Overview: Pathways for Survival
The algal ancestors of plants obtained water, minerals and CO2 from the water in which they were completely immersed.
For vascular plants, the evolutionary journey onto land involved the differentiation of the plant body into roots, which absorb water and minerals from the soil, and shoots, which absorb light and atmospheric CO2 for photosynthesis.
This morphological solution created a new problem: the need to transport materials between roots and shoots.
Xylem transports water and minerals from the roots to the shoots.
Phloem transports sugars from the site of production to the regions that need them for growth and metabolism.
Concept 36.1 Physical forces drive the transport of materials in plants over a range of distances
Transport in plants occurs on three levels:
1. The uptake and loss of water and solutes by individual cells, such as root hairs.
2. Short-distance transport of substances from cell to cell at the level of tissues or organs, such as the loading of sugar from photosynthetic leaf cells into the sieve tubes of phloem.
3. Long-distance transport of sap within xylem and phloem at the level of the whole plant. Transport at the cellular level depends on the selective permeability of membranes.
The selective permeability of a plant cell’s plasma membrane controls the movement of solutes between the cell and the extracellular solution.
Molecules tend to move down their concentration gradient. Diffusion across a membrane is called passive transport and occurs without the direct expenditure of metabolic energy by the cell.
Active transport is the pumping of solutes across membranes against their electrochemical gradients, and requires expenditure of energy by the cell.
The cell must expend metabolic energy, usually in the form of ATP, to transport solutes “uphill.”
Transport proteins embedded in the membrane can speed movement across the membrane.
Some transport proteins bind selectively to a solute on one side of the membrane and release it on the opposite side.
Others act as selective channels, providing a selective passageway across the membrane.
For example, the membranes of most plant cells have potassium channels that allow potassium ions (K+) to pass, but not similar ions, such as sodium (Na+).
Some channels are gated, opening or closing in response to certain environmental or biochemical stimuli. Proton pumps play a central role in transport across plant membranes.
The most important active transport protein in the plasma membrane of plant cells is the proton pump.
It hydrolyzes ATP and uses the released energy to pump hydrogen ions (H+) out of the cell.
This creates a proton gradient because the H+ concentration is higher outside the cell than inside.
It also creates a membrane potential or voltage, a separation of opposite charges across a membrane.
Both the concentration gradient and the membrane potential are forms of potential (stored) energy that can be harnessed to perform cellular work.
This potential energy is used to drive the transport of many different solutes.
For example, the membrane potential generated by proton pumps contributes to the uptake of potassium ions (K+) by root cells.
The proton gradient also functions in cotransport, in which the downhill passage of one solute (H+) is coupled with the uphill passage of another, such as NO3− or sucrose.
The role of proton pumps in transport is a specific application of the general mechanism called chemiosmosis, a unifying process in cellular energetics.
In chemiosmosis, a transmembrane proton gradient links energy-releasing processes to energy-consuming processes.
The ATP synthases that couple H+ diffusion to ATP synthesis during cellular respiration and photosynthesis function somewhat like proton pumps.
However, proton pumps normally run in reverse, using ATP energy to pump H+ against its gradient. Differences in