Section 1: Homeostasis Preview Bellringer Key Ideas A Changing Environment Responses to Change Humans and Homeostasis Summary

Bellringer Imagine you are playing softball on a warm summer day. Write down two ways that your body would adjust to keep you cool and to keep your muscles working.

Key Ideas How does a change in the external environment affect an organism? How does an organism respond to changes, and what happens if the organism fails to respond to changes? What organ systems do humans need in order to survive?

A Changing Environment The external environment around an organism is constantly changing. These changes threaten the stability of an organism’s internal environment. The maintenance of a relatively stable internal environment is called homeostasis.

A Changing Environment, continued Change in seasons, light level, water availability, or other changes to the external environment can change an organism’s internal environment. Different organisms deal with these changes in different ways. In a colder environment, for example, organisms may eat more food, puff out fur or feathers, fly to warmer areas, or hibernate.

Responses to Change Organisms must carry out many chemical reactions to grow, obtain energy, and reproduce. Many organisms also move, breathe, produce heat, and do other tasks. All of these activities require the organism to maintain a relatively stable internal environment.

Responses to Change, continued Organisms detect and respond to both internal and external environments in a variety of ways. Detection and response can take place at both the cellular and organismal level. Failure to respond to change can result in an organism’s death.

Responses to Change, continued From Cells to Organ Systems All organisms respond to changes in the environment. The organelles in some single-celled organisms work in a way that is similar to the way tissues and systems in multicellular systems work. This allows single-celled organisms to perform all the life processes needed for homeostasis.

Responses to Change, continued A Euglena is a single-celled photosynthetic protist. If you place the Euglena in a darkened tank of water and turn a light on at one end of the tank, the Euglena’s eyespot will sense the light. In response, the protist’s flagellum will propel the Euglena toward the light.

Responses to Change, continued The chloroplast in the Euglena will begin to perform photosynthesis. By moving toward the light, the Euglena is able to produce enough energy for its cell to maintain homeostasis and stay alive. Multicellular organisms can respond to change at the cellular, tissue, organ system, or organismal levels.

Responses to Change, continued The pores on a plant’s leaf help the plant to regulate water loss and gas exchange. During the day, tissues perform photosynthesis. Guard cells near these active tissues swell with water. This opens pores, letting in the carbon dioxide that is needed for photosynthesis

Responses to Change, continued Open pores also allow water to exit the leaf. At night, when photosynthesis slows, water exits the guard cells. The cells shrink and close the pores, preventing excess water loss at night.

Control of Stomatal Opening

Responses to Change, continued If an organism is not able to respond to changes in its environment, disease or death can result. Consider what would happen if a plant could not slow water loss. When the amount of water in a plant’s tissues got too low, the plant would wilt, collapse, and die.

Responses to Change, continued Humans must also control internal water levels. About 2/3 of the human body is made of water. If a person loses too much water, by sweating or urination, dehydration will occur. Dehydration causes blood pressure to drop significantly until death occurs.

Responses to Change, continued Some deep-water fish must be adapted to deal with high water pressure. The coelacanth is a fish that lives in the deep waters off the coast of Africa. At the depth where the fish lives, water pressure is several hundred times greater than the water pressure at the ocean’s surface.

Responses to Change, continued If a person tried to swim at this pressure, the person’s body would be crushed. The coelacanth will actually die in the low-pressure environment at the water’s surface. The coelacanth’s body is adapted to require high pressure, in order to keep gases from bubbling out of the fish’s bloodstream and blocking flow to the fish’s heart.

Humans and Homeostasis Humans are constantly subjected to environmental stresses that threaten to upset the delicate balance that exists within our cells and tissues. Humans require multiple systems for digestion, respiration, reproduction, circulation, movement, coordination, and immunity. These systems interact to maintain homeostasis.

Humans and Homeostasis, continued The organs and systems of the human body help to provide all of the cells with their basic needs. Each cell needs a constant supply of nutrients and oxygen, removal of waste materials, and protection from disease. With the exception of the reproductive system, each of the human body systems performs a task that helps stabilize the body’s internal environment.

Visual Concept: Overview of Organ Systems Click the button below to watch the Visual Concept.

Humans and Homeostasis, continued In the human body, the main components of homeostasis are the following: the concentration of salts The pH of the internal environment The concentration of nutrients and waste products The concentration of oxygen and carbon dioxide The volume and pressure of extracellular fluid

Humans and Homeostasis, continued When these five components are adequately controlled, homeostasis is maintained and the body is most likely healthy. No organ system can maintain homeostasis by itself. Organ systems must work together in a synchronized manner.

Humans and Homeostasis, continued For example, the concentration of oxygen and carbon dioxide is controlled by four different body systems. First, the respiratory system brings oxygen into and carbon dioxide out of the body. The circulatory system distributes the oxygen to the body’s tissues and picks up carbon dioxide.

Humans and Homeostasis, continued If the level of carbon dioxide gets too high, the nervous system instructs the muscular system to make the muscles around the rib cage work harder. This action allows the lungs of the respiratory system to breathe faster, to get rid of excess carbon dioxide.

Summary The external environment around an organism is constantly changing. These changes threaten the stability of an organism’s internal environment. Organisms detect and respond to change in a variety of ways: both at the cellular level and at the organismal level. Failure to respond to change can result in an organism’s death.

Summary, continued Humans require multiple systems for digestion, respiration, reproduction, circulation, movement, coordination, and immunity. The systems interact to maintain homeostasis.