Chapter 6 Photosynthesis

Section 1: The Light Reactions Autotrophs: use energy from sunlight or chemical bonds of inorganic molecules to make organic substances MOST autotrophs use PHOTOSYNTHESIS Other autotrophs use energy from broken chemical bonds to make organic molecules Heterotrophs: get energy from food instead of from sunlight or inorganic substances

Autotrophs Heterotrophs

PHOTOSYNTHESIS 6CO2 + 6H2O + light energy C6H12O6 + 6O2 Plants can use sunlight convert carbon dioxide and water into glucose and oxygen Autotrophs and heterotrophs will then use the oxygen released from photosynthesis for cellular respiration (Chapter 7) Photosynthesis is broken into: THE LIGHT REACTION and THE CALVIN CYCLE

THE LIGHT REACTION Requires light in order to occur Light is absorbed in chloroplasts: thylakoids grana stroma

Light and Pigments Sunlight is made of a variety of colors, even though it looks white If we pass white light through a prism, we can separate it into its component colors (red, orange, yellow, green, blue, indigo and violet) We call this the visible light spectrum and each color can be reflected, transmitted or absorbed Pigments: compounds that absorb light; some pigments absorb certain colors more strongly than other colors

If a color is absorbed, it takes that color OUT of the spectrum; if blue and red are absorbed, orange, yellow, green, indigo and violet will be reflected

Chloroplast Pigments Located in the thylakoid membranes of chloroplasts are several pigments; the most important of these pigments are called chlorophylls The most common types are chlorophyll a (absorbs more red light) and chlorophyll b (absorbs more blue light) Neither a or b absorbs much green light, so it is usually reflected (that’s why leaves LOOK green)

Chlorophyll a is directly involved in the light reaction, but chlorophyll b aids chlorophyll a in capturing light (called an accessory pigment); Other accessory pigments found in the thylakoid membrane: yellow, orange and brown carotenoids; accessory pigments allow plants to capture much more of the light spectrum

In leaves, chlorophyll is the most abundance pigment, but other plant parts the colors of OTHER pigments are visible (petals and fruits); During fall, plant leaves being to lose their chlorophyll and the pigments of the carotenoids are shown in the leaves

Light Energy to Chemical Energy Once the pigments in chloroplasts have captured the light energy, it has to be converted to chemical energy This energy is TEMPORARILY stored in ATP and NADPH; O2 is given off during this reaction Chlorophylls and carotenoids are clustered in the thylakoid membrane; These clusters of pigments and proteins are known as photosystems: photosystem I and photosystem II Start here Wednesday Oct 19

Photosystem II The light reaction begins when a photosystem absorbs light and that energy is bounced around until it reaches a pair of chlorophyll a pigments The light reaction starts in photosystem II where light “excites” electrons and forces them to enter a higher energy level After the chlorophyll a molecules lose electrons (oxidation), the electrons MUST go to a primary electron acceptor (reduction)

Photosystem II (cont) The primary electron acceptor will donate the electrons to molecules called the electron transport chain (they transfer electrons from one molecule to the next) As electrons move down the ETC, they lose energy while moving hydrogen protons (H+) into the thylakoid The electron is then moved into photosystem I

Photosystem I Light is absorbed by chloroplast pigments in photosystem I This happens at the same time that light is absorbed in photosystem II Electrons move from a part of chlorophyll a molecules in PS1 to another primary electron acceptor The electrons lost by the chlorophyll a pair to the electron acceptor are replaced by the electrons that have passed through the photosystem II complex

Photosystem I (cont) The electrons are then donated by the primary electron acceptor to ANOTHER electron transport chain This ETC brings electrons to the side of the thylakoid membrane that faces the stroma The electrons then combine with a proton and NADP+ (an organic electron acceptor) This causes NADP+ to be reduced to NADPH

Replacing Electrons Electrons from photosystem II replace the electrons lost in photosystem I If this did not occur, the BOTH electron transport chains would stop and photosynthesis would not occur Electrons in photosystem II are replaced by the splitting of water 2H2O 4H+ + 4e- +O2 The protons are left inside of the thylakoid membrane, the oxygen diffuses out of the chloroplasts and out of the plant

Making ATP Synthesis of ATP is VERY important in the light reaction This occurs through a process called chemiosmosis: this is reliant on the concentration gradient of protons across the thylakoid membrane Some protons are created from splitting of water, but others are “pumped” from the stroma to inside of the thylakoid membrane This creates a concentration gradient and ATP synthase uses the diffusion of protons to make ATP from ADP and a phosphate group

Section 2: The Calvin Cycle During the Calvin cycle, plants use products of the light reaction to make sugars from CO2 The Calvin cycle produces a three-carbon sugar through carbon fixation: incorporation of CO2 into organic compounds A total of 3CO2 must enter the Calvin cycle to produce a three-carbon sugar

Alternative Pathways There are plant species that have evolved in hot, dry climates These plants MUST fix carbon, but they can lose water very rapidly through stomata (small pores in leaves) Stomata allow CO2 in and O2 out so if the stomata are closed, the level of CO2 will be too low and O2 will be too high How do they avoid this?

C4 Pathway This allows plants to fix CO2 into four carbon compounds Plants that use this are called C4 plants During the hottest part of the day, the stomata are partially closed, CO2 is fixed into the four carbon compound The CO2 is then released in other cells and it enters the Calvin cycle Ex: corn, sugar cane, and crab grass

The CAM Pathway Also adapted in a hot, dry climate These plants fix carbon through a CAM pathway (crassulacean acid metabolism) Plants that use the CAM pathway keep their stomata open at night and close them during the day At night, CAM plants take in CO2 and fix it into a variety of organic compounds During the day (when there is SUNLIGHT), CO2 is released from these organic compounds and put into the Calvin cycle

Summary of Photosynthesis Photosynthesis happens in two stages: The light reactions: energy is absorbed from sunlight and converted to chemical energy The Calvin cycle: CO2 and the chemical energy stored in ATP and NADPH are used to form organic compounds Photosynthesis is an ongoing cycle: some of the products of the light cycle are used in the Calvin cycle and some of the products of the Calvin cycle are used in the light cycle

What affects photosynthesis? Light Intensity: the rate of photosynthesis increases as the intensity of light increases (more electrons are excited) CO2 levels: increasing carbon dioxide levels will stimulate photosynthesis until the rate levels off Temperature: Increasing temperature will accelerate photosynthesis until a certain point; past this point the enzymes used in photosynthesis start to denature and will not work; the stomata also close in hot temperatures to prevent water loss