Photosynthesis
(i) Photosynthesis. Energy capture by photosynthetic pigments to generate ATP and for photolysis. Transmission and reflection of light that is not absorbed by pigments. Absorption spectra of chlorophyll a and b and carotenoids compared to the action spectra for photosynthesis. Carotenoids extend the range of wavelengths absorbed by photosynthesis and pass the energy to chlorophyll. Absorbed energy excites electrons in the pigment molecule. Transfer of these high-energy electrons through electron transport chains releases energy to generate ATP by ATP synthase. Energy is also used for photolysis, in which water is split into oxygen, which is evolved, and hydrogen, which is transferred to the coenzyme NADP. The enzyme RuBisCO fixes carbon dioxide by attaching it to ribulose bisphosphate (RuBP) in the Calvin cycle. The 3-phosphoglycerate produced is phosphorylated by ATP and combined with hydrogen from NADPH to form glyceraldehyde-3-phosphate (G3P). G3P is used to regenerate RuBP and for the synthesis of glucose, which may be synthesised into starch or cellulose or pass to other biosynthetic pathways to form a variety of metabolites.
(abridged from Photosynthetic Cells (Nature Scitable)
Photosynthesis Overview
This video gives a clear discussion of photosynthesis. The details of Photosystems I and II and the proton gradients (4:25 – 7:00), and Photorespiration/ C4 & CAM plants (8:30 – end) are beyond the scope of the Higher syllabus.
Photosynthesis is a complex chemical process which occurs in all green plants, and is responsible for converting light energy into chemical energy. Most living things depend on photosynthetic cells to manufacture the molecules, such as sugars, which they require as a source of energy. During the process of photosynthesis, cells use carbon dioxide and energy from the Sun to make sugar molecules and oxygen. This occurs in a two steps, the light dependent reaction and the Calvin cycle or carbon fixation. The sugar molecules produced are the basis for more complex molecules made by the photosynthetic cell, such as starch, cellulose, proteins and lipids.
Absorption Spectra
The Chloroplast
Photosynthetic cells contain special pigments that absorb light energy. Different pigments respond to different wavelengths of visible light. Chlorophyll, the primary pigment used in photosynthesis, reflects green light and absorbs red and blue light most strongly. In plants, photosynthesis takes place in chloroplasts, which contain the chlorophyll. Chloroplasts are surrounded by a double membrane and contain a third inner membrane, called the thylakoid membrane, which forms stacks of green discs called thylakoid. In electron micrographs, thylakoid membranes look like stacks of coins, although the compartments they form are connected like a maze of chambers. The green pigment chlorophyll is located within the thylakoid membrane, and the space between the thylakoid and the chloroplast membranes is called the stroma.
Chlorophyll A is the major pigment used in photosynthesis, but there are several types of chlorophyll and numerous other pigments that respond to light, including red, brown, and blue pigments. These other pigments may help channel light energy to chlorophyll A or protect the cell from photo-damage. For example, the photosynthetic protists called dinoflagellates, which are responsible for the “red tides” that often prompt warnings against eating shellfish, contain a variety of light-sensitive pigments, including both chlorophyll and the red pigments responsible for their dramatic colouration.
Steps of Photosynthesis
Photosynthesis consists of both light-dependent reactions and light-independent reactions. In plants, the so-called “light” reactions occur within the chloroplast thylakoids, where the aforementioned chlorophyll pigments reside. When light energy reaches the pigment molecules, it excites electrons within them, and these electrons are shunted to an electron transport chain in the thylakoid membrane. The electron is moved through a series of steps, losing energy at each step, producing ATP using the enzyme ATP synthase. At the same time, energy is used to split a water molecule to produce oxygen, which is released as a waste product, and hydrogen which is captured by the coenzyme, NADP (forming NADPH) – the electron originally lost from the chlorophyll molecule is also replaced at this point. Both ATP and NADPH are produced on the stroma side of the the thylakoid membrane (the other side of the membrane is called the lumen) and are therefore perfectly placed to supply the Calvin Cycle.
Once the light reactions have occurred, the light-independent reactions occur. These reactions occur in the chloroplast stroma. During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions are used by the enzyme RuBisCo to attach the carbon in carbon dioxide (from the atmosphere) to a 5 carbon molecule called Ribulose – 1, 5- bisphosphate (RuBP). The resultant 6 carbon molecule is unstable and immediately breaks into two, 3 carbon molecules called, glyceraldehyde-3-phosphate (G3P). The cells use some G3P to regenerate RuBP and use the remainder to build a wide variety of molecules, including sugars (such as glucose) and other organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma. The products of these reactions are then transported to other parts of the cell, including the mitochondria, where they can be broken down to release their energy (captured by ATP). Some sugar molecules are stored as sucrose or starch.
