Photosynthesis 2023 PDF

Photosynthesis An overview What is photosynthesis? The conversion of light energy, carbon dioxide and water into high energy compounds (e.g. glucose). General equation for photosynthesis is: visible light CO2(g) + H2O(l) → [...

Photosynthesis An overview What is photosynthesis? The conversion of light energy, carbon dioxide and water into high energy compounds (e.g. glucose). General equation for photosynthesis is: visible light CO2(g) + H2O(l) → [C6H12O6](aq) + O2(g) chlorophyll Photosynthesis can be thought of as the reverse of cellular respiration. Why ? Photo = light & Synthesis = production of carbs  What organisms use photosynthesis? These organisms are known as autotrophs because they can create their own food. Why study photosynthesis? Without autotrophs, heterotrophs (organisms that eat other organisms) would not have a source of food/energy Where does photosynthesis take place? Cross section of a leaf Mesophyll cell chloroplast In the chloroplast! Parts of a Leaf Cuticle: water resistant waxy layer on the surface of a leaf Epidermis Layer: transparent layer of cells below cuticle of a leaf Mesophyll Layers: cells that form the bulk of a plant leaf Vascular Bundle (Vein): transports water and minerals from roots to leaves and carries carbohydrates from leaves to other parts of plants Guard Cells: epidermal cells of a leaf that regulate the size of an opening known as stoma Stoma(sl.)/ Stomata: openings on the surface of a leaf that allow for exchange of gases between air spaces in the leaf interior and the atmosphere Plant Transport: https://www.youtube.com/watch?v=u_8fOz_R-l0 Vascular Bundle is made of xylem (transports water) and phloem (transports food “sugar”) How do guard Cells control Stomata opening/closing? Guard cells control the diameter of the stoma The stoma opens during the day and closes at night (usually) Sunlight activates a proton pump in the guard cells causing H+ to exit the cells. flaccid The electrical potential of the cells becomes negative K+ ions flow in to correct this difference The influx of K+ ions draws water into the guard cell making them turgid The swelling of the guard cells opens the stoma Turgid Chloroplast Photosynthesis takes place in the chloroplast. Each photosynthetic cell contains anywhere from 40-200 chloroplasts. A leaf may have 500,000 chloroplast /mm2! The Raw Materials Light passes through the upper epidermis to reach the chloroplast in the mesophyll cells; captured by pigments (i.e. chlorophyll in thylakoid membrane). Water is absorbed by plant roots, and is delivered to the leaf via the vascular bundle. Carbon Dioxide gets into the leaf through the stomata (see picture below) Absorption of Light Pg. 158 What is Light? Light is a form of electromagnetic radiation Only certain waves are visible to the naked eye. These are known as the visible spectrum (ROYGBIV)  What is Light? Light travels in photons (packets of energy) Longer wavelengths of light carry less energy.  When a photon is absorbed by an atom, it “excites” an electron. Pigments Pigments are molecules that absorb light Pigments are located on the thylakoid membrane Colour of pigments depends on the light they reflect. for example, ❑ chlorophyll a reflects green ❑ chlorophyll b reflect green ❑ Beta-carotene reflects yellow-orange Accessory pigments ❑ Xanthophyll reflects yellow Absorption Spectrum Why are leaves green? Why do they change colour in the fall? Other Accessory Pigments… Absorption Spectrum Action Spectrum Photoexcitation When photons strike pigments, an electron is excited. The excited electrons is passed along until it reaches a reaction center where its energy is used to start photosynthesis. If the energy is not captured, it is lost as heat and light (fluorescence). Photosystem A photosystem is made of a cluster of pigments (on thylakoid membrane) that capture light. made of an antenna complex + reaction centre Chloroplast have 2 photosystems: ❑ PI or P700 ❑ PII or P 680 Overview of Photosynthesis Photosynthesis - Overview Light Reactions - Overview Light Reactions: light is absorbed by chlorophyll which drives the transfer of electrons to (NADP+). NADP+ is reduced to form NADPH. ATP is also produced during the light reactions in a process known as photophosphorylation Take home point: Light reactions produce ATP and NADPH which are used in the Calvin Cycle to make sugar. Oxygen is also produced (as a result of the water splitting) as a by-product. Why does the cyclic pathway exist? 1. chloroplasts switches from linear to cyclic electron flow when too little NADP+ is available to accept electrons 2. may be common in photosynthetic cell types with especially high ATP needs (such as the sugar-synthesizing bundle-sheath cells of plants that carry out C4 photosynthesis) 3. may play a role in preventing excess light from damaging photosystem proteins and promoting repair of light-induced damage  Calvin Cycle - Overview Calvin Cycle: begins with carbon fixation. The Calvin Cycle uses NADPH and ATP from the light reactions to reduce carbon dioxide to carbohydrates (e.g. Glucose). The Calvin Cycle is sometimes referred to as the “light independent reactions”. Why?