Chapter 5 - Photosynthesis.pdf

Photosynthesis Lecturer: Ms Thabisa Mdlangu [email protected] Life Depends On Photosynthesis Photosynthesis - process by which plants, algae and some microorganisms harness solar energy and convert it to chemical energy. With a few exceptions, all life on Earth depends on photosynthesis Pigment mol...

Photosynthesis Lecturer: Ms Thabisa Mdlangu [email protected] Life Depends On Photosynthesis Photosynthesis - process by which plants, algae and some microorganisms harness solar energy and convert it to chemical energy. With a few exceptions, all life on Earth depends on photosynthesis Pigment molecules in plant cells capture energy from the sun, in a series of chemical reactions that energy is used to build carbohydrate glucose (C6H12O6) from carbon dioxide (CO2) molecules, plant uses water in this process and releases oxygen gas (O2) as a by-product Light energy 6 CO2 + 6 H2O C6H12O6 + 6 O2 Photosynthesis is an Oxidation-Reduction (Redox) process Glucose produced has several functions in the cell: Used for energy, as raw material to build cellulose walls, for manufacturing of additional compounds etc. If a plant produces more glucose than it immediately needs it may store the excess as starch (or sometimes sucrose). Some plant organs are used for storage such as tubers Photosynthesis fulfills the needs of the plant in terms of energy and growth, but also produces oxygen Plants can feed other organisms, such as animals, are called Autotrophs, meaning they can use inorganic substances such as water and carbon dioxide to produce organic compounds, using mostly light as their energy source Heterotrophs obtain their carbon by consuming pre-existing organic molecules. Sunlight Is The Energy Source For Photosynthesis Visible light a small sliver of a much larger electromagnetic spectrum, the range of possible frequencies of radiation All electromagnetic radiation, including light, consists of photons, discrete packets of kinetic energy A photon’s wavelength is the distance it moves during a complete vibration The shorter a photon’s wavelength, the more energy it contains Sunlight that reaches Earth’s surface consists of three main components: ultraviolet radiation, visible light and infrared radiation Ultraviolet Radiation - shortest wavelength In the middle range is Visible Light, provides energy that powers photosynthesis We perceive visible light of different wavelengths as distinct colours Infrared Radiation - longest wavelength and contains too little energy per photon to be useful to organisms. Most of its energy converted immediately to heat Plant cells contain several pigment molecules that capture light Most abundant, Chlorophyll a, green photosynthetic pigment in plants, algae and cyanobacteria Photosynthetic organisms also have several Accessory Pigments - energy-capturing pigments molecules other than chlorophyll a, e.g. chlorophyll b and carotenoids Photosynthetic pigments have distinct colours because they absorb only some wavelengths of visible light, while transmitting or reflecting others, e.g. chlorophyll a and b absorb blue and red wavelengths but reflect green light Only absorbed light useful in photosynthesis, accessory pigments absorb wavelengths that chlorophyll a cannot, extend range of light wavelengths that cell can harness Leaves are the main organs of photosynthesis Broad, flat surfaces expose abundant surface area to sunlight Water essential for photosynthesis and roots responsible for its absorption Certain gasses also important for photosynthesis, carbon dioxide and oxygen enter and exit through Stomata, tiny openings in epidermis of leaf or stem. Stomata allow gas exchange but water evaporated through the same openings (transpiration) Stomata have guard cells that can close stomata when too much water is evaporating Most photosynthesis occurs in Mesophyll, collective term for internal cells filling leaf’s interior Leaf mesophyll cells contain abundant Chloroplasts, organelles of photosynthesis Each chloroplast contains tremendous surface area for reactions of photosynthesis Two membranes enclose Stroma, gelatinous fluid containing ribosomes, DNA, enzymes Suspended in stroma of each chloroplast are between 10 and 100 Grana, each composed of a stack of 10 to 20 disc-shaped thylakoids. Each Thylakoid consists of a membrane studded with photosynthetic pigments and enclosing a volume called Thylakoid Space. Pigments and proteins that participate in photosynthesis grouped into photosystems in thylakoid membrane One Photosystem consists of chlorophyll a aggregated with other pigment molecules and proteins that anchor entire complex in membrane. Within each photosystem are some 300 chlorophyll molecules and 50 accessory pigments. Although all pigment molecules absorb light energy, only one chlorophyll a molecule per photosystem uses energy in photosynthetic reactions. Photosystem’s Reaction Center is this chlorophyll molecule and its associated proteins. All other pigments in photosystem called Antenna Pigments because they capture photon energy and funnel it to reaction center. Photosynthesis Occurs In Two Stages Photosynthesis occurs in two stages in chloroplast, the light reactions and the carbon or light-independent reactions Light Reactions convert solar energy to chemical energy. In chloroplast’s thylakoid membranes pigment molecules in two linked photosystems capture kinetic energy from photons and store it as potential energy in chemical bonds of two molecules, ATP and NADPH (a coenzyme that carries pairs of energized electrons). In photosynthesis these electrons come from chlorophyll. Once light reactions are underway the chlorophyll replaces its lost electrons by splitting water molecules yielding oxygen as a by-product ATP and NADPH are resources used in Carbon Reactions or Light Independent Reactions. These reactions occur in the stroma and use ATP and high energy electrons in NADPH to reduce carbon dioxide to glucose molecules. The carbon dioxide comes from the atmosphere