Biology

Learninsta presents the core concepts of Biology with high-quality research papers and topical review articles.

Photosynthesis in Bacteria

Though we study about bacterial photosynthesis as the last part, bacterial photosynthesis formed first and foremost in evolution. Bacteria does not have specialized structures like chloroplast. It has a simple type of photosynthetic apparatus called chlorosomes and chromatophores (Table 13.6). Van Neil (1930) discovered a bacterium that releases sulphur instead of oxygen during photosynthesis.

Photosynthesis in Plants	Photosynthesis in Bacteria
1. Cyclic and non-cyclic phosphorylation takes place	1. Only cyclic phosphorylation takes place
2. Photosystem I and II involved	2. Photosystem I only involved
3. Electron donor is water	3. Electron donor is H<sub>2</sub>S
4. Oxygen is evolved	4. Oxygen is not evolved
5. Reaction centres are P700 and P680	5. Reaction centre is P<sub>870</sub>
6. Reducing agent is NADPH + H+	6. Reducing agent is NADH + H+
7. PAR is 400 to 700 nm	7. PAR is above 700 nm
8. Chlorophyll, carotenoid and xanthophyll	8. Bacterio chlorophyll and bacterio viridin
9. Photosynthetic Apparatus – Chloroplast	9. It is chlorosomes and chromatophores

Here, electron donor is hydrogen sulphide (H₂S) and only one photosystem is involved (PS I) and the reaction centre is P₈₇₀. Pigments present in bacteria are bacteriochlorophyll a, b, c, d, e and g and carotenoids. Photosynthetic bacteria are classified into three groups:

1.Green Sulphur Bacteria.
Example: Chlorobacterium and Chlorobium.

2. Purple Sulphur Bacteria.
Example: Thospirillum and Chromatium.

3. Purple Non-Sulphur Bacteria.
Example: Rhodopseudomonas and Rhodospirillum.

Cyanobacteria contain chlorophyll while other forms of bacteria contain bacteriochlorophyll. Cyanobacteria perform photosynthesis using water as an electron donor in a similar manner to plants. This results in the production of oxygen and is known as oxygenic photosynthesis.

There are several groups of bacteria that undergo anoxygenic photosynthesis: green sulfur bacteria, green and red filamentous anoxygenic phototrophs (FAPs), phototrophic purple bacteria, phototrophic acidobacteria, and phototrophic heliobacteria.

There are several groups of bacteria that undergo anoxygenic photosynthesis: green sulfur bacteria, green and red filamentous anoxygenic phototrophs (FAPs), phototrophic purple bacteria, phototrophic acidobacteria, and phototrophic heliobacteria.

The main purpose of photosynthesis is to convert radiant energy from the sun into chemical energy that can be used for food. Cellular respiration is the process that occurs in the mitochondria of organisms (animals and plants) to break down sugar in the presence of oxygen to release energy in the form of ATP.

Oxygenic photosynthetic bacteria perform photosynthesis in a similar manner to plants. They contain light-harvesting pigments, absorb carbon dioxide, and release oxygen. Cyanobacteria or Cyanophyta are the only forms of oxygenic photosynthetic bacteria known to date.

An example of photosynthesis is how plants convert sugar and energy from water, air and sunlight into energy to grow. The water from the leaves evaporates through the stomata, and filling its place, entering the stomata from the air, is carbon dioxide. Plants need carbon dioxide to make food.

Algae are sometimes considered plants and sometimes considered “protists” (a grab-bag category of generally distantly related organisms that are grouped on the basis of not being animals, plants, fungi, bacteria, or archaeans).

In all phototrophic eukaryotes, photosynthesis takes place inside a chloroplast, an organelle that arose in eukaryotes by endosymbiosis of a photosynthetic bacterium (see Unique Characteristics of Eukaryotic Cells). These chloroplasts are enclosed by a double membrane with inner and outer layers.

The most influential bacteria for life on Earth are found in the soil, sediments and seas. Well known functions of these are to provide nutrients like nitrogen and phosphorus to plants as well as producing growth hormones. By decomposing dead organic matter, they contribute to soil structure and the cycles of nature.

Bacteria are classified into five groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios) or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains or clusters.

Photosynthesis converts solar energy into chemical energy. Photosynthesis produces carbohydrates. Plants need sunlight, carbon dioxide, water, nutrients, and chlorophyll to complete photosynthesis. Plants use chlorophyll, water, and carbon dioxide to make sugar.

Photosynthesis is the biochemical process in which energy from sunlight is converted by plants, algae, and some bacteria into sugars, which are used by the organism as food. However, there is a least one exception: a little bacterium deep under the Pacific Ocean which manages photosynthesis without sunlight.

Cyanobacteria are oxygenic photosynthetic bacteria. They harvest the sun’s energy, absorb carbon dioxide, and emit oxygen. Like plants and algae, cyanobacteria contain chlorophyll and convert carbon dioxide to sugar through carbon fixation.

According to the diagram of photosynthesis, the process begins with three most important non-living elements: water, soil, and carbon dioxide. Plants begin making their ‘food’, which basically includes large quantities of sugars and carbohydrate, when sunlight falls on their leaves.

The reactants for photosynthesis are light energy, water, carbon dioxide and chlorophyll, while the products are glucose (sugar), oxygen and water.

Learninsta presents the core concepts of Biology with high-quality research papers and topical review articles.

Factors Affecting Photosynthesis

In 1860, Sachs gave three cardinal points theory explaining minimum, optimum and maximum factors that control photosynthesis. In 1905, Blackman put forth the importance of smallest factor. Blackman’s law of limiting factor is actually a modified Law proposed by Liebig’s Law of minimum. According to Blackman, “When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the lowest factor”.

To conclude in an easy way “at any given point of time the lowest factor among essentials will limit the rate of photosynthesis”. For example, when even sufficient light intensity is available, photosynthesis may be low due to low CO₂ in the atmosphere.

Here, CO₂ acts as a limiting factor. If CO₂ is increased in the atmosphere the rate of photosynthesis also increases. Further increase in photosynthesis is possible only if the available light intensity is also increased proportionately (Figure 13.21).

Factors affecting photosynthesis are further grouped into External or Environmental factors and Internal factors.

I. External Factors:
Light, carbon dioxide, temperature, water, mineral and pollutants.

II. Internal Factors:
Pigments, protoplasmic factor, accumulation of carbohydrates, anatomy of leaf and hormones.

External Factors

1. Light

Energy for photosynthesis comes only from light. Photooxidation of water and excitation of pigment molecules are directly controlled by light. Stomatal movement leading to diffusion of CO₂ is indirectly controlled by light.

a. Intensity of Light:

Intensity of light plays a direct role in the rate of photosynthesis. Under low intensity the photosynthetic rate is low and at higher intensity photosynthetic rate is higher. It also depends on the nature of plants. Heliophytes (Bean Plant) require higher intensity than Sciophytes (Oxalis).

b. Quantity of Light:

In plants which are exposed to light for longer duration (Long day Plants) photosynthetic rate is higher.

c. Quality of light:

Different wavelengths of light affect the rate of photosynthesis because pigment system does not absorb all the rays equally. Photosynthetic rate is maximum in blue and red light. Photosynthetically Active Radiation (PAR) is between 400 to 700 nm. Red light induces highest rate of photosynthesis and green light induces lowest rate of photosynthesis.

2. Carbon Dioxide

CO₂ is found only 0.3% in the atmosphere but plays a vital role. Increase in concentration of CO₂ increases the rate of photosynthesis (CO₂ concentration in the atmosphere is 330 ppm). If concentration is increased beyond 500ppm, rate of photosynthesis will be affcted showing the inhibitory effect.

3. Oxygen

The rate of photosynthesis decreases when there is an increase of oxygen concentration. This Inhibitory effect of oxygen was first discovered by Warburg (1920) using green algae Chlorella.

4. Temperature

The optimum temperature for photosynthesis varies from plant to plant. Temperature is not uniform in all places. In general, the optimum temperature for photosynthesis is 25°C to 35°C. This is not applicable for all plants.

The ideal temperature for plants like Opuntia is 55°C, Lichens 25°C and Algae growing in hot spring photosynthesis is 75°C. Whether high temperature or low temperature it will close the stomata as well as inactivate the enzymes responsible for photosynthesis (Figure 13.22).

5. Water

Photolysis of water provides electrons and protons for the reduction of NADP, directly. Indirect roles are stomatal movement and hydration of protoplasm. During water stress, supply of NADPH + H⁺ is affected.

6. Minerals

Deficiency of certain minerals affect photosynthesis e.g. mineral involved in the synthesis of chlorophyll (Mg, Fe and N), Phosphorylation reactions (P), Photolysis of water (Mn and Cl), formation of plastocyanin (Cu).

7. Air pollutants

Pollutants like SO₂, NO₂, O₃ (Ozone) and Smog affects rate of photosynthesis.

Internal Factors

1. Photosynthetic Pigments
It is an essential factor and even a small quantity is enough to carry out photosynthesis.

2. Protoplasmic Factor
Hydrated protoplasm is essential for photosynthesis. It also includes enzymes responsible for Photosynthesis.

3. Accumulation of Carbohydrates
Photosynthetic end products like carbohydrates are accumulated in cells and if translocation of carbohydrates is slow then this will affect the rate of photosynthesis.

4. Anatomy of Leaf
Thickness of cuticle and epidermis, distribution of stomata, presence or absence of Kranz anatomy and relative proportion of photosynthetic cells affect photosynthesis.

5. Hormones
Hormones like gibberellins and cytokinin increase the rate of photosynthesis.

Learninsta presents the core concepts of Biology with high-quality research papers and topical review articles.

Photorespiration or Cycle or Photosynthetic Carbon Oxidation (PCO) Cycle

Respiration is a continuous process for all living organisms including plants. Decker (1959) observed that rate of respiration is more in light than in dark. Photorespiration is the excess respiration taking place in photosynthetic cells due to absence of CO₂ and increases of O₂ (Table 13.5). This condition changes the carboxylase role of RUBISCO into oxygenase.

Photorespiration	Dark respiration
1. It takes place in photosynthetic green cells	1. It takes place in all living cells
2. It takes place only in the presence of light	2. It takes place all the time
3. It involves chloroplast, peroxisome and mitochondria	3. It involves only mitochondria
4. It does not involve Glycolysis, Kreb’s Cycle, and ETS	4. It involves glycolysis, Kreb’s Cycle and ETS
5. Substrate is glycolic acid	5. Substrate is carbohydrates, protein or fats
6. It is not essential for survival	6. Essential for survival
7. No phosphorylation and yield of ATP	7. Phosphorylation produces ATP energy
8. NADH2 is oxidised to NAD+	8. NAD+ is reduced to NADH2
9. Hydrogen peroxide is produced	9. Hydrogen peroxide is not produced
10. End products are CO2 and PGA	10. End products are CO2 and water

C₂ Cycle takes place in chloroplast, peroxisome and mitochondria. RUBP is converted into PGA and a 2C-compound phosphoglycolate by Rubisco enzyme in chloroplast. Since the first product is a 2C-compound, this cycle is known as C₂ Cycle. Phosphoglycolate by loss of phosphate becomes glycolate. Glycolate formed in chloroplast enters into peroxisome to form glyoxylate and hydrogen peroxide.

Glyoxylate is converted into glycine and transferred into mitochondria. In mitochondria, two molecules of glycine combine to form serine. Serine enters into peroxisome to form hydroxy pyruvate. Hydroxy pyruvate with help of NADH + H⁺ becomes glyceric acid.

Glyceric acid is cycled back to chloroplast util ising ATP and becomes Phosphoglyceric acid (PGA) and enters into the Calvin cycle (PCR cycle). Photorespiration does not yield any free energy in the form of ATP. Under certain conditions 50% of the photosynthetic potential is lost because of Photorespiration (Figure 13.20).

Significance of Photorespiration

1. Glycine and Serine synthesised during this process are precursors of many biomolecules like chlorophyll, proteins, nucleotides.
2. It consumes excess NADH + H⁺ generated.
3. Glycolate protects cells from Photo oxidation.

Carbon Dioxide Compensation Point

When the rate of photosynthesis equals the rate of respiration, there is no exchange of oxygen and carbon dioxide and this is called as carbon dioxide compensation point. This will happen at particular light intensity when exchange of gases becomes zero. When light is not a limiting factor and atmospheric CO₂ concentration is between 50 to 100 ppm the net exchange is zero.

Learninsta presents the core concepts of Biology with high-quality research papers and topical review articles.

Crassulacean Acid Metabolism or CAM Cycle

It is one of the carbon pathways identified in succulent plants growing in semi-arid or xerophytic condition. This was first observed in crassulaceae family plants like Bryophyllum, Sedum, Kalanchoe and is the reason behind the name of this cycle. It is also noticed in plants from other families Examples: Agave, Opuntia, Pineapple and Orchids.

The stomata are closed during day and are open during night (Scotoactive). This reverse stomatal rhythm helps to conserve water loss through transpiration and will stop the fixation of CO₂ during the day time. At night time CAM plants fix CO₂ with the help of Phospho Enol Pyruvic acid (PEP) and produce oxalo acetic acid (OAA). Subsequently OAA is converted into malic acid like C₄ cycle and gets accumulated in vacuole increasing the acidity.

During the day time stomata are closed and malic acid is decarboxylated into pyruvic acid resulting in the decrease of acidity. CO₂ thus formed enters into Calvin Cycle and produces carbohydrates (Figure 13.19).

Significance of CAM Cycle

1. It is advantageous for succulent plants to obtain CO₂ from malic acid when stomata are closed.
2. During day time stomata are closed and CO₂ is not taken but continue their photosynthesis.
3. Stomata are closed during the day time and help the plants to avoid transpiration and water loss.

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation to periodic water supply, occurring in plants in arid regions (e.g., cacti) or in tropical epiphytes (e.g., orchids and bromeliads). CAM plants close their stomata during the day and take up CO₂ at night, when the air temperature is lower.

Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions that allows a plant to photosynthesize during the day, but only exchange gases at night.

In CAM plants, carbon dioxide is only gathered at night, when the stomata open. Click for more detail. During the day, the malic acid is converted back to carbon dioxide. This type of photosynthesis is known as Crassulacean Acid Metabolism because of the storage of carbon dioxide at night as an acid.

Crassulacean acid metabolism (CAM) is an important elaboration of photosynthetic carbon fixation that allows chloroplast-containing cells to fix CO₂ initially at night using phosphoenolpyruvate carboxylase (PEPC) in the cytosol.

Some plants that are adapted to dry environments, such as cacti and pineapples, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. This name comes from the family of plants, the Crassulaceae, in which scientists first discovered the pathway. Image of a succulent.

Biochemical studies indicate that photorespiration consumes ATP and NADPH, the high-energy molecules made by the light reactions. Thus, photorespiration is a wasteful process because it prevents plants from using their ATP and NADPH to synthesize carbohydrates.

The CAM plants close their stomata at the time of day and open it in night. When the stomata remains closed then this will prevent the loss of water by the process of transpiration. This process also prevents the carbon dioxide gas being entering into the plant leaves.

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation to periodic water supply, occurring in plants in arid regions (e.g., cacti) or in tropical epiphytes (e.g., orchids and bromeliads). CAM plants close their stomata during the day and take up CO₂ at night, when the air temperature is lower.

Unlike plants in wetter environments, CAM plants absorb and store carbon dioxide through open pores in their leaves at night, when water is less likely to evaporate. During the day, the pores, also called stomata, stay closed while the plant uses sunlight to convert carbon dioxide into energy, minimizing water loss.

Without enough light, a plant cannot photosynthesise very quickly – even if there is plenty of water and carbon dioxide and a suitable temperature. Increasing the light intensity increases the rate of photosynthesis, until some other factor – a limiting factor – becomes in short supply.

But there are few plants like Peepal which gives out Oxygen at night by CAM photosynthesis which are the plants the Crassulaceae family. During daylight hours, plants take in carbon dioxide and release oxygen through photosynthesis, and at night only about half that carbon is then released through respiration.

Learninsta presents the core concepts of Biology with high-quality research papers and topical review articles.

Hatch & Slack Pathway or Cycle or Dicarboxylic Acid Pathway or Dicarboxylation Pathway

Till 1965, Calvin cycle is the only pathway for CO₂ fixation. But in 1965, Kortschak, Hart and Burr made observations in sugarcane and found C₄ or dicarboxylic acid pathway. Malate and aspartate are the major labelled products. This observation was confirmed by Hatch & Slack in 1967.

This alternate pathway for the fixation of CO₂ was found in several tropical and sub-tropical grasses and some dicots. C₄ cycle is discovered in more than 1000 species. Among them 300 species belong to dicots and rest of them are monocots.

C₄ plants represent about 5% of Earth’s plant biomass and 1% of its known plant species. Despite this scarcity, they account for about 30% of terrestrial carbon fixation. Increasing the proportion of C₄ plants on earth could assist biosequestration of CO₂ and represent an important climate change avoidance strategy.

C₄ pathway is completed in two phases, first phase takes place in stroma of mesophyll cells, where the CO₂
acceptor molecule is 3-Carbon compound, phosphoenol pyruvate (PEP) to form 4-carbon Oxalo acetic acid (OAA). The first product is a 4-carbon and so it is named as C₄ cycle.

oxalo acetic acid is a dicarboxylic acid and hence this cycle is also known as dicarboxylic acid pathway (Figure 13.18). Carbon dioxide fixation takes place in two places one in mesophyll and another in bundle sheath cell (di carboxylation pathway).

It is the adaptation of tropical and sub tropical plants growing in warm and dry conditions. Fixation of CO₂ with minimal loss is due to absence of photorespiration. C₄ plants require 5 ATP and 2 NADPH + H⁺ to fix one molecule of CO₂.

Stage: I Mesophyll Cells

Oxaloacetic acid (OAA) is converted into malic acid or aspartic acid and is transported to the bundle sheath cells through plasmodesmata.

Stage: II Bundle Sheath Cells

Malic acid undergoes decarboxylation and produces a 3 carbon compound Pyruvic acid and CO₂. The released CO₂ combines with RUBP and follows the calvin cycle and finally sugar is released to the phloem. Pyruvic acid is transported to the mesophyll cells.

Significance of C₄ Cycle

1. Plants having C₄ cycle are mainly of tropical and sub-tropical regions and are able to survive in environment with low CO₂ concentration.
2. C₄ plants are partially adapted to drought conditions.
3. Oxygen has no inhibitory effect on C₄ cycle since PEP carboxylase is insensitive to O₂.
4. Due to absence of photorespiration, CO₂ Compensation Point for C₄ is lower than that of C₃ plants (C₄ Cycle) are given in table 13.4