants lack. Movement entails cells that are flexible—not stiffened with strong walls of cellulose, as plant cells are. Since animals eat plants, or eat other animals that eat plants, it can be said that nearly all the biosphere derives its energy directly from the sun. The only exceptions are in volcanic areas in the dark ocean depths, where the energy to power life is obtained from the erupting chemicals.

The closely packed layers called thyiakoids, or granae, are the sites within chloroplasts in which the light reaction of photosynthesis takes place.

The raw materials for photosynthesis are carbon dioxide from the air (taken in through the leaves) and water, usually from the soil (taken in through the roots). The two combine initially to produce simple sugars and oxygen. Sunlight is the energy source, and the green pigment chlorophyll is the means whereby the sunlight can be used. In biochemical terms, the whole process is the reduction of carbon dioxide (to simple sugars) by hydrogen obtained from the breakdown of water mediated by chlorophyll. Photosynthesis takes place as a large number of interrelated stages, but can be summed up overall by the simple equation:

6CO2 + 12H2O + (light + chlorophyll)

-> C6H12O6 + 6O2 + 6H2O

The green color of plants is due to the presence of pigments in the chloroplasts. Chief of these pigments is chlorophyll a, which is found in all green plants and converts light energy in the red and blue wavelengths into chemical form. Most plants also contain other pigments—chlorophyll b, xanthophyll, carotene, and pheophytin—each of which absorbs light of different wavelengths. Light energy in the presence of chlorophyll splits the water molecule. This releases hydrogen (H), which combines with the hydrogen-carrier (NADP), converting it to NADPH2. The energy released (electrons) is transferred to the phosphorus (P) carrier, converting ADP (adenosine diphosphate) to ATP (adenosine triphosphate). The oxygen split from water is released to the atmosphere through the stomates. This entire process, which releases energy stored in ATP and the transfer of hydrogen to NADPH2, is called the “light reaction.”

The next state, the “dark reaction,” is so called because it does not require light. This cyclic process, called the Calvin cycle, combines the energy of ATP with NADPH2 and carbon dioxide to form a simple carbohydrate (glucose). The cycle begins when a carbon dioxide (C02) molecule combines with a molecule of 5-carbon sugar (RUBP-ribulose bisphosphate), which then splits into two molecules of a 3-carbon compound, PGA (phos-phoglyceric acid). These two 3-carbon PGA molecules combine with the transfer of hydrogen (H) from NADPH2 and the transfer of the electron energy from ATP to form the simple 6-carbon sugar, glucose.

In many tropical grasses and plants that grow in arid areas the cycle is modified so that the first photosynthetic product is not a 3-carbon molecule but a 4-carbon one (a dicarbox-ylic acid). This alternative method, the so-called C4 pathway, is more efficient than the C3 pathway.

The rate at which photosynthesis takes place is affected by various external factors. The most obvious of these is the availability of light. At low light levels the rate of photosynthesis is proportional to the light intensity. At higher light levels, a point of light saturation is reached beyond which any increase in light does not increase the rate of photosynthesis. Then other factors become limiting. One of these limiting factors is the carbon dioxide concentration of the air; an increase in carbon dioxide speeds up photosynthesis. Temperature is a limiting factor only to the dark reaction, because it is a chemical process (the light reaction is photochemical, and therefore largely independent of temperature). Water is a limiting factor in that lack of it causes the stomata of the leaves to close, thereby preventing carbon dioxide from entering.

Plant respiration

Photosynthesis effectively traps the energy of sunlight and stores it (as food materials) for future use. The stored energy is released in the process of respiration, which chemically is an oxidation reaction in which sugars are converted to carbon dioxide and water, with the release of energy. Like photosynthesis, respiration is a stepwise process that can be summed up by the simple equation:

C6H12O2 + 6O2 -enzymes-> 6CO2 + 6H2O + energy

which is the exact opposite of the photosynthesis equation. At a certain light level, the rates of photosynthesis and respiration are in balance. This is known as the light compensation point.

Transpiration—the loss of water vapor through the stomata located primarily on the underside of a leaf-acts rather like a suction pump that draws water up a plant’s xylem vessels from the roots to the leaves. The tension created is sufficient to transport water to the top of the tallest tree. Stomata (shown close-up, below left) are pores, each one edged with two guard cells that enable them to open and close, depending on external conditions such as humidity and temperature.

Respiration takes place in the cytoplasm of the cell, principally in the mitochondria. The major sequence of reactions follows a cyclic path known as the Krebs cycle (or citric acid cycle), which is preceded by a shorter metabolic pathway called glycolysis. Using glucose as an example, the first stages involve the splitting of the 6-carbon glucose molecule (by hydrolysis) in a series of reactions to produce pyruvic acid. Next, carbon dioxide is lost from the two 3-carbon pyruvic acid molecules, and the resultant 2-carbon acetyl group combines with a carrier enzyme to form acetyl coenzyme A.-Then at the start of the Krebs cycle the acetyl group is transferred to a 4-carbon molecule (oxaloacetic acid) to produce the 6-carbon citric acid. Finally this acid is broken down in a series of steps to regenerate the 4-carbon molecule, oxaloacetic acid. Some steps involve the release of carbon dioxide and hydrogen, which is passed via a hydrogen acceptor such as NAD. Energy is released during hydrogen transfer, and some of it converts ADP into ATP.

Some organisms such as yeasts and bacteria can derive their energy in the absence of oxygen, by anaerobic respiration or fermentation, from glycolysis. The route is pyruvic acid to carbon dioxide and acetaldehyde, which is converted to ethyl alcohol (as in winemaking), which in turn may be oxidized to acetic acid. Only a small amount of energy is released, and some of the products are toxic. When oxygen is in short supply—for example, in waterlogged roots—plants may be forced to use anaerobic respiration. Some have developed systems to convert the poisonous product alcohol into less toxic substances (such as organic acids) so that they can employ anaerobic respiration to get through difficult times.

A mitochondrion is the microscopic body most concerned with cell respiration, as explained in the diagram below.

Synthesis of complex compounds

The energy released by respiration is used by plants for various purposes, including the build-up of complex compounds such as carbohydrates, fats, and proteins and to power energy-consuming processes that take place within the cells. A simple sugar, such as glucose, is a basic end product of photosynthesis and is the starting material for a large number of different chemicals. Chemically, glucose is a monosaccharide whose molecules contain six carbon atoms, five of which together with an oxygen atom are arranged in a ring—it is termed a hexose. Each carbon has a hydrogen atom and a hydroxyl group attached to it. It is the replacement of the attached groups or one of the carbon atoms that alters the chemistry of the molecule and creates new compounds. Other important plant monosaccharides include 3-carbon (triose) and 5-carbon (pentose) sugars.

When two hexose molecules join together they form a disaccharide such as sucrose (table sugar), which is formed and stored in plants such as sugar beet and sugar cane. When large numbers of hexose molecules join together, a polysaccharide results. Polysaccharides may be structural, such as the natural polymer—and valuable raw material—cellulose, formed from glucose units. Other structural polysaccharides include mannan from yeast (mannose units), xylan (xylose units), and pectin, materials that form cell walls.

Cell respiration is a complex process that begins with glycolysis, the breakdown of glucose. The 2-car-bon compound acetyl coenzyme A is produced, which then initiates a series of reactions within a mitochondrion. In this series, called the Krebs cycle, more carbon dioxide is released and energy stored in the form of ATP (adenosine triphosphate), 12 molecules of which are produced during each cycle.

Other groups of polysaccharides have nutrient functions, acting as food stores in plants. Among the most important are starches and inulins. Starch in a storage organ such as a potato tuber, for example, occurs as grains containing two different molecules, amylose and amylopectin. Amylose consists of long chains of glucose units, whereas amylopectin has a very branched structure. Inulin is made up mainly of fructose units, with some glucose.

Many seeds have fats and oils as storage material rather than carbohydrates such as saccharides and polysaccharides. The fats are esters of a high molecular weight fatty acid (such as stearic or oleic acid) and an alcohol (usually glycerol). Many plant oils are of economic importance: examples include the oils from olives and groundnuts.

The living parts of cell protoplasm consist of proteins, rather complex molecules containing nitrogen and built up from amino acids. There are 20 naturally occurring amino acids in proteins, and any given protein depends on the number and arrangement of some of these. Important proteins include enzymes, the biological catalysts that bring about and control most of the chemical processes in the cells; some work only in conjunction with a (nonprotein) coenzyme. There are various types of enzymes. Carbohydrases, for example, break down carbohydrates; transferase enzymes transfer various groups—for instance, carboxylases add or subtract carbon dioxide. The synthesis of enzymes and other proteins, and the passing on of hereditary information, is controlled by nucleic acids. There are two kinds found in the nuclei of cells: the complex DNA (deoxyribonucleic acid) and the simpler RNA (ribonucleic acid).

Secondary plant products

Sugar (sucrose) is a carbohydrate made and stored by plants, from which it is extracted for human consumption. In temperate regions farmers grow sugar beets, seen being harvested; sugar cane is the equivalent crop in tropical areas.

In addition to carbohydrates, fats, and proteins, there are several other classes of chemicals produced by plants, many of commercial value. The nitrogen-containing alkaloids, for instance, are toxic compounds used as drugs. Their function in plants is probably to deter grazing animals. Most occur in members of three families: the poppies (Papaveraceae), buttercups (Ranunculaceae), and the nightshade family (Solanaceae). Examples of alkaloids include atropine, colchicine, morphine, nicotine, opium, and strychnine.

Tannins are a group of substances frequently found in the vacuoles of cells. Most are polymers of carbohydrates and phenolic acids and have a bitter, astringent taste. Their function, like alkaloids, may be to deter herbivores. Cultivated plants containing them include tea, coffee, and bilberries.

Terpenes are aromatic chemicals that are important constituents of essential oils. Together with aldehydes, ketones, and alcohols they form the scents of plants and give rise to resins and balsams. They are often produced in response to an injury from special cells and have a protective function.

Belladonna (Atropa belladonna) is the source of the alkaloid atropine. Like other such compounds it is highly poisonous and is prescribed as a drug in carefully controlled doses.