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Energy in a Cell. Chapter 9. Cell Energy. Energy is essential to life Plants trap light energy in sunlight and store it in the bonds of certain molecules to use later Other organisms get energy from eating those green plants What processes can you name that require energy?.

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Energy in a cell

Energy in a Cell

Chapter 9

Cell energy
Cell Energy

Energy is essential to life

Plants trap light energy in sunlight and store it in the bonds of certain molecules to use later

Other organisms get energy from eating those green plants

What processes can you name that require energy?

Adenosine triphosphate atp
Adenosine Triphosphate (ATP)

An energy molecule in the cell that allows for quick and easy access to energy when needed by the cell’s organelles.

A type of chemical energy

Releases energy when the chemical bonds are broken


Forming atp
Forming ATP

Phosphate groups are negatively charged

Negative doesn’t like being next to negative

A small amount of energy is required to attach one phosphate group to adenosine (AMP)

When a second phosphate group is added, this requires a lot more energy (ADP)

When a third phosphate group is added, this requires an even greater amount of energy (ATP)

The process of forming ATP requires much energy

Breaking down atp
Breaking down ATP

Energy of ATP becomes available to a cell when the molecule is broken down

When a cell requires energy, ATP goes to the cell, attaches to the binding site, and a phosphate group is broken off – this gives off energy for the cell and the ATP molecule becomes ADP (fig. 9.2 pg. 223)

In order for ADP to become ATP again it goes to the mitochondria and gets recharged (another phosphate group gets attached)

Uses of cell energy
Uses of cell energy

  • Energy is VERY important on the cellular level

    • Making new molecules

    • Building membranes and cell organelles

    • Cells use energy to maintain homeostasis

    • Kidneys use energy to move molecules and ions in order to eliminate waste substances while keeping needed substances in the bloodstream.


  • In this section…

    • What is photosynthesis?

    • Where photosynthesis happens

    • Color: How it works

    • The two phases of photosynthesis


  • A process of taking light energy and converting it into chemical energy

  • This energy is stored as carbohydrates in plants

  • Happens in two phases:

    • Light-dependent reactions- converts light energy into chemical energy-molecules of ATP produced fuel light-independent reactions

    • Light-independent reactions- produce glucose

Where does photosynthesis occur
Where does photosynthesis occur?

In chloroplasts there are thylakoid disks/grana

Light-dependent reactions happen in the thylakoid membranes


To trap the energy in the sunlight, the thylakoid membranes contain pigments.

Pigments are molecules that absorb specific wavelengths of sunlight

Chlorophyll is the most common type of pigment in chloroplasts

Why we see color
Why we see color

We see colors that are reflected and not absorbed. Green pigments absorb all light except green (chlorophyll)

In the fall, trees stop producing chlorophyll, which results in the different colors seen.

The big picture
The Big Picture


Light-Dependent Reactions

Light-Independent Reactions

(Calvin Cycle)

Stored Energy

(stored as glucose)

Light dependent reactions
Light-Dependent Reactions

First phase of photosynthesis requires sunlight.

A light-dependent reaction involves sunlight striking molecules of chlorophyll and exciting an electron.

Excited electrons are passed from chlorophyll to an electron transport chain

Electron transport chain- a series of proteins embedded in the thylakoid membrane

Light dependent reactions1
Light-dependent reactions

Once in the electron transport chain, each protein in the chain passes the energized electrons along to the next protein-

some energy is lost during each pass-

lost energy can be used to form ATP from ADP OR to pump hydrogen ions into the center of the thylakoid disc.

After electrons have traveled down the electron transport chain, they are re-energized in a second photosystem and passed down a second electron transport chain- electrons are still very energized

Light dependent reactions2
Light-Dependent reactions

Electrons are then transferred to the stroma of the chloroplast

Transferred by an electron carrier molecule called NADP+ (nicotinamide adenine dinucleotide phosphate)

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

NADPH stores the energy until it can transfer it to the stroma- this is where NADPH will play an important role in the light-independent reaction.

Restoring electrons
Restoring Electrons

  • The chlorophyll needs to replace the electrons that were lost at the beginning of photosynthesis in order to absorb additional light to keep the process going.

  • To replace lost electrons, the molecules of water are split in the first photostem- reaction called photolysis

  • For every water molecule that is split, 1 Oxygen, 2 electrons, and 2 Hydrogen ions are formed

    • Oxygen produced by photolysis is released into the air-supplies oxygen for air we breathe

    • Electrons are returned to the chlorophyll

    • H+ ions are pumped into the thylakoid-> they accumulate in high concentrations which causes a concentration gradient- H+ ions diffuse out of thylakoid and provide energy for production of ATP (called chemiosmosis)

Light independent reactions
Light-Independent reactions

2nd phase of photosynthesis

Does NOT require light

Takes place in the stroma of the chloroplast

Aka Calvin cycle- called a cycle bc one of the products is needed to start the cycle over

Follow the cycle on pg. 229

The calvin cycle
The Calvin Cycle

1) CARBON FIXATION-The carbon atom from CO2 bonds with a five-carbon sugar called ribulosebiphosphate (RuBP) to form an unstable six carbon sugar.

2) FORMATION OF 3-CARBON MOLECULES-The six-carbon sugar immediately splits to form two three-carbon molecules.

3) 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.

4) 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.

5) 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.

Getting energy to make atp
Getting Energy to Make ATP

  • Cellular Respiration- The process by which mitochondria break down food molecules to produce ATP.

  • There are 3 stages of cellular respiration

    • 1) Glycolysis- anaerobic (no oxygen required)

    • 2) Citric acid cycle- aerobic (oxygen required)

    • 3) Electron transport chain- aerobic (oxygen required)


a series of reactions in the cytoplasm of a cell in which glucose (a 6 carbon molecule) is broken down into two molecules of pyruvic acid (3 carbon molecules).

ATP- it takes 2 molecules of ATP to start the process of glycolysis, and only 4 ATPs are made, therefore this process is not very energy efficient.


*only 2 molecules of ATP are produced from the breakdown of one glucose molecule.

NAD+ (nicotinamidedinucleotide) - just as photosynthesis has the energy carrier NADP+; glycolysishas an energy carrier called NAD+.

*NAD+ forms NADH when carrying an electron.

At the end of glycolysis the pyruvicacid molecules produced move to the mitochondria,thepowerhouses or ATP producers of the cell.

Glycolysis on a molecular level
Glycolysis- on a molecular level

Post glycolysis

  • Post-glycolysisreactions - before the pyruvicacid molecules can enter the citric acid cycle (the next stage of cellular respiration) some modifications need to be done.

    • pyruvicacid loses a molecule of CO2and combines with Coenzyme A to form a molecule of Acetyl-CoA.

    • the rxn w/ Coenzyme A makes a molecule of NADH+ H+

The citric acid cycle
The Citric Acid Cycle

The Citric Acid Cycle: “The breakdown of Glucose”-a series of chemical reactions similar to the Calvin Cycle, but opposite in purpose.

Calvin Cycle - forms glucose in photosynthesis

Citric Acid Cycle - breaks down glucose in cellular respiration

Materials needed :

to break down glucose, two electron carriers are


a) NAD+

b) FAD (flavin adenine dinucleotide)

The citric acid cycle1
The Citric Acid cycle

The Citric Acid Cycle (CAC) produces a number of molecules:

a) 1 ATP is produced

b) 3 NADH + H+ are produced

c) 1 FADH2 molecule is produced

Steps of the citric acid cycle
Steps of the Citric Acid Cycle

1) formation of citric acid- a 2 carbon acetyl CoAcombines with a 4 carbon compound called oxaloaceticacid, forming a 6 carbon molecule called citric acid.

2) formation of CO2- one molecule of CO2 is formed from the citric acid cycle which reduces the citric acid molecule to a 5 carbon molecule called ketoglutaric acid.*from this rxn, one molecule of NADH +H+ is made from one NAD+

3) formation of second CO2- another molecule of CO2 is formed and released from the ketoglutaric acid; this results in a 4 carbon compound called succinic acid.*from this rxn, one molecule of ATP and one molecule of NADH + H+ are formed.

4) recycling of oxaloacetic acid- succinic acid undergoes a series of rxns which form FADH and NADH + H+ and oxaloacetic acid; this is then available for the next cycle to occur.

Succinic -> fumaric -> malic -> oxaloacetic

Electron transport chain
Electron Transport Chain

Function- move energized molecules; NADH & FADH2 pass energized molecules from protein to protein releasing small amounts of energy with each pass.

Location - the inner membrane of the mitochondria

Electron transport chain1
Electron Transport Chain

The Process:

a) NADH & FADH2 pass energized molecules from protein to protein; small amounts of energy are released with each pass.

b) some energy is used to form ATP, while some is used to pump H+ ions into the center of the mitochondria.

c) as H+ ions are pumped into the center of the mitochondria, the center becomes more (+),while the outside becomes more (-). Since the outside is more (-) it will attract more (+)’s or more H+ ions,creating an electrochemical gradient.

d) The electrochemical gradient drives the inner membrane of the mitochondria to form ATP.

e) The final electron acceptor in the ETC is Oxygen. The oxygen reacts with H+ ions to form water molecules.

ETC Importance

The importance of Oxygen (O2)

If oxygen is not available for the ETC, then the chain cannot pass along energized electrons; if electrons cannot be passed, then there is no room to accept more electrons and a blockage results. Therefore, cellular respiration cannot occur.

Overall production

  • The ETC results in the production of 32 ATP molecules

  • This is the most efficient means for production of ATP

  • Think: Aerobic (jogging) vs, Anaerobic (sprinting) - which can be done longer?


  • sometimes your cells may be deprived of oxygen for a short time

  • fermentation can occur during extremely strenuous activities

  • Fermentation - anaerobic process that occurs when your cells are w/o O2 for a short time. It occurs after glycolysis and provides a way to continue producing ATP until oxygen is available again.

  • 2 main types of fermentation:

    a) lactic acid fermentation

    b) alcoholic fermentation

Lactic Acid Fermentation

*occurs during anaerobic conditions when oxygen is not available as the final electron acceptor in the ETC, therefore a “back-up” occurs.

What happens:

a) as NADH and FADH2 try to pass their

energized electrons onto the next protein in the

ETC, they are rejected.

b) if NADH and FADH2 cannot pass on their

energized electrons, then NADH and FADH2

cannot be converted back to NAD+ & FAD, which

are needed to keep the CAC and glycolysis


Alcoholic Fermentation

*often used by yeast cells to produce CO2 and ethyl alcohol.

*anaerobic process - used to make bread dough “rise” and brew alcohols.

Comparing Photosynthesis and Cellular Respiration

  • Both use an ETC to form ATP

  • Do opposite jobs

  • Photosynthesis - produces high energy carbohydrates and O2 from the sun’s energy

  • Cellular respiration - uses O2 to break down carbohydrates with much lower energy level