Chapter 3 - CELLS UNDERGO CHEMICAL REACTIONS

METABOLISM

Metabolism is made up of two different types of chemical reaction:

• Catabolic metabolism is the reactions in which large molecules are broken down to smaller ones. This process is known as catabolism. Digestion is an example of catabolism

• Anabolic metabolism is the reactions in which small molecules are built up into larger ones. This process is also referred to as anabolism. Protein synthesis is an example of anabolism.

Catabolic reactions release energy, whereas anabolic reactions require energy. Thus, metabolism is concerned with maintaining a balance between energy release and energy utilisation.

Nutrients

A nutrient is any substance in food that is used for growth, repair or maintaining the body; that is, any substance required for metabolism. There are six groups of nutrients: water, carbohydrates, lipids, proteins, minerals and vitamins.

Organic compounds

are molecules that have a carbon chain. They also contain a number of hydrogen atoms and may include atoms of oxygen, nitrogen and sulfur.

Carbohydrates

Carbohydrates are the main source of energy for cells. Simple sugars, particularly glucose, are used in cellular respiration to release energy. Complex carbohydrates, such as starch, are broken down to simple sugars. All carbohydrates contain atoms of CARBOHYDRATES Carbohydrates always contain carbon, hydrogen and oxygen. There are always twice as many hydrogen atoms as oxygen atoms. Monosaccharides are simple sugars or single-unit sugars; examples are glucose, fructose and galactose. carbon, hydrogen and oxygen; there are twice as many hydrogen atoms as oxygen atoms

Simple sugars are called

monosaccharides, Glucose, fructose and galactose are examples of monosaccharides.

Disaccharides

such as sucrose, maltose and lactose, are formed when two simple sugars join together.

Polysaccharides

are larger carbohydrate molecules formed when many simple sugars join together. Glycogen, cellulose and starch are examples of polysaccharides

Lipids

include fats and oil and are another important energy source. They are broken down to fatty acids and glycerol. The glycerol can then enter the glycolysis pathway of cellular respiration and is broken down to release energy in a similar way to glucose. Other examples of lipids are phospholipids, which are important in the cell membrane, and steroids, including cholesterol and the sex hormones. Each lipid molecule consists of one molecule of glycerol and one, two or three fatty acid molecules

The most common fat, including the fat that is stored in the body, is

triglyceride, which is composed of glycerol and three fatty acid molecules.

Inorganic compounds

Inorganic compounds are not based on a carbon chain. Most do not contain carbon atoms at all, but those that do, such as carbon dioxide, are small molecules. Some important inorganic compounds are water, minerals and vitamins.

• Water is important in metabolism because it is the fluid in which other substances are dissolved. Some of the cell’s chemical reactions occur in water, and in others water molecules actually take part in the reaction.

• Minerals are important for metabolism because they may be a part of enzymes, may function as cofactors for enzymes, or may be a part of substances such as adenosine triphosphate (ATP) that are involved in metabolism.

• Vitamins act as coenzymes for many of the chemical reactions of metabolism.

The reacting particles need to collide with enough energy to break the bonds; this is the

activation energy.

Chemicals called catalysts are able to

decrease the amount of energy needed to break the bonds. This means that the activation energy will be lower, and more particles will have enough energy to react, making the reaction happen at a faster rate

Enzymes

Enzymes are biological catalysts that are able to speed up chemical reactions by lowering the activation energy. They are not consumed or altered in the reaction.

he molecule on which an enzyme acts is called the

substrate

enzyme–substrate complex

Each enzyme will combine with only one particular substrate and is therefore involved in only one specific reaction. This occurs because the enzyme and its substrate have characteristics that are complementary to one another; that is, the enzyme and the substrate have a shape and a structure that allow them to fit together. The part of the enzyme molecule that combines with the substrate is called the active site. When the enzyme and substrate are combined, they are called an enzyme–substrate complex

Cellular respiration

is one of the most important metabolic processes in any cell. It is the process by which organic molecules, taken in as food, are broken down in the cells to release energy for the cell’s activities – activities such as the movement of the cell, uptake of materials from the surroundings, or production and secretion of new chemical compounds.

Cellular respiration can release

energy from glucose, amino acids, fatty acids and glycerol. However, the main food material utilised is glucose, and the discussion here will therefore be confined to the respiration of glucose.

The remaining energy from cellular respiration is used to form a compound called…

adenosine triphosphate (ATP)

ATP is composed of…

ATP is composed of:

• adenosine, which is made up of the nucleic acid base adenine and the sugar ribose

• three phosphate groups.

ATP is formed when an inorganic phosphate group is joined to a molecule of adenosine diphosphate (ADP). The phosphate groups in ATP are joined by high-energy chemical bonds. Some of the energy from cellular respiration is stored in the bond between the ADP molecule and the third phosphate group. This bond is more easily broken than the bond between the first and second phosphate groups, allowing the energy to be released when needed. ATP can thus be used to transfer the energy released in cellular respiration to processes in the cell that require energy. The ADP formed when the energy is released can be reused to store some more of the energy from cellular respiration.

Glycolysis

The first phase in the breakdown of glucose does not require oxygen. It is called glycolysis, which means ‘splitting glucose’. A glucose molecule is broken down, in a series of 10 steps, to two molecules of pyruvate. Sometimes this molecule is called pyruvic acid; however, the two substances differ slightly in their structure.

Anaerobic respiration

If no oxygen is available, the pyruvate produced in glycolysis is then converted to lactic acid by fermentation. The production of lactic acid from glucose is called anaerobic respiration, which means respiration without oxygen.

The enzymes required for anaerobic respiration are available in the

cytosol cell

Aerobic respiration occurs in the

mitochondria of the cell

aerobic respiration

The complete breakdown of glucose to carbon dioxide and water requires oxygen. The pyruvate produced from glycolysis is completely broken down to carbon dioxide and water. This is known as

Steps for cellular respiration

1 For the pyruvate to enter the next pathway it is first converted to acetyl coenzyme A (acetyl CoA). To do this, a carbon dioxide molecule is removed from the pyruvate and the remaining two-carbon structure joins to coenzyme A. No ATP is produced during this process.

2 The acetyl CoA then enters the citric acid cycle, also known as the Krebs cycle. Here the carbon atoms in the acetyl CoA are released in carbon dioxide. For every acetyl CoA that enters the citric acid cycle, one molecule of ATP is also produced. This means that two ATP molecules are produced per glucose molecule.

3 The final stage of cellular respiration is the electron transport system; the only stage that uses oxygen. This stage is also called oxidative phosphorylation. Here electrons are passed between molecules, finally resulting in oxygen molecules forming water. There is some debate regarding the exact number of ATP molecules that are produced during this process. Estimates range between 26 and 34 molecules.

Thus, aerobic respiration of one molecule of glucose has the potential to generate up to 38 molecules of ATP – two from glycolysis, two from the citric acid cycle and up to 34 from the electron transport mechanism. This can be represented

In cellular respiration, only about

40% of the energy released is incorporated into ATP; the other 60% is lost as heat. Therefore, energy must be continually consumed in the form of food to replace what is lost as heat and utilised for other purposes.

The reactions of cellular respiration are

that is, they release energy as larger molecules are broken down into smaller ones. ATP may be used to transfer energy produced in catabolic reactions to anabolic reactions that require energy. For example, when lactic acid is recombined with oxygen in the liver to form glucose, or when glucose molecules are joined to form glycogen, the energy required comes from the breakdown of ATP to ADP. Similarly, energy for the build-up of proteins, lipids and other molecules is transferred from cellular respiration by ATP. Figure 3.23 models how ATP is able to transfer energy from one reaction to another.