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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Unicellular vs Multicellular Transport
All living organisms need to transport substances like nutrients, oxygen and waste products in and out of their cells. However, the way this happens varies dramatically between unicellular (single-celled) and multicellular organisms. Let's explore why these differences exist and how they work!
Key Definitions:
Single-celled organisms like amoeba, paramecium and bacteria rely entirely on their cell membrane for transport. They have a high surface area to volume ratio, which means substances can easily move in and out of the cell through simple diffusion, osmosis and active transport.
Organisms with many cells, like humans, plants and animals, have specialised transport systems (like blood vessels or xylem/phloem) because many of their cells are not in direct contact with the external environment. They have a lower surface area to volume ratio.
As organisms get larger, they face a fundamental challenge: volume increases faster than surface area. This creates a transport problem that affects how efficiently substances can move in and out of cells.
Imagine a tiny cube with sides 1mm long. Now imagine a larger cube with sides 10mm long:
Surface area: 6mm²
Volume: 1mm³
Surface area to volume ratio: 6:1
Surface area: 600mm²
Volume: 1000mm³
Surface area to volume ratio: 0.6:1
The larger cube has a much lower surface area to volume ratio, which means it would struggle to get enough substances in and out if it were a single cell. This is why larger organisms need specialised transport systems!
An elephant has a much lower surface area to volume ratio than a mouse. This affects many aspects of their biology:
Single-celled organisms have it relatively simple when it comes to transport. Their entire cell is in direct contact with the environment, so they can rely on these basic processes:
Oxygen and nutrients diffuse directly into the cell, while carbon dioxide and waste products diffuse out. No energy required!
Water moves in and out of the cell depending on the concentration of dissolved substances inside and outside the cell.
When needed, the cell can use energy (ATP) to move substances against their concentration gradient.
Multicellular organisms face a much bigger challenge. Many of their cells are buried deep inside the body, far from the external environment. They need specialised transport systems to move substances to and from these cells.
The human circulatory system is a perfect example of a specialised transport system in a multicellular organism:
Acts as a pump to move blood around the body. It creates the pressure needed to transport substances to distant cells.
A network of arteries, veins and capillaries that reach every cell in the body. Capillaries have thin walls to allow efficient exchange of substances.
A specialized fluid containing red blood cells (carrying oxygen), white blood cells (for immune response), platelets (for clotting) and plasma (carrying dissolved substances).
Specialized areas like the lungs and small intestine that have a large surface area and thin exchange surface to maximize the rate of transport.
Multicellular organisms have evolved various adaptations to overcome the surface area to volume problem:
Flat leaves with a large surface area for gas exchange, extensive root systems to absorb water and minerals and specialised xylem and phloem vessels for transport.
Gills with many thin filaments creating a large surface area for gas exchange and a streamlined body to reduce the distance oxygen needs to travel.
Lungs with millions of alveoli for gas exchange, a four-chambered heart for efficient circulation and a branching network of blood vessels.
Flatworms (planarians) are interesting because they're at the boundary between simple and complex animals. They're multicellular but don't have a circulatory system! How do they manage?
This shows how body shape can be adapted to solve transport problems in simpler multicellular organisms.
Let's compare how different transport processes work across organisms:
Diffusion and Osmosis:
• Used by all organisms
• No energy required
• Only works over short distances
• Moves substances from high to low concentration
• Main method for unicellular organisms
Energy-Driven Movement:
• Used by all organisms
• Requires ATP energy
• Can move substances against concentration gradients
• Used for specific substances
• Essential for both unicellular and multicellular organisms
Circulatory Systems:
• Only in complex multicellular organisms
• Requires specialized organs (heart, blood vessels)
• Can transport substances over long distances
• Moves many substances simultaneously
• Examples: human blood circulation, plant xylem/phloem
Specialized Areas:
• Found in multicellular organisms
• Large surface area
• Thin exchange barrier
• Often moist
• Examples: lungs, gills, intestines, leaves
The key difference between transport in unicellular and multicellular organisms comes down to the surface area to volume ratio problem. As organisms get larger, they need specialized systems to ensure all cells receive what they need and can get rid of waste products efficiently.
Remember these key points:
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