Water and ions move across the root cell’s cell membrane

737

Now that you’ve seen how water can move into plant cells by osmosis, what about ions? And what is the role of the cell membrane? The movement of water and mineral ions across a root cell membrane can be impeded for two reasons:

  1. The interior of the membrane is hydrophobic, whereas water and mineral ions are polar.

  2. Some mineral ions must be moved against their concentration gradients.

However, as you saw in Chapter 6, membrane proteins assist with the movement of materials across membranes:

Electric charge differences also play a role in the uptake of mineral ions. For example, a negatively charged ion that moves into a negatively charged compartment is moving against an electrical gradient, and this requires energy. Concentration and electrical gradients combine to form an *electrochemical gradient. Uptake against an electrochemical gradient involves active transport, which requires energy and specific transport proteins.

*connect the concepts Electrochemical gradients are important in many biological systems. Learn more about electrochemical gradients, and their role in the animal nervous system, in Key Concept 44.2.

Unlike animals, plants do not have a sodium–potassium pump (described in Key Concept 6.4) to drive active transport. Rather, plants have a proton pump, which uses energy obtained from ATP to move protons out of the cell against a proton concentration gradient (Figure 34.4, Step 1). Because protons (H+) are positively charged, their accumulation outside the cell has two results:

  1. An electrical gradient is created, with the region outside the cell more positively charged than the inside.

  2. A proton concentration gradient develops, with more protons outside the cell than inside.

image
Figure 34.4 The Proton Pump in Transport of K+ and Cl The active transport of hydrogen ions (H+) out of the cell by the proton pump (1) drives the movement of both cations (2) and anions (3) into the cell.

Both the electrical gradient and the concentration gradient assist with the movement of other ions into the cell. Because the inside of the cell is more negative than the outside, cations (positively charged ions) such as potassium (K+) can move into the cell by facilitated diffusion through specific membrane channels (Figure 34.4, Step 2). In addition, the proton concentration gradient can be harnessed to drive secondary active transport, in which anions (negatively charged ions) such as chloride (Cl) are moved into the cell. These ions can move against the electrochemical gradient because symport proteins couple their movement with that of H+ (Figure 34.4, Step 3).