How do water, sugar and other substances move around in a plant?  Ch. 4 p. 50-54

All molecules are in constant motion.

Solids: molecules vibrate in place

Liquids: molecules slide around within the liquid

Gases: molecules bump around freely

1. Diffusion p. 22

As molecules bump into each other, they move from areas of high to areas of low concentration. This is called diffusion. Concentration is the number of molecules in a given area. For example, if someone is popping popcorn down the hall, you begin to smell it. That is because the popcorn molecules are bouncing around down the hall. If somebody else at the other end of the hall starts frying bacon, the bacon molecules move away from that area. Each type of molecule diffuses independently. For example, the bacon molecules will not prevent the diffusion of the popcorn molecules.

When stomata open, this allows carbon dioxide to move in. As the carbon dioxide is converted to glucose. This lowers the carbon dioxide concentration in the leaf and maintains the concentration gradient so that carbon dioxide gas continues to diffuse into the leaf.

At the same time that carbon dioxide is moving into the leaf, water vapor is moving out of the leaf. This is because the air has less water vapor than the the leaf. This is especially true in a dry climate like Arizona. As water vapor diffuses out, columns of water are pulled along the plant vascular bundles. The vascular bundles in plants that carry water are called xylem. The movement of water through the xylem is analogous to water being sucked through a straw.

Water loss by evaporation from stomata is called transpiration (fig. 4.1)

Based on the above table, would you consider palm trees to be a low water plant?  Palm trees tend to be tropical and when found in the desert are often at the "oasis".  So while they are heat tolerant, they are certainly NOT low water users!

Transpiration of water helps keep plants cool by evaporative cooling. This cooling affect in plants is important since many enzyme-mediated chemical reactions work best at certain temperatures. Examples of chemical reactions we have looked at are photosynthesis, cellular respiration, the production of many of the herbal medicines that we discussed.

2. How do sugars move from leaves to roots? (fig. 4.4)

Phloem cells carry sugar from the leaves down to the roots. Cells that make the sugars pump them across the plasma membrane into the phloem cells. Water follows the sugar because of osmosis. Osmosis is the movement of water across membranes from areas of low solute concentration to areas of high solute concentration. Solutes are substances dissolved in the water.

At the root end, sugars are pumped out of the phloem into root cells. Water follows the movement of the sugars. Thus, water carries the sugar along like a garden hose with higher pressure on one end and lower pressure on the other.

Osmosis is really just a specialized case of diffusion. Why? Because when water is bound to particles in solution, it is no longer free to move across membranes. Therefore, there is a difference in the concentration of free water molecules on different sides of a membrane. What direction do these free water molecules move? From areas where there are lots of them (high concentration of free water molecules) to areas where there are less of them (low concentration of free water molecules). The availability of the water molecules depends on how many are bound to the solute molecules.

The opening of stomata is caused by the pumping of K⁺ and Cl^- into the guard cells that surround them. This pumping is followed by the movement of water by osmosis and leads to expansion of the guard cells. The guard cells have thicker cell walls on the inside so they tend to bulge out when they expand.

Halophytes store large amounts of salt in their vacuoles. By doing this, they are able to pull in water by osmosis.