Hand out penny candy to the students. Inquire as to what is in the candy. The students will identify sugar. Ask them if candy gives them an energy boost. If yes, ask them to explain how that happens. They will probably say that they digest the sugar. Shift the discussion to plants. Ask how plants get their energy. They might know about the production of food through photosynthesis but not how manufactured food is used.

1 Describe cellular respiration.

2 Describe the processes of cellular respiration.

3 Identify factors that affect cellular respiration.

_ aerobic respiration _ anabolism _ anaerobic respiration _ catabolism _ cellular respiration _ chemiosmosis _ citric acid cycle _ cytosol

_ electron transport system (ETS) _ fermentation _ glycolysis _ metabolism _ mitochondria _ oxidation _ redox reactions _ reduction

I. All living plant cells respire and use the energy released to live and function. Cellular respiration is the process in which chemical energy stored in certain foods is converted to ATP, or high-energy compounds.

A. The cells that make up organisms are at constant work producing materials for growth, reproduction, movement, and maintenance of life. All these chemical processes are called metabolism. Converting substances to make ATP and other energy-rich molecules is one vital process. Metabolic processes entail both the combining and the splitting of molecules.

1. The assembly of complex molecules from simple molecules is a process called anabolism. Anabolism is the process by which plants store energy during photosynthesis, produce new materials for cellular activities, and grow.

2. The breakdown of larger molecules into smaller molecules is an element of metabolism known as catabolism. An example of a catabolic reaction is the breakdown of carbohydrates manufactured during photosynthesis to release ATP.

B. The most common catabolic reaction in plants involves the breakdown of glucose. This process requires oxygen and is known as aerobic respiration. The process of respiration takes place in complex organelles known as mitochondria.

1. Aerobic respiration is essentially the reverse of photosynthesis. In aerobic respiration, sugars made in photosynthesis are broken down into energy-rich molecules through a long sequence of reactions.

2. Oxygen and water are critical ingredients to the reaction.

3. In the process, chemical energy is released when the molecular bonds of the sugar molecules are broken. The extracted energy, ATP, drives a variety of chemical reactions important in the growth and development of the plant. Byproducts of the reaction are carbon dioxide and water.

C. Some cells perform a less efficient form of respiration called anaerobic respiration. Anaerobic respiration does not require oxygen. Cells that rely on anaerobic respiration are found in soil or water where oxygen is in short supply.

1. Some bacteria and fungi adapted to anaerobic respiration convert energy by fermentation. Fermentation is a type of cellular respiration that produces ethyl alcohol or lactic acid and is anaerobic.

2. Humans recognize the value of fermentation in the making of silage, beer, and wine.

D. Plant growth takes place primarily at night when photosynthesis is shut down. It is fueled by aerobic respiration. With signals from hormones, enzymes or chemical activators are produced. Each enzyme has one specific job. With split-second timing, they break down sugars and recombine them with nitrogen and other minerals. Many complex molecules are produced, including starches; pectin to bind cells; lignin, which is a tough, durable substance; cellulose; lipids; proteins; pigments; hormones; vitamins; and alkaloids and tannins, materials that protect plants from pests and diseases.

II. Aerobic respiration of glucose involves four primary stages.

A. The first stage of aerobic respiration is glycolysis. Glycolysis is the splitting of sugar. A specific enzyme is the catalyst for this reaction that involves about ten steps. Glycolysis takes place in the cytosol, the fluid of a cell in which organelles are suspended. Six-carbon glucose molecules are split into two, three-carbon pyruvate molecules with a net profit of two ATP and two NADH molecules. Glycolysis does not require oxygen.

B. Stage two involves the formation of acetyl coenzyme A (acetyl CoA). Pyruvate molecules enter the mitochondria. Each pyruvate molecule becomes a two-carbon molecule, acetate, through oxidation. The acetate combines with coenzyme A to form acetyl CoA. In the process, carbon dioxide is removed, and hydrogen atoms are contributed to the production of NADH.

C. The third stage is the citric acid cycle, also known as the Kreb’s cycle. It consists of eight steps that take place in the mitochondria. Acetyl CoA combines with a four-carbon molecule to form a six-carbon citrate molecule. In a series of events, the citrate reforms a four-carbon molecule. With each turn of the cycle, one ATP and two carbon dioxide molecules are released. NADH and FADH2 are also produced. The cycle must turn twice to process one glucose molecule.

D. Throughout the first three steps, hydrogen atoms were transferred to hydrogen acceptors NAD and FAD, forming NADH and FADH2. In the final stage of aerobic respiration, these reduced compounds enter the electron transport system.

1. The electron transport system (ETS) is a series of chemical reactions by which electrons are passed from one acceptor molecule to another. Oxygen is the final hydrogen acceptor. During this pathway, ATP is synthesized by the process of chemiosmosis.

2. Chemiosmosis is the process in which hydrogen atoms cross the thylakoid membrane and travel down a protein gradient, producing ATP. This fourth stage sees the production of 32 ATP molecules. Water is also made and released.

E. Energy in cells is transferred by the flow of protons and electrons.

1. A chemical reaction in which a substance loses electrons is called oxidation. Substances gain electrons through the chemical process of reduction. Electrons released during oxidation cannot exist independently. Therefore, every oxidation reaction is accompanied by a reduction reaction. These oxidation-reduction reactions are called redox reactions. Oxidation-reduction reactions are crucial to photosynthesis, cellular respiration, and other chemical reactions that occur in cells.

2. During aerobic respiration, glucose or other forms of carbohydrates are broken down in the presence of water and oxygen. The products are energy in the form of ATP, carbon dioxide, and water. In aerobic respiration, glucose is oxidized, and oxygen is reduced. In the multi-step reaction, hydrogen is transferred from the glucose molecule to oxygen. Energy from the hydrogen electrons is used for ATP synthesis.

III. Nearly all energy used to maintain life originates from the sun. Plants convert the solar energy to chemical energy through photosynthesis. Plants and animals then release the chemical energy for their use through respiration. Various factors influence respiration.

A. Respiration increases as temperatures rise and decreases as temperatures drop.

B. Oxygen is required for aerobic respiration to occur. If levels of oxygen are reduced in the atmosphere, respiration slows.

C. Soils saturated with water lack oxygen. In the absence of oxygen, respiration does not occur. This explains why root cells and, ultimately, root systems die in waterlogged soil.

D. Under low light conditions, fewer carbohydrates are produced by plants. Low levels of carbohydrates, in turn, mean low rates of respiration.

E. The stage of growth and the age of the plant influences the rate of respiration needed to maintain life processes. Young actively growing plants tend to have a higher rate of respiration.

F. Water is essential for cellular respiration as well as photosynthesis. Without water, enzyme activity grinds to a halt. The plant essentially stops growing. The lack of available water during drought conditions can cause respiration to stop.

1. What is cellular respiration?

2. What are the processes of cellular respiration?

3. What factors affect cellular respiration?