SERIES 5: METABOLISM
HOW DO PLANTS MAXIMIZE THEIR CELLULAR EFFICIENCY? WHAT CAN WE LEARN ABOUT PLANTS AND APPLY TO TECHNOLOGY AND FUTURE INNOVATIONS?
Understanding Plant Metabolism: The Essential Processes for Growth and Survival
Plant metabolism is a nuanced and important aspect of plant biology, encompassing a wide range of biochemical processes that sustain growth, development, and adaptation to environmental changes. Let’s delve into the fundamental aspects of plant metabolism, including photosynthesis, respiration, secondary metabolism, and their roles in plant adaptation and overall health.
Photosynthesis
Photosynthesis is where green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. This glucose is important as it is a key reactant in respiration! This process primarily occurs in the chloroplasts of plant cells, which contain the key pigment chlorophyll, and can be divided into two main stages:
Light Dependent Reactions: These occur in the thylakoid membranes of the chloroplasts and involve the absorption of light by chlorophyll. These chlorophylls have light catching receptors, that take the light energy and convert it into chemical energy. This is what allows us biologists to call plants autotrophs! The energy from light is used to split water molecules into oxygen, protons, and electrons. The electrons are transferred through a series of proteins known as the electron transport chain, leading to the production of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are energy carriers. This formation of NADPH and ATP is caused by the process of phosphorylation. This is where an additional phosphate is added to the molecules of ADP and NADH, which are ‘energy currencies’. The forming and breaking of this bond within these molecules allows energy to be both stored (saved) and released (spent).
Calvin Cycle (Dark Reactions): This cycle occurs in the stroma of the chloroplasts and does not require light directly. Instead, it uses the ATP and NADPH generated during the light reactions to convert carbon dioxide (CO₂) into glucose. This allows the plant to produce glucose at all times of the day, to allow the plant to respire.
RESPIRATION
Cellular respiration is the process by which plants break down glucose to release energy stored in its chemical bonds. This process occurs in the mitochondria and can be divided into several stages:
Glycolysis: This initial stage takes place in the cytoplasm and breaks down one molecule of glucose into two molecules of pyruvate, producing a small amount of ATP and NADH in the process. Pyruvate is a common intermediate in metabolic reactions and contains a carboxylic acid and ketone group. It consists of 3 carbons.
Citric Acid Cycle (Krebs Cycle): The pyruvate is transported into the mitochondria, where it is converted into acetyl-CoA and enters the citric acid cycle. This cycle generates ATP, NADH, and FADH₂ (flavin adenine dinucleotide in its reduced form)
Electron Transport Chain and Oxidative Phosphorylation: In the inner mitochondrial membrane, electrons from NADH and FADH₂ are transferred through proteins, leading to the production of a large amount of ATP. Oxygen acts as the last electron acceptor, fusing with protons to form water.
Respiration allows plants to harness the energy stored in glucose for a wide range cellular activities, including growth, reproduction, and defense.
Secondary Metabolism: Beyond Basic Nutrients
In addition to primary metabolic processes, plants engage in secondary metabolism, which produces compounds not directly involved in growth and development but crucial for interactions with their environment. Secondary metabolites can be classified into three main groups:
Phenolics: This category consists of flavonoids, tannins, and lignins, which play roles in plant coloration, defence against herbivores, and structural support.
Alkaloids: These nitrogen-containing compounds often have pharmacological effects and can act as deterrents to herbivores or pathogens. Examples include nicotine, caffeine, and morphine.
Terpenoids: These compounds are derived from five-carbon isoprene units and include essential oils, which can attract pollinators, repel herbivores, or inhibit the growth of competing plants. Isoprenes are branched – chain unsaturated hydrocarbons.
Plant metabolism is very receptive to changes in environmental conditions. For example, under drought stress, plants may alter their metabolic pathways to conserve water and energy. This includes the compounding of osmoprotectants like proline and the adjustment of photosynthetic processes to minimize water loss.
Additionally, plants have evolved complex regulatory mechanisms to coordinate metabolic pathways with developmental processes to develop effective communication between different systems in the plant. Hormones such as abscisic acid, gibberellins, and auxins play essential roles in modulating metabolism in response to internal and external signals. Check out our article on ‘Plant Hormones’ for more information!
Interested in finding out more? Check out these resources!
Plant Metabolomics: Current Initiatives and Future Prospects - PMC (nih.gov)
Exploring the Diversity of Plant Metabolism - ScienceDirect
Recent Advances in Plant Metabolomics: From Metabolic Pathways to Health Impact - PMC (nih.gov)
Lecture 23 - Plant Metabolism (youtube.com)
Plant Secondary Metabolism: Terpenoids, Lectins and Glycosides (youtube.com)
March 2024