TRANSPORT IN PLANTS Flashcards
What are the two transport tissues in plants and what do they transport?
Xylem: Conducting water and dissolved mineral salts from the roots to the stems and leaves. (INSIDE FACING OF STEM)
Phloem: Phloem conducts manufactured food (sucrose and amino acids) from the green parts of the plant, especially the leaves, to other parts of the plant. (OUTSIDE FACING OF STEM)
What kind of cells make up xylem tissue?
What are adaptations of the xylem?
Dead Cells.
The xylem vessel has an empty lumen without protoplasm or ‘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).
What are the adaptations of the phloem?
- 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.
How are the vascular tissues organised in stems?
- The xylem and phloem are grouped together to form vascular bundles.
- Arranged in a ring around a central region called the pith.
- The phloem lies outside the xylem with a tissue called the 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 the pith serve to store up food substances.
- The stem is covered by a layer of cells called the epidermis.
How are aphids used in translocation studies?
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.
How are isotopes used in translocation?
Carbon-14 (14C) 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, 14C. 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 present in the phloem.
HOW DOES WATER ENTER A PLANT?
Each root hair is a fine tubular outgrowth of an epidermal cell. It grows between the 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.
(ROOT HAIR CELL OSMOSIS)
The entry of water dilutes the sap. The sap of the root hair cell (cell A) now has a higher water potential than that of the next cell. Hence, water passes by osmosis from the root hair cell into the inner cell.
Similarly, water passes from cell B into the next cell of the cortex. This process continues until the water enters the xylem vessels and moves up the plant.
How do root hairs absorb ions or mineral salts?
- By active transport, when the concentration of ions in the salt solution is lower than that in the root hair cell sap.
- By diffusion, when the concentration of cer tain ions in the soil solution is higher than that in the root hair cell.
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 by osmosis.
-The root hair cell contains many mitochondria. Aerobic respiration in the mitochondria releases energy for the active transport of ions into the cell.
What is root pressure?
The living cells around the xylem vessels in the root use active transport to pump ions into the vessels. This lowers the water potential in the xylem vessels. Water therefore passes from the living cells into the xylem vessels by osmosis and flows upwards.
What is capillary action?
Water tends to move up inside very narrow tubes (capillary tubes) due to the interactions between water molecules and the surfaces of the tubes. Since the xylem vessels in the plant are very narrow tubes, capillary action helps in moving water up the vessels.
How is transpiration involved in moving water against gravity?
(Transpiration pull)
The evaporation of water from the leaves removes water from the xylem vessels. This results in a suction force which pulls water up the xylem vessels. This suction force due to transpiration is known as transpiration pull.
Explain how water moves from a mesophyll cell in the leaf into the atmosphere.
- Water continuously moves out of the mesophyll cells to form a thin film of moisture over their surfaces.
- Water evaporates from this thin film of moisture and moves into the intercellular air spaces. Water vapour accumulates in the large air spaces near the stomata (sub-stomatal air spaces).
- Water vapour then diffuses through the stomata to the drier air outside the leaf. THIS IS TRANSPIRATION.
- As water evaporates from the mesophyll cells, the water potential of the cell sap decreases. The mesophyll cells begin to absorb water by osmosis from the cells deeper inside the leaf. These cells, in turn, remove water from the veins, that is, from the xylem vessels.
- This results in transpiration pull, a suction force which pulls the whole column of water up the xylem vessels.
What is the importance of transpiration?
- Transpiration pull draws water and mineral salts from the roots to the stems and leaves.
- Evaporation of water from the cells in the leaves removes latent heat of vaporisation. This cools the plant, preventing it from being scorched by the hot sun.
- Water transpor ted to the leaves can be used in photosynthesis; to keep cells turgid; and to replace water lost by the cells. Turgid cells keep the leaves spread out widely to trap sunlight for photosynthesis.
What does a potometer measure?
Directly measure the rate of absorption of water
by the plant and not the rate of transpiration.
