Unit 1 Matter & Energy for Life Biology 2201 3.2 The Process of Photosynthesis Photosynthesis The process by which an organism captures the energy of the sun to convert CO 2 and water into glucose. Light energy is converted into chemical energy. The process looks as follows: carbon dioxide + water glucose + oxygen 6CO2 + 6H2O 6C 6H12O6 + O2 The actual process of photosynthesis is actually more complex than the general equation shown above indicates. As well, specific pigments and organelles are needed to perform photosynthesis. 1
Importance of Photosynthesis It is the chief source of energy on earth. It supplies most of the oxygen found in the atmosphere. Who Does It? Who Doesn t? Organisms that carry on photosynthesis are called autotrophs or producers. They contain chlorophyll inside cell organelles called chloroplasts. Organisms that are not able to carry on photosynthesis are called heterotrophs and capture their food. They depend on plants directly or indirectly as their source of food. 2
Light Light is a form of energy that must be transferred in some fashion. However, scientists do not always agree about the nature of light. Hence, there are 2 models to describe the nature and behaviour of light: Wave model of light Photon model of light Wave Model of Light Radiation such as light consists of energy waves with both electrical and magnetic properties. This electromagnetic radiation travels outward from its source in regular waves or pulses of energy, rather than in a constant unchanging flow The entire range of radiation produced by natural or human made sources is called the electromagnetic spectrum (see fig 3.5, p. 74) All forms of electromagnetic radiation travel at 300 000 000 m/s, but properties vary according to the frequency with which the waves of that type pulse, or vibrate We perceive different frequencies of visible radiation (light) as different colours. Light with the greatest frequency is perceived as violet while light with the lowest frequency is red. A combination of all frequencies is white. 3
Photon Model of Light Light travels through space in the form of individual energy packets which are called photons. Like waves, photons travel at 300 000 000 m/s The amount of energy in a photon depends on the frequency of the light. The higher the frequency, the more energy the photon is able to deliver. Violet light has a higher frequency than red light does. This means that there is more energy in a photon of violet light than there is a photon of red light. For explaining photosynthesis, the photon model of light is more appropriate. Each model has its strengths and weaknesses, but the photon model is better for photosynthesis. Stages of Photosynthesis Stages of photosynthesis: 1. Photo stage (Light dependent reactions) 2. Synthesis stage (Light independent reactions) 4
Photo stage is the stage where light is collected requires special chemicals called pigments pigments substances that absorb specific wavelengths of visible light main pigments of photosynthesis: chlorophyll green photosynthetic pigment within chloroplasts that absorbs light energy (chlorophyll a absorbs red light, chlorophyll b absorbs blue light) for photosynthesis. It is the main pigment. carotene orange pigment of photosynthesis, also absorbs light but is an accessory pigment xanthophyll yellow pigment; also an accessory pigment The Chloroplast (see fig 3.12, p. 80 structure of a chloroplast) thylakoid a disc like structure containing chlorophyll; photo reactions occur here grana a stack of thylakoids stroma space between grana in chloroplasts. The synthesis stage occurs here 5
Photosystems photosystem cluster of a few hundred pigment molecules (mostly chlorophylls) within the thylakoid membrane that act together during photosynthesis (see fig 3.11, p. 79) there are 2 photosystems that act in succession in photosynthesis: Photosystem II captures light first and uses it to split H 2 O into oxygen and hydrogen atoms. Photosystem I captures more light to produce ATP. Synthesis stage take place in the stroma of the chloroplast produces glucose (food) from carbon dioxide most photosynthetic organisms use a pathway called the Calvin cycle (C3 pathway) some organisms use other glucose making pathways ex. C 4 pathway used by sugar cane; cactus use CAM (Craussic acid metabolism) 6
Rate of Photosynthesis 4 Factors 1. Light Intensity The greater the amount of light, the more photosynthesis occurs 2. Temperature below 0 o C and above 35 o C there is little photosynthesis 3. Water when in short supply, photosynthesis slows down 4. Minerals When in short supply, photosynthesis slows down 7