3.2 Light Energy and Photosynthetic Pigments Pages 147- 154

Photosynthesis….. In plants and algae, all the reactions of photosynthesis take place within the chloroplasts.

The 3 Stages of Photosynthesis Stage 1: Photoexcitation: Capturing of light energy Stage 2: ETC & Chemiosmosis: Using the light energy to make ATP and reduced NADP+ Stage 3: Calvin Cycle: Synthesis of organic compounds from CO2

The first two stages involve a series of reactions that are directly energized by light, so we call them LIGHT DEPENDENT REACTIONS They occur on the thylakoid membranes of chloroplasts. The third stage occurs in the stroma and is LIGHT INDEPENDENT (or Dark Reactions)

Light (Electromagnetic Radiation) Light behaves as if it were composed of “units” or “packets” of energy that travels in waves. These packets are PHOTONS. Photons are characterized by a wavelength that is inversely proportional to their energy. Therefore, photons with short wavelengths have high energy and those with long wavelengths have low energy.

The wavelength of light determines its colour The wavelength of light determines its colour. For example, the wavelength of red light is about 700nm and the wavelength of blue light is about 470nm. 700nm RED BLUE 470nm

The Electromagnetic Spectrum Figure 3, p 148

red-orange-yellow-green-blue-indigo-violet Most of the photons in the electromagnetic spectrum are invisible to humans. Visible light ranges from 750 nm (red light) to 380 nm (violet light). The colours visible are: red-orange-yellow-green-blue-indigo-violet (ROY G BIV)

Red light has a wavelength of 750 nm and thus has the longest wavelength (of visible light) with the lowest energy. Violet light has wavelengths of 380 nm and thus has the shortest wavelength (of visible light) with the highest energy.

Wavelengths in Photosynthesis The best wavelengths for promoting photosynthesis are RED and BLUE. This was discovered by T.W. Engelmann in 1881. He put green algae (Spirogyra) under a microscope in a liquid medium and illuminated it with visible light. He then put motile bacteria in the medium.

After a few minutes the bacteria tended to congregated around the sections illuminated with blue and red light. He concluded that these sections must be putting out the greatest concentration of oxygen. https://www.youtube.com/watch?v=Rt37Hyn4Qv4

Importance of Pigments Substances can do one of two things when they are struck by a particular colour of light: absorb that wavelength – allows energy to be absorbed and possibly used reflect that wavelength – energy is not absorbed and that colour will be seen

The Importance of Pigments Pigments are molecules that absorb light. Ex. Chlorophyll When a photon of light strikes a photosynthetic pigment, an electron in an atom contained within the molecule becomes excited.

Electrons of the excited molecule are energized and move further from the nucleus of the atom. The excited (energized) molecule can pass the energy to another molecule or release it in the form of light or heat.

Chlorophyll a is the main photosynthetic pigment in all organisms except bacteria. It absorbs blue light and red light. It REFLECTS green light! It is the only pigment that can transfer the energy of light to the carbon fixation reactions of photosynthesis.

Ex. Chlorophyll b REFLECTS yellow-green light. Other pigments, called accessory pigments absorb slightly different wavelengths of light: Ex. Chlorophyll b REFLECTS yellow-green light. It absorbs blue-green and red light Carotenoids REFLECT yellow-orange light It absorbs blue-green light.

The combination of all of the pigments increases the range of colours (of light) that plants can use in photosynthesis.

Chlorophyll Chlorophyll a and b have 2 basic parts. A long hydrocarbon tail (embedded in the lipid bilayer of the thylakoid membrane) A hydrophilic porphyrin ring that has alternating single and double bonds

When light is absorbed by chlorophyll, it excites electrons from magnesium within the porphyrin ring to begin the process of photosynthesis.

Carotenoids Carotenoids do not participate directly in photosynthetic reactions but are able to pass their energy to chlorophyll a. Carotenoids also have two basic parts: Carbon rings Hydrocarbon chains containing alternating single and double bonds

Carbon Ring Hydrocarbon Chain When light is absorbed by carotenoids, it excites electrons in the hydrocarbon chain

Why Do Leaves Change Colour in the Fall? Chlorophyll is the main photosynthetic pigment in green plants, which is why plants usually look green. But often, leaves turn other colours (orange, red, yellow) in autumn. Why?

Leaves Changing Colours The production of chlorophyll is temperature dependent. At the end of summer, early autumn the temperature drops (in climates such as ours in the Temperate Zone) Plants stop producing chlorophyll

Leaves Changing Colour As a result they are not reflecting as much green light as before. Now, the other pigments are more abundant which is why we see the colours they reflect instead.

Absorption and Action Spectra Action Spectrum: a graph showing the rate of photosynthesis at each wavelength of light. Absorption Spectrum: a graph showing the percentage of light absorbed at each wavelength

Absorption and Action Spectra You need to be able to draw an absorption spectrum for chlorophyll and an action spectrum for photosynthesis

Absorption Spectrum of Chlorophyll See page 133

Action Spectrum of a Plant Pigment

Why are they so similar? Photosynthesis can only occur in wavelengths of light that chlorophyll or other photosynthetic pigments can absorb.

Photosynthesis and O2 Remember, one of the waste products produced from photosynthesis is Oxygen gas. The oxygen can be used by the cell for cellular respiration. Some of the oxygen is released by the cell as a waste product.

Where did all the oxygen come from? 1/5 of the air you are breathing right now is oxygen. However, there was none at all present 4 billion years ago. The earliest life forms on Earth were bacteria and they lived in an environment with an atmosphere of mostly CO2 Thus, early life forms were anaerobic cells

These single-celled organisms would consume organic molecules (i. e These single-celled organisms would consume organic molecules (i.e. simple sugars) that were forming from chemical reactions on Earth The more they reproduced, the more food that was consumed.

Oxygen Pollution Oxygen gas is toxic to the kinds of bacteria which preceded photosynthetic ones, so this pollution would have eventually killed off large populations of anaerobes. Anaerobic bacteria that survived would live in mud of places protected from the new oxygen-rich atmosphere.

The ability of an organism to make its own food gives it a distinct advantage over those that cannot. As a result, photosynthetic bacteria proliferated and produced more and more oxygen

The Great Oxidation Event The Great Oxidation Event: The rise in atmospheric oxygen by 2% 2.4 bya – 2.2 bya At the same time, the Earth experience its first glaciation (likely due to the reduction in the greenhouse effect since photosynthesis will decrease carbon dioxide concentrations)

During this time there is an increase of oxygen in oceans also occurred, causing the oxidation of dissolved iron in the water. This causes a distinctive rock formation “the banded iron formation”.

Levels of atmospheric oxygen remained at about 2% until 750-635 mya. At this time, many groups of multicellular organisms were evolving.

Remember, the increase in Oxygen in the atmosphere allowed for the evolution or aerobic respiration. Aerobic respiration provides far more ATP energy than anaerobic and thus allowed for more complex, multicellular organisms to evolve.