What You’ll Learn

You will recognize why organisms need a constant supply of energy and where that energy comes from.

You will identify how cells store and release energy as ATP.

You will describe the pathways by which cells obtain energy.

What You’ll Learn

You will compare ATP production in mitochondria and in chloroplasts.

• Explain why organisms need a supply of energy.

Section Objectives:

• Describe how energy is stored and released by ATP.

• All living organisms must be able to obtain energy from the environment in which they live.

• Plants and other green organisms are able to trap the light energy in sunlight and store it in the bonds of certain molecules for later use.

Cell EnergyCell Energy

• Other organisms cannot use sunlight directly.

• They eat green plants. In that way, they obtain the energy stored in plants.

Cell EnergyCell Energy

• Active transport, cell division, movement of flagella or cilia, and the production, transport, and storage of proteins are some examples of cell processes that require energy.

Work and the need for energyWork and the need for energy

• There is a molecule in your cells that is a quick source of energy for any organelle in the cell that needs it.

• The name of this energy molecule is adenosine triphosphate or ATP for short.

• ATP is composed of an adenosine molecule with three phosphate groups attached.

Work and the need for energyWork and the need for energy

• The charged phosphate groups act like the positive poles of two magnets.

• Bonding three phosphate groups to form adenosine triphosphate requires considerable energy.

Forming and Breaking Down ATPForming and Breaking Down ATP

• When only one phosphate group bonds, a small amount of energy is required and the chemical bond does not store much energy. This molecule is called adenosine monophosphate (AMP).

Forming and Breaking Down ATPForming and Breaking Down ATP

• When a second phosphate group is added, more energy is required to force the two groups together. This molecule is called adenosine diphosphate, or ADP.

• An even greater amount of energy is required to force a third charged phosphate group close enough to the other two to form a bond. When this bond is broken, energy is released.

Forming and Breaking Down ATPForming and Breaking Down ATP

• The energy of ATP becomes available to a cell when the molecule is broken down.

Adenosine

Adenosine

P P P

P

P

P P

Adenosine triphosphate (ATP)

Adenosine diphosphate (ADP)

Forming and Breaking Down ATPForming and Breaking Down ATP

• When ATP is broken down and the energy is released, the energy must be captured and used efficiently by cells.

• Many proteins have a specific site where ATP can bind.

How cells tap into the energy stored in ATPHow cells tap into the energy stored in ATP

• Then, when the phosphate bond is broken and the energy released, the cell can use the energy for activities such as making a protein or transporting molecules through the plasma membrane.

ATP

ADP

ADP

Protein P Energy

How cells tap into the energy stored in ATPHow cells tap into the energy stored in ATP

• When ATP has been broken down to ADP, the ADP is released from the binding site in the protein and the binding site may then be filled by another ATP molecule.

How cells tap into the energy stored in ATPHow cells tap into the energy stored in ATP

• Relate the structure of chloroplasts to the events in photosynthesis.

Section Objectives:

• Describe light-dependent reactions.

• Explain the reactions and products of the light-independent Calvin cycle.

Trapping Energy from SunlightTrapping Energy from Sunlight• The process that uses the sun’s energy to

make simple sugars is called photosynthesis.

1. The light-dependent reactions convert light energy into chemical energy.

• Photosynthesis happens in two phases.

2. The molecules of ATP produced in the light-dependent reactions are then used to fuel the light-independent reactions that produce simple sugars.

• The general equation for photosynthesis is written as 6CO2 + 6H2O→C6H12O6 + 6O2

Trapping Energy from SunlightTrapping Energy from Sunlight

Click image to view movie.

Trapping Energy from SunlightTrapping Energy from Sunlight

The chloroplast and pigmentsThe chloroplast and pigments• To trap the energy in the sun’s light, the

thylakoid membranes contain pigments, molecules that absorb specific wavelengths of sunlight.

• Although a photosystem contains several kinds of pigments, the most common is chlorophyll.

• Chlorophyll absorbs most wavelengths of light except green.

Light-Dependent ReactionsLight-Dependent Reactions

• As sunlight strikes the chlorophyll molecules in a photosystem of the thylakoid membrane, the energy in the light is transferred to electrons.

• These highly energized, or excited, electrons are passed from chlorophyll to an electron transport chain, a series of proteins embedded in the thylakoid membrane.

Light-Dependent ReactionsLight-Dependent Reactions

Sun

Chlorophyll passes energy down through the electron transport chain.

for the use in light-independent reactions

bonds P to ADP

forming ATPoxygen

released

splitsH2O

H+

NADP+

NADPH

Light energy transfers to chlorophyll.

Energized electrons provide energy that

• At each step along the transport chain, the electrons lose energy.

• This “lost” energy can be used to form ATP from ADP, or to pump hydrogen ions into the center of the thylakoid disc.

• Electrons are re-energized in a second photosystem and passed down a second electron transport chain.

Light-Dependent ReactionsLight-Dependent Reactions

• The electrons are transferred to the stroma of the chloroplast. To do this, an electron carrier molecule called NADP is used.

• NADP can combine with two excited electrons and a hydrogen ion (H+) to become NADPH.

• NADPH will play an important role in the light-independent reactions.

Light-Dependent ReactionsLight-Dependent Reactions

Restoring electronsRestoring electrons• To replace the lost electrons, molecules of

water are split in the first photosystem. This reaction is called photolysis.

Sun

Chlorophyll

2e-

H2O

O2 + 2H+1_2

H2O 2

_1 O2 + 2e-

• The oxygen produced by photolysis is released into the air and supplies the oxygen we breathe.

• The electrons are returned to chlorophyll.

• The hydrogen ions are pumped into the thylakoid, where they accumulate in high concentration.

Restoring electronsRestoring electrons

The Calvin Cycle

The Calvin Cycle

(CO2)

(Unstable intermediate)

ATP

ADP +

ADP +

(Sugars and other carbohydrates)

NADPH

NADP+

(PGAL)

(PGAL)

ATP

(PGAL)

(RuPB)

• Carbon fixation The carbon atom from CO2 bonds with a five-carbon sugar called ribulose biphosphate (RuBP) to form an unstable six-carbon sugar.

(CO2)

(RuBP)

• The stroma in chloroplasts hosts the Calvin cycle.

The Calvin CycleThe Calvin Cycle

• Formation of 3-carbon molecules The six-carbon sugar formed in Step A immediately splits to form two three-carbon molecules.

(Unstable intermediate)

The Calvin CycleThe Calvin Cycle

The Calvin CycleThe Calvin Cycle• Use of ATP and NADPH

A series of reactions involving ATP and NADPH from the light-dependent reactions converts the three-carbon molecules into phosphoglyceraldehyde (PGAL), three-carbon sugars with higher energy bonds.

ATP

NADPH

NADP+

(PGAL)

ADP +

• Sugar production One out of every six molecules of PGAL is transferred to the cytoplasm and used in the synthesis of sugars and other carbohydrates. After three rounds of the cycle, six molecules of PGAL are produced.

(PGAL)

(Sugars and other carbohydrates)

The Calvin CycleThe Calvin Cycle

• RuBP is replenished Five molecules of PGAL, each with three carbon atoms, produce three molecules of the five-carbon RuBP. This replenishes the RuBP that was used up, and the cycle can continue.

PADP+

ATP

(PGAL)

The Calvin CycleThe Calvin Cycle

• Compare and contrast cellular respiration and fermentation.

Section Objectives:

• Explain how cells obtain energy from cellular respiration.

Cellular RespirationCellular Respiration

• The process by which mitochondria break down food molecules to produce ATP is called cellular respiration.

• There are three stages of cellular respiration:

glycolysis, the citric acid cycle, and the electron transport chain.

Cellular RespirationCellular Respiration

• The first stage, glycolysis, is anaerobic—no oxygen is required.

• The last two stages are aerobic and require oxygen to be completed.

GlycolysisGlycolysis• Glycolysis is a series of chemical reactions in

the cytoplasm of a cell that break down glucose, a six-carbon compound, into two molecules of pyruvic acid, a three-carbon compound.

Glucose

2ATP2ADP

2PGAL

4ADP + 4P

2NAD+

2NADH + 2H+

4ATP

2 Pyruvic acid

• Glycolysis is not very effective, producing only two ATP molecules for each glucose molecule broken down.

Glucose

2ATP2ADP

2PGAL

4ADP + 4P

2NAD+

2NADH + 2H+

4ATP

2 Pyruvic acid

GlycolysisGlycolysis

• Before citric acid cycle and electron transport chain can begin, pyruvic acid undergoes a series of reactions in which it gives off a molecule of CO2 and combines with a molecule called coenzyme A to form acetyl-CoA.

Pyruvic acid

Outside the mitochondrion

Mitochondrial membrane

Inside the mitochondrion

Pyruvic acid

Intermediate by-product NAD+

NADH + H+

CO2

Coenzyme A

- CoAAcetyl-CoA

GlycolysisGlycolysis

The citric acid cycleThe citric acid cycle

• The citric acid cycle, also called the Krebs cycle, is a series of chemical reactions similar to the Calvin cycle in that the molecule used in the first reaction is also one of the end products.

• For every turn of the cycle, one molecule of ATP and two molecules of carbon dioxide are produced.

The Citric Acid CycleThe Citric Acid Cycle

(Acetyl-CoA)

Citric acid NAD+

NADH + H+

O= =O(CO2)

NAD+

O= =O

(CO2)ADP +

ATP

FADFADH2

Citric Acid Cycle

NAD+

NADH + H+ Oxaloacetic acid

The mitochondria host the citric acid cycle.

NADH + H+

The citric acid cycleThe citric acid cycle• Citric acid The

two-carbon compound acetyl-CoA reacts with a four-carbon compound called oxaloacetic acid to form citric acid, a six-carbon molecule.

Acetyl-CoA

Citric acidOxaloacetic acid

• Formation of CO2 A molecule of CO2 is formed, reducing the eventual product to a five-carbon compound. In the process, a molecule of NADH and H+ is produced.

NAD+

NADH + H+

O= =O(CO2)

The citric acid cycleThe citric acid cycle

• Formation of the second CO2 Another molecule of CO2 is released, forming a four-carbon compound. One molecule of ATP and a molecule of NADH are also produced.

NAD+

NADH + H+

O= =O

(CO2)ADP +

ATP

The citric acid cycleThe citric acid cycle

FADH2

NADH + H+

• Recycling of oxaloacetic acid The four-carbon molecule goes through a series of reactions in which FADH2, NADH, and H+ are formed. The carbon chain is rearranged, and oxaloacetic acid is again made available for the cycle.

NAD+

FAD

The citric acid cycleThe citric acid cycle

The electron transport chainThe electron transport chain• In the electron transport chain, the carrier

molecules NADH and FADH2 gives up electrons that pass through a series of reactions. Oxygen is the final electron acceptor.

EnzymeElectron carrier proteins

e -

NADH

FADH2

NAD+

FAD

Electron pathway

4H+ + O2

+ 4 electrons

H2O

H2O

ADP + ATP

Inner membrane

Center of mitochondrion

Space between inner and outer

membranes

• Overall, the electron transport chain adds 32 ATP molecules to the four already produced.

The electron transport chainThe electron transport chain

FermentationFermentation

• During heavy exercise, when your cells are without oxygen for a short period of time, an anaerobic process called fermentation follows glycolysis and provides a means to continue producing ATP until oxygen is available again.

Lactic acid fermentationLactic acid fermentation

• Lactic acid fermentation is one of the processes that supplies energy when oxygen is scarce.

• In this process, the reactions that produced pyruvic acid are reversed.

• Two molecules of pyruvic acid use NADH to form two molecules of lactic acid.

Lactic acid fermentationLactic acid fermentation

• This releases NAD+ to be used in glycolysis, allowing two ATP molecules to be formed for each glucose molecule.

• The lactic acid is transferred from muscle cells, to the liver that converts it back to pyruvic acid.

Alcoholic fermentationAlcoholic fermentation

• Another type of fermentation, alcoholic fermentation, is used by yeast cells and some bacteria to produce CO2 and ethyl alcohol.

Comparing Photosynthesis and Cellular RespirationComparing Photosynthesis and Cellular Respiration

Photosynthesis Cellular RespirationFood synthesized Food broken down

Energy from sun stored in glucose Energy of glucose released

Carbon dioxide taken in Carbon dioxide given off

Oxygen given off Oxygen taken in

Produces sugars from PGAL Produces CO2 and H2O

Requires light Does not require light

Occurs only in presence of chlorophyll

Occurs in all living cells

Table 9.1 Comparison of Photosynthesis and Cellular Respiration

• ATP is the molecule that stores energy for easy use within the cell.

The Need for Energy

• ATP is formed when a phosphate group is added to ADP. When ATP is broken down, ADP and phosphate are formed and energy is released.

• Green organisms trap the energy in sunlight and store it in the bonds of certain molecules for later use.

• Organisms that cannot use sunlight directly obtain energy by consuming plants or other organisms that have consumed plants.

The Need for Energy

• Photosynthesis is the process by which cells use light energy to make simple sugars.

• Chlorophyll in the chloroplasts of plant cells traps light energy needed for photosynthesis.

• The light reactions of photosynthesis produce ATP and result in the splitting of water molecules.

Photosynthesis: Trapping the Sun’s Energy

Photosynthesis: Trapping the Sun’s Energy• The reactions of the Calvin Cycle make

carbohydrates using CO2 along with ATP and NADPH from the light reactions.

Getting Energy to Make ATP

• In cellular respiration, cells break down carbohydrates to release energy.

• The first stage of cellular respiration, glycolysis, takes place in the cytoplasm and does not require oxygen.

• The citric acid cycle takes place in mitochondria and requires oxygen.