Life Processes Exercise Solutions | KalviGavani

In unicellular organisms, the entire cell surface is in direct contact with the environment, so simple diffusion can meet oxygen needs. In multicellular organisms, cells are organised into tissues, organs, and systems. Cells deep inside the body are far from the external environment. The distance over which oxygen must diffuse is too large, and the rate of simple diffusion is far too slow to supply oxygen to billions of metabolically active cells. Therefore, multicellular organisms need specialised systems — a respiratory system (to take in O₂ and release CO₂) and a circulatory system (to transport O₂ to every cell and carry away CO₂).

The fundamental criterion is the presence of life processes — visible or molecular-level activities that maintain the living state. These include:

• Movement — not just locomotion, but molecular movement (metabolism) inside cells.
• Nutrition — taking in and processing food for energy and materials.
• Respiration — releasing energy from food.
• Growth — increase in size or cell number.
• Response to stimuli — reacting to environmental changes.
• Reproduction — producing offspring.
• Excretion — removing metabolic wastes.

Even when visible movement is absent (e.g., a dormant seed), metabolic reactions continue at the molecular level, confirming life.

Organisms require external raw materials for maintaining life processes:

• Food (organic compounds) — source of carbon, energy, and building materials for growth and repair. Autotrophs use CO₂ as the carbon source; heterotrophs consume ready-made organic food.
• Oxygen — required as the terminal electron acceptor in aerobic respiration to oxidise food and release energy.
• Water — essential solvent for all biochemical reactions, raw material for photosynthesis, and medium for transport within the body.

The essential life processes are:

1. Nutrition — obtaining and processing food to provide energy and raw materials for the body. 2. Respiration — the chemical process of breaking down food to release energy in the form of ATP. 3. Transportation — moving nutrients, oxygen, waste, and other substances to and from all parts of the body. 4. Excretion — removing harmful metabolic waste products (urea, CO₂, etc.) from the body to maintain homeostasis.

Without any one of these, the organism cannot survive for long.

There are two main modes:

1. Autotrophic nutrition — organisms synthesise their own food from simple inorganic substances (CO₂ and water) using an external energy source. Example: green plants perform photosynthesis; some bacteria perform chemosynthesis. 2. Heterotrophic nutrition — organisms cannot make their own food and depend on autotrophs or other heterotrophs. Types include:

• Holozoic — ingestion, digestion, and absorption of solid food (e.g., humans, Amoeba).
• Saprophytic — feeding on dead and decaying organic matter (e.g., fungi, bacteria).
• Parasitic — deriving nutrition from a living host organism, harming it (e.g., Cuscuta, Plasmodium, tapeworm).
• Symbiotic — mutual benefit between two organisms (e.g., Rhizobium in root nodules of legumes fixes nitrogen; plant provides shelter and food).

Air (atmosphere) contains a much higher concentration of oxygen (~21%) than water (dissolved oxygen is very low and varies with temperature). Terrestrial organisms breathe oxygen-rich air directly, so they obtain oxygen more efficiently and with less effort. Aquatic organisms must breathe water and extract dissolved oxygen, which requires faster breathing rates and more energy because the oxygen content of water is far lower. Also, as temperature rises, dissolved oxygen in water decreases, making it even harder for aquatic organisms.

Glucose is oxidised through two main pathways depending on oxygen availability:

1. Aerobic respiration (oxygen present):

• Glycolysis in cytoplasm: Glucose → 2 Pyruvate + 2 ATP
• In mitochondria: Pyruvate → CO₂ + H₂O + 34–36 ATP
• Total: 36–38 ATP per glucose molecule

2. Anaerobic respiration (oxygen absent):

• Glycolysis in cytoplasm: Glucose → 2 Pyruvate + 2 ATP
• In yeast: Pyruvate → Ethanol + CO₂ + Energy (2 ATP total) — fermentation
• In muscle cells: Pyruvate → Lactic acid + Energy (2 ATP total) — lactic acid pathway

Additionally, some organisms can switch between aerobic and anaerobic based on oxygen availability (facultative anaerobes).

Oxygen transport:

• Oxygen from alveolar air diffuses into the blood capillaries surrounding the alveoli.
• About 97% of oxygen is carried by haemoglobin in RBCs as oxyhaemoglobin (HbO₂).
• About 3% is dissolved directly in blood plasma.

Carbon dioxide transport:

• CO₂ is more soluble in water, so about 70% is transported as bicarbonate ions (HCO₃⁻) dissolved in plasma. The reaction: CO₂ + H₂O → H₂CO₃ → HCO₃⁻ + H⁺ (catalysed by carbonic anhydrase in RBCs).
• About 20–25% is carried by haemoglobin as carbaminohaemoglobin.
• About 5–7% is dissolved directly in plasma.

The lungs maximise gas exchange through structural specialisations:

• Branching airways: The trachea divides into two bronchi, which branch repeatedly into smaller bronchioles, ending in tiny air sacs called alveoli.
• Enormous number of alveoli: Each lung contains about 300 million alveoli, providing a total surface area of approximately 70–80 m² (nearly the size of a badminton court).
• Thin walls: Each alveolus has a wall only one cell thick, and the surrounding capillary wall is also one cell thick — the total diffusion barrier is just two cells thick, minimising the distance for gas exchange.
• Dense capillary network: Each alveolus is surrounded by a dense mesh of blood capillaries, ensuring a large area of contact between blood and air.
• Moist inner surface: The inner lining of alveoli is moist, allowing gases to dissolve before diffusing.
• Elastic fibres: Allow alveoli to expand during inhalation and recoil during exhalation.

This design follows the principle that the rate of diffusion is proportional to surface area and inversely proportional to the diffusion distance.

Plants do not have specialised excretory organs. They use the following methods:

• Oxygen produced during photosynthesis is released into the atmosphere through stomata — this is an excretory product of photosynthesis.
• Carbon dioxide produced during respiration is also released through stomata and lenticels.
• Excess water is removed by transpiration through stomata.
• Waste products like tannins, resins, gums, and latex are stored in dead cells of bark, old leaves, or vacuoles. When leaves fall or bark is shed, these wastes are eliminated.
• Some wastes are stored as crystals (e.g., calcium oxalate crystals in some plant cells) in inactive forms.
• Some nitrogenous wastes are recycled — plants convert them into amino acids or other useful compounds.

Fats are digested in the small intestine through the following steps:

1. Emulsification by bile: The liver secretes bile, which is stored in the gallbladder and released into the small intestine. Bile salts emulsify large fat globules — breaking them into numerous tiny droplets. This dramatically increases the surface area available for enzyme action. Bile does not chemically digest fats; it is a physical process.

2. Enzymatic digestion by lipase: The pancreas secretes pancreatic lipase into the small intestine. Lipase acts on the emulsified fat droplets and hydrolyses them into fatty acids and glycerol:

• Fats (triglycerides) → Fatty acids + Glycerol

3. Absorption: The products of fat digestion (fatty acids and glycerol) are absorbed into the lacteals (lymphatic capillaries) within the villi of the small intestine, from where they enter the lymphatic system and eventually the bloodstream.

The small intestine is the principal site of fat digestion because it is where both bile and pancreatic lipase are available.

Saliva is secreted by three pairs of salivary glands (parotid, submandibular, sublingual) and performs the following roles:

1. Enzymatic digestion: Saliva contains salivary amylase (ptyalin), which begins the chemical digestion of starch. It hydrolyses starch into maltose (a disaccharide):

• Starch + Water → Maltose (salivary amylase, pH ~6.8)
• This is the only carbohydrate digestion that occurs in the mouth.

2. Lubrication: Saliva moistens and lubricates food, making it easier to chew (masticate) and form into a soft ball called a bolus for swallowing.

3. Protection: Saliva contains lysozyme, which has antibacterial properties and helps maintain oral hygiene by killing some bacteria.

4. Taste: Dissolved food molecules interact with taste buds on the tongue, enabling the sensation of taste.

5. pH maintenance: Saliva keeps the mouth slightly alkaline, which is optimal for salivary amylase activity.

Necessary conditions for autotrophic nutrition (photosynthesis):

1. Sunlight — the energy source that drives the reaction; trapped by chlorophyll. 2. Carbon dioxide (CO₂) — the carbon source; enters the leaf through stomata from the atmosphere. 3. Water (H₂O) — the hydrogen source; absorbed by roots from the soil and transported through xylem. 4. Chlorophyll — the green pigment in chloroplasts that absorbs light energy (especially red and blue wavelengths). 5. Suitable temperature — enzyme activity is temperature-sensitive (optimal around 25–35°C).

Byproducts of photosynthesis:

• Oxygen (O₂) — released into the atmosphere as a byproduct of the photolysis of water. This is the primary byproduct.
• Water vapour — released through stomata during transpiration (which accompanies gas exchange for photosynthesis).

Organisms using anaerobic respiration:

• Yeast (Saccharomyces cerevisiae) — anaerobic respiration is called fermentation; produces ethanol and CO₂ (used in brewing and baking).
• Some bacteria — e.g., lactic acid bacteria (Lactobacillus) used in curd making; Clostridium.
• Muscle cells in humans — during strenuous exercise when oxygen supply is insufficient; produces lactic acid, causing muscle cramps.
• Parasitic worms — e.g., Ascaris (tapeworm) lives in the oxygen-poor intestine and respires anaerobically.

The lungs are designed to maximise gas exchange through the following structural features:

1. Extensive branching: The trachea splits into two bronchi, which branch into progressively smaller bronchioles, finally terminating in tiny sac-like structures called alveoli. This branching vastly increases the number of terminal units.

2. Enormous surface area: There are approximately 300 million alveoli in both lungs combined, providing a total surface area of about 70–80 m² — roughly the size of a tennis court. This large area maximises the rate of gas exchange.

3. Ultra-thin diffusion barrier: Each alveolus is lined by a single layer of flattened epithelial cells (squamous epithelium). The adjacent capillary wall is also one cell thick. The total diffusion distance (alveolar wall + capillary wall) is only about 0.5 micrometres, allowing rapid diffusion of gases.

4. Rich capillary network: Each alveolus is wrapped tightly in a dense mesh of blood capillaries. This ensures that a large volume of blood is always in close contact with alveolar air, maintaining a steep concentration gradient for both O₂ and CO₂.

5. Moist inner surface: The inner lining of alveoli is coated with a thin film of water (surfactant reduces surface tension and prevents collapse). Gases must dissolve in this moisture before diffusing.

6. Elastic recoil: Elastic fibres in alveolar walls allow the lungs to stretch during inhalation and recoil during exhalation, maintaining efficient ventilation.

These design features ensure that the rate of gas exchange meets the high metabolic demand of the human body.

Haemoglobin (Hb) is the iron-containing respiratory pigment in red blood cells that binds oxygen and transports it from the lungs to all body tissues. A deficiency of haemoglobin causes a condition called anaemia, with the following consequences:

1. Reduced oxygen-carrying capacity of blood — less haemoglobin means less oxygen can be transported per unit volume of blood. 2. Fatigue and weakness — body cells receive insufficient oxygen for aerobic respiration, resulting in less ATP production and low energy levels. 3. Breathlessness (dyspnoea) — the body compensates by increasing the breathing rate to take in more oxygen, leading to shortness of breath even during mild exertion. 4. Pale skin and mucous membranes — haemoglobin gives blood its red colour; reduced Hb causes pallor. 5. Rapid heartbeat (tachycardia) — the heart pumps faster to circulate the limited oxygen more rapidly, straining the cardiovascular system. 6. Dizziness and headache — the brain is highly sensitive to oxygen deprivation. 7. Impaired growth in children — cells cannot grow and divide normally without adequate energy supply.

Common causes of haemoglobin deficiency include iron deficiency, vitamin B₁₂ deficiency, blood loss, and genetic disorders like sickle cell anaemia.

Double circulation means that blood passes through the heart twice during one complete cycle through the body. The two circuits are:

• Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cava.
• The right atrium contracts, pushing blood into the right ventricle through the tricuspid valve.
• The right ventricle contracts, pumping deoxygenated blood through the pulmonary artery to the lungs.
• In the lungs, CO₂ is released and O₂ is picked up.
• Oxygenated blood returns to the left atrium via the pulmonary veins.

• Oxygenated blood from the left atrium enters the left ventricle through the bicuspid (mitral) valve.
• The left ventricle contracts forcefully, pumping oxygenated blood through the aorta to all body organs.
• Oxygen is delivered to tissues; CO₂ is collected from tissues.
• Deoxygenated blood returns to the right atrium via the vena cava, completing the cycle.

Why is double circulation necessary?

• It keeps oxygenated and deoxygenated blood completely separate, ensuring that only fully oxygenated blood is sent to body tissues.
• Mammals and birds are warm-blooded (homeothermic) and maintain a constant body temperature, which requires a high metabolic rate and large oxygen supply.
• Double circulation allows blood to be pumped at high pressure to the body (systemic) while protecting the delicate lung capillaries from damage (pulmonary circuit operates at lower pressure).
• It ensures efficient delivery of oxygen and nutrients to all cells, supporting the high energy demands of warm-blooded animals.

Both alveoli and nephrons exemplify the biological principle of increasing surface area to maximise exchange, and both are surrounded by extensive capillary networks to maintain close contact between the exchange surface and blood.