34.12:
Short-distance Transport of Resources
When roots take up nutrients and water from the soil, it is distributed across diverse tissues within the whole plant. The products of photosynthesis, likewise, must travel throughout the plant to cells with storage functions or energy-requiring processes, crossing cell walls, membranes, and the cytoplasm of many cells along the way.
Plants can move solutes using three general pathways. Transport via the apoplastic pathway occurs through the extracellular space including cell walls, while transport via the symplastic pathway occurs through plasmodesmata, pores that directly connect the cytoplasm of neighboring cells.
A third pathway, the transmembrane pathway, moves substances in and out of cells through the plasma membrane. Repeatedly moving substances across the plasma membrane is sufficiently fast over 2 or 3 cells, but much slower over longer distances.
Transport across membranes in plant cells bears some general similarities to transport in animal cells. The selectively-permeable plasma membrane allows some substances, such as carbon dioxide and oxygen gas, to diffuse through passively, moving along their concentration gradients from areas of high concentration to areas of low concentration.
Other substances cannot diffuse across the membrane due to charge or size, such as ions and larger molecules like sugars. Instead, cells actively take in these solutes using specific membrane proteins, such as ion channels and transporter proteins.
Proton pumps use the chemical energy from ATP to create an electrochemical gradient of hydrogen ions across the cell membrane. Many transporters in plants use this hydrogen gradient to move resources into the cell. For instance, the nitrate transporter in roots moves one nitrate along with one hydrogen ion, even against the concentration gradient of nitrate.
34.12:
Short-distance Transport of Resources
Short-distance transport refers to transport that occurs over a distance of just 2-3 cells, crossing the plasma membrane in the process. Small uncharged molecules, such as oxygen, carbon dioxide, and water, can diffuse across the plasma membrane on their own. In contrast, ions and larger molecules require the assistance of transport proteins due to their charge or size. Transport across membranes also occurs within individual cells, playing a variety of essential roles for the plant as a whole.
Resources are transported into and out of the central vacuole within each plant cell
One of the roles of the large central vacuole of a plant cell is the storage of resources. Active and passive transport proteins are found in the vacuolar membrane, or tonoplast, just as they are found in the plasma membrane of the cell, and they regulate the movement of solutes between the cytoplasm and vacuole. Sugar can be stored for later, ions are sequestered from the cytoplasm, and protons, in particular, are pumped into the vacuole, creating an acidic environment for breaking down unwanted or toxic substances that enter the cell.
Movement across the tonoplast controls turgor pressure
In addition to its role in storage, the vacuole generates turgor pressure – a force that pushes the plasma membrane against the cell wall – contributing to the structure of the plant. The size of the vacuole is regulated by the movement of solutes across the tonoplast by channels and transporters. Water diffuses passively across the tonoplast to balance out a difference in solute concentration across the membrane, and it can also move more rapidly through aquaporins, water channels that can open and close in response to cellular signals. Under drought conditions, a lack of water will result in a loss of turgor pressure within individual cells as the vacuole shrinks. On a macroscopic level, the plant will appear wilted when turgor pressure is low.