Configuración en base común

Objectives :

1. To define metabolism

2. To differentiate between catabolism and anabolism

3. To explain the different metabolic processes involved in the various organ systems

Metabolism

Metabolism is from the Greek word metabolē , meaning "change" It is a set of life-sustaining chemical trans- formations within the cells of living organisms. The three main purposes of metabolism are the conversion of food/fuel to energy to run cellular processes, the conversion of food/fuel to building blocks for proteins, lipids, nucleic acids, and some carbohydrates, and the elimination of nitrogenous wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments.

The word metabolism can also refer to the sum of all chemical reactions that occur in living organisms, including digestion and the transport of substances into and

between different cells, in which case the set of reactions within the cells is called intermediary metabolism.

Metabolism is usually divided into two categories :

a). catabolism - the breaking down of organic matter by way of cellular respiration b). anabolism - the building up of components of cells such as proteins and nucleic acids.

Usually, breaking down releases energy and building up consumes energy. The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, by a sequence of enzymes. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts that allow the reactions to proceed more rapidly.

Metabolic Processes

Metabolic processes are sequences of biochemical reactions that take place within living cells to maintain life.

They can be divided into two main types :

A. Catabolic processes involve the breakdown of complex molecules from food into smaller units that can be used as building blocks for new molecules or to provide energy

B. Anabolic processes involve the use of energy to build new chemicals that become components of cells. These reactions are made possible by a number of organic catalysts known as enzymes.

Together, the two types of metabolic processes allow the transformation of the raw materials, or nutrients, that are taken in by an organism into tissue. One compound, common to all cellular life, is essential to these trans- formations. Adenosine triphos- phate (ATP) is used to store energy obtained from nutrients, such as carbohydrates, and to release energy when it is required for the building of new molecules.

Catabolic Processes

Some organisms, such as green plants, make their own food from inorganic materials, while others, such as animals, consume organic materials to obtain their nutrition. The food consumed by animals can be broken down into three main types — carbohydrates, lipids (fats and oils), and proteins. Digestion involves catabolic processes that break these down into simpler components. For example, relatively complex carbohydrates, such as polysaccharides and disaccharides are broken down into glucose, and proteins are broken down into amino acids. These simpler compounds may be used by anabolic processes to build new materials, or they may be further broken down to provide energy.

Cellular respiration is the process by which the carbohydrate glucose (C6H12O6) is broken down into carbon dioxide (CO2) and water (H2O), producing energy that is stored in ATP. The procedure involves oxidation, and where there is atmospheric

oxygen available it is used in what is known as aerobic respiration. This is the process that takes place in animals, plants and some microorganisms.

In conditions where no free oxygen is present, anaerobic respiration takes place. This is found only in certain microorganisms that live in soil, decaying organic matter, under the sea, deep underground, and in the intestines of animals. These

of free oxygen. Anaerobic respiration is much less efficient than the aerobic process, and produces much less ATP .

In animals, lipids are also oxidized to carbon dioxide and water, but the first few steps are different. The chemistry of organisms takes place in a water-based envi- ronment, but fats and oils do not mix with water. The first step is to emulsify these substances, which means converting them into a form that will mix with water, in the same way that detergents can help clean up oil spills. This is done by soap-like substances contained in bile released by the gall bladder into the small intestine. The lipids are then broken down into fatty acids and glycerol, which can be absorbed through the intestines, and which can then undergo oxidation reactions similar to those performed on carbohydrates.

Proteins are very large, complex molecules, made up of building blocks known as amino acids. They are metabolized by various reactions that split them up into their amino acids, which can be absorbed, and then used within cells. Generally, proteins are not used to provide energy, but instead the amino acids are utilized to manufacture new proteins to build tissue and muscle. In cases where no carbohydrate or fat is available in the diet, and the body has used up its fat reserves, proteins may be used to generate energy, by oxidation of their amino acids. In these cases, the body may start breaking down muscle proteins.

Anabolic Processes

Also known as biosynthesis, these are reactions that use up the energy stored in ATP by catabolic processes. They include the building of proteins from amino acids and the construction of DNA from nucleotides. In animals, the muscle contractions that power movement can also be included, as these require the use of stored energy. In plants, the synthesis of glucose from carbon dioxide and water through photosynthesis is another anabolic pathway.

How do animal cells get food

Food to an animal cell is glucose. Glucose is a monosaccharide (a simple sugar) produced by plants in the process known as photosynthesis. Glucose is used to produce the energy to make another molecule called ATP (adenosine triphosphate) which is the "energy currency" of the cell (This process is called cellular respiration and this process takes place in the mitochondria of the cell.) All cells, regardless of what living thing they are in, require glucose to make ATP in sufficient quantities to run their metabolism.

Animals get this glucose by eating plants or animals that eat plants. The glucose that the cells do not use is stored in the tissues of the plants.

When we eat plants or animals, the glucose is often stored in larger molecules called polysaccharides (starches). During the process of digestion, these molecules are broken down to glucose. Other molecules made from these simple sugars include the disaccharides, or double sugars (table sugar, sucrose, is one of these). Once these molecules are broken down, the glucose enters the cell via diffusion. Once the cell has the glucose, it is transported to the mitochondria where it is then processed to form the ATP.

How Cells Obtain Energy from Food

Cells require a constant supply of energy to generate and maintain the biological order that keeps them alive. This energy is derived from the chemical bond energy in food molecules, which thereby serve as fuel for cells.

Sugars are particularly important fuel molecules, and they are oxidized in small steps to carbon dioxide (CO2) and water . In this section we trace the major steps in the

breakdown, or catabolism, of sugars and show how they produce ATP, NADH, and other activated carrier molecules in animal cells. We concentrate on glucose breakdown, since it dominates energy production in most animal cells. A very similar pathway also operates in plants, fungi, and many bacteria. Other molecules, such as fatty acids and proteins, can also serve as energy sources when they are funneled through appropriate enzymatic pathways.

Lesson 7.2: GAS EXCHANGE WITH THE ENVIRONMENT