Characteristics of Living Organisms - Grade 9 Lesson 1 Notes

Overview of Unit 1: Characteristics of Living Organisms

This study guide is based on the curriculum materials for Grade 9, specifically Lesson 1, as organized by Manivannan lalajini on October 1, 2025. The primary focus of this unit is to establish the fundamental criteria that distinguish living organisms from non-living matter. The core content is delivered through the document titled "Grade 9 Lesson 1.docx" and forms the basis of the Integrated Science curriculum for the Year 9I class of 2025/26.

The Seven Characteristics of Living Organisms

To be classified as a living organism, an entity must demonstrate seven specific biological processes. These are often summarized by the mnemonic MRS GREN, representing Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition. These processes are essential for the survival and propagation of species within the biosphere. Each characteristic is defined by specific biological parameters that ensure the organism can maintain its internal environment and interact effectively with its external environment.

Movement

Movement is defined as an action performed by an organism, or a part of an organism, that causes a change in position or place. While this is often obvious in animals through locomotion (the movement from one location to another), it is also present in plants. In plants, movement is typically slower and involves parts of the organism, such as leaves turning toward light ($\text{phototropism}$) or roots growing toward water ($\text{hydrotropism}$). In multicellular organisms, movement may involve the contraction of muscle fibers, while in unicellular organisms, it may involve the use of cilia, flagella, or pseudopodia.

Respiration

Respiration describes the complex chemical reactions that occur within cells to break down nutrient molecules and release energy for metabolism. Most living organisms carry out aerobic respiration, which requires oxygen to break down glucose according to the following chemical equation:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}$

This biological energy, stored in the form of Adenosine Triphosphate ($ATP$), is utilized for various cellular processes, including growth, repair, and active transport. In the absence of oxygen, some organisms can perform anaerobic respiration, although this process yields significantly less energy and often produces metabolic byproducts like lactic acid or ethanol.

Sensitivity

Sensitivity, also referred to as irritability, is the ability of an organism to detect and respond to changes in the internal or external environment. These changes are known as stimuli (singular: stimulus). Organisms possess various receptors to sense these changes, such as light, temperature, chemicals, and physical pressure. By responding to these stimuli, organisms can navigate toward favorable conditions (such as food or light) and away from unfavorable conditions (such as predators or extreme heat), thereby increasing their chances of survival.

Growth

Growth is defined as a permanent increase in size and dry mass. This expansion is achieved through an increase in cell number, an increase in cell size, or both. In multicellular organisms, growth involves the process of mitosis, where cells divide to produce genetically identical daughter cells. It is important to distinguish between temporary increases in size (such as swelling due to water absorption) and true biological growth, which involves the synthesis of new proteins and structures that contribute to the organism's total dry biomass.

Reproduction

Reproduction is the process by which organisms produce more of their own kind, ensuring the continuity of the species. This can occur through two primary methods: asexual reproduction, involving a single parent and producing genetically identical offspring (clones), and sexual reproduction, involving the fusion of male and female gametes ($\text{fertilization}$) to produce genetically diverse offspring. Without reproduction, a population would eventually face extinction as older individuals die off.

Excretion

Excretion is the removal from organisms of the waste products of metabolism, toxic materials, and substances that are in excess of requirements. Metabolism refers to the sum of all chemical reactions occurring within a cell, and many of these reactions produce byproducts that would be harmful if allowed to accumulate. For example, carbon dioxide ($CO_2$) is a waste product of aerobic respiration, and urea is a waste product of the breakdown of excess proteins. Excretion differs from egestion, which is the removal of undigested food (feces) that never actually entered the body's cells.

Nutrition

Nutrition involves the intake of materials for energy, growth, and development. Organisms require specific nutrients to maintain health and facilitate biological processes. Moving through the food chain, organisms obtain these nutrients in different ways. Plants are autotrophic, meaning they create their own organic nutrients (like glucose) from inorganic materials using light energy through the process of photosynthesis:

$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$

Animals, conversely, are heterotrophic and must consume organic compounds (carbohydrates, proteins, and fats) by eating other organisms. Both plants and animals require inorganic ions and water to support their physiological functions.

Classroom Context and Course Information

This material is a component of the iLower Secondary curriculum at CIS (Prasanna Weerasinghe). The Lesson 1 documentation was shared within the Year 9I 2025 digital classroom environment. The class is part of a broader academic schedule that includes several other Year 9 courses, such as:

• Year 9 German (DM 2025/26)

• Mrs. Madurika's Geography Class

• Year 9I [Art & Design] with Ms. Gandhari

• Year 9I Computing

• 8I - Integrated Science (transitional context)

• Year 9I Class 2025/26