Biology 1Chapter 8&9

Energy in a Cell

• 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. Autotrophs

Cell Energy

• Other organisms cannot use sunlight directly.• They eat green plants. In that way, they obtain

the energy stored in plants. Heterotrophs

• 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 energy

• There is a molecule in your cells that is a quick source of energy for any organelle in the cell that needs it. If cells don’t have this molecule, they will die.

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

• ATP is composed of an adenosine (an adenine and a ribose together) molecule with three phosphate groups attached.

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

• Bonding a third phosphate group onto ADP to form adenosine triphosphate requires considerable energy.

Forming and Breaking Down ATP

• When ATP is formed, the energy is stored in the bond of the molecule. When this bond is broken, the energy is released.

• A molecule with only 2 phosphate groups attached is called adenosine diphosphate, or ADP, and it is formed when ATP is broken down to release energy.

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

Adenosine

Adenosine

P P P

PP

P P

Adenosine triphosphate (ATP)

Adenosine diphosphate (ADP)

Forming and Breaking Down ATP

• When ATP is broken down it releases energy and a phosphate group and reforms ADP which can then be reused to make ATP again.

+ Energy

+ Energy

• When ATP is broken down and the energy is released, ADP is formed and a phosphate group is released. The ADP and phosphate is then available to be reused again. It is a cycle between ATP and ADP.

• 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.

Trapping Energy from Sunlight• All biological energy originally comes from the sun• The process that uses the sun’s energy to make simple

sugars is called photosynthesis.

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

Why are carbs important?

Carbohydrates are the main energy molecules used by all organisms. They are what we break down to make ATP!!!

• Photosynthesis occurs in the chloroplast (which contains the light trapping pigment - chlorophyll).

• Photosynthesis happens in two phases.

1. The light-dependent reactions convert light energy into chemical energy. Occurs in the thylakoid.

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 Calvin Cycle). Occurs in the stroma.

•Photosynthesis depends on 3 things: temperature, light and availability of raw materials.

Stroma

Thylakoid

• To trap the energy in the sun’s light, the thylakoid membranes contain pigments, molecules that absorb specific wavelengths of sunlight.

• Although there are several kinds of pigments, the most common is chlorophyll.

• Chlorophyll absorbs most wavelengths of light except green. Since green is reflected, that is the color our eyes see.

Light-Dependent Reactions

Light-Dependent Reactions• As sunlight strikes the chlorophyll molecules the

thylakoid membrane, the energy in the light is transferred to electrons.

• These highly energized, or excited, electrons leave the chlorophyll and are passed down an electron transport chain, a series of proteins embedded in the thylakoid membrane. (bucket brigade)

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

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

• Then, the electrons are transferred to the stroma of the chloroplast. To do this, an electron carrier molecule (coenzyme – biological carrier molecules) called NADP+ is used.

• NADP+ picks up two excited electrons and a hydrogen ion (H+) to become NADPH which carriers them to the light independent reactions where they will be used.

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

Light-Dependent Reactions• The electrons are re-energized and passed down a

second electron transport chain.

Restoring electrons• To replace the electrons lost by the chlorophyll,

molecules of water are split. This reaction is called photolysis.

• 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.

Click image to view movie.

Light-Dependent Reactions

Sun

Chlorophyll passes energy down through the electron transport chain.

for the use in light-independent reactions

bonds

P

for the use in light-independent reactions

to ADP

forming ATP

bonds

oxygenreleased

splitsH2O

H+

NADP+

NADPH

Light energy transfers to chlorophyll.

Energized electrons provide energy that

Chlorophyll passes energy down through the electron transport chain.

Sun

• Result in:

1. Electrons

2. ATP

3. Hydrogen

4. Oxygen

• 1,2,3 used in the light independent reactions

• 4 is released as waste

The Calvin Cycle

(CO2)

(Unstable intermediate)

ATP

ADP +

ADP +

(Sugars and other carbohydrates)

NADPH

NADP+

(PGAL)

(PGAL)

ATP

(PGAL)

(RuPB)

• The stroma in the chloroplasts hosts the Calvin cycle.

• 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 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 Cycle

The 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 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 Cycle

The Calvin Cycle

(CO2)

(Unstable intermediate)

ATP

ADP +

ADP +

(Sugars and other carbohydrates)

NADPH

NADP+

(PGAL)

(PGAL)

ATP

(PGAL)

(RuPB)

•The Calvin Cycle results in the production of simple sugar molecules called carbohydrates that are the major source of energy for most organisms.

Cellular Respiration

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

Full Cellular Respiration – Aerobic

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

• The citric acid cycle and the e.t.c. are aerobic (require oxygen to be completed).

Glycolysis

• 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

Glycolysis

• 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

Preparatory Step

The 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 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 cycle• Citric acid forms.• 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 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 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 cycle

The 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

NADH + H+

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

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

EnzymeElectron carrier proteins

e -

NADH

FADH2

NAD+

FAD

Electron pathway

4H+ + O2

+ 4 electronsH2O

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.

Fermentation• If there is no oxygen present after

glycolysis, aerobic respiration cannot occur. Cells can continue producing ATP by using another process (anaerobic, does not require oxygen) called fermentation.

• 2 types: 1. lactic acid fermentation – makes lactic acid 2. alcoholic fermentation – makes alcohol

(ethanol)

• Fermentation is a quick process, but does not produce any additional ATP.

• No additional ATP are produced, but NAD+ is recycled allowing glycolysis to continue.

• The lactic acid - muscle cells

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

Comparing Photosynthesis and Cellular Respiration

Photosynthesis Cellular RespirationFood synthesized Food broken downEnergy from sun stored in glucose Energy of glucose releasedCarbon 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

•Notice, the end products of one are the starting products of the other.

•Also, photosynthesis takes place in the chloroplast while respiration takes place in the mitochondria.