Photosynthesis Lecture Notes 2024-2025 PDF

PHOTOSYNTHESIS Hazwani Mat Saad / Nurul Nadiah Abd Razak PHOTOSYNTHESIS 1.1 Organ and organelle of photosynthesis. Absorption and action spectra, Photosystems. 1.2 The chemistry of photosynthesis : The light reaction. The light independent reaction. 1.3 C4 and CAM Plants. Factors limiti...

PHOTOSYNTHESIS Hazwani Mat Saad / Nurul Nadiah Abd Razak PHOTOSYNTHESIS 1.1 Organ and organelle of photosynthesis. Absorption and action spectra, Photosystems. 1.2 The chemistry of photosynthesis : The light reaction. The light independent reaction. 1.3 C4 and CAM Plants. Factors limiting the rate of photosynthesis. 1.4. Application of photosynthesis.  1.1 Organ and organelle of photosynthesis. LECTURE 1 The leaf as an organ of photosynthesis Structure of the chloroplast The chloroplast pigments - chlorophyll - accessory pigments Absorption & Action Spectrum Photosystems OBJECTIVES Describe the structure of leaves and function in photosynthesis. Describe structure and function of chloroplast. Describe photosynthetic pigments and function in photosynthesis. Describe photosystem.  All green parts of a plant have chloroplast, but leaves are the major site of photosynthesis in most plants. In eukaryotes, photosynthesis takes place in an organelle called chloroplasts. The leaf structure is highly adapted for photosynthesis to take place at optimum condition. 1.1 Organ & organelle of sunlight photosynthesis CO + H O --------->(CH O)n +. O 2 2 2 2 chlorophyll carbohydrate Carbon dioxide from the atm diffuses through the stomata into the sub-stomatal air chambers, then via the intercellular air spaces to the chloroplasts. The chloroplasts are located in the spongy mesophyll and palisade cells.  Water drawn up from the soil via the conducting tissues of the roots and stem, passes out of the xylem elements in the veins to the surrounding cells. Water contains mineral salts (including nitrates, sulphates and phosphates) required for the synthesis of proteins and other compounds. Oxygen and excess water vapour diffuse out of the leaf via the intercellular air spaces and stomata. The products of photosynthesis (sugar) are transported to other parts of the plant via the sieve tubes.  Leaves are generally thin and flat and collectively present a large surface area to the light. The leaf is covered on both sides by a layer of epidermal cells. On the outer surface is the cuticle. The inside of the leaf is filled with cells containing chloroplasts. Those immediately beneath the 1.1.1 Structure upper epidermis are called the palisade cells. of the leaf - elongated - long axes perpendicular to the surface. Filling the leaf between the palisade layer and the lower epidermis is the spongy mesophyll. The lower epidermis is pierced by numerous pores called stomata (singular stoma). The palisade cells They are elongated with their long axes perpendicular to the surface. They are separated from each other by narrow air spaces. Densely packed with chloroplasts. The chloroplasts tend to arrange themselves in the upper part of the cell which receives maximum illumination. The palisade cells collectively form the palisade mesophyll which may be one or several cells in thickness.  Spongy mesophyll chloroplast Its cells are irregular in shape and arrangement. Also contain chloroplasts (but fewer than the palisade cells). Between the spongy mesophyll cells are large air spaces which communicate with each other and with the much narrower air spaces between the palisade cells. This system of air spaces allows gases to diffuse freely between the cells within the leaf. Stomata (singular stoma) The upper epidermis contain fewer stomata than the lower epidermis. Each stoma opens into a sub-stomatal air chamber which connects with the intercellular air spaces. Bordered by guard cells which can open or close the pore, the stomata regulate the passage of carbon dioxide and water vapour across the surface of the leaf. On the outer surface is the cuticle The cuticle is generally thicker on the upper surface of the leaf than on the lower surface. Adaptation of leaf as a photosynthetic organ : 1. The thinness minimizes the distance of carbon dioxide to diffuse into the cells. 2. Even though leaves are thin and flat, which makes them liable to sag but the shape is maintained by the turgor of the living cells inside them. 3. The midrib and veins provide tissue strengthening, suitable position of leaf to capture maximum light. 4. Large surface area allowing maximum gaseous diffusion. Even though it also leads to an increase in evaporative water lost but this was reduced by having impermeable cuticle on the leaf surface. Cross-section of leaf  1.1.2 The structure of the chloroplast A chloroplast of a higher plant is biconvex in shape. Two outer Chloroplasts are surrounded by two membranes, layers of membrane which form the chloroplast envelope. The inner membrane encloses a fluid-filled region called the stroma. Inner membrane Stroma contain most of the enzymes required to system – tylakoids, produce carbohydrate molecules. all interconnecting by channels Suspended in the stroma is the third system of membranes that forms an interconnected set of flat, disc-liked sacs called thylakoids.  The function of the thylakoid membranes is to hold the chlorophyll molecules in a suitable position for trapping the maximum amount of lamellae light energy. At some region, tylakoid sacs are arranged in stacks, called grana (sing. granum) at intervals, with lamellae (layers) between the grana. Each thylakoid consists of a pair of membranes encloses a fluid-filled interior space called thylakoids lumen. CHLOROPLAST  Numbers of chloroplast varying from one species to another. They are about 3 to 10 micrometer (average 5 Example: micrometer) in diameter, and so are visible with a light microscope. 1 in the unicellular alga Chlorella The thylakoid membrane system is the site of the 100 in palisade light-dependent reactions in photosynthesis. mesophyll cells. The membranes are covered with chlorophyll and other photosynthetic pigments, enzymes and electron carriers.  The thylakoids are surrounded by the stroma (a protein-rich matrix) The stroma is the site of the light- independent reactions of photosynthesis. The structure is gel-like, containing soluble enzymes, particularly those of the Calvin cycle, and other chemicals such as sugars and organic acids. The enzymes responsible for the reduction of carbon dioxide, together with starch grains and numerous ribosomes.  Chlorophylls – green plants 1.1.3 Photosynthetic pigments Carotenoids – green plants The photosynthetic pigments of higher plants fall into several classes, the chlorophylls, Anthocyanins – red algae, purple carotenoids, anthocyanins and phycobilins. mushroom caps, blueberries The role of these pigments is to absorb light energy, thereby converting it to chemical Phycobilins – red algae, energy. cyanobacteria They are located on the thylakoids. The chloroplasts are usually arranged within the cells so that the membranes are at right-angles to the light source for maximum absorption. Chloroplast is in fact a mixture of various pigments These pigments can be extracted from leaves with propanone and separated by chromatography. At least five pigments can be identified: chlorophyll a (blue-green), chlorophyll b (yellow-green), xanthophyll (yellow) and carotene (yellow). The fifth pigment, phaeophytin (grey), is a breakdown product of chlorophyll.  Chlorophyll 1. Chlorophyll is the main photosynthetic pigment. Functions to absorb light and use it in the manufacture of carbohydrate. Several types - Chlorophyll a and b. Chlorophyll a is the most important and abundant photosynthetic pigment which initiates the light dependent reaction. 2. Chlorophylls absorb mainly red and blue region of the Figure 2 : Why leaves are green: visible spectrum. interaction of light with chloroplasts. These pigments are reflecting green light and therefore The pigment molecules of chloroplasts absorb blue and red light and reflect or giving plants their characteristic green colour, unless transmit green light. masked by other pigments. 3. The chlorophyll molecule has a flat, light-absorbing head end which contains a magnesium atom at its centre. This explains the need for magnesium by plants and the fact that magnesium deficiency reduces chlorophyll production and causes yellowing. The chlorophyll molecule also has a long hydrocarbon tail which is hydrophobic (water-hating). Chlorophyll a differ from chlorophyll b only in the funtional group on the porphyrin ring. - methyl group (-CH3) : chlorophyll a - carbonyl group (-CHO) : chlorophyll b  This difference shifts the wavelengths of light absorbed and reflected by both pigments giving slightly different in colour for chlorophyll a and b. Chlorophyll a – appear bright/blue green Chlorophyll b – appear yellow-green Figure 3 : Location and structure of chlorophyll molecules in plants.  Carotenoids 1. Carotenoids are yellow and orange pigments that absorb strongly in the blue-violet range. They are called accessory pigments because they transfer energy, they absorbed from light on to chlorophyll. 2. Carotenoids consist of carbon rings linked to chains in which single and double bonds alternate. 3. Carotenoids have three absorption peaks in the blue-violet range of the spectrum. They may also protect chlorophylls from excess light and from oxidation by oxygen produced in photosynthesis. 4. Carotenoids consist of two types : carotenes and xanthophylls. The most widespread and important carotene is beta- carotene, which is familiar as the orange pigment in carrots. Two carbon rings are connected by a chain of 18 carbon atoms connected alternately by single and double bonds. Vertebrates are able to break the molecule into two during digestion forming two molecules of vitamin A. When vitamin A is oxidized in turn, retinal, the pigment used in human vision, is produced. That explains the connection between carrots (rich in beta-carotene), vitamin A and vision. Chromatography A technique used to separate individual substances from a mixture. Many types : paper chromatography, TLC, HPLC, GCMS Paper chromatography/TLC A solution is made, loaded onto a strip of absorbent chromatography paper (stationary phase) by applying it at a point just above the bottom. The paper is then suspended so that its end dips into a suitable solvent (mobile phase). The solvent moves up the paper by capillary action taking the substances which were loaded on as it goes. These substances will then separate out at various levels. The places where they end up depends on their solubility in the moving solvent and the ease by which they are absorbed by the paper. It is then possible to identify the individual components, either by comparing their position on the chromatogram with standard solutions, or by calculating their Rf (retardation factor) values. Rf value = Distance from baseline travelled by solute --------------------------------------------------------------------------- Distance from baseline travelled by solvent (solvent front) HPLC (High-performance liquid chromatography) : technique in analytic chemistry used to separate the components in a mixture, to identify each component, and to quantify each component. Retardation factor (Rf) In chromatography, the retardation factor (R) is the fraction of an analyte in the mobile phase of a chromatographic system. In planar chromatography in particular, the retardation factor Rf is defined as the ratio of the distance traveled by the center of a spot to the distance traveled by the solvent front.  LIGHT - Visible light represents a very small 1.1.4 Absorption & portion of a vast, continuous of range action spectra radiation called the electromagnetic spectrum. - All radiation in this spectrum travels as waves. - A wavelength is the distance from one peak to the next. - Visible spectrum: 380-760nm. - Light is composed of small particles (or packets) of energy called photon. Figure 4: Electromagnetic spectrum  Radiation within the visible light portion of the spectrum excites certain type of biological molecules, moving the electron into higher level. Why does Radiation with wavelength longer photosynthesis than visible light doesn’t have enough depend on energy to excite these molecules. visible light? Radiation with wavelengths shorter than visible light is so energetic, disrupts the the bonds of many biological molecules.  1. An action spectrum is a graph showing the effectiveness of different wavelengths of light in stimulating the process being investigated. Use to identify the pigments involved in photosynthesis. 2. An absorption spectrum is a graph of the relative amounts of light absorbed at different wavelengths by a pigment. Differences between absorption & action Shown that chlorophyll a and b absorb light from both the spectra red and blue / violet parts of the spectrum, whereas xanthophyll and carotene absorb light only from the blue / violet part. Absorption spectrum of chlorophyll a and b Action spectrum of photosynthesis  1. The chlorophyll and accessory pigment molecules are located in two types of photosystem, known as photosystems I and II (PSI and PSII). These photosystems are visible as particles in the thylakoid membranes. 2. Each contains an antenna complex, or light-harvesting complex, of pigment molecules. The light-harvesting complex contains 200-300 pigment molecules and collects light energy. Different pigments collect light of different 1.4.5 Photosystems wavelengths, making the process more efficient. 3. All the energy is transferred from molecule to molecule, and finally to a specialised form of chlorophyll a known as P700 in PSI and P680 in PSII. Pwavelengths stands for pigment; their absorption peaks are at of 700 nm and 680 nm respectively (both red light). The chlorophylls P700 and P680 become 'excited' by the energy they absorb and release high energy electrons.  Figure 5: How a photosystem harvests light  Figure 5: How a photosystem harvests light