Human Body System Organization

Organs are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants. An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs. It includes organs such as the heart and blood vessels. The digestive system consists of several organs, including the stomach, intestines, liver, and pancreas. Organ systems can come together to create an entire organism.

There are eleven distinct organ systems in the human body (Figure 20.1). Assigning organs to organ systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system. In later chapters, we will discuss some of these organ systems in more depth.

Plant Structure and Function

Plant tissues form organs (such as leaves, stems, or roots), each of which perform a specific set of functions. Together, organs often work to form organ systems. Vascular plants have two distinct organ systems: a shoot system, and a root system. The shoot system consists of two portions: the vegetative (non-reproductive) parts of the plant, such as the leaves and the stems, and the reproductive parts of the plant, which include flowers and fruits. The shoot system generally grows above ground, where it absorbs the light needed for photosynthesis. The root system, which anchors the plant into the ground, absorbs water and minerals, and serves as a storage site for food is usually underground. (Figure 20.2) shows the organ systems of a typical plant.

Stem Structure and Function

Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes (Figure 20.3). Nodes are points of attachment for leaves. Leaves often consist of a thin region that attaches to the stem (the petiole) and a broader blade (see Leaves). The stem region between two nodes is called an internode. An axillary bud is usually found in the axil—the area between the base of a leaf and the stem—where it can give rise to a branch called an axillary shoot (Figure 20.4). The shoot apex at the tip shoot contains the shoot apical meristem surrounded by developing leaves called leaf primordia (see Meristems).

The primary stem refers to the herbaceous (non-woody) stem, which has not undergone secondary growth (the growth that produces bark and wood). Some species (all monocots and some eudicots) remain herbaceous for their entire lives, maintaining the primary stem. Other species of eudicots initially form a primary stem but later become woody, replacing the primary stem with the secondary stem. The anatomy of the stem (internal structure) can be examined through longitudinal sections (cutting the stem lengthwise) or in cross sections (cutting a slice of the stem perpendicular to the length).

All three tissue types are represented in the primary stem. The epidermis is the dermal tissue that surrounds and protects the stem. The epidermis typically consists of one layer of cells. A waxy cuticle on the outside of these cells limits water loss. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. Pores in the epidermis called stomata (singular: stoma) allow for gas exchange. Each stoma is bordered by a pair of guard cells, which regulate stomatal opening. While stomata are present in stems, they occur at higher densities in leaves. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores (Figure 20.5).

Ground tissue fills much of the stem, forming the cortex directly within the epidermis and the pith (if present) in the center. The outermost portion of the cortex is usually a few layers of collenchyma cells. The remainder of the cortex and pith consist of parenchyma cells. The arrangement of vascular tissue in the stem depends on the species (see below).

Cross sections reveal three possible arrangements of vascular tissue (steles) in the stem. The first arrangement (solenostele) is present in a few eudicots, such as basswood (Tilia). In the solenostele, the vascular tissue appears as a continuous ring (vascular cylinder; Figure 20.6). The second arrangement (eustele) is present in most eudicots such as sunflower (Helianthus) and buttercup (Ranunculus). In the eustele, vascular tissue is clustered into distinct vascular bundles arranged in a ring, allowing for thicker interfascicular regions in between them (Figure 20.8). In these solenosteles and eusteles, the vascular tissue separates ground tissue into an outer cortex and central pith. The third arrangement (atactostele) is present in most monocots, such as corn (Zea mays) and a few eudicots. In the atactostele, vascular bundles are scattered throughout the stem (Figures 20.7 & 20.9). While vascular bundles near the outside of the stem are packed more densely in this third arrangement, their distribution is somewhat disorderly. There is no distinction between the cortex and pith in the third arrangement.

Stem Modifications

Stems (or entire shoots) of some species deviate in structure and function from a typical stem. These are called stem modifications (shoot modifications). While leaves are part of the shoot, modifications involving just the leaf are discussed separately.

Rhizomes and stolons are horizontal stems that function in propagation (Figure 20.10). Rhizomes are belowground stems that burrow into the ground just below the soil surface. They have short internodes and usually have small, scale-like leaves that are not photosynthetic. Buds from the axils of the leaves make new branches that will grow to become aboveground shoots. Ginger (Zingiber) and Johnson grass (Sorghum halepense) form rhizomes. Stolons (runners) are aboveground horizontal shoots, which sprout and produce a new plants. Compared to rhizomes, stolons have long internodes. Examples include strawberry (Fragaria), spider plants (Chlorophytum), and Bermuda grass (Cynodon dactylon).

Tubers, corms, and bulbs are modified for starch storage. Tubers, such as potatoes, are thick, belowground stems found at the tips of rhizomes or stolons (figure 20.11). The “eyes” of potato are actually lateral buds, and the tuber body is comprised of many parenchyma cells that contain amyloplasts with starch. Corms and bulbs are modified shoots. A corm is a short, thick underground storage stem with thin scaly leaves (for example, Gladiolus and Crocus; Figure 20.12). A bulb, such as an onion, differs from a corm in the fact that it stores its nutrients in its fleshy leaves (Figure 20.12). Lilies and tulips also form bulbs.

Some plants have sharp, generally narrow projections that function in defense against herbivores. Such structures are called thorns when they arise from an entire stem. Hawthorn (Crataegus) and Bougainvillea produce thorns. In contrast, prickles form from the surface tissues (epidermis and cortex) of the stem rather than the whole organ (figure 20.13). Rose (Rosa) and blackberry (Rubus) produce prickles. Spines are similar structures derived from leaves.

Tendrils are thin, string-like structures that allow the shoot to attach to other surfaces to access light. Tendrils can be derived from stems, leaves, or leaflets, and they are common in vines. Morning glory and sweet potato (Ipomoea), grapes (Vitis), and many members of the cucumber family (Cucurbitaceae) such as cucumbers, pumpkin, and squash have tendrils that arise from stems (Figure 20.14).

Structure and Function of Leaves

Each leaf typically has a flat, wide portion called the blade (lamina), which is also the widest part of the leaf (Figure 20.15). Some leaves are attached to the plant stem by a stalklike petiole and are called petiolate leaves (Figure 20.16). Although petioles are narrow and often resemble stems, they are considered part of the leaf. A petiolate leaf thus consists of the blade and the petiole. Petioles usually attach at to the margin (edge) of the blade along the base, but in peltate leaves, the petiole is attached underneath the blade (Figure 20.17). Leaves that do not have a petiole and are directly attached to the plant stem are called sessile (apetiolate) leaves (Figure 20.18). In a special type of sessile leaves called perfoliate leaves, the stem passes through the center of the blade (Figure 20.18). Many leaves have a midrib, which travels the length along the center of the leaf. The midrib contains the main vein (primary vein) of the leaf as well as supportive ground tissue (collenchyma or sclerenchyma).

Tissue Organization in Leaves

All three tissue types are represented in leaves. The epidermis represents the dermal tissue, the mesophyll that fills the leaf is ground tissue, and the vascular bundles that form the leaf veins represent vascular tissue (Figure 20.19). These three tissues will be discussed using a eudicot leaf that is adapted to a moderate amount of water (mesophytic leaf). Variations in leaf structure are discussed later on this page.

 

Leaf Adaptations

Pine Leaves

Pines evolved during a period in Earth’s history when conditions were becoming increasingly dry, and pine needles have many adaptations to deal with these conditions. Many of these adaptations are similar the xerophytic leaves of some angiosperms (described above) because pines themselves are xerophytes.

The epidermis of the leaf seems to be more than one cell layer thick (figure 20.20). These subsequent layers of epidermis-like tissue under the single, outer layer of true epidermis are called the hypodermis , which offers a thicker barrier and helps prevent water loss. The epidermis itself is coated on the outside by a thick layer of wax called the cuticle. Because waxes are hydrophobic, this also helps prevent water loss through the epidermis. The stomata are typically sunken, occurring within the hypodermis instead of the epidermis. Sunken stomata create a pocket of air that is protected from the airflow across the leaf and can aid in maintaining a higher moisture content (figure 20.21).

Within the mesophyll, there are several canals that appear as large, open circles in the cross section of the leaf. These are resin canals. The cells lining them secrete resin (the sticky stuff that coniferous trees exude, often called pitch), which contains compounds that are toxic to insects and bacteria. When pines evolved, not only was the Earth becoming drier, but insects were evolving and proliferating. These resin canals are not features that help the plant survive dry conditions, but they do help prevent herbivory. In addition to prevention of herbivory, resin can aid in closing wounds and preventing infection at wound sites.

There are two bundles of vascular tissue embedded within a region of cells called transfusion tissue, which functions in transporting materials to and from the mesophyll cells. The transfusion tissue and vascular bundles are surrounded by a distinct layer of cells called the endodermis. This is similar to the tissue of the same name in the root, but the cells are not impregnated with the water-repelling compound suberin.

Finally, the overall shape of the leaf allows for as little water loss as possible by decreasing the relative surface area, taking a rounder shape as opposed to a flatter one. This low surface area-to-volume ratio is characteristic of xerophytes.

Corn Leaves

The model organism for monocots in botany is usually corn (Zea mays). In corn, there are approximately the same number of stomata on both the upper and lower epidermis. The mesophyll is not divided into two distinct types. The vascular bundles all face the same directly (appearing circular in cross section) because they run parallel to each other.

Corn is not necessarily a xerophyte, but it is adapted to deal with high temperatures. One of these adaptations, C₄ type photosynthesis is discussed in Photorespiration and Photosynthetic Pathways and results in a cell arrangement called Kranz anatomy. The vascular bundles are surrounded by obviously inflated parenchyma cells that form a structure called a bundle sheath, and these are packed with chloroplasts (Figure 20.22). (Bundle sheaths surround vascular bundles of other types of leaves as well, but the bundle sheath cells are much smaller). Mesophyll cells encircle the bundle sheath cells. In C₄ photosynthesis, carbon dioxide is first gathered by the mesophyll cells and temporarily stored as a four-carbon sugar. This four-carbon sugar is transferred to the bundle sheath cells, where it is broken down to release carbon dioxide. It is in the bundle sheath cells where a process called the Calvin cycle, and glucose is ultimately produced. C₄ photosynthesis concentrates carbon dioxide inside the bundle sheath cells, reducing the need to frequently open stomata for gas exchange. This helps conserve water.

When moisture is plentiful, the corn leaves are fully expanded and able to maximize photosynthesis. When moisture is limited, the leaves roll inward, limiting both moisture loss and photosynthetic capacity. This is accomplished by the presence of bulliform cells in the upper epidermis (Figure 20.23). These clusters of enlarged cells are swollen with water when there is abundant water available. As the water content in the plant decreases, these cells shrivel, causing the upper epidermis to curl or fold inward at these points. This adaptation to sun exposure can be found in many other grasses, as well (corn is a member of the Poaceae, the grass family).

Sun and Shade Leaves

The light intensity experienced by a developing leaf influences its structure. Leaves that develop when consistently exposed to direct sunlight (sun leaves) thus differ from leaves exposed to low light intensities (shade leaves) in several ways (Figure 20.24). Relative to shade leaves, sun leaves are smaller and thicker. This reduces surface area relative to volume, conserving water, which would otherwise be easily lost under bright sunlight and resultantly warmer temperatures. In contrast, the broad, thin shape of shade leaves helps capture sufficient light when light intensity is low. The thicker cuticle of sun leaves also limits water loss. They have more palisade parenchyma and more vascular tissue. Sun leaves can maintain a high photosynthetic rate at high light intensities, but shade leaves cannot.

Leaf Modifications

The structure and function of a leaf can be modified over the course of evolution as a plant adapts to a particular environment (Figure 20.25). When function of the leaf blade is no longer primarily photosynthesis, some other plant part is usually modified to take its place. Storage leaves are thick leaves underground that store starch (as with a bulb; see Stem Modifications). Succulent leaves are also thickened leaves, but they are found above ground, still conduct photosynthesis, and function primarily in water storage (Figure 20.26).

Spines are sharp projections derived from leaves that function in plant defense. Almost all cacti (in the plant family Cactaceae) have spines (Figure 20.27). Other examples include barberry and some Acacia species, in which large spines house mutualist ants. Spines can also be formed from stipules (stipular spines) or bud scales. Recall from Stem Modifications that thorns and prickles have the same function, but they are derived from whole stems or the outer tissue layers of stems, respectively.

Tendrils are another structure that can originate from multiple structures, including stems, leaves, or leaflets. These are narrow, coiling structures that climbing plants attach to nearby structures for support (Figure 20.28).

Showy bracts are brightly colored leaves function in attracting pollinators. From a distance, showy bracts often appear like the petals of a flower. However, the actual flowers are typically small and in a cluster surrounded by the bracts (Figure 20.29).

Carnivorous plants grow in bogs where the soil is low in nitrogen, and their leaves are adapted to help them to survive this nutrient-poor environment. In these plants, leaves are modified as traps to capture insects. While they still conduct photosynthesis to capture energy and synthesize sugars (and are thus autotrophs), they rely on insect-capturing leaves as a supplementary source of much-needed nitrogen, similar to fertilizer. Several examples of carnivorous plants are the cobra lily (Darlingtonia), various pitcher plants (Nepenthes [Figure 20.30], Cephalotus, Sarracenia), the butterwort (Utricularia), the sundew (Drosera; Figure 20.31), and the best known, the Venus flytrap (Dionaea).

Root Structure and Function

There are two types of root systems. The first is a fibrous root system which has multiple big roots that branch and form a dense mass which does not have a visible primary root (“grass-like”). The other is the tap root system which has one main root that has branching into lateral roots (“carrot-like”).

Root systems are mainly of two types (Figure 20.32). Eudicots have a tap root system, while monocots have a fibrous root system. A tap root system has a main root that grows down vertically, and from which many smaller lateral roots arise. Dandelions are a good example; their tap roots usually break off when trying to pull these weeds, and they can regrow another shoot from the remaining root). A tap root system penetrates deep into the soil. In contrast, a fibrous root system is located closer to the soil surface, and forms a dense network of roots that also helps prevent soil erosion (lawn grasses are a good example, as are wheat, rice, and corn). Some plants have a combination of tap roots and fibrous roots. Plants that grow in dry areas often have deep root systems, whereas plants growing in areas with abundant water are likely to have shallower root systems.

Along with having different systems, there are primary root that originated from the root of the seedling (Figure 20.33) and secondary (lateral) roots originate from the primary roots, and adventitious roots originate on stems or leaves, rather than from the base of the embryo. Adventitious roots can grow if plant cuttings are placed in water (Figure 20.33).

References

Adapted from Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-2-themes-and-concepts-of-biology?query=%22organ%20system%22&target=%7B%22type%22%3A%22search%22%2C%22index%22%3A0%7D#fs-id2155753

Adapted from Ha, M., Morrow, M., & Algiers, K. (n.d.). Botany. Retrieved from https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/03%3A_Plant_Structure

Barrickman, N., Bell, K., and Cowan, C. (n.d.) Human Biology. Pressbooks. Retrieved from https://slcc.pressbooks.pub/humanbiology/chapter/chapter-12-organ-systems-of-the-human-body/

Lumen Learning. (2021). Fundamentals of Biology I. https://library.achievingthedream.org/herkimerbiologyfundamentals1/chapter/plant-sensory-systems-and-responses/

5.1: Organs and systems of the human organism. (2019, February 18). Medicine LibreTexts; Libretexts. https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Human_Anatomy_and_Physiology_Preparatory_Course_(Liachovitzky)/05%3A_Higher_Levels_of_Complexity-_Organs_and_Systems/5.01%3A_Organs_and_Systems_of_the_Human_Organism