Plant Biology - GRE Subject Test: Biology

Chloroplasts are organelles in plant cells that conduct photosynthesis; therefore they are unique to plant cells. All the other mentioned organelles can be found in both animal and plant cells.

Chloroplasts, the site of photosynthesis, are only in plant cells and are not found in animal cells. Ribosomes, a cell membrane, and a mitochondria, however, can be found in both animal and plant cells.

The other structure that may be found in plant cells, but not animal cells, is a cell wall.

The Calvin cycle takes place in the stroma of the chloroplast, which is the region in the chloroplast lumen outside of the thylakoids. It does not actually matter whether or not light is present for the reactions of the Calvin cycle to take place. They are light independent, but light will not prevent the reactions from occurring, similar to how glycolysis is independent of oxygen. As long as the appropriate nutrients and reactants are present, including ATP and NADPH generated from the light reactions, the Calvin cycle will occur.

The Calvin Cycle occurs in the stroma, the aqueous fluid-filled area of the chloroplast. The stroma can be seen as analogous to the cytoplasm of a cell, in that it is the liquid in which all other substructures reside. The other processes of photosynthesis, the light-dependent reactions, take place in the thylakoid, a membrane-bound substructure within the chloroplast.

Photosystem II splits water into hydrogen and oxygen. Hydrogen ions accumulate in the thylakoid space, creating an electrochemical concentration gradient. Due to this gradient, hydrogen ions pass through ATP sythase, powering the synthesis of ATP from ADP + Pi.

Plants do not have the ability to actively transport water to their respective cells. Instead, water undergoes capillary action, which allows it to flow upward against gravity. When the water is located in a very narrow chamber, such as the xylem of a plant, it creates intermolecular interactions with the walls of the chamber. These interactions allow small amounts of the water to "climb" the chamber walls. Due to the cohesion of water, whereby it is attracted to itself, more water molecules follow the "climbing" adhesion molecules. This subsequently allows the adhering molecules to climb higher, and the joint interaction of the adhesion and cohesion eventually allow the water to reach the topmost region of the plant (the leaves). Water is then released from the stomata, furthering the pull of water to the region of low pressure.

One of water's most distinctive properties is cohesion—that is, the tendency of water molecules to "stick" to one another. In plants, this cohesion results in columns of water that stretch through the plant's xylem (the vascular tissue responsible for transport of water), from the roots all the way to the leaves. During transpiration, water evaporates from plants' leaves. Because of the cohesion of water, whenever water evaporates, more molecules are "pulled" into the roots to maintain the column of water. This is the transpirational pull-cohesion tension theory.

In contrast, adhesion is the tendency of water molecules to "stick" to other substances, such as the walls of a glass. Adhesion is responsible for the curved meniscus of water in a graduated cylinder. Phloem is responsible for sugar and carbohydrate transport in plants, while xylem transports water.

The stoma allows for gas exchange and transpiration. The stoma usually opens following stimulation by sunlight and closes in low water environments. Other answers are parts of a typical plant, however do not play a role in gas exchange or transpiration.

A plant's vascular tissues transport nutrients throughout the plant, just as the circulatory system transports nutrients throughout human bodies. While blood is the primary solvent for nutrients in humans, water is the primary solvent for nutrients in plants. Animals, however, use blood pressure to propel nutrients throughout the body while plants use gravity and the cohesive properties of water to transport nutrients.

The two primary types of plant vascular tissue are xylem, which transports water, and phloem, which transports organic molecules like glucose.

Plants grow so as to maximize the elongation of their stems as much as possible. Cells on the lighter side of the stem are already being provided with photosynthetic energy, while cells on the darker side are receiving less of this energy input. This causes the cells on the darker side to elongate toward the energy source. When one side of the stem is longer than the other, it causes a curve in the growth, resulting in a directionality of the growth of the stem.

In addition to growing in height, woody plants also grow in thickness. This is the function of lateral meristems. Lateral meristems are comprised of the vascular cambrium, and by cork cambrium that form vascular cylinders. The vascular cambrium adds layers of secondary xylem and phloem (wood), whereas the cork cambrium replaces the outer epidermis with a thicker and tougher layer called periderm.

The vascular system in plants is designed to transport materials (water, nutrients, food) between the roots and shoots. There are two primary types of tissue dedicated to these processes. Xylem transports water and dissolved minerals upward from the roots; phloem transports sugars—the products of photosynthesis—from where they are synthesized to where they are needed, such as roots and new growth areas of leaves and fruits.

Both xylem and phloem are comprised of a variety of cell types that are specialized for transport and support.

Fibrous root systems are common in seedless vascular plants and in most monocots, such as grasses. Many small roots grow from the stem of the plant and are considered adventitious (a term describing a plant organ that grows in an unusual location).

Fibrous roots have no main root and do not penetrate deeply into the soil, usually penetrating only a few centimeters. As such, fibrous root systems are best adapted to shallow soil. This also helps prevent erosion, as the shallow, highly-branched roots hold the topsoil in place.

Stems are one of the three basic plant organs, and consist of an alternating system of nodes (where leaves attach) and internodes (regions of the stem that span between nodes).

Some plants have evolved to have stems with additional functions, such as the ability to store food or to participate in asexual reproduction. These modified stems include rhizomes, bulbs, tubers and stolons.

A rhizome is a horizontal shoot of the plant that grows just below the surface. Vertical shoots (and resulting leaves) grow from axillary buds on the rhizome. Examples of plants with rhizomes include irises, hops, and asparagus.

Xylem is dead at maturity, while phloem is living. All other answer choices are true. Xylem is also thicker and more rigid, which allows for greater pressure during water transport. It provides a strong support structure for the plant, enabling taller growth.

The periderm forms a protective layer around the outside of many stems and roots and consists of cork cambium, cork, and phelloderm. Cork cambium is the site of active secondary growth within the periderm of vascular plants. Phloem is a type of vascular tissue that directs the flow of nutrients and metabolic products from the leaves down to the roots.

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.

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.

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.

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.