NAME ____________________________ Period___________Photosynthesis and Cellular Respiration Notes Outline

Energy in Living Systems You get __________ from the ________ you _______.

Directly or indirectly, almost all of the ____________ in living systems needed

for __________________ comes from the _________.

___________ from the _______ enters living systems when __________,

__________, and certain _________________ absorb ________________.

Some of the __________ in ______________ is captured and ________ to

make _______________ __________________.

These _______________ ________________ store chemical _________ and

can serve as ___________ for ____________________.

Building Molecules that Store Energy ___________________ involves either using _________ to build __________

or breaking __________ molecules in which ____________ is ___________.

____________________ is the process by which _________ energy is

converted to _____________ energy.

___________________ that use energy from ______________ or from

chemical __________ in _____________ substances to make organic

______________ are called _________________.

Most _______________ (usually _________) are ________________

organisms.

Some _____________, including certain ________________, use chemical

__________ from _______________ substances to make ___________

compounds.

_______________ found near deep-sea _____________ vents live in

perpetual _________________.

o ______________ does not reach the bottom of the __________.

These ________________ get ____________ from ______________ flowing

out of the ___________.

Breaking Down Food for Energy The chemical ___________ in organic _______________ can be transferred

to other ____________ compounds or to ___________ that consume ______.

______________ that must get _________ from _________ instead of

directly from ____________ or inorganic ______________ are called

______________________.

o ________________, including __________, get __________ from

________ through the process of _____________ ________________.

_____________ ________________ is a _______________ process similar

to burning ________.

While burning ____________ almost all of the ____________ in a fuel to

________, cellular ______________

releases much of the energy in

________ to make __________.

________ provides ________ with the

___________ they need to carry out the

_______________ of _________.

Transfer of Energy to ATP The word ________ is often used to describe how _______ get _________

from __________.

o The overall process is similar. However, the “__________” of _______

in living _________ differs from the burning of a log in a campfire.

o When a log ________, the energy __________ in wood is __________

quickly as ________ and _________.

o In ________, chemical energy _________ in food molecules is

___________ gradually in a series of __________ assisted chemical

______________.

As shown in the above diagram, the ______________ of one chemical

_________ becomes a ____________ in the next _____________.

When _______ break down food ____________, some of the __________ in

the _____________ is released as _________.

Much of the remaining _________ is stored temporarily in molecules of ____.

______ delivers __________ wherever ________ is needed in a _______.

o The __________ released from _______ can be used to _______ other

chemical _____________, such as those that build ________________.

o Most chemical ______________ require _______ energy than is

_____________ from ________.

ATP ________ (____________ ______________) is a

____________ with two extra ___________-storing

______________ groups.

The __________ phosphate groups in _______ form a chain that branches

from a five-___________ ________ (_________).

o The _____________ “______” is _________ because the phosphate

groups are ______________ charged and _______ each other.

o The ___________ groups store _______ like a compressed ________.

o The ________ is ____________ when the _________ that hold the

____________ groups together are _________.

ATP Þ ADP __________ the outer phosphate ________ requires an input of __________.

o However, much more ________ is ____________, than is ___________

by the ______________.

The ________________ of a phosphate _________ from _______ produces

_______ (adenosine _____________________).

o This reaction _____________ energy in a way that enables _______ to

use the ___________.

________ use this __________ released by this ____________ to _________

__________________.

Photosynthesis _________, _________, and some ____________

capture about __% of the _______ in the

______________ that reaches the ________ and

convert it to ___________ energy through the

process of ____________________.

_______________ is the process that provides ______________ for almost

all ______.

________________ occurs in the ______________ of ________ and

_______ cells and in the cell ____________ of certain __________________.

The Steps of PhotosynthesisStep 1 ____________ is captured from ___________________.

Step 2 __________ energy is converted to ____________ energy, which is

temporarily stored in ________ and the energy carrier molecule ______.

Step 3 The ____________ energy stored in ______ and ________ powers the

formation of organic ____________, using _________________ (_______).

_________________ can be summarized by the following ______________:

________ + ________ → __________ + _______

Step One: Absorption of Light Energy The _____________ reactions that occur in the

______ and ___________ steps of

_________________ are sometimes called

“_______ __________,” or _______

________________ reactions.

o Without the ______________ of _______,

these reactions could _______ occur.

_________ energy is used to make _______-storing __________________.

_________ is a form of _____________, ________ in the form of

__________ that travel from our _____ through _________.

Different types of ____________ (______ and _____) have different

_________________ (the distance between two consecutive __________).

When the ________ shines on you, your _______ is bombarded by many

kinds of ____________ from the ________.

However, we only can see ____________ known as _________ light.

Electromagnetic Spectrum

Pigments The ______________ containing _______-absorbing

substances are called ___________.

____________ absorb only certain _______________

and ___________ all the others.

_________________, the primary __________ in ___________________,

absorbs mostly _______ and ________ light and reflects _________ and

________ light.

This _____________ of ________ and _________ light makes many

__________, especially their leaves, look ___________.

Plants contain ______ types of _____________, chlorophyll ____ and

chlorophyll ____.

o Both types of _____________ play an important role in plant

__________________.

The ____________ that produce

_________ and __________ fall

______ colors, as well as the

________ of many ________,

_____________, and

________________, are called

__________________.

_______________ absorb

__________________ of light

different from those absorbed by __________________, so having both

______________ enables _________ to absorb more _________ energy

during _______________________.

Production of Oxygen _____________ involved in plant ______________ are located in the

___________________ of leaf ________.

Clusters of ______________ are embedded in the ______________ of disk-

shaped structures called ________________.

When _________ strikes a _____________ in a ______________, ________

is transferred to _______________ in ______________________.

This _____________ transfer causes the _____________ to _________ to a

__________ energy __________.

o _____________ with extra ___________ are said to be “___________.”

_____________ electrons jump from ____________ molecules to other

nearby ______________ in the ________________ membrane, where the

_______________ are used to __________ the second step in

___________________.

The ____________ electrons that leave _________ molecules must be

___________ by other ____________.

__________ get these replacement _____________ from ____________

molecules, ________.

________ molecules are ________,

____________ molecules _________ the

______________ from the ______________

atoms, ___, leaving hydrogen _______,

_____.

The remaining __________ atoms, ____,

(from the water molecules) ___________ to

form oxygen _______, _____.

Step Two: Conversion of Light Energy ___________ electrons that leave ________________ molecules are used to

produce new ______________, including _______, that temporarily store

chemical ___________.

First, the excited ____________ jumps to a nearby ___________ in the

_______________ membrane.

Then, the ___________ is passed through a __________ of molecules along

the ________________ membrane.

o The series of ___________ are called _____________ _____________

__________.

Electron Transport Chains of Photosynthesis

Electron Transport Chains The first ____________ transport chain lies _____________ the two large

green clusters of _____________ molecules.

This type of ___________ transport chain contains a ____________ (the

large orange molecule) that acts as a membrane ________.

Excited ____________ lose some of their _________ as they each ________

through this ____________.

The ____________ lost by the _____________ is used to pump __________

ions, H+, __________ the ______________.

o ____________ ions are also ____________ when ________ molecules

are ___________ inside the ____________.

As the process continues, ___________ ions become more ______________

inside the ____________ than outside, producing a _________________

gradient across the _______________ membrane.

As a result, ____________ ions have a tendency to __________ back out of

the ___________ down their concentration gradient through specialized

carrier _____________ (pink molecule).

o These carrier _________ are unusual because they function both as an

______ channel and as an _____________.

As ____________ ions pass through the ____________ portion of the protein,

the protein ____________ a reaction in which a _____________ group is

added to a molecule of _________, making __________.

o The _____________ of hydrogen ions across the _____________

membrane through these ____________ provides the __________

needed to make ______, which is used to ________ the third step of

_________________.

While one ____________ transport chain provides _____________ used to

make _______, a second ____________ transport chain

provides ________ used to make ___________.

o ____________ is an electron __________ that provides

the high-_________ electrons needed to make

_________-_____________ bonds in the third

step of ____________________.

The second _______________ transport chain is shown to the right

of the second green molecule.

In this second chain, ____________ electrons ___________ with _________

ions as well as an electron _____________ called _____________, forming

_____________________.

Light-Dependent Reaction Summary _______________ molecules in the _____________ of _______________

absorb _________ energy.

____________ in the _____________ are excited by ________ and move

through _____________ transport chains in thylakoid membranes.

These ____________ are replaced by electrons from __________ molecules,

which are _________ by an ____________.

____________ atoms from _________ molecules combine to form

___________ gas.

_____________ ions accumulate inside ____________, setting up a

________________ gradient that provides the _________ to make _______.

Step Three: Storage of Energy In this final stage of _________________, _________ atoms from carbon

dioxide in the ____________ are used to make organic ____________ in

which _____________ energy is ___________.

The transfer of ___________ __________ to organic

compounds is called ______________

_______________ _______________.

The reactions that “_______” carbon dioxide are

sometimes called “________ _____________,” or

___________-_____________________ reactions.

The Calvin Cycle The most common method of carbon dioxide _____________ is the

_____________ __________.

The __________ cycle is a series of __________-assisted chemical

____________ that produces a _________-carbon _________.

The Calvin Cycle is named for ________________, the American biochemist

who worked out the ______________ reactions of the cycle.

The _______________ are ___________; they ___________ the _______-

carbon _______________ needed to ___________ the cycle again.

In carbon dioxide ____________, each molecule of carbon dioxide (______)

is added to a __________-carbon compound by an __________.

The resulting ________-carbon compound ___________ into _____ three-

carbon _____________. _____________ groups form _________ and

_____________ from ____________ are added to the _________-carbon

compounds, forming _________-carbon ____________.

One of the resulting ________-carbon __________ is used to make organic

________________, including _________ and __________, in which

_____________ is ___________ for later use by the ______________.

The other __________-carbon ___________ are used to _____________ the

initial ___________-carbon compound, completing the __________.

A total of _____________ carbon dioxide ____________ must enter the

______________ cycle to produce each ________-carbon _________ that

will be used to make other ___________ compounds.

These ___________ compounds provide the ___________ with _________

for ___________ and _________________.

The _________ used in the __________ cycle is supplied by _______ and

_______ made during the second step of ________________.

Stages of Photosynthesis

Used Produced

Step 1Step 2

Step 3

Factors that Affect Photosynthesis

_________________ is directly affected by ________________ factors.

The most obvious factor is __________.

o In general, the _________ of _________________ increase as _______

intensity ____________ until all the _____________ are being _______.

o At this ________________ point, the ___________ of the Calvin cycle

cannot proceed any __________.

o The overall ________ of _______________ is limited by the

__________ step, which occurs in the __________ cycle.

The carbon dioxide ______________ also affects the rate of _____________.

o Once a certain __________________ of carbon dioxide is __________,

_________________ cannot proceed any ___________.

___________________ is most ____________ within a certain range of

_________________.

o _________________ involves many ____________-assisted chemical

______________ and unfavorable ______________ may inactivate

certain _______________.

Cellular Energy Most of the __________ we eat contain usable ____________.

o Much of the _________ is stored in ____________, ______________,

and __________.

__________ transfer ____________ in organic compounds to _______

through a process called ___________________________.

o ____________ in the air makes the production of ________ more

___________.

______________ processes that require ___________ are called _________.

___________ processes that do _____ require _________ are called

____________ (_________ _______).

The Steps of Cellular Respiration

____________ respiration is the process _______ use to produce the

____________ in organic _____________.

_____________ respiration can be summarized by the following __________:

____________ + ________ → _________ + _________ + ________

___________ respiration occurs in ______ steps.

Step 1 – _________ is converted to ___________, producing a small amount of

_______ and ____________.

Step 2 – When ____________ is present, ____________ and _________ are

used to make large amounts of _________ (aerobic respiration). ___________

respiration occurs in the _________________ of all ________. When ________

is ______ present, ____________ is converted to either _____________ or

______________ and __________________________.

Step One: Breakdown of Glucose The primary ______ for ___________ respiration is __________, which is

formed when ________________ such as ________ and _________ are

broken down.

If too few ________________ are available to meet an organism’s glucose

_________, other molecules, such as _______, can be broken down to make

________.

o One gram of ______ contains ________ energy than ______ grams of

___________________.

________________ and _________ _________ can also be used to make

_______, but they are usually used for ____________ important cell _______.

Glycolysis

In the first ________ of ____________ respiration, ____________ is broken

down in the _____________ during a process called ______________.

_______________ is an ___________-assisted ___________ process that

breaks down one ______-carbon molecule of ___________ to two ______-

carbon ____________ ions.

o ______________ is the ion of a ________-carbon organic acid called

______________ ________.

The ____________ produced during ________________ still contains some

of the ______________ that was __________ in the ____________ molecule.

As _____________ is broken down, some of its ____________ atoms are

transferred to an ____________ acceptor called __________.

This forms an ____________ carrier called ___________.

The ______________ carried by __________ are eventually ___________ to

other ______________ compounds.

This recycles _______, making it ____________ to accept more __________.

_____________ uses two ______ molecules but produces four _____

molecules, yielding a net _____ of ______ ATP molecules.

_____________ is followed by another set of reactions that use the ________

temporarily stored in ____________ to make more ________.

Step Two: Production of ATP When ____________ is present, __________ produced during __________

enters a _________________ and is converted to a _____-carbon compound.

This reaction produces one _____________ molecule, one ______ molecule,

and one ______-__________ __________ group.

The __________ group is attached to a molecule called ____________ A

(_______), forming a compound called _________-______.

Krebs Cycle ___________-_______ enters a series of __________-assisted reactions

called the _________ cycle.

The cycle is named for the biochemist ____________, who first described the

cycle in ________.

After the ___________ cycle, ________ and _________ now contain much of

the ___________ that was previously stored in _________ and ___________.

When the _________ cycle is completed, the __________-carbon compound

that began the cycle has been ______________, and _________-______ can

enter the cycle _________.

Electron Transport Chain In _____________ respiration, _____________ donated by _______ and

____________ pass through an _______________ transport chain.

In ________________ cells, the electron transport chain is located in the

inner _________________ of _____________________.

The ___________ of these _____________ is used to pump ___________

ions _____ of the inner ____________________ compartment.

________________ ions accumulate in the _________ compartment,

producing a _______________ gradient across the inner _______________.

_______________ ions diffuse back into the inner ______________ through

a carrier ___________ that adds a __________ group to _____, making ___.

At the end of the _____________ transport chain, _____________ ions and

spent _____________ combine with _____________ molecules, O2, forming

_____________ molecules, _______.

Respiration in the Absence of Oxygen What happens when there is not enough ____________ for ___________

respiration to occur?

o The _________ transport chain does ______ function because

__________ is not available to serve as the final electron _________.

o _________ are not transferred from _________, and ________

therefore they cannot be ____________.

o When __________ is not present, NAD+ is __________ in another way.

Under ____________ conditions, ___________ carried by ________ are

transferred to ____________ produced during _____________.

This process recycles ________needed to continue making ______ through

_______________.

The recycling of _________ using an organic ___________ acceptor is called

_____________________.

o _______________ carry out more than a dozen kinds of

_______________ all using some form of organic ____________

acceptor to recycle _______+.

Two important forms of ______________ are _______________ fermentation

and _______________ fermentation.

o _______________ fermentation by some ____________ and ________

is used in the production of __________ such as ___________ and

some _____________.

Lactic Acid Fermentation In some _____________, a _______-carbon ___________ is converted to a

________-carbon ________ through lactic acid __________________.

o __________ is the ion of an ____________ acid called _________ acid.

During vigorous ___________, pyruvate in ___________ is converted to

___________ when ________ cells must operate __________ enough

___________.

______________ enables _____________ to continue producing ______ in

___________ as long as the ____________ supply lasts.

____________ removes excess ________ from ____________.

o ___________ can build up in _________ cells if it is not removed

______________ enough, sometimes causing muscle _____________.

Alcoholic Fermentation In other ______________, the ________-carbon ____________ is broken

down to ___________, a ______-carbon compound, through ___________

fermentation.

______________________ is released during the process.

First, _____________ is converted to a _______-carbon compound, releasing

________________________.

Second, _____________ are transferred from a molecule of _________ to the

__________-carbon compound, producing ___________.

As in lactic acid _________________, ______+ is ____________, and

_________________ can continue to produce _________.

Alcoholic ________________ by ____________, a ___________, has been

used in the preparation of many ____________ and _________________.

__________ and _______ contain ___________ made during ____________

fermentation by ___________.

_____________________ released by the _________ causes the rising of

___________ dough and the ____________________ of some alcoholic

beverages, such as ___________.

___________ is actually __________ to yeast.

o At a concentration of about ______percent ethanol _______ yeast.

o Therefore, naturally ____________ wine contains about ____% ethanol.

Production of ATP

The ________ amount of _______ that a cell is able to harvest from each

___________ molecule that enters ___________ depends on the presence or

absence of ___________.

o When Oxygen is __________, __________ respiration occurs.

o When Oxygen is __________, ____________ occurs.

Photosynthesis and Cellular Respiration OutlineTeacher Guide (Key) © Lisa Michalek

Energy in Living Systems You get energy from the food you eat. Directly or indirectly, almost all of the energy in living systems needed for metabolism

comes from the sun. Energy from the sun enters living systems when plants, algae, and certain prokaryotes

absorb sunlight. Some of the energy in sunlight is captured and used to make organic compounds. These organic compounds store chemical energy and can serve as food for organisms.

Building Molecules that Store Energy Metabolism involves either using energy to build molecules or breaking down molecules in

which energy is stored. Photosynthesis is the process by which light energy is converted to chemical energy. Organisms that use energy from sunlight or from chemical bonds in inorganic substances

to make organic compounds are called autotrophs. Most autotrophs (usually plants) are photosynthetic organisms. Some autotrophs, including certain prokaryotes, use chemical energy from inorganic

substances to make organic compounds. Prokaryotes found near deep-sea volcanic vents live in perpetual darkness.

o Sunlight does not reach the bottom of the ocean. These prokaryotes get energy from chemicals flowing out of the vents.

Breaking Down Food for Energy The chemical energy in organic compounds can be transferred to other organic compounds

or to organisms that consume food. Organisms that must get energy from food instead of directly from sunlight or inorganic

substances are called heterotrophs.o Heterotrophs, including humans, get energy from food through the process of

cellular respiration. Cellular respiration is a metabolic process similar to burning fuel. While burning converts almost all of the energy in a fuel to heat, cellular respiration

releases much of the energy in food to make ATP. ATP provides cells with the energy they need to carry out the activities of life.

Transfer of Energy to ATP The word burn is often used to describe how cells get energy from food.

o The overall process is similar. However, the “burning” of food in living cells differs from the burning of a log in a campfire.

o When a log burns, the energy stored in wood is released quickly as heat and light.o In cells, chemical energy stored in food molecules is released gradually in a series

of enzyme assisted chemical reactions. As shown in the above diagram, the product of one chemical reaction becomes a reactant

in the next reaction. When cells break down food molecules, some of the energy in the molecules is released as

heat. Much of the remaining energy is stored temporarily in molecules of ATP. ATP delivers energy wherever energy is needed in a cell.

o The energy released from ATP can be used to power other chemical reactions, such as those that build molecules.

o Most chemical reactions require less energy than is released from ATP.

ATP ATP (adenosine triphosphate) is a nucleotide with two extra energy-storing phosphate

groups. The three phosphate groups in ATP form a chain that branches from a five-carbon sugar

(ribose).o The phosphate “tail” is unstable because the phosphate groups are negatively

charged and repel each other.o The phosphate groups store energy like a compressed spring.o The energy is released when the bonds that hold the phosphate groups together are

broken.

ATP Þ ADP Breaking the outer phosphate bond requires an input of energy.

o However, much more energy is released, than is consumed by the reaction. The removal of a phosphate group from ATP produces ADP (adenosine diphosphate).

o This reaction releases energy in a way that enables cells to use the energy. Cells use this energy released by this reaction to power metabolism.

Photosynthesis Plants, algae, and some bacteria capture about 1% of the energy in the sunlight that

reaches the Earth and convert it to chemical energy through the process of photosynthesis. Photosynthesis is the process that provides energy for almost all life. Photosynthesis occurs in the chloroplasts of algae and plant cells and in the cell membrane

of certain prokaryotes.

The Steps of PhotosynthesisStep 1 Energy is captured from sunlight.Step 2 Light energy is converted to chemical energy, which is temporarily stored in ATP

and the energy carrier molecule NADPH.Step 3 The chemical energy stored in ATP and NADPH powers the formation of organic

compounds, using carbon dioxide (CO2).

Photosynthesis can be summarized by the following equation:

6CO2 + 6H2O → C6H12O6 + 6O2

Carbon water Sugars Oxygen Dioxide Gas

Step One: Absorption of Light Energy The chemical reactions that occur in the first and second steps of photosynthesis are

sometimes called “light reactions,” or light dependent reactions.o Without the absorption of light, these reactions could not occur.

Light energy is used to make energy-storing compounds. Light is a form of radiation, energy in the form of waves that travel from our Sun through

space. Different types of radiation (light and heat) have different wavelengths (the distance

between two consecutive waves). When the sun shines on you, your body is bombarded by many kinds of radiation from the

Sun. However, we only can see radiation known as visible light.

Pigments The structures containing light-absorbing substances are called pigments. Pigments absorb only certain wavelengths and reflect all the others. Chlorophyll, the primary pigment in photosynthesis, absorbs mostly blue and red light and

reflects green and yellow light. This reflection of green and yellow light makes many plants, especially their leaves, look

green. Plants contain two types of chlorophyll, chlorophyll a and chlorophyll b.

o Both types of chlorophyll play an important role in plant photosynthesis. The pigments that produce yellow and orange fall leaf colors, as well as the colors of many

fruits, vegetables, and flowers, are called carotenoids. Carotenoids absorb wavelengths of light different from those absorbed by chlorophyll, so

having both pigments enables plants to absorb more light energy during photosynthesis.

Production of Oxygen Pigments involved in plant photosynthesis are located in the chloroplasts of leaf cells. Clusters of pigments are embedded in the membranes of disk-shaped structures called

thylakoids. When light strikes a thylakoid in a chloroplast, energy is transferred to electrons in

chlorophyll. This energy transfer causes the electrons to jump to a higher energy level.

o Electrons with extra energy are said to be “excited.” Excited electrons jump from chlorophyll molecules to other nearby molecules in the

thylakoid membrane, where the electrons are used to power the second step in photosynthesis.

The excited electrons that leave chlorophyll molecules must be replaced by other electrons. Plants get these replacement electrons from water molecules, H2O. Water molecules are split, chlorophyll molecules take the electrons from the

hydrogen atoms, H, leaving hydrogen ions, H+.

The remaining oxygen atoms, O, (from the water molecules) combine to form oxygen gas, O2.

Step Two: Conversion of Light Energy Excited electrons that leave chlorophyll molecules are used to produce new molecules,

including ATP, that temporarily store chemical energy. First, the excited electron jumps to a nearby molecule in the thylakoid membrane. Then, the electron is passed through a series of molecules along the thylakoid membrane.

o The series of molecules are called electron transport chains.

Electron Transport Chains The first electron transport chain lies between the two large green clusters of pigment

molecules. This type of electron transport chain contains a protein (the large orange molecule) that

acts as a membrane pump. Excited electrons lose some of their energy as they each pass through this protein. The energy lost by the electrons is used to pump hydrogen ions, H+, into the thylakoid.

o Hydrogen ions are also produced when water molecules are split inside the thylakoid.

As the process continues, hydrogen ions become more concentrated inside the thylakoid than outside, producing a concentration gradient across the thylakoid membrane.

As a result, hydrogen ions have a tendency to diffuse back out of the thylakoid down their concentration gradient through specialized carrier proteins (pink molecule).

o These carrier proteins are unusual because they function both as an ion channel and as an enzyme.

As hydrogen ions pass through the channel portion of the protein, the protein catalyzes a reaction in which a phosphate group is added to a molecule of ADP, making ATP.

o The movement of hydrogen ions across the thylakoid membrane through these proteins provides the energy needed to make ATP, which is used to power the third step of photosynthesis.

While one electron transport chain provides energy used to make ATP, a second electron transport chain provides energy used to make NADPH.

o NADPH is an electron carrier that provides the high-energy electrons needed to make carbon-hydrogen bonds in the third step of photosynthesis.

The second electron transport chain is shown to the right of the second green molecule. In this second chain, excited electrons combine with hydrogen ions as well as an electron

acceptor called NADP+, forming NADPH.

Light-Dependent Reaction Summary Pigment molecules in the thylakoids of chloroplasts absorb light energy. Electrons in the pigments are excited by light and move through electron transport chains

in thylakoid membranes. These electrons are replaced by electrons from water molecules, which are split by an

enzyme. Oxygen atoms from water molecules combine to form oxygen gas.

Hydrogen ions accumulate inside thylakoids, setting up a concentration gradient that provides the energy to make ATP.

Step Three: Storage of Energy In this final stage of photosynthesis, carbon atoms from carbon dioxide in the atmosphere

are used to make organic compounds in which chemical energy is stored. The transfer of carbon dioxide to organic compounds is called carbon dioxide fixation. The reactions that “fix” carbon dioxide are sometimes called “dark reactions,” or light-

independent reactions.

The Calvin Cycle The most common method of carbon dioxide fixation is the Calvin Cycle. The Calvin cycle is a series of enzyme-assisted chemical reactions that produces a three-

carbon sugar. The Calvin Cycle is named for Melvin Calvin, the American biochemist who worked out the

chemical reactions of the cycle. The reactions are cyclic; they recycle the five-carbon compound needed

to begin the cycle again. In carbon dioxide fixation, each molecule of carbon dioxide (CO2) is added to a five-carbon

compound by an enzyme. The resulting six-carbon compound splits into two three-carbon compounds. Phosphate

groups form ATP and electrons from NADPH are added to the three-carbon compounds, forming three-carbon sugars.

One of the resulting three-carbon sugars is used to make organic compounds, including starch and sucrose, in which energy is stored for later use by the organism.

The other three-carbon sugars are used to regenerate the initial five-carbon compound, completing the cycle.

A total of three carbon dioxide molecules must enter the Calvin cycle to produce each three-carbon sugar that will be used to make other organic compounds.

These organic compounds provide the organism with energy for growth and metabolism. The energy used in the Calvin cycle is supplied by ATP and NADPH made during the

second step of photosynthesis.

Stages of PhotosynthesisUsed Produced

Step 1 Light, Water Oxygen, Hydrogen Ions

Step 2 Electrons, Hydrogen Ions

ATP, NADPH

Step 3 ATP, NADPH, Carbon Dioxide

Organic Compounds

Factors that Affect Photosynthesis Photosynthesis is directly affected by environmental factors. The most obvious factor is light.

o In general, the rate of photosynthesis increase as light intensity increases until all the pigments are being used.

o At this saturation point, the reactions of the Calvin cycle cannot proceed any faster.o The overall rate of photosynthesis is limited by the slowest step, which occurs in the

Calvin cycle. The carbon dioxide concentration also affects the rate of photosynthesis.

o Once a certain concentration of carbon dioxide is present, photosynthesis cannot proceed any faster.

Photosynthesis is most efficient within a certain range of temperatures.o Photosynthesis involves many enzyme-assisted chemical reactions and unfavorable

temperatures may inactivate certain enzymes.

Cellular Energy Most of the foods we eat contain usable energy.

o Much of the energy is stored in proteins, carbohydrates, and fats. Cells transfer energy in organic compounds to ATP through a process called cellular

respiration.o Oxygen in the air makes the production of ATP more efficient.

Metabolic processes that require oxygen are called aerobic. Metabolic processes that do not require oxygen are called anaerobic (without air).

The Steps of Cellular Respiration Cellular respiration is the process cells use to produce the energy in organic compounds. Cellular respiration can be summarized by the following equation:

enzymes C6H12O6 + 6O2 → 6CO2 + 6H2O + energyglucose oxygen carbon water ATP

gas dioxide

Cellular respiration occurs in two steps.

Step 1 – Glucose is converted to pyruvate, producing a small amount of ATP and NADH.

Step 2 – When oxygen is present, pyruvate and NADH are used to make large amounts of ATP (aerobic respiration). Aerobic respiration occurs in the mitochondria of all cells. When oxygen is not present, pyruvate is converted to either lactate or ethanol and carbon dioxide.

Step One: Breakdown of Glucose The primary fuel for cellular respiration is glucose, which is formed when carbohydrates

such as starch and sucrose are broken down.

If too few carbohydrates are available to meet an organism’s glucose needs, other molecules, such as fats, can be broken down to make ATP.

o One gram of fat contains more energy than two grams of carbohydrates. Proteins and nucleic acids can also be used to make ATP, but they are usually used for

building important cell parts.

Glycolysis In the first step of cellular respiration, glucose is broken down in the cytoplasm during a

process called glycolysis. Glycolysis is an enzyme-assisted anaerobic process that breaks down one six-carbon

molecule of glucose to two three-carbon pyruvate ions.o Pyruvate is the ion of a three-carbon organic acid called pyruvic acid.

The pyruvate produced during glycolysis still contains some of the energy that was stored in the glucose molecule.

As glucose is broken down, some of its hydrogen atoms are transferred to an electron acceptor called NAD+.

This forms an electron carrier called NADH. The electrons carried by NADH are eventually donated to other organic compounds. This recycles NAD+, making it available to accept more electrons. Glycolysis uses two ATP molecules but produces four ATP molecules, yielding a net gain

of two ATP molecules. Glycolysis is followed by another set of reactions that use the energy temporarily stored in

NADH to make more ATP.

Step Two: Production of ATP When oxygen is present, pyruvate produced during glycolysis enters a mitochondrion and

is converted to a two-carbon compound. This reaction produces one carbon dioxide molecule, one NADH molecule, and one two-

carbon acetyl group. The acetyl group is attached to a molecule called coenzyme A (CoA), forming a compound

called acetyl-CoA.

Krebs Cycle Acetyl-CoA enters a series of enzyme-assisted reactions called the Krebs cycle. The cycle is named for the biochemist Hans Krebs, who first described the cycle in 1937. After the Krebs cycle, NADH and FADH2 now contain much of the energy that was

previously stored in glucose and pyruvate. When the Krebs cycle is completed, the four-carbon compound that began the cycle has

been recycled, and acetyl-CoA can enter the cycle again.

Electron Transport Chain In aerobic respiration, electrons donated by NADH and FADH2 pass through an electron

transport chain. In eukaryotic cells, the electron transport chain is located in the inner membranes of

mitochondria. The energy of these electrons is used to pump hydrogen ions out of the inner mitochondrial

compartment. Hydrogen ions accumulate in the outer compartment, producing a concentration gradient

across the inner membrane.

Hydrogen ions diffuse back into the inner compartment through a carrier protein that adds a phosphate group to ADP, making ATP.

At the end of the electron transport chain, hydrogen ions and spent electrons combine with oxygen molecules, O2, forming water molecules, H2O.

Respiration in the Absence of Oxygen What happens when there is not enough oxygen for aerobic respiration to occur?

o The electron transport chain does not function because oxygen is not available to serve as the final electron acceptor.

o Electrons are not transferred from NADH, and NAD+ therefore they cannot be recycled.

o When Oxygen is not present, NAD+ is recycled in another way. Under anaerobic conditions, electrons carried by NADH are transferred to pyruvate

produced during glycolysis. This process recycles NAD+ needed to continue making ATP through glycolysis. The recycling of NAD+ using an organic hydrogen acceptor is called fermentation.

o Prokaryotes carry out more than a dozen kinds of fermentation all using some form of organic hydrogen acceptor to recycle NAD+.

Two important forms of fermentation are lactic acid fermentation and alcoholic fermentation.

o Lactic acid fermentation by some prokaryotes and fungi is used in the production of foods such as yogurt and some cheeses.

Lactic Acid Fermentation In some organisms, a three-carbon pyruvate is converted to a three-carbon lactate through

lactic acid fermentation.o Lactate is the ion of an organic acid called lactic acid.

During vigorous exercise, pyruvate in muscles is converted to lactate when muscle cells must operate without enough oxygen.

Fermentation enables glycolysis to continue producing ATP in muscles as long as the glucose supply lasts.

Blood removes excess lactate from muscles.o Lactate can build up in muscle cells if it is not removed quickly enough, sometimes

causing muscle soreness.

Alcoholic Fermentation In other organisms, the three-carbon pyruvate is broken down to ethanol, a two-carbon

compound, through alcoholic fermentation. Carbon dioxide is released during the process. First, pyruvate is converted to a two-carbon compound, releasing carbon dioxide. Second, electrons are transferred from a molecule of NADH to the two-carbon compound,

producing ethanol. As in lactic acid fermentation, NAD+ is recycled, and glycolysis can continue to produce

ATP. Alcoholic fermentation by yeast, a fungus, has been used in the preparation of many foods

and beverages. Wine and beer contain ethanol made during alcoholic fermentation by yeast. Carbon dioxide released by the yeast causes the rising of bread dough and the carbonation

of some alcoholic beverages, such as beer.

Ethanol is actually toxic to yeast. o At a concentration of about 12 percent ethanol kills yeast. o Therefore, naturally fermented wine contains about 12% ethanol.

Production of ATP The total amount of ATP that a cell is able to harvest from each glucose molecule that

enters glycolysis depends on the presence or absence of oxygen.o When Oxygen is present, aerobic respiration occurs.o When Oxygen is absent, fermentation occurs.