What Are Life Processes?

Life processes are the fundamental activities that living organisms carry out to maintain their survival, prevent damage, and sustain their organised structure. These processes are essential for the continuation of life.

These maintenance processes require **energy**. This energy is obtained from outside the organism's body, typically in the form of **food** (which contains carbon-based molecules). Organisms also need external raw materials for growth and synthesis of body components.

Food sources from the environment are often complex and need to be broken down into simpler substances. This is achieved through a series of chemical reactions, often involving oxidising-reducing reactions. Many organisms use oxygen from outside for this breakdown process.

The process of acquiring energy sources (food) and raw materials from outside the body and transferring them inside is called **nutrition**.

The process of acquiring oxygen from outside the body and using it in the breakdown of food sources for cellular energy needs is called **respiration**.

When chemical reactions involved in energy generation produce by-products that are useless or harmful to the body, these waste materials must be removed. The process of removing these harmful metabolic wastes from the body is called **excretion**.

In single-celled organisms, the entire surface is in contact with the environment, so simple diffusion across the surface can handle the uptake of food, exchange of gases, and removal of wastes.

However, in multicellular organisms, not all cells are in direct contact with the environment. As the body size and complexity increase, diffusion alone is insufficient to meet the requirements of all cells. This leads to **specialisation** of tissues and organs for specific functions (e.g., specialised tissues for taking in food, specialised organs for gas exchange).

This specialisation, where food and oxygen are taken up at one location and waste is produced in cells throughout the body, creates a need for a **transportation system**. A transportation system is required to carry food and oxygen from the points of uptake to all cells in the body, and to transport waste products from the cells to the excretory organs.

Thus, the processes essential for maintaining life in multicellular organisms are:

1. **Nutrition:** Obtaining food and energy from outside.
2. **Respiration:** Utilizing oxygen for the breakdown of food to release energy.
3. **Transportation:** Moving substances (food, oxygen, water, waste) within the body.
4. **Excretion:** Removing metabolic waste products from the body.

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Nutrition

Nutrition is the process by which an organism obtains and utilizes food (sources of energy and raw materials) necessary for its survival, growth, development, and maintenance.

Organisms obtain food in different ways:

• **Autotrophic Nutrition:** Organisms that use simple inorganic materials (carbon dioxide and water) from the environment and convert them into complex organic food substances using an external energy source (like sunlight). These organisms are called **autotrophs**. Green plants and some bacteria are autotrophs. They essentially produce their own food.
• **Heterotrophic Nutrition:** Organisms that obtain complex organic food substances by consuming other organisms (plants, animals, or their products). These complex substances are broken down into simpler ones using enzymes before absorption. These organisms are called **heterotrophs**. Animals and fungi are heterotrophs. Their survival depends directly or indirectly on autotrophs for food.

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Autotrophic Nutrition (Photosynthesis)

**Photosynthesis** is the process by which autotrophs (green plants and some bacteria) carry out autotrophic nutrition. They convert simple inorganic substances (carbon dioxide and water) into complex carbohydrates (food) using light energy absorbed by chlorophyll.

Equation for photosynthesis:

6CO$_2$ + 12H$_2$O $\xrightarrow[\text{Chlorophyll}]{\text{Sunlight}}$ C$_6$H$_{12}$O$_6$ + 6O$_2$ + 6H$_2$O

(Carbon dioxide) (Water) (Glucose) (Oxygen) (Water)

Key events that occur during photosynthesis:

1. **Absorption of light energy:** Chlorophyll, the green pigment present in chloroplasts (cell organelles in plant leaves), absorbs sunlight.
2. **Conversion of light energy to chemical energy and splitting of water:** The absorbed light energy is converted into chemical energy. This energy is used to split water molecules (H$_2$O) into hydrogen and oxygen.
3. **Reduction of carbon dioxide:** Carbon dioxide is reduced to carbohydrates (like glucose) using the chemical energy and hydrogen produced from water splitting.

These steps don't always happen immediately one after the other. For example, desert plants open their stomata at night to take in CO$_2$ (to minimize water loss during the hot day), store it as an intermediate, and then use the light energy absorbed during the day to carry out the later steps of photosynthesis.

To demonstrate that chlorophyll and carbon dioxide are necessary for photosynthesis, experiments involving variegated leaves (to show starch production only in green parts containing chlorophyll) and plants covered with bell jars (one with potassium hydroxide to absorb CO$_2$, one without) followed by starch tests are performed.

Plants obtain carbon dioxide from the atmosphere primarily through tiny pores on the leaf surface called **stomata**. Gas exchange (CO$_2$ intake and O$_2$ release) occurs through stomata. Stomata are surrounded by **guard cells**, which regulate their opening and closing. Guard cells swell when they absorb water, opening the pore; they shrink when they lose water, closing the pore. Plants close stomata when they don't need CO$_2$ to conserve water. Gas exchange also occurs across the surfaces of stems and roots.

Water used in photosynthesis is absorbed from the soil by the roots. Other raw materials like nitrogen, phosphorus, iron, and magnesium are also absorbed from the soil. Nitrogen is crucial for synthesizing proteins and other compounds and is absorbed as inorganic nitrates or nitrites, or organic compounds formed by bacteria from atmospheric nitrogen.

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Heterotrophic Nutrition

**Heterotrophic nutrition** involves obtaining nutrition from other organisms. Organisms adapt their methods of obtaining and using food based on the food source (stationary vs. mobile) and its availability. Different strategies exist:

• Some organisms (e.g., fungi like bread moulds, yeast, mushrooms) break down the food material **outside** their body and then absorb the simpler products.
• Some organisms (e.g., animals) take in the food material as a whole and break it down **inside** their bodies. The ability to do this depends on the organism's body design and digestive system.
• Some organisms (e.g., parasitic organisms like Cuscuta (amar-bel), ticks, lice, leeches, tapeworms) derive nutrition from plants or animals **without killing them**. This is called parasitic nutrition.

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How Do Organisms Obtain Their Nutrition?

The method of obtaining food varies with the complexity of the organism. In single-celled organisms, food uptake can occur through the entire cell surface. For example, **Amoeba** takes in food particles using temporary finger-like extensions of the cell membrane called **pseudopodia**. The pseudopodia fuse around the food particle, forming a **food vacuole**. Inside the food vacuole, complex food substances are broken down into simpler ones by enzymes and then diffuse into the cytoplasm. Undigested waste is expelled from the cell surface.

**Paramoecium**, another unicellular organism, has a definite shape. Food is taken in at a specific spot on the cell surface. The cell is covered with cilia, which move in a coordinated manner to sweep the food particles into this oral groove or food inlet.

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Nutrition In Human Beings

Nutrition in human beings occurs through a complex system involving the **alimentary canal** (a long tube from mouth to anus) and associated digestive glands. The alimentary canal has different parts specialised for different functions (ingestion, digestion, absorption, egestion).

The process begins in the **mouth**:

• Food is ingested and mechanically broken down into smaller particles by **teeth**.
• Food is mixed with **saliva**, a fluid secreted by **salivary glands**. Saliva contains an enzyme called **salivary amylase** (or ptyalin) which begins the chemical digestion of **starch** (a complex carbohydrate) into simple sugars (maltose).
• The muscular **tongue** mixes the food with saliva and helps in swallowing.

Food moves from the mouth to the stomach through the **oesophagus** (food pipe) by **peristaltic movements** (rhythmic contractions of the muscular walls of the alimentary canal).

In the **stomach**:

• The stomach is a large, muscular J-shaped organ that receives food from the oesophagus.
• Its muscular walls contract and relax to mix food with **gastric juices** secreted by **gastric glands** in the stomach wall.
• Gastric juice contains:
  • **Hydrochloric acid (HCl):** Creates an acidic medium (pH 1.5-3.5) which activates the enzyme pepsin and kills bacteria present in food.
  • **Pepsin:** A protein-digesting enzyme that begins the breakdown of proteins into smaller molecules (peptides).
  • **Mucus:** Protects the inner lining of the stomach wall from the corrosive action of HCl and pepsin under normal conditions.
• Food is released from the stomach into the small intestine in small amounts regulated by a **sphincter muscle**.

The **small intestine** is the longest part of the alimentary canal (about 6-7 meters in adults), highly coiled to fit into the abdomen. Its length varies depending on the diet (longer in herbivores to digest cellulose, shorter in carnivores).

• The small intestine is the primary site for **complete digestion** of carbohydrates, proteins, and fats, and the main region for **absorption** of digested food.
• It receives secretions from two accessory digestive glands:
  • **Liver:** Secretes **bile juice**, stored in the gallbladder. Bile is alkaline and neutralizes the acidic food coming from the stomach, making it alkaline for pancreatic enzymes to act. Bile salts also emulsify fats (break down large fat globules into smaller ones), increasing the surface area for enzyme action.
  • **Pancreas:** Secretes **pancreatic juice**, containing enzymes like **trypsin** (for protein digestion) and **lipase** (for emulsifying fats).
• The walls of the small intestine also contain glands that secrete **intestinal juice**, which contains enzymes that complete the digestion: proteins $\to$ amino acids, complex carbohydrates $\to$ glucose, fats $\to$ fatty acids and glycerol.

Absorption of digested food:

• The inner lining of the small intestine is folded into numerous finger-like projections called **villi** (singular: villus). Villi greatly increase the surface area for efficient absorption of digested food.
• Villi are richly supplied with **blood vessels** that absorb glucose, amino acids, salts, etc., and **lymphatic vessels** (lacteals) that absorb fatty acids and glycerol.
• The absorbed food is transported by the blood to all cells of the body for energy, growth, and repair.

The unabsorbed food passes into the **large intestine**.

• The large intestine absorbs most of the remaining water from the undigested food material.
• The remaining waste material (faeces) is stored temporarily and then eliminated from the body through the **anus**. The exit is regulated by the **anal sphincter muscle** (which is under nervous control, allowing voluntary control of defecation).

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Respiration

**Respiration** is the process by which organisms break down glucose (a simple carbohydrate derived from food) to release **energy** for various life processes. This process typically occurs within cells.

The first step in the breakdown of glucose (a six-carbon molecule) is its conversion into a three-carbon molecule called **pyruvate**. This process occurs in the **cytoplasm** and is common to all types of respiration.

Further breakdown of pyruvate depends on whether oxygen is available:

• **Anaerobic Respiration:** Takes place in the **absence of oxygen**. Pyruvate is converted into products like **ethanol and carbon dioxide** (e.g., in yeast during fermentation) or **lactic acid** (e.g., in muscle cells during strenuous activity when oxygen is limited). This process yields a relatively **small amount of energy**.

• **Aerobic Respiration:** Takes place in the **presence of oxygen**. Pyruvate is completely broken down into **carbon dioxide and water**. This process occurs in the **mitochondria**. Aerobic respiration yields a **much larger amount of energy** compared to anaerobic respiration.

The energy released during cellular respiration is stored in the form of a molecule called **ATP (Adenosine Triphosphate)**. ATP is the **energy currency** of the cell and is used to power most cellular activities (e.g., muscle contraction, protein synthesis, nerve impulse conduction). When ATP is broken down (e.g., terminal phosphate bond broken), a specific amount of energy is released to drive endothermic reactions in the cell.

Organisms need to ensure sufficient intake of oxygen for aerobic respiration and removal of carbon dioxide produced. Different organisms have evolved different mechanisms for gas exchange:

• **Plants:** Exchange gases (CO$_2$ and O$_2$) primarily through stomata on leaves by diffusion. Large intercellular spaces in plants facilitate gas exchange with all cells. Gas exchange direction depends on environmental conditions and the plant's needs (CO$_2$ intake/O$_2$ release during day when photosynthesis occurs, O$_2$ intake/CO$_2$ release during night due to respiration).
• **Aquatic animals:** Animals living in water need to obtain dissolved oxygen. They often have specialized organs like **gills**, which have a large surface area for diffusion of oxygen from water into the blood. The concentration of dissolved oxygen in water is relatively low, so aquatic organisms often have a much faster breathing rate than terrestrial organisms to obtain enough oxygen.
• **Terrestrial animals:** Obtain oxygen from the atmosphere. They have evolved different respiratory organs (lungs, tracheae, skin) that provide a large, fine, delicate surface area in contact with the atmosphere for efficient gas exchange. These organs are often located inside the body for protection, with passages to bring air to them.

In **human beings**, the respiratory system (Fig 5.9 in textbook) facilitates gas exchange:

• Air is inhaled through nostrils, filtered by nasal hairs and mucus.
• It passes through the throat and trachea (windpipe), which has cartilage rings to prevent collapse.
• The trachea branches into bronchi, which divide into smaller tubes called bronchioles.
• Bronchioles terminate in tiny balloon-like structures called **alveoli** (in the lungs). Alveoli provide a vast surface area for gas exchange.
• Alveolar walls are very thin and richly supplied with blood capillaries. Oxygen from the inhaled air diffuses across the alveolar walls into the blood. Carbon dioxide from the blood (transported from body tissues) diffuses into the alveoli to be exhaled.
• Breathing movements (involving ribs and diaphragm) create pressure differences that draw air into and out of the lungs.
• Lungs retain a residual volume of air after exhalation to allow continuous gas exchange.

In large animals, diffusion alone is insufficient for oxygen transport to all cells. **Respiratory pigments** are used for oxygen transport. In humans, the respiratory pigment is **haemoglobin**, present in red blood cells. Haemoglobin has a high affinity for oxygen and carries it from the lungs to oxygen-deficient tissues. Carbon dioxide is more soluble in water and is mostly transported in the blood in a dissolved form.

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Transportation

Transportation is the process of moving substances within the body of a multicellular organism. In complex organisms, specialized transport systems are needed to deliver food, oxygen, water, and other necessary materials to all cells and to remove waste products.

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Transportation In Human Beings (Circulatory System)

In human beings, the transport system is the **circulatory system**, which consists of the **heart**, **blood vessels** (arteries, veins, capillaries), and **blood**. Blood transports food, oxygen, carbon dioxide, nitrogenous wastes, hormones, and other substances.

• **Blood:** A fluid connective tissue consisting of **plasma** (fluid medium) and suspended cells (red blood cells, white blood cells, platelets). Plasma transports dissolved substances like food, CO$_2$, and nitrogenous wastes. Red blood cells (containing haemoglobin) transport oxygen.
• **Blood Vessels:** A network of tubes throughout the body:
  • **Arteries:** Thick, elastic-walled vessels carrying oxygenated blood (mostly) away from the heart to various organs under high pressure. They divide into smaller arterioles and eventually capillaries.
  • **Veins:** Thinner-walled vessels (blood is under less pressure) collecting deoxygenated blood (mostly) from organs and bringing it back to the heart. Veins have valves to ensure blood flows only in one direction (towards the heart). They are formed by the joining of capillaries.
  • **Capillaries:** Smallest, one-cell thick vessels forming networks within tissues. Exchange of materials (oxygen, nutrients, waste, CO$_2$) between blood and surrounding cells occurs across their thin walls.
• **Heart:** A muscular pumping organ (about the size of a fist) located in the chest. It has four chambers (two atria and two ventricles) in humans, which prevents the mixing of oxygenated and deoxygenated blood.

Blood flow through the heart in humans is a **double circulation**:

• **Pulmonary Circulation:** Deoxygenated blood from the body enters the **right atrium**, goes to the **right ventricle**, which pumps it to the **lungs** for oxygenation. Oxygenated blood returns from the lungs to the **left atrium**.
• **Systemic Circulation:** Oxygenated blood from the lungs enters the **left atrium**, goes to the **left ventricle**, which pumps it to the **rest of the body**. Deoxygenated blood returns from the body to the right atrium.

Double circulation ensures efficient supply of oxygenated blood to the body, which is necessary for animals with high energy needs (like mammals and birds) that maintain a constant body temperature. Ventricles have thicker muscular walls than atria because they pump blood to greater distances (lungs or body). Valves in the heart and veins prevent the backflow of blood.

**Blood pressure** is the force blood exerts against vessel walls. Systolic pressure (during ventricular contraction) is about 120 mmHg, and diastolic pressure (during ventricular relaxation) is about 80 mmHg. High blood pressure (hypertension) can be harmful.

If blood vessels develop leaks (e.g., due to injury), blood loss is minimised, and pressure is maintained by **platelets**, which are cells in the blood that help in clotting at the site of injury.

Besides blood, **lymph** (or tissue fluid) is another fluid involved in transport. Lymph is a colorless fluid formed when some plasma, proteins, and blood cells leak out of capillaries into the intercellular spaces in tissues. Lymph drains into lymphatic capillaries, which form larger lymphatic vessels. Lymph transports digested fats from the intestine and drains excess fluid from tissue spaces back into the blood circulation, also playing a role in the immune system.

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Transportation In Plants

Plants absorb raw materials (water and minerals) from the soil through their roots and synthesise food (carbohydrates) in their leaves through photosynthesis. If the plant body is small, diffusion can transport materials sufficiently. However, in larger plants, a dedicated transport system is needed.

Plants have a relatively low energy demand as they are stationary and have a large proportion of dead cells in some tissues. Their transport systems are relatively slow compared to animals but operate over potentially large distances (e.g., tall trees).

Plant transport systems consist of **vascular tissues**: **xylem** and **phloem**. They are organized as continuous conducting tubes.

• **Xylem:** Transports **water and dissolved minerals** from the **roots upwards** to the leaves and other aerial parts. It consists of interconnected vessels and tracheids forming continuous channels. Absorption of ions by root cells creates a concentration difference, causing water to move into the roots by osmosis (root pressure), which pushes water up the xylem, especially significant at night. During the day, when stomata are open, the major driving force for water movement is **transpiration pull**.

Water loss in the form of vapour from the aerial parts (mostly through stomata) is called **transpiration**. Evaporation from leaf cells creates a suction (transpiration pull) that pulls water up the xylem vessels from the roots. Transpiration also helps in temperature regulation.

• **Phloem:** Transports **products of photosynthesis (food, mainly sugars)** from the **leaves** (where synthesized) to other parts of the plant, including roots, fruits, seeds, and growing regions. This transport of soluble food is called **translocation**. Phloem also transports amino acids and other substances.

Translocation in phloem requires **energy** (from ATP). Sucrose, for example, is actively transported into phloem tissue, increasing osmotic pressure, causing water to move into the phloem. This pressure gradient drives the movement of materials in the phloem to areas of lower pressure (where food is consumed or stored), allowing transport in both upward and downward directions according to the plant's needs.

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Excretion

**Excretion** is the biological process of removing harmful metabolic waste products from the body. These wastes are generated from various metabolic activities, including respiration and the breakdown of nitrogen-containing substances.

Different organisms have different strategies for excretion:

• **Unicellular organisms:** Remove wastes by simple diffusion across their body surface into the surrounding water.
• **Complex multicellular organisms:** Use specialized organs or organ systems for excretion due to their large size and the fact that not all cells are in direct contact with the environment.

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Excretion In Human Beings (Excretory System)

The excretory system in human beings (Fig 5.13 in textbook) is primarily responsible for removing nitrogenous wastes (like urea and uric acid) from the blood and eliminating excess water and salts. The main components are:

• **Kidneys:** A pair of bean-shaped organs located in the abdomen on either side of the backbone. They are the main filtration units, removing waste products from the blood and producing urine.
• **Ureters:** A pair of tubes that carry urine from each kidney to the urinary bladder.
• **Urinary bladder:** A muscular bag-like structure that stores urine temporarily.
• **Urethra:** A tube that carries urine from the urinary bladder out of the body. The exit is controlled by sphincter muscles.

**Urine formation** occurs in the kidneys. The basic filtration unit in the kidney is the **nephron** (Fig 5.14 in textbook). Each kidney contains millions of nephrons.

Structure and Function of a Nephron:

• A nephron consists of a **glomerulus** (a cluster of thin-walled blood capillaries) associated with a cup-shaped structure called **Bowman's capsule**.
• Blood enters the glomerulus under pressure, and small molecules (water, glucose, amino acids, salts, urea, etc.) are filtered from the blood into Bowman's capsule. This fluid is called the **initial filtrate**.
• The Bowman's capsule leads into a long, coiled **renal tubule**. As the filtrate flows along the tubule, useful substances (glucose, amino acids, salts, and a major amount of water) are selectively re-absorbed back into the blood capillaries surrounding the tubule. Waste products like urea and excess water remain in the filtrate.
• The amount of water re-absorbed depends on the body's hydration level (amount of excess water) and the amount of dissolved waste that needs to be excreted.
• The remaining fluid is urine, which is collected from the nephrons into collecting ducts, which drain into the ureter.

Urine is stored in the muscular urinary bladder until it is released through the urethra. The bladder is under nervous control, allowing conscious control over the urge to urinate.

An **artificial kidney** (hemodialysis) is a device used for patients with kidney failure to remove nitrogenous wastes from their blood through dialysis. It mimics the filtration function of the kidney but lacks the selective re-absorption process.

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Excretion In Plants

Plants use different strategies for excretion compared to animals, as they have different metabolic needs and body structures.

• **Gaseous wastes:** Oxygen produced during photosynthesis and carbon dioxide produced during respiration are released through stomata on leaves and lenticels on stems by diffusion.
• **Excess water:** Removed by transpiration, mainly through stomata.
• **Other wastes:**
  • Many waste products are stored in cellular vacuoles.
  • Waste products may be stored in tissues consisting of dead cells, or in plant parts that are eventually shed, like falling leaves.
  • Some waste products are stored as resins and gums, especially in older xylem tissue.
  • Some waste substances are excreted by plants into the soil around their roots.

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