Energy Flow In Plant Cells

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Categories: Cellular biology; photosynthesis and respiration; physiology

Life on earth is dependent on the flow of energy from the sun. A small portion of the solar energy, captured in the process of photosynthesis, drives many chemical reactions associated with living systems.

In living organisms, energy flows through chemical reactions. Each chemical reaction converts one set of substances, called the reactants, into another set, the products. All chemical reactions are essentially energy transformations, in which energy stored in chemical bonds is transferred to other, newly formed chemical bonds. Exergonic reactions release energy, whereas endergonic reactions

Transformation of Sunlight into Biochemical Energy

Energy Flow Marine
Chloroplasts in plant cells convert light, water, and carbon dioxide into carbohydrates via photosynthesis

Oxygen, as a by-product of photosynthesis, is released

Respiration by-products are carbon dioxide, water, and energy dissipated as heat

Mitochondria in plant cells perform cellular respiration

Mitochondria in plant cells perform cellular respiration

Respiration results in energy storage in ATP molecules require an input of energy for a reaction to occur.

In plants, such reactions occur during the process whereby plant cells convert the energy of sunlight into chemical energy that fuels plant growth and other processes. During this process, called photosynthesis, carbon dioxide combines with simple sugars to form more complex carbohydrates in special structures called chloroplasts. These chloro-plasts are membrane-bound organelles that occur in the cells of plants, algae, and some protists. The energy that drives the photosynthetic reaction is derived from the photons of sunlight; hence it is an endergonic reaction (it requires energy). Because plants, algae, and certain protists are the only living organisms that can produce their chemical energy using sunlight, they are called producers; all other life-forms are consumers. During seed germination, simple sugars, such as glucose, are broken down in a series of reactions called respiration. Energy is released to power the growth of embryo and young seedlings; hence, the reaction is exergonic. Within plant cells, both reactions occur.

In many reactions, electrons pass from one atom or molecule to another. These reactions, known as oxidation-reduction (or redox) reactions, are of great importance in living systems. The loss of an electron is known as oxidation, and the atom or molecule that loses the electron is said to be oxidized. Reduction involves the gain of an electron. Oxidation and reduction occur simultaneously; the electron lost by the oxidized atom or molecule is accepted by another atom or molecule, which is thus reduced.

Within plant cells, the energy-capturing reactions (photosynthesis) and the energy-releasing reactions (respiration) are redox reactions. Furthermore, all chemical reactions are orderly, linked and intertwined into sequences called metabolic pathways. All metabolic pathways in plant cells are finely tuned in three ways: the chemical reactions are regulated through the use of enzymes, exergonic reactions are always coupled with ender-gonic reactions, and energy-carrier molecules are synthesized and used for effective energy transfer.

Enzymes and Cofactors

Enzymes are biological catalysts, usually proteins, synthesized by plant cells. A number of characteristics make enzymes an essential component for energy flow in plant life. Enzymes dramatically speed up chemical reactions. Enzymes are normally very specific, catalyzing, in most cases, a single reaction that involves one or two specific mole-

cules but leaves quite similar molecules untouched. In addition, enzyme activity is well regulated.

Many enzymes require a nonprotein component, or cofactor, for their optimal functions. Co-factors may be metal ions, part of or independent of the enzyme itself. Magnesium ions (Mg2+), for example, are required in many important reactions in energy transfer, including photosynthesis and respiration. The two positive charges often hold the negatively charged phosphate group in position and help in moving it from one molecule to another. In other cases, ions may help enzymes maintain their proper three-dimensional conformation for optimal function. Some organic molecules can also be cofactors, including vitamins and their derivatives, and are usually called coenzymes. One example is the electron carrier nicotinamide adenine dinucleotide (NAD+). NAD+ is derived from nucleotide and vitamin-niacin. When NAD+ accepts electrons, it is converted into NADH + H+, which passes its electrons to another carrier; hence, NAD+ is regenerated.

Plant cells regulate the amount and activity of their enzymes through various mechanisms. First, they control the synthesis of particular enzymes to meet their needs. They limit or stop the production of enzymes not needed by metabolic reactions and, hence, conserve energy. Second, plant cells may synthesize an enzyme in an inactive form and activate it only when needed. Third, plant cells can employ a feedback regulation mechanism by which an enzyme's activity is inhibited by an adequate amount of the enzyme's product. Furthermore, the activities of enzymes are affected by the environment, including temperature, pH (a measure of acidity versus alkalinity), and the presence of other chemicals. Different enzymes may require a slightly different physical environment for optimal function.

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