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BIOC40H3 (14)

Lecture 5

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University of Toronto Scarborough
Biological Sciences
Connie Soros

Plant Physiology – Lecture 5  Photosynthesis is probably the single most important process on earth o It is the only process that can harvest the energy of the sun  Photosynthesis literally means to „make something using light‟  It is the conversion of light energy to chemical energy stored in the bonds of carbohydrates and consists of two linked sets of reactions  The first set of reactions (the light reactions) is initiated by light, and the second set (the Calvin cycle) requires products of the light capturing reactions  The light capturing reactions produce oxygen and energy from water and light and the Calvin cycle produces sugars from carbon dioxide PROPERTIES OF LIGHT  Light is both wave-like and particle-like  Similar to waves of water or airwaves, electromagnetic radiation characterized by its wavelength – the distance between two successive wave the crests o The wavelength determines the type of electromagnetic radiation  The visible light spectrum that humans can see is from is about 400-710nm  Shorter wavelengths of electromagnetic radiation contain more than longer wavelengths (therefore blue light and ultraviolet light contain much more energy than red light infrared light)  Light is also particle-like nature, in that it occurs in discrete packets called photon  When a photon of light strikes an object, the photon may be absorbed, transmitted or reflected  Sunlight includes white light which consists of all wavelengths in the visible portion of those electromagnetic spectrum at once THE CHLOROPLAST  Photosynthesis takes place in the chloroplast  The chloroplast consists of an extensive system of internal membranes called thylakoids  All chlorophyll is contained within this membrane system and is the site of all photosynthesis  The second step of photosynthesis, the carbon reactions, takes place in the stroma, the region of the chloroplast outside the thylakoids  The stacked membranes of thylakoids are known as grana lamellae and exposed membranes that are not stacked are called stroma lamellae  The chloroplast surrounded by two separate membranes each consisting of a lipid bilayer called the envelope PIGMENTS  In order for light to be used by living organisms, it must first be absorbed  A molecule that absorbs light is called a pigment  Most pigments absorb only certain wavelengths and transmit or reflect the wavelengths they don‟t absorb  The photosynthetic pigment chlorophyll appears green to our eyes because it absorbs light mainly in the red and blue parts of the spectrum, so the light enriched in green wavelengths (about 550nm) is reflected into our eyes  The light absorption pattern of a pigment is known as the absorption spectrum for that substance  In action spectrum is the relative efficiency of different wavelengths of light in driving a light dependent process such as photosynthesis  The action spectrum of photosynthesis was first shown by T.W. Engelmann in 1882 o He performed an experiment in which he projected a tiny spectrum of colours onto a slide under the microscope using a prism o He then arranged a filament of the green algae Spirogyra parallel to the spread of the spectrum o He used an indicator species of oxygen seeking motile bacteria on the slide and noticed that they congregated mostly in the areas where the violet and red wavelengths projected onto the algae, (ie. where the chlorophyll pigments absorb) o This action spectrum gave the first indication of the effectiveness of light absorbed by pigments in driving photosynthesis  The main photosynthetic pigments are chlorophylls, carotenoids and phvcobilins  There are a number of different types of chlorophyll which differ from one another in their molecular structure and specific absorption properties  All photosynthetic eukaryotes contain chlorophyll a and it is essential for the oxygen generating photosynthesis carried out by photosynthetic organisms o Chlorophyll a contains a magnesium ion held in a nitrogen containing porphyrin ring o Attached to the ring is a long hydrocarbon chain, forming a hydrophobic tail that serves to an anchor the molecule to specific hydrophobic proteins of the thylakoid membranes within the chloroplast  Chlorophyll b is an accessory pigment (ie. Not directly involved in energy transduction but broadens the range of light lat can be used in photosynthesis) o It has a slightly different absorption spectrum than chlorophyll a  Chlorophyll c and d can take the place of chlorophyll b in some groups of algae  Two other classes of accessory pigments that are involved in the capture of light energy are the carotenoids and the phycobilins  Carotenoids are red, orange or yellow lipid soluble pigments, and like the chlorophylls are associated with hydrophobic proteins in the thylakoid membranes o They are linear polyenes  Two groups of carotenoids are present in chloroplasts, carotenes and xanthophylls o Beta-carotene found in plants is the main source of vitamin A for humans and other animals  In green leaves, the colour of carotenoids is usually masked by the more abundant green of 
 chlorophyll, but becomes visible when chlorophyll breaks down in autumn  Although carotenoids are involved in the capture of light energy for photosynthesis as antenna pigments, their primary function is of anti-oxidants, preventing oxidative damage to the chlorophyll molecules by light (ie. as photoprotective agents)  The third class of accessory pigments are the phycobilins (found in red algae and cyanobacteria (the blue-green algae)) and unlike the other pigments, they are water-soluble. o They are open-chain tetrapyroles found in antenna structures called phycobilisomes. PHOTOSYSTEMS  In the chloroplast, light energy is converted into chemical energy by two different specialized units of organization, which operate in series called photosystems  The absorbed light energy is used to power the transfer of electrons through a series of compounds that act as electron donors and electron acceptors +  The majority of these electrons oxidize H O2to O an2 eventually reduce NADP to NADPH  Light energy is also used to generate a proton motive force across the thylakoid membrane, which is used to synthesize ATP REVIEW REDOX REACTION  Reduction-oxidation reactions are a class of chemical reactions that involve the loss or gain of one or more electrons  They are very important in biology because they drive the formation of ATP  The atom that loses one or more electrons is oxidized, and the atom that gains electrons is reduced, one reactant is always an electron donor and is associated with an electron acceptor  Electrons are often transferred from one molecule to a different molecule, when this happens, the electron is usually accompanied by a proton (H ) +  Molecules that are reduced, (ie.gain a hydrogen (H) atom) tend to have a high potential energy because the electrons in C-H bonds are relatively far away from the positive charges in the nucleus  This is why molecules that have a large number of C-H bonds, like carbohydrates and fats store a lot of potential energy  Conversely, molecules that are oxidized often lose a proton along with an electron  Oxidized molecules tend to have many C-O bonds  Oxidized molecules tend to have lower potential energy because oxygen atoms have very high electronegativity and hold onto electrons very tightly  Reduction means “adding H‟s” and oxidation means “removing H‟s”. OIL RIG  In each of the photosystems, the chlorophyll and other pigments are embedded in the thylakoids of chloroplasts.  Photosystem I preferentially absorbs far-red light of wavelengths greater than 680nm (called P for “pigment” 700 nm the optimal absorption peak) while Photosystem II preferentially 700 absorbs
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