PHOTOSYNTHESIS AND CELLULAR RESPIRATION

PHOTOSYNTHESIS

• Method of converting sun energy into chemical energy usable by cells

• Autotrophs: self feeders, organisms capable of making their own food– Photoautotrophs: use sun energy e.g. plants

photosynthesis-makes organic compounds (glucose) from light

– Chemoautotrophs: use chemical energy e.g. bacteria that use sulfide or methane chemosynthesis-makes organic compounds from chemical energy contained in sulfide or methane

Photosynthesis is a anabolic process and respiration is a catabolic process.

• Anabolism: It includes the building up process. it is used and stored as potential energy. The overall reaction are:

6CO2 + 12 H2O + light energy → C6H12O6 + 6O2+ 6H2O• Catabolism: It includes breakdown process. Energy released

as kinetic energy.Overall Reaction:

C6H12O6 + 6O2 → 6CO2 + 6H2O

Photosynthetic apparatus

• Photosynthesis takes place only in green parts of the plant in the subcellular organelles called chloroplast.

• The organelles contain green pigments, the chlorophyll absorbs light for photosynthetic reaction.

• The higher plant chloroplast is generally lens or disc shaped and ranges in size from 3 to 10µm in diameter.

Cont….• The chloroplast have two layered membrane.• The inner portion of the chloroplast is divided into two parts:1) Stroma : Colourless. It contain rich in protein, ribosome, DNA and starch

grains 2) Grana : Green coloured granular parts. It is made up of flattened sacs called

thylakoid. The thylakoid are packed together in such a way that look like a pile of 25 paisa coin. There may be 40 to 60 grana per chloroplast and each granum may be made up of 2 to 100 or more thylakoid. Grana are linked together by intergranal lamella or stromal thylakoid called frets. The area between the two thylakoid (in grana) is termed as partition.

• Lipid and proteins are also present in the thylakoid membrane.

Chloroplast Pigments

• Chlorophyll pigments are responsible for the efficient capture of solar energy.

• They absorb visible and some infra-red radiation , chlorophyll a is the major light harvesting pigment of green plants.

• Chloroplasts contain several pigments:-– Chlorophyll a:all oxygen evolving organism. It is

bluish-green in pure state – Chlorophyll b: higher plants and green algae . It is

olive green in pure state.– Carotenoids: red colour. Carotenoid have

conjugate double bond – Xanthophyll: yellow colour

Function of these pigments:

a) Carotenoid function as accessory pigment. They absorb radiant energy in the mid region of visible spectrum.

b) They protect the chloroplast constituent from nascent oxygen released during photolysis of water.

c) By their colour, the carotenoid make the flower and fruit conspicuous to animals for pollination and dispersal.

d) Β-carotene produce vitamin A in animals.

There are two types of photosynthetic reaction

1) Light reaction2) Dark reaction

Light-dependent Reactions

• Overview: light energy is absorbed by chlorophyll molecules-this light energy excites electrons and boosts them to higher energy levels. They are trapped by electron acceptor molecules that are poised at the start of a neighboring transport system. The electrons “fall” to a lower energy state, releasing energy that is harnessed to make ATP

The two groups of pigments have been called photosystem:

1) PSI: It is maximally absorbs light of wavelength longer than 680nmdesignated as P700. it is located on both nonappressed part of grana thylakoid and stroma thalakoid. It has more of chlorophyll a. it takes part in both cyclic and nocyclic photophosphorylation.

2) PSII: This pigment absorbs light at 680nm and lower wavelength and the system as designated as P680. it is located in the appressed part of grana thylakoid. It has chlorophyll a, b and carotenoids. PSII contain Mn2+ , Cl, quencher molecule Q, plastoquinone(PQ), cytochrome complex and plastocyanin.

Z –Scheme reaction• PSI and PSII photosynthesis act in a series to produce NADPH and ATP,

which are the products of the light reaction of photosynthesis.• In this process, the electron flow from water to reduced NADP to

NADPH.• The energy needed to move electrons is provided by the light quanta

absorbed.• Some energy electron transport is utilized in the regeneration of ATP

from ADP.• The pathway of electron transport from water to NADP involving two

photosystem is often called ‘Z Scheme’ as graphic representation of the transport pathway according to the redox potential of the various molecules involved in the process, gives an appearance of alphabet ‘Z’.

Emerson enhancement effect or, Red-drop

• The most effective light in photosynthesis is red(650-680) and the photosynthetic efficiency decreases in far-red.

• In 1950’s, Robert Emerson and his associates found that if light of shorter wavelength was superimposed with the light of far-red wavelength, the rate of photosynthesis was greater than could be expected from adding the rates found when either light was given alone.

• He found that rate of photosynthesis can be enhanced if monochromatic beam of two wavelengths(long and short) are applied simultaneously.

• The enhancement of photosynthesis in far-red light by a light of shorter wavelength was called Emerson enhancement effect.

Light independent reaction or, Dark reaction

1) Calvin Benson Cycle or, C3- cycle:• From 1946 to 1953, M. Calvin and his co-workers, used 14C as

radioactive label to follow the path of photosynthetic C- fixation.• When photosynthesizing algae was exposed to radioactive CO2 for only

5 seconds or less, it was found that the 1st compound to become radioactive was phosphoglyceric acid(Calvin and Benson, 1948)

• This product was isolated and chemically degraded and it was found that most of the 14C was in carboxyl position of the molecule;

(P)-O-CH2-CHOH-COOH • Since the 1st product in the series of reaction is a three carbon

compound(phosphoric acid) the pathway is called C3 pathway or c3 cycle.

2) Crassulacean AcidMetabolism(CAM)

• The term ‘Crassulacean acid metabolism’ (CAM) derives from the large succulent plant family crassulaceae, since all species of this family possese CAM and also 25 to 30 flowering plant families that have showing CAM.

• During night when stomata are open, CO2 is fixed through the action of PEP carboxylase to malic acid i.e. malic acid is formed in the night by the carboxylation of phosphoenolpyruvic acid in the presence of enzyme PEP carboxylase.

• This phase is completed in two steps.• In the 1st, phosphoenolpyruvic acid fixes CO2 and is converted into malic

acid by the enzyme malic dehydrogenase.• The leaves of CAM plants also contain enzymes of calvin cycle.• The CO2 fixed in the malic acid during dark period is ultimately converted

to hexose or other carbohydrate.

3) Hatch and Slack or, C4 pathway• Kortschak and his coworkers(1954) reported the formation of

dicarboxylic acid(C4) as primary products of photosynthesis in sugarcane which was confirmed in several other plants by M.D. Hatch and C.R. Slack(1966) of Australia.

• They then proposed an alternative pathway, C4 cycle.• C4 plants including also families of graminae, angiosperm like

zygophyllaceae, Asceraceae etc.• These plants have a peculiar type of leaf anatomy called “Kranz

anatomy”.• In “Kranz anatomy” of C4 species, the vascular bundles of leaf are

bounded by large chloroplast containing bundle sheath cells and also have mesophyll cell.

CELLULAR RESPIRATION

Cellular Respiration Overview

• It is a catabolic process. Transformation of chemical energy in food into ATP.

These reactions proceed the same way in plants and animals. Process is called cellular respiration

• Overall Reaction:– C6H12O6 + 6O2 → 6CO2 + 6H2O

• Breakdown of glucose begins in the cytoplasm: the liquid matrix inside the cell

• At this point life diverges into two forms and two pathways– Anaerobic cellular respiration (aka fermentation)– Aerobic cellular respiration

GLYCOLYSIS

• The step-wise degradation from glucose to pyruvic acid is termed as glycolysis.

• After the name of its tracers, the glycolytic pathway is also known as Embden Meyerhof Paranas pathway(EMP pathway).

• In this state a molecule of glucose which is a 6-carbon compound is broken down into two molecules of pyruvic acid which is a 3-carbon compound through a large number of step-wise closely integrated reaction.

It occurs in following 3 important phases:• In the 1st phase of glycolysis, the glucose molecule is phosphorylated

with the introduction of two phosphate group into its structure. For its phase 2 molecules of ATP are needed.

• The 2nd phase involves the breaking up of 6- carbon compound fructose 1,6-diphosphate into 2 molecules of 3-carbon compounds, i.e. 3-phosphoglyceraldehyde and dihydroxyacetone phosphate. These two 3-carbon compounds are interconvertible.

• The 3rd phase involves degradation of 3-PGAld into pyruvic acid with the production of 4 molecules of ATP. As in the phosphorylation of glucose during the 1st phase 2 molecules of ATP have already been used up, there is a net gain of only 2 molecules of ATP during glycolytic reaction