PHOTOSYNTHESIS

1 EJC H2 Biology T3W5 Photosynthesis Energy & Equilibrium – Photosynthesis 1. Overview of Topic All living things require energy to grow and survive. Most living organisms get their energy either directly or indirectly from sunlight. Producers derive their energy from the sun via photosynthesis while consumers such as herbivores, carnivores and omnivores get their energy from eating other organisms. Photosynthesis is the process by which plants, some bacteria and some protists use light energy to produce glucose from CO2 and water. The oxidation of glucose during respiration can yield ATP to fulfil an organism’s energy needs. Photosynthesis is also a source of carbon, which is needed for the synthesis of other organi c compounds, e.g. pro teins, lipids and nucleic acids. In this chapter , we will look at the location, importance and mechanisms of photosynthesis. 2. Learning Outcomes a. Identify components of chloroplasts an d mitochondria in drawings, photomicrographs and electron micrographs. b. Explain the absorption and action spectra of photosynthetic pigments. c. With reference to the chloroplast structure, describe and explain how light energy is harnesses and converted into chemical energy during the light-dependent reactions of photosynthesis. d. Outline the three phases of the Calvin cycle in C3 plants: i. CO2 fixation, ii. PGA reduction and iii. Ribulose bisphosphate (RuBP) regeneration, indicating the roles of rubisco, ATP and reduced NADP in these processes that ultimately allow synthesis of sugars. e. Discuss limiting factors in photosynthesis and carry out investigations on the effect of limiting factors such as temperature, light intensity and carbon dioxide concentration on the rat e of photosynthesis. 3. References Campbell, N.A. and Reece, J.B. (2008). Biology, 9th edition. Pearson.

2 EJC H2 Biology T3W5 Photosynthesis Contents 1. Overview of Topic ...................................................................................................... 1 2. Learning Outcomes.................................................................................................... 1 3. References ................................................................................................................. 1 4. Introduction ............................................................................................................... 3 5. Chloroplast – The Site of Photosynthesis .................................................................. 3 6. Light & Photosynthetic Pigments .............................................................................. 4 7. Absorption & Action Spectrum.................................................................................. 6 8. Photoactivation of Photosynthetic Pigments ............................................................ 7 9. Photosystems ............................................................................................................ 8 10. Light Dependent Stage of Photosynthesis............................................................. 9 a. Non-cyclic Light-dependent Reaction ................................ ................................ .. 10 b. Cyclic Light-dependent Reaction................................ ................................ .......... 12 c. Synthesis of ATP via Photophosphorylation ................................ ......................... 13 11. Light-independent Stage of Photosynthesis - Calvin Cycle ................................ 16 12. Limiting Factors in Photosynthesis ..................................................................... 20

3 EJC H2 Biology T3W5 Photosynthesis 4. Introduction An autotroph is an organism that can produce its own food and are therefore also called producers. Most autotrophs use a process called photosynthesis to make their own food – energy from the sun is used to convert CO 2 and water into glucose. Plants are one of the most well-known examples of autotrophs, but there are many other autotrophic organisms such as phytoplankton and some bacteria. Photosynthesis takes place in the chloroplast in two stages: Light-independent stage will continue to take place so long as the products of the light-dependent stage have not run out. 5. Chloroplast – The Site of Photosynthesis Structure of chloroplast. PHOTO SYNTHESIS  Light-dependent stage  Light energy is converted into chemical energy in the form of NADPH and ATP. These products are then used to drive the reactions in the next stage of photosynthesis.  light-independent stage (a.k.a Calvin cycle)  Fixing atmospheric carbon in the form of CO2 into sugar molecules using NADPH and ATP from light-dependent reaction.

4 EJC H2 Biology T3W5 Photosynthesis Electron micrograph of chloroplast. Chloroplast envelope (double membrane) separates the inside of the chloroplast and the cytoplasm of the cell. The dense fluid within the chloroplast containing soluble enzymes and other organic substances is called the stroma. Stacks of thylakoids form a granum (singular) and are found within the chloroplast. Intergranal lamella link the grana (plural). Photosynthetic pigments (e.g. chlorophyll and carotenoids) are located on the thylakoid membrane and allow plant cells to capture light energy for photosynthesis. 6. Light & Photosynthetic Pigments The electromagnetic spectrum. The sun gives off a full range of light of varying wavelengths called the electromagnetic spectrum. However, Earth’s atmosphere only allows certain wavelengths of light through, one of which is visible light . Out of all the light wavelengths that gets pass the Earth’s atmosphere, only visible light is absorbed by the photosynthetic pigments in plants for photosynthesis. Notes to self

5 EJC H2 Biology T3W5 Photosynthesis There are several types of photosynthetic pigments but we will focus only on the following: o Chlorophyll a is the most abundant photosynthetic pigment. They exist as several forms (e.g. P700 and P680) with different absorption peaks. They are the only pigments that participate directly in photosynthesis. o Chlorophyll b and carotenoids are accessory pigments . They play an indirect role in photosynthesis and channel light energy absorbed to chlorophyll a. Other than their indirect role in photosynthesis, carotenoids have a photoprotective role . They absorb and dissipate excess light energy that could damage chlorophyll or quench reactive oxidative molecules that are dangerous to the cell. Additionally, they add colour to fruits and flowers which help in disper sal and pollination respectively. Structure of a chlorophyll molecule. Porphyrin ring: light-absorbing “head” of molecule Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts (Note: H atoms are not shown) CH3 in chlorophyll a CHO in chlorophyll b Notes to self Notes to self

6 EJC H2 Biology T3W5 Photosynthesis 7. Absorption & Action Spectrum The ability of a pigment to absorb various wavelengths of light can be measured by a spectrophotometer. A spectrophotometer is a device that directs beams of light of different wavelengths through a solution of pigment and measures the fraction of the light transmitted at each wavelength. Absorption spectrum and action spectrum of photosynthetic pigments. Q: Note down any significant observations about the absorption spectrum.  Each pigment has its own specific spectrum  Each pigm