Chapter 15: Life Processes in Living Organisms
Comprehensive chapter summary with detailed explanations and examples.
Grade 9 Learning: Chapter 15: Life Processes in Living Organisms
Introduction
Living organisms perform various essential functions to sustain life. These include obtaining nutrients, transporting substances, removing waste, responding to stimuli, and coordinating different activities. This chapter explores key life processes like transportation and excretion in both plants and animals, and the crucial process of coordination that ensures the smooth functioning of complex organisms.
Transportation in Plants
Need for Transportation
Plants absorb water and minerals through their roots, but these substances are needed by all parts, especially the leaves for photosynthesis. Similarly, the food synthesized in the leaves needs to be distributed throughout the plant for energy and growth. Therefore, plants require an efficient transport system.
Water and Mineral Transport (Xylem)
The transport of water and dissolved minerals from the roots to the leaves occurs through specialized tissue called Xylem.
- Absorption: Root cells are in contact with soil water and minerals. Due to differences in concentration, water enters the root cells by osmosis, and minerals are often actively absorbed.
- Root Pressure: The continuous absorption of water by root cells creates a positive pressure, known as root pressure, which helps push the water column upwards to a certain height.
- Transpiration Pull: The primary mechanism for water movement over long distances is transpiration pull. Transpiration is the evaporation of water from the plant surface, mainly through stomata in the leaves. This water loss creates a suction force that pulls the continuous column of water upwards through the xylem vessels from the roots to the leaves.
- Xylem Tissue: Consists of tracheids, vessels, xylem parenchyma, and xylem fibers. Tracheids and vessels form continuous channels for water transport.
Food Transport (Phloem)
The transport of soluble food materials (mainly sucrose) synthesized during photosynthesis from the leaves to other parts of the plant (like roots, fruits, seeds, storage organs) is called translocation. This occurs through another specialized tissue called Phloem.
- Mechanism: Translocation in phloem is an active process requiring energy (ATP). Food is loaded into the phloem tubes (sieve tubes) in the leaves, increasing the solute concentration. Water enters the phloem from adjacent xylem by osmosis, creating high pressure. This pressure drives the phloem sap towards regions of lower pressure (sink regions like roots or fruits), where the food is unloaded and utilized or stored.
- Direction: Unlike xylem transport (which is unidirectional upwards), phloem transport can occur in both upward and downward directions, depending on the plant's needs (from source to sink).
- Phloem Tissue: Consists of sieve tubes, companion cells, phloem parenchyma, and phloem fibers. Sieve tubes are the main conducting channels, supported by companion cells.
Excretion
Definition
Excretion is the process by which metabolic waste products, which can be harmful if accumulated, are removed from the body of an organism.
Excretion in Plants
Plants lack specialized excretory organs like animals. They use various strategies to get rid of waste products:
- Gaseous Wastes: Oxygen (by-product of photosynthesis) and Carbon Dioxide (by-product of respiration) are released through stomata and lenticels.
- Water: Excess water is removed through transpiration.
- Stored Wastes: Many waste products are stored in cell vacuoles, or in tissues like leaves, bark, fruits, or seeds, which are then shed from the plant. Some wastes like resins, gums, latex, and essential oils are stored in old xylem or bark.
- Raphides: Some plants store waste as calcium oxalate crystals (raphides), which can cause irritation.
- Excretion into Soil: Some waste substances are excreted by roots into the surrounding soil.
Excretion in Humans
The human excretory system is responsible for eliminating nitrogenous wastes (like urea), excess salts, water, and other metabolic by-products from the blood.
- Main Organs: The system primarily consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.
- Kidneys: These bean-shaped organs filter blood. Each kidney contains millions of microscopic filtering units called nephrons.
- Nephron Structure: Each nephron has a cup-shaped Bowman's capsule containing a network of capillaries called the glomerulus, attached to a long, coiled renal tubule (including the Loop of Henle) that leads to a collecting duct.
- Urine Formation:
- Glomerular Filtration (Ultrafiltration): Blood enters the glomerulus under high pressure, forcing water, urea, glucose, salts, and other small molecules from the blood into Bowman's capsule, forming the glomerular filtrate.
- Selective Reabsorption: As the filtrate passes through the renal tubule, useful substances like glucose, amino acids, most salts, and a significant amount of water are reabsorbed back into the blood capillaries surrounding the tubule.
- Tubular Secretion: Some waste products (like potassium ions, hydrogen ions, certain drugs) are actively secreted from the blood into the tubule.
- Urine Pathway: Urine from nephrons collects in the collecting ducts, flows into the renal pelvis, down the ureters to the urinary bladder for temporary storage, and is finally expelled from the body through the urethra.
- Dialysis: In case of kidney failure, an artificial kidney (dialysis machine) is used to filter waste products from the blood.
Coordination
Definition
Coordination is the systematic integration and regulation of various activities and processes within an organism to ensure efficient functioning, respond to environmental changes, and maintain a stable internal state (homeostasis).
Coordination in Plants
Plants lack a nervous system but coordinate their activities using chemical signals (hormones) and growth movements in response to stimuli.
Plant Movements
- Tropic Movements (Growth-dependent): Directional growth responses.
- Phototropism: Growth towards light (e.g., shoot bending towards window).
- Gravitropism (Geotropism): Growth in response to gravity (roots grow down, shoots grow up).
- Hydrotropism: Growth towards water.
- Chemotropism: Growth in response to chemicals (e.g., pollen tube growth towards ovule).
- Thigmotropism: Growth in response to touch (e.g., tendrils coiling).
- Nastic Movements (Growth-independent): Non-directional responses, often rapid changes in turgor pressure.
- Thigmonasty: Response to touch (e.g., folding of Mimosa leaves).
- Nyctinasty: 'Sleep' movements in response to light/dark cycles (e.g., legume leaves folding at night).
Plant Hormones (Phytohormones)
These chemical messengers regulate growth, development, and responses to stimuli.
Auxins (e.g., IAA)
Functions: Promote cell elongation, apical dominance, root initiation, phototropism, gravitropism.
Gibberellins (e.g., GA3)
Functions: Promote stem elongation (bolting), break seed and bud dormancy, promote flowering and fruit development.
Cytokinins (e.g., Zeatin)
Functions: Promote cell division (cytokinesis), promote lateral bud growth, delay leaf senescence (aging).
Abscisic Acid (ABA)
Functions: Inhibits growth, promotes seed and bud dormancy, causes stomatal closure during water stress, promotes leaf senescence and abscission (falling off).
Ethylene (Gaseous hormone)
Functions: Promotes fruit ripening, promotes senescence and abscission of leaves and flowers.
Coordination in Humans
Coordination in humans is achieved through the combined action of the Nervous System and the Endocrine System.
Nervous System
Provides rapid, short-lived coordination through electrical and chemical signals.
- Neuron (Nerve Cell): The basic structural and functional unit. Composed of a cell body (cyton), dendrites (receive signals), and an axon (transmits signals). The junction between two neurons is a synapse, where signals are transmitted chemically via neurotransmitters.
- Types of Neurons:
- Sensory Neurons: Carry signals from sense organs/receptors to the CNS.
- Motor Neurons: Carry signals from the CNS to muscles or glands (effectors).
- Association Neurons (Interneurons): Found within the CNS, connect sensory and motor neurons.
- Divisions of the Nervous System:
- Central Nervous System (CNS): Consists of the brain and spinal cord. Acts as the main processing center.
- Peripheral Nervous System (PNS): Consists of all the nerves that branch out from the CNS to connect it to other body parts (sense organs, muscles, glands). Includes cranial nerves (from brain) and spinal nerves (from spinal cord).
- Autonomic Nervous System (ANS): Part of the PNS that controls involuntary functions (heart rate, digestion, breathing). Has two divisions: Sympathetic (prepares for action) and Parasympathetic (promotes rest and digest).
- Brain: The main control center, protected by the skull (cranium) and meninges (membranes). Cushioned by cerebrospinal fluid.
- Cerebrum: Largest part, responsible for voluntary actions, thought, memory, learning, intelligence, and processing sensory information. Divided into two hemispheres.
- Cerebellum: Located below the cerebrum, coordinates voluntary muscle movements, maintains posture and balance.
- Medulla Oblongata (Brain Stem): Connects the brain to the spinal cord. Controls vital involuntary functions like heartbeat, breathing rate, blood pressure, swallowing, vomiting.
- Spinal Cord: A cylindrical structure extending from the brain stem down the vertebral column. Transmits nerve impulses between the brain and the rest of the body and controls reflex actions.
- Reflex Action: A rapid, automatic, involuntary response to a stimulus, processed at the level of the spinal cord (or brain stem). The pathway is called the reflex arc (Receptor -> Sensory Neuron -> Interneuron (in CNS) -> Motor Neuron -> Effector).
Endocrine System (Chemical Coordination)
Provides slower, longer-lasting coordination through chemical messengers called hormones, secreted by endocrine (ductless) glands directly into the bloodstream.
- Hormones: Act on specific target cells or organs to regulate processes like growth, metabolism, reproduction, mood, and stress response.
- Major Endocrine Glands and Hormones:
- Hypothalamus: Links nervous and endocrine systems, controls pituitary gland.
- Pituitary Gland (Master Gland): Secretes hormones controlling other glands (TSH, ACTH, FSH, LH) and growth (GH), water balance (ADH), etc.
- Thyroid Gland: Secretes Thyroxine, regulating metabolism.
- Parathyroid Glands: Secrete Parathormone, regulating calcium levels.
- Adrenal Glands: Secrete Adrenaline (emergency hormone) and Corticosteroids (stress response, metabolism).
- Pancreas (Islets of Langerhans): Secretes Insulin (lowers blood sugar) and Glucagon (raises blood sugar).
- Testes (Males): Secrete Testosterone (male characteristics, sperm production).
- Ovaries (Females): Secrete Estrogen and Progesterone (female characteristics, menstrual cycle, pregnancy).
- Thymus: Secretes Thymosin (immune system development).