Plant Uptake and Transport

Plants are anchored into the ground by structures called roots. A root is an organ of a vascular plant that provides water and mineral support and anchors a plant to the soil. Plants can have one or many roots. They are the primary site of water and nutrient absorption. It is essential the soil provide adequate amounts of water and nutrients for the plant to survive. Water is a main component in the process of photosynthesis, the process by which plants make food from sunlight, as well as in the transport of materials from the ground up to the leaves and from the leaves back down to the roots. Differences in solute (dissolved solids) concentration between the inside and outside of roots result in osmosis (movement of water across a membrane, down a concentration gradient). To increase their surface area, roots have many tiny projections all over them called root hairs. These allow for more water uptake and also offer more support to anchor the plant into the soil.

The movement of water molecules across a semipermeable membrane from an area of lower concentration of solute to higher concentration of solute is called osmosis. Water and ions (charged particles) move across the cell membranes of the root cells in this manner. This action can sometimes be disrupted. Thus, there are special channel proteins that help facilitate the movement of water and ions. An aquaporin is a transport protein in a cell membrane that allows for osmosis, the diffusion of water across a semipermeable membrane. Ions are moved by other channel proteins when they must move across a membrane against their concentration gradient. The molecular pumps that move these ions consume energy in a process known as active transport, the movement of material across the cell membrane against its concentration gradient, requiring the cell to expend energy. Plants use a proton pump that uses energy from ATP to move protons out of the cell against their gradient. This action forms electrical and proton gradients, which means there are more protons outside the cell than inside. This makes the inside of the cell more negative than the outside, so positive ions, such as potassium, can then move into the cell.

Once water has been taken into the roots, it has to be transported to the different areas of the plant where it is needed, especially into the leaves. To do this, plants have evolved a series of tubes called xylem, a vein-like tissue that carries water from roots and stems to leaves.

Water and ions pass from the roots into the xylem through two different pathways. Each pathway may occur independently or at the same time. The first path is called the apoplast, the continuous network of cell walls and extracellular spaces in plants through which materials can pass without having to go into the cell itself. It looks like a mesh network through which the water can pass. If this is used, the water and nutrients never have to enter the cell itself. Apoplasts work very quickly.

The other pathway uses a symplast, the network in cell interiors of plant cells through which materials can pass uninterrupted via plasmodesmata. Using this pathway involves the water and minerals passing through the cytoplasm (watery interior) of the plant cells. The pathway is continuous because of openings in the cell walls and membranes. A plasmodesma (plural, plasmodesmata) is a small channel between mesophyll cells and bundle-sheath cells through which molecules pass between carbon fixation in the mesophyll cells and the Calvin cycle in the bundle-sheath cells. Plasmodesmata extend through the cell wall of a plant cell and directly connect the cytoplasm of adjacent plant cells. Here, the water passes through each membrane and through the cell itself until it reaches its destination.

After passing through one (or both) of these pathways, the water and minerals reach the endodermis (inner layer) of the root, called the cortex. A special hydrophobic structure called the Casparian strip, a water-impermeable ring within the roots that regulates water uptake, surrounds each endodermal cell. Because it is waterproof, the Casparian strip forces the water and minerals into the symplast, so all the water and nutrients are now being channeled through it. This action pushes the solution into the xylem so that it can then be transported up through the xylem toward the leaves. At this point, it is called xylem sap. The largest issue facing moving water in the xylem is that the water has to move against gravity. Scientists have developed a model that shows how water is able to move from the soil into the roots, up the stem, and to the leaves. The model used to describe how water is pulled from the roots, up the stems, and out of the leaves is called the transpiration-cohesion-tension model, and it consists of three main parts.

• Transpiration is the method by which water exits the leaves through small openings called stomata. It evaporates from the leaves because of temperature, humidity, and other environmental factors.
• Cohesion is a property of water molecules that allows them to stick together. As one water molecule moves up through the xylem, it pulls another one along with it.
• Tension is created by negative pressure in the xylem resulting from the act of transpiration from the leaves.

Thus, transpiration, the loss of water from the plant leaves through the stomata as a result of temperature and humidity (faster at higher temperatures and slower in humid conditions), creates negative pressure at the top of the plant, resulting in tension, which draws water upward. Water's cohesion means each water molecule that rises because of this tension pulls another molecule along. When these water molecules reach the leaves, they evaporate through transpiration, continuing the process.

There is a large concentration difference between the amount of water vapor in the air and the amount present within a leaf. As a result, water from the leaves is constantly evaporating into the atmosphere. This is called transpiration. Located in large numbers on the undersides of leaves, as well as on many other parts of the plant, is a small opening that allows for gas exchange between the plant and the external environment called stoma (plural, stomata). Water exits the moist cells of the leaf by evaporating out of these stomata when they are open.

Each stoma is surrounded by two guard cells. These cells swell and relax depending on the amount of water they contain. If they are full of water and thus swollen, the stoma is open. When they contain less water and thus are flaccid, the stoma is closed. Several environmental factors determine the opening and closing of the stomata. These include temperature, light availability, levels of carbon dioxide, and the amount of water available. The plant's response to these stimuli can occur very quickly, as environmental conditions are always changing. For example, light causes the stomata to open. Opening them allows carbon dioxide to enter the leaves, while water vapor and oxygen are expelled. To do this, the guard cells absorb blue light waves, which activate the proton pump. When light is not present, the guard cells close, shutting down the proton pump. This reduces the water potential, the tendency of water to move from areas of high concentration to those of low concentration of the cells, and closes the stomata. Not all plants respond to environmental stimuli in the same manner. For example, cacti living in the desert are exposed to very high temperatures during the day, when there is a lot of light, and much cooler temperatures when it is dark. Because water conservation is a primary concern for these desert plants, having their stomata open during the day would result in excessive water loss. Instead, these plants keep their stomata closed during the day and open them at night.

Having too many stomata can be a detriment. Tall trees can lose up to 2 liters of water per hour through the stomata. There are more than 250,000 stomata per square inch on the underside of any given leaf. Large trees can have tens of thousands of leaves. This equates to an enormous amount of water potentially being lost every day. To compensate for this, plants are able to reduce the number of stomata they use. Trees do this by dropping excess leaves in certain environmental conditions. Other plants can regulate the formation of new stomata as new leaves develop.

Water is brought from the roots up to the leaves by way of the xylem for the purpose of carrying out photosynthesis. During this process the water is broken apart into hydrogen ions (H⁺) and oxygen atoms. The oxygen is released through the stomata as a waste product, and the hydrogen ions are used to fix carbon atoms into sugar molecules. This sugar is synthesized to be used as an energy source for the plants. Because photosynthesis takes place in the leaves and glucose is stored in the roots, there needs to be a mechanism in place to transport glucose back down the plant from the leaves to the roots. This is performed by a series of tissues that transport sugars and other materials from the leaves of plants to their roots called phloem.

The newly created sugars diffuse into the nearest phloem tubes. The transport of materials through the phloem of vascular plants is called translocation, and phloem sap is the product being moved. These materials move from a source, the location in plants where the synthesized materials originate, such as leaves or roots that make the products, to a sink, the location in plants where the synthesized materials are transported, such as the roots, fruits, or flowers, where the products are consumed. Notice that roots can be both a source and sink. This is because roots store the sugars made from photosynthesis in the fall (functioning as a sink) and then send them back up the plant to be used as an energy source when new leaves form in the spring (functioning as a source).

Phloem sap moves from areas of high pressure to areas of low pressure. The pressure flow model refers to the idea of plant transport based on the osmotically generated pressure that moves materials between sources and sinks. This gives those cells a higher sugar concentration than surrounding cells. This results in water moving into the cells by osmosis, causing higher pressure inside these cells than outside. This moves the sap to the sink end of the tubes. Here, the sugars are unloaded into storage areas, such as fruits, and the water moves back into the xylem to be used again.

Like water moving into the xylem, the sugars in the phloem move through two main pathways: apoplastic and symplastic. The apoplastic pathway moves the sugars from the central tissues of the leaves into the apoplast (the space outside the cell membrane). Specific sugars and amino acids (the building blocks of proteins) allow the plants to monitor which sugars and how much of them are moved in this manner. The symplastic pathway moves the sugars from the interior tissues of the leaves directly into sieve tubes, which are structures in the plant that move sugar. Once the sugars reach the sink, they are moved into the surrounding tissues for storage and maintenance.