Macrostructures - AP Biology

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.

The outer layer of cells on a leaf is called the epidermis. It is composed of the upper epidermis and the lower epidermis and the two have somewhat different structures and function, but both serve to protect the leaf from the outside environment. Spongy mesophyll and bundle sheath cells are both structures on the interior of the leaf, which house chloroplasts. Stomata are found on the outside layer of a leaf, but they are pores that allow gas exchange. Xylem is a transport structure that is used to conduct water and nutrients through a plant.

The mesophyll of a leaf contains palisade parenchyma cells (along with other cell types) which contain a high concentration of chloroplasts and are located near the edge of the leaf. These parenchyma cells in the mesophyll perform the bulk of the photosynthesis in a plant, though any part of a plant that contains chlorophyll can photosynthesize, including the spongy mesophyll.

The petiole attaches the leaf to the stem. Xylem is a tissue that transports water and minerals from the roots to other areas of a plant. Phloem is a tissue that transport water and nutrients from the leaves down to other areas of the plant. A filament is a part of a flower, not a leaf. Mesophyll is the tissue that makes up much of the body of the leaf and is where most photosynthesis takes place, but does not attach the leaf to the stem.

Parenchyma cells are the main photosynthetic cells of a plant, but also function in storage of water and nutrients. Parenchyma cells are usually soft cells that are located in leaves and fruit. Phloem cells contain sieve-tube members, and function in transport of water and nutrients throughout the plant. Schlerenchyma cells are strengthened with lignen, and are support cells.

The mesophyll is the interior of the leaf. Chloroplasts are mainly found within the mesophyll and leaves are the major sites of photosynthesis in plants.

Stomata are the pores on the leaves that allow for carbon dioxide to enter and oxygen to leave the leaf. Stroma, thylakoids, and chlorophyll are all components of the chloroplasts.

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.

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.

Roots are usually underground and not exposed to sunlight, do not usually contain chlorophyll, thus they do not perform photosynthesis. However, roots do provide stabiity, obtain water and nutrients from the soil, and store carbohydrates.

The root cap occurs on the end of any root. It protects the tip of a plant's root. Specifically, it protects the apical meristem of the plant, which lengthens the root and helps it grow. Also root cap secretes a polysaccharide slime called mucilage. The zone of elongation does occur in roots, but it is not contained to the root cap. The root cap grows towards the pull of gravity. The root cap occurs only on the tip of the root, not over the entire surface of the root. These cells only protect the root; they are not xylem cells, which function in transport of water and nutrients from the roots up to the leaves.

Parenchyma cells make up a type of ground tissue in plants. Parenchyma cells compose most of the soft tissues in plants including leaves, roots, stems, and seeds. The functions of parenchyma cells include photosynthesis, storage, and wound repair.

Meristem tissue contains undifferentiated cells and allows for plant growth. Apical meristems are found in areas of growth, which include the shoot and roots—cells in these tissues divide rapidly.

The vascular cambium is a type of secondary meristem tissue located between the xylem and phloem of the stems and roots. Secondary meristem tissues are responsible for lateral growth; more specifically, the vascular cambium is responsible for secondary xylem and phloem development.

The cork cambium is a type of secondary meristem tissue found in the periderm. Cork cambium controls lateral growth; specifically, it is the source of secondary epidermis growth.

Cutting into the trunk of a tree to create a horizontal cross section reveals rings, called annual growth rings. These rings show new vascular cambium growth over the course of a year. Differences in growth speed change the size of the growth rings.

The receptacle is the part of an angiosperm out of which the flower grows; thus, the receptacle is at the top of the stem.

The carpel is a part of the female angiosperm reproductive system. The carpel includes the ovary, ovules, style, and stigma. Flowers may have several carpels clustered into a pistil.