In order for a tree to carry out photosynthesis and maintain its overall health, water and nutrients need to move throughout the entire tree, even against gravity. How does a plant manage this without an organ like a heart that pumps fluids? As we will see, water is pulled from the roots to the leaves through a process called transpiration. In addition, water potential drives the movement of water from one area of the plant to another, using osmosis, gravity and the surface tension of water. Transpiration begins in the leaves. The arrangement of cells and structures in the leaf facilitate the movement of gases and water into and out of the leaf. A leaf contains several layers of specialized cells. The upper epidermis is one cell layer thick and provides a protective covering. Below that layer is mesophyll tissue. Cells in the Palisade Mesophyll layer are sites for photosynthesis and secretion, as well as storage of food and water. The spongy mesophyll layer contains a looser arrangement of cells, where spaces between cells aid in gas exchange and the passage of water vapor from the leaves. Throughout the lower epidermal tissue are stomata, which are microscopic openings flanked by guard cells. Gasses pass into and out of the leaf through these openings, as well as water vapor evaporating from the leaf, a process known as transpiration. The spongy mesophyll layer contains arrangements of vascular tissue, consisting of xylem and phloem that are specialized for the transport of water and nutrients throughout the plant. The vascular tissue extends from the leaves through the stem to the roots. Water is transported in xylem from the roots, where the water potential is higher, up to the leaves where the water potential is lower. The arrangement of the tissues, the functions of the cells and water potential determine the direction in which water will move through a plant. Water passes out of the leaf as water vapor through the stomata. The water vapor lost from the leaves is replaced with water that enters through the roots, and is brought up through the stem in xylem. Xylem is composed of vessels, which are continuous tubes formed from dead hollow cylindrical cells, arranged end-to-end, and tracheads, which are dead cells that taper as the ends overlap. This arrangement and the polar nature of water molecules allow water to pass in an unbroken stream through the xylem, from the roots, up through the chute, and into the leaves. Adhesion is the attraction of water molecules to a surface, such as the wall of the xylem. Cohesion is hydrogen bonding between water molecules. Together, adhesion and cohesion allow water to move through the xylem in a continuous stream, from the roots, up through the stem, to replace water lost from the leaves through the stomata. Water enters the plant through the epidermal cells of the roots, and travels into the xylem. Water potential in the cells of the roots increases when Simporter pumps in the plasma membrane, allow protons to pass into the cell, traveling down their concentration gradient. These pumps couple the transport of protons with the transport of minerals and other solutes into the cell. Water follows into the cell driven by osmosis. The presence of aquaporin channels in the membrane enhances osmosis, allowing bulk flow of water from the soil into the roots. The other main vascular tissue is phloem. Flowem transports carbohydrates and amino acids that are produced in the leaves to cells in the roots and stems where they are used and stored. Conduction in phloem is carried out through two kinds of elongated cells, Siv cells and Siv tube members. Most angiosperms contain Siv tube members. Both types of cells have clusters of pores known as Siv areas that are abundant on the overlapping ends of cells. These structures aid in the movement of carbohydrates, like sucrose, that are manufactured in the leaves and carried in the phloem throughout the plant, a process called translocation. Terger pressure increases in the Siv tube members, as sucrose from surrounding cells is brought into phloem through active transport. Water then enters phloem from xylem biosmosis, which drives the transport of carbohydrates in the phloem. Water movement in vessels is one way, while transport in Siv tube members can go in both directions. Water potential is an important driver in both xylem and phloem transport, but only phloem transport utilizes both active and passive transport. Our heart pumps blood throughout our bodies to provide nutrients and water to our cells. Vascular plants can accomplish this same feat without a heart, using transpiration, water potential, and translocation to move water, nutrients, and minerals to all cells of the plant.