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Chapter 10 Respiration In Organisms
We all need energy to perform daily activities. Have you ever wondered why we breathe faster after strenuous exercise like running? This is closely related to the process of **respiration**, which provides energy to all living organisms.
Why Do We Resp ire?
All living organisms are composed of tiny units called **cells**, which are the smallest structural and functional units. Each cell within an organism carries out essential life functions such as nutrition, transport, excretion, and reproduction. To perform these functions, cells constantly require **energy**. This need for energy exists even when we are resting, eating, sleeping, or reading.
The energy required for life processes is stored in the **food** we eat. This stored energy is released through the process of **respiration**. Therefore, all living organisms must respire to obtain energy from their food.
During the act of breathing, we take in air, which contains oxygen. This inhaled oxygen is transported to all parts of the body, eventually reaching every single cell. Inside the cells, the oxygen helps in the breakdown of food (specifically glucose, a type of carbohydrate from digested food).
The process where food is broken down within the cell, releasing energy, is called **cellular respiration**. Cellular respiration occurs in the cells of all organisms.
The most common form of cellular respiration involves the use of oxygen:
- Aerobic Respiration: This is the breakdown of glucose in the presence of oxygen. It yields a significant amount of energy, along with carbon dioxide and water.
$\textsf{Glucose} \xrightarrow{\textsf{in the presence of oxygen}} \textsf{carbon dioxide} + \textsf{water} + \textsf{energy}$
Some organisms can survive and obtain energy even without air (oxygen). These are called **anaerobes**, and they perform **anaerobic respiration**.
- Anaerobic Respiration: This is the breakdown of glucose without using oxygen. It releases energy, but generally much less than aerobic respiration. The products vary depending on the organism.
For example, in organisms like **yeast** (single-celled), anaerobic respiration breaks down glucose into alcohol and carbon dioxide. Yeast is used in the production of wine and beer due to its ability to produce alcohol anaerobically.
$\textsf{Glucose} \xrightarrow{\textsf{in the absence of oxygen}} \textsf{alcohol} + \textsf{carbon dioxide} + \textsf{energy}$
Our own **muscle cells** can also respire anaerobically, but only for short periods when the supply of oxygen is temporarily insufficient, such as during heavy physical activity.
During intense exercise (fast running, cycling, heavy weight lifting), the body's demand for energy is very high. However, the supply of oxygen delivered by breathing and circulation might not be enough to meet this demand through aerobic respiration alone. In such situations, muscle cells resort to anaerobic respiration to produce extra energy quickly:
$\textsf{Glucose} \xrightarrow{\textsf{in the absence of oxygen (in muscle)}} \textsf{lactic acid} + \textsf{energy}$
The accumulation of **lactic acid** in muscle cells during this temporary anaerobic respiration causes muscle cramps after heavy exercise. Getting relief from cramps through a hot water bath or massage is because these methods improve blood circulation. Enhanced blood flow increases the supply of oxygen to the muscle cells. This increased oxygen allows the lactic acid to be completely broken down into carbon dioxide and water, removing the cause of the cramps.
Breathing
**Breathing** is the process by which an organism takes in air rich in oxygen and gives out air rich in carbon dioxide. It is a vital part of respiration.
- **Inhalation:** The process of taking in air rich in oxygen into the body.
- **Exhalation:** The process of giving out air rich in carbon dioxide from the body.
Breathing is a continuous process that occurs throughout an organism's life.
Activity 10.1
Trying to hold your breath by closing your nostrils and mouth demonstrates that breathing is essential for survival and that the body needs to breathe regularly. The urge to breathe after holding your breath shows the body's need for oxygen and the accumulation of carbon dioxide.
The **breathing rate** is defined as the number of times a person breathes in a minute. One breath consists of one inhalation followed by one exhalation. The breathing rate is not constant; it changes depending on the body's demand for oxygen.
An average adult human at rest typically breathes 15 to 18 times per minute. However, during heavy physical activity, the breathing rate can significantly increase, up to 25 times or more per minute.
Activity 10.2
Measuring one's breathing rate at rest and after different levels of physical activity (brisk walk, running) shows that the breathing rate increases with increasing activity level. This demonstrates that when the body needs more energy (during exercise), the breathing rate speeds up to supply more oxygen to the cells. More oxygen leads to faster breakdown of food and greater energy release, explaining why activities make us breathe faster and feel hungry.
Table 10.1: Changes in breathing rate under different conditions (Example Structure):
| Name of the classmate | Breathing rate (breaths/minute) | |||
|---|---|---|---|---|
| Normal | After a brisk walk for 10 minutes | After running fast 100 m | At rest | |
| Self | ... | ... | ... | ... |
| Classmate 1 | ... | ... | ... | ... |
| ... | ... | ... | ... | ... |
Activity 10.3
Ordering images depicting various activities (sleeping, reading, walking, running) based on how they would affect breathing rate (from slowest to fastest) reinforces the relationship between activity level and breathing needs.
Feeling drowsy or sleepy can slow down breathing, which might affect the oxygen supply to the body cells. Yawning is sometimes a response to reduced oxygen levels or increased carbon dioxide in the blood.
How Do We Breathe?
Breathing in humans involves a specific mechanism involving several organs and structures.
Air is typically inhaled through the **nostrils**. From the nostrils, it passes into the **nasal cavity**. It then travels down the **windpipe** to reach the **lungs**, which are located within the **chest cavity**.
The chest cavity is surrounded by the **ribs** on the sides. Forming the floor of the chest cavity is a large, muscular sheet called the **diaphragm**. The process of breathing (inhalation and exhalation) is driven by the coordinated movements of the diaphragm and the rib cage.
- **Inhalation:** During inhalation, the **ribs move up and outwards**, and the **diaphragm moves downwards**. This combined movement increases the volume of the chest cavity. As the volume increases, the pressure inside the chest cavity decreases, causing air from outside (where pressure is higher) to rush into the lungs, filling them with air.
- **Exhalation:** During exhalation, the **ribs move down and inwards**, and the **diaphragm moves upwards** to its original position. This movement decreases the volume of the chest cavity. As the volume decreases, the pressure inside increases, forcing the air out of the lungs.
These movements cause a noticeable change in the size of the chest cavity, which can be felt by placing a hand on the chest or abdomen while breathing deeply.
Activity 10.4
Measuring the size of the chest using a measuring tape during both deep inhalation (chest expanded) and complete exhalation (chest relaxed) quantifies the change in chest size caused by breathing. Comparing the measurements among classmates highlights individual differences in lung capacity or breathing effort.
Table 10.2: Effect of breathing on chest size (Example Structure):
| Name of the classmate | Size of the chest (cm) | Difference in size (cm) | |
|---|---|---|---|
| During inhalation | During exhalation | ||
| Self | ... | ... | ... |
| Classmate 1 | ... | ... | ... |
| ... | ... | ... | ... |
Filtering Inhaled Air: The nasal cavity has fine hair and mucus that trap unwanted particles (smoke, dust, pollen) from the inhaled air, preventing them from entering the respiratory system. If particles bypass this filter, they can irritate the nasal lining, triggering a sneeze to expel them. Covering your nose while sneezing prevents expelling particles onto others.
Smoking damages the lungs and is linked to serious diseases like cancer, so it should be avoided.
Activity 10.5
Building a simple model of the respiratory system using a plastic bottle, Y-shaped tube, balloons, and a rubber sheet helps visualise the mechanism of breathing. The bottle represents the chest cavity, the Y-tube represents the windpipe and bronchi, the balloons represent the lungs, and the rubber sheet tied at the bottom represents the diaphragm. Pulling the rubber sheet downwards simulates diaphragm movement during inhalation, causing the volume inside the bottle to increase and the balloons (lungs) to inflate as air is drawn in. Pushing the rubber sheet upwards simulates diaphragm movement during exhalation, decreasing the volume and forcing air out of the balloons (lungs).
What Do We Breathe Out?
The air we breathe out is not just 'used' air; it has a different composition than the air we inhale.
Activity 10.6
Blowing exhaled air through freshly prepared lime water demonstrates that exhaled air is rich in **carbon dioxide**. Lime water (calcium hydroxide solution) turns milky or cloudy when carbon dioxide is passed through it, due to the formation of calcium carbonate. This reaction is a standard test for carbon dioxide and confirms its presence in the air we breathe out (as also seen in Chapter 6).
Exhaled air is a mixture of gases. While it still contains oxygen, the concentration is lower than inhaled air, and the concentration of carbon dioxide is significantly higher.
- Approximate composition of **Inhaled Air**: Oxygen ~21%, Carbon dioxide ~0.04%.
- Approximate composition of **Exhaled Air**: Oxygen ~16.4%, Carbon dioxide ~4.4%.
Exhaled air also contains **water vapour**, which is why breathing onto a cold surface like a mirror causes a film of moisture to appear.
Breathing In Other Animals
While many larger animals breathe using lungs similar to humans, different organisms have evolved diverse respiratory organs and mechanisms adapted to their environment and lifestyle.
- Most larger terrestrial animals (elephants, lions, cows, goats, frogs, lizards, snakes, birds) have **lungs** located in their chest cavity.
- **Earthworms** breathe through their **moist and slimy skin**. Gases (oxygen and carbon dioxide) pass directly through their thin, moist skin.
- **Frogs** are amphibians; they have **lungs**, but can also breathe through their **moist and slippery skin**, both on land and in water.
- **Insects** like cockroaches have a network of air tubes called **tracheae** running throughout their body for gas exchange. Air enters and leaves these tracheal tubes through small openings on the sides of their body called **spiracles**. This tracheal system is unique to insects.
Breathing Under Water
Organisms living in water obtain oxygen that is **dissolved** in the water.
- **Fish** use special respiratory organs called **gills** to breathe underwater. Gills are projections of the skin that are richly supplied with blood vessels. As water passes over the gills, the blood vessels in the gills absorb dissolved oxygen from the water and release carbon dioxide.
- Marine mammals like **whales and dolphins** do not have gills. They have lungs and breathe air directly from the atmosphere through nostrils or a blowhole located on the top of their head. They need to come to the water surface periodically to inhale air.
Do Plants Also Resp ire?
**Yes**, plants are living organisms and require energy for their survival, growth, and processes. Like animals, plants also perform **respiration**. They take in **oxygen** from the air and give out **carbon dioxide**.
In plants, each part can generally exchange gases independently with the environment:
- Leaves have tiny pores called **stomata** (singular: stoma) on their surface, primarily for the exchange of oxygen and carbon dioxide (as discussed in Chapter 1). During respiration, plants take in oxygen and release carbon dioxide through stomata. (Note: Photosynthesis uses carbon dioxide and releases oxygen, but this mainly occurs in sunlight. Respiration occurs continuously, day and night).
- Roots, which are underground, also need oxygen for respiration. Root cells absorb oxygen from the **air spaces** present between the soil particles.
If a potted plant is overwatered, the water fills up the air spaces in the soil, reducing the availability of oxygen to the roots. This can lead to the roots suffocating and potentially damaging or killing the plant.
The process of glucose breakdown in plant cells to release energy is similar to that in other living organisms, involving aerobic respiration where oxygen is used to produce carbon dioxide and water, releasing energy.
Respiration is a fundamental biological process necessary for all living organisms to obtain the energy required for life.