Photosynthesis

Photosynthesis is the process that converts solar energy to chemical energy. Directly or indirectly, photosynthesis nourishes almost the entire living worlds

-Autotrophs sustain themselves without eating anything derived from other organisms

-Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules.

-Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2.

-Heterotrophs obtain their organic material from other organisms.

-Heterotrophs are the consumers of the biosphere.

-Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2

                                                 Photosynthesis

-Photosynthesis converts light energy to the chemical energy of food.

-Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria.

-The structural organization of these cells allows for the chemical reactions of photosynthesis.

-The chloroplasts have their own DNA and their own ribosomes.

               Chloroplasts: the site of photosynthesis in plants

-Leaves are the major locations of photosynthesis

-Their green color is from chlorophyll, the green pigment within chloroplasts

-Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast

-CO2 enters and O2 exits the leaf through microscopic pores called stomata 

-Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

 STOMATA

-A typical mesophyll cell has 30-40 chloroplasts.

-The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast),thylakoids may be stacked in columns called Grana.

-Chloroplasts also contain stroma a dense fluid

Tracking atoms through photosynthesis

– Photosynthesis can be summarized as the following equation:

              6CO2 + 12H2O + light energy  → C6H12O6 + 6O2 + 6H2O

– Chloroplasts, split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

CO2 + H2O→⌊ch2o⌋+O2

CO2 + H2S →⌊CH2O⌋+2S

The two stages of photosynthesis

Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part).

The light reactions (in the thylakoids)

• Split H2O
• Release O2
• Reduce NADP+ to NADPH
• Generate ATP from ADP by photo phosphorylation.

The Calvin cycle (in the stroma) forms sugar from CO2 using ATP and NADPH.

• The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules.
• Chloroplasts are solar–powered chemical factories.
• Their thylakoids transform light energy into the chemical energy of ATP and NADPH

The nature of sunlight

 – Light is a form of electromagnetic energy, also called electromagnetic radiation

– Like other electromagnetic energy light travels in rhythmic waves

– Wavelength is the distance between crests of waves

– Wavelength determines the type of electromagnetic energy

– the electromagnetic spectrum is the entire range of electromagnetic energy, or radiation.

– Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see.

– Light also behaves as though it consists of discrete particles called photons.

 

                                     Photosynthetic pigments

Pigments are substances that absorb visible light

Different pigments absorb different wavelengths

Wavelengths that are not absorbed are reflected or transmitted

Leaves appear green because chlorophyll reflects and transmits green light

 A spectrophotometer measures a pigment’s ability to absorb various wavelengths. This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

 

                                               spectrophotometer

An absorption spectrum is a graph plotting a pigment light absorption versus wavelength .The absorption spectrum of chlorophyll a suggests, that violet-blue and red light, work best, for photosynthesis . An action spectrum, profiles the relative effectiveness of different wavelengths of radiation in driving a process. The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W.Engelmann. In his experiment he exposed different segments of a filamentous algae to different wavelengths.Areas receiving wavelengths favorable to photosynthesis produced excess O2. He used the wavelength of aerobic bacteria clustered along the algae as a measure of O2 production.Chlorophyll a is the main photosynthesis pigment.Accessory pigments such as chlorophyll b broaden the spectrum used for photosynthesis. Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll b

                                                        Structure of pigments

Excitation of chlorophyll by light

When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable. When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence. If illuminated an isolated solution of chlorophyll will fluoresce, giving off light and heat.