Department of Botany The Open University of Sri
Department of Botany The Open University of Sri Lanka 1
Content v Learning Outcomes v Autotrophic organisms v Heterotrophs v Photosynthesis v Light absorption by the chloroplast v The stages of photosynthesis v The C 4 plants v Factors affecting photosynthesis v. The measurement of photosynthesis 2
Learning Outcomes After studying this session, you should be able to v explain the mode of obtaining their energy requirement for maintenance of their biological activities. v describe the structure of leaf, chloroplast and different pigments associated with photosynthesis v describe the portions of photosynthetically active areas in the electromagnetic spectrum v explain that light energy is converted to chemical energy, and carbon is fixed into organic compounds. 3
v describe how light is captured by the plants by means of pigments and the structural adaptation to do so. v distinguish between the two processes involved in the photosynthesis i. e. , Phase I and Phase II. v explain photophosphorylation, the process in which NADPH and ATP are generated. v briefly describe the Calvin cycle and its important reactions. Distinguish between the C 3 and C 4 pathways of photosynthesis. 4
How do we get energy to do our work? v From food of course. Can we synthesize our food within our bodies? v No we cannot. So we are referred to as “heterotrophs”. But there are organisms having the ability to synthesize their own food within their bodies. They are called “autotrophs”. 5
Categorization of organisms depending on their food habits Organisms Autotrophic Phototropic Use energy of sun light for the synthesis of food Eg. All green plants, Cyanobacteria And some bacteria Chemotropic Use energy of a chemical reaction for the synthesis of food Eg. Some bacteria like Ferrous bacteria Heterotrophic Parasitic Saprophytic Live within another organism and obtain nutrition from the host Obtain food from decaying matters 6
What is this process of synthesis of food within the bodies of all plants? v It is called “photosynthesis”. And it occurs within cells of plants. Sun light 6 CO 2 Carbon dioxide + C 6 H 12 O 6 6 H 2 O Water Chlorophyll Glucose + 6 O 2 Oxygen 7
Where does photosynthesis occur? v In higher green plants or eukaryotes this reaction takes place in the“chloroplast”. v But in prokaryotic organism photosynthesis occur in photosynthetic lamella dispersed in the cytoplasm. v As you know prokaryotes like bacteria and cyanobacteria lack membrane bound organelles. 8
What is the structural unit of photosynthesis? v It is the“thylakoid” v This structure has the form of a flattened sac or vesicle. 9
Do prokaryotes have thylakoids? v In eukaryotes the thylakoid may form a part of the internal membrane structure of the chloroplast. v To give you a clear picture of the thylakoid let’s look at the detailed structure of the, chloroplast. 10
Do prokaryotes have thylakoids? v Yes. They do, however their thylakoid membrane may form a part of the cell membrane or they may occur in the cytoplasm v In cyanobacteria the thylakoid may be a part of the elaborate internal structure. 11
Chloroplast Starch grain External membrane Stroma Inter Membranous space Internal membrane Grana Channel interconnecting thylakoid Thylakoid 12
Chloroplast v Chloroplasts like mitochondria are bound by double membranes which are separated by intermembrane space. v Look at the picture once again and trace the membrane. v Inner membrane is smooth in chloroplast unlike that of mitochondria. 13
Chloroplast v Thylakoids are formed by a third membranous system which is the interior of the chloroplast. v Surrounding the thylakoid is a dense solution which fills the interior of the chloroplast. v. This is called the stroma and it’s composition differs from that of the solution surrounding other organelles in the cytoplasm. 14
Chloroplast v You know that thylakoids are flattened sacs. They are enclosed by additional compartments called thylakoid spaces which also contain another solution with yet another composition. 15
Chloroplast v Under the high power of a light microscope this is how the chloroplasts appear. Chloroplasts v But under an electron microscope we can see that thylakoids are present in stacks. 16
Chloroplast v They are called “grana” v In some thylakoid membranes there are extensions that interconnect grana through stroma which separate them. v Now look at the diagram of the chloroplast again and try to distinguish all the parts. 17
Chloroplast v. The thylakoid of the chloroplast is oriented parallel to each other. Thus by swinging towards the light chloroplast simultaneously aims all of its million of pigment molecules for optimum reception. Photosynthetic pigments v You have observed that most of the leaves are green in color. 18
Why is that ? v Because leaves have pigments What are pigments? v. They are substances which absorb light 19
What are these pigments? v They are , 1. Chlorophyll a 2. Chlorophyll b 3. Carotene 4. Xanthophylls 20
What are these pigments? v Among these, chlorophyll is the major and the most important pigment for photosynthesis. v Out of chlorophyll, chlorophyll a is present in all photosynthesizing organisms and is essential for the process of photosynthesis. 21
Where are these pigments present? v They are arranged on the lamella of chloroplast in a very thin layer to facilitate maximum absorption of light. v They are more concentrated in the thylakoid disc of the grana. 22
How can you relate the presence of pigments and photosynthesis? v Presence of pigment is essential for the photosynthesis to take place. Because these pigments must absorb light. Mainly sunlight to provide energy for the reaction to proceed. 23
Transverse section of a plant leaf Cuticle Upper epidermis Palisade sheath cell Spongy mesophyll cell Lower epidermis Stoma Sub-stomatal air space Guard cell 24
Absorption Spectrum of Chlorophyll a 400 Chlorophyll b 500 600 700 800 Wavelength (nm) 25
Process of photosynthesis inside the Chloroplast CO 2 H 2 O NADP+ ADP RUBP 3 -Phosphoglycerate ATP NADPH G 3 P O 2 Starch Amino acid Fatty acid Sucrose 26
Photosynthesis in summarized form CO 2 Light Reaction ATP NADPH Dark Reaction Glucose O 2 H 22 O O 27
CO 2 1 C Ribulose 1, 55 C biphosphate 33 C phosphoglycerate 3 ATP 3 ADP Ribulose 55 C phosphate 2 Pi C 3 Cycle Glyceraldehyde 3 C 3 -phosphate 6 AT P 6 ADP 1, 33 C diphosphoglycerate 6 NADP H 6 NADP Glyceraldehyde 3 C 3 -phosphate Glyceraldehyde 3 -phosphate 3 C 6 Pi Sugar, Fatty acid, Amino acids 28
Electromagnetic spectrum Visible range Gamma rays 10 -14 X-rays 10 -12 Ultraviolet rays 10 -10 10 -8 Infrared rays 10 -6 Radar 10 -4 10 -2 FM TV Short wave AM 102 1 Wavelength (m) 400 500 600 700 Wavelength (nm) 29
Rate of photosynthesis CO 2 concentration and rate of photosynthesis Carbon dioxide concentration Rate of photosynthesis Photosynthetic rate in relation to light intensity Light intensity 30
Cyclic electron flow The phase of light reaction ATP - 0. 4 ADP + P X FRS Ferredoxin reductase Cytochrome 0 b b PS II Q NADP- Cyto. Chrome f Plastocyanin P 700 Sun Non cyclic electron flow ligh t ADP + P P 680 ligh t Z B ATP Sun +0. 8 PS I +0. 4 2 H 2 O O 2 + 4 H+ Plasto -Quinone 31
Structure of C 4 plant leaf Vascular membrane (Vein) Bundle sheath cell Air space beneath stoma Mesophyll cells 32
Measurement of Photosynthesis There are two methods by which the rate of photosynthesis can be measured. 1. Harvest Method 2. Gas Exchange method 33
Harvest Method v In this method the rate is determined by the increase of dry weight of the plant, assuming that when CO 2 is fixed to form sugars the dry weight is increased. v By measuring the dry weight of the plant at the beginning and at the end of a given period the rate can be determined. 34
Harvest Method v To get accurate results weights have to be obtained over a long period of time. v However this method has its draw backs because respiration, the oxidation of the produced sugars can take place simultaneously, leading to decrease in dry weight. 35
Gas Exchange method Air bubbles Hydrilla plant 36
Course Team Author Ms. Indrani Amarasinghe Web Content Developer Mr. Chameera Kendaragama OER Transformation 2014 © 2014, Open University of Sri Lanka (OUSL). OUSL OER is developed by the Centre for Educational Technology and Media. Except where otherwise noted, content on this site is licensed under a Creative Commons Attribution-Non Commercial- Share Alike 3. 0 License Produced by The Open University of Sri Lanka 2014 37
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