Edexcel iGCSE Biology-2.1 levels of organisation in organisms- Study Notes- New Syllabus

Levels of Organisation

📝 Introduction

Living organisms are made of different structural levels, starting from the smallest units (organelles) and building up to whole systems.

Each level works together, forming a hierarchy.

👉 Think of it like Lego blocks – tiny pieces (organelles) combine to make bigger structures (cells → tissues → organs → systems → organism).

🔬 1. Organelles

Tiny structures inside cells that carry out specific functions.

• Nucleus → controls cell activities, stores DNA.
• Mitochondria → site of respiration, releases energy.
• Chloroplasts → photosynthesis (plants only).
• Ribosomes → protein synthesis.

🧩 2. Cells

The basic unit of life. All living organisms are made of cells.
Each cell type is adapted for its function.

• Root hair cell → absorbs water/minerals.
• Red blood cell → carries oxygen.
• Muscle cell → contracts for movement.

🧵 3. Tissues

A group of similar cells working together to perform a function.

• Xylem tissue → transports water in plants.
• Phloem tissue → transports sugars.
• Muscle tissue → contracts to cause movement.
• Nervous tissue → transmits electrical impulses.

🏥 4. Organs

Different tissues working together for a specific task.

• Heart → pumps blood (muscle + nerve + blood vessel tissue).
• Leaf → photosynthesis (palisade tissue, xylem, phloem, guard cells).
• Stomach → digestion (muscle, glandular, epithelial tissue).

⚙️ 5. Organ Systems

A group of organs working together to carry out a big function.

• Digestive system → breaks down food + absorbs nutrients.
• Circulatory system → transports substances around the body.
• Respiratory system → gas exchange.
• Nervous system → communication and response.
• Vascular system (plants) → transport of water (xylem) + food (phloem).

📊 Summary Table – Levels of Organisation