Transport Systems Essay
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...
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