9.1 Cellular Respiration: An Overview

Chemical Energy and Food

• Food gives living things the energy they need to grow

  • Autotrophs make their own food through photosynthesis

  • Heterotrophs must eat other living things for food. All living things,

  • Food molecules store chemical energy for all living things; that energy is released when they break those food molecules down

• Energy in food can be measured in units called calories

  • A calorie is the amount of energy needed to raise the temperature of 1 gram of water 1 degree Celsius

• Cells break down food molecules over time, getting a little bit of chemical energy at key steps

Overview of Cellular Respiration

• Cellular respiration is the process that releases energy by breaking down glucose and other food molecules in the presence of oxygen

  • Cellular respiration gives off carbon dioxide, water, and energy and the process can be summarized like this:

• The first step of cellular respiration is glycolysis

  • Only a small amount of energy is used to make ATP during this stage; the rest is still locked in the bonds of a molecule called pyruvic acid

• The second step of cellular respiration is the Krebs cycle

  • Pyruvic acid enters the Krebs cycle, where a little more energy is given off

• The third step of cellular respiration is the electron transport chain

  • Most of the energy from cellular respiration comes from the electron transport chain; this stage uses oxygen and reactants from the other two stages to finish the job

• Oxygen is used at the end of the electron transport chain; any time a cell needs more energy, it needs more oxygen, too

• Chemical pathways that need oxygen are called aerobic

  • The Krebs cycle and electron transport chain are both aerobic

• A chemical process that does not need oxygen is called anaerobic

  • Glycolysis is an anaerobic process

• Mitochondria are the organelles most important in cellular respiration

  • The Krebs cycle and electric transport chain take place inside the mitochondria

Comparing Photosynthesis and Cellular Respiration

• Photosynthesis and cellular respiration can be thought of as opposite processes, as the equations for photosynthesis and cellular respiration are the reverse of each other

• Photosynthesis produces food molecules and removes carbon dioxide from the air, and cellular respiration puts it back

• Photosynthesis gives off oxygen, and cellular respiration uses that oxygen to release energy from food

9.2 The Process of Cellular Respiration

Glycolysis

• Glycolysis is the first stage of cellular respiration

  • During glycolysis, glucose is broken down into 2 molecules of pyruvic acid' ATP and NADH are also made

• 1 molecule of glucose, which has 6 carbon atoms, is changed into 2 molecules of pyruvic acid, which each have 3 carbon atoms

• One of the steps of glycolysis passes 4 electrons to an electron carrier called NAD+,  or nicotinamide adenine dinucleotide

• Glycolysis does not need oxygen, meaning that glycolysis can quickly get chemical energy to cells when there is not any oxygen present

  • When oxygen is present, however, the pyruvic acid and NADH made during glycolysis become the materials needed for the other stages of cellular respiration

The Krebs Cycle

• The Krebs cycle takes place within the matrix - the innermost space of the mitochondrion

• During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of steps that release chemical energy

  • Step 1: Pyruvic acid from glycolysis enters the mitochondrion’s matrix

  • Step 2: Enzymes split CO2 off from pyruvic acid, leaving a 2-carbon molecule, and NADH is produced from NAD

  • Step 3: The 2-carbon-atom molecule joins a 4-carbon-atom molecule to become citric acid

  • Step 4: More CO2 and NADH are made as citric acid becomes a 4-carbon-atom molecule

  • Step 5: More reactions make high-energy molecules of ATP, FADH2, and NADH

  • Step 6: The 4-carbon atom molecule can go through the cycle again

• Because glycolysis makes 2 molecules of pyruvic acid from each glucose molecule, the Krebs cycle “turns” twice for each glucose molecule that enters glycolysis

Electron Transport and ATP Synthesis

• Carriers from glycolysis and the Krebs cycle go into the last stage of cellular respiration, the electron transport chain

• The NADH made during glycolysis can enter the mitochondrion to join the NADH and FADH2 made by the Krebs cycle

  • Electrons are then passed from all of those carriers to the electron transport chain, which uses them to change ADP into ATP

• Together, glycolysis, the Krebs cycle, and the electron transport chain make about 36 molecules of ATP per molecule of glucose

  • Those 36 ATP molecules represent about 36 percent of the total energy of glucose, meaning that the cell is actually better at using food than a car’s engine is at burning gasoline!

9.3 Fermentation

Fermentation

• Fermentation is the process by which cells release energy in the absence of oxygen

• Fermentation is an aerobic process and takes place in the cytoplasm

  • There are two different kinds—alcoholic fermentation and lactic acid fermentation

• In alcoholic fermentation, pyruvic acid from glycolysis is changed into alcohol and carbon dioxide

  • Yeasts and a few other microorganisms use alcoholic fermentation

  • A summary of alcoholic fermentation after glycolysis is as follows:

• In lactic acid fermentation, pyruvic acid from glycolysis is changed into lactic acid

  • Most living things carry out fermentation by changing pyruvic acid into lactic acid

  • Unlike alcoholic fermentation, lactic acid fermentation does not give off carbon dioxide

  • Like alcoholic fermentation, lactic acid fermentation makes NAD+ so that glycolysis can continue

  • Lactic acid fermentation after glycolysis can be written as:

Energy and Exercise

• You have three main sources of ATP: ATP already in muscles, ATP made by lactic acid fermentation, and ATP made by cellular respiration

  • At the beginning of a race, the body uses all three sources; however, stored ATP and lactic acid fermentation can give energy only for a short time

• For quick bursts of energy, the body uses ATP already in muscles and ATP from lactic acid fermentation

• For exercise longer than 90 seconds, the body uses cellular respiration