CHAPTER 8 PHOTOSYNTHESIS 8-1 Energy and Life Energy - the ability to do work - all activities depend on some kind of energy; this includes the activities of living things - without the ability to obtain and use energy, life would cease to exist

Where does the energy that living things need come from? A. Autotrophs and Heterotrophs Plants (autotrophs) use light energy from the sun to produce food Animals (heterotrophs) obtain energy from food they consume Mushrooms (heterotrophs) obtain food from organisms they decompose *****to live, all organisms (plants and animals) must release the energy in sugars and other compounds

B. Chemical Energy and ATP Forms of energy include: light, heat, electricity - energy is stored in chemical compounds - all cellular activities are powered by chemical fuel 1. ATP and ADP ATP (adenosine triphosphate) - a chemical compound that living things use to store energy - consists of: adenine – a nitrogen-containing compound ribose – a 5-carbon sugar 3 phosphate groups

ADP (adenosine diphosphate) - structurally similar to ATP, but contain s only 2 phosphate groups - small amounts of energy can be stored by adding a phosphate group to ADP and converting it to ATP

C. ATP and Glucose - cells regenerate ATP from ADP by using energy stored in carbohydrates like glucose - ATP moves energy more efficiently than it stores it - One molecule of glucose stores 90 times more chemical energy than one molecule of ATP

8-2 An overview of Photosynthesis Photosynthesis - the process by which plants use the energy from sunlight to convert CO2 and H2O to O2 and sugars and starches A. Investigating photosynthesis Experiments of many scientists contributed to our understanding of photosynthesis 1. Van Helmont’s experiment (1643) - carefully measured a plant’s intake and weight increase and concluded that trees gain most of their mass from water - his experiment accounts for the water portion of the carbohydrate produced by photosynthesis

2. Priestley’s experiment (1771) - using a bell jar, candle, and a living plant, he found that plants release O2

3. Jan Ingenhousz (1779) - found that aquatic plants produce O2 bubbles in the light, but not in the dark, concluding that plants need sunlight to produce O2

*****These 3 experiments and others performed later, revealed that in the presence of light, plants change CO2 and H2O into carbohydrates and release O2

B. The Photosynthesis Equation Photosynthesis uses the energy of sunlight to convert H2O and CO2 into O2 and high-energy sugars light Carbon dioxide + water  glucose + oxygen 6CO2 + H2O  C6H12O6 + 6O2 - plants use the sugars to produce more complex carbohydrates (starches) - plants obtain CO2 from the air or water in which they grow

C. Light and Pigments - you have to understand how plants capture the energy of sunlight in order to understand how they use raw materials to produce complex sugars - photosynthesis requires light and chlorophyll, the pigmented molecule found in chloroplasts - there are 2 types of chlorophyll; chlorophyll a and chlorophyll b - chlorophyll absorbs light energy at different wavelengths within the visible spectrum chlorophyll a (violet and red) chlorophyll b (red and blue) - chlorophyll does not absorb light in the green spectrum (this is why plants are green) and very little in the yellow and orange spectrum.

- Any compound that absorbs light also absorbs the energy from that light which is transferred directly to electrons in the chlorophyll molecule - This increases the energy level in the chlorophyll electrons and these high-energy electrons make photosynthesis work

8-3 The Reactions of Photosynthesis A. Inside the Chloroplast - where photosynthesis takes place - thylakoids are sac-like membranes arranged in stacks called grana - the thylakoids contain clusters of chlorophyll and other pigments and proteins known as photosystems that capture the energy of sunlight - photosynthesis takes place in two stages; the light-dependent reaction which takes place in the thylakoids and the light-independent reaction (Calvin cycle) which takes place in the region outside the thylakoid membranes (stroma)

The Reactions of Photosynthesis

B. NADPH (nicotinamide adenine dinucleotide phosphate) - a special carrier molecule that transports the high-energy electrons that result from the capture of energy from sunlight in chlorophyll - NADP+ accepts and holds 2 high-energy electrons with a H+ - This converts NADP+ to NADPH trapping the energy of sunlight in chemical form - NADPH carries this energy to chemical reactions elsewhere in the cell such as building needed carbohydrates like glucose

C. Light-dependent Reactions - require sunlight - produce O2 gas and convert ADP and NADP+ into the energy carriers ATP and NADPH

STEPS Step A - Pigments in photosystem II absorb light - energy from the light is absorbed into electrons increasing their energy levels - these high-energy electrons move into the electron transport chain Step B - high-energy electrons move from the electron transport chain to photosystem I. - Electron energy is used to move hydrogen ions from the stroma into the inner thylakoid

Step C - Pigments in photosystem I use light energy to reenergize the electrons. - NADP+ picks up these high-energy electrons at the outer thylakoid surface along with a hydrogen ion and becomes NADPH Step D - Inside of the thylakoid membrane fills up with positively charged hydrogen ions making the outside of the thylakoid membrane negatively charged and the inside positively charged

Step E - Hydrogen ions cannot cross the membrane but requires a membrane protein called ATP synthase which allows them too pass through it. - As they pass through it, ATP synthase converts ADP to ATP

D. The Calvin Cycle (light independent or dark reaction) - the Calvin Cycle uses ATP and NADPH to produce high-energy sugars - it takes place in the stroma of chloroplasts - it does not require direct light

STEPS Step A - Six CO2 molecules enter from the atmosphere and combine with six 5-carbon molecules to form twelve 3-carbon molecules Step B - The twelve 3-carbon molecules are converted to higher energy forms powered by ATP and NADPH Step C - Two of the 3-carbon molecules are converted to sugars and other compounds

Step D - the remaining ten 3-carbon molecules are converted back to 5-carbon molecules to be used in the next cycle ***** Six CO2 molecules are used to produce one 6-carbon sugar molecule

D. Factors Affecting Photosynthesis 1. Shortage of water - can slow or stop photosynthesis 2. Temperature - photosynthetic enzymes work best at 0-35 degrees Celsius - above or below that, enzymes could be damaged and at very low temperatures, photosynthesis could stop completely

3. Light intensity - increase in light intensity increases the rate of photosynthesis - at a certain intensity level, plants will reach a maximum rate of photosynthesis - this maximum intensity level will vary from plant type to plant type