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