Chapter 5

Energy Flow in the Life of a Cell

5.1 What Is Energy?

• Energy is the capacity to do work.– Synthesizing molecules– Moving objects– Generating heat and light

5.1 What Is Energy?• Types of energy

– Kinetic: energy of movement– Potential: stored energy

Fig. 5-1

5.1 What Is Energy?• First Law of Thermodynamics

– “Energy cannot be created nor destroyed, but it can change its form.”

– Example: potential energy in gasoline can be converted to kinetic energy in a car, but the energy is not lost

5.1 What Is Energy?• Second Law of Thermodynamics

– “When energy is converted from one form to another, the amount of useful energy decreases.”

– No process is 100% efficient.– Example: more potential energy is in the

gasoline than is transferred to the kinetic energy of the car moving

– Where is the rest of the energy? It is released in a less useful form as heat—the total energy is maintained.

5.1 What Is Energy?• Matter tends to become less organized.

– There is a continual decrease in useful energy, and a build up of heat and other non-useful forms of energy.

– Entropy: the spontaneous reduction in ordered forms of energy, and an increase in randomness and disorder as reactions proceed

– Example: gasoline is made up of an eight-carbon molecule that is highly ordered

– When broken down to single carbons in CO2, it is less ordered and more random.

5.1 What Is Energy?

• In order to keep useful energy flowing in ecosystems where the plants and animals produce more random forms of energy, new energy must be brought in.

5.1 What Is Energy?

• Sunlight provides an unending supply of new energy to power all plant and animal reactions, leading to increased entropy.

Fig. 5-2

5.2 How Does Energy Flow In Chemical Reactions?

• Chemical reaction: the conversion of one set of chemical substances (reactants) into another (products)– Exergonic reaction: a reaction that releases

energy; the products contain less energy than the reactants

energyreleased

reactants

productsExergonic reaction

++

(a)

5.2 How Does Energy Flow In Chemical Reactions?

• Exergonic reaction

Fig. 5-3a

5.2 How Does Energy Flow In Chemical Reactions?

• Endergonic reaction: a reaction that requires energy input from an outside source; the products contain more energy than the reactants

energyused

products

reactantsEndergonic reaction

++

(b)

5.2 How Does Energy Flow In Chemical Reactions?

• Endergonic reaction

Fig. 5-3b

5.2 How Does Energy Flow In Chemical Reactions?

• Exergonic reactions release energy.– Example: sugar burned by a flame in the presence

of oxygen produces carbon dioxide (CO2) and water

– Sugar and oxygen contain more energy than the molecules of CO2 and water.

– The extra energy is released as heat.

5.2 How Does Energy Flow In Chemical Reactions?

• Burning glucose releases energy.

Fig. 5-4

energyreleased

C6H12O6 6 O2

(glucose) (oxygen)+

6 CO2

(carbondioxide)

6 H2O(water)

+

5.2 How Does Energy Flow In Chemical Reactions?

• Endergonic reactions require an input of energy.– Example: sunlight energy + CO2 + water in

photosynthesis produces sugar and oxygen– The sugar contains far more energy than the CO2

and water used to form it.

5.2 How Does Energy Flow In Chemical Reactions?

• Photosynthesis requires energy.

Fig. 5-5

C6H12O6 6 O2

(glucose) (oxygen)+

6 CO2

(carbondioxide)

6 H2O(water)

+

energy

high

low

progress of reaction progress of reaction

energycontent

ofmolecules

Activation energy neededto ignite glucose

Energy level of reactants

glucose + O2

CO2 + H2O CO2 + H2O

glucose

Activationenergycapturedfromsunlight

Energy level of reactants

Burning glucose (sugar): an exergonic reaction Photosynthesis: an endergonic reaction(a) (b)

5.2 How Does Energy Flow In Chemical Reactions?

• All reactions require an initial input of energy.– The initial energy input to a chemical reaction is called

the activation energy.

Fig. 5-6

5.2 How Does Energy Flow In Chemical Reactions?

• The source of activation energy is the kinetic energy of movement when molecules collide.

• Molecular collisions force electron shells of atoms to mingle and interact, resulting in chemical reactions.

5.2 How Does Energy Flow in Chemical Reactions?

• Exergonic reactions may be linked with endergonic reactions.– Endergonic reactions obtain energy from

energy-releasing exergonic reactions in coupled reactions.

– Example: the exergonic reaction of burning gasoline in a car provides the endergonic reaction of moving the car

– Example: exergonic reactions in the sun release light energy used to drive endergonic sugar-making reactions in plants

5.3 How Is Energy Carried Between Coupled Reactions?

• The job of transferring energy from one place in a cell to another is done by energy-carrier molecules.– ATP (adenosine triphosphate) is the main energy

carrier molecule in cells, and provides energy for many endergonic reactions.

ADP

ATP

phosphate

energy

+

A P P P

A P P P

5.3 How Is Energy Carried Between Coupled Reactions?

• ATP is made from ADP (adenosine diphosphate) and phosphate plus energy released from an exergonic reaction (e.g., glucose breakdown) in a cell.

Fig. 5-7

5.3 How Is Energy Carried Between Coupled Reactions?

• ATP is the principal energy carrier in cells.– ATP stores energy in its phosphate bonds and

carries the energy to various sites in the cell where energy-requiring reactions occur.

– ATP’s phosphate bonds then break yielding ADP, phosphate, and energy.

– This energy is then transferred to the energy-requiring reaction.

phosphateADP

energy

ATP+

A

A P P P

P P P

5.3 How Is Energy Carried Between Coupled Reactions?

• Breakdown of ATP releases energy.

Fig. 5-8

5.3 How Is Energy Carried Between Coupled Reactions?

• To summarize:– Exergonic reactions (e.g., glucose breakdown)

drive endergonic reactions (e.g., the conversion of ADP to ATP).

– ATP moves to different parts of the cell and is broken down exergonically to liberate its energy to drive endergonic reactions.