20 Chapter 20: Sensing and Responding to Stimuli: Body System Organization and Plant Responses

Mason Tedeschi

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, 20.2). 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.


Fig 20.1-20.2 Each organ system works together to perform a specific function
Table 20.1

Reading Question #1

Which body system functions to deliver air to sites where gas exchange occur?

A. Reproductive

B. Urinary

C. Nervous

D. Respiratory

Reading Question #2

Which body system functions to provide communication within the body via hormones and directs long-term change in other organ systems to maintain homeostasis?

A. Reproductive

B. Endocrine

C. Integumentary

D. Digestive

Reading Question #3

Which body system functions break down and absorb food so the body can acquire the nutrients it needs?

A. Reproductive

B. Urinary

C. Digestive

D. Muscular

Plant Responses to Light

Plants have a number of sophisticated uses for light that go far beyond their ability to photosynthesize low-molecular-weight sugars using only carbon dioxide, light, and water. Phototropism is a directional response that allows plants to grow towards, or even away from, light. Positive phototropism is growth towards a light source (Figure 20.3), while negative phototropism (also called skototropism) is growth away from light. The sensing of light in the environment is important to plants; it can be crucial for competition and survival. The response of plants to light is mediated by different photoreceptors.

Figure 20.3 Azure bluets (Houstonia caerulea) display a phototropic response by bending toward the light. (credit: Cory Zanker)

Plant Responses to Gravity

Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, and roots grow downward into the ground. A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth of the shoot apical tip upward is called negative gravitropism, whereas growth of the roots downward is called positive gravitropism.

Amyloplasts (also known as statoliths) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap. When a plant is tilted, the statoliths drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction.

The mechanism that mediates gravitropism is reasonably well understood. When amyloplasts settle to the bottom of the gravity-sensing cells in the root or shoot, they physically contact the endoplasmic reticulum (ER), causing the release of calcium ions from inside the ER. This calcium signaling in the cells causes polar transport of the plant hormone IAA to the bottom of the cell. In roots, a high concentration of IAA inhibits cell elongation. The effect slows growth on the lower side of the root, while cells develop normally on the upper side. IAA has the opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion, causing the shoot to grow up. After the shoot or root begin to grow vertically, the amyloplasts return to their normal position. Other hypotheses—involving the entire cell in the gravitropism effect—have been proposed to explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response.


The term auxin is derived from the Greek word auxein, which means “to grow.” Auxins are the main hormones responsible for cell elongation in phototropism and gravitropism. They also control the differentiation of meristem into vascular tissue, and promote leaf development and arrangement. While many synthetic auxins are used as herbicides, IAA is the only naturally occurring auxin that shows physiological activity. Apical dominance—the inhibition of lateral bud formation—is triggered by auxins produced in the apical meristem. Flowering, fruit setting and ripening, and inhibition of abscission (leaf falling) are other plant responses under the direct or indirect control of auxins.

Commercial use of auxins is widespread in plant nurseries and for crop production. IAA is used as a rooting hormone to promote growth of adventitious roots on cuttings and detached leaves. Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Outdoor application of auxin promotes synchronization of fruit setting and dropping to coordinate the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.

Plant Responses to Wind and Touch

The shoot of a pea plant winds around a trellis, while a tree grows on an angle in response to strong prevailing winds. These are examples of how plants respond to touch or wind.

The movement of a plant subjected to constant directional pressure is called thigmotropism, from the Greek words thigma meaning “touch,” and tropism implying “direction.” Tendrils are one example of this. The meristematic region of tendrils is very touch sensitive; light touch will evoke a quick coiling response. Cells in contact with a support surface contract, whereas cells on the opposite side of the support expand. Application of jasmonic acid is sufficient to trigger tendril coiling without a mechanical stimulus.

A thigmonastic response is a touch response independent of the direction of stimulus. In the Venus flytrap, two modified leaves are joined at a hinge and lined with thin fork-like tines along the outer edges. Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more of them in succession, the leaves close quickly, trapping the prey. Glands on the leaf surface secrete enzymes that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal.

Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens. Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind’s force. Researchers hypothesize that mechanical strain induces growth and differentiation to strengthen the tissues. Ethylene and jasmonate are likely involved in thigmomorphogenesis.

Reading Question #4

Plant growth towards sunlight is best described as

A. Phototropism

B. Gravitropism

C. Negative feedback

D. Positive feedback

Reading Question #5

Plant roots growing downward is best described as

A. Phototropism

B. Gravitropism

C. Negative feedback

D. Positive feedback



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

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


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Introductory Biology 2 Copyright © 2023 by Mason Tedeschi is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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