SL Topic 2.8 Photosynthesis

The Chloroplast Chloroplast surrounded by two plasma membranes Thylakoids- disk shaped structures with membrane surrounding chlorophyll Grana (granum) – stacks of thylakoids Stroma – fluid that fills in the chloroplast space Lamellae – connect neighboring stacks of granum

Light energy is turned into chemical energy Producers (plants, algae and some bacteria) can contain chlorophyll, mostly in organelles called chloroplasts Chlorophyll can trap light energy (photons-packets of light) Chlorophyll converts light energy into ATP, chemical energy These energy molecules are then converted into carbohydrates

White light from the sun is composed of a range of wavelengths (colors) Red light has a longer wavelength with less energy Blue light has shorter wavelengths with more energy

Chlorophyll is the main photosynthetic pigment There are other pigments that can be seen by chromatography or more naturally by leaves turning colors in the fall Structure of the pigment molecule allows it to absorb light For chlorophyll, the tail region is embedded in the thylakoid membrane and protein on top traps the photons

Absorption of light Shows the rate of photosynthesis at different wavelengths of light Maximum rates of photosynthesis are at the blue ends and red ends of the visible light spectrum The lowest rates are in the green and yellow wavelengths Thus, red and blue light is absorbed by pigments to do photosynthesis, while green and yellow are reflected (that’s why plants look green) Draw this but don’t copy wavelengths

Photolysis (aka Light Reactions) Light is absorbed the chlorophyll molecules embedded in the thylakoid membranes of the chloroplast Light energy is converted to chemical energy that splits incoming water molecules into H+ and O2 These split molecules then give rise to the production of ATP, cell energy and NADPH

ATP-ADP cycle ADP (adenosine di-phosphate) takes in energy from food or sunlight and uses energy to form a high energy phosphate bond Now becomes ATP (adenosine tri-phosphate) When cellular work needs to be done, high energy bond is broken to release the energy Process repeats

Making Organic Molecules (Dark Reactions) H+, in the form of NADP-H, and ATP molecules made during photolysis in the thylakoids now move out into the chloroplast space, stroma CO2 is brought into plant through stomata on leaves and makes its way into the chloroplasts In the stroma, a cyclic metabolic pathway called the Calvin cycle takes place

Calvin cycle takes energy molecules made in photolysis and Carbon from CO2 and fixes them into organic molecules (carbohydrates, proteins and lipids) Chemical energy from photolysis is transferred to chemical bonds in these organic molecules which can be used in the future for energy

Measuring the Rate of Photosynthesis CO2 + H2O ---- C6H12O6 + O2 Rate can be directly measured by the use of CO2 or the production of O2 Rate can be measured indirectly by increase in Biomass (organic molecules will build up during plant growth)

Factors affecting rate of photosynthesis Light Intensity: (temperate) a) As the intensity of light is increased the rate of reaction of photosynthesis increases. (b) Light intensity has saturated the plants. That is the rate now remains constant for any further increase in light intensity. (c) Note that light intensity to achieve maximum rate of photosynthesis is less than the intensity of light in summer.

Carbon Dioxide Levels (a). As the concentration of carbon dioxide is increased the rate of photosynthesis increases. (b).The concentration of carbon dioxide has saturated the process. The maximum rate of reaction has been achieved. Further increases in carbon dioxide do not increase the rate. The rate is now constant. (c) Note this is the normal concentration of carbon dioxide in the atmosphere.

Temperature Photosynthesis has many enzyme molecules. The optimum temperature in a temperate climate is about 25° C. However. Temperature has many effects on a plant and the graph should be treated with caution. Temperature has just as many effects on respiration (changing food to energy), transpiration (intake of gases) and translocation (movement of electrons) all of which in turn affect photosynthesis.