Plants – Lecture 1, Revision question answers
Plant cells and tissues
1. a. Describe the structure of a primary cell wall. b. How does a secondary cell wall differ? a. The primary cell wall consists of strands of cellulose that run parallel to each other to form the microfibrils. Various molecules including glycoprotein and pectin form crosslinks between the microfibrils.
b. Compared with the primary cell wall the secondary cell wall is thicker and more rigid.
The linking material is lignin which gives the secondary wall its additional strength.
2. Name the three tissue types found in plants and the general function of each.
Ground tissue makes up most of the plant body and has a number of functions including photosynthesis, storage, structural support and strength/protection.
Dermal tissue (includes epidermis and periderm) forms the outer protective layer of the plant. Vascular tissue (including xylem and phloem) is responsible for the transport of materials throughout the plant body.
3. Draw a simple diagram to represent the epidermis of a leaf. How would you expect the root epidermis to be different? Explain.
See the iBook, Plants p12.
The leaf epidermis has a cuticle to reduce water loss from the leaf. There is no cuticle on the root epidermis. One of the functions of the root is water absorption and the presence of a cuticle would impede (slow or stop) this.
The leaf epidermis has stomata to allow gas exchange and transpiration. Stomata are absent in the root epidermis – transpiration does not occur and gas exchange to support photosynthesis is not necessary.
The leaf epidermis has trichomes that act as leaf hairs. They function in providing protection and reduce water loss. In the root trichomes are present but as root hairs they are responsible for absorption of water from the soil.
4. In flowering plants, water movement involves specialised cells- tracheids and vessel elements. Describe the structure of each of these cell types at maturity.
Tracheids are elongated cells with pointy ends. They contain many pits, which allow movement of water from one cell to another. At maturity they are dead and contain no organelles. A vessel element at maturity is a cylindrical shaped cell with no end cell walls. At maturity it is dead so has no organelles.
5. Name and briefly describe the model used to explain the rise of water from the roots to the leaves in plants.
Cohesion-Tension model - water is pulled up the stem as a result of tension created at the top of the plant. This tension is caused by the evaporative pull of transpiration. (So as water evaporates from the leaf surface water is drawn up from the soil). The upward pull of the water is possible as long as there is a continuous unbroken column of water in
the xylem. The adhesive and cohesive properties of water allow this column of water to be maintained. The adhesive property of water means that water sticks to surfaces eg., to the inner walls of the vessels or tracheids. The cohesive property means that water molecules stick to each other.
6. Both sieve cells and companion cells have an important role in the transport of sugars in solution. What is the function of each?
Sieve cells stack on top of each other