Photosynthesis

1. Photosynthesis IB topic 3.8 (page 83)

2. What is photosynthesis? • Photosynthesis is the production of organic compounds (carbohydrates, lipids, and proteins) using light energy and simple inorganic substances such as carbon dioxide and water • It is an example of energy conversion • Light energy is converted into chemical energy

3. First organisms • Prokaryotes were the first organisms to perform photosynthesis • About 3500 million years ago • They were joined billions of years later by algae and plants • One consequence of photosynthesis is the rise in oxygen concentration of the atmosphere, which began about 2300 million years ago

4. Why is this important? • Photosynthetic organisms produce foods that begin food chains • We count on the Sun as a constant energy source for warmth and food • Sunlight must be converted into a useful form of chemical energy for non-photosynthetic organisms • The most common chemical energy produced from photosynthesis is glucose • Most common molecule for fuel in cell respiration (remember?)

5. Pigments and light absorption • The vast majority of plants appear green to our eyes • Zoom into a leaf cell … the only structures in a leaf that are actually green are the chloroplasts • Plants contain a variety of pigments in chloroplasts • The photosynthetic pigment that dominates in most plant species is the molecule chlorophyll

6. Light absorption (cont.) • Light absorption in photosynthesis involves chemical substances called pigments • A white or transparent substance does not absorb visible light • Pigments are substances that do absorb light and therefore appear colored to us

7. Pigments • Pigments that absorb all colors appear black (b/c they emit no light) • Pigments that, for example, absorb all colors but blue appear blue to us (b/c this part of the sunlight is not absorbed) • Instead, the blue light is reflected and can pass into our eye

8. Pigments and light absorption • The 1st stage in photosynthesis is absorption of sunlight • Visible light is a form of electromagnetic radiation • Sunlight is a mixture of different wavelengths of visible light • Which we see at different colors • We call this the visible portion of the spectrum • Blue, green, and red

9. The electromagnetic spectrum

10. Absorption spectrum of several plant pigments

11. Visible light spectrum • Substances can do one of two things when they are struck by a particular wavelength (color) of light: • Absorb that wavelength (energy is being absorbed/used) • Reflect that wavelength (energy is not being absorbed/used; you will see that color)

12. Let’s Check In – Turn and Talk • You are walking outside with a friend who is wearing a red and white shirt. Explain why the shirt appears red and white • Sunlight is mixture of visible light • When sunlight hits the red pigments in the shirt, the blue and green wavelengths of light are absorbed • The red wavelengths are reflected, thus, our eyes see red • White  all wavelengths of light are reflected; eyes and brain interpret this mixture as white

13. How does this apply to photosynthesis? • Chlorophyll is a green pigment • Chlorophyll reflects green light and must absorb other wavelength of visible light • When a plant is hit by sunlight, the red and blue wavelengths of light are absorbed by chlorophyll and used for photosynthesis • Almost all of the energy of the green wavelengths is reflected, not absorbed • Do not try to grow plants in only green light

14. Fun Fact • Chlorophyll is not the only pigment type found in green plants • Most green plants also contain other pigments in smaller quantities that reflect colors like red and yellow • We see this in foliage – turning of the leaves in the fall • End of growing season, • Preparation for cold weather • These colors were always there in the leaf, but were hidden by the dominant color of chlorophyll until late in the season

15. Stages of photosynthesis • Photosynthesis produces sugar molecules as food source for the plant • Sugars (glucose) are held together by covalent bonds • Requires energy to create those covalent bonds • Sun is the source of that energy

16. Two stages of photosynthesis • 1st stage: • Set of reactions that “trap” light energy and convert it to ATP (chemical energy) • 2nd stage: • Set of reactions in which ATP is used to help bond carbon dioxide and water molecules together to create a sugar (glucose)

17. 1st stage – a deeper look • Called light-dependent reactions • Chlorophyll (and other pigments) absorb light energy and convert to ATP • Light energy is also used to accomplish a reaction called photolysis of water • Water molecule is split into hydrogen and oxygen • The oxygen that is split away is released from the plant (waste product) • Good for us & millions of other organisms that need oxygen for aerobic respiration • Useful products for the plant formed during this stage are ATP and hydrogen

18. 2nd stage – a deeper look • Called light independent reactions • ATP and hydrogen are used as forms of chemical energy to convert carbon dioxide and water into useful organic molecules for the plant • Remember? Carbon dioxide is one of the few inorganic molecules that contain carbon • Glucose (6C) is organic • Product of photosynthesis

19. 2nd stage – a deeper look (cont.) • Equation: • 6CO2 + 6H2O  C6H12O6 + 6O2 • Requires 6 molecules of carbon dioxide to form 1 glucose molecule • This conversion of an inorganic form of an element to an organic form is known as fixation

20. Fixation • Therefore, photosynthesis can be described as a series of reactions in which carbon dioxide and water are fixed into glucose, and oxygen is produced as a by-product • This fixation reaction requires energy • The energy to create the glucose comes directly from the ATP and hydrogen created in the 1st stage

21. Important to Note • Energy traced back to sunlight • Glucose is one of many possible organic molecules that can form from photosynthesis

22. Measuring the rate of photosynthesis • Revisit equation: • 6CO2 + 6H2O  C6H12O6 + 6O2 • How does this equation compare to what you know about cellular respiration? • Products and reactants are reversed

23. Measuring the rate (cont.) • At any given time of year, a plant has a fairly consistent rate of cell respiration • Consistent night and day AND relatively low • Plants do not have muscle and other ATP demanding tissues like animals • They need ATP for various biochemical processes, but not as much

24. Measuring the rate (cont.) • The same consistency is not true for the rate of photosynthesis • Rate of photosynthesis depends on environmental factors • Intensity of light, temperature

25. Measuring the rate (cont.) • Daytime, especially on a warm, sunny day = high rate of photosynthesis • Rate of CO2 taken in and O2 released will both be very high • The plant is also doing cell respiration • Correction should be made for both CO2 and O2 levels • At night, the rate of photosynthesis may drop to zero • Giving off CO2 and taking in O2 just to maintain relatively low rate of cell respiration

26. Measuring the rate (cont.) • Measuring the rate of oxygen production or carbon dioxide intake is considered a direct measurement • As long as correction is made for cell respiration • Another common approach: • Keeping track of the change in biomass of experimental plants • Mass of plants is considered to be an indirect measurement • Increase or decrease in biomass may be traced to a whole variety of factors besides photosynthetic rate. • See Figure 3.26 in your text

27. Factors that affect rate of photosynthesis • Light intensity • Carbon dioxide concentration • Temperature • See page 87

28. Photosynthesis – the effect of light intensity • At low light intensities the rate of photosynthesis increases as light intensity is increased • But at high light intensities, further increases in light intensity have not effect • Several essential steps in photosynthesis are achieved using the light energy • ATP is produced (from ADP + phosphate) • Water molecules are split to release hydrogen • Needed later to make carbohydrate

29. Photosynthesis – the effect of CO2 concentration • Although CO2 concentration has been increasing over the past 100 years, it is still low enough to affect the rate of photosynthesis • At low CO2 levels, increases in concentration cause the rate of photosynthesis to rise • At high CO2 levels, further increases in concentration do not affect the rate of photosynthesis

30. Photosynthesis – the effects of temperature • At low temps, the rate of photosynthesis is very low or zero • As temp increases the rate increases until it reaches its maximum (optimum) • Above this temperature, the rate decreases and eventually becomes zero • Like enzymes, right? • Not coincidental; fixation of carbon dioxide it catalyzed by enzymes • In most plants the optimum temperature is between 25-35*C

31. Recap • Photosynthetic organisms produce organic molecules (glucose) to begin food chain • Photosynthetic pigments (chlorophyll) are used to absorb light energy to being the process • Most plants are green because chlorophyll reflect the green wavelengths of light and absorbs the red and blue wavelengths

32. Recap (cont.) • Photosynthesis occurs in 2 stages: light dependent and light independent • Light dependent • Produce ATP and hydrogen; gives off O2 • Light independent • “fix” carbon dioxide and water into organic molecules (such as glucose) using ATP and hydrogen molecules produced during light dependent reactions

33. Recap (cont.) • The rate of photosynthesis can be measured directly by measuring either the production of oxygen or uptake of carbon dioxide • The rate of photosynthesis can be measured indirectly by measuring the biomass increase of a plant • Temperature, light intensity, and carbon dioxide can affect rate of photosynthesis