Edexcel iGCSE Biology-3.2 Transport system in multicellular organisms- Study Notes- New Syllabus

Transport System in Multicellular Organisms

📝 Introduction

All living organisms need nutrients, gases, and waste removal.
In unicellular organisms, diffusion alone is enough.
But in multicellular organisms (e.g., humans, plants), diffusion is too slow and inefficient.
That’s why they evolved specialised transport systems.

🔑 Reasons Why Transport Systems Are Needed

• Large Size
  Multicellular organisms are made of millions of cells.
  Cells deep inside the body are far from the surface → diffusion distance is too long.
• Small Surface Area to Volume Ratio (SA:V)
  As size increases, SA:V decreases.
  Surface area is not enough to supply all cells just by diffusion.
• High Metabolic Demand
  Active tissues (e.g., muscles, brain, roots, leaves) need lots of oxygen and nutrients quickly.
  Diffusion is too slow to meet this demand.
• Waste Removal
  Substances like carbon dioxide and urea must be removed efficiently.
  Otherwise, toxic build-up could harm the organism.
• Specialisation of Cells
  Multicellular organisms have many types of specialised cells.
  They depend on each other → nutrients and oxygen must be transported to where they are needed.

🚩 Examples of Transport Systems

• Animals → Circulatory system (heart + blood vessels)
• Plants → Xylem (water, minerals) and Phloem (sugars)

📊 Summary Table

⚡ Quick Recap 
Unicellular → diffusion works (small, large SA:V).
Multicellular → need transport system because:
– Too big
– Small SA:V
– High energy demand
– Need waste removal
Examples: Circulatory system (animals), Xylem + Phloem (plants).