Plant Nutrition

Photosynthesis

  Photosynthesis, process by which green plants and certain other organisms use the energy of light to convert carbon dioxide and water into the simple sugar glucose. In so doing, photosynthesis provides the basic energy source for virtually all organisms. An extremely important byproduct of photosynthesis is oxygen, on which most organisms depend.

Photosynthesis occurs in green plants, seaweeds, algae, and certain bacteria. These organisms are veritable sugar factories, producing millions of new glucose molecules per second. Plants use much of this glucose, a carbohydrate, as an energy source to build leaves, flowers, fruits, and seeds. They also convert glucose to cellulose, the structural material used in their cell walls. Most plants produce more glucose than they use, however, and they store it in the form of starch and other carbohydrates in roots, stems, and leaves. The plants can then draw on these reserves for extra energy or building materials. Each year, photosynthesizing organisms produce about 170 billion metric tons of extra carbohydrates, about 30 metric tons for every person on earth.

Photosynthesis has far-reaching implications. Like plants, humans and other animals depend on glucose as an energy source, but they are unable to produce it on their own and must rely ultimately on the glucose produced by plants. Moreover, the oxygen humans and other animals breathe is the oxygen released during photosynthesis. Humans are also dependent on ancient products of photosynthesis, known as fossil fuels, for supplying most of our modern industrial energy. These fossil fuels, including natural gas, coal, and petroleum, are composed of a complex mix of hydrocarbons, the remains of organisms that relied on photosynthesis millions of years ago. Thus, virtually all life on earth, directly or indirectly, depends on photosynthesis as a source of food, energy, and oxygen, making it one of the most important biochemical processes known.

Site of Occurence:

Plant photosynthesis occurs in leaves and green stems within specialized cell structures called chloroplasts. One plant leaf is composed of tens of thousands of cells, and each cell contains 40 to 50 chloroplasts. The chloroplast, an oval-shaped structure, is divided by membranes into numerous disk-shaped compartments. These disklike compartments, called thylakoids, are arranged vertically in the chloroplast like a stack of plates or pancakes. A stack of thylakoids is called a granum (plural, grana); the grana lie suspended in a fluid known as stroma.

Embedded in the membranes of the thylakoids are hundreds of molecules of chlorophyll, a light-trapping pigment required for photosynthesis. Additional light-trapping pigments, enzymes (organic substances that speed up chemical reactions), and other molecules needed for photosynthesis are also located within the thylakoid membranes. The pigments and enzymes are arranged in two types of units, Photosystem I and Photosystem II. Because a chloroplast may have dozens of thylakoids, and each thylakoid may contain thousands of photosystems, each chloroplast will contain millions of pigment molecules.

III

HOW PHOTOSYNTHESIS WORKS

Photosynthesis is a very complex process, and for the sake of convenience and ease of understanding, plant biologists divide it into two stages. In the first stage, the light-dependent reaction, the chloroplast traps light energy and converts it into chemical energy contained in nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), two molecules used in the second stage of photosynthesis. In the second stage, called the light-independent reaction (formerly called the dark reaction), NADPH provides the hydrogen atoms that help form glucose, and ATP provides the energy for this and other reactions used to synthesize glucose. These two stages reflect the literal meaning of the term photosynthesis, to build with light.

Photosynthesis relies on flows of energy and electrons initiated by light energy. Electrons are minute particles that travel in a specific orbit around the nuclei of atoms and carry a small electrical charge. Light energy causes the electrons in chlorophyll and other light-trapping pigments to boost up and out of their orbit; the electrons instantly fall back into place, releasing resonance energy, or vibrating energy, as they go, all in millionths of a second. Chlorophyll and the other pigments are clustered next to one another in the photosystems, and the vibrating energy passes rapidly from one chlorophyll or pigment molecule to the next, like the transfer of energy in billiard balls.

Light contains many colors, each with a defined range of wavelengths measured in nanometers, or billionths of a meter. Certain red and blue wavelengths of light are the most effective in photosynthesis because they have exactly the right amount of energy to energize, or excite, chlorophyll electrons and boost them out of their orbits to a higher energy level. Other pigments, called accessory pigments, enhance the light-absorption capacity of the leaf by capturing a broader spectrum of blue and red wavelengths, along with yellow and orange wavelengths. None of the photosynthetic pigments absorb green light; as a result, green wavelengths are reflected, which is why plants appear green.

Photosynthesis begins when light strikes Photosystem I pigments and excites their electrons. The energy passes rapidly from molecule to molecule until it reaches a special chlorophyll molecule called P700, so named because it absorbs light in the red region of the spectrum at wavelengths of 700 nanometers.

Until this point, only energy has moved from molecule to molecule; now electrons themselves transfer between molecules. P700 uses the energy of the excited electrons to boost its own electrons to an energy level that enables an adjoining electron acceptor molecule to capture them. The electrons are then passed down a chain of carrier molecules, called an electron transport chain. The electrons are passed from one carrier molecule to another in a downhill direction, like individuals in a bucket brigade passing water from the top of a hill to the bottom. Each electron carrier is at a lower energy level than the one before it, and the result is that electrons release energy as they move down the chain. At the end of the electron transport chain lies the molecule nicotine adenine dinucleotide (NADP⁺). Using the energy released by the flow of electrons, two electrons from the electron transport chain combine with a hydrogen ion and NADP⁺ to form NADPH.

When P700 transfers its electrons to the electron acceptor, it becomes deficient in electrons. Before it can function again, it must be replenished with new electrons. Photosystem II accomplishes this task. As in Photosystem I, light energy activates electrons of the Photosystem II pigments. These pigments transfer the energy of their excited electrons to a special Photosystem II chlorophyll molecule, P680, that absorbs light best in the red region at 680 nanometers. Just as in Photosystem I, energy is transferred among pigment molecules and is then directed to the P680 chlorophyll, where the energy is used to transfer electrons from P680 to its adjoining electron acceptor molecule.

Complete Reaction: 6CO2 (CarbonDioxide) + 12H2O (Water) = C6H12O6 (Sugar) + 6O2 (Oxygen) + 6H2O (Water)

The Light Dependant Stage:

From the Photosystem II electron acceptor, the electrons are passed through a different electron transport chain. As they pass along the cascade of electron carrier molecules, the electrons give up some of their energy to fuel the production of ATP, formed by the addition of one phosphorus atom to adenosine diphosphate (ADP). Eventually, the electron transport carrier molecules deliver the Photosystem II electrons to Photosystem I, which uses them to maintain the flow of electrons to P700, thus restoring its function.

P680 in Photosystem II is now electron deficient because it has donated electrons to P700 in Photosystem I. P680 electrons are replenished by the water that has been absorbed by the plant roots and transported to the chloroplasts in the leaves. The movement of electrons in Photosystems I and II and the action of an enzyme split the water into oxygen, hydrogen ions, and electrons. The electrons from water flow to Photosystem II, replacing the electrons lost by P680. Some of the hydrogen ions may be used to produce NADPH at the end of the electron transport chain, and the oxygen from the water diffuses out of the chloroplast and is released into the atmosphere through pores in the leaf.

The transfer of electrons in a step-by-step fashion in Photosystems I and II releases energy and heat slowly, thus protecting the chloroplast and cell from a harmful temperature increase. It also provides time for the plant to form NADPH and ATP. In the words of American biochemist and Nobel laureate Albert Szent-Gyorgyi, “What drives life is thus a little electric current, set up by the sunshine.”

The Light-Independent ReactionThe chemical energy required for the light-independent reaction is supplied by the ATP and NADPH molecules produced in the light-dependent reaction. The light-independent reaction is cyclic, that is, it begins with a molecule that must be regenerated at the end of the reaction in order for the process to continue. Termed the Calvin cycle after the American chemist Melvin Calvin who discovered it, the light-independent reactions use the electrons and hydrogen ions associated with NADPH and the phosphorus associated with ATP to produce glucose. These reactions occur in the stroma, the fluid in the chloroplast surrounding the thylakoids, and each step is controlled by a different enzyme.

The light-independent reaction requires the presence of carbon dioxide molecules, which enter the plant through pores in the leaf, diffuse through the cell to the chloroplast, and disperse in the stroma. The light-independent reaction begins in the stroma when these carbon dioxide molecules link to sugar molecules called ribulose bisphosphate (RuBP) in a process known as carbon fixation.

With the help of an enzyme, six molecules of carbon dioxide bond to six molecules of RuBP to create six new molecules. Several intermediate steps, which require ATP, NADPH, and additional enzymes, rearrange the position of the carbon, hydrogen, and oxygen atoms in these six molecules, and when the reactions are complete, one new molecule of glucose has been constructed and five molecules of RuBP have been reconstructed. This process occurs repeatedly in each chloroplast as long as carbon dioxide, ATP, and NADPH are available. The thousands of glucose molecules produced in this reaction are processed by the plant to produce energy in the process known as aerobic respiration, used as structural materials, or stored. The regenerated RuBP is used to start the Calvin cycle all over again.

Bacteria lack chloroplasts, and instead use structures called chromatophores—membranes formed by numerous foldings of the plasma membrane, the membrane surrounding the fluid, or cytoplasm, that fills the cell. The chromatophores house thylakoids similar to plant thylakoids, which in some bacteria contain chlorophyll. For these bacteria, the process of photosynthesis is similar to that of plants, algae, and seaweed. Many of these chlorophyll-containing bacteria are abundant in oceans, lakes, and rivers, and the oxygen they release dissolves in the water and enables fish and other aquatic organisms to survive.A majority of plants use these steps in photosynthesis. Plants such as corn and crabgrass that have evolved in hot, dry environments, however, must overcome certain obstacles to photosynthesis. On hot days, they partially close the pores in their leaves to prevent the escape of water. With the pores only slightly open, adequate amounts of carbon dioxide cannot enter the leaf, and the Calvin cycle comes to a halt. To get around this problem, certain hot-weather plants have developed a way to keep carbon dioxide flowing to the stroma without capturing it directly from the air. They open their pores slightly, take in carbon dioxide, and transport it deep within the leaves. Here they stockpile it in a chemical form that releases the carbon dioxide slowly and steadily into the Calvin cycle. With this system, these plants can continue photosynthesis on hot days, even with their pores almost completely closed. A field of corn thus remains green on blistering days when neighboring plants wither, and crabgrass thrives in lawns browned by the summer sun.

Certain archaebacteria, members of a group of primitive bacteria-like organisms, carry out photosynthesis in a different manner. The mud-dwelling green sulfur and purple sulfur archaebacteria use hydrogen sulfide instead of water in photosynthesis. These archaebacteria release sulfur rather than oxygen, which, along with hydrogen sulfide, imparts the rotten egg smell to mudflats. Halobacteria, archaebacteria found in the salt flats of deserts, rely on the pigment bacteriorhodopsin instead of chlorophyll for photosynthesis. These archaebacteria do not carry out the complete process of photosynthesis; although they produce ATP in a process similar to the light-dependent reaction and use it for energy, they do not produce glucose. Halobacteria are among the most ancient organisms, and may have been the starting point for the evolution of photosynthesis.

While it may seem that we understand photosynthesis in detail, decades of experiments have given us only a partial understanding of this important process. A more thorough understanding of the details of photosynthesis may pave the way for development of crops that are more efficient at using the sun’s energy, producing food for increasingly bountiful harvests.

Leaf Structure

I

INTRODUCTION

Leaf, part of a plant that serves primarily as the plant's food-making organ in a process called photosynthesis. Leaves take part in other plant functions as well, including transpiration and guttation, both of which remove excess water from the plant, and respiration, the process by which a plant obtains oxygen and energy. Leaves also may store food and water and provide structural support.

A leaf is an extension of a plant's stem. Although most leaves are flat, broad, or bladelike, they also may be many other shapes, including round, oval, or feathery. In general, the leaves of trees such as hardwoods tend to be broad and relatively large, and the leaves of conifers, or cone-bearing trees, are usually small and needlelike in shape. In size, leaves range from only several millimeters (a fraction of an inch) long, as in the water plant Elodea, to 15 to 18 m (15 to 60 ft) long, as in some palm trees.

Green leaves derive their color from a green pigment called chlorophyll. The presence of additional pigments causes other leaf colors such as red in coleus and purple in cabbage. In temperate regions of the world, the leaves of some plants change color in autumn. Leaves of most garden plants turn yellow in the autumn, but those of many trees take on brilliant orange or red colors.

Most plants whose leaves change color also lose their leaves in the autumn. Such plants are called deciduous. In other plants, such as laurels and pines, the leaves do not change color and do not fall off in autumn. Such plants are called evergreens.

THE PARTS OF A TYPICAL LEAF

Leaf External Structure (specular parts)

Leaf internal structure (microscopic)

The typical green leaf is called a foliage leaf. It usually consists of two basic parts: a petiole and a blade.

The petiole is a stalklike structure that supports the leaf blade on the stem. It also serves as a passageway between the stem and the blade for water and nutrients. Another function of the petiole is to move the leaf into the best position for receiving sunlight. Most petioles are long, narrow, and cylindrical.

Many plants, such as grasses and corn, do not have petioles. In these plants the base of the blade is attached directly to the stem—the base encircles the stem as a sheath. Such leaves are called sessile leaves.

The leaf blade is usually a thin, flat structure. Its margins, or edges, may be smooth, as in the dogwood; jagged or toothed, as in the elm; or lobed, as in the oak and maple. The surface of the blade may be smooth, fuzzy, sticky, dull, or shiny. In most plants the leaves have a single blade and are referred to as simple. In other plants, such as clover, the blade is divided into separate leaflets. This kind of leaf is called a compound leaf. Most of the functions carried on by leaves take place in the blade.

A

Epidermis

The blade consists of an upper and lower epidermis and a spongy layer of tissue, called the mesophyll. Running through the mesophyll is a branching system of veins.

The epidermis is the leaf blade's skin. It is a thin, usually transparent, colorless layer of cells that covers both the upper and lower surfaces of the blade. The epidermis prevents the leaf from losing excessive amounts of water and protects it against injury.

In most plants the epidermis is covered with cutin, a waxy substance secreted by the epidermal cells. The layer of cutin, called the cuticle, is responsible for the glossy appearance of some leaves. The cuticle gives the leaf additional protection by slowing down the rate at which water is lost. Generally, the cuticle is thinner on the epidermis on the underside of the leaf than on the upper epidermis, which is exposed to the sun.

In many kinds of leaves, hairs grow from the epidermis. The soft hairs of plants such as the mullein give the leaves a woolly or feltlike texture. In some plants the epidermal hairs secrete fluids. For example, in geraniums and petunias the hairs secrete a fluid that gives the leaves a clammy texture. The strong-smelling oils of the peppermint and spearmint plants come from epidermal hairs. In other plants, such as the nettle, the epidermal hairs are stiff and contain a poisonous fluid that produces a skin irritation when a person is pricked by them.

B

Guard Cells

Scattered throughout the epidermis are pairs of bean-shaped cells, called guard cells. Guard cells contain chloroplasts, which are tiny granules filled with the green pigment chlorophyll. Chlorophyll gives leaves their characteristic green color. Chloroplasts enable leaves to carry on photosynthesis because they are able to absorb carbon dioxide and sunlight, which are required for the food-making process. In response to heat and light, each pair of guard cells pulls apart, and a tiny pore forms between them. The pores, called stomata, open to the outside atmosphere.

When the stomata are open, carbon dioxide and oxygen pass either in or out—when carbon dioxide enters, it takes part in photosynthesis, the food-making process that releases oxygen as a waste product. This oxygen passes out of the leaf. At the same time, oxygen also enters the leaf, where it takes part in respiration, a process that forms carbon dioxide as a waste product. This carbon dioxide passes out through the stomata. Water also passes out of the open stomata in the form of a vapor. This process is called transpiration. Generally, there are more stomata on the under surface of a leaf than on the upper surface. This prevents water from evaporating too quickly or in excessive amounts from the leaf's upper side, which is exposed to the sun. Stomata close at night, providing another level of water conservation.

C

Water Pores

In addition to the stomata, many kinds of leaves have large specialized water pores in their epidermis. These pores, called hydathodes, permit guttation, the process by which a plant loses liquid water. Unlike the stomata, hydathodes remain open all the time.

Guttation takes place only when water is being rapidly absorbed by the roots, such as after a heavy rainfall, and when transpiration slows down, as on cool, humid nights. When these conditions occur together, droplets of water can be seen on the leaf early in the morning before they evaporate in the heat of the day. Unlike dew, which condenses on leaves from water vapor in the air and covers the entire leaf surface, guttation droplets form only on the edges and tips of leaves. Generally, the droplets are noticeable only on the leaves of strawberries and a few other kinds of plants.

D

Mesophyll

The mesophyll, sandwiched between the upper and lower epidermis, consists of many thin-walled cells that are usually arranged in two layers. The palisade layer is next to the upper epidermis. It consists of cylindrical cells that are packed closely together. Next to the palisade layer and making up most of the thickness of the leaf blade is the spongy layer. The spongy layer consists of roundish cells that are packed loosely together and have numerous air spaces between them. In most plants the spongy layer extends down to the lower epidermis. However, in certain grasses, irises, and other plants whose leaves grow straight up and down, the spongy layer is wedged between two palisade layers of mesophyll. Like the guard cells, all the cells of the mesophyll contain chloroplasts.

E

Veins

Running through the middle of the mesophyll and branching out to all of its cells are veins. The veins extend into the petiole and connect with other veins in the stem of the plant. A major function of the veins is to help support the leaf blade. Each type of plant has a characteristic pattern of veins forming lines and ridges in the blade.

The veins of a leaf are made up of two specialized tissues, xylem and phloem. Xylem usually forms the upper half of the vein. It consists of tubular open-ended cells that are arranged end to end. The walls of the cells are thick and rigid. Xylem conducts water and dissolved minerals to the leaf blade from the rest of the plant.

Phloem lies on the underside of the vein. It is made up of thin-walled tubular cells with tiny openings at their ends, somewhat like a sieve. These cells are also arranged end to end. Phloem carries food manufactured in the blade to the rest of the plant.

III

LEAF GROWTH AND LEAF FALL

A leaf has a limited life span, usually living for only a single growing season in most deciduous plants and seldom more than a few years in evergreen plants. In temperate regions, leaves develop and grow during spring and early summer. In autumn they grow old, change color, and die. In nonwoody plants (low in xylem) the leaves wither and fall away because of decay and various external conditions. Woody plants (rich in xylem) lose their leaves as a result of characteristic changes in the base of the leaf. In tropical regions that have distinct wet and dry seasons, the formation and fall of leaves depend on moisture conditions rather than temperature. Contrary to popular belief, evergreen plants also shed their leaves. However, evergreens are never bare because the old leaves are pushed out only as new leaves develop.

A

Budding

All leaves develop from buds, which are located at the nodes, or joints, of a plant stem and at the end of a plant stem. Contained in the buds are areas of rapidly growing tissue, called the meristem. The meristem gives rise to the first recognizable signs of the leaf. In the spring the buds shed their outer covering and open, exposing the leaves.

As leaves develop, they are arranged on the stem in one of three ways: alternate, opposite, or whorled. The arrangement provides an equal distribution of leaf weight on the stem. It also prevents overlapping so that each leaf can receive adequate sunlight.

B

Color Changes

In addition to chlorophyll, leaf cells also may contain other pigments. These pigments account for the color of autumn leaves. Among the pigments found in leaves are yellow xanthophylls, yellowish-orange carotenes, and red and purple anthocyanins. Leaves also may contain tannins, which give them a golden-yellow color in autumn.

Like chlorophyll, xanthophylls and carotenes are contained in tiny granules in some leaf cells. Although these pigments are present throughout the leaf's lifetime, their colors are usually masked by the green of chlorophyll. In the autumn, however, chlorophyll production decreases, and the yellow and orange pigments become visible in the leaves. Eventually all pigment production stops, and the leaves turn brown.

Unlike xanthophylls and carotenes, anthocyanins are not contained in granules but are dissolved in the liquid part of leaf cells. In some plants, such as coleus and red cabbage, anthocyanins are always present, giving the leaves a reddish or purplish color. In other plants, anthocyanins are not present throughout the life of the leaf, but are produced only under certain conditions. In oak and maple leaves, for example, sugar accumulates in autumn. This accumulation is believed to result in the formation of anthocyanins and the production of vivid colors in the leaves.

C

Leaf Fall

The leaves of evergreens continue to function and manufacture food throughout the year. In deciduous plants, however, the leaves stop functioning in the autumn and drop off. Leaves may be killed by frost, but changes due to age and growing conditions occur well before then. Decreased day length, reduced light intensity, lower temperatures, lack of water, and decrease of growth-promoting substances in the plant all contribute to the decline of the leaves. The changes start in the weakest part of the petiole, at the base. During autumn the cells in the base of the petiole begin to disintegrate and die. As a result, the leaf blade is supported only by the veins in the petiole. Soon the vascular bundles become plugged, decreasing the flow of water, food, and minerals to and from the leaf blade. When the blade is disturbed, as by wind, it breaks off the plant at the base of the petiole.

IV

IMPORTANCE

Unlike leaf-bearing plants, animals cannot manufacture their own food. For this reason, animals must receive their nourishment, either directly or indirectly, from leaf-bearing plants. For example, cattle, sheep, and horses eat the leaves of grasses and other plants. These animals, in turn, are consumed by various carnivorous animals, including humans. Humans also eat many kinds of leaves directly, including artichoke, cabbage, lettuce, and spinach.

Plant Nutrition

Nitrogen composes about four-fifths (78.03 percent) by volume of the atmosphere. Nitrogen is inert and serves as a diluent for oxygen in burning and respiration processes. It is an important element in plant nutrition; certain bacteria in the soil convert atmospheric nitrogen into a form, such as nitrate, that can be absorbed by plants, a process called nitrogen fixation. It ensures healthy growth of plants. Nitrogen in the form of protein is an important constituent of animal tissue. The element occurs in the combined state in minerals, of which saltpeter (KNO₃) and Chile saltpeter (NaNO₃) are commercially important products. 

Magnesium, is the centre of chlorophyll, hence a very important nutrient for plants photosynthesis.

Phosphorus: is necessary for healthy roots.

Potassium: necessary for plants fruits.