1. Energy Flow in the Life of a Cell6
Energy Flow in the Life of a Cell 1

2. Chapter 6 At a Glance 6.1 What Is Energy?
6.2 How Is Energy Transformed During Chemical Reactions?
6.3 How Is Energy Transported Within Cells?

3. 6.1 What Is Energy? Energy is the capacity to do work
Work is a transfer of energy to an object, which causes the object to move

4. 6.1 What Is Energy?
Chemical energy is the energy that is contained in molecules and released by chemical reactions
Molecules that provide chemical energy include sugar, glycogen, and fat
Cells use specialized molecules such as ATP to accept and transfer energy from one chemical reaction to the next

5. 6.1 What Is Energy? There are two fundamental types of energy
Potential energy is stored energy
For example, the chemical energy in bonds, the electrical charge in a battery, and a penguin poised to plunge
Kinetic energy is the energy of movement
For example, light, heat, electricity, and the movement of objects

6. Figure 6-1 Converting potential energy to kinetic energy

7. 6.1 What Is Energy?
The laws of thermodynamics describe the basic properties of energy
The laws describe the quantity (the total amount) and the quality (the usefulness) of energy
Energy can neither be created nor destroyed (the first law of thermodynamics), but can change form
The first law is often called the law of conservation of energy
The total amount of energy within a closed system remains constant unless energy is added or removed from the system

8. 6.1 What Is Energy?
The laws of thermodynamics describe the basic properties of energy (continued)
The amount of useful energy decreases when energy is converted from one form to another (the second law of thermodynamics)
Entropy (disorder) is the tendency to move toward a loss of complexity and of useful energy and toward an increase in randomness, disorder, and less-useful energy

9. 6.1 What Is Energy?
The laws of thermodynamics describe the basic properties of energy (continued)
Useful energy tends to be stored in highly organized matter, and when energy is used in a closed system (such as the world in which we live), there is an overall increase in entropy
For example, when gasoline is burned, the orderly arrangement of eight carbons bound together in a gasoline molecule are converted to eight randomly moving molecules of carbon dioxide

10. Figure 6-2 Energy conversions result in a loss of useful energy
Combustion
by engine
gas
100 units chemical
energy
80 units
heat energy 
20 units kinetic energy 10

11. 6.1 What Is Energy?
Living things use the energy of sunlight to create the low-entropy conditions of life
The highly organized low-entropy systems of life do not violate the second law of thermodynamics because they are achieved through a continuous influx of usable light energy from the sun
In creating kinetic energy in the form of sunlight, the sun also produces vast entropy as heat

12. 6.2 How Is Energy Transformed During Chemical Reactions?
A chemical reaction is a process that forms or breaks the chemical bonds that hold atoms together
Chemical reactions convert one set of chemical substances, the reactants, into another set, the products
All chemical reactions either release energy or require a net input of energy
Exergonic reactions release energy
Endergonic reactions require an input of energy

13. Figure 6-3 An exergonic reaction
energy
reactants
products 13

14. Figure 6-4 An endergonic reaction
energy
products
reactants 14

15. 6.2 How Is Energy Transformed During Chemical Reactions?
Exergonic reactions release energy
Reactants contain more energy than products in exergonic reactions
An example of an exergonic reaction is the burning of glucose
As glucose is burned, the sugar (C6H12O6) combines with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O), releasing energy
Because molecules of sugar contain more energy than do molecules of carbon dioxide and water, the reaction releases energy

16. Figure 6-5 Reactants and products of burning glucose
energy
C6H12O6 
6 O2
(glucose)
(oxygen)
6 CO2 
6 H2O
(carbon
dioxide)
(water) 16

17. 6.2 How Is Energy Transformed During Chemical Reactions?
Endergonic reactions require a net input of energy
The reactants in endergonic reactions contain less energy than the products
An example of an endergonic reaction is photosynthesis
In photosynthesis, green plants add the energy of sunlight to the lower-energy reactants water and carbon dioxide to produce the higher-energy product sugar

18. Figure 6-6 Photosynthesis
energy
C6H12O6 
6 O2
(glucose)
(oxygen)
6 CO2 
6 H2O
(carbon
dioxide)
(water) 18

19. 6.2 How Is Energy Transformed During Chemical Reactions?
Endergonic reactions require a net input of energy (continued)
All chemical reactions require an initial energy input (activation energy) to get started
The negatively charged electron shells of atoms repel one another and inhibit bond formation
Molecules need to be moving with sufficient collision speed to overcome electronic repulsion and react
Increasing the temperature will increase kinetic energy and, thus, the rate of reaction

20. Figure 6-7 Activation energy in an exergonic reaction
Activation energy required
to start the reaction
high
energy level of reactants
reactants
energy
content of
molecules
energy level of products
products
low
progress of reaction
An exergonic reaction
Sparks ignite gas 20

21. 6.3 How Is Energy Transported Within Cells?
Most organisms are powered by the breakdown of glucose
Energy in glucose cannot be used directly to fuel endergonic reactions
Energy released by glucose breakdown is first transferred to an energy-carrier molecule
Energy-carrier molecules are high-energy, unstable molecules that are synthesized at the site of an exergonic reaction, capturing some of the released energy

22. 6.3 How Is Energy Transported Within Cells?
ATP and electron carriers transport energy within cells
Adenosine triphosphate (ATP) is the most common energy-carrying molecule
ATP is composed of the nitrogen-containing base adenine, the sugar ribose, and three phosphates
ATP is sometimes called the “energy currency” of cells

23. 6.3 How Is Energy Transported Within Cells?
ATP and electron carriers transport energy within cells (continued)
Energy is released in cells during glucose breakdown and is used to combine the relatively low-energy molecules adenosine diphosphate (ADP) and phosphate (P) into ATP
Energy is stored in the high-energy phosphate bonds of ATP
The formation of ATP is an endergonic reaction

24. Figure 6-8 The interconversion of ADP and ATP
energy
ATP
phosphate
ADP
ATP synthesis: Energy is stored in ATP
energy
ATP
phosphate
ADP
ATP breakdown: Energy is released 24

25. 6.3 How Is Energy Transported Within Cells?
ATP and electron carriers transport energy within cells (continued)
At sites in the cell where energy is needed, ATP is broken down into ADP  P and its stored energy is released
This energy is then transferred to endergonic reactions through coupling
Unlike glycogen and fat, ATP stores energy very briefly before being broken down

26. 6.3 How Is Energy Transported Within Cells?
ATP and electron carriers transport energy within cells (continued)
ATP is not the only energy-carrier molecule in cells
Energy can be transferred to electrons in glucose metabolism and photosynthesis
Electron carriers such as nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD) transport high-energy electrons
Electron carriers donate their high-energy electrons to other molecules, often leading to ATP synthesis

27. 6.3 How Is Energy Transported Within Cells?
Coupled reactions link exergonic with endergonic reactions
In a coupled reaction, an exergonic reaction provides the energy needed to drive an endergonic reaction
Sunlight energy stored in glucose by plants is transferred to other organisms by the exergonic breakdown of the sugar and its use in endergonic processes such as protein synthesis
The two reactions may occur in different parts of the cell, so energy-carrier molecules carry the energy from one to the other

28. Animation: Coupled Reactions

29. Figure 6-9 Coupled reactions within living cells
high-energy
reactants
(glucose)
high-energy
products
(protein)
ATP
exergonic
(glucose breakdown)
endergonic
(protein synthesis)
low-energy
products
(CO2, H2O)
ADP  Pi
low-energy
reactants
(amino acids) 29