3.2 PHOTOSYNTHESIS 2

Photosynthesis Can be broken down into three stages. LIGHT REACTIONS (in thylakoids) 1) Capturing light energy. 2) Using captured light energy to make ATP and reduce NADP + (nicotinamide adenine dinucleotide phosphate) to NADPH CARBON FIXATION (in stroma) 3) Uses the energy of ATP and the reducing power of NADPH to drive Calvin Cycle, which incorporates CO2 into carbon compounds such as glucose.

Properties of Light Of the total solar energy that reaches the earth, 60% is lost to the atmosphere. Of the 40% which reaches plants, only 5% is used in photosynthesis. Light is also known as electromagnetic (EM) radiation that travels at 300,000,000m/s

Properties of Light That light comes in packets known as photons (or quanta). Photons of light energy come in different wavelengths of light energy.  The longer the wavelength, the lower the energy.  The shorter the wavelength, the higher its energy.  Visible light is between 380 nm (violet) and 750nm (red).

Photosystems Are clusters of photosynthetic pigments embedded in the thylakoid membranes which absorb photons of particular wavelengths. transfers energy to ADP, P, and NADP +, forming ATP and NADPH. The electrons that reduce NADP + to NADPH are supplied by water molecules that enter the thylakoids through the stroma. ATP and NADPH are synthesized in the stroma, where carbon fixation reactions occur.

Scientists of Photosynthesis Helmont (willow tree + soil) Priestly (candle + plant) Ingenhousz (sunlight, CO2, O2) Niel (O2 due to splitting of H2O) Ruben & Kamen (confirmed Niel’s theory using isotopes and algae) Blackmann (light-dependant and light-independant stages of photosynthesis) Engelmann (wavelengths of light that best support photosynthesis)

J.B. Helmont Early 1600s Question: plants obtain all of their food from soil? Method:  Measured the initial mass of a willow tree  Measured the initial mass of the soil  planted the willow tree in soil.  Watered the plant (for five years)  Measured the final mass of the of the willow tree and soil separately. Results:  Tree mass increased by 74.4 kg  Soil’s mass decreased by only 60 g. Conclusion  Drastic increase in tree mass is due to the absorption of water.

Joseph Priestly 1771 Candle enclosed in bell jar  candle burns out  “candle injures the air” Candle + plant enclosed in bell jar  candle burns  “plant restores the air after being injured”

Jan Ingenhousz 1796 CRUCIAL to the understanding of photosynthesis.  First to realize that sunlight is essential to photosynthesis  First to realize that CO2 is the source of carbon in plants.  identified the gas released by plants: oxygen! Falsely assumed that the oxygen given off was due to CO2 being split apart:  “Sunshine splits apart the carbon dioxide that a plant has absorbed from the air; the plant throws out at that time the oxygen alone and keeps the carbon to itself as nourishment”

T.W. Engelmann 1882 What was known:  Photosynthesis depends on light energy  Photosynthetic organisms produce O2  Algae are photosynthetic  Aerobic bacteria require O2 Question: do all colours of the visible spectrum carry out photosynthesis equally?

Method:  Place a long filament of Spirogyra (algae) across a microscopic slide.  Add aerobic bacteria to the length of the algae.  Expose the length of the filament to different wavelengths of light (via a prism) Prediction:  The aerobic bacteria will thrive in areas where the algae photosynthesizes (produces oxygen). This will depend on the wavelengths of light that the algae is exposed to. Results:  Very few bacteria grew in the area illuminated with green light (approx. The middle of the visible spectrum) Conclusion:  Red and blue-violet light best support photosynthesis  oxygen production.

F.F. Blackman 1905 Determined that photosynthesis occurs in two stages  Light-dependant  Light-independant Also determined that the rate of photosynthesis depends on the concentration of carbon dioxide.  Rate of photosynthesis decreases with lower CO2 concentration.

... The 1930s Van Niel Oxygen in photosynthesis is produced by splitting water (not CO2) Ruben & Kamen Confirmed Niel’s findings.  Method  Grew Chlorella two different mediums: Heavy water ( 18 O isotope) + normal CO2 Normal water + heavy CO2 ( 18 O isotope)  Prediction  If water was the source of O2, Chlorella grown in heavy water would produce heavy O2 gas.  If carbon dioxide was the source of O2, Chlorella grown in heavy CO2 would produce heavy O2 gas.  Results:  Chlorella grown in heavy water produced heavy O2 gas.  Conclusion:  Water is the source of the oxygen given off in photosynthesis!  Detected 18 O in the oxygen molecules released by the photosynthetic organism.

Absorption Spectrum The wavelengths of light absorbed by a pigment. Chlorophylls a and b absorb blue-violet and red and transmit green nm. Chlorophyll a is the pigment that transfers energy from light to the carbon fixation reactions.

Action Spectrum Graph illustrating the effectiveness with which different wavelengths of light produce photosynthesis.

Accessory Pigments Carotenoids: Absorb in the blue-violet range ( nm). Contain two hydrocarbon rings connected by an alternating single and double-bond hydrocarbon chain. Reflect red and yellow. Dispersed in the thylakoid membranes. Some don’t participate in photosynthesis, but rather absorb energy that can damage chlorophyll release that energy as heat.

Accessory Pigments Carotenoids: (yellow-red) Xanthophylls – pigments in chloroplast (thylakoid) membranes that give rise to yellow colour in leaves Anthocyanins – pigments in vacuoles that give rise to the red colour in autumn leaves. So, why do plants appear to be green in the spring/summer? Why do leaves turn colour in the fall?

Photosynthetically Active Radiation (PAR) The wavelengths from 400 nm to 700 nm that support photosynthesis. Chlorophylls a and b combined with other pigments pretty much absorb the entire visible spectrum.

Summary

Seatwork/Homework Reread section 3.2 Answer PPs on page 154:  #1,2,3, 5, 7 (read text), 8.