Biology: Photosynthesis PDF

Biology Topic D: Photosynthesis By: Assoc. Prof. Rasha M. Abu El Khair Associate professor of Pharmacognosy and Vice Dean for Education affairs, College of Pharmacy, AASTMT An outline of photosynthesis  Photosynthesis involves taking in carbon dioxide and reducing it – that is, adding hydrogen...

Biology Topic D: Photosynthesis By: Assoc. Prof. Rasha M. Abu El Khair Associate professor of Pharmacognosy and Vice Dean for Education affairs, College of Pharmacy, AASTMT An outline of photosynthesis  Photosynthesis involves taking in carbon dioxide and reducing it – that is, adding hydrogen to it – to produce carbohydrate.  The hydrogen for this process comes from water.  The energy that drives the reactions comes from light, which is absorbed by a green pigment called chlorophyll.  The two products of the reaction are carbohydrate and oxygen.  Oxygen is a waste product of photosynthesis.  It may be lost from the cells, or it may be used in respiration  The reaction shown in the overall equation is actually made up of many smaller steps.  These can be divided into two main stages  the light-dependent stage  and the light-independent stage. Light-dependent stage  In the light-dependent stage, pigments including chlorophyll absorb energy from light.  Some of this energy is used to split water molecules into hydrogen and oxygen.  This is called photolysis (which means light splitting).  The energy in the hydrogen is used make ATP, in a process called photophosphorylation.  The hydrogen is eventually picked up by a coenzyme called NADP, to make reduced NADP. Light-independent stage  In the light-independent stage, the energy in the ATP, and the energy and hydrogens in the reduced NADP are used to reduce carbon dioxide and produce carbohydrates.  This stage is also known as the Calvin cycle. Structure and function of chloroplasts  Both the light-dependent stage and the light independent stage take place inside chloroplasts.  Chloroplasts are found in only some types of plant cell – mainly in palisade mesophyll and spongy mesophyll tissues in leaves.  Each cell may contain many chloroplasts. Stroma  Each chloroplast is surrounded by an envelope of two membranes.  Inside the chloroplast is a watery material that makes up the stroma.  There are many enzymes and other substances in the stroma which, as you will see, are required for the reactions of the light-independent stage of photosynthesis.  The stroma of a chloroplast contains small ribosomes and small circles of DNA, used to synthesise proteins.  The stroma also contains starch grains, which store some of the carbohydrate made, in an insoluble form Grana  Apart from the two membranes of the envelope, there are more membranes inside the chloroplast.  These membranes are called lamellae.  The membranes are arranged so that they produce fluid-filled sacs.  The membranes that form these sacs are called thylakoid membranes, and the spaces inside the sacs are thylakoid spaces.  In some parts of the chloroplast, the thylakoids are stacked up like a pile of coins or pancakes, and these stacks are called grana Grana  The membranes of the lamellae and thylakoids hold carrier molecules that work as an electron transport chain,  Embedded tightly in the lamellae and thylakoid membranes are several different photosynthetic pigments.  These are coloured substances that absorb energy  from certain wavelengths (colours) of light.  The most abundant pigment is chlorophyll, which comes in two forms – chlorophyll a and chloropyll b.  Other pigments include carotene and xanthophyll. Functions of chloroplast pigments  The sun emits energy that spans a broad spectrum of electromagnetic radiation.  The electromagnetic spectrum ranges from short-wavelength gamma rays, through ultraviolet, visible, and infrared light, to very long-wavelength radio waves  Light and all other electromagnetic waves are composed of individual packets of energy called photons.  The energy of a photon corresponds to its wavelength:  Short-wavelength photons, such as gamma and X-rays, are very energetic, whereas long-wavelength photons, such as microwaves and radio waves, carry lower energies.  When a specific wavelength of light strikes an object such as a leaf, one of three events occurs: The light may be reflected (bounced back), transmitted (passed through), or absorbed (captured).  Wavelengths of light that are reflected or transmitted can reach the eyes of an observer; these wavelengths are seen as the color of the object.  Light energy that is absorbed can drive biological processes such as photosynthesis.  Chloroplasts contain a variety of pigment molecules that absorb different wavelengths of light.  Chlorophyll a, the key light-capturing pigment molecule in chloroplasts, strongly absorbs violet, blue, and red light, but reflects green, thus giving green leaves their color.  Chloroplasts also contain other molecules, collectively called accessory pigments, which absorb additional wavelengths of light energy and transfer their energy to chlorophyll a.  Accessory pigments include chlorophyll b, a slightly different form of chlorophyll a that reflects yellow-green light and absorbs some of the blue and red-orange wavelengths of light that are missed by chlorophyll a.  Carotenoids are accessory pigments found in all chloroplasts. They absorb blue and green light and therefore appear mostly yellow or orange  Carotenoid accessory pigments include beta-carotene, which gives many vegetables and fruits (including carrots, squash, oranges, and cantaloupes) their orange colors.  Although carotenoids are present in leaves, their color is usually masked by the more abundant green chlorophyll.  In temperate regions, as leaves begin to die in autumn, chlorophyll breaks down before carotenoids do, revealing these bright yellow and orange pigments as fall colors  The majority of pigments in a chloroplast are chlorophyll a and chlorophyll b.  This is why chloroplasts and the cells that contain them, and the leaves in which these cells are found, look green.  Chlorophyll a absorbs slightly longer wavelengths of light than chlorophyll b.  The range of wavelengths absorbed by these three pigments is called an absorption spectrum  The pigments in a thylakoid membrane are arranged in clusters called photosystems.  Each photosystem is made up of large numbers of pigment molecules, plus some proteins.  The pigments absorb energy from light, and channel it to a reaction centre.  There are two types of photosystem, photosystem I and photosystem II.  The main wavelength of light absorbed by photosystem I is 700 nm, while photosystem II absorbs mainly light of wavelength 680 nm.  The reaction centres of both photosystems contain two molecules of chlorophyll a.  All the other pigments -chlorophyll b, carotene and xanthophyll – help to channel energy harvested from light to these chlorophyll a molecules.  This increases the energy level of the electrons in the chlorophyll a molecules.  It is these high-energy electrons that drive the steps that take place in the light-dependent stage of photosynthesis. Photosystems Practical: Measurement of transpiration rate using potometer