To travel from A to B, we use specially designed modes of transport like cars, and multicellular and single-celled organisms move substances the same way!
Single-celled organisms move substances across their body surface, but when there are many cells, the volume increases while the surface area stays the same, so substances have to supply more needs and travel farther, slowing down diffusion and causing cell death.
To battle this, multicellular organisms increase their SURFACE AREA TO VOLUME RATIO by using specialised exchange surfaces and transport mediums to take substances in and out and around organisms.
1. Gas Exchange
In mammalian gas exchange, specially designed ALVEOLI increase the surface area of the lung, aiding the uptake of oxygen and the removal of carbon dioxide.
In plants, however, carbon dioxide diffuses into the cells to be used for photosynthesis, while the oxygen made diffuses out into the environment.
Diffusion also helps remove waste like urea, where blood transports it from the liver to the kidneys to be made into urine and excreted out of the body.
3. Dissolved Food
Dissolved food molecules diffuse across the wall of the small intestine and into the bloodstream using MICROVILLI that increase the gut's surface area for absorption.
4. Water transport via OSMOSIS
Osmosis is the diffusion of water down a concentration gradient, and when plant cells are placed in a dilute solution they take water in and become TURGID, and the only thing that stops them bursting is their strong permeable cell walls.
But if plant cells are placed in highly concentrated solutions, meaning lots of solute but little water, cells release water and they shrink away from the cell wall, becoming PLASMOLYSED.
Animals cells change shape during osmosis as they don't have cell walls, sometimes affecting their function like red blood cells expanding or shrivelling up.
5. Active Transport of Mineral Ions
In plants, the uptake of MINERAL IONS by root hair cells uses active transport to pull mineral salts from the low concentration in the soil into the already highly saturated cells AGAINST the concentration gradient, using ENERGY from respiration.