Photosynthesis

Using Light to Make Food

PhotosynthesisPhotosynthesis

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Light Light ReactionReaction

Dark ReactionDark Reaction

Light is AdsorbedLight is AdsorbedBy By

ChlorophyllChlorophyll

Which splitsWhich splitswaterwater

ChloroplastChloroplast

ATP andATP andNADPHNADPH22

ADPADPNADPNADP

Calvin CycleCalvin Cycle

EnergyEnergy

Used Energy and is Used Energy and is recycled.recycled.

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Plants and Photosynthesis Plants are ‘autotrophs’, ie. they can make their

own food. Plants are ‘producers of the Biosphere’, ie. They

produce food to sustain other living organisms as well.

Photo = light; Synthesis = put together. Plant cells have chloroplasts which can capture

light energy and convert it into chemical energy – stored in glucose.

The Flow Of Biological Energy

Photosynthesis cellular respiration

carbohydrates/

energy rich O2

chloroplast mitochondria

CO2 + H2O ATP for:

(energy poor)movement/cell activity

Chemical Equation for Photosynthesis

Chloroplast – the Photosynthetic Organelle in Plant Cells

Chloroplast

The chloroplast is the site of photosynthesis.

Chloroplasts are concentrated in the mesophyll tissue found in the leaf. Mesophyll tissue is present right under the epidermis of the leaf and consists of Palisade tissue + spongy tissue.

Gases: Where Does the O2 Come From?

CO2 is taken into the leaf through stomata for photosynthesis.

O2 is released out of the leaf through stomata as a product of photosynthesis.

In order to determine whether the O2 gas product came from the reactant CO2 or from H2O, scientists used tracer (radio-isotopic) Oxygen, first in CO2 and then in H2O as follows:

Fate of Atoms in Photosynthesis

It was proved from the above experiment that the O2 product came from splitting of the reactant H2O molecule.

Consequently, the fate of all the atoms in the chemical reaction of photosynthesis were determined:

Photosynthesis and Cellular Respiration Both Photosynthesis and Cellular Respiration

are Redox reactions, but they are reverse of each other.

Whereas Photosynthesis is an endergonic reaction, Cellular Respiration is an exergonic reaction.

Two Stages of Photosynthesis Photosynthesis occurs in two stages, viz. Light

reactions (during day-time only, in the thylakoid membrane) and Calvin Cycle (both day and night, in the stroma), which are linked together by NADPH and ATP.

Light Reaction Light reaction – is dependent on light and

occurs only during the day in nature. It takes place in the thylakoid membrane of

the chloroplast. Light reactions involve a) Splitting of water to produce oxygen, b) Energy production (ATP)

and c) Reduction of NADP+ to NADPH.

Calvin Cycle Calvin Cycle – is independent of light

and occurs during the day and the night. It takes place in the stroma of the

chloroplast. Calvin Cycle involves the production of

‘food’ as glucose, with the utilization of CO2, energy and NADPH.

Light Used in Photosynthesis During Photosynthesis, the chlorophyll pigments

can absorb only some wavelengths of the visible light from the electromagnetic spectrum.

Visible light consists of the following colors or wavelengths in order of increasing wavelengths / decreasing energy:

Violet, Indigo, Blue, Green, Yellow, Orange and Red.

The grana of the chloroplasts absorb mainly blue-violet and red-orange lights.

Green light is reflected and transmitted by green plants – hence, they appear green.

Light

Photon = a discrete packet of light energy. The shorter the wavelength, the greater the

energy….and vice-versa.

Pigments of the Chloroplast Chlorophyll a directly participates in

Light reactions. Chlorophyll b and carotenoids do not

directly participate in light reactions.

Basic steps of a Light Reaction Light reaction involves the following basic steps: 1) Absorption of light 2) Excitation of Chlorophyll a and emission of

electron 3) Formation of ATP and NADPH via the

electron transport chain

Components of Light Reaction Light / Solar Energy / Photon Reaction Center = Chlorophyll a and primary electron

acceptor Antenna = Reaction center and other pigment molecules

that gather light. Photosystem = Antenna + Reaction center.

Photosystem I There are two types of Photosystems, (viz.

Photosystem I and Photosystem II) occuring within the thylakoid membrane.

Its excited state emits electrons which are accepted by the primary electron acceptor and passed down an electron transport chain, eventually reducing NADP+ to NADPH.

Photosystem II In Photosystem II, chlorophyll a molecule is the reaction

center. This molecule absorbs orange-red light which has a higher energy level.

Its excited state (due to light absorption) emits electrons that are accepted by a primary electron acceptor and passed down an electron transport chain, eventually replenishing the lost electrons from Photosystem I.

H20 is split into H+ (protons), electrons and Oxygen gas. These electrons replenish the lost electrons from PhotosystemII.

Photosystems I and II

Calvin Cycle

The Calvin Cycle can take place during the day and the night in the stroma of the chloroplast.

It is powered by ATP and NADPH, synthesized during the light reactions – therefore, Calvin cycle depends upon the products of Light reaction.

Calvin Cycle - Continued

The Calvin Cycle utilizes CO2 in ‘carbon fixation’ which is finally converted into Glucose.

An excess of glucose is converted into starch – stored in roots, tubers and fruits.