Chapter 6:Cell Energy: Photosynthesis

and Respiration

Section 1:

Photosynthesis: Capturing and Converting Energy

Photosynthesis

In the process of photosynthesis, plants convert the energy of sunlight into the energy in the chemical bonds of carbohydrates – sugars, and starches

Put more simply, plants use the energy of sunlight to produce carbohydrates in a process called photosynthesis

Photosynthesis

An understanding of photosynthesis was developed from studies of plant growth

Dutch physician Jan Van Helmont devised an experiment to determine how plant growth actually works Found the mass of a pot of dry soil and a small seedling Planted the seedling in the pot of soil Watered regularly for 5 years Gained about 75 kilograms Mass of soil was unchanged Concluded that most of the mass must come from water

because that is all he added to the pot

Photosynthesis

Although Van Helmont did not realize it, carbon dioxide in the air made a major contribution to the mass of his tree

It is the carbon in carbon dioxide that is used to make carbohydrates in photosynthesis

Photosynthesis

Almost 100 years after Van Helmont’s experiment, Joseph Priestley performed an experiment that would give another insight into the process of photosynthesis Took a candle, placed a glass jar over it, and

watched as the flame gradually died out Something in the air was necessary to keep a

candle burning When that substance was used up, the candle

went out oxygen

Photosynthesis

Priestley then placed a spring of mint under the jar and allowed a few days to pass, the candle could be relighted and would remain lighted for awhile

The mint had produced the substance required for burning oxygen

Later, Dutch scientist Jan Ingenhousz showed that this only occurred when the plant was exposed to light

Requirements for Photosynthesis

These experiments reveal that in the presence of light, plants transform carbon dioxide and water into carbohydrates and release oxygen

Usually produces the sugar glucose 6CO2 + 6H2O C6H12O6 + 6O2light

Sunlight

Nearly all organisms on Earth depend on the sun for energy

Autotroph – organisms that are able to use a source of energy, such as sunlight, to produce food directly from simple inorganic substances in the environment

Heterotroph – organisms that obtain energy from the foods they eat

The sun bathes the Earth in a steady stream of light We see colorless “white” light but it is actually a

mixture of different wavelengths of light Visible spectrum

Pigments

Process of photosynthesis begins when light is absorbed by pigments in the plant cell Colored substances that absorb or reflect light Principal pigment in green plants is chlorophyll

Absorbs red and blue light but does not absorb light in the middle region of the spectrum very well

These wavelengths are reflected

Energy-Storing Compounds

In a green plant, the energy of sunlight is transferred to electrons, raising them to a higher energy level The electrons belong to the pigment

chlorophyll High-energy electrons are trapped in

chemical bonds Two ways in which energy of sunlight is

trapped in chemical bonds

Energy-Storing Compounds

First way sunlight is trapped in chemical bonds Simpler of the two A pair of high-energy electrons are passed directly to an

electron carrier A molecule that can accept a pair of electrons and later

transfer them along with most of their energy to another compound

Plants use the electron carrier NADP+ When NADP+ accepts a pair of high-energy electrons,

it is converted to NADPH ONE WAY IN WHICH SOME OF THE ENERGY

OF SUNLIGHT CAN BE TRAPPED IN CHEMICAL FORM

Energy-Storing Compounds

Second way sunlight is trapped in chemical bonds Involves adenosine triphosphate (ATP)

Consists of adenine, a 5-carbon sugar called ribose, and three phosphate groups

During photosynthesis, green plants produce ATP, which is an energy-storing compound used by every living cell

•As one might suspect, there are 3 phosphate groups.

•There is a high E bond between the 2nd and 3rd P group.

•When cells need E this high E bond is broken and E is released. It’s not ATP anymore. What is the new molecule formed?? ADP

•Notice the other two components of the the ATP molecule.

Adenine and Ribose

•E from the food a cell takes in is used to convert ADP back to ATP.

ADP + phosphate    ATP by the enzyme ATP synthetase

Chapter 6:Cell Energy: Photosynthesis and

Respiration

Section 2:

Photosynthesis: The Light and Dark Reactions

Photosynthesis: The Light and Dark Reactions

The production of NADPH and ATP requires sunlight Light reactions – the energy of sunlight is

captured and used to make energy-storing compounds

Another set of reactions called the dark reactions uses the energy stored in NADPH and ATP to produce glucose

Do not require light However, they can and do occur in the

light also

The Light Reactions

Photosynthesis takes place in the chloroplast Within the chloroplast are saclike

photosynthetic membranes that contain chlorophyll

Light reactions take place in these membranes Can be divided into four basic processes: light

absorption, electron transport, oxygen production, and ATP formation

Light Absorption Photosynthetic membranes contain clusters of pigment

molecules, or photosystems, that are able to capture the energy of sunlight Two photosystems in plants

Photosystem I Photosystem II

Each contains several hundred chlorophyll molecules as well as other accessory pigments

Absorb light in the regions of the spectrum where chlorophyll does not

Light Absorption

After light energy is absorbed by one of the pigment molecules in a photosystem, the energy is passed from one pigment molecule to the next until it reaches a special pair of chlorophyll molecules in the reaction center of the photosystem In the reaction center, high-energy electrons

are released and are passed to the first of many electron carriers

Electron Transport

High-energy electrons are transferred along a series of electron carriers Electron transportthe electron carriers

themselves are known as the electron transport chainAt the end of the chain, the electrons are

passed to NADP+, converting it to NADPH

Oxygen Production

The photosynthetic membrane contains a system that provides new electrons to chlorophyll to replace the ones that wound up in NADPH Four electrons are removed from two water

molecules 4 H+ ions 2 O atoms

Form a single molecule of oxygen gas Released into the air

ATP Formation

H+ ions are released inside the photosynthetic membrane as well as being pumped across the membrane

The inside of the membrane fills up with H+ ions

Makes the outside negatively charged and the inside positively charged Forms ATP

A Summary of the Light Reactions

Use water, ADP, NADP+ Produce O2, ATP and NADPH

The dark reactions will convert these energy-storing molecules to a more convenient form

The Dark Reactions

Light does not play a role in the dark reactions The series of chemical changes that make up

the dark reactions is critical to living things Carbon dioxide is used to make organic

compounds The dark reactions form a cycle called the

Calvin cycle

The Calvin Cycle

5 carbon sugar (C5) combines with CO2 to form two 3 carbon compounds (C3) Relatively slow

Uses the enzyme rubisco to speed up the process

Using ATP and NADPH, the 3 carbon compounds are converted to PGAL (phosphoglyceraldehyde) 6 turns of the cycle to make one molecule of

glucose

Chapter 6: Cell Energy: Photosynthesis and

Respiration

Section 3:

Glycolysis and Respiration

Glycolysis – Breaking Down Glucose

C6H12O6 + 6O2 6CO2 + 6H2O Gives off 3811 calories Glycolysis takes place in the cytoplasm

of a cell In glycolysis, a series of enzymes

catalyzes chemical reactions that change glucose, one step at a time, into different molecules

Respiration

If oxygen is available, respiration can take place Aerobic process Respiration is the process that involves oxygen

and breaks down food molecules to release energy Uses the pyruvic acid formed in glycolysis Often used as a synonym for breathing Takes place in the cell’s mitochondria

The Krebs Cycle

First set of reactions in respiration Krebs cycle

2 carbon atoms added (from the breakdown of pyruvic acid)

2 carbon atoms removed (in 2 molecules of CO2) 3 molecules of NAD+ converted to NADH 1 molecule of FAD converted to FADH2

1 molecule of GDP converted to GTP

Electron Transport in the Mitochondrion

High energy electrons from NADH and FADH2 are passed to electron transport enzymes in the mitochondrion

Form an ETC along which electrons are passed Enzyme at the end of the chain combines e- from

ETC, H+ ions from fluid inside the cell, and O2 to form H2O

Oxygen is the final electron acceptor in respiration Is essential for obtaining energy from both NADH

and FADH2

ATP Formation

Electron transport involves the movement of hydrogen ions

As enzymes accept electrons, they pump a hydrogen from the inside to the outside

This movement powers the formation of ATP On average, the movement of a pair of electrons

down the ETC produces enough energy to form 3 ATP from ADP

More H+ ions outside This imbalance supplies the energy to make ATP

from ADP

The Totals

Glycolysis and respiration together produce a total of 36 ATP molecules

Obtaining Energy From Food

Complex carbohydrates are broken down into simple sugars that are then converted into glucose The pathways we have discussed can be

used to produce energy The cell can generate chemical energy in

the form of ATP from just about any source

Breathing and Respiration

Final acceptor for all electrons in respiration is oxygen

Without oxygen, electron transport cannot operate, Krebs cycle stops, and ATP production stops

With each breath we take, air flows into our lungs Oxygen has a critical role to play in the

mitochondria of every cell

Energy in Balance

Photosynthesis and respiration can be thought of as opposite processes Photosynthesis deposits energy Respiration withdraws energy

Chapter 6: Cell Energy: Photosynthesis and

Respiration

Section 4:

Fermentation

Fermentation

Fermentation is a process that enables cells to carry out energy production in the absence of oxygen Breakdown of glucose and release of energy in which

organic substances are the final electron acceptors Fermentation is anaerobic—it does not require oxygen Fermentation enables cells to carry out energy

production in the absence of oxygen Produces 2 ATP

Lactic Acid Fermentation

In many cells, the pyruvic acid that accumulates as a result of glycolysis can be converted to lactic acid

Lactic acid fermentation Pyruvic acid + NADH lactic acid + NAD+ Lactic acid is produced in muscles during rapid

exercise when the body cannot supply enough oxygen to tissues to produce all of the ATP that is required

Causes a burning, painful sensation Large muscles quickly run out of oxygen

Muscle cells begin to rapidly produce ATP by fermentation

Alcoholic Fermentation

Another type of fermentation occurs in yeasts and a few other microorganisms

Pyruvic acid is broken down to produce a 2 carbon alcohol and carbon dioxide

Alcoholic fermentation Pyruvic acid + NADH alcohol + CO2 + NAD+

Alcoholic Fermentation

Particularly important to bakers and brewers Causes dough to rise and forms bubbles in beer and

wine To brewers, alcohol is a welcomed byproduct of

fermentation However, it is not desirable from a yeast cell’s

point of view Alcohol is toxic When the level of alcohol reaches about 12

percent, yeast cells die Thus alcoholic beverages must be processed if

higher concentrations of alcohol are desired