1. Energy and Life

2. The Flow of Energy in Living Cells Energy is the ability to do work Energy is considered to exist in two states  kinetic energy  the energy of motion  potential energy  stored energy that can be used for motion All the work carried out by living organisms involves the transformation of potential energy to kinetic energy

3. The Flow of Energy Cellular activity requires energy. Energy is defined as the capacity to do work. Kinetic energy Potential energy The study of energy is called thermodynamics.

4. Figure 6.1 Potential and kinetic energy

5. The Flow of Energy in Living Things There are many forms of energy but all of them can be converted to heat Heat energy is the most convenient form of energy to measure Thermodynamics is the study of energy or heat changes

6. Laws of Thermodynamics Laws of thermodynamics govern the energy changes that are involved with any activity by an organism The First Law of Thermodynamics:  Energy cannot be created nor destroyed; it can undergo conversion from form to another.  Energy is lost during the conversion. The Second Law of Thermodynamics  Disorder (entropy) in the universe is increasing.  Energy from the sun is converted to heat or random molecular motion.

7. The Flow of Energy in Living Things Energy from the sun is captured by some types of organisms and is used to build molecules These molecules then posses potential energy that can be used to do work in the cell Chemical reactions involve the making and breaking of chemical bonds

8. Chemical Reactions The starting molecules of a chemical reaction are called the reactants or, sometimes, substrates The molecules at the end of a reaction are called the products There are two kinds of chemical reactions  endergonic reactions have products with more energy than the reactants  these reactions are not spontaneous  exergonic reactions have products with less energy than the reactants  these reactions are spontaneous

9. Chemical Reactions All chemical reactions require an initial input of energy called the activation energy  the activation energy initiates a chemical reaction by destabilizing existing chemical bonds Reactions become more spontaneous if their activation energy is lowered  this process is called catalysis  catalyzed reactions proceed much faster than non- catalyzed reactions

10. Chemical reactions and activation energy Figure 6.4 (a)Figure 6.4 (b)

11. Chemical Reactions Reactions that occur on their own are called exogonic and release energy Reactions that need assistance to start are endogonic and require energy. (Activation energy) Activation energy is needed by endogonic reactions to destabilize bonds and cause the reaction to occur. Catalysis is the process of lowering activation energy…helps both exogonic and endogonic reactions.

12. (c) Catalyzed reaction

13. How Enzymes Work Enzymes are the catalysts used by cells to perform particular reactions  enzymes bind specifically to a molecule and stress the bonds to make the reaction more likely to proceed  active site is a site on the surface of the enzyme that binds to a reactant  the site on the reactant that binds to an enzyme is called the binding site

14. Enzymes Allosteric sites are the points where signal molecules bind to control the rate of enzyme activity. Metal ions act as cofactors to aid catalysis. Nonprotein organic molecules called coenzymes aid catalysis. Coenzymes carry energy-bearing electrons in biochemical reactions (NAD  NADH) Enzymes need optimal temperature and pH to operate effectively..these are specific to each enzyme.

15. How Enzymes Work The binding of a reactant to an enzyme causes the enzyme’s shape to change slightly  this leads to an “induced fit” where the enzyme and substrate fit tightly together as a complex  the enzyme lowers the activation energy for the reaction while it is bound to the reactant  the enzyme is unaffected by the chemical reaction and be re- used

16. Figure 6.6 How Enzymes Work

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19. How Enzymes Work Catalyzed reactions may occur together in sequence  the product of one reaction is the substrate for the next reaction until a final product is made  the series of reactions is called a biochemical pathway Figure 6.7

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21. How Enzymes Work Temperature and pH affect enzyme activity  enzymes function within an optimum temperature range  when temperature increases, the shape of the enzyme changes due to unfolding of the protein chains  enzymes function within an optimal pH range  the shape of enzymes is also affected by pH  most enzymes work best within a pH range of 6 - 8 exceptions are stomach enzymes that function in acidic ranges

22. How Cells Regulate Enzymes Cells can control enzymes by altering their shape  allosteric enzymes are affected by the binding of signal molecules  the signal molecules bind on a site on the enzyme called the allosteric site  some signals act as repressors inhibit the enzyme when bound  other signals act as activators change the shape of the enzyme so that it can bind the substrate

23. Figure 6.9 Allosteric enzyme regulation

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25. 6.5 How Cells Regulate Enzymes Feedback inhibition is a form of enzyme inhibition where the product of a reaction acts as a repressor  competitive inhibition  the inhibitor competes with the substrate for the active site  the inhibitor can block the active site so that it cannot bind substrate  non-competitive inhibition  the inhibitor binds to the allosteric site and changes the shape of the active site so that no substrate can bind

26. How enzymes can be inhibited

27. ATP: The Energy Currency of the Cell The energy from the sun or from food sources must be converted to a form that cells can use  adenosine triphosphate (ATP) is the energy currency of the cell

28. ATP: The Energy Currency of the Cell The structure of ATP suits it as an energy carrier  each ATP molecule has three parts 1. a sugar 2. an adenine nucleotide 3. a chain of three phosphate groups  the phosphates are negatively charged and it takes a lot of chemical energy to hold them together  the phosphates are poised to come apart

29. ATP: The Energy Currency of the Cell ATP cycles in the cell with respect to its energy needs  photosynthesis  some cells convert energy from the sun into ATP and then use it to make sugar where it is stored as potential energy  cellular respiration  cells break down the potential energy in sugars and convert it ATP

30. Chapter 7 Photosynthesis

31. An Overview of Photosynthesis Most of the energy used by almost all living cells ultimately comes from the sun  plants, algae, and some bacteria capture the sunlight energy by a process called photosynthesis  only about 1% of the available energy in sunlight is captured

32. An Overview of Photosynthesis The leaf cells of plants contain chloroplasts  the chloroplast contains internal membranes called thylakoids  the thylakoids are stacked together in columns called grana  the stroma is a semiliquid substance that surrounds the thylakloids

33. An Overview of Photosynthesis The photosystem is the starting point of photosynthesis  it is a network of pigments in the membrane of the thylakoid  the primary pigment of a photosystem is chlorophyll  the pigments act as an antenna to capture energy from sunlight  individual chlorophyll pigments pass the captured energy between them

34. An Overview of Photosynthesis Photosynthesis takes places in three stages 1. capturing energy from sunlight 2. using the captured energy to produce ATP and NADPH 3. using the ATP and NADPH to make carbohydrates from CO 2 in the atmosphere

35. An Overview of Photosynthesis The overall reaction for photosynthesis may be summarized by this equation 6 CO 2 + 12 H 2 O + light  C 6 H 12 O 6 + 6 H 2 O + 6 O 2 The process of photosynthesis is divided into two types of reactions  light-dependent reactions  take place only in the presence of light and produce ATP and NADPH  light-independent reactions  do not need light to occur and result in the formation of organic molecules  more commonly known as the Calvin cycle

36. How Plants Capture Energy from Sunlight  Light is comprised of packets of energy called photons sunlight has photons of varying energy levels ○ the possible range of energy levels is represented by an electromagnetic spectrum human eyes only perceive photons of intermediate energy levels ○ this range of the spectrum is known as visible light

37. Figure 7.1 Photons of different energy: the electromagnetic spectrum

38. How Plants Capture Energy from Sunlight  Pigments are molecules that absorb light energy the main pigment in plants is chlorophyll ○ chlorophyll absorbs light at the ends of the visible spectrum, mainly blue and red light plants also contain other pigments, called accessory pigments, that absorb light levels that chlorophyll does not ○ these pigments give color to flowers, fruits, and vegetables ○ they are present in leaves too but are masked by chlorophyll, until the fall when the chlorophyll is broken down

39. Absorption spectra of chlorophylls and carotenoids

40. Organizing Pigments into Photosystems  In plants, the light-dependent reactions occur within a complex of proteins and pigments called a photosystem light energy is first captured by any one of the chlorophyll pigments the energy is passed along to other pigments until it reaches the reaction center chlorophyll molecule the reaction center then releases an excited electron, which is then transferred to an electron acceptor the excited electron that is lost is then replaced by an electron donor

41. Figure 7.7 How a photosystem works

42. Organizing Pigments into Photosystems Plants use two photosystems in series to generate power to reduce NADP + to NADPH with enough energy left over to generate ATP.

43. Photosystem Conversion of Light to Chemical Energy Plants use two photosystems in a two-stage process called noncyclic photophosphorylation. For every pair of electrons obtained from water, one molecule of NADPH and a little over one molecule of ATP are produced.

44. Photosystem Conversion of Light to Chemical Energy Photosystem II  Reaction center is called P 680  Oxygen atoms of two water molecules bind to magnesium, causing water to split  Oxygen is released while electrons from water are used to replace those that are boosted from the reaction center by sunlight.

45. Photosystem Conversion of Light to Chemical Energy Path to Photosystem I:  The electron taken from the Photosystem II is carried to photosystem I by several intermediates. Making ATP: Chemiosmosis  Protons cross the thylakoid membranes at embedded proton pumps causing ADP to be phosphorylated to ATP.

46. Photosystem Conversion of Light to Chemical Energy Photosystem I:  The reaction center is P 700  The electron from photosystem II is boosted to an even higher energy level as light strikes Photosystem I.  The electron is passed to another carrier.

47. Photosystem Conversion of Light to Chemical Energy Making NADPH  Electrons transported from Photosystem I are used to reduce NADP + to NADPH. Making More ATP  While the electron is passed from water to NADPH, one molecule of NADPH and more than one molecule of ATP are generated.  If more ATP is needed, some plants can by-pass Photosystem I and switch to cyclic photophosphorylation.

48. Figure 7.6 Plants use two photosystems

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50. Photorespiration: Putting the Brakes on Photosynthesis Many plants have trouble carrying out C 3 photosynthesis when it is hot  plants close openings in their leaves, called stomata (singular, stoma), in order to prevent water loss  the closed stoma also prevent gas exchange  O 2 levels build up inside the leaves while the concentrations of CO 2 fall  the enzyme rubisco fixes oxygen instead of carbon  this process is called photorespiration and short-circuits the Calvin cycle and photosynthesis

51. Plant response in hot weather

52. Photorespiration: Putting the Brakes on Photosynthesis Some plants have adapted to hot climates by performing C 4 photosynthesis  these C 4 plants include sugarcane, corn, and many grasses  they fix carbon using different types of cells and reactions than C 3 plants and do not run out of CO 2 even in hot weather  CO 2 becomes trapped in cells called bundle-sheath cells

53. Figure 7.12 Carbon fixation in C 4 plants

54. Photorespiration: Putting the Brakes on Photosynthesis Another strategy to avoid a reduction in photosynthesis in hot weather occurs in many succulent (water-storing) plants, such as cacti and pineapples  these plants undergo crassulacean acid metabolism (CAM)  photosynthesis occurs via the C 4 pathway at night and the C 3 pathway during the day

55. A Review of Cellular Respiration Through respiration, one molecule of glucose generates a total of 36 ATP. The control of the process works through a system of feedback inhibition in which key enzymes in the Krebs Cycle become stuck. The binding of the ATP causes the enzyme to change its form and not function as an enzyme.