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Respiration in Organisms for Class 7: The Complete CBSE Guide (2026-27)

When you run fast during a football match, your breathing becomes rapid and heavy — but why? Respiration in organisms class 7 answers this question by exploring how every living cell extracts energy from food. Unlike the common confusion between breathing (inhaling oxygen, exhaling carbon dioxide) and respiration (the cellular chemical process breaking down glucose), this CBSE Science chapter reveals that respiration happens inside mitochondria of every cell, whether in a towering banyan tree, a darting housefly, or your own muscle tissue. The 2024-25 NCERT textbook for Class 7 Science structures this chapter around cellular respiration fundamentals, aerobic versus anaerobic pathways, the human breathing mechanism, and the fascinating diversity of respiratory organs across the animal and plant kingdoms.

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Key takeaways

  • ✓Respiration in organisms class 7 distinguishes breathing (physical gas exchange) from cellular respiration (chemical energy release in mitochondria).
  • ✓Aerobic respiration requires oxygen and yields 38 ATP molecules per glucose, while anaerobic respiration occurs without oxygen and produces only 2 ATP plus lactic acid or ethanol.
  • ✓The human breathing mechanism involves inhalation through diaphragm contraction (expanding chest cavity) and exhalation through diaphragm relaxation (reducing volume).
  • ✓Plants respire through stomata in leaves and lenticels in stems, conducting gas exchange 24 hours despite photosynthesis occurring only in daylight.
  • ✓Insects use a tracheal system with spiracles and air tubes delivering oxygen directly to cells without needing blood transport.
  • ✓Fish extract dissolved oxygen from water using gills with thin, blood-rich filaments that maximize surface area for gas diffusion.
  • ✓Earthworms and frogs can respire through moist skin, with oxygen dissolving in the mucus layer and diffusing into blood capillaries beneath the surface.

What is Respiration in Organisms? Breaking Down the NCERT Definition

Respiration in organisms class 7 defines respiration as a biochemical process occurring in living cells where food (primarily glucose) is broken down with or without oxygen to release energy in the form of ATP (adenosine triphosphate). The NCERT textbook emphasizes that respiration is NOT the same as breathing. Breathing is the mechanical process of taking in air and expelling it, whereas respiration is the chemical oxidation of glucose happening inside mitochondria. Every organism — from single-celled amoeba to blue whales — respires continuously to fuel cellular activities like growth, repair, movement and reproduction. The energy currency ATP powers everything from muscle contraction to nerve impulse transmission. This distinction is tested repeatedly in CBSE exams: a typical 2-mark question asks students to differentiate breathing from respiration with two points each. Understanding that respiration is universal (all living cells do it) while breathing mechanisms vary (lungs in mammals, gills in fish, skin in earthworms) forms the conceptual spine of this chapter.
  • Respiration = cellular chemical process releasing energy from glucose inside mitochondria
  • Breathing = physical gas exchange moving air in and out of the body
  • Respiration occurs 24×7 in every living cell, including plant cells during day and night
  • The general equation: Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
  • Mitochondria are called powerhouses because aerobic respiration happens on their inner membrane folds (cristae)

Aerobic Respiration: The Oxygen-Dependent Energy Pathway

Aerobic respiration is the complete breakdown of glucose in the presence of oxygen, yielding maximum energy output. Respiration in organisms class 7 notes must highlight the chemical equation: C₆H₁₂O₆ (glucose) + 6O₂ → 6CO₂ + 6H₂O + 38 ATP. This process occurs in three stages: glycolysis in the cytoplasm (breaking glucose into pyruvate, yielding 2 ATP), the Krebs cycle in the mitochondrial matrix (processing pyruvate to produce electron carriers), and the electron transport chain on the inner mitochondrial membrane (generating the bulk of the 38 ATP molecules). For Class 7, NCERT keeps the focus on the inputs (glucose and oxygen) and outputs (carbon dioxide, water and energy), reserving deeper biochemical steps for Class 10. The carbon dioxide produced is carried by blood to the lungs and exhaled, which is why we breathe out more CO₂ than we breathe in. During vigorous exercise when oxygen supply is adequate, muscles rely entirely on aerobic respiration. A common 3-mark exam question asks students to write the balanced equation and state where in the cell aerobic respiration occurs.

Anaerobic Respiration: Survival Without Oxygen

Anaerobic respiration occurs when cells break down glucose without using oxygen, yielding far less energy. Respiration in organisms class 7 distinguishes two types: lactic acid fermentation (in animal muscle cells and some bacteria) and alcoholic fermentation (in yeast and some plant tissues). In muscle cells during intense exercise — sprinting, heavy lifting — oxygen supply cannot keep pace with demand. Glycolysis continues, but pyruvate is converted to lactic acid instead of entering the Krebs cycle. The equation: C₆H₁₂O₆ → 2C₃H₆O₃ (lactic acid) + 2 ATP. The accumulation of lactic acid causes muscle cramps and fatigue; once exercise stops and oxygen is available, the lactic acid is transported to the liver and converted back to glucose. In yeast, anaerobic respiration produces ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH (ethanol) + 2CO₂ + 2 ATP. This principle underlies bread-making (CO₂ makes dough rise) and brewing (ethanol is the alcohol in beer and wine). A popular 5-mark question asks students to compare aerobic and anaerobic respiration in a tabular format.

The Human Breathing Mechanism: Inhalation and Exhalation Explained

The CBSE class 7 science respiration in organisms chapter details the human respiratory system: nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and alveoli in the lungs. Breathing involves two phases. During inhalation, the diaphragm (a dome-shaped muscle below the lungs) contracts and flattens, while intercostal muscles between ribs contract, pulling the rib cage upward and outward. This increases the chest cavity volume, decreasing internal air pressure below atmospheric pressure, so air rushes into the lungs. During exhalation, the diaphragm relaxes and moves upward, intercostal muscles relax, the rib cage moves downward and inward, reducing chest volume and increasing pressure, forcing air out. The NCERT diagram of the human respiratory system is a frequent 3-mark sketch question. Alveoli are tiny air sacs (approximately 300 million in adult lungs) where actual gas exchange occurs: oxygen diffuses from alveolar air into capillary blood, and carbon dioxide diffuses from blood into alveolar air. The total surface area of alveoli is roughly 70 square meters, comparable to a badminton court, maximizing diffusion efficiency.
  • Diaphragm contracts → chest expands → pressure drops → air flows in (inhalation)
  • Diaphragm relaxes → chest contracts → pressure rises → air flows out (exhalation)
  • Trachea divides into two bronchi, each entering one lung and branching into bronchioles
  • Alveoli are surrounded by a network of capillaries for oxygen-carbon dioxide exchange
  • Breathing rate increases during exercise to supply more oxygen for aerobic respiration in muscles

Respiration in Plants: Stomata, Lenticels and Continuous Gas Exchange

Respiration in organisms class 7 emphasizes that plants respire 24 hours a day, even though photosynthesis (which produces oxygen) happens only during daylight. Plants take in oxygen and release carbon dioxide continuously through stomata (tiny pores on leaf surfaces, mostly on the underside) and lenticels (small openings in the bark of woody stems and roots). Each stoma is flanked by two guard cells that open and close the pore. During the day, the rate of photosynthesis exceeds respiration, so net gas exchange shows oxygen release; at night, only respiration occurs, so plants release CO₂ and take in O₂ just like animals. Roots, which are underground and lack chlorophyll, depend entirely on oxygen diffusing through soil air spaces. Waterlogged soil suffocates roots because water displaces air, cutting off oxygen supply — a fact that explains why overwatering kills potted plants. The NCERT diagram of a stoma with guard cells is a standard 2-mark labeling question in the CBSE exam. A common misconception is that plants do not respire; this chapter corrects that by stressing respiration is universal in all living cells.

Respiration in Insects: The Tracheal System and Spiracles

Insects such as grasshoppers, cockroaches and butterflies do not have lungs. Instead, respiration in organisms class 7 introduces the tracheal system: a network of air-filled tubes (tracheae) opening to the outside through small holes called spiracles located along the sides of the insect's body. Air enters spiracles, travels through progressively finer branches (tracheoles) and delivers oxygen directly to every tissue and cell. This direct delivery system eliminates the need for blood to transport oxygen, which is why insect blood (hemolymph) does not carry respiratory gases. The tracheal system is highly efficient for small body sizes but limits how large insects can grow — the reason why giant dragonflies existed millions of years ago when atmospheric oxygen was higher. During flight, insects pump their abdomen to push air through the tracheal network, increasing oxygen supply to flight muscles. A typical 3-mark question asks students to draw and label the tracheal system or explain how insects breathe without lungs.
  • Spiracles are the external openings for air entry, often guarded by valves to prevent water loss
  • Tracheae branch into finer tracheoles that penetrate tissues and reach individual cells
  • No respiratory pigment (like hemoglobin) needed because oxygen diffuses directly through air tubes
  • Muscle contractions during movement help pump air in and out of the tracheal system
  • This system is efficient for insects but would not scale to support large body mass

Respiration in Fish: Gills and Dissolved Oxygen Extraction

Fish live in water and extract dissolved oxygen using gills, specialized organs located on either side of the head beneath protective bony flaps called opercula. Respiration in organisms class 7 NCERT describes the gill structure: each gill consists of numerous thin filaments rich in blood capillaries. Water enters the fish's mouth, flows over the gill filaments and exits through the gill slits. As water passes over the gills, oxygen dissolved in the water diffuses into the blood, and carbon dioxide from the blood diffuses into the water. The large surface area of gill filaments and the counter-current flow mechanism (blood flows in the opposite direction to water flow) maximize oxygen uptake efficiency. Fish cannot survive out of water for long because gill filaments collapse and stick together in air, drastically reducing surface area for gas exchange, and the fish suffocates despite air containing far more oxygen than water. A 2-mark question might ask: 'Why do fish die when taken out of water even though air has more oxygen than water?' The answer hinges on gill structure collapse.

Respiration in Earthworms and Frogs: Skin as a Respiratory Surface

Some organisms use their skin for gas exchange. Earthworms have no lungs or gills; they respire through their moist skin. Oxygen dissolves in the thin layer of mucus on the skin surface and diffuses into capillaries just beneath the epidermis. Carbon dioxide diffuses out in the opposite direction. This cutaneous respiration requires the skin to remain moist at all times — if an earthworm dries out, it suffocates. That is why earthworms emerge from soil during heavy rain: waterlogged soil has little oxygen, so they come to the surface where oxygen is more available. Frogs have a dual system: they possess lungs for breathing air (especially on land and when active) but can also respire through their moist skin, particularly when underwater or during hibernation. The skin of a frog is richly supplied with blood vessels and must stay wet to facilitate gas diffusion. Respiration in organisms class 7 students often face a 2-mark question asking how earthworms breathe. The key points are moist skin, diffusion through skin into blood, and the necessity of moisture for this process.
  • Earthworms secrete mucus to keep skin moist, enabling oxygen dissolution and diffusion
  • Skin must be thin and well-supplied with capillaries for efficient gas exchange
  • Frogs use lungs when on land and active; skin respiration dominates when submerged or at rest
  • Cutaneous respiration is only effective for small organisms with high surface-area-to-volume ratios
  • Drying out is fatal for earthworms because it halts gas exchange across the skin

Respiration Rate and Factors Affecting It in Organisms

Respiration rate (the number of breaths per minute or the rate of oxygen consumption) varies across organisms and situations. In humans, the resting breathing rate is about 12-20 breaths per minute for adults and 20-30 for children. During exercise, this can rise to 40-60 breaths per minute to meet increased oxygen demand for aerobic respiration in muscles. Factors affecting respiration rate in organisms class 7 include body size (smaller animals like mice have faster rates), activity level (active animals respire faster), temperature (cold-blooded animals like fish and frogs have respiration rates that increase with temperature), and availability of oxygen (lower oxygen triggers faster breathing to compensate). Plants respire faster at higher temperatures and during active growth phases. The NCERT textbook describes a simple activity: count your breathing rate at rest for one minute, then run in place for two minutes and count again. The increase demonstrates the link between physical activity and oxygen demand. A common 3-mark application question might present a scenario — a student's breathing rate at rest is 18/min, after exercise it is 45/min — and ask students to explain the physiological reason.

Common Misconceptions in Respiration in Organisms Class 7

Several misconceptions repeatedly appear in CBSE Class 7 student answers. First, many students believe plants do not respire or respire only at night. The truth: plants respire continuously (day and night), but during the day photosynthesis rate exceeds respiration rate, so the net gas exchange is oxygen release. Second, students often confuse breathing with respiration, using the terms interchangeably. Breathing is mechanical (inhaling/exhaling air), while respiration is biochemical (glucose oxidation in mitochondria). Third, there is confusion that anaerobic respiration is 'bad' or occurs only in 'lower organisms'. In fact, human muscle cells routinely switch to anaerobic respiration during intense exercise when oxygen supply is insufficient. Fourth, some students think fish breathe oxygen from H₂O molecules. Fish extract dissolved O₂ gas in water, not oxygen atoms from water molecules. Fifth, students may think only animals respire and plants only photosynthesize. Both processes occur in plants, serving different functions: photosynthesis builds glucose using light energy, respiration breaks down glucose to release energy for cellular work. Addressing these misconceptions directly improves conceptual clarity and exam scores.
  • Misconception 1: Plants respire only at night. Reality: Plants respire 24×7; photosynthesis happens only during daylight.
  • Misconception 2: Breathing equals respiration. Reality: Breathing is gas exchange; respiration is cellular glucose oxidation.
  • Misconception 3: Anaerobic respiration is abnormal. Reality: It is a normal backup pathway during oxygen shortage.
  • Misconception 4: Fish split water molecules for oxygen. Reality: Fish extract dissolved O₂ gas from water via gills.
  • Misconception 5: Respiration and photosynthesis are opposites in plants. Reality: They are complementary — photosynthesis stores energy, respiration releases it.

Important Diagrams and Labeling Questions in Respiration in Organisms Class 7

CBSE Class 7 Science exams regularly include 2-3 mark diagram-based questions from the respiration in organisms chapter. Students must be able to draw and label: (1) the human respiratory system showing nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, lungs, diaphragm and alveoli; (2) a simplified representation of an alveolus with surrounding capillaries illustrating gas exchange; (3) the structure of a stoma with guard cells on a leaf surface; (4) the tracheal system of an insect showing spiracles, tracheae and tracheoles; (5) the gill structure in a fish with filaments and blood flow direction. The NCERT textbook provides clear diagrams for each. When practicing, students should use a sharp pencil, draw neat lines, label with arrows pointing precisely to structures (not vague regions), and write labels horizontally for readability. A labeling mistake such as marking bronchi as bronchioles or confusing lenticels with stomata costs marks. Teachers often recommend students create flashcards with blank diagrams on one side and labeled versions on the other for repeated self-testing. In the 2024-25 exam pattern, diagram questions typically appear in Section B (2-3 marks each).

Formula Sheet and Key Equations for Respiration in Organisms Class 7

While respiration in organisms class 7 is more conceptual than formula-heavy, students must memorize and be able to write the chemical equations for aerobic and anaerobic respiration. (1) Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (38 ATP). Verbally: Glucose plus oxygen yields carbon dioxide plus water plus energy. (2) Anaerobic respiration in muscle cells: C₆H₁₂O₆ → 2C₃H₆O₃ + Energy (2 ATP). Verbally: Glucose yields lactic acid plus energy. (3) Anaerobic respiration in yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy (2 ATP). Verbally: Glucose yields ethanol plus carbon dioxide plus energy. Students should write these equations with correct chemical formulas and arrow notation. A common 2-mark question is: 'Write the word equation for aerobic respiration.' Another asks for the balanced chemical equation. Additionally, understanding that respiration is essentially the reverse of photosynthesis (6CO₂ + 6H₂O + Light energy → C₆H₁₂O₆ + 6O₂) helps conceptual integration across chapters. NCERT does not require students to memorize molecular structures or reaction intermediates at this level, but accurate recall of these three main equations is essential.
  • Aerobic respiration produces the most ATP (38 per glucose molecule) and requires oxygen
  • Anaerobic respiration in animals produces lactic acid and only 2 ATP per glucose
  • Anaerobic respiration in yeast/plants produces ethanol and CO₂, used commercially in fermentation
  • All equations must be written with correct chemical symbols: C for carbon, H for hydrogen, O for oxygen
  • Energy is often written as 'ATP' or 'Energy' in word equations for Class 7 level

Exam Strategy and Important Questions for Respiration in Organisms Class 7

Respiration in organisms carries approximately 8-10 marks in the CBSE Class 7 Science annual exam. Question types include: 1-mark MCQs or true/false (e.g., 'Yeast respires aerobically — True or False?'), 2-mark short answers (e.g., 'State two differences between breathing and respiration'), 3-mark questions (e.g., 'Explain the breathing mechanism in humans with reference to the role of the diaphragm'), and 5-mark long answers (e.g., 'Describe aerobic and anaerobic respiration with equations and examples'). Diagram questions appear regularly (2-3 marks). To prepare, students should first read the NCERT chapter carefully, highlight key terms like stomata, spiracles, alveoli, and memorize the three main chemical equations. Next, practice writing short definitions without looking at the book. Then, draw each diagram at least five times to build muscle memory. Solve at least 20 previous years' and sample questions to identify recurring patterns. Time management is critical: allocate roughly 1 minute per mark, so a 3-mark question should be answered in 3 minutes. Many students lose marks by writing vague answers ('Respiration gives energy') instead of precise ones ('Respiration is the biochemical breakdown of glucose in the presence or absence of oxygen to release ATP, occurring in mitochondria of every living cell'). Precision and use of correct NCERT terminology earn full marks.
  • 1-mark questions test definitions and true/false recall — answer in one crisp sentence
  • 2-mark questions require two clear points or a labeled diagram with 2-3 labels
  • 3-mark questions need structured answers: define the term, explain the process, give an example
  • 5-mark questions often ask for comparisons (aerobic vs anaerobic) or detailed descriptions — use subheadings and bullet points for clarity
  • Diagram questions: draw large enough, use pencil, label with straight arrows, underline labels

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Frequently asked questions

What is the difference between breathing and respiration in organisms class 7?+
Breathing is the physical process of inhaling air (taking oxygen in) and exhaling air (releasing carbon dioxide out) through lungs or other respiratory organs. Respiration is the biochemical process occurring inside every living cell where glucose is broken down in the presence or absence of oxygen to release energy in the form of ATP. Breathing is an external mechanical activity; respiration is an internal chemical reaction happening in mitochondria. All organisms respire, but not all organisms breathe (e.g., single-celled amoeba respires but does not breathe in the way humans do).
Do plants respire during the day or only at night?+
Plants respire continuously, both during the day and at night. Respiration is the process of breaking down glucose to release energy, which every living cell requires 24 hours a day. During daylight, plants also perform photosynthesis, which produces oxygen at a rate higher than the oxygen consumed by respiration, so the net gas exchange appears to be oxygen release. At night, photosynthesis stops (no light), but respiration continues, so plants take in oxygen and release carbon dioxide just like animals. The misconception arises because we notice CO₂ release from plants only at night when photosynthesis is absent.
Why does anaerobic respiration produce less energy than aerobic respiration?+
Aerobic respiration completely breaks down one glucose molecule into carbon dioxide and water through glycolysis, the Krebs cycle and the electron transport chain, producing 38 ATP molecules. Anaerobic respiration only completes glycolysis (because oxygen is absent to run the Krebs cycle and electron transport chain), converting glucose to lactic acid or ethanol and yielding only 2 ATP per glucose molecule. The incomplete breakdown means much of the energy in glucose remains locked in the end products (lactic acid or ethanol), which is why anaerobic respiration is far less efficient. Organisms use it only as a short-term backup when oxygen is unavailable.
How do insects breathe without lungs or gills?+
Insects breathe using a tracheal system consisting of air-filled tubes called tracheae that open to the outside through small pores called spiracles located along the sides of their body. Air enters through spiracles, travels through branching tracheae and finer tracheoles, and delivers oxygen directly to every cell and tissue. This direct delivery eliminates the need for blood to transport oxygen, which is why insect blood does not carry respiratory gases. During movement or flight, insects can pump their abdomen to actively push air through the tracheal network, increasing oxygen supply.
Why do fish die when taken out of water even though air has more oxygen?+
Fish die out of water because their gills are adapted to extract dissolved oxygen from water, not gaseous oxygen from air. Gill filaments are thin, delicate structures that remain spread out and functional when surrounded by water. When a fish is removed from water, the gill filaments collapse and stick together in air, drastically reducing the surface area available for oxygen diffusion. Additionally, the gills dry out quickly, further impairing gas exchange. Even though air contains more oxygen than water, the fish cannot access it because its respiratory organ (gills) does not function properly in air.
What happens to lactic acid produced during anaerobic respiration in muscles?+
When muscles perform intense activity and oxygen supply cannot meet demand, they switch to anaerobic respiration, producing lactic acid as a byproduct. The accumulation of lactic acid in muscle tissue causes muscle fatigue and cramps. Once the intense activity stops and oxygen becomes available again, the lactic acid is transported via the bloodstream to the liver. In the liver, lactic acid is either converted back into glucose (through a process called gluconeogenesis) or fully oxidized into carbon dioxide and water using the now-available oxygen. This is why you continue to breathe heavily even after you stop exercising — you are repaying the 'oxygen debt' and clearing lactic acid.
How do earthworms respire if they have no lungs or gills?+
Earthworms respire through their moist skin, a process called cutaneous respiration. Oxygen from the air or water in the soil dissolves in the thin mucus layer covering the earthworm's skin. The dissolved oxygen then diffuses through the skin into blood capillaries located just beneath the epidermis. Carbon dioxide from the blood diffuses outward through the skin into the environment. This method requires the skin to remain continuously moist; if an earthworm dries out, it cannot absorb oxygen and will suffocate. That is why earthworms live in damp soil and emerge during heavy rain when waterlogged soil reduces oxygen availability underground.
Which organisms use both lungs and skin for respiration?+
Frogs are a classic example of organisms that use both lungs and skin for respiration. When frogs are on land and active, they breathe air using their lungs, taking in oxygen and expelling carbon dioxide. When frogs are underwater or hibernating, they rely on cutaneous respiration (breathing through their skin), which is moist and richly supplied with blood vessels. The ability to use both methods allows frogs to thrive in aquatic and terrestrial environments. Adult frogs have lungs, whereas tadpoles (the larval stage) have gills and respire like fish until they undergo metamorphosis.
Why do we breathe faster during exercise?+
During exercise, muscle cells require significantly more energy to contract and sustain movement. This energy comes from aerobic respiration, which consumes oxygen and produces carbon dioxide. To meet the increased oxygen demand and remove the excess carbon dioxide produced, the brain signals the breathing rate to increase. Faster breathing brings more oxygen into the lungs and blood, ensuring muscles receive sufficient oxygen for aerobic respiration. The heart rate also increases to pump oxygen-rich blood more rapidly to the muscles. If oxygen supply still cannot keep pace with demand during very intense exercise, muscles switch partially to anaerobic respiration, producing lactic acid.
Can respiration in organisms class 7 chapter help in understanding Class 10 biology?+
Absolutely. Respiration in organisms class 7 lays the conceptual foundation for Class 10 Chapter 'Life Processes', which covers respiration in much greater biochemical detail including glycolysis, the Krebs cycle and the electron transport chain. Understanding the basic definitions (breathing vs respiration), the chemical equations for aerobic and anaerobic respiration, the role of mitochondria, and the diversity of respiratory organs in Class 7 makes the Class 10 content far easier to grasp. Students who master this chapter in Class 7 find the CBSE Class 10 board exam questions on respiration more manageable because the core logic and terminology are already familiar.
What are stomata and lenticels, and how do they differ?+
Stomata are tiny pores found primarily on the underside of leaves, each surrounded by two guard cells that regulate opening and closing. They facilitate gas exchange (oxygen in for respiration, carbon dioxide in for photosynthesis, oxygen out from photosynthesis, carbon dioxide out from respiration) and also allow water vapor to escape (transpiration). Lenticels are small, raised pores found in the bark of woody stems, roots and some fruits. They permit gas exchange in parts of the plant covered by thick, non-porous bark. While stomata can actively open and close, lenticels are generally open continuously. Both serve the same function — allowing oxygen in and carbon dioxide out for respiration in plant tissues.
Is yeast respiration aerobic or anaerobic, and why is it important?+
Yeast can respire both aerobically and anaerobically depending on oxygen availability. In the presence of oxygen, yeast performs aerobic respiration, breaking down glucose completely into carbon dioxide and water, releasing 38 ATP per glucose. In the absence of oxygen, yeast switches to anaerobic respiration (alcoholic fermentation), converting glucose into ethanol and carbon dioxide, releasing only 2 ATP per glucose. Anaerobic respiration in yeast is commercially important: the carbon dioxide produced makes bread dough rise (baking industry), and the ethanol produced is the basis of alcoholic beverages like beer and wine (brewing industry). This dual capability makes yeast one of the most economically significant microorganisms.

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