Living Organisms: Characteristics, Cell Structure, and Organization

Biology is the study of life. To understand living organisms, we must first distinguish them from non-living things and explore their fundamental building blocks: the cell.

1. Characteristics of Living Things

All living organisms share common characteristics, often remembered by the acronym MR NIGER D:

• Movement: The ability to change position or posture. Animals move their whole body (locomotion), while plants show restricted movement (e.g., phototropism).
• Respiration: The breakdown of food substances to release energy.
• Nutrition: The process by which organisms obtain and utilize food.
• Irritability/Sensitivity: The ability to respond to external stimuli.
• Growth: An irreversible increase in size and dry mass.
• Excretion: The removal of metabolic waste products from the body.
• Reproduction: The ability to produce offspring to ensure continuity of the species.
• Death: All living things eventually cease to function and die.

2. Cell Structure and Functions

The cell is the basic structural and functional unit of life. Cells contain organelles that perform specific tasks.

• Nucleus: Contains genetic material (DNA) and controls all cellular activities.
• Mitochondria: The 'powerhouse' of the cell where aerobic respiration occurs.
• Ribosomes: Sites for protein synthesis.
• Endoplasmic Reticulum (ER): Rough ER (with ribosomes) transports proteins; Smooth ER synthesizes lipids.
• Cell Membrane: A semi-permeable layer that regulates what enters and leaves the cell.
• Cell Wall: (Plant cells only) Provides structural support and protection, made of cellulose.
• Chloroplasts: (Plant cells only) Contain chlorophyll for photosynthesis.
• Vacuoles: Large and central in plants (for storage/turgidity); small or absent in animals.

3. Levels of Organization

Living things are organized in a hierarchy of increasing complexity:

• Cell Level: The simplest level. Examples include Euglena and Paramecium (unicellular organisms).
• Tissue Level: A group of similar cells performing a specific function. Examples: Epithelial tissue, Hydra.
• Organ Level: Different tissues working together. Example: Onion bulb, Heart, Leaf.
• System Level: A group of organs working together. Examples: Digestive system, Excretory system.
• Organism Level: The complete living being. Example: Chlamydomonas, Humans.