Living things contain many complex systems of chemical reactions called biochemical pathways.

All these chemical systems are important, but the two most important are photosynthesis and cellular respiration.

 
Photosynthesis is the chemical process by which green plants use light energy to produce food.

This process supports virtually all life on Earth. Sugars, the first products of photosynthesis, are converted into starch, protein, cellulose, and thousands of other chemicals used by living things.

Photosynthesis is also the only source to replenish Earth's oxygen. Before plants appeared, the atmosphere was high in carbon dioxide, but contained little or no oxygen. The present atmosphere is about 0.035% CO₂ and 21% O₂ - thanks to photosynthesis.

Photosynthesis is commonly represented by this chemical equation.

6CO₂ + 6H₂O C₆H₁₂O₆ + 6O₂

It is actually a complex series of chemical reactions.

• Light reactions: H₂O O₂

  The series of reactions in which energy from sunlight is used to split water molecules - requiring a LOT of energy.

• Dark reactions: CO₂ C₆H₁₂O₆

  The series of reactions in which glucose is synthesized.

  It does not have to be DARK for these reactions to occur. They just do not require as much energy as the light reactions.

Chlorophyll is the chemical that makes it all possible.

Chlorophyll is a very large molecule with a chemical formula of C₅₅H₇₀MgN₄O₆.

This molecule is called a pigment because it absorbs certain wavelengths of light. Its color represents the colors of light that it reflects. Therefore, green light provides the least energy for the process of photosynthesis.

Red and blue are the wavelengths absorbed, and provide the energy for photosynthesis.

Types of Chlorophyll:

• Chlorophyll a - a bright green pigment that is indispensable to photosynthesis.
   
• Chlorophyll b - an olive-green pigment that contributes to photosynthesis.
   
• Chlorophyll c - a yellow-green accessory pigment, contributing little to photosynthesis.

The structure of chloroplasts is quite complex, but these are the major structures:

• The organelle is surrounded by a double membrane.
   
• Inside the inner membrane is a complex mix of enzymes and water called stroma.
   
• Embedded in the stroma is a network of stacked sacs. Each stack is called a granum.
   
• Each of the flattened sacs which make up the granum is called a thylakoid.

Only 1 in 250 chlorophyll molecules actually converts quanta, units of light energy, into usable energy. These molecules are called reaction-center chlorophyll. The other molecules absorb light energy and deliver it to the reaction-center molecule.

These bulk chlorophyll molecules are known as antenna pigments because they collect and channel energy. A unit of several hundred antenna pigment molecules plus a reaction center is called a photosynthetic unit.

The large number of antenna pigment molecules in each photosynthetic unit enables its reaction center to be constantly supplied with quanta of energy.

 
Factors Determining the Rate of Photosynthesis

Light intensity:

• light-limited - At low light intensities photosynthesis is starved for energy. The system uses most of the quanta the pigments capture and is therefore maximally efficient, but because there are few quanta, the rate is low.

  Under these conditions the rate may only slightly exceede the rate of cellular respiration, so the net photosynthetic production by the cells is actually very poor.
   

• light saturation - As the light intensity is raised, the rate of photosynthesis increases. However, a plateau is reached at about one-fourth the intensity of full sunlight.

  Light saturation does not result from a limitation in the capacity of chlorophyll to absorb light. It represents the maximum rate at which the dark reactions of photosynthesis can use energy from chlorophyll. A further increase in the energy supply becomes excess energy and it converted to heat and wasted.

• The light reactions of photosynthesis are NOT temperature dependent.
   
• The dark reactions of photosynthesis are temperature dependent enzymatic processes.

  These reactions have an optimum temperature. The rate of photosynthesis in most plants increases only up to about 25 ^oC (77 ^oF). The rate levels out and then actually drops as the temperature approaches human body temperature.

  This may seem odd because we normally think of human body temperature as a physiological optimum temperature.

• Length of day - the more hours of sunlight, the longer the light reactions take place. In fact, the decreasing hours of sunlight in the fall cause the breakdown of chlorophyll and the change in leaf color.
   
• Amount of CO₂ - up to a point, the rate of photosynthesis increases with CO₂ concentration in the atmospheree.
   
• Air pollution - most types of air pollution will decrease the rate of photosynthesis, either by decreasing sunlight penetration or by interferring with the chemical reactions themselves.

• At least 10 moles of quanta are required in photosynthesis.
  One mole equals 6.02 X 10²³

• Red light contains about 40 kcal/mole of quanta, therefore 10 moles of quanta would contain 400 kcal of energy.
   
• One mole of glucose (C₆H₁₂O₆) is known to contain 686 kcal of stored energy.
   
• One-sixth mole of glucose is formed from one mole of CO₂. This amount contains 114 kcal of stored energy.
   
• The maximum efficiency of photosynthetic energy conversion is therefore 114/400, or about 28.5%.

Most agricultural and forestry measurements in the field give efficiencies at or below 1%.

C₃ Plants:

• Most plants are C₃ type.
   
• They form 3-carbon organic acids as their first stable products.

• Include sugar cane, corn, and sorghum.
   
• They form 4-carbon organic acids as their first stable products.
   
• These plants are more efficient in producing sugars because they have little or no sugar loss during respiration.

 
Significance of Photosynthesis:

Photosynthesis is responsible for the conversion of carbon from carbon dioxide into organic compounds in plants. It allows the plant to make organic building blocks, new cells, starch, and proteins.

Without photosynthesis, life as we know it would not exist. Plants provide, directly or indirectly, ALL the food and atmospheric oxygen used by Earth's animals.

During cellular respiration, food molecules are broken down to release the energy in their chemical bonds.

There are two types of respiration:

• Aerobic respiration requires molecular oxygen to break down organic molecules.
  C₆H₁₂O₆ + 6O₂ 6CO₂ + 6H₂O

• Anaerobic respiration does not require molecular oxygen to break down organic molecules.
  C₆H₁₂O₆ 2C₂H₅OH + 2CO₂

adenosine triphosphate, ATP

ATP + H₂O ADP + HPO₄

When energy is not immediately needed, the reverse reaction takes place and the phosphate group is reattached to the molecule. The ATP molecule acts like a chemical "battery", storing energy when it is not needed, but able to release it instantly when it is required.

 
The organelle where cellular respiration takes place is the mitochondria.

The mitochondria are surrounded by two lipid-protein membranes, the outer and inner membranes. These are separated by the inter-membrane space.

The outer membrane is smooth, while the inner membrane is highly folded. The folds are called cristae, and project into the internal membrane space.

The internal space of the mitochondrion (enclosed within the folded inner membrane) is filled with a dense protein-like material and is called the matrix. It contains ribosomes, mitochondrial DNA, and enzymes.

 
Chemically, cellular respiration is the exact opposite of photosynthesis.

Photosynthesis: 6CO₂ + 6H₂O C₆H₁₂O₆ + 6O₂

Respiration: C₆H₁₂O₆ + 6O₂ 6CO₂ + 6H₂O

While photosynthesis occurs only in green plant cells, respiration occurs in all cells.

 
The energy sequence for life goes something like this:

1. Plants use photosynthesis to store the Sun's energy in chemical form in glucose.
    
2. Cellular respiration releases the energy stored in the chemical bonds of glucose.
    
3. The energy from glucose is used to form ATP.
    
4. The conversion of ATP to ADP in the cell's mitochondria produces energy to fuel cell activity.

Respiration is completed in two phases:

• Phase I is called glycolysis and is the first step in both types of respiration.
   
• Phase II depends upon whether O₂ is used or not.

Phase I - Glycolysis:

• These reactions are completed in the cell cytoplasm (cytosol). Therefore, glycolysis is also called cytoplasmic respiration.
   
• Glycolysis involves a series of reactions in which a molecule of 6-C (6 carbon) glucose is broken down into two molecules of 3-C pyruvic acid.

C₆H₁₂O₆ 2CH₃COOH + 2NADH₂ + 2ATP

• Each step is controlled by a specific enzyme and is reversible.
   
• Reactions in the glycolysis are anaerobic in nature, they do not require molecular oxygen.

Phase II - Aerobic Respiration:

• The first set of aerobic reactions is called the Kreb's Cycle or Citric Acid Cycle.

Pyruvic acid is broken down to form the end products of aerobic respiration.

The Krebs Cycle produces 2 ATP molecules, 10 carrier molecules, and CO₂ from each glucose molecule.

• The second set of aerobic reactions is called the Electron Transport Chain (ETS) or Respiratory Chain.

Hydrogen ions, H⁺, and the electrons associated with them, react with molecular oxygen to form water.

The ETS reactions are oxidation-reduction (Redox) reactions. During the transfer of hydrogen/electrons through the chain, each carrier molecule is alternately reduced and oxidized.

• When a carrier accepts hydrogen/electron it gets reduced.
   
• The reduced carrier is oxidized when it transfers the hydrogen/electron to the next carrier in the chain.

Phase II - Anaerobic Respiration:

• The complete anaerobic process takes place in the cell cytoplasm only.
   
• Mitochondria are not involved.
   
• Oxygen is not used.
   
• The Kreb's Cycle and ETS reactions are not involved.
   
• Phase I consists of the same reactions of glycolysis found in aerobic respiration.
   
• Phase II consists of converting the pyruvic acid into ethyl alcohol.
   
• Anaerobic respiration only allows glycolysis to continue.

C₆H₁₂O₆ 2C₂H₅OH + 2CO₂ + 2ATP

Aerobic respiration is almost 20 times more efficient than anaerobic respiration.

• Aerobic respiration, with oxygen, produces 38 ATP molecules from each glucose molecule.
   
• Anaerobic respiration, without oxygen, produces 2 ATP molecules from each glucose molecule.

 
Lactic Acid Fermentation:

The expression "lactic acid" is commonly used by athletes to describe the cause of "the burn" felt during exhaustive exercise.

The buildup of lactic acid (C₃H₆O₃) causes a decrease in blood pH - interferring with muscle performance.

Hydrogen ions are not being removed by aerobic respiration because of a lack of oxygen. This is an indication the body is beginning to run on anaerobic energy.