Ergastic Substances of Plant Cells

What are Ergastic Substances?

• Ergastic substances are non-living materials found in the cells, distinct from the living protoplasmic content, also known as bioplasm. These substances can be organic or inorganic and are typically by-products of metabolism. Their presence in cells serves various functions, including storage, structural support, and defense. Ergastic substances can be found in the protoplasm, vacuoles, or cell walls, depending on their type and role within the organism.
• In plant cells, ergastic substances can be stored in different forms, both soluble and insoluble. They may include crystals, oil drops, gums, tannins, resins, and other metabolic products. Each of these has a specific role, such as storing energy, protecting the cell from external harm, or maintaining structural integrity. For example, tannins and resins often act as defense mechanisms by deterring herbivores or pathogens, while oils and starches serve as energy reserves.
• Ergastic substances are a critical part of cellular function, particularly in plants, where they help in the organism’s growth and survival. By acting as storage materials, they ensure the cell has the necessary resources during periods of dormancy or stress. Inorganic ergastic substances, like crystals, often contribute to the structural stability of cells. Therefore, the presence of ergastic substances is essential for the overall functionality of the cell, even though they are not part of the living protoplasm.

Types of Ergastic Substances

Ergastic substances of plants exist in three forms:

1. Reserve Materials

Reserve materials are non-living inclusions in plants that serve as essential nutritional components for plant growth and development. They are categorized into three main types: carbohydrates, nitrogenous matters, and fats and oils. Each type plays a distinct role in plant nutrition, aiding in energy storage and metabolic processes.

• Carbohydrates:
  • These organic compounds consist of carbon, hydrogen, and oxygen, with hydrogen and oxygen occurring in the same ratio as water. Upon heating, carbohydrates leave behind carbon as a black residue.
    1. Sugars:
       • Simple, soluble carbohydrates. Glucose (C6H12O6) is produced in chloroplasts with sunlight, while fructose is found in many fruits. Sucrose (C12H22O11), a more complex sugar, is found abundantly in sugarcane and beets, commonly known as table sugar.
    2. Inulin:
       • A soluble carbohydrate, (C6H10O5)n, found in root tubers of plants like Dahlia. When immersed in alcohol or glycerin, inulin precipitates into fan-shaped crystals.
    3. Starch Grains:
       • These are complex carbohydrates found in most plants, excluding fungi and bacteria. Starch, produced by chloroplasts (assimilatory starch) or amyloplasts (reserve starch), is a major energy reserve. The grains differ in shape and have a central point called the hilum, around which starchy layers are deposited. Grains may be simple (one hilum), compound (multiple hila), or semi-compound. Starch grains turn blue when treated with iodine, and their formula is (C6H10O5)n.
    4. Glycogen:
       • An insoluble carbohydrate found in fungi, glycogen is similar to animal starch and shares the same formula as starch, (C6H10O5)n.
• Nitrogenous Reserve Materials:
  • Chemically complex, these materials contain nitrogen, sulfur, and sometimes phosphorus, alongside carbon, hydrogen, and oxygen. They are divided into two types: insoluble proteids and soluble amino acids.
    1. Proteids:
       • These are typically insoluble in water but dissolve in strong acids and alkalis. A notable proteid form, called aleurone grains, is present in the endosperm of seeds like castor oil. Each grain contains a crystalloid (the nitrogenous component) and a globoid (a double phosphate of calcium and magnesium). Proteids convert into amino acids, which travel to other parts of the plant for assimilation. A common proteid, gliadin, found in wheat, has the formula C685H1068N198O211S6.
• Fats and Oils:
  • These reserve materials are energy-rich and consist of carbon, hydrogen, and oxygen, though in different proportions compared to carbohydrates. Fats (solid at room temperature) and oils (liquid) are prevalent in seeds. For example, palmitin, a common vegetable fat, has the formula C51H98O6. Fats are lighter than water, leaving greasy marks on paper, and are insoluble in water but dissolve in solvents like ether and chloroform. Castor-oil fat, for instance, is sparingly soluble in alcohol. Fats can be saponified when treated with caustic soda or potash, producing soap and glycerine.

Thus, reserve materials, comprising carbohydrates, nitrogenous compounds, and fats and oils, are integral to plant metabolism, providing energy storage and facilitating essential physiological processes.

2. Secretory Materials

Secretory Materials play a crucial role in the physiological functions of plants, although they are not classified as food. These substances, such as pigments and enzymes, indirectly contribute to various essential processes that support plant life, from food production to metabolism. Their significance lies in their ability to facilitate and enhance the plant’s internal mechanisms, without themselves serving as a direct source of nutrition.

• Coloring Matters:
  • Pigments like chlorophyll, xanthophyll, and carotin are examples of secretory materials that are not considered plant food. Chlorophyll, the green pigment, is essential for photosynthesis, as it enables plants to produce food by capturing light energy. Despite this crucial role, chlorophyll itself is not a nutrient but rather a facilitator in the food production process. Other pigments, such as xanthophyll (yellow) and carotin (orange), also play supportive roles, contributing to the plant’s health and functioning by interacting with light and protecting the plant from damage.
• Enzymes:
  • Enzymes are biological catalysts produced by the plant’s protoplasm, functioning as digestive agents. Their primary function is to break down complex, insoluble food substances into simpler, soluble forms that the plant can assimilate. For instance, enzymes convert starch into sugar, proteids into amino acids, and fats into fatty acids and glycerin. Despite being crucial in these reactions, enzymes remain unchanged throughout the process.
    • Specificity: Enzymes are highly specific in their action, meaning each enzyme targets a particular substrate. For example, enzymes that break down starch into sugar do not act on proteids or fats. This specificity ensures that the right chemical reactions occur in the right places, maintaining the plant’s metabolic balance.
    • Reversibility: In some cases, enzyme reactions are reversible. An enzyme that helps convert starch into sugar can also facilitate the reverse reaction, turning sugar back into starch. This ability to catalyze reversible reactions adds flexibility to the plant’s metabolic processes, allowing it to adapt to changing conditions.

3. Excretory Materials or Waste Products

Excretory Materials or Waste Products are substances produced by plants that serve no beneficial purpose for their survival or growth. Unlike animals, plants lack specialized excretory systems; however, they possess various mechanisms to eliminate waste products. A selection of significant excretory materials includes alkaloids, tannins, latex, essential oils, mineral crystals, and organic acids. Each of these materials has specific characteristics and functions that contribute to the plant’s overall physiology.

• Alkaloids:
  • Alkaloids are complex nitrogenous compounds found in various plants. They are often characterized by their bitter taste and can be extremely toxic. Many alkaloids have medicinal properties and are active components in herbal remedies. Common examples include quinine from the bark of the Cinchona tree, morphine from the poppy plant, caffeine found in coffee beans, and nicotine in tobacco. These compounds are typically extracted through dissolution in alcohol, emphasizing their importance in both ecological and pharmacological contexts.
• Tannins:
  • Tannins are bitter substances present in the cell sap of numerous plants, as well as in the cell walls of dead tissues, such as bark. They are notably abundant in the fruits of myrobalans (e.g., Amlaki, Bahera) and are present in varying quantities in bananas, guavas, and mangoes, often disappearing as the fruit ripens. Tannins react with iron salts, turning black, and are utilized in ink production and leather tanning, showcasing their economic significance.
• Latex:
  • Latex is a milky or watery fluid found in many plants and is an emulsion of diverse substances suspended in a watery matrix. It serves as the primary commercial source of rubber, demonstrating its economic importance. Additionally, the latex from the papaw plant contains the enzyme papain, which aids in the digestion of protein, illustrating its functional role within the plant’s biology.
• Essential Oils:
  • Essential oils are volatile compounds present in various plants, contributing to their characteristic fragrances. The pleasant aromas of flowers such as rose, jasmine, and lotus are attributable to these oils, which are secreted in specialized glands. Unlike fixed oils, essential oils are typically extracted through distillation, emphasizing their aromatic and culinary applications.
• Mineral Crystals:
  • Mineral crystals can be found within plant cells, occurring either singularly or in clusters, forming distinctive shapes. Calcium oxalate crystals are common, especially in underground plant structures. These solitary crystals may appear as rod-like, cubical, prismatic, or octahedral forms and are prevalent in dry onion scales. Two notable types of crystal aggregates are raphides, which resemble needle bundles, and sphaeraphides, which have a star-like appearance. These structures can be found in plants such as Pistia and arum. Additionally, calcium carbonate crystals often aggregate on the epidermis of leaves in species like banyan and India-rubber trees, forming cystoliths, which resemble clusters of grapes.
• Organic Acids:
  • Organic acids are another class of excretory materials found in the cell sap of many plants, especially in unripe fruits that exhibit a sour taste. Common examples include tartaric acid in tamarind, oxalic acid in Oxalis species, and citric acid in citrus fruits such as lemons. These acids contribute to the taste profile of fruits and play roles in various metabolic processes.