Respiration in Plants Class 11 – Easy NEET Notes
1. Introduction With 💖Learn Sufficient Notes💖
- All of us breathe to stay alive, but why is breathing so important? What actually happens when we breathe? Do all living things like plants and microbes also breathe? If yes, then how do they do it? All living organisms need energy to do everyday activities like absorption, transport, movement, reproduction, and even breathing itself. But where does this energy come from? We know that we eat food for energy, but how is this energy taken from food and used by the body? Do all foods give the same amount of energy? Do plants eat? How do they get their energy? And what about micro-organisms—do they also need food for energy?
- These may seem like separate questions, but in reality, they are all related. The process of breathing is closely connected to the process of releasing energy from food. All the energy needed for life comes from the oxidation of large molecules called food. Green plants and cyanobacteria can make their own food. They use photosynthesis to capture light energy and turn it into chemical energy, which is stored in carbohydrates like glucose, sucrose, and starch. But not all parts of a green plant can do photosynthesis—only the cells with chloroplasts, usually found in the outer layers of the plant, can perform it. So, other non-green parts of the plant need to receive food made during photosynthesis. This is why food must be moved to all parts of the plant.
- Animals cannot make their own food. They are heterotrophs and get food either directly from plants (as herbivores) or indirectly by eating other animals (as carnivores). Saprophytes, like fungi, get their food from dead and decaying matter. In the end, almost all the food used in respiration comes from photosynthesis. This chapter explains cellular respiration, which is the process of breaking down food inside cells to release energy. This energy is then used to make ATP, the molecule that stores usable energy.
- Photosynthesis happens inside the chloroplasts in eukaryotic cells, while the breakdown of complex food molecules for energy takes place in the cytoplasm and mitochondria. The process of breaking C–C bonds in food molecules through oxidation inside the cell, which releases a large amount of energy, is called respiration. The substances used for this process are called respiratory substrates. Most of the time, carbohydrates are used to release energy, but under certain conditions, proteins, fats, and organic acids can also be used by some plants.
- Inside the cell, this oxidation does not happen all at once. The energy is not released in a single step, and it is not released freely. Instead, the energy is given out slowly in small steps, controlled by enzymes, and is stored as chemical energy in the form of ATP. This energy cannot be used directly, so it is first used to make ATP, and later, ATP is broken down wherever the cell needs energy. Because of this, ATP is called the energy currency of the cell. The energy stored in ATP is used in different processes that need energy, and the remaining carbon skeletons produced during respiration are used to build other important molecules in the cell.
2. Understanding Plant Breathing Mechanism
- Do plants breathe? The answer is not very simple. Yes, plants need oxygen (O₂) for respiration and they also release carbon dioxide (CO₂). So, they do have ways to make oxygen available to their cells. But unlike animals, plants do not have special organs for gas exchange. Instead, they use small openings called stomata and lenticels for this purpose. There are some important reasons why plants can live without respiratory organs. First, each part of the plant handles its own gas exchange. There is no need to move gases from one part to another. Second, plants do not need as much gas exchange as animals. The roots, stems, and leaves respire much more slowly than animal cells. Only during photosynthesis, a large amount of gas is exchanged, and each leaf is built to take care of its own needs. Also, when a leaf cell is doing photosynthesis, it produces oxygen inside itself, so there is no shortage of oxygen.
- Third, even in large plants, the distance that gases must travel is very small. Almost all living cells in a plant are near the surface, so gases can reach them easily. You may wonder about thick stems and roots. In these parts, the living cells are in thin layers just under the bark, and they have lenticels to allow gas exchange. The inner cells are dead and only give support, so they don’t need oxygen. In addition, the parenchyma cells in leaves, stems, and roots are loosely packed, making a network of air spaces that helps gases move inside the plant.
- When glucose is completely broken down (or combusted), it gives carbon dioxide (CO₂), water (H₂O), and energy, most of which comes out as heat:
- C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
- But if a plant cell wants to use this energy, it should not be lost as heat. So, the plant breaks glucose in a special way, in small steps, so that the energy can be stored in the form of ATP. This is the main idea of respiration.
- During respiration, the cell uses oxygen and produces carbon dioxide, water, and energy. This process needs oxygen, but some cells live in places where oxygen is not available. Can you think of such situations or organisms? Scientists believe that the first living cells on Earth lived in an environment without oxygen. Even today, some organisms are adapted to live without it. Some are called facultative anaerobes (they can live with or without oxygen), and others are obligate anaerobes (they cannot survive with oxygen). All living organisms, even today, have enzymes that can partially break glucose without using oxygen. This process, where glucose is broken down into pyruvic acid, is called glycolysis.
3. Breakdown of Glucose in Cytoplasm
- The word glycolysis comes from two Greek words: “glycos”, meaning sugar, and “lysis”, meaning splitting. The process of glycolysis was explained by scientists Gustav Embden, Otto Meyerhof, and J. Parnas, so it is often called the EMP pathway. In organisms that do not use oxygen, glycolysis is the only way they can respire. Glycolysis happens in the cytoplasm of the cell and takes place in all living organisms. During this process, glucose is partially broken down to form two molecules of pyruvic acid. In plants, the glucose used in this process comes either from sucrose, which is made during photosynthesis, or from stored carbohydrates. The enzyme invertase breaks sucrose into glucose and fructose, which can easily enter the glycolysis pathway. The sugars glucose and fructose are first changed into glucose-6-phosphate by the enzyme hexokinase. Then, this compound is converted into fructose-6-phosphate. After this, both glucose and fructose follow the same set of reactions in glycolysis.
- Glycolysis has ten steps, each controlled by a different enzyme, and leads to the formation of pyruvate (or pyruvic acid) from glucose. During this process, we must notice where ATP and NADH + H⁺ are either used or made. ATP is used up in two steps: one, when glucose becomes glucose-6-phosphate, and two, when fructose-6-phosphate becomes fructose-1,6-bisphosphate. The fructose-1,6-bisphosphate is then split into two three-carbon molecules — dihydroxyacetone phosphate and glyceraldehyde-3-phosphate (PGAL). These two forms are easily interchanged. A key point in glycolysis is when PGAL is converted into 1,3-bisphosphoglyceric acid (BPGA). In this step, NAD⁺ is converted into NADH + H⁺ as hydrogen atoms are removed from PGAL and passed to NAD⁺. This is an oxidation reaction. Then, BPGA is changed into 3-phosphoglyceric acid (PGA), and this step produces ATP. Another ATP is made when phosphoenolpyruvate (PEP) is changed into pyruvic acid. So, from one molecule of glucose, you get two molecules of ATP from the BPGA step and two more from the PEP step, making a total of four ATPs made, but since two ATPs were used at the beginning, the net gain is two ATPs. You also get two NADH molecules.
- The end product of glycolysis is pyruvic acid, and what happens to it depends on the type of cell and its need. There are three main fates of pyruvic acid. First is lactic acid fermentation, second is alcoholic fermentation, and third is aerobic respiration (which happens in the presence of oxygen). Fermentation occurs in organisms that live without oxygen, like some bacteria and unicellular fungi (e.g., yeast). But for the complete breakdown of glucose into carbon dioxide (CO₂) and water (H₂O), living beings use aerobic respiration, which includes the Krebs cycle and requires oxygen.
4. Fermentation: An Oxygen-Independent Respiration
Fermentation is a biological process that occurs in the absence of oxygen (anaerobic conditions), where glucose is only partially oxidized. In organisms like yeast, glucose undergoes glycolysis to form pyruvic acid, which is then converted into ethanol and carbon dioxide. This conversion is facilitated by two key enzymes: pyruvic acid decarboxylase and alcohol dehydrogenase. In contrast, certain bacteria convert pyruvic acid into lactic acid through different biochemical reactions. Even in animal muscle cells, during intense physical activity when oxygen is limited, pyruvic acid is converted into lactic acid. This process is catalyzed by the enzyme lactate dehydrogenase and involves NADH + H⁺ as a reducing agent, which gets reoxidised back to NAD⁺ so that glycolysis can continue. Both types of fermentation – alcoholic and lactic acid – yield very little energy. In fact, they release less than 7% of the total energy present in a glucose molecule, and only a small fraction of that is stored in the form of ATP. These processes also produce by-products that can be harmful in high concentrations – for example, acids or alcohols. Yeast, during alcohol fermentation, can tolerate only up to about 13% alcohol concentration, after which the environment becomes toxic to them, and they die. Therefore, alcoholic beverages made through natural fermentation cannot have alcohol content above this level. Higher concentrations are achieved through processes like distillation. Since fermentation does not allow complete breakdown of glucose, the energy extracted is limited. For organisms to completely oxidise glucose and extract maximum energy, the process of aerobic respiration is used. This occurs in the mitochondria of eukaryotic cells and requires molecular oxygen (O₂). In aerobic respiration, glucose is broken down into carbon dioxide and water, releasing a large amount of energy, most of which is captured in the form of ATP, the energy currency of the cell. This complete oxidation makes aerobic respiration far more efficient and is the primary mode of respiration in higher organisms.
5. Cellular Energy Release via Aerobic Mechanism
Aerobic respiration is a biological process that occurs within the mitochondria of cells, where the final product of glycolysis—pyruvate—is transported from the cytoplasm into the mitochondrial matrix. This process involves two key stages. First, pyruvate undergoes complete oxidation through a series of reactions, during which all its hydrogen atoms are systematically removed, resulting in the formation of three molecules of carbon dioxide (CO₂) per pyruvate molecule. Second, the electrons removed along with hydrogen atoms are transferred to molecular oxygen (O₂), and this transfer is coupled with the synthesis of ATP, the energy currency of the cell. These two stages are compartmentalized within the mitochondria—while the oxidation of pyruvate occurs in the mitochondrial matrix, the electron transport and ATP synthesis processes take place on the inner mitochondrial membrane. The pyruvate formed from the glycolysis of carbohydrates in the cytosol is transported into the mitochondria, where it undergoes oxidative decarboxylation, a complex process catalyzed by an enzyme complex known as pyruvate dehydrogenase. This enzyme requires the help of coenzymes such as NAD⁺ (Nicotinamide Adenine Dinucleotide) and Coenzyme A, along with the presence of magnesium ions (Mg²⁺). During this reaction, pyruvic acid reacts with NAD⁺ and Coenzyme A to produce acetyl CoA, CO₂, NADH, and H⁺. From the metabolism of two pyruvate molecules (derived from one glucose molecule), two NADH molecules are generated. The resulting acetyl CoA then enters the tricarboxylic acid cycle (TCA cycle), also known as the Krebs cycle, which was first described by the scientist Hans Krebs. This cycle plays a vital role in the complete breakdown of acetyl CoA and the production of additional NADH, FADH₂, and ATP through a series of enzymatic reactions that are central to aerobic energy production in living organisms.
5. Metabolic Pathway Releasing Energy and CO₂
The Tricarboxylic Acid Cycle (TCA cycle) begins when an acetyl group combines with oxaloacetic acid (OAA) and water to form citric acid. This process is helped by an enzyme called citrate synthase, and Coenzyme A (CoA) is released. The citric acid is then changed into isocitrate, which goes through two steps where carbon dioxide (CO₂) is removed. These steps result in the formation of α-ketoglutaric acid, and then succinyl-CoA. After this, succinyl-CoA is converted into oxaloacetic acid (OAA) again, which helps the cycle continue. During this step, a molecule of GTP (guanosine triphosphate) is made, which is a type of substrate-level phosphorylation. GTP is then changed into GDP, and at the same time, ATP is formed from ADP. There are three steps in this cycle where NAD⁺ becomes NADH + H⁺, and one step where FAD⁺ becomes FADH₂. For this cycle to keep going, the cell must keep making oxaloacetic acid and also must change NADH back into NAD⁺, and FADH₂ back into FAD⁺. The overall result of this part of respiration is that from one molecule of glucose, the cell produces CO₂, eight molecules of NADH + H⁺, two molecules of FADH₂, and only two molecules of ATP in the TCA cycle. At this point, oxygen (O₂) has not been used, and not many ATP molecules have been made yet. You might be wondering why this is still called respiration. The reason is that the NADH + H⁺ and FADH₂ made during this cycle will play a big role in the next step, where oxygen is involved and a large amount of ATP is made.
6. Energy Generation by Electron Carriers
The next step in respiration is to release and use the energy stored in NADH + H⁺ and FADH₂. This happens when they are oxidised in the Electron Transport System (ETS). In this process, electrons are passed through a series of molecules and finally to oxygen (O₂), which results in the formation of water (H₂O). The path through which the electrons move is called the Electron Transport System (ETS), and it is located in the inner membrane of the mitochondria. The electrons from NADH made in the mitochondrial matrix during the citric acid cycle are passed to a protein called NADH dehydrogenase (complex I). From there, the electrons move to a molecule called ubiquinone, which is found in the inner membrane. Ubiquinone also takes electrons from FADH₂ through complex II, which comes from the oxidation of succinate in the TCA cycle. The reduced form of ubiquinone, called ubiquinol, then passes the electrons to cytochrome c through the cytochrome bc₁ complex (complex III). Cytochrome c is a small protein that carries electrons to complex IV, which is the cytochrome c oxidase complex. This complex contains cytochromes a and a₃ and two copper centers. As the electrons move through these complexes from I to IV, they are linked with ATP synthase (complex V), which makes ATP from ADP and inorganic phosphate. The amount of ATP made depends on the type of electron donor. One molecule of NADH gives 3 ATP, and one molecule of FADH₂ gives 2 ATP. Even though this whole process happens only when oxygen is present (aerobic respiration), oxygen’s role is limited to the final stage. Still, it is very important because oxygen removes hydrogen from the system and works as the final hydrogen acceptor. Unlike in photosynthesis, where light energy creates the proton gradient, in respiration, the energy from oxidation-reduction reactions does this work. That is why the process is called oxidative phosphorylation. You have already learned earlier about how ATP is made using the chemiosmotic hypothesis. The energy from the electron transport system is used by ATP synthase (complex V) to make ATP. This enzyme has two parts: F₁ and F₀. The F₁ part is outside the membrane and is the site where ATP is made from ADP and inorganic phosphate. The F₀ part is in the membrane and forms a channel for protons (H⁺) to pass through. The flow of protons through F₀ drives the F₁ part to make ATP. To make one molecule of ATP, four protons (4H⁺) move from the intermembrane space into the mitochondrial matrix following the proton gradient.
7. Energy Accounting of Cellular Respiration
- We can try to calculate the net gain of ATP from the complete breakdown of one molecule of glucose, but this is only a theoretical estimate. These calculations are based on a few assumptions. First, it is assumed that all the steps of glycolysis, TCA cycle, and Electron Transport System (ETS) happen one after another in a perfect order. Second, it is assumed that the NADH made in glycolysis is sent into the mitochondria and used in oxidative phosphorylation. Third, it is assumed that none of the substances made during these processes are used for making other compounds. Lastly, it is assumed that only glucose is being used for respiration and no other substances are entering or exiting the cycle. But in real living cells, these conditions are not fully true. All the metabolic pathways work at the same time, not in a strict order. Substances can join or leave the pathways whenever needed. ATP is made and used depending on the cell’s needs, and the speed of enzyme actions is controlled in many ways. Still, doing this calculation helps us understand how well the cell can extract and store energy. Based on the theoretical model, one molecule of glucose can give a total of 38 ATP molecules during aerobic respiration.
- Now, let us look at the differences between fermentation and aerobic respiration. In fermentation, glucose is only partly broken down, while in aerobic respiration, glucose is fully broken down into carbon dioxide (CO₂) and water (H₂O). In fermentation, only 2 ATP molecules are made from each molecule of glucose, but in aerobic respiration, many more ATP molecules are produced. Also, in fermentation, NADH changes back into NAD⁺ very slowly. But in aerobic respiration, this reaction happens much more quickly and efficiently.
8. Link Between Catabolism and Anabolism
Glucose is the main and most commonly used substrate for respiration. Usually, all other carbohydrates are first converted into glucose before they can be used in respiration. However, other types of molecules like fats and proteins can also be used, but they enter the respiratory pathway at later steps, not at the beginning. For example, fats must first be broken into glycerol and fatty acids. Fatty acids are then converted into acetyl CoA before they enter the respiration process. Glycerol changes into PGAL (phosphoglyceraldehyde) and joins the pathway. Proteins are broken by protease enzymes into amino acids, and after a process called deamination, they enter the pathway either as pyruvate, acetyl CoA, or at various steps in the Krebs’ cycle depending on their type. Since respiration mainly involves the breakdown of food molecules to release energy, it is often called a catabolic process, and the respiratory pathway is usually considered a catabolic pathway. But is this idea fully correct? We have already seen that different substrates enter the respiratory pathway at different points to release energy. But the same compounds can also be taken out from the pathway and used to make other molecules. For example, when fatty acids are used for energy, they are broken down into acetyl CoA. But when the body needs to make fatty acids, it uses acetyl CoA from the respiration pathway. So the pathway helps in both breaking down and building of fatty acids. The same is true for proteins – the intermediate compounds in respiration are used during both the breakdown and the making of proteins. The process of breaking down molecules is called catabolism, and the process of making new molecules is called anabolism. Since the respiratory pathway helps in both catabolism and anabolism, it is better to call it an amphibolic pathway, not just a catabolic one.
9. Ratio of CO₂ Released to O₂ Consumed
Let us now understand another important part of respiration. In aerobic respiration, oxygen (O₂) is used, and carbon dioxide (CO₂) is released. The respiratory quotient (RQ), also called the respiratory ratio, is the ratio of the volume of CO₂ produced to the volume of O₂ used during respiration. It is calculated using this formula:
RQ = volume of CO₂ evolved / volume of O₂ consumed.
The value of RQ depends on the type of respiratory substrate used by the organism. If carbohydrates like glucose are used and completely broken down, the amount of CO₂ released and O₂ consumed is the same. So the RQ = 1. For example, in the reaction:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy,
we get RQ = 6/6 = 1.
But when fats are used in respiration, they need more oxygen and release less carbon dioxide, so the RQ is less than 1. For example, when a fat like tripalmitin (C₅₁H₉₈O₆) is used, the equation is:
2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O + energy,
so RQ = 102/145 ≈ 0.7.
When proteins are used as the respiratory substrate, the RQ is about 0.9.
But in real living organisms, usually a mixture of substrates like carbohydrates, fats, and proteins are used together in respiration. Pure fats or pure proteins are almost never used alone as the only respiratory source.
10. Key Concept Recap
Unlike animals, plants do not have special breathing systems or organs for gaseous exchange. Instead, they use small openings called stomata and lenticels to allow gases to move in and out by diffusion. Most of the living cells in plants are close to air and can exchange gases easily. The process where carbon-carbon (C–C) bonds in large organic molecules are broken by oxidation to release energy is called cellular respiration. The most preferred substrate for respiration is glucose, but fats and proteins can also be used to release energy. The first step of respiration takes place in the cytoplasm, where glucose is broken into two molecules of pyruvic acid through enzyme-controlled steps. This is known as glycolysis. What happens to pyruvate depends on whether oxygen is available and on the type of organism. If there is no oxygen (anaerobic conditions), lactic acid fermentation or alcohol fermentation may occur. Fermentation takes place in anaerobic conditions in many prokaryotes, unicellular eukaryotes, and germinating seeds. In eukaryotic organisms, if oxygen is present, aerobic respiration happens. In this case, pyruvic acid enters the mitochondria and is changed into acetyl CoA, releasing carbon dioxide (CO₂). This acetyl CoA goes into the tricarboxylic acid (TCA) cycle, also known as the Krebs’ cycle, which happens in the mitochondrial matrix. During the Krebs’ cycle, NADH + H⁺ and FADH₂ are formed. The energy stored in these molecules, along with the NADH + H⁺ from glycolysis, is used to make ATP. This happens through a system called the Electron Transport System (ETS) found in the inner membrane of mitochondria. As electrons move through the ETS, they release energy that is used to make ATP. This process is known as oxidative phosphorylation. In this step, oxygen (O₂) acts as the final electron acceptor and becomes water. The respiratory pathway is called an amphibolic pathway because it helps in both breaking down (catabolism) and building up (anabolism) of substances. The value of the Respiratory Quotient (RQ) depends on the type of substrate being used during respiration.
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