lesson overview lesson overview cellular respiration: an overview chemical energy and food where do...

Lesson OverviewLesson Overview Cellular Respiration: An OverviewCellular Respiration: An Overview

Chemical Energy and FoodWhere do organisms get energy?

Organisms get the energy they need from food.


Chemical Energy and FoodFood provides living things with the chemical building blocks they need to grow and reproduce.

Food molecules contain chemical energy that is released when its chemical bonds are broken.


Chemical Energy and FoodEnergy 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.

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.

Cells break down food molecules gradually and use the energy stored in the chemical bonds to produce compounds such as ATP that power the activities of the cell.


Overview of Cellular RespirationWhat is cellular respiration?

Cellular respiration is the process that releases energy from food in the

presence of oxygen.


Overview of Cellular RespirationIf oxygen is available, organisms can obtain energy from food by a process called cellular respiration. The summary of cellular respiration is presented below.

In symbols:

6 O2 + C6H12O6 6 CO2 + 6 H2O + Energy

In words:

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.


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


Stages of Cellular RespirationGlycolysis produces only a small amount of energy. Most of glucose’s energy (90%) remains locked in the chemical bonds of pyruvic acid at the end of glycolysis.


Stages of Cellular RespirationDuring the Krebs cycle, a little more energy is generated from pyruvic acid.


Stages of Cellular RespirationThe electron transport chain produces the bulk of the energy in cellular respiration by using oxygen, a powerful electron acceptor.


Oxygen and EnergyPathways of cellular respiration that require oxygen are called aerobic. The Krebs cycle and electron transport chain are both aerobic processes. Both processes take place inside the mitochondria.


Oxygen and EnergyGylcolysis 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. Glycolysis takes place in the cytoplasm of a cell.


Comparing Photosynthesis and Cellular Respiration

What is the relationship between 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.



Photosynthesis and cellular respiration are opposite processes.

The energy flows in opposite directions. Photosynthesis “deposits” energy, and cellular respiration “withdraws” energy.

The reactants of cellular respiration are the products of photosynthesis and vice versa.



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.


THINK ABOUT ITFood burns! How does a living cell extract the energy stored in food without setting a fire or blowing things up?


GlycolysisWhat happens during the process of glycolysis?

During glycolysis, 1 molecule of glucose, a 6-carbon compound, is transformed into 2 molecules of pyruvic acid, a 3-carbon compound.


GlycolysisGlycolysis is the first stage of cellular respiration.

During glycolysis, glucose is broken down into 2 molecules of the 3-carbon molecule pyruvic acid. Pyruvic acid is a reactant in the Krebs cycle.

ATP and NADH are produced as part of the process.


ATP ProductionThe cell “deposits” 2 ATP molecules into its “account” to get glycolysis going.


ATP ProductionGlycolysis then produces 4 ATP molecules, giving the cell a net gain of 2 ATP molecules for each molecule of glucose that enters glycolysis.


NADH ProductionDuring glycolysis, the electron carrier NAD+ (nicotinamide adenine dinucleotide) accepts a pair of high-energy electrons and becomes NADH.


NADH ProductionNADH carries the high-energy electrons to the electron transport chain, where they can be used to produce more ATP.

2 NADH molecules are produced for every molecule of glucose that enters glycolysis.


The Advantages of GlycolysisGlycolysis produces ATP very fast, which is an advantage when the energy demands of the cell suddenly increase.

Glycolysis does not require oxygen, so it can quickly supply energy to cells when oxygen is unavailable.


The Krebs CycleWhat happens during the Krebs cycle?

During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions.


The Krebs CycleDuring the Krebs cycle, the second stage of cellular respiration, pyruvic acid produced in glycolysis is broken down into carbon dioxide in a series of energy-extracting reactions.

The Krebs cycle is also known as the citric acid cycle because citric acid is the first compound formed in this series of reactions.


Citric Acid ProductionPyruvic acid from glycolysis enters the matrix, the innermost compartment of the mitochondrion.


Citric Acid ProductionOnce pyruvic acid is in the mitochondrial matrix, NAD+ accepts 2 high-energy electrons to form NADH. One molecule of CO2 is also produced.

The remaining 2 carbon atoms react to form acetyl-CoA.


Citric Acid ProductionAcetyl-CoA combines with a 4-carbon molecule to produce citric acid.


Energy ExtractionCitric acid is broken down into a 5-carbon compound and then a 4-carbon compound. Two molecules of CO2 are released. The 4-carbon compound can then start the cycle again by combining with acetyl-CoA.


Energy ExtractionEnergy released by the breaking and rearranging of carbon bonds is captured in the forms of ATP, NADH, and FADH2.


Energy ExtractionFor each turn of the cycle, one ADP molecule is converted into ATP. ATP can directly power the cell’s activities.


Energy ExtractionThe electron carriers NAD+ and FAD each accept pairs of high-energy electrons to form NADH and FADH2. NADH and FADH2 are used in the electron transport chain to generate ATP.


Energy ExtractionRemember! Each molecule of glucose results in 2 molecules of pyruvic acid, which enter the Krebs cycle. So each molecule of glucose results in two complete “turns” of the Krebs cycle.

Therefore, for each glucose molecule, 6 CO2 molecules, 2 ATP molecules, 8 NADH molecules, and 2 FADH2 molecules are produced.


Electron Transport and ATP SynthesisHow does the electron transport chain use high-energy electrons from glycolysis and the Krebs cycle?

The electron transport chain uses the high-energy electrons from glycolysis and the Krebs cycle to convert ADP into ATP.


Electron TransportNADH and FADH2 pass their high-energy electrons to electron carrier proteins in the electron transport chain.


Electron TransportAt the end of the electron transport chain, the electrons combine with H+ ions and oxygen to form water.


Electron TransportEnergy generated by the electron transport chain is used to move H+ ions against a concentration gradient across the inner mitochondrial membrane and into the intermembrane space.


ATP ProductionH+ ions pass back across the mitochondrial membrane through the ATP synthase, causing the ATP synthase molecule to spin. With each rotation, the ATP synthase attaches a phosphate to ADP to produce ATP.


The TotalsHow much energy does cellular respiration generate?


Energy TotalsIn the presence of oxygen, the complete breakdown of glucose through cellular respiration results in the production of 36 ATP molecules.

This represents about 36 percent of the total energy of glucose. The remaining 64 percent is released as heat.


Energy TotalsThe cell can generate ATP from just about any source, even though we’ve modeled it using only glucose. Complex carbohydrates are broken down into simple sugars like glucose. Lipids and proteins can be broken down into molecules that enter the Krebs cycle or glycolysis at one of several places.


FermenationHow do organisms generate energy when oxygen is not available?

In the absence of oxygen, fermentation releases energy from food

molecules by producing ATP.


FermentationFermentation is a process by which energy can be released from food molecules in the absence of oxygen. Fermentation occurs in the cytoplasm of cells.


FermentationUnder anaerobic conditions, fermentation follows glycolysis. During fermentation, cells convert NADH produced by glycolysis back into the electron carrier NAD+, which allows glycolysis to continue producing ATP.


Alcoholic FermentationYeast and a few other microorganisms use alcoholic fermentation that produces ethyl alcohol and carbon dioxide.

This process is used to produce alcoholic beverages and causes bread dough to rise.


Alcoholic FermentationChemical equation:

Pyruvic acid + NADH Alcohol + CO2 + NAD+


Lactic Acid FermentationMost organisms, including humans, carry out fermentation using a chemical reaction that converts pyruvic acid to lactic acid.

Chemical equation:

Pyruvic acid + NADH Lactic acid + NAD+


Energy and ExerciseHow does the body produce ATP during different stages of exercise?

For short, quick bursts of energy, the body uses ATP already in muscles as

well as ATP made by lactic acid fermentation.

For exercise longer than about 90 seconds, cellular respiration is the only

way to continue generating a supply of ATP.


Quick EnergyCells normally contain small amounts of ATP produced during cellular respiration, enough for a few seconds of intense activity.

Lactic acid fermentation can supply enough ATP to last about 90 seconds. However, extra oxygen is required to get rid of the lactic acid produced. Following intense exercise, a person will huff and puff for several minutes in order to pay back the built-up “oxygen debt” and clear the lactic acid from the body.


Long-Term EnergyFor intense exercise lasting longer than 90 seconds, cellular respiration is required to continue production of ATP.

Cellular respiration releases energy more slowly than fermentation does.

The body stores energy in the form of the carbohydrate glycogen. These glycogen stores are enough to last for 15 to 20 minutes of activity. After that, the body begins to break down other stored molecules, including fats, for energy.

Hibernating animals like this brown bear rely on stored fat for energy when they sleep through the winter.

lesson overview lesson overview cellular respiration: an overview chemical energy and food where do...

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