PowerPoint Presentation

The general structure of a plant can be divided into 2 systems the aboveground shoot system and the below ground root system. The shoot system is specialized for reproduction and to harvest light and CO2 for use in photosynthesis (which makes sugars). The root system is specialized to absorb water and nutrients such as nitrogen, phosphorus and potassium. In this PowerPoint we will study how plants transport water, nutrients and sugars throughout the different structures of the plant.1

There are 3 major types of tissue in plants dermal, ground and vascular tissue. Dermal tissue is a single layer of cells the covers the plant body and protects it. The ground tissue includes cells that perform photosynthesis and store substances for the plant such as starch. The vascular tissue provides support and transports water, nutrients and sugars for the plant. The figure above shows the location of these three types of tissues in the root, the stem and the leaves of a plant. As you can see the vascular tissue is colored red in the figure and it is always surrounded by the ground tissue. This arrangement facilitates the efficient transport of substances between these two tissue types. The ground tissue in the leaves and stem photosynthesize and produce sugars, which can then be transported to vascular tissue so they can delivered throughout the plant. Similarly the vascular tissue carries water and nutrients from the roots to the ground tissue, which needs these substances for photosynthesis and to synthesize molecules needed by the plant.2

The arrangement of vascular and ground tissue can vary between plant types. The slide above shows micrographs of cross-sections through two different plant stems. The plant on the left is a dicot and it has large vascular tissue bundles arranged in a ring around the outer edge of the stem. The plant on the right is a monocot and it was small vascular tissue bundles arranged randomly throughout the ground tissue.3

There are two types of vascular tissue xylem and phloem. Xylem transports water and nutrients absorbed by the roots. Phloem transport sugars synthesized by the ground tissue. We will look at the structure of xylem first.

Within xylem tissue there are two types of cells tracheids and vessel elements but all xylem cells share the following traits: 1) they transport water and dissolved nutrients in one direction, from the roots to the tips of the shoots, 2) xylem cells are dead when they start to transport water and nutrients and 3) the xylem cells have secondary cell walls consisting of the strong, structural molecule lignin (in addition to their primary cell walls that consist of cellulose).

Xylem cells originate from meristem (growth) areas of the plant. The xylem cells are formed, they grow to their mature size and then they die. After they die, the soft tissue of the cell is reabsorbed by the plant and the primary and secondary cell walls remain, forming a hollow tube like structure of lignin and cellulose that can transport water. Tracheids and vessel elements differ from each other by their shape. Tracheids are long, slender cells with tapered ends. Along the side of the tracheids are pits, where the secondary cell wall is missing and only the primary cell wall remains. Water is able to enter tracheids through the pits. Tracheids are stacked together in bundles and water is able to travel both vertically and horizontally between tracheids through the pits. Vessel elements are shorter and wider xylem cells, which are stacked on top of each other to form tubes. Water enters vessel elements through pits, and then moves between vessel elements through perforations in the ends of the vessel elements.4

This slide shows a micrograph of a vascular bundle that contains both tracheids and vessel elements.5

Now we will look at the process by which water moves from the ground into the roots and up to the tips of the shoots even up to the tips of trees that can be 300 feet tall! Basically, water is pulled up to the tops of plants, especially tall plants, from the roots, by forces generated by transpiration at the surface of leaves. Transpiration is the loss of water from the surface of leaves via evaporation. So lets analyze the steps in this process.

The figure above shows a stoma. These are the structures on leaf cells where O2 and CO2 can enter cells. The stoma has guard cells that can change shape to control the size of the opening in the stoma. When the pore is open, water can also be lost from the inside of the cell via evaporation.

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Steps 1 and 2 in the figure above (a close up of the cells around the stomatal opening) show that when the stomata are open water will diffuse from the leaf cells around the stoma opening and evaporate through the opening created by the guard cells.

Step 3 (a cross-section of a leaf) shows that when water diffused from the leaf cells, these cells will have a lower water concentration than the xylem cells near them. Therefore, by the force of diffusion, water will then move (or be pulled) from the xylem cells to the leaf cells.

Step 4 shows that when water is pulled out of the xylem cells in the leaves (by the force of diffusion), it will pull water up the xylem tube because there is a strong cohesive force between individual water molecules. See next slide.7

When this water cell diffuses from the xylem to the ground tissue cells of the leaf, it pulls the column of water in the xylem up because each of the water molecules have strong hydrogen bonds between them.xylemWater

Water moves up the xylemWater diffuses into ground tissueWater molecules are polar molecules, meaning that the oxygen atoms in a water molecule (shown as red circles) have partially negative charges and the hydrogen atoms in a water molecule (shown as white circles) have partially positive charges. The partial negative and positive charges attract one another forming a hydrogen bond. The hydrogen bond is a cohesive force that holds water molecules together. The slide explains how this cohesive force causes a column of water to move up the xylem tubes.8

Steps 5 and 6 in the figure above shows that as water is pulled up the xylem, it creates a diffusive force that causes water to be pulled out of the soil into the roots. The next slide will show how this happens.9

This micrograph shows that in roots, the xylem is located in the middle of the root cell and is surrounded by ground tissue. Therefore, the water must diffuse from the soil across the epidermis of the root (dermal tissue) and through the ground tissue to the xylem cells.10

There are three ways that water travels through the ground tissue. In the transmembrane route, water flows through water channels in the membranes of cells (known as aquaporins), then out of the cell into the space between cells, then back into the next cell via an aquaporin. It continues to travel this way until it reaches the xylem. In apoplastic route, water flows through spaces in between the cells. In the symplastic route, water flows through cells via pores between cells known as plasmodesmata.11

When the water reaches the xylem in the roots, it diffuses into the xylem to fill the space that is created as the water is pulled up the xylem column.12

So to summarize, water evaporates from the surface of the leaves (transpiration), causing water to diffuse from the xylem into ground tissue to replace the water that evaporated. The loss of water from the xylem, causes water to be pulled up the xylem because of the cohesive force between water molecules (created by hydrogen bonding). The as the water moves up the xylem, it creates a diffusive force that pulls water into the roots from the soil, where it diffuses through the ground tissue and then enters the xylem.13

Now we will look at the process of translocation movement of sugar within the plants. Sugars are formed during the process of photosynthesis, which occurs in the leaves and, to a lesser extent, the stem of plants. Therefore, as the figure above shows, the leaves and stems are sources of sugar. The flowers and leaves need the sugars as a source of energy, so these structures are sinks for the sugars, or structures to which the sugars must travel. 14

This figure shows that paths that sugars will travel from the sources to the sinks.15

Phloem is the vascular tissue through which the sugars are transported from source cells to sink cells. Phloem consists of cells known as sieve tube members and companion cells. Sieve tube members are the cells through which the sugars are transported, and unlike xylem cells, they are alive at maturity. The sieve tube members are filled with cytosol (watery matrix inside of cells) like normal cells, but they contain few organelles. The sieve tube members have pores in them called plasmadesmata that connect them to companion cells. The sieve-tube members are stacked on top of one another to form a tube, and substances pass from one sieve-tube member to the next via perforations in the sieve plate.

Companion cells have a full complement of organelles and provide the attached sieve-tube members with the proteins and other molecules that they need to remain alive.16

This slide shows how sugars produced by source cells travel to sink cells that need the sugars.Leaf cells produce sugars. Recent research has shown that the sugars are actively transported (which requires ATP energy) from the leaf cells, to the companion cells and then to the phloem cells. The sugar diffuses through the phloem sieve tube members.The sugar is unloaded from the phloem into the sink cells by passive diffusion or active transport, depending on the structure of the plant where it is being unloaded.17