GRE Subject Test: Biology : Understanding Plant Microstructures
Study concepts, example questions & explanations for GRE Subject Test: Biology
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Example Questions
Example Question #2 : Plant Structures
Plant cells differentiate to perform different functions and enable the plant to grow. One cell type is present in young stems and petioles and functions to provide flexible support. This cell type is less resistant to bending forces because it lacks a secondary cell wall and the protein lignin, which causes rigidity in other plant cells.
What differentiated plant cell is being described?
Parenchyma cells
Collenchyma cells
Sclerenchyma cells
Sieve plate cells
Collenchyma cells
As described in the beginning of this question, collenchyma cells are found in young stems and petioles (leaves) and function to provide flexible support to the plant. This is because chollenchyma cells lack secondary cell walls and do not produce lignin to harden them—this protein is characteristic of sclerenchyma cells, which are also used to provide support/strength to the plant.
Due to their lack of rigidity, collenchyma cells a also capable of elongating with the stems and leaves they support, allowing them to remain alive at maturity. Sclerenchymal cells lack this ability.
Example Question #302 : Gre Subject Test: Biology
Plant cells differentiate to be able to perform different functions and enable it to grow. One cell type has a critical job in supporting the plant. These cells have secondary walls that are further strengthened by a glue-like substance called lignin, which increases the cell's rigidity. At maturity, these cells cannot elongate and are found in regions of the plant that have stopped growing, forming a "skeleton" for the plant.
What type of differentiated plant cell is described?
Sclerenchyma cells
Secondary meristems
Parenchyma cells
Collenchyma cells
Sclerenchyma cells
As described in the background to the question, sclerenchyma cells are specialized to support the plant as it grows. These cells have thick secondary walls that are further strengthened by the hardening agent called lignin. As a result, these cells are highly rigid and inflexible.
At maturity, these cells cannot elongate and are found in regions of the plant that have stopped growing. In some parts of the plant, the sclerenchyma cells may even be dead; however, the rigid walls remain and act like a skeleteon that supports the remainder of the plaint over its lifetime.
Sclerenchyma cells can also further differentiate into two types called sclereids and fibers. Sclerids can provide hardness to nut shells. Fibers, as their name suggests, are usually arranged in long threads and have commercial uses, such as being made into rope.
Example Question #1 : Understanding Leaves
In plants, leaves contain specialized pores used for gas exchange. Each pore is formed by a pair of cells that control its closing and opening. What are these cells called?
Stoma cells
Cuticle cells
Epidermal cells
Guard cells
Guard cells
For proper functioning, plants must take in carbon dioxide, expel oxygen, and limit the loss of water vapor. This gas exchange takes place via pores called stomata. These pores are formed by a pair of adjacent cells that can open and close in response to a number of factors. These cells are called guard cells.
The cuticle and epidermis are layers of leaf structure, and do not correspond to specific cell types. The stoma is the name of a single pore itself, not its surrounding cells.
Example Question #2 : Understanding Plant Microstructures
What is the main structural component of a plant cell wall?
Cellulose
Actin and myosin
Peptidoglycan
Chitin
Collagen
Cellulose
Cellulose, a polymer of glucose, is the main component of plant cell walls.
Collagen is found in the connective tissues of animals. Chitin is found in the cell walls of fungi. Actin and myosin are the proteins responsible for contraction in muscle cells; actin is also a microfilament in the cytoskeleton. Peptidoglycan is found in the cell walls of bacteria.
Example Question #2 : Understanding Plant Microstructures
What are the protein channels in plants that allow high rates of water flow through the membrane via passive transport?
Aquaporins
Plasmodesmata
Xylem
Carrier proteins
Water does not need a protein channel to pass through the membrane
Aquaporins
The correct answer is aquaporins. While water can move across a membrane via simple diffusion, these transmembrane proteins increase the flow of water. Remember that water is a polar molecule, and is thus relatively impermeable to the plasma membrane despite its small size.
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