Photosynthesis

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Water

Photosynthesis (An Overview)Plants, certain protists and photosynthetic bacteria have photosynthetic

activities. They make their own food. This makes them Autotrophs (self-feeders) or Producers (produce their own food).

Photosynthesis is the process by which light energy is converted to chemical bond energy and carbon is fixed into organic compounds.

Photosynthesis involves the uptake of carbon dioxide and water (both low energy compounds). Light energy is the energy source, which drives the process. Products are glucose (a sugar; stored chemical energy), oxygen gas and water. Photosynthesis is the primary energy-storing process on which almost all life, both plant and animal depends.

Complete Photosynthesis Equation

Developing a General Equation For Photosynthesis

Plants are made up of mostly carbon atoms, which are obtained from carbon dioxide.

Water, an input to photosynthesis is split and oxygen gas is given off, an output of photosynthesis.

Pictured is Elodea sp., an aquatic plant, which is photosynthesizing and giving off O2 bubbles.

(Plural Grana).

Chloroplasts: Sites of Photosynthesis

Chloroplast – the site of photosynthesis. Chloroplasts are found in the green parts of plants. They contain the pigment chlorophyll, which takes in the light energy.

Chloroplasts

Photosynthesis Occurs in 2 Main StagesTwo Main Photosynthesis Stages:

1) The Light ReactionThe Light Reaction2) The Calvin CycleThe Calvin Cycle (a.k.a. Enzymatic Reaction or Dark

Reaction)

The light reaction uses light energy to directly produce ATP, while the Calvin Cycle produces sugar. To power the production of sugar, the Calvin Cycle uses the ATP & NADPH formed during the light reaction.

Stage Location in Chloroplast

Energy to Run Stage

Other Chemical Inputs

Chemical Outputs

Light Light ReactionReaction

Light H2O, NADP+ H+ ADP P+

Thylakoids (In Grana)

ATP NADPH O2

The The Calvin Calvin CycleCycle

Stroma CO2C6H12O6 (Sugar)

NADP+ H+ ADP P+

ATP NADPH

Photosynthesis Occurs in 2 Main Stages

The Light Reaction (Photosynthesis Stage 1):Converting Light Energy to Chemical Energy

- Chlorophylls (Green in Color)Chlorophyll a absorbs light in the violet, blue and red ranges. It

reflects green, yellow and orange light. Chlorophyll a participates directly in the light reaction. It is a large molecule with a single magnesium (Mg) atom in the middle.Chlorophyll b absorbs mainly blue and orange light. It reflects yellow-green. Chlorophyll b passes energy to Chlorophyll a. Chlorophyll b broadens the range of light that a plant can use.

- Carotenoids (Yellow, Orange and Red in Color)Carotenoids absorb light in the blue, green and violet ranges. Carotenoids are present during the entire life of the leaf. They are easily seen in the fall when chlorophyll is broken down and leaves the leaf. Carotenoids help protect the plant against harmful UV radiation.

In order to capture necessary light energy, chloroplasts have Photosynthetic Pigments:

Absorption Spectra of Chlorophylls a & b at Different Wavelengths of Light.

This graph represents the visible light section of the Electromagnetic Energy Spectrum, which is a graph showing the intensity of light (in energy units) as a function of wavelength.

Violet Blue Green Yellow Orange Red

Engelmann’s experiment, demonstrating that red and blue light are effective in photosynthesis. The bacteria (rice-shaped) migrate to that portion of the algal filament (algae In a wide strip) where oxygen is produced.

The Light Reaction (The Photosystems)

The light reaction is broken up into 2 pigment systems called Photosystem I & Photosystem II. Photosystems are made up of light harvesting complexes in the thylakoid membranes of chloroplasts.Light harvesting complexes are made up of carotenoids and chlorophyll pigment molecules which capture light energy. These pigment molecules funnel energy to a chlorophyll a reaction center. The chlorophyll a molecule in the reaction center becomes excited and loses an electron to a primary electron acceptor.

As covered earlier, the light reaction involves light energy, thylakoids, certain inputs (H2O, NADP+, H+, ADP, P+) and certain outputs (ATP, NADPH, O2). The photosystems dive deep into specifics about the light reaction.

The green circles on the below figure symbolize the light-harvesting complex from the previous slide. Do you see how it fits on this slide? Two light-harvesting complexes are shown on this slide. The light harvesting complexes are within photosystems.

Photosystem I’s reaction center is P700 because it absorbs light maximally at 700 nm. Photosystem I has more chlorophyll a.

Photosystem II’s reaction center is P680 because it absorbs light maximally at 680 nm. Photosystem II has equal amounts of chlorophyll a and b, which makes it sensitive to a somewhat shorter wavelength than Photosystem I.

Primary Electron Acceptors

Note: Do you see how the previous slide’s figure fits into this one? We have zoomed out.

Can you find all of the important components of the light reaction on the below figure? Light energy, thylakoid, inputs (H2O, NADP+, H+, ADP, P+) and outputs (ATP, NADPH, O2), light harvesting

complexes and photosystems.

Now to explain the entire photosystem process. Starting at the left, light is absorbed at photosystem II and electrons are excited at the chlorophyll a reaction center and transferred to the primary electron acceptor. Water is split and oxygen is given off. The splitting of water provides electrons to replace those lost from chlorophyll a. The electrons travel from molecule to molecule (electron transport chain) in the thylakoid membrane (down like a slinky), which pumps H+ into the thylakoid. Electrons eventually reach photosystem I, where they are again excited and transferred to the primary electron acceptor. Electrons join NADP+ and H+ and NADPH is formed. A greater concentration of H+ is now in the thylakoid and H+ travels by diffusion into the stroma and ATP is formed.

(For every 2 photons of light, one water molecule is split, which gives off 2 electrons, which will eventually form one NADPH molecule. It takes two water molecules to make one oxygen gas (O2)molecule). So how many photons of light does it take to make one O2 molecule?

This

production

of ATP by

light energy

is called Photosphorylation.

This

protein is

called

ATP

Synthase. It

synthesizes

ATP.

Light Reaction

Calvin Cycle

This Figure illustrates the photosystems (light reaction) & how the photosystems relate to the Calvin Cycle, which is up next.

Can you see how the previous figures are within this figure?

BIO 141 Botany with Laboratory

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