transport in plants

How can we study the movement of water in plants?

1. Take a celery plant and wash it with water to remove the soil. 2. Allow the plant to stand with its stem immersed in dilute red ink (or methylene blue solution) 3. After a few hours, you can see that the red ink has risen up the plant. Cut thin transverse sections of the stem. 4. Place the sections on a glass slide. Examine the sections under a microscope. => Xylem tissue should be stained red, and we can thus conclude from the investigation that xylem vessels transport water key idea: The path that water takes through a plant can be denonstrated by letting a plant take in coloured water. Only the xylem tissue will be stained

How does water enter a plant?

Absorption of water takes place through the root hairs. Root hairs also absorb ions or mineral salts from the soil solution Each root hair is a fine tubular outgrowth of an epidermal cell. It grows between soil particles, coming into close contact with the soil solution surrounding them The thin film of liquid surrounding each soil particle is a dilute solution of mineral salts The sap in the root hair cell is a relatively concentrated solution of sugars and various salts. Thus, the root hair cell sap has a lower water potential than the soil solution. These two solutions are separated by the partially permeable cell surface membrane of the root hair cell. Water enters the root hair via osmosis. The entry of water dilutes the sap. The sap of the root hair cell now has a higher water potential than that of the next cell. Hence, water passes by osmosis from the root hair cell to the inner cell Similarly, water passes from the inner cell to the next inner cell of the cortex. This process continues until the water reaches and enters the xylem vessels snd moves up the plant.

How do root hairs absorb mineral salts or ions?

By active transport, when the concentration of ions in the soil solution is lower than that in the root hair cell sap. -The root hairs have to absorb ions against a concentration gradient, that is, by active transport. -The energy needed for this process comes from cellular respiration in the root hair cell. By diffusion, when the concentration of certain ions in the soil is higher than that in the root hair cell.

Using isotopes in translocation studies

Carbon-14 (^14 C) is a radioactive carbon isotope. Its presence can be detected by an X-ray photographic film. A leaf is provided with carbon dioxide containing the radioactive carbon 14^C. When photosynthesis takes place, the sugars formed will contain radioactive carbon. The stem is then cut and a section of it is exposed onto an X-ray photographic film. It is found that radioactive substances are found in the phloem, since radioactive substances cause the X-ray film to darken. key idea: Radioactive 14^C can also be used to show that manufactured food substances are transported by the phloem.

How is phloem adapted for its function?

Companion cells have many mitochondria, which provide the energy needed for the companion cells to load sugars from the mesophyll cells into the sieve tubes by active transport. The holes in the sieve plates allow rapid flow of manufactured food substances through the sieve tubes.

Phloem function

Conducts (translocates) manufactured food (sucrose and amino acids) from the green parts of the plant, especially the leaves, to the other parts of the plant. Translocation expends energy.

How is water transported up the stem without using any energy

Despite moving against gravity, the water transport does not expend energy. The xylem cells are dead an do not transport water, but rather the channel these dead cells form that transports the water. (channel wall made of dead xylem cells). Water flows through the channel (one-way path; upwards) through transpirational pull and cohesion tension. Transpiration is the evaporation of water from the plant. Water molecules have strong cohesion with each other. Hence water molecules at the top of the plant are pulled through the leaf during transpiration, and the other molecules attached to them below will get pulled up as well. When water is absorbed by the roots, pressure creates an upward force which pushes the water up through the stem. These forces along with the cohesion forces draw water up the stem and veins without using energy. However, as this is a form of passive transport, flow of water is affected by factors that affect the rate of transpiration.

transport structures of flowering plants

Flowering plants have a system of vessels that runs up and down the plants carrying materials. These vessels are called transport or vascular tissues. Vascular tissues are a system of vessels that transports materials in plants. There are two types of transport tissues in plants: the xylem (or wood) and phloem.

How are the vascular tissues organised in roots?

In a dicotyledonous root, the xylem and phloem are not bundled together. Instead, they alternate with each other. The cortex of the root is also a storage tissue (from endodermis to epidermis) The epidermis of the root is the outermost layer of cells. It bears root hairs. It is also called the piliferous layer. Each root hair is a tubular outgrowth of an epidermal cell. This outgrowth increases the surface area to volume ratio of the root hair cell. The absorption of water and mineral salts is increased through this adaptation.

How are the vascular tissues organised in stems ?

In a dicotyledonous stem, the xylem and phloem are grouped together to form a vascular bundle. The vascular bundles are arranged in a ring around a central region called the pith. The phloem lies outside the xylem with a tissue called cambium between them. Cambium cells can divide and differentiate to form new xylem and phloem tissues, giving rise to a thickening of the stem. The region between the pith and the epidermis is the cortex. Both the cortex and pith serve to store up food substances, such as starch. The stem is covered by a layer of cells called the epidermis. The epidermal cells are protected by a waxy waterproof cuticle that greatly reduces evaporation of water from the stem.

Phloem structure

Phloem consists of mainly sieve tubes and companion cells. A sieve tube consists of columns of elongated, thin-walled living cells called sieve tube cells or sieve tube elements. Sieve tube cells are placed end to end to form sieve tubes. The partition walls ("end-walls") separating the cells have a lot of minute pores. The cross-walls look like a sieve and so are called sieve plates. A mature sieve tube has only a thin layer of cytoplasm inside the cell. This cytoplasm is connected to cells above and below through the sieve plates. Each sieve tube cell has lost its central vacuole, nucleus and most organelles. Each sieve tube cell also has a companion cell beside it, which carries out metabolic processes needed to keep the sieve tube cell alive. Each companion cell is a narrow, thin-walled cell with many mitochondria, cytoplasm and a nucleus. Companion cells provide nutrients and help the sieve tube cells to transport manufactured food.

How is the root hair cell adapted to its function of absorption

The root hair is long and narrow. This increases the surface area to volume ratio which in turn increases the rate of absorption of water and mineral salts by the root hair cell. The cell surface membrane prevents the cell sap from leaking out. The cell sap contains sugars, amino acids and salts. It has a lower water potential than the soil solution. This results in water entering the root hair via osmosis. The root hair cell contains many mitochondria. Aerobic respiration in the mitochondria release energy for the active transport of ions into the root hair cell.

How can we study the path food substances take through a plant?

The transport of manufactured food substances such as sugars and amino acids in plants is known as translocation. What evidence is there to show that transport of manufactured food substances occurs through the phloem? 1. Using aphids in translocation studies Insects such as aphids feed on plant juices. The long mouthpart of each aphid penetrates the leaf or stem. The aphid can be anaesthetised with carbon dioxide while it is feeding. The body of the aphid is then cut off, leaving only the feeding stylet in the plant tissues. A liquid will exude from the cut end of the proboscis. An analysis of this liquid shows that it contains sucrose and amino acids. If the stem is sectioned and examined under a microscope, you will see that the feeding stylet of the aphid is inserted into the phloem sieve tube. This shows that translocation of sugars and amino acids occurs in the phloem. key idea: Studies show that aphids extend their feeding stylets into the phloem and the liquid they take in contains manufactured food substances. This demonstrates that phloem carries manufactured food. 2. Using the 'ringing' experiment - Cut off a complete ring of bark including the phloem and cambium from the main stem of a woody twig. (e.g. hibiscus). This will leave the xylem exposed. Place the twig in water with the cut ring above water level. - Set up a control using an unringed twig -Observe the twigs daily. Note where the roots or swellings appear. Make drawings of your observations. key idea: Removing the outer layer of phloem tissue from a plant (ringing) will cause the plant to swell just above the cut area.

How is a xylem vessel adapted for its function?

The xylem vessel has an empty lumen without protoplasm or partition walls ("end-walls") . This reduces resistance to water flowing through the xylem. Its walls are thickened with lignin. Lignin is a hard and rigid substance. It prevents collapse of the vessel (all the xylem vessels together provide mechanical support to the plant)

Structure of Xylem, consisting of (xylem vessels)

Xylem tissue consists mainly of xylem vessels. A xylem vessel is a long hollow tube stretching from the root to the leaf (hollow part a.k.a. lumen). The xylem vessel is a structure made of many dead cells The inner walls of xylem vessels are strengthened by deposits of a substance called lignin. Lignin may be deposited in the form of rings, spirals or the whole vessel is lignified except in regions called pits

Functions of Xylem

Xylem tissue has two functions: - Conducting water and dissolved mineral salts from the roots of the plant to the stems and leaves. It does not expend any energy while doing this. - Providing mechanical support to the plant