9.1 Cellular Respiration: An Overview Lesson Overview 9.1 Cellular Respiration: An Overview

Chemical Energy and Food Food provides living things with the chemical building blocks they need to grow and reproduce. Recall, that some organisms are autotrophs (they make their own food through photosynthesis) and others are heterotrophs (they rely on other organisms for food). For all organisms, food molecules contain chemical energy that is released when its chemical bonds are broken.

Chemical Energy and Food How much energy is actually stored in food? Actually, quite a bit, although it varies with the type of food. Energy stored in food is expressed in units of calories. A Calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. 1000 calories = 1 kilocalorie, or Calorie. This is the Calorie used on food labels.

Chemical Energy and Food Cells use all sorts of molecules for food, including fats, proteins, and carbohydrates. The energy stored in each of these molecules varies because their chemical structures, and therefore their energy-storing bonds, differ. For example, one gram of the sugar glucose releases 3811 calories of heat energy when it is burned. In contrast, one gram of triglyceride fats in beef releases 8893 calories of heat energy when its bonds are broken. In general, carbohydrates and proteins contain about 4000 calories (4 Calories) of energy per gram, while fats contain about 9000 calories (9 Calories) per gram.

Chemical Energy and Food Cells don’t burn food and release energy as heat. Cells break down food molecules gradually, capturing a little bit of chemical energy at key steps in the process. This allows the cell to use the energy stored in the chemical bonds of food (like glucose) to produce compounds such as ATP that directly power the activities of the cell.

Overview of Cellular Respiration If oxygen is available, organisms can obtain energy from food by a process called cellular respiration. Cellular respiration is the process that releases energy from food in the presence of oxygen. The summary of cellular respiration is presented below. 6 O2 + C6H12O6  6 CO2 + 6 H2O + Energy Oxygen + Glucose  Carbon dioxide + Water + Energy The cell has to release the chemical energy in food molecules (like glucose)\ gradually, otherwise most of the energy would be lost in the form of heat and light. The cell needs to find a way to trap those little bits of energy by using them to make ATP.

Stages of Cellular Respiration The three main stages of cellular respiration are: glycolysis, the Krebs cycle, and the electron transport chain.

Stages of Cellular Respiration Glucose first enters glycolysis. Only a small amount of energy is captured to produce ATP during this stage. Most of glucose’s energy (90%) remains locked in the chemical bonds of pyruvic acid at the end of glycolysis. The end product of glycolysis is two molecules of pyruvate.

Stages of Cellular Respiration Pyruvic acid enters the second stage of cellular respiration, the Krebs cycle. During the Krebs cycle, a little more energy is generated from pyruvic acid.

Stages of Cellular Respiration The final stage, the electron transport chain, produces the bulk of the energy in cellular respiration. This stage requires reactants from the other two stages of the process. The electron transport chain is able to extract so much energy from these reactants by using oxygen, a powerful electron acceptor.

Oxygen and Energy Oxygen is required at the very end of the electron transport chain. Anytime a cell’s demand for energy increases, its use of oxygen increases too.

Oxygen and Energy The term respiration is synonymous with breathing. That is why the term cellular respiration is used to refer to energy releasing pathways within the cell. The double meaning of respiration points out a crucial connection between cells and organisms. Most of the energy-releasing pathways within the cell require oxygen, and that is the reason we need to breathe (or respire).

Oxygen and Energy Pathways of cellular respiration that require oxygen are called aerobic. The Krebs cycle and electron transport chain are both aerobic processes. Even though the Krebs cycle doesn’t directly require oxygen, it is considered aerobic because it can’t run without the oxygen-requiring electron transport chain. Both processes take place inside the mitochondria.

Oxygen and Energy Gylcolysis is an anaerobic process. It does not directly require oxygen, nor does it rely on an oxygen-requiring process to run. However, it is still considered part of cellular respiration because its final products are key reactants for the aerobic stages. Glycolysis takes place in the cytoplasm of a cell.

Oxygen and Energy If oxygen is not present, another anaerobic pathway, fermentation, makes it possible for the cell to keep glycolysis running, generating ATP to power cellular activity.

Comparing Photosynthesis and Cellular Respiration Consider the following questions - If nearly all organisms break down food by the process of cellular respiration, why doesn’t the Earth run out of oxygen? Where does all the carbon dioxide waste product go? How does the chemical energy stored in food get replaced? The relationship between photosynthesis and cellular respiration answers these questions.

Comparing Photosynthesis and Cellular Respiration are opposite processes. The energy flows in opposite directions. Photosynthesis “deposits” energy, and cellular respiration “withdraws” energy. The chemical equations for photosynthesis and cellular respiration are the reverse of each other. The reactants of cellular respiration are the products of photosynthesis and vice versa.

Comparing Photosynthesis and Cellular Respiration Photosynthesis removes carbon dioxide from the atmosphere, and cellular respiration puts it back. Photosynthesis releases oxygen into the atmosphere and cellular respiration uses that oxygen to release energy from food.

Comparing Photosynthesis and Cellular Respiration The release of energy by cellular respiration takes place in plants, animals, fungi, protists, and most bacteria. Energy capture by photosynthesis occurs only in plants, algae, and some bacteria.