Energy flow in plant cells

Transformation of sunlight into biochemical energy

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 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 chloroplasts 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 eactions 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 endergonic 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, inmost cases, a single reaction that involves one or two specific molecules 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. Cofactors 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.

ATP: The Energy Carrier

During seed germination, stored glucose is broken down, making chemical energy available for movement, cellular repair, growth, and development. However, plant embryos cannot directly use the chemical energy derived from the breakdown of glucose.

Within plant cells, most energy is transferred through a carrier—adenosine triphosphate, or ATP, known as the universal currency for energy transfer. Whether helping to convert light energy respiration, ATP acts as an agent to carry and transfer energy into chemical energy during photosynthesis or breaking down glucose in glycolysis and aerobic.

ATP is a nucleotide composed of the nitrogen containing base adenine, the sugar ribose, and three phosphate groups. Energy released through glucose breakdown is used to drive the synthesis of ATP from adenosine diphosphate, or ADP, and inorganic phosphate (Pi):

ADP + Pi + energy → ATP

The energy is largely stored in the bonds linking the phosphate groups. In reactions or processes where energy is needed, ATP releases energy through the hydrolysis and hence the removal of phosphate group:

ATP + H20 → ADP + Pi + energy (7.3 kilocalories per mole)

Sometimes the second phosphate group may also be removed via hydrolysis to generate the same amount of energy and adenosine monophosphate (AMP):

ADP + H20 → AMP + Pi + energy (7.3 kilocalories per mole)

The terminal phosphate group of ATP is not simply removed inmost cases but is transferred to another molecule within a plant cell. This addition of a phosphate group to a molecule is defined as phosphorylation. The enzymes that catalyze such transfers are named kinases.

The following two examples of energy transfer involve ATP. The first is synthesis of sucrose by sugarcane:

glucose + fructose + 2 ATP + 2 H20 → sucrose + H20 + 2 ADP + 2 Pi

The second example is the complete breakdown of glucose during cellular respiration:

glucose + 6 O2 + 36-38 ADP + 36-38 Pi → 6 C02 + 6 H20 + 36-38 ATP

Either ADP or ATP can be recycled through endergonic or exergonic reactions intertwined in the metabolic pathways. In the plant kingdom, energy flow begins with photosynthesis, through which ATP and then high-energy bonds are formed as sugar by the conversion of light energy from the sun.

In respiration, these bonds are broken down to carbon dioxide and water, and energy is released. Some of this energy is used to power cellular processes, but some energy is lost in each energy-conversion step. The energy flow among all other organisms also starts fromphotosynthesis or plants, either directly or indirectly.