Definitions [10]
Define the following term:
Photolysis of water
Releasing electrons and dividing the water molecule (H2O) into its two components (Hydrogen and Oxygen). Photolysis is the term used to describe this reaction, which is characterised by the fracturing of molecules by light (photo = light, lysis = breaking).
Photosynthesis is the process by which living plant cells, containing chlorophyll, produce food substances (glucose and starch) from carbon dioxide and water by using light energy. Plants release oxygen as a byproduct during photosynthesis.
The phenomenon that is based on the fact that light waves are transverse electromagnetic waves is called polarisation.
The branch of optics that is based on rectilinear propagation of light and deals with mirrors, lenses, reflection, refraction, etc. is called ray optics.
The branch of optics that considers light as a wave which can bend around objects, diffract and interfere, etc. is called wave optics.
Coupled time-varying electric and magnetic fields that propagate in space are called electromagnetic waves.
Photolysis occurs in the grana of a chloroplast and is defined as the splitting of H2O molecules into hydrogen ions and oxygen in the presence of light.
Photophosphorylation is the process of converting ADP into energy-rich ATP by adding an inorganic phosphate (Pi), using energy from light (photons).
Define the following term:
Polymerisation
Most green plants use glucose as soon as it occurs during photosynthesis to make starch. Polymerisation is the process by which several glucose molecules are turned into one starch molecule.
The process by which monomer molecules combine together to form a polymer is called polymerisation.
The process by which monomer molecules combine together to form a polymer is called polymerisation.
The process of conversion of many simpler and smaller molecules into a complex, bigger molecule is termed as polymerisation. For example, conversion of several glucose molecules into a starch molecule.
Key Points
- Photosynthesis converts sunlight into chemical energy using CO₂ and water, producing glucose and releasing oxygen as a byproduct.
- Green plants are autotrophs (make their own food); all other organisms are heterotrophs. All life depends on sunlight for energy.
- Three essentials for photosynthesis: chlorophyll, light, and CO₂.
- Experiment 1 - Variegated leaf tested for starch showed that photosynthesis occurs only in green parts in the presence of light.
- Experiment 2 - Leaf part enclosed with KOH-soaked cotton (absorbs CO₂) tested negative for starch → proved CO₂ is necessary for photosynthesis.
- Light consists of energy-carrying photons guided by the rules of electromagnetic (EM) waves.
- Commonly observed phenomena of light are broadly classified into three categories: Ray optics, Wave optics, and Particle nature of light.
- Light thus exhibits a dual nature — it behaves both as a wave (wave optics) and as a particle (photon/particle nature), depending on the phenomenon observed.
- Light Harvesting Complexes (LHC) - Made up of hundreds of pigment molecules bound to proteins. Found in PS I and PS II. Help absorb different wavelengths of light for efficient photosynthesis.
- Antennae System - In each photosystem, all pigments except one chlorophyll a molecule form the antennae (light-harvesting system). They absorb light and pass energy to the reaction centre.
- Reaction Centre - The single chlorophyll a molecule that directly participates in the photochemical reaction. It is different in PS I and PS II.
- PS I - Reaction centre = P700 (absorbs light at 700 nm).
- PS II - Reaction centre = P680 (absorbs light at 680 nm).
- Steps of Light Reaction - Light absorption → Water splitting → Oxygen release → ATP and NADPH production.
| Characteristics | C₃ Plants | C₄ Plants |
|---|---|---|
| Cell type in which the Calvin cycle takes place | Mesophyll | Bundle sheath |
| Cell type in which the initial carboxylation reaction occurs | Mesophyll | Mesophyll |
| How many cell types fix CO₂ | One: Mesophyll | Two: Mesophyll and Bundle sheath |
| Primary CO₂ acceptor | RuBP | PEP |
| Number of carbons in the primary CO₂ acceptor | 5 | 3 |
| Primary CO₂ fixation product | PGA | OAA |
| Number of carbons in the primary fixation product | 3 | 4 |
| Presence of RuBisCO | Yes | Yes |
| Presence of PEP Carboxylase | No | Yes |
| Cells containing RuBisCO | Mesophyll | Bundle sheath |
| CO₂ fixation rate under high light | Low | High |
| Photorespiration at low light | High | Negligible |
| Photorespiration at high light | High | Negligible |
| Photorespiration at low CO₂ | High | Negligible |
| Photorespiration at high CO₂ | High | Negligible |
| Optimum temperature | 20–25°C | 30–40°C |
| Examples | Wheat, rice | Maize, sugarcane |
- Photorespiration is a process where O₂ is used, and CO₂ is released, opposite to photosynthesis, making it a wasteful process.
- It occurs when O₂ concentration is high and CO₂ is low, causing RuBisCO to act as oxygenase instead of carboxylase.
- In this process, RuBP reacts with O₂ to form one molecule of PGA and one molecule of phosphoglycolate (2C).
- Photorespiration does not produce ATP or NADPH; instead, it uses ATP and releases CO₂, reducing photosynthetic efficiency.
- It mainly occurs in C₃ plants, leading to decreased carbon fixation and lower productivity.
- C₄ plants do not show photorespiration because they increase CO₂ concentration at the RuBisCO site, ensuring proper functioning of the Calvin cycle.
- Blackman’s Law of Limiting Factors states that the rate of photosynthesis is controlled by the factor in the least supply.
- Light affects photosynthesis through intensity, quality, and duration; it shows a linear increase at low intensity and saturation at about 10% of full sunlight.
- Carbon dioxide is the major limiting factor; increasing CO₂ concentration increases photosynthesis up to a limit, after which it may become harmful.
- Temperature controls enzymatic reactions (dark reactions); C₄ plants work better at higher temperatures, while C₃ plants have a lower optimum temperature.
- Water affects photosynthesis indirectly; water stress causes stomatal closure, reducing CO₂ availability and decreasing photosynthesis.
- All factors work together, but usually one limiting factor determines the overall rate of photosynthesis.
