Cellular Energy

Energy in the World of Life

• Sustaining life’s organization requires ongoing energy inputs

• Assembly of the molecules of life starts with energy input into

living cells

Energy Conversion

• Only about 10% of the

energy in food goes

toward building body

mass, most is lost in

energy conversions

Energy’s One Way Flow

• The total amount of energy available in the universe to do

work is always decreasing

• Each time energy is transferred, some energy escapes as

heat (not useful for doing work)

• On Earth, energy flows from the sun, through producers, then

consumers

• Living things need a constant input of energy

The Energy of Life

• Energy can come in many forms…Light, heat, electricity, etc.

• Cells receive their energy from certain chemical fuels.

• The main type of chemical fuel that provides energy for all

types of cells is ADENOSINE TRIPHOSPHATE. (ATP)

• ATP is composed of a nitrogen-

containing compound called

ADENINE, a 5-carbon sugar

called RIBOSE and three

PHOSPHATE GROUPS.

ADP

• Cells can also use a compound called ADENOSINE

DIPHOSPHATE, which is identical to ATP, but with one less

phosphate group.

Energy from ATP & ADP

• Energy is released into the cell by breaking the bonds

between the phosphate groups in ATP.

• When bonds break, energy is released and the ATP becomes

ADP.

• Cells can also store energy by

“charging” ADP with phosphate

groups. Think of ADP and ATP

as a rechargeable battery.

ATP as Stored as Energy

• The body contains only a small amount of ATP at a time.

Only enough to run cellular functions that need to be taken

care of immediately.

• Instead, cells use large organic molecules such as glucose to

store mass amounts of energy that will later be turned into

ATP.

• A single molecule of glucose has over 90 times the energy

of a single ATP molecule.

Autotrophs and Heterotrophs

• Autotrophs harvest energy directly from the environment,

and obtain carbon from inorganic molecules

• Plants and most other autotrophs make their own food by

photosynthesis, a process which uses the energy of sunlight

to assemble carbohydrates from carbon dioxide and water

• Animals and other heterotrophs get energy and carbon by

breaking down organic molecules assembled by other

organisms

Two Main Metabolic Pathways

• Aerobic metabolic pathways (using oxygen) are used by

most eukaryotic cells

• Anaerobic metabolic pathways (which occur in the absence

of oxygen) are used by prokaryotes and protists in anaerobic

habitats

Aerobic Respiration

• In modern eukaryotic cells, most of the aerobic respiration

pathway takes place inside mitochondria

• Like chloroplasts, mitochondria have an internal folded

membrane system that allows them to make ATP efficiently

Overview of

Carbohydrate Breakdown Pathways

• Photoautotrophs (make their own energy via the sun) make

ATP during photosynthesis and use it to synthesize glucose

and other carbohydrates

• Most organisms, including photoautotrophs, make ATP by

breaking down glucose and other organic compounds

Figure 7-2 p118

energy

Photosynthesis

CO2glucose

H2O O2

Aerobic Respiration

energy

Overview of Aerobic Respiration

• Three stages

• Glycolysis

• The Krebs cycle

• Electron Transport Chain (ATP formation)

C6H12O6 (glucose) + O2 (oxygen) →CO2 (carbon dioxide) + H2O (water) + energy (ATP)

Figure 7-3 p119

Aerobic Respiration

glucose

Glycolysis

2 NADH 2 pyruvate

Krebs

Cycle

8 NADH, 2 FADH2

Electron Transfer

Phosphorylationoxygen

2 ATP4 ATP (2

net)

6 CO2

2 ATP

H2O

32 ATP

In the Cytoplasm

In the Mitochondrion

Glycolysis –

Glucose Breakdown Starts

• The reactions of glycolysis convert one molecule of glucose to

two molecules of pyruvate (pyruvic acid) for a net yield of two

ATP

• An energy investment of ATP is required to start glycolysis

Glycolysis

• GLYCOLYSIS is the process in which one molecule of glucose is broken

in half, producing two molecules of PYRUVIC ACID, a 3-carbon

compound.

• What do we put into Glycolysis?

• 2 Molecules ATP

• ATP helps break apart glucose

with their energy.

• 2 Molecules of NAD+

• NAD+ is a special chemical that helps

carry high-energy electrons to other

parts of the cellular respiration

process to make ATP.

• Once NAD+ has electrons, it

becomes known as NADH

Glycolysis

• Two ATP are used to split glucose (1)

• Enzymes react with this newly spit

glucose, and released electrons, to form

2 NADH (2)

• Four ATP are formed in this process

from ADP (3)

• Glycolysis ends with the formation

of two three-carbon pyruvic acid

molecules (4)

1

2

3

4

Second Stage of Aerobic Respiration

• The second stage of aerobic respiration completes the

breakdown of glucose that began in glycolysis

• Occurs in mitochondria

• Includes a set of reactions known as the KREBS CYCLE

The Krebs Cycle

• At the end of glycolysis, about 90% of the chemical energy

that was available in glucose is still unused…still locked in the

high-energy electrons trapped in the newly-formed pyruvic

acid.

• To extract that remaining energy, the body uses oxygen to

take these remaining electrons and convert them to energy

for the cell to use.

• This is WHY we need to breathe to stay alive!

The Krebs Cycle

• During the KREBS CYCLE, pyruvic acid is broken down into

carbon dioxide in a series of energy-extracting reactions.

• During the Krebs Cycle, the first compound that is made is

CITRIC ACID, so therefore, the Kreb Cycle is also often

referred to as the CITRIC ACID CYCLE.

• The Krebs Cycle begins when the pyruvic acid made in

glycolysis enters the mitochondria of the cell.

• One carbon from the pyruvic acid breaks off and becomes a part of a

molecule of carbon dioxide, CO2 (1).

• Pyruvic acid then loses high-energy

electrons, NAD+ comes along and grabs

electrons and becomes NADH. (2) The

pyruvic acid goes through a few more

chemical reactions and

becomes CITRIC ACID. (3)

• SO FAR, WHAT IS MADE?

CITRIC ACID, CO2

The Krebs Cycle

1

2

3

The Krebs Cycle

• As the Kreb Cycle continues, the CITRIC ACID

is broken down into a 5-carbon compound,

then a 4-carbon compound. Along the

way, two more molecules of CO2

are released. (4)

• Along the way, high-energy electron

carriers NAD+ and a new carrier

FAD, gain electrons and become

NADH and FADH2. (5)

• In the end, only one molecule of

ATP is generated. (6)

• SO…

FROM ONE MOLECULE OF

PYRUVIC ACID, 4 NADH, 1 FADH2

AND 1 ATP ARE PRODUCED.

4

5

6

Aerobic Respiration’s Big Energy Payoff

• Many ATP are formed during the third and final stage of

aerobic respiration

• Electron Transport Chain

• Occurs in mitochondria

• Results in attachment of many phosphates to many ADPs

to form many ATPs

Electron Transport Chain

• From the Kreb Cycle, we obtain the high-energy electron

carriers NADH and FADH2.

• These electron carries make their way from the Kreb Cycle to

the next and final portion of the cellular respiration

process…the ELECTRON TRANSPORT CHAIN (E.T.C.)

• The ELECTRON TRANSPORT

CHAIN uses the high-energy

electrons from the Kreb Cycle to

convert ADP to ATP.

Electron Transport Chain

• High-energy electrons from NADH and FADH2 are passed into and along

the E.T.C. (1)

• The E.T.C. is composed of a series of special proteins that are located in

the inner membrane of the mitochondria.

• These electrons are

passed from one protein

to the next

• The energy from the electrons,

force hydrogen from one side

of membrane to the next. (2)

• In this process, electrons react

with hydrogen, combine them

with oxygen to form water. (3)1

2

3

Electron Transport Chain

• All these hydrogen ions that have moved across the membrane

build up a high positive charge and cause the opposite side to

become oppositely charged.

• Because of this difference

of charges, hydrogen ions

want to escape and do so

through special proteins

called ATP SYNTHASE. (4)

4

Electron Transport Chain

• Every time a hydrogen ion passes through ATP SYNTHASE,

the protein spins, attaching a phosphate group to ADP,

producing ATP. (5)

• On average, every pair of high-energy electrons that moves

down the electron

transport chain provides

enough energy to convert

3 ADP molecules into

3 ATP molecules.

5

Summary: The Energy Harvest

• Typically, the breakdown of one glucose molecule yields 36

ATP

• Glycolysis: 2 ATP

• The Krebs cycle: 2 ATP

• Electron Transport Chain: 32 ATP

Fermentation

Fermentation

• When oxygen is not present, glycolysis turns to a new

pathway known as fermentation, instead of the Krebs Cycle.

• FERMENTATION releases energy from food molecules in the

absence of oxygen.

• During fermentation, cells convert NADH back into NAD+ by

giving high-energy electrons back to pyruvic acid.

• Now, with more NAD+, more ATP can finally be made.

• Fermentation is an ANAEROBIC process, which means

oxygen is not used.

Two Fermentation Pathways

• ALCOHOLIC FERMENTATION

• Yeasts and a few other microorganisms use this process.

Ethyl alcohol and carbon dioxide are given off as wastes.

• Pyruvic Acid + NADH Alcohol + CO2 + NAD+

• Alcoholic fermentation is what causes bread dough to rise.

• When the yeast in bread runs out of oxygen, it begins to

ferment, giving off carbon dioxide bubbles.

• Usually all alcohol given off is evaporated off during the

baking process.

Fermentation

Figure 7-10b p127

Two Fermentation Pathways

LACTIC ACID FERMENTATION

• In many cells, pyruvic acid built up by fermentation can be

converted to a new substance called LACTIC ACID.

• LACTIC ACID helps convert NADH back to NAD+ to make ATP.

• Pyruvic Acid + NADH Lactic Acid + NAD+

• LACTIC ACID is produced in your muscles during rapid exercise

when oxygen is not readily available to your muscle tissues.

Sunlight as an Energy Source

• Energy flow through nearly all ecosystems on Earth begins

when photosynthesizers intercept energy from the sun

• Photosynthetic organisms use pigments to capture the energy

of sunlight and convert it to chemical energy – the energy

stored in chemical bonds

Pigments: The Rainbow Catchers

• Different wavelengths of light form colors of the rainbow

• Pigment

• An organic molecule that selectively absorbs light of

specific wavelengths

• Chlorophyll a

• The most common photosynthetic pigment

• Absorbs violet and red light (appears green)

Photosynthetic Pigments

• Collectively, chlorophyll and accessory pigments absorb most

wavelengths of visible light

• Certain electrons in pigment molecules absorb photons of

light energy, boosting electrons to a higher energy level

• Energy is captured and used for photosynthesis

Some Photosynthetic Pigments

6.4 Overview of Photosynthesis

• In plants and other photosynthetic eukaryotes, photosynthesis

occurs in chloroplasts

• Photosynthesis occurs in two stages

Two Stages of Photosynthesis

• Light-dependent reactions

• First stage of photosynthesis

• Light energy is transferred to ATP and NADPH

• Water molecules are split, releasing O2

• Occurs in the grana of a chloroplast

• Light-independent reactions

• Second stage of photosynthesis

• Energy in ATP and NADPH drives the creation of glucose

and other carbohydrates from CO2 and water

• Occurs in the stroma of a chloroplast

Chloroplast

Photosynthesis

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

3D ANIMATION: Photosynthesis Bio

Experience 3D

Light-Dependent Reactions

• Light dependent reactions use energy from the sun to

produce ATP and NADPH.

• Light Dependent reactions take place using two special

proteins that are trapped in the membrane of chloroplasts…

• PHOTOSYSTEM II

• PHOTOSYSTEM I

(Photosystem II comes first but it is named “II” because it

was discovered after Photosystem I)

Light-Dependent Reactions

1. Photons of light hit PSII (1) and excite electrons (2). These

electrons, travel out of PSII where it interacts with hydrogen ions

and forces hydrogen ions (3) through the membrane and into the

thylakoid (4). To replace the lost electrons in PSII, water is split and

oxygen is given off (5).

1 3

2

4

5

Light-Dependent Reactions

2. The excited electron, travels through the membrane, through

other proteins and eventually reaches PS1(6). Light hits PS1,

re-energizing that electron(7) which will help create the

electron carrier NADPH (8).

6

78

Light-Dependent Reactions

3. Finally, hydrogen ions, which are trapped in the thylakoid (9),

find their way to ATP synthase (10). The ions travel up ATP

synthase, spinning it and giving it energy to turn ADP into

useful ATP (11).

9

11

10

Light-Independent Reactions

• Light-Independent Reactions are also known as the

CALVIN CYCLE.

• So what’s carried over from the light-dependent reactions?

• NADPH

• ATP

• What’s needed for the light-independent reactions?

• CO2

• What’s going to be produced?

• Glucose (C6H12O6)

• GLUCOSE is used as an energy storage molecule for the

plant.

Light-Independent Reactions

• 6 carbon dioxide (CO2) molecules enter the reaction

from the atmosphere (1) and combine with other

molecules to form special 3-carbon molecules.1

Light-Independent Reactions

• These 3-carbon molecules are bombarded with energy from

ATP and electrons from NADPH. (2)

• This excites the 3-carbon molecules and pumps them full

of energy.

• ATP turns into

ADP and the

NADPH turns

back into

NADP+.

2

Light-Independent Reactions

• Two of the 3-carbon molecules go on to produce a 6-carbon

sugar, called GLUCOSE (C6H12O6). (3)

• The remaining 3-carbon molecules get recycled and start the

Calvin Cycle over

again. (4)

3

4