Core Steps & Mechanisms of Photosynthesis in Higher Plants
We all have learnt in our junior classes about photosynthesis. It is a method which helps plants to prepare their food by themselves. It is one of the most important processes and our food and life also depends on this. But do you know how photosynthesis occurs in higher plants? How is it different from normal photosynthesis?
In this article, we are going to understand the process of photosynthesis in higher plants, reaction of photosynthesis, steps of photosynthesis and full process of photosynthesis.
Definition of Photosynthesis
The process by which green plants, in the presence of light, combine water and carbon dioxide to form carbohydrates is termed as photosynthesis. It occurs in green parts of the plant, mostly the leaves, sometimes the green stems and floral buds. The leaves contain specialised cells called mesophyll cells which contain the chloroplast– the pigment containing organelle. These are the actual sites for photosynthesis.
Structure of a Leave Cell
What is Photosynthesis in Biology?
In Biology, photosynthesis is defined as the process by which green plants, in the presence of light, combine water and carbon dioxide to form carbohydrates. Oxygen is released as a by-product of photosynthesis.
Steps of Photosynthesis
The steps of photosynthesis are explained below:
Step-1 Green plants possess the green pigment, chlorophyll which can capture, transform, translocate and store energy that is readily available for all forms of life on this planet.
Step-2 Photosynthesis is a process in which light energy is converted into chemical energy.
Step-3 Except green plants, no other organism can directly utilise solar energy to synthesise food, hence they are dependent on green plants for their survival.
Step-4 Green plants which can prepare organic food from simple inorganic elements are called autotrophic while all other organisms which cannot prepare their own food are called heterotrophic.
Step-5 During photosynthesis, oxygen liberated into the atmosphere makes the environment livable for all aerobic organisms.
Step-6 Simple carbohydrates produced in photosynthesis are transformed into lipids, proteins, nucleic acids, and other organic molecules.
Step-7 Plants and plant products are the major food sources of almost all organisms on the earth.
Step-8 Fossil fuels like coal, gas, and oil represent the photosynthetic products of the plants belonging to early geological periods.
Photosynthesis in Higher Plants
Photosynthesis is a method which helps plants to prepare their food by themselves. It is considered as one of the most essential processes on earth which is responsible for the existence of human beings and almost all other organisms.
In higher plants, the process of photosynthesis can be explained through two processes, which are light reactions and dark reactions. Now, let's understand what is a light reaction and what is a dark reaction.
Light Reaction
Photosynthesis begins with the light reactions. During this reaction, the energy from the sun is absorbed by the pigment chlorophyll within the thylakoid membranes of the chloroplast. The energy is then quickly transferred to two molecules, ATP and NADPH, which are utilised in the second stage of the chemical process. ATP and NADPH are generated by 2 electron transport chains. During the light reactions, water is used and O2 is formed. These reactions can only occur during daylight because the process needs sunlight to start.
Dark Reaction
Dark reaction is additionally referred to as carbon-fixing reaction. It's a light-independent process within which sugar molecules are formed from the CO2 and water molecules.
The dark reaction occurs within the stroma of the chloroplast, where they utilise the products of the light reaction.
Plants capture the CO2 from the atmosphere through stomata and proceed to the Calvin cycle.
In the Calvin cycle, the ATP and NADPH formed throughout the light reaction drive the reaction and convert six molecules of CO2 into one sugar molecule, i.e. glucose.
Reaction of Photosynthesis
Photosynthesis is the process by which green plants, in the presence of sunlight, combine water and CO2 to make carbohydrates. O2 is formed as a by-product of a chemical process.
Photosynthesis is represented by the following overall chemical equation:
6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2
Interesting Facts
Plants perform photosynthesis in organelles called chloroplasts.
- Chlorophyll isn't a single pigment molecule, but rather is a family of connected molecules that share an analogous structure. These squares measure different pigment molecules that absorb/reflect completely different wavelengths of light.
Conclusion
Photosynthesis is the most vital process on earth without which the existence of human beings and most other living organisms is not possible. It's a process by which green plants, algae and chlorophyll containing microorganisms utilise the energy of sunlight to synthesise their own food (organic matter) from simple inorganic molecules.
The oxidation of organic compounds releases stored energy which is used by the living organisms to carry out essential metabolic processes. It's important to note that photosynthesis is the only natural process that liberates oxygen which is used by all living forms for the process of aerobic respiration.
FAQs on Photosynthesis in Higher Plants: Complete Study Guide
1. What is photosynthesis and why is it a crucial process for higher plants?
Photosynthesis is a physicochemical process by which higher plants, algae, and some bacteria use light energy to synthesize organic compounds (food) from inorganic substances like carbon dioxide and water. It is crucial for two primary reasons: first, it is the primary source of all food on Earth, converting light energy into chemical energy stored in glucose. Second, it is responsible for releasing oxygen into the atmosphere, which is essential for the respiration of most living organisms.
2. Where in the plant cell does photosynthesis occur?
Photosynthesis in higher plants occurs within specialized organelles called chloroplasts. These are mainly located in the mesophyll cells of leaves. Inside the chloroplast, the process is divided into two parts:
- Light-dependent reactions take place in the thylakoid membranes, which are organized into stacks called grana.
- Light-independent reactions (Calvin cycle) occur in the stroma, the fluid-filled space surrounding the thylakoids.
3. What is the main difference between the light-dependent and light-independent reactions?
The primary difference lies in their dependence on light and their end products. The light-dependent reactions directly use light energy to produce ATP and NADPH, which are energy-carrying molecules, and release oxygen as a byproduct. The light-independent reactions (or Calvin cycle) do not directly require light but use the ATP and NADPH produced during the light reactions to fix atmospheric CO₂ into organic sugar molecules like glucose.
4. What are the three main stages of the Calvin Cycle?
The Calvin Cycle, or C3 cycle, is the primary pathway for carbon fixation in plants. It involves three distinct stages:
- Carboxylation: This is the carbon fixation step where CO₂ is combined with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP), to form two molecules of 3-phosphoglycerate (3-PGA). This reaction is catalysed by the enzyme RuBisCO.
- Reduction: In this stage, the 3-PGA molecules are converted into carbohydrates (triose phosphates) using the ATP and NADPH produced during the light-dependent reactions.
- Regeneration: The cycle concludes with the regeneration of the initial CO₂ acceptor, RuBP, from the triose phosphates. This step also requires ATP.
5. Why is the C4 pathway considered more efficient than the C3 pathway in certain environments?
The C4 pathway is an adaptation found in plants in hot, dry climates (e.g., maize, sugarcane). It is more efficient than the C3 pathway under these conditions because it minimises a process called photorespiration. C4 plants have a special leaf anatomy ('Kranz' anatomy) and use an enzyme called PEP carboxylase, which has a high affinity for CO₂ and is unaffected by oxygen levels. This allows C4 plants to concentrate CO₂ in their bundle sheath cells, ensuring that RuBisCO works efficiently to fix carbon, even when stomata are partially closed to conserve water.
6. What is the difference between cyclic and non-cyclic photophosphorylation?
Both processes generate ATP using light energy, but they differ in their electron flow and products. In non-cyclic photophosphorylation, electrons excited from Photosystem II (PS II) pass to Photosystem I (PS I) and are ultimately used to reduce NADP⁺ to NADPH. It produces both ATP and NADPH and involves the splitting of water (photolysis) to replace electrons in PS II. In cyclic photophosphorylation, electrons excited from PS I are cycled back to it through an electron transport chain. This process only involves PS I and produces only ATP, with no formation of NADPH or release of oxygen.
7. How do factors like light intensity, CO₂ concentration, and temperature limit the rate of photosynthesis?
The rate of photosynthesis is governed by the Law of Limiting Factors, meaning the rate is determined by the factor available at the lowest level.
- Light Intensity: At low intensities, the rate is directly proportional to the light available. At very high intensities, the rate plateaus as other factors (like CO₂) become limiting, and photo-oxidation can damage pigments.
- CO₂ Concentration: Increasing CO₂ concentration generally increases the rate of photosynthesis, especially for C3 plants, until another factor becomes limiting. C4 plants show saturation at lower CO₂ levels.
- Temperature: Photosynthesis involves enzymes, which have an optimal temperature range. For C3 plants, this is typically 20-25°C, while for C4 plants, it is higher (30-40°C). Temperatures outside this range can denature enzymes and slow the rate.
8. Is photorespiration just a wasteful process, or does it serve any purpose?
Photorespiration is a process where the enzyme RuBisCO binds with O₂ instead of CO₂, which occurs under high temperature and high oxygen concentration. It is often considered wasteful because it uses ATP and releases previously fixed CO₂, reducing the photosynthetic output by up to 25% in C3 plants. However, some research suggests it may have a protective role. By consuming excess energy (ATP and NADPH) under high light and low CO₂ conditions, photorespiration can prevent photo-oxidative damage to the photosynthetic machinery, thus serving a photoprotective function.
