Photosynthesis: Light Dependent Reactions PDF

Photosynthesis Overview Sunlight provides the energy. Photosynthetic organisms serve as the ultimate source of food for life. Food chains start with photosynthesizers. It is an endergonic process! Light energy is...

Photosynthesis Overview Sunlight provides the energy. Photosynthetic organisms serve as the ultimate source of food for life. Food chains start with photosynthesizers. It is an endergonic process! Light energy is captured and transferred to the chemical potential energy in the bonds of a glucose molecule! It is an anabolic process! Where does photosynthesis take place? 1. Photosynthesis Background Information: Solar Radiation - Solar radiation is described in terms of its wavelength. -Photons are discrete packets of radiant energy that travel in waves. 1. Visible Light: The portion of the electromagnetic spectrum as shown here. This is also known as the PAR region as these are the wavelengths of light that are captured to drive photosynthesis. Only 42% of solar radiation that hits earth’s atmosphere reaches surface, most as visible light. Higher energy wavelengths, like gamma rays are screened out by ozone layer in upper atmosphere. Lower energy wavelengths, like radio waves are screened out by water vapour and carbon dioxide. (remember, the energy content of photons is inversely proportional to wavelength) Light may be: 1. transmitted (passed through matter) 2. reflected (bounced) or 3. Absorbed (changed from light energy to another form pigment a compound that absorbs certain wavelengths of visible light while reflecting others. Photons are absorbed by chlorophyll molecules embedded in the thylakoid membranes of the grana. The polar “heads” of the chlorophyll molecules are positioned on the outside of the membrane. Photosynthetic pigments include: the chlorophylls, and to a lesser extent, the carotenoids. PHOTONS of specific wavelengths are absorbed by specific plant pigments. Pigment = a molecule capable of absorbing light energy. chlorophyll a - "team captain“, primary photosynthetic pigment chlorophyll b - accessory (antenna) pigments carotenoids - accessory (antenna) pigments Chlorophyll structure  The dual nature of chlorophyll consists of a “porphyrin ring” and a “phytol tail”.  The tails are non-polar and the heads are polar which accounts for its positioning in the thylakoid membrane.  At the centre of the molecule is an atom of magnesium. This accounts for the molecule’s instability.  Chlorophyll a is a green pigment that absorbs red and blue- violet light. It appears green because it reflects green light. Chlorophyll b absorbs wavelengths in the blue and red-orange parts of the spectrum. It also appears green All photoautotrophs contain chlorophyll because it reflects green a. light. Depending on their habitat, they will contain a variety of accessory pigments. Absorbance Spectrum including accessory pigments. A variety of accessory pigments help absorb light that chlorophyll would absorb poorly or not at all. They pass this energy on to chlorophyll to support photosynthesis.  The carotenoids are less abundant than chlorophylls in green leaves. However, they become more visible in autumn. This is why leaves change color. Chlorophylls degrade as daylight hours and temperature get lower. Tannins are waste products of cell processes found in vacuoles and cell walls, giving brown colors. Anthocyanins are complex and numerous pigments generally produced in the fall from sugars and responsible for pink to purple colors. When chlorophyll absorbs a photon, one of its electrons is boosted to a higher energy state. This can result in: i) Photo-excitation or ii) Photo-oxidation  Photo-oxidation- the excited electrons Photo-excitation – the electrons in are removed from the chlorophyll chlorophyll are excited but return to molecule by a primary electron acceptor their ground state. (PEA) molecule located in the thylakoid Energy is released as heat and light membrane. The PEA is reduced. Absorption of light by the chlorophyll molecules trigger the process of PHOTOSYNTHESIS. There are 2 stages to photosynthesis: 1. The light dependent reactions( Light stage ) in photosynthesis: the reaction that traps solar energy and uses it to generate ATP and NADPH (form of energy that the cell can use!) Photosynthesis can only take place during daylight hours because light must be absorbed before anything else can happen. 2. The light independent reactions ( Dark stage/Calvin cycle) in photosynthesis: the reaction that assimilates carbon dioxide to produce an organic molecule that can be used to produce carbohydrates (glucose) 2.Light-Dependent Reactions of Photosynthesis Occur in thylakoid membranes and require 2 PHOTOSYSTEMS Photosystem: A collection of accessory pigments surrounding a reaction centre. Accessory pigments = antenna complex Chlorophyll a molecules = reaction centre The 2 photosystems work together to harvest light energy = a photosynthetic unit. Chlorophyll a is the only pigment in the entire photosystem capable of transferring an excited electron to a large protein embedded in the membrane. This protein is called the Primary Electron Acceptor (P.E.A.). CHLOROPHYLL a Photosystem I and II Photosystem I (PSI or P700)- chlorophyll a maximum absorbance at 700nm Photosystem II (PSII or P680)- chlorophyll a maximum absorbance at 680 nm Chlorophyll b and carotenoids are also busy absorbing photons but instead of passing their excited electrons to the PEA, they pass them to each other and on to chlorophyll a, which becomes photo-oxidized. In this way, they are acting as the ANTENNA COMPLEX. Light Dependent Summary 1. A photon strikes a pigment molecule (photosystems I and II) 2. Energy is instantaneously changed from light energy into chemical potential energy as electrons are lost from photo- oxidized chlorophyll a molecule (reaction centre). 3. Using a series of redox reactions and an ETC, the energy is "packaged" as chemical energy in the form of ATP and another "energy ( electron ) carrier" molecule called NADPH. This can be NADP⁺(ox)or NADPH (red) by the gain of 2e. Chemiosmosis in the chloroplast  The energy released as electrons pass down the gradient between photosystem II and plastocyanin (PC) is harnessed by the cytochrome b6/f complex to pump protons (H+) against their concentration gradient from the stroma of the chloroplast into the interior of the thylakoid.  As their concentration increases inside , a strong electrochemical gradient is set up. The only exit for these protons is through the ATP synthase complex. As in mitochondria, the energy released as these protons flow down their gradient is harnessed for the synthesis of ATP. The process is an example of facilitated diffusion. Light Stage Dark Stage Requirements Requirements  Sunlight  NADPH  Water  ATP  Chlorophyll  NADP⁺  CO₂  ADP + P Products Products  Oxygen ADP + Pi  NADPH NADP⁺  ATP PGAL ( glucose) Chloroplast vs. Mitochondrion  Chloroplasts carry out their energy conversions by chemiosmotic mechanisms in much the same way that mitochondria do. Although much larger, they are organized on the same principles.  They have a highly permeable outer membrane; a much less permeable inner membrane, in which membrane transport proteins are embedded; and a narrow inter membrane space in between.  The inner membrane surrounds a large space called the stroma, which is analogous to the mitochondrial matrix and contains many metabolic enzymes.  Like the mitochondrion, the chloroplast has its own genome and genetic system. The stroma therefore also contains a special set of ribosomes, RNAs, and the chloroplast DNA. Helpful Links  https://www.youtube.com/watch?v=KfvYQgT2M-k (do not focus on the Calvin Cycle, that is our next lesson)  https://www.youtube.com/watch?v=SnnmmKApT-c  https://www.youtube.com/watch?v=hj_WKgnL6MI Textbook Work Do: pg. Learning Check pg. 159 1, 2, 4, 6 Pg. 165 Q 2, 4, 5, 10, 15