In single celled organisms, nutrients, water and various substances that are need for basic cellular processes can easily be transported in and out through the cell membrane. This is an effective transport system for microscopic organisms, but multicellular organisms require a more complex transport system to sustain itself. Take humans for example; unlike a cell, only the outer layer of skin is in contact with the body’s outside environment. It’s not possible for substances to diffuse all the way through every layer of cells in the human body and reach an organ deep within it, like the heart.
The movement of waste from inside the body to outside the body is also not possible without a transport system. For this very reason, large multicellular organisms such as plants and mammals must have multiple specialised transport systems. Many large multicellular organisms contain complex transport systems. In mammals, a circulatory system containing the heart, arteries, veins, blood and lungs allow cells in the organism to gain oxygen from red blood cells, release carbon dioxide and dissolved nutrients from the plasma in blood.
Hormones, proteins and white blood cells from the immune system are other substances that can be found in blood. This system is known as a “circulatory” system because fluids containing the nutrients (blood) travel around it in a closed circuit around the body. An organ called the heart, which is a large muscle, continuously pumps blood through the circuit. The vessels that make up the circuit are thick veins and arteries, or thin one-cell thick capillaries. Dissolved nutrients and oxygen are contained in the blood, which travel around arteries and into the smaller capillaries.
As the capillaries are only one cell thick, it is possible for the oxygen and the nutrients to diffuse through to the cells in need. After the oxygen diffuses through, the blood becomes deoxygenated and travels through veins, back into the heart and then pumped to the lungs to be oxygenated again. It also releases carbon dioxide waste from cells, and the gas is exhaled out of the body. The process repeats itself infinitely, until the organism dies, and effectively transports nutrients and gases. In plants, the circulatory system isn’t used mainly for gas exchange like it is in animals.
The main purpose of the circulatory system in plants is for the transport of water, sugars and minerals. There are two types of tissues that are involved in a plant’s circulatory system. These long and tube-like connective cells are called xylem and phloem. Together they are grouped into long vessels called “vascular bundles”, and are found throughout the plant. Xylem tissues are live cells when they are first created, but die every year to become support for the plant, as well as the original purpose of transpiration.
Together these cells form a one-way system that transports water and minerals. Phloem tissues on the other hand, are cells that make up a two-way system which transport a fluid made up of water and dissolved sugars. As not all cells of a plant have the ability to carry out photosynthesis, it is necessary for sugars to be transported from the main site of photosynthesis (which are the leaves) to areas where there are no cells capable of photosynthesis (such as the roots or the stems of a plant). This is also why phloem is a two-way system, unlike the one-way system of xylem.
Xylem cells only need to travel one way as the water it carries eventually reaches the leaves and are released out of the plant through the stomata on the leaves. That is also why xylem is mainly made up of dead cells and phloem is made up of living cells. Xylem uses passive transport to move water through a plant, where as phloem uses active transport and requires energy, and therefore living cells, to move substances. The cells found in phloem tissues include sieve tubes, companion cells, phloem fibres and phloem parenchyma cells. Sieve tubes are long cells that connect and direct the flow of the fluid within the phloem.
They lose some organelles like the nucleus as they mature, making the insides of the cell hollow. As the sieve tube can’t carry out its regular cellular processes, a companion cell does it instead. These live cells are used in the process of translocation in plants. The circulatory system in mammals is closely related to another system, called the lymphatic system. Capillaries connect these two systems, as the “lymph” (a fluid which travels around the lymphatic system to remove waste and fight infection) travels through capillaries as well as through the fluid between cells.
It also drains excess liquid from tissues so that they don’t swell up. The circulatory system is much different from the lymphatic system, because it is a closed system. The blood in the vessels don’t leave to flow around cells, where as lymph does. Not like the circulatory system, the lymphatic system doesn’t have a pump to move the lymph around. However, lymph can enter the bloodstream, so it can “hitch a ride” with the blood. Also, the lymphatic vessels are not like veins and arteries. The system is made of various organs, ducts and nodes. There isn’t a specific “circuit” like there is for blood.
However different these two systems are, as they work together, the body can efficiently remove waste products from cells. Likewise in plant cells, there are two transport systems that are very different but cooperate with each other in the organism. In xylem cells, water and minerals travel along a one-way system. The water is used in cellular processes, to maintain cell turgor as well as to replace the water that is lost through the transpiration process. Unlike mammals, plants don’t have a “pump” to enable the fluids to travel around the plant. Instead, fluid moves through the xylem with transpiration pull and root pressure.
As water is evaporated from the plant and into the atmosphere, it creates a negative pressure and draws up the water from the root to the leaves. The natural surface tension and cohesion of water molecules help the transpiration process to draw water upwards in the plant. The translocation in phloem cells is not like the passive transport of transpiration. It is a type of active transport requiring energy and can be explained with the pressure flow hypothesis. Water flow through the phloem while under pressure caused by the difference in water molecule amounts in the xylem tissue next to the phloem.
Meanwhile, sugars are moved into the sieve tubes as the water moves in by osmosis. The turgor pressure that builds up in the tubes moves the sugar and water along, both up and down the tubes. Therefore, this active transport process is much different from the transpiration process. Some plants don’t have the need for xylem cells, such as the elodea plant. In a way they are without half of their circulatory system, but they are constantly surrounded by water so it is not needed. However, mammals don’t have the ability to “leave out” parts of the circulatory system and all components are essential for it to work.
Both plants and mammals have transport systems that require energy. Both of the mammalian systems need energy, while only the phloem system need energy in plants. However, plants are autotrophs and animals are heterotrophs. They acquire their energy from very different sources. Plants use photosynthesis to make their own energy. They use products such as water and carbon dioxide to produce oxygen and glucose. Mammals can’t use photosynthesis; they eat other organisms and digest them to obtain sugars. Both plants and animals use cellular respiration to convert the sugars into ATP for cellular processes.
Although the transport systems of mammals and plants are very different, they both serve a similar purpose of transporting substances around the organism. The major differences between them are that mammals have a closed circulatory system, where as plants constantly lose water as it exits the plant by evaporation. Blood remains within the vessels in the system. Also, mammals have a pump (the heart), which pushes the fluids around the body. Plants don’t have this and rely on transpiration and translocation for the movement of fluids.
Some similarities include how both systems transport nutrients and water, as well as how they both incorporate active and passive transport. Even if it is active transport (in phloem or in the mammalian transport systems), the nutrients usually diffuse into the cells from the transport vessels, which is a type of passive transport. Both transport systems in plants and mammals are an essential part of large multicellular organisms and without them there would be no way of transporting much needed substances to cells in need.