What Are Life Processes?

Life processes are the basic functions performed by living organisms to maintain life and prevent damage and breakdown of their structures. These processes continue even when the organism is not actively doing anything, such as when sleeping.

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Since maintenance requires energy, living organisms need a way to obtain energy from outside their bodies. This process is called nutrition, where food (a source of energy and raw materials, typically carbon-based molecules) is taken in from the environment.

Different organisms use different nutritional processes depending on the complexity of the food source and their body design. Food needs to be broken down or converted into a uniform source of energy and molecules for growth and maintenance. This involves a series of chemical reactions, often including oxidation-reduction reactions.

Many organisms use oxygen from the outside to break down food sources. The process of acquiring oxygen and using it for cellular energy needs is called respiration.

In single-celled organisms, the entire surface is in contact with the environment, allowing simple diffusion for processes like taking in food, gas exchange (oxygen and carbon dioxide), and waste removal.

However, in multi-cellular organisms, the body is larger and more complex, and not all cells are in direct contact with the environment. Simple diffusion is insufficient to meet the needs of all cells. This necessitates specialised tissues and organs for specific functions (e.g., uptake of food, gas exchange). This creates a need for a transportation system to carry food, oxygen, and other necessary substances to all parts of the body.

Chemical reactions for energy generation also produce waste products that can be harmful. Organisms need to remove these wastes from the body through a process called excretion. In multi-cellular organisms, specialised tissues or organs are developed for excretion, and the transportation system also carries wastes to these excretory organs.

The essential life processes include: Nutrition, Respiration, Transportation, and Excretion.

Nutrition

Nutrition is the process by which organisms obtain and utilise food, which provides energy and materials for growth, development, and maintenance.

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Organisms exhibit different modes of nutrition:

• Autotrophic nutrition: Organisms produce their own food from simple inorganic substances (carbon dioxide and water) using an external energy source (usually sunlight). Includes green plants and some bacteria.
• Heterotrophic nutrition: Organisms obtain complex food substances from other organisms (plants or animals). They rely directly or indirectly on autotrophs. Includes animals and fungi. Heterotrophs use enzymes to break down complex food into simpler molecules.

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Autotrophic Nutrition

Autotrophs, primarily green plants, fulfil their energy and carbon requirements through photosynthesis. This is the process of converting simple inorganic substances from the environment into stored forms of energy (carbohydrates) using light energy.

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The overall equation for photosynthesis is:

$6\text{CO}_2 \text{(Carbon dioxide)} + 12\text{H}_2\text{O} \text{(Water)} \xrightarrow{\text{Sunlight}}{\text{Chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 \text{(Glucose)} + 6\text{O}_2 \text{(Oxygen)} + 6\text{H}_2\text{O}$

Key events during photosynthesis:

1. Absorption of light energy by chlorophyll (a green pigment found in chloroplasts, organelles within plant cells).
2. Conversion of light energy into chemical energy, and splitting of water molecules into hydrogen and oxygen.
3. Reduction of carbon dioxide to carbohydrates (glucose).

These steps don't necessarily happen immediately one after another (e.g., desert plants take up $\text{CO}_2$ at night and process it with daytime energy).

Carbohydrates produced are used for energy or stored as starch. Starch is an internal energy reserve for plants. Animals store energy as glycogen.

Raw materials for photosynthesis:

• Carbon dioxide: Taken in from the atmosphere through tiny pores on the leaf surface called stomata. Gas exchange also occurs through the surface of stems and roots. The opening and closing of stomata are regulated by guard cells.
• Water: Absorbed from the soil by the roots, especially in terrestrial plants.
• Sunlight: Energy source, absorbed by chlorophyll.
• Chlorophyll: Pigment present in chloroplasts, absorbs light energy.

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Heterotrophic Nutrition

Heterotrophic organisms obtain nutrients from consuming other organisms. The specific method of obtaining food depends on the food source's availability (stationary or mobile) and the organism's body design.

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Different strategies for heterotrophic nutrition:

• Saprotrophic nutrition: Organisms break down food material outside their body and then absorb it (e.g., fungi like bread moulds, yeast, mushrooms).
• Holozoic nutrition: Organisms take in whole food material and break it down inside their bodies (e.g., animals).
• Parasitic nutrition: Organisms derive nutrition from plants or animals without killing them (e.g., Cuscuta (amar-bel), ticks, lice, leeches, tapeworms).

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How Do Organisms Obtain Their Nutrition?

The complexity of the digestive system varies depending on the organism's body design and the type of food it consumes.

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• Single-celled organisms: Food is taken in by the entire cell surface (e.g., Amoeba forms food vacuoles using temporary finger-like extensions called pseudopodia; Paramoecium takes food at a specific spot using cilia).
• Multi-cellular organisms: Different parts of the body specialise in different functions, including obtaining and digesting food.

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Nutrition In Human Beings

In humans, nutrition occurs through the alimentary canal, a long tube extending from the mouth to the anus, with specialised regions for different digestive processes.

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The digestive process involves breaking down complex food into smaller molecules that can be absorbed:

1. Mouth: Food is mechanically crushed by teeth. Saliva (secreted by salivary glands) wets the food for smooth passage and contains salivary amylase, an enzyme that begins breaking down starch into simple sugars. The tongue mixes food with saliva and helps in swallowing.
2. Oesophagus (Food-pipe): Food moves from the mouth to the stomach through peristaltic movements (rhythmic contractions and relaxations of muscles in the canal lining).
3. Stomach: A large, muscular organ that expands to hold food. Muscular walls mix food with gastric juices secreted by gastric glands in the stomach wall. Gastric juice contains hydrochloric acid (creates an acidic medium for enzyme action, also kills bacteria), pepsin (a protein-digesting enzyme), and mucus (protects the stomach lining from acid).
4. Small Intestine: The longest part of the alimentary canal, highly coiled. It is the site of complete digestion of carbohydrates, proteins, and fats. It receives secretions from:
   • Liver: Secretes bile juice, which emulsifies large fat globules into smaller ones (increasing enzyme efficiency) and makes the acidic food from the stomach alkaline.
   • Pancreas: Secretes pancreatic juice, containing enzymes like trypsin (for protein digestion) and lipase (for digesting emulsified fats).
   • Walls of the small intestine: Secrete intestinal juice, containing enzymes that complete the digestion of proteins (to amino acids), complex carbohydrates (to glucose), and fats (to fatty acids and glycerol).
5. Absorption: Digested food is absorbed by the walls of the small intestine. The inner lining has finger-like projections called villi, which greatly increase the surface area for efficient absorption. Villi are richly supplied with blood vessels that transport absorbed food to all body cells.
6. Large Intestine: Receives unabsorbed food from the small intestine. Its walls absorb excess water from this material.
7. Anus: Undigested waste material is removed from the body via the anus. This process is regulated by the anal sphincter muscle.

Respiration

Respiration is the process by which organisms break down food material (primarily glucose) in cells to release energy, which is then used to fuel various life processes.

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The first step in respiration, common to all pathways, is the breakdown of glucose (a 6-carbon molecule) into pyruvate (a 3-carbon molecule). This occurs in the cytoplasm.

Breakdown of glucose (6-carbon molecule) $\longrightarrow$ Pyruvate (3-carbon molecule) + Energy

From pyruvate, energy can be released through different pathways depending on the presence or absence of oxygen:

• Anaerobic Respiration: Occurs in the absence of oxygen.
  • In yeast during fermentation: Pyruvate is converted into ethanol and carbon dioxide. This process yields relatively less energy.
  • In muscle cells during strenuous activity (lack of oxygen): Pyruvate is converted into lactic acid. Build-up of lactic acid can cause muscle cramps.
• Aerobic Respiration: Occurs in the presence of oxygen.
  • Takes place in the mitochondria.
  • Pyruvate is completely broken down into carbon dioxide and water.
  • This process releases a much larger amount of energy compared to anaerobic respiration.

The energy released during respiration is used to synthesise ATP (Adenosine Triphosphate) molecules. ATP is considered the energy currency of the cell. The energy stored in ATP is used to drive endothermic reactions and other cellular activities like muscle contraction, protein synthesis, and nerve impulse conduction.

Organisms need to ensure sufficient intake of oxygen for aerobic respiration. Plants exchange gases ($\text{O}_2$ and $\text{CO}_2$) by diffusion through stomata and large intercellular spaces. Animals have evolved specific respiratory organs.

Respiration in Animals:

• Aquatic animals: Obtain oxygen dissolved in water. The amount of dissolved oxygen is low, so aquatic organisms (like fish) have faster breathing rates and special organs (gills) to absorb oxygen efficiently from water.
• Terrestrial animals: Obtain oxygen from the atmosphere. They have respiratory organs with large surface areas in contact with air (e.g., lungs in humans).

Respiration in Humans:

• Air enters through nostrils, filtered by hairs and mucus in the nasal passage.
• Passes through the throat (pharynx and larynx) and into the lungs via the trachea and bronchi. Rings of cartilage prevent the air passage from collapsing.
• Within the lungs, bronchi divide into smaller tubes (bronchioles) terminating in tiny balloon-like structures called alveoli.
• Alveoli provide a vast surface area for gas exchange. Their walls are very thin and have an extensive network of blood vessels.
• Oxygen from the inhaled air in the alveoli diffuses into the blood. Carbon dioxide from the blood diffuses into the alveolar air to be exhaled.

Oxygen is transported by the respiratory pigment haemoglobin (present in red blood cells), which has high affinity for oxygen. Carbon dioxide is mostly transported in blood in dissolved form.

Transportation

In multi-cellular organisms, a transportation system is essential to move substances like food, oxygen, water, minerals, and waste products between different parts of the body, as not all cells are directly in contact with the external environment.

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Transportation In Human Beings

The transport system in human beings is the circulatory system, which includes the heart, blood, and blood vessels.

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• Blood: A fluid connective tissue. Consists of plasma (fluid medium) and suspended cells (red blood cells, white blood cells, platelets). Plasma transports food, $\text{CO}_2$, and nitrogenous wastes in dissolved form. Red blood cells carry oxygen (bound to haemoglobin). Blood also transports hormones and salts.
• Heart: A muscular pumping organ (about the size of a fist) with four chambers (two atria and two ventricles). It pumps blood throughout the body. The chambers prevent the mixing of oxygenated (from lungs) and deoxygenated (from body) blood, ensuring efficient oxygen supply.
  • Oxygenated blood from lungs enters the left atrium, then to the left ventricle, which pumps it to the body.
  • Deoxygenated blood from the body enters the right atrium, then to the right ventricle, which pumps it to the lungs for oxygenation.

  Ventricles have thicker muscular walls than atria as they pump blood to longer distances. Valves in the heart prevent blood backflow.

  Humans and other mammals/birds have a four-chambered heart for efficient double circulation, necessary for high energy needs to maintain body temperature. Amphibians and reptiles have three-chambered hearts and tolerate some mixing. Fish have two-chambered hearts with single circulation (blood goes through heart once per cycle).

• Blood Vessels: Network of tubes through which blood flows.
  • Arteries: Carry blood away from the heart (mostly oxygenated, except pulmonary artery). Have thick, elastic walls as blood is under high pressure.
  • Veins: Carry blood towards the heart (mostly deoxygenated, except pulmonary vein). Have thinner walls and valves to prevent backflow as blood pressure is lower.
  • Capillaries: Smallest, thin-walled vessels (one-cell thick) connecting arteries and veins. Exchange of materials (oxygen, food, waste) between blood and body cells occurs across their walls.
• Platelets: Cell fragments in blood that help in clotting at injury sites to prevent excessive blood loss and maintain pressure in the circulatory system.

Blood pressure: The force blood exerts against vessel walls. Systolic pressure (during ventricular contraction, $\approx 120$ mmHg), Diastolic pressure (during ventricular relaxation, $\approx 80$ mmHg).

Lymph: Another fluid involved in transport, also called tissue fluid. Forms from blood plasma, proteins, and cells that leak from capillaries into intercellular spaces. Similar to plasma but colourless and with less protein. Lymph drains into lymphatic capillaries, which form larger vessels that eventually return lymph to the blood. Lymph carries absorbed fat from the intestine and drains excess fluid from tissues.

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Transportation In Plants

Plants also need a transport system to move water, minerals, and food (products of photosynthesis) between different parts of the plant body, as distant parts cannot rely on simple diffusion.

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Plants have lower energy needs than animals and use relatively slow transport systems. The transport system in highly organised plants consists of vascular tissue: xylem and phloem.

• Xylem: Transports water and dissolved minerals from the roots (where they are absorbed from the soil) upwards to the leaves and other aerial parts. Xylem tissue (vessels and tracheids) forms a continuous network from roots to leaves.
  • Water enters root cells due to a concentration difference caused by active uptake of ions from the soil.
  • This creates root pressure, which pushes a column of water up the xylem.
  • Transpiration pull is the major force for water movement in tall plants, especially during the day when stomata are open. Loss of water vapour from leaves through stomata (transpiration) creates a suction force that pulls water up the xylem from the roots. Transpiration also helps in temperature regulation.
• Phloem: Transports soluble products of photosynthesis (food, mainly sugars like sucrose), amino acids, and other substances from the leaves (where photosynthesis occurs) to other parts of the plant (roots for storage, fruits, seeds, growing regions). This process is called translocation.
  • Translocation in phloem requires energy (ATP).
  • Material (e.g., sucrose) is loaded into sieve tubes of phloem using ATP.
  • This increases the osmotic pressure in the phloem tissue, causing water to move into it by osmosis.
  • The resulting pressure gradient drives the movement of material in the phloem to areas of lower pressure (where food is needed or stored). This allows phloem to transport material according to the plant's needs, both upwards and downwards.

Transport in xylem is largely driven by physical forces (root pressure and transpiration pull), while transport in phloem involves active processes requiring energy.

Excretion

Excretion is the biological process of removing harmful metabolic waste products from the body. Waste products are generated from various metabolic activities.

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Organisms have different strategies for excretion:

• Single-celled organisms: Remove waste products by simple diffusion across the body surface into the surrounding water.
• Multi-cellular organisms: Use specialised organs and systems for excretion due to their complex body design and inability of all cells to directly access the environment for waste removal.

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Excretion In Human Beings

The excretory system in human beings is responsible for filtering waste products, especially nitrogenous wastes, from the blood and removing them from the body as urine.

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The human excretory system consists of:

• A pair of kidneys: Bean-shaped organs located in the abdomen, one on each side of the backbone. They filter waste from the blood.
• A pair of ureters: Tubes connecting each kidney to the urinary bladder, transporting urine.
• A urinary bladder: Muscular organ that stores urine.
• A urethra: Tube that carries urine from the urinary bladder out of the body.

Urine Formation:

The basic filtration unit in the kidney is the nephron. Each kidney contains a large number of nephrons (Fig. 6.14). Each nephron consists of:

• A cluster of thin-walled blood capillaries called the glomerulus.
• A cup-shaped structure called Bowman's capsule that surrounds the glomerulus and collects the filtrate.
• A long, coiled renal tube.

Steps in urine formation:

1. Filtration: Blood flows through the glomerulus under pressure. Water, salts, glucose, amino acids, urea, and other small molecules are filtered from the blood into Bowman's capsule, forming the initial filtrate. Blood cells and large proteins remain in the blood.
2. Selective Re-absorption: As the filtrate flows through the renal tubule, useful substances like glucose, amino acids, essential salts, and a large amount of water are selectively re-absorbed back into the blood capillaries surrounding the tubule.
3. Tubular Secretion: Some additional waste substances from the blood are secreted into the tubule.
4. Urine Formation: The remaining fluid, containing metabolic wastes (mainly urea) and excess water, forms urine.

The amount of water re-absorbed is regulated based on the body's hydration level and the amount of dissolved waste to be excreted. Urine from each kidney flows down the ureter to the urinary bladder. The bladder stores urine until sufficient pressure builds up, triggering the urge to urinate. Urination is regulated by muscular sphincters and is under nervous control.

Artificial Kidney (Hemodialysis): In cases of kidney failure, an artificial kidney machine is used to filter the blood. The patient's blood is passed through tubes immersed in a dialysing fluid. Waste products diffuse from the blood into the fluid, and purified blood is returned to the body. This process is similar to kidney function but lacks the selective re-absorption process of a healthy kidney.

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Excretion In Plants

Plants use different strategies to excrete waste products compared to animals.

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Methods of Excretion in Plants:

• Gaseous wastes: Oxygen (produced during photosynthesis) and carbon dioxide (produced during respiration) are removed by diffusion through stomata (in leaves) and lenticels (in stems).
• Excess water: Removed by transpiration (loss of water vapour through stomata).
• Solid and liquid wastes: Plants store waste products in various ways:
  • In cellular vacuoles (especially in dead cells).
  • In leaves that are shed (waste products accumulate in old leaves before they fall).
  • As resins and gums (especially in old xylem tissue).
  • Excreting some waste substances into the soil around their roots.

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