1. Photosynthesis and Cellular Respiration
© Lisa Michalek

2. 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.

3. Energy in Living Systems
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.

4. 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.

5. Building Molecules that Store Energy
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.
Sunlight does not reach the bottom of the ocean.
These prokaryotes get energy from chemicals flowing out of the vents.

6. 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.
Heterotrophs, including humans, get energy from food through the process of cellular respiration.

7. Breaking Down Food For Energy
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.

8. Transfer of Energy to ATP
The word burn is often used to describe how cells get energy from food.
The overall process is similar. However, the “burning” of food in living cells differs from the burning of a log in a campfire.
When a log burns, the energy stored in wood is released quickly as heat and light.
In cells, chemical energy stored in food molecules is released gradually in a series of enzyme assisted chemical reactions.

9. Transfer of Energy to ATP
As shown in the above diagram, the product of one chemical reaction becomes a reactant in the next reaction.

10. Transfer of Energy to ATP
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.
The energy released from ATP can be used to power other chemical reactions, such as those that build molecules.
Most chemical reactions require less energy than is released from ATP.

11. 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).
The phosphate “tail” is unstable because the phosphate groups are negatively charged and repel each other.
The phosphate groups store energy like a compressed spring.
The energy is released when the bonds that hold the phosphate groups together are broken.

12. ATP  ADP
Breaking the outer phosphate bond requires an input of energy.
However, much more energy is released, than is consumed by the reaction.
The removal of a phosphate group from ATP produces ADP (adenosine diphosphate).
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.

13. 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.

14. Photosynthesis occurs in the chloroplasts of algae and plant cells and in the cell membrane of certain prokaryotes.

15. The Steps of Photosynthesis
Step 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).

16. The Steps of Photosynthesis

17. Photosynthesis Can be summarized by the following equation:
6CO2 + 6H2O Light C6H12O6 + 6O2
Carbon water Sugars Oxygen
Dioxide Gas

18. 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.
Without the absorption of light, these reactions could not occur.
Light energy is used to make energy-storing compounds.

19. Step One: Absorption of Light Energy
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.

20. Electromagnetic Spectrum

21. 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.

22. Pigments
Plants contain two types of chlorophyll, chlorophyll a and chlorophyll b.
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.

23. 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.

24. Production of Oxygen
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.
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.

25. Production of Oxygen
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.

26. 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.
The series of molecules are called electron transport chains.

27. Electron Transport Chains of Photosynthesis

28. 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.

29. Electron Transport Chains
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.
Hydrogen ions are also produced when water molecules are split inside the thylakoid.

30. Electron Transport Chains
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).
These carrier proteins are unusual because they function both as an ion channel and as an enzyme.

31. Electron Transport Chains
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.
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.

32. Electron Transport Chains
While one electron transport chain provides energy used to make ATP, a second electron transport chain provides energy used to make NADPH.
NADPH is an electron carrier that provides the high-energy electrons needed to make carbon-hydrogen bonds in the third step of photosynthesis.

33. Electron Transport Chains
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.

34. 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.

35. 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.

36. 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.

37. The Calvin Cycle
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.

38. The Calvin Cycle
In carbon dioxide fixation, each molecule of carbon dioxide (CO2) is added to a five-carbon compound by an enzyme.

39. The Calvin Cycle
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.

40. The Calvin Cycle
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.

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

42. The Calvin 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.

43. Stages of Photosynthesis
Used
Produced
Step 1
Light, Water
Oxygen, Hydrogen Ions
Step 2
Electrons, Hydrogen Ions
ATP, NADPH
Step 3
ATP, NADPH, Carbon Dioxide
Organic Compounds

44. Factors that Affect Photosynthesis
Photosynthesis is directly affected by environmental factors.
The most obvious factor is light.
In general, the rate of photosynthesis increase as light intensity increases until all the pigments are being used.
At this saturation point, the reactions of the Calvin cycle cannot proceed any faster.
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.
Once a certain concentration of carbon dioxide is present, photosynthesis cannot proceed any faster.
Photosynthesis is most efficient within a certain range of temperatures.
Photosynthesis involves many enzyme-assisted chemical reactions and unfavorable temperatures may inactivate certain enzymes.

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

46. 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
C6H12O O → 6CO H2O + energy
glucose oxygen carbon water ATP
gas dioxide

47. The Steps of Cellular Respiration
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.

48. 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.
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.

49. 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.
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.

50. Glycolysis
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.

51. Glycolysis

52. Glycolysis
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.

53. 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.

54. 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.

56. Krebs Cycle
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.

57. 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.

58. Electron Transport Chain
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.

59. Electron Transport Chain

60. Respiration in the Absence of Oxygen
What happens when there is not enough oxygen for aerobic respiration to occur?
The electron transport chain does not function because oxygen is not available to serve as the final electron acceptor.
Electrons are not transferred from NADH, and NAD+ therefore they cannot be recycled.
When Oxygen is not present, NAD+ is recycled in another way.

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

62. Lactic Acid Fermentation
In some organisms, a three-carbon pyruvate is converted to a three-carbon lactate through lactic acid fermentation.
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.
Lactate can build up in muscle cells if it is not removed quickly enough, sometimes causing muscle soreness.

63. Lactic Acid Fermentation

64. 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.

65. Alcoholic Fermentation

66. Alcoholic Fermentation
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.
At a concentration of about 12 percent ethanol kills yeast.
Therefore, naturally fermented wine contains about 12% ethanol.

67. 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.
When Oxygen is present, aerobic respiration occurs.
When Oxygen is absent, fermentation occurs.

68. Production of ATP