2023-09-12-BIOL254-W9-Notes .docx

LECTURE 13: PRINCIPLES OF PLANT PHYSIOLOGY Photosynthesis This is all about taking CO2 from thin air and converting the carbon atoms to organic form of carbohydrates. For the synthesis of carbohydrate to occur, you need an energy input and that comes from sunlight. Sunlight coming from the top of th...

LECTURE 13: PRINCIPLES OF PLANT PHYSIOLOGY Photosynthesis This is all about taking CO2 from thin air and converting the carbon atoms to organic form of carbohydrates. For the synthesis of carbohydrate to occur, you need an energy input and that comes from sunlight. Sunlight coming from the top of the plant. Photosynthesis involves a series of photochemical and biochemical reactions. The Photochemical Reactions The photochemical reactions involve harnessing the sunlight. Plant cells contain a range of pigments molecules which plant use to capture photons. In plant, these pigments are coloured organic substances and the main one involves in capturing the photons is called chlorophyll. Sunlight is a mixture of radiations ranging from gamma ray to radio waves. Some of these radiations have short wavelengths and some have long wavelengths. The type of radiation of interest to plants is photoactive radiations (400 to 700nm). These involves the blue light and red light, as they are more useful for photosynthesis. Chloroplast When the photoactive radiation hit the surface of the leaves , some of it is absorbed by the chloroplast and some will be reflected away. The radiation that gets trapped inside the chloroplast are the blue and red light because of the properties of the plant pigments. Plants are green because they harvest the red and blue lights. Within the mesophyll cells, you find the chloroplast, inside the chloroplast are some membrane systems called the thylakoids. The thylakoids are the membrane system that harvest the light. The first part of the thylakoids is stack of tubing called the granum. The stack of tubing is interconnected by other tubing, the interconnecting tubing is called the stroma lamella. The whole apparatus is suspended in some liquid called the stroma. Inside each tubing of granum is called the thylakoid space or lumen and we have some liquid in it as well. The pigment molecules/ chlorophyll are embedded in the membrane of the stroma lamella, granum and thylakoids. Light Absorption and Emission by Chlorophyll From the Practical in the Laboratory Each molecule has the most stable structure called the ground state. The photons from blue light hitting the plant cells are absorbed by the chlorophyll molecule and this disturbs the ground state of the molecule making it excited. The chlorophyll molecule in this unstable state will want to return to the ground state and this will make it lose some heat and emitting a light (photons) and lose the extra energy in the environment. The chlorophyl molecule absorbs red light, becomes excited to return in its ground state, it loses energy and loses pink glow to the environment and returns to its ground state. In real life, these heat loss and fluorescence is minimized because of the way the chlorophyll molecule is imbedded. Pigments molecules that can absorb red and blue light are not in isolation and can absorb light together. The pigment molecule on top will absorb the light and becomes excited and pass energy to the nearby pigment molecules. The first step in energy harnessing is the cooperation between the pigment molecules passing energy from one pigment to another using the antenna complex. Funnelling of excitation from the antenna system towards the reaction centre Light lands on the carotenoids, then from there to the chlorophyll b and later to chlorophyll a and then to P680 (a very important molecule) Z Scheme of Photosynthesis 2 specialized chlorophyll molecules called P680 and P700. P680 Can get extra energy from sunlight by direct absorption of photon from red light or by the internal system. The P680 got excited and lose electron to attain molecules and go back to the ground state. Due to the lose of electron, the P680 then gets some other electron from somewhere else such as water. the water molecule then split into proton hydrogen and electrons. The by-product of photosynthesis is oxygen, and this is because of splitting up of water molecule. The electron ejected by the P680 pass on from down the chain to the other part of the membrane. P700 This molecule can absorb the photon in far red light directly. It can also get energy from internal system. P700 get extra energy from light. P700 also gets excited, lose electron which will be donated to convert NADP to NADPH. The P700 is dependent on the function of P680 cos it needs the electron from P680 to be able to go back to ground stat again. The P700 part is called Photosystem I – this is where you see the formation of the NADPH. In Photosystem II – you the see the splitting up of the water molecules. This is where the P680 is. Where is the ATP Formed ? The ATP is formed during photosynthesis in the light reaction is seen at the surface in the stroma of the chloroplast. This possible because of the splitting up of water molecules. Mechanisms of Action of Important Herbicides These herbicides are effective because the target the electron transport chain in the weeds. Example of herbicides are DCMU and paraquat.