1. Chapter 6: Cell Energy: Photosynthesis and Respiration
Section 1:
Photosynthesis: Capturing and Converting Energy

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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 + 6O2
light

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. E from the food a cell takes in is used to
convert ADP back to ATP.
ADP + phosphate    ATP by the enzyme ATP synthetase

15. Chapter 6: Cell Energy: Photosynthesis and Respiration
Section 2:
Photosynthesis: The Light and Dark Reactions

16. 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

17. 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

18. 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

19. 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

20. Electron Transport
High-energy electrons are transferred along a series of electron carriers
Electron transportthe electron carriers themselves are known as the electron transport chain
At the end of the chain, the electrons are passed to NADP+, converting it to NADPH

21. 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

22. 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

23. 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

24. 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

25. 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

26. Chapter 6: Cell Energy: Photosynthesis and Respiration
Section 3:
Glycolysis and Respiration

27. 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

28. 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

29. 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

31. 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

32. 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

33. The Totals
Glycolysis and respiration together produce a total of 36 ATP molecules

34. 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

35. 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

36. Energy in Balance
Photosynthesis and respiration can be thought of as opposite processes
Photosynthesis deposits energy
Respiration withdraws energy

37. Chapter 6: Cell Energy: Photosynthesis and Respiration
Section 4:
Fermentation

38. 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

39. 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

40. 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+

41. 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