1. Life processes require a constant supply of energy
Life processes require a constant supply of energy. Cells use energy that is stored in the bonds of certain organic molecules. Adenosine triphosphate (ATP) is a molecule that transfers energy from the breakdown of food molecules to cell processes. Adenosine triphosphate (ATP) is the most important biological molecule that supplies energy to the cell. A molecule of ATP is composed of three parts:  A nitrogenous base (adenine)  A sugar (ribose)  Three phosphate groups (therefore the name triphosphate) bonded together by “high energy” bonds

2. The ATP-ADP cycle  The energy stored in ATP is released when a phosphate group is removed from the molecule. ATP has three phosphate groups, but the bond holding the third phosphate groups is very easily broken.  When the phosphate is removed, ATP becomes ADP - adenosine diphosphate, a phosphate is released into the cytoplasm and energy is released.  ADP is a lower energy molecule than ATP, but can be converted to ATP by the addition of a phosphate group.

3. ATP → ADP + phosphate + energy available for cell processes  To supply the cell with energy, ADP is continually converted to ATP by the addition of a phosphate during the process of cellular respiration. ATP carries much more energy than ADP.  As the cell requires more energy, it uses energy from the breakdown of food molecules to attach a free phosphate group to an ADP molecule in order to make ATP.

4. ADP + phosphate + energy from breakdown of food molecules→ ATP ATP is consumed in the cell by energy-requiring processes and can be generated by energy-releasing processes. In this way ATP transfers energy between separate biochemical reactions in the cell. ATP is the main energy source for the majority of cellular functions. This includes the synthesis of organic molecules, including DNA and, and proteins. ATP also plays a critical role in the transport of organic molecules across cell membranes, for example during exocytosis and endocytosis

5. All organisms need a constant source of energy to survive
All organisms need a constant source of energy to survive. The ultimate source of energy for most life on Earth is the Sun. Photosynthesis, which occurs in the chloroplast, is the overall process by which solar energy (sunlight) is used to chemically convert water and carbon dioxide into chemical energy stored in simple sugars (such as glucose). This process occurs in two stages.  The first stage is called the light-dependent reactions because they require solar energy.  Sugars are not made during the light-dependent reactions.  During the light-dependent reactions, solar energy is absorbed by chloroplasts and two energy-storing molecules (ATP and NADPH) are produced.  The solar energy is used to split water molecules that results in the release of oxygen as a waste product. The splitting of water molecules allows for the temporary transfer of the solar energy to electrons released by the broken bonds. This energy is used to make ATP and NADPH.  The second stage is called the Calvin cycle or the light-independent reactions because they do not require solar energy.  During the Calvin cycle (light-independent reactions), carbon dioxide from the atmosphere and energy carried by ATP and NADPH is used to make simple sugars (such as glucose). These simple sugars store chemical energy.

6. The process photosynthesis is generally represented using a balanced chemical equation. However, this equation does not represent all of the steps that occur during the process of photosynthesis. Solar energy 6CO2 + 6H2O  C6H12O6 + 6O2  In general, six carbon dioxide molecules and six water molecules are needed to produce one glucose molecule and six oxygen molecules. The reactants, water and carbon dioxide are input during different stages of the process. Of the reactants water is used during the light-dependent reactions and carbon dioxide is used during the Calvin cycle.  The products (oxygen and glucose) are outputs of different stages of the process. Oxygen is released during the light-dependent reactions and glucose is formed during the Calvin cycle.  Solar energy is needed to split the water molecules.

7. Photosynthesis is the overall process by which solar energy (sunlight) is used to chemically convert water and carbon dioxide into chemical energy stored in simple sugars (such as glucose). Solar Energy 6CO2 + 6H2O  C6H12O6 + 6O2 The simple sugars produced by the fixation of atmospheric carbon (from carbon dioxide) are mostly recycled to keep the Calvin cycle (light-independent reactions) going. Some of these sugars, however, are converted to form other carbohydrates such as glucose, starch and cellulose.

8. Glucose can be used by the cell for energy to make ATP during cellular respiration or it can be converted into starch or cellulose. The sugars produced by photosynthesis also provide carbon skeletons that can interact with elements such as nitrogen, sulfur, and phosphorus to make other organic molecules such as amino acids, lipids or nucleic acids

9. The ultimate goal of cellular respiration is to convert the chemical energy in food to chemical energy stored in adenosine triphosphate (ATP). ATP can then release the energy for cellular metabolic processes, such as active transport across cell membranes, protein synthesis, and muscle contraction.  Any food (organic) molecule, including carbohydrates, fats/lipids, and proteins can be broken down into smaller molecules and then used as a source of energy to produce ATP molecules.

10. To transfer the energy stored in glucose to the ATP molecule, a cell must break down glucose slowly in a series of steps and capture the energy in stages.  The first stage is glycolysis. o In the process of glycolysis a glucose molecule is broken down into pyruvic acid molecules with a net gain of two ATP molecules. o Glycolysis is a series of reactions using enzymes that takes place in the cytoplasm and does not need oxygen. Glucose pyruvic acid + ATP (small amount)

11. If oxygen is available, the two-stage process of aerobic respiration occurs, primarily in the mitochondria of the cell.  The first stage of aerobic respiration is called the Krebs cycle. o The pyruvic acid, produced by glycolysis, travels to the mitochondria where it is broken down in a cycle of chemical reactions, from which carbon dioxide and energy (used to form a small number of ATP molecules) are released. pyruvic acid carbon dioxide + ATP (small amount)  The second stage of aerobic respiration is the electron transport chain. o The electron transport chain is a series of chemical reactions in which energy is transferred to form a large number of ATP molecules. o At the end of the chain oxygen enters the process and is combined with hydrogen to form water.

12. In general, one glucose molecule and six oxygen molecules are needed to produce six carbon dioxide molecules and six water molecules.  Each of the reactants (glucose and oxygen) is used during different stages of cellular respiration. Glucose is an input of glycolysis and oxygen is an input of the electron transport chain of aerobic respiration.  Each of the products (carbon dioxide and water) is formed during different stages of the process. Carbon dioxide is released from the Krebs cycle and water is released at the end of the electron transport chain.  Up to 38 molecules of ATP are made from the breakdown of one glucose molecule: 2 from glycolysis and up to 36 from aerobic respiration.  Most of the energy released by cellular respiration, that is not used to make ATP, is released in the form of heat.

13. If no oxygen is available, cells can obtain energy through the process of anaerobic respiration. Fermentation is an anaerobic process that allows glycolysis (which is also anaerobic) to continue making ATP in the absence of oxygen.  Fermentation is not an efficient process and results in the formation of far fewer ATP molecules than aerobic respiration.  Two fermentation processes that occur in many organisms are: o Lactic acid fermentation occurs, for example, in muscle tissues during rapid and vigorous exercise when muscle cells may be depleted of oxygen. Lactic acid fermentation is also used by bacteria in the production of food products such as yogurt and sauerkraut. o The pyruvic acid formed during glycolysis is broken down to lactic acid, and in the process energy is released, which can be used in glycolysis to make ATP. Glucose  Pyruvic acid  Lactic acid + energy o Once oxygen becomes available again, muscle cells return to using aerobic respiration

14. THINK ABOUT IT
Homeostasis is hard work. Organisms and the cells within them have to grow and develop, move materials around, build new molecules, and respond to environmental changes.
What powers so much activity, and where does that power come from?

15. Chemical Energy and ATP
Why is ATP useful to cells?

16. Chemical Energy and ATP
Why is ATP useful to cells?
ATP can easily release and store energy by breaking and re-forming the bonds between its phosphate groups. This characteristic of ATP makes it exceptionally useful as a basic energy source for all cells.

17. Chemical Energy and ATP
One of the most important compounds that cells use to store and release energy is adenosine triphosphate (ATP).
ATP consists of adenine, a 5-carbon sugar called ribose, and three
phosphate groups.

18. Storing Energy
Adenosine diphosphate (ADP) looks almost like ATP, except that it has two phosphate groups instead of three. ADP contains some energy, but not as much as ATP.
When a cell has energy available, it can store small amounts of it by adding phosphate groups to ADP, producing ATP.
ADP is like a rechargeable battery that powers the machinery of the cell.

19. Releasing Energy
Cells can release the energy stored in ATP by breaking the bonds between the second and third phosphate groups.
Because a cell can add or subtract these phosphate groups, it has an efficient way of storing and releasing energy as needed.

20. Using Biochemical Energy
One way cells use the energy provided by ATP is to carry out active transport.
Many cell membranes contain sodium-potassium pumps. ATP provides the energy that keeps these pumps working, maintaining a balance of ions on both sides of the cell membrane.

21. Using Biochemical Energy
ATP powers movement, providing the energy for motor proteins that contract muscle and power the movement of cilia and flagella.

22. Using Biochemical Energy
Energy from ATP powers the synthesis of proteins and responses to chemical signals at the cell surface.

23. Using Biochemical Energy
ATP is not a good molecule for storing large amounts of energy over the long term.
It is more efficient for cells to keep only a small supply of ATP on hand.
Cells can regenerate ATP from ADP as needed by using the energy in foods like glucose.

24. Heterotrophs and Autotrophs
What happens during the process of photosynthesis?

25. Heterotrophs and Autotrophs
What happens during the process of photosynthesis?
In the process of photosynthesis, plants convert the energy of sunlight into chemical energy stored in the bonds of carbohydrates.

26. Heterotrophs and Autotrophs
Organisms that obtain food by consuming other living things are known as heterotrophs.
Some heterotrophs get their food by eating plants.
Other heterotrophs, such as this cheetah, obtain food from plants indirectly by feeding on plant-eating animals.
Still other heterotrophs, such as mushrooms, obtain food by decomposing other organisms.

27. Heterotrophs and Autotrophs
Organisms that make their own food are called autotrophs.
Plants, algae, and some bacteria are able to use light energy from the sun to produce food. The process by which autotrophs use the energy of sunlight to produce high-energy carbohydrates that can be used for food is known as photosynthesis.

28. Light Energy from the sun travels to Earth in the form of light.
Sunlight is a mixture of different wavelengths, many of which are visible to our eyes and make up the visible spectrum.

29. Pigments
Plants gather the sun’s energy with light-absorbing molecules called pigments.
The plants’ principal pigment is chlorophyll.

30. Energy Collection
Because light is a form of energy, any compound that absorbs light absorbs energy. Chlorophyll absorbs visible light especially well.
When chlorophyll absorbs light, a large fraction of the light energy is transferred to electrons. These high-energy electrons make photosynthesis work.

31. High-Energy Electrons
Think of a high-energy electron as being similar to a hot potato. If you wanted to move the potato from one place to another, you would use an oven mitt—a carrier—to transport it.
Plants use electron carriers to transport high-energy electrons from chlorophyll to other molecules.

32. High-Energy Electrons
NADP+ (nicotinamide adenine dinucleotide phosphate) is a carrier molecule.
NADP+ accepts and holds two high-energy electrons, along with a hydrogen ion (H+). In this way, it is converted into NADPH.
The NADPH can then carry the high-energy electrons to chemical reactions elsewhere in the cell.

33. An Overview of Photosynthesis
Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high-energy sugars and oxygen.
In symbols:
6 CO2 + 6 H2O  C6H12O6 + 6 O2
In words:
Carbon dioxide + Water  Sugars + Oxygen

34. Light-Dependent Reactions
Photosynthesis involves two sets of reactions.
The first set of reactions is known as the light-dependent reactions because they require the direct involvement of light and light-absorbing pigments.

35. Light-Dependent Reactions
The light-dependent reactions use energy from sunlight to produce ATP and NADPH.
These reactions take place within the thylakoid membranes of the chloroplast.
Water is required as a source of electrons and hydrogen ions. Oxygen is released as a byproduct.

36. Light-Independent Reactions
Plants absorb carbon dioxide from the atmosphere and complete the process of photosynthesis by producing sugars and other carbohydrates.
During light-independent reactions, ATP and NADPH molecules produced in the light-dependent reactions are used to produce high-energy sugars from carbon dioxide.

37. Light-Independent Reactions
No light is required to power the light-independent reactions.
The light-independent reactions take place outside the thylakoids, in the stroma.

38. The Light-Dependent Reactions: Generating ATP and NADPH
The light-dependent reactions use energy from sunlight to produce oxygen
and convert ADP and NADP+ into the energy carriers ATP and NADPH.
The light-dependent reactions encompass the steps of photosynthesis that directly involve sunlight.
The light-dependent reactions occur in the thylakoids of chloroplasts.

39. Summary of Light-Dependent Reactions
The light-dependent reactions produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH.
ATP and NADPH provide the energy needed to build high-energy sugars from low-energy carbon dioxide.

40. The Light-Independent Reactions: Producing Sugars
What happens during the light-independent reactions?
During the light-independent reactions, ATP and NADPH from the light
dependent reactions are used to produce high-energy sugars.

41. The Light-Independent Reactions: Producing Sugars
During the light-independent reactions, commonly referred to as the Calvin cycle, plants use the energy that ATP and NADPH contains to build stable high-energy carbohydrate compounds that can be stored for a long time.

42. Summary of the Calvin Cycle
The Calvin cycle uses 6 molecules of carbon dioxide to produce a single 6-carbon sugar molecule.

43. Summary of the Calvin Cycle
The energy for the reactions is supplied by compounds produced in the light-dependent reactions.

44. Summary of the Calvin Cycle
The plant uses the sugars produced by the Calvin cycle to meet its energy needs and to build macromolecules needed for growth and development.
When other organisms eat plants, they can use the energy and raw materials stored in these compounds.

45. The End Results
The two sets of photosynthetic reactions work together—the light-dependent reactions trap the energy of sunlight in chemical form, and the light-independent reactions use that chemical energy to produce stable, high-energy sugars from carbon dioxide and water.
In the process, animals, including humans, get food and an atmosphere filled with oxygen.

46. Overview of Cellular Respiration
If oxygen is available, organisms can obtain energy from food by a process called cellular respiration. The summary of cellular respiration is presented below.
In symbols:
6 O2 + C6H12O6  6 CO2 + 6 H2O + Energy
In words:
Oxygen + Glucose  Carbon dioxide + Water + Energy
The cell has to release the chemical energy in food molecules (like glucose) gradually, otherwise most of the energy would be lost in the form of heat and light.

47. Stages of Cellular Respiration
The three main stages of cellular respiration are glycolysis, the Krebs cycle, and the electron transport chain.

48. Stages of Cellular Respiration
Glycolysis produces only a small amount of energy. Most of glucose’s energy (90%) remains locked in the chemical bonds of pyruvic acid at the end of glycolysis.

49. Stages of Cellular Respiration
During the Krebs cycle, a little more energy is generated from pyruvic acid.

50. Stages of Cellular Respiration
The electron transport chain produces the bulk of the energy in cellular respiration by using oxygen, a powerful electron acceptor.

51. Oxygen and Energy
Pathways of cellular respiration that require oxygen are called aerobic. The Krebs cycle and electron transport chain are both aerobic processes. Both processes take place inside the mitochondria.

52. Oxygen and Energy
Glycolysis is an anaerobic process. It does not directly require oxygen, nor does it rely on an oxygen-requiring process to run. However, it is still considered part of cellular respiration. Glycolysis takes place in the cytoplasm of a cell.

53. Glycolysis What happens during the process of glycolysis?
During glycolysis, 1 molecule of glucose, a 6-carbon compound, is transformed into 2 molecules of pyruvic acid, a 3-carbon compound.

54. Glycolysis Glycolysis is the first stage of cellular respiration.
During glycolysis, glucose is broken down into 2 molecules of the 3-carbon molecule pyruvic acid. Pyruvic acid is a reactant in the Krebs cycle.
ATP and NADH are produced as part of the process.

55. ATP Production
The cell “deposits” 2 ATP molecules into its “account” to get glycolysis going.

56. ATP Production
Glycolysis then produces 4 ATP molecules, giving the cell a net gain of 2 ATP molecules for each molecule of glucose that enters glycolysis.

57. NADH Production
NADH carries the high-energy electrons to the electron transport chain, where they can be used to produce more ATP.
2 NADH molecules are produced for every molecule of glucose that enters glycolysis.

58. The Advantages of Glycolysis
Glycolysis produces ATP very fast, which is an advantage when the energy demands of the cell suddenly increase.
Glycolysis does not require oxygen, so it can quickly supply energy to cells when oxygen is unavailable.

59. The Krebs Cycle What happens during the Krebs cycle?
During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions.

60. The Krebs Cycle
During the Krebs cycle, the second stage of cellular respiration, pyruvic acid produced in glycolysis is broken down into carbon dioxide in a series of energy-extracting reactions.
The Krebs cycle is also known as the citric acid cycle because citric acid is the first compound formed in this series of reactions.

61. Electron Transport and ATP Synthesis
How does the electron transport chain use high-energy electrons from glycolysis and the Krebs cycle?
The electron transport chain uses the high-energy electrons from glycolysis and the Krebs cycle to convert ADP into ATP.

62. Energy Totals
In the presence of oxygen, the complete breakdown of glucose through cellular respiration results in the production of 36 ATP molecules.
This represents about 36 percent of the total energy of glucose. The remaining 64 percent is released as heat.

63. Comparing Photosynthesis and Cellular Respiration
Photosynthesis and cellular respiration are opposite processes.
The energy flows in opposite directions. Photosynthesis “deposits” energy, and cellular respiration “withdraws” energy.
The reactants of cellular respiration are the products of photosynthesis and vice versa.

64. Comparing Photosynthesis and Cellular Respiration
The release of energy by cellular respiration takes place in plants, animals, fungi, protists, and most bacteria.
Energy capture by photosynthesis occurs only in plants, algae, and some bacteria.

65. Fermentation
Fermentation is a process by which energy can be released from food molecules in the absence of oxygen. Fermentation occurs in the cytoplasm of cells.

66. Fermentation
Under anaerobic conditions, fermentation follows glycolysis. During fermentation, cells convert NADH produced by glycolysis back into the electron carrier NAD+, which allows glycolysis to continue producing ATP.

67. Alcoholic Fermentation
Yeast and a few other microorganisms use alcoholic fermentation that produces ethyl alcohol and carbon dioxide.
This process is used to produce alcoholic beverages and causes bread dough to rise.

68. Lactic Acid Fermentation
Most organisms, including humans, carry out fermentation using a chemical reaction that converts pyruvic acid to lactic acid.
Chemical equation:
Pyruvic acid + NADH  Lactic acid + NAD+