Lecture 28 – Lecture 29 Photosynthesis Ozgur Unal NIS - BIOLOGY Lecture 28 – Lecture 29 Photosynthesis Ozgur Unal

6CO2 + 6H2O -------------> C6H12O6 + 6O2 Photosynthesis Photosynthesis: 6CO2 + 6H2O -------------> C6H12O6 + 6O2 How does photosynthesis benefit humans and other animals? How does it benefit plants?

Photosynthesis Photosynthesis occurs in two phases: Phase One: Light dependent reactions where light energy is converted into chemical energy in the form of ATP and NADPH Phase Two: Light independent reactions where the ATP and NADPH are used to make glucose. Glucose  Complex carbohydrates, Proteins, Lipids, Nucleic Acids

Phase One – Light Reactions Chloroplasts are special organelles to capture light energy. Mainly found in the cells of leaves. Disc-shaped organelles that contain two main compartments: Thylakoids: Flattened saclike membranes arranged in stacks (called grana). Stroma: The fluid filled space outside the grana.

Phase One – Light Reactions Pigments: Light absorbing colored molecules found in the thylakoid membranes of chloroplasts. Different pigments absorb specific wavelengths of light. The major light absorbing pigment in chloroplast is called chlorophyll. Several types of chlorophyll: Chlorophyll a and Chlorophyll b. Check out Figure 8.6. The structure of different chloropyhlls differ from one another. This is why they absorb different wavelengths of light. In general chlorophylls reflect green light. www.science.smith.edu/departments/Biology/Bio231/ltrxn.html

Phase One – Light Reactions Pigments

Phase One – Light Reactions Accessory pigments help plants trap additional light energy. Example: Carotenoids absorb light mainly in the blue and green regions of the spectrum, while reflecting most light in the yellow, orange and red regions. Why do the leaves of some trees change color in fall?

Phase One – Light Reactions Electron Transport: In the first phase, electrons released due to light are transported along some molecules. Photosystem I and photosystem II contain light absorbing pigments that play an important role in light reactions. Follow along Figure 8.8 as we go over the text. Chemiosmosis is the mechanism by which ATP is produced as a result of the flow of electrons down a concentration gradient. The channels through which H+ passes are enzymes that synthesize ATP. These enzymes are called ATP synthase.

Phase Two – The Calvin Cycle ATP and NADPH provide cells with large amounts of energy. But these molecules are not stable enough to store the chemical energy for long periods of time. In the second phase of photosynthesis, called the Calvin cycle, the chemical energy is stored in organic molecules. The reactions in the Calvin cycle do not require light. Go over Figure 8.9 as you read the steps in the Calvin cycle.

Alternative Pathways The environment in which an organism lives can impact the organism’s ability to carry out photosynthesis. Example: Lack of sufficient water in an environment forces plants to have alternative photosynthesis pathways to maximize energy conversion. C4 plants: Example: Sugarcane and corn These plants are called C4 because they fix CO2 in to four carbon compounds. These plants keep their stomata (plant cell pores) closed during hot days to minimize water loss.

Alternative Pathways CAM plants: Crassulacean acid metabolism (CAM) Example: Pineapple and other plants that need to conserve water (i.e. in deserts). These plants allow CO2 enter the leaves only at night when the atmosphere is cooler and more humid. At night, they fix CO2 into organic compounds. This pathway allows sufficient CO2 uptake while minimizing water loss.