All cells, whether free living or part of a larger organism, need certain basic things to survive. If they are animal cells, they need oxygen for respiration and they must rid themselves of the waste gas, carbon dioxide. Plant cells must have access to carbon dioxide for photosynthesis and they also need oxygen when they are breaking down sugars to release energy in the process of cellular respiration. Animal cells, not able to make their own sugars through photosynthesis, also need nutrients to burn for energy and in order to grow and eventually reproduce. These activities produce waste products which must be voided from the cell. The build up of waste products can poison a cell and eventually kill it.
All this is easy for a single celled organism. Protozoa and single celled algae inhabit aquatic and marine environments, so they are surrounded by water. Single celled organisms are defined by the cellular membrane, which separates them from the surroundings, Being semipermeable, it also allows gases and nutrients to flow into the cell as needed, and waste products to move out. The movement does not have to be forced. Simple diffusion means that nutrients and gases will flow from areas of high concentration to areas of low. If the cell needs oxygen, it will move inward. If the levels of carbon dioxide go up, it will move out. The same is true of nutrients and waste products.
But what about cells buried deep in a multicellular organism? How do these cells, which need them as much as single cells, get oxygen and nutrients. How do they get rid of waste products? This is where transport systems become so necessary in higher animals and plants.
The simplest multicellular animals do not have special transport systems. Jellyfish and sponges pump water through and around their cells and no cell is buried so deep that it cannot get access to food, water and oxygen. Even the smallest and simplest worms do not need special transport systems, but this limits how big they can get. Flatworms, phylum Platyhelminthes, have only two cell layers, ectoderm and endoderm. The cells on the outside, which are muscular and move the animal about, are in contact with water and so can access food and oxygen and get rid of wastes. Platyhelminthes have one opening that serves as both mouth and anus and leads to a two way digestive system. By swallowing water and food, the cells on the inside can access the nutrients and oxygen and rid themselves of wastes. They are in direct contact with the ectoderm and so can pass broken down nutrients on to the outer cells as well.
But get a little bigger, add a mesoderm or middle layer, and the game plan has to change. In order to get bigger, organisms had to develop transport systems to supply cells that were no longer in direct contact with the environment. My first paid research work as a biologist was to study an ectoparasite of oysters, the polychaete worm, Polydora websteri, which makes mud burrows on the insides of the oyster shells, thus ruining a potentially valuable food item. When I looked at them under the microscope, I discovered that they had red blood, just like we do. They don't have a heart but move the blood around the body with muscular movements. The food they eat and the oxygen they need moves in the blood cells to all the cells of the body. Then the blood moves waste products back out to the gut where they can be voided through the anus, just as in higher animals
By the time we get to arthropods and vertebrates, we find that these two very different animal groups have separately evolved an organ to move the blood, a muscular heart that pumps the blood around. This is the basic animal transport system: heart and blood vessels. In aquatic and marine animals, the blood passes through gills which puts the blood in contact with oxygen-rich water. In land animals, lungs have evolved and the blood capillaries pass through the oxygen-rich cells of the lungs where they can pick up oxygen and release carbon dioxide, still simply by the process of diffusion. Nutrients are picked up from the digestive system and waste products delivered to the excretory system. All this is only possible with an efficient transport system.
What about plants? How have they solved the problems of feeding their cells and removing waste products? Primitive plants are limited in size and cannot leave aquatic or marine environments because they lack transport systems. Higher plants have specialised tissues called xylem and phloem that perform the functions of the blood vessels in higher animals. Xylem carries water up and down the stems of higher plants while phloem carries nutrients. Phloem is responsible for moving sugars from the leaves down to the stem and root cells while moving minerals from the roots up to the above-ground cells.
How does this movement occur without a heart? It all has to do with the flow of water in the process of diffusion. Leaves have large cells called stomates from which water transpires into the surrounding environment. This causes water to be drawn up from the soil, through the roots and up xylem till it replaces the water that has been lost. Nutrients continue to move by diffusion from cells of high concentration (phloem) to other cells with low concentrations of nutrients. Waste products flow the opposite direction for the same reason.
Multicellular organisms need to provide their cells with water, oxygen and nutrients in order to maintain healthy cells. They do this with transport systems because those cells are no longer able to collect these ingredients or release waste products themselves. Without transport systems, higher organisms from redwood trees to humans, would not be possible.