The transportation system is essential for all organisms. Because, transportation allow 2-way movement in which nutrients and essential materials are absorbed and the wastes are excreted away. Transportation mechanism vary in plants and animals as the animals have the capacity to move from one place to another to reach their food source whereas plants are stationary making. However, the deeply rooted plant roots help in accomplishing their mineral and water requirements despite their stationary nature. On the other hand, the animal`s body system is able to carry out the transportation within their body through osmosis and diffusion. For example, diffusion of gases in the lungs, blood circulation, etc. Since the roots and leaves in a plant stay far enough, it is very difficult for the plants to adapt the diffusion as a transport mechanism. On the other side, plants have low energy needs when compared to animals. This is because, as the plants grow, they form a large proportion of dead cells on some layers. There are 3 pathways in the plant transportation system:
The uptake and release of water and solutes by individual cells.
Transportation through the short distance between one cell to another.
Transportation through the long-distance of sap within xylem and phloem.
Role, of Xylem and Phloem in transportation
They are the 2 important channels of plant transportation. The xylem moves carries water and minerals obtained from the soil towards the upper parts of the plant. Phloem is responsible for the transportation of sugars from source tissues such as from photosynthetic leaf cells towards sink tissues (ex. non-photosynthetic root cells or developing flowers). Phloem also help in transporting proteins and mRNAs throughout the plant.
Translocation is the term used to explain the movement of necessary materials from leaves to rest of the plant tissues. Plants demand minerals, sunlight and carbon dioxide, out of which minerals are transported through translocation. Sunlight and carbon dioxide are absorbed directly through chlorophylls found in the leaves. Plants carry out the transportation in different directions. In the rooted plants, the xylem help in a transportation in a unidirectional fashion, i, e from roots to the stems. Furthermore, the minerals and organic nutrients undergo multidirectional transport. Elements such as growth regulators, hormones and chemical stimuli are moved in a polarized(dispersed) manner from the location where they are formed.
Mechanism of transportation in plants
Plants use diffusion, facilitated diffusion and active transport mechanisms in order to keep the transportation system in balance. Diffusion is a passive process as it doesn`t require energy to transport the materials. Diffusion is a gradual process of moving the substances from a higher concentration region to a lower concentration region. The rate of diffusion rely upon the concentration gradient, systemic pressure, temperature and permeability of membrane( if found) separating them. Substances with high hydrophilic moiety are difficult to move passively via membranes hence facilitated diffusion comes into picture. Facilitated diffusion is the passive movement of molecules along their concentration gradient guided by the presence of another molecule( hence the name facilitated ). Facilitated diffusion is facilitated by an integral membrane protein forming an easy channel. Three types of transport proteins are Uniport, Symport, and Antiport proteins. As the name suggests, the uniport proteins carry a single solute across the membrane. Symport proteins help to translocate 2 different solutes at a time in the same direction. Antiport proteins help to exchange the solutes by transporting one towards the cell and the other away from the cell. Since active transport works against the gravity, it needs ATP. ATP help in the process of pumping the necessary materials against the concentration gradient. The ATP donates a phosphate to a particular gateway. Phosphate movement help in pumping the desired molecules across the membrane, despite the lower concentration gradient.
Forces required for transportation in plants
The frequent evaporation of water molecules through the water film, the curvature of the meniscus goes high resulting in surface tension. Surface tension help in drawing water from their surrounding cells.
Water Potential Gradient
The water potential gradient is highest in the roots than the air space. This is known as the water potential gradient. Water potential gradient help in moving water from the roots to the leaves through the spongy parenchymal cells.
The force of Hydrogen Bonding between Water Molecules
The water molecules stick to each other by hydrogen bonds. As the molecules are pushed by diffusion, the is a mechanical push against the water molecules. This helps the movement of water molecules within the xylem.
It slightly differs from diffusion because unlike diffusion, osmosis is carried out in the presence of a medium (semi-permeable membrane). Solutes and water molecules move from high concentration areas to the low concentration areas through the semi-permeable membrane. The plant cell membrane is a permeable layer allowing free movement of substances. The plant cell contains large vacuole, with a vascular sap assist in gaining the solute potential required by the cell. In-plant cells, the membrane of the vacuole, cell membrane and tonoplasts are the important determinants of the movement of molecules.
It is the contraction of the protoplasm of cells as a result of the loss of water through osmosis. 3 features of solutes (solutions) responsible for plasmolysis are isotonic, hypertonic and hypotonic nature of solutes. Isotonic solutions have the same pressure levels across both the sides of the cell (outside and inside the cell wall). When the cytoplasmic cells have a high concentration than the solutes found outside, it is called hypotonic. If the solution found outside is more concentrated, it is called hypertonic solution. Depending upon the nature of the solution, a fluid shift drives the cell into either dehydration or swelling as shown below.
Water Movement against the gravity in a Plant
Water movement against gravity is enabled by active transport with the help of ATPs. However, factors such as root pressure also drive the water from the bottom upwards. The root pressure is not a primary force behind the upward water movement, however, the root pressure assist in re-establishing the chain of water molecules in the xylem resulting in transpiration. Hence, the transpiration pull is the root cause of water movement in the tall trees.
Plants do not have any circulatory system, but the flow of water upward via xylem is achieved by fairly high rates and that is up to 15 mts/ hr. According to researchers, water is “Pulled” through the plant and this driving force is transpiration from the leaves. It is termed as cohesion – tension – transpiration pull model of water transport.