04.04 Homeostatic Mechanisms

Disruptions to Dynamic Homeostasis

Homeostasis is the process of maintaining a relatively constant internal environment despite changing external conditions. The various homeostatic conditions in your body are said to be dynamic, meaning that constant adjustments are being made to maintain overall balance by continuously opposing outside forces that tend to change that environment. Feedback mechanisms play an important role in maintaining this dynamic balance, but sometimes a disruption is beyond what can be counteracted and controlled within the body. Such disruptions, whether at the molecular, cellular, or ecosystem level, can negatively affect the health of the organism.

Homeostatic Mechanisms

Our planet is home to a large variety of living organisms. Scientists have identified and formally named about 1.8 million species, but they believe millions more remain to be discovered. With such rich biological diversity on this planet, these organisms share some fundamental functions and needs. All organisms must obtain nutrients, eliminate wastes, and produce offspring. The mechanisms used by plants, animals, bacteria, and other organisms on Earth to perform these functions share many similarities. This continuity of homeostatic mechanisms reflects the common ancestry of all life, with the differences and changes resulting as adaptations to diverse environmental conditions.

Osmoregulation:

Plants and single-celled organisms also have mechanisms for maintaining internal solute and water levels -The single-celled Paramecium caudatum has a contractile vacuole that can contract and force water out of the cell as quickly as it enters by osmosis.

Gas Exchange

Gas exchange is another example of how living organisms maintain an important internal balance. The majority of living cells on Earth use cellular respiration to metabolize energy-rich compounds. Cellular respiration requires oxygen and releases carbon dioxide, so gas exchange is another necessary homeostatic function. All gas exchange involves diffusion of the gases across a moist semipermeable membrane. Single-celled organisms, such as protozoa and bacteria, are in constant contact with their external environment. This means gas exchange can occur through diffusion directly across the cell membrane. In larger multicelled organisms, adaptations can help bring the environment closer to the individual cells. Sponges and hydra have water-filled central cavities that allow gas exchange to occur directly across each cell's membrane. In a similar adaptation, planaria have branches in their gastrovascular cavity that connect with all parts of the body.

The structure of plants allows most living cells to have at least part of

their surface exposed to the surrounding environment. The loose packing of parenchyma cells in leaves, stems, and roots provides an interconnecting system of air spaces, an adaptation that facilitates gas exchange without the need of a system to transport gases throughout the plant internally. In aquatic plants, water passes along the tissues to create a moist medium available for gas exchange. Terrestrial plants take in air through the tissues of the roots, stem, and leaves, and then gases diffuse into the moisture surrounding the internal cells. Because terrestrial plants must balance gas exchange with protection against excess water loss, pores called stomata are found on the lower surface of leaves. They are open during the day when the rate of photosynthesis is highest but close at night to seal in moisture.

In simple animals, cells exchange oxygen and carbon dioxide directly with

the surrounding environment. In more complex animals, gas exchange occurs between the environment and the bloodstream. Blood then carries the oxygen to deeply embedded cells that would not be able to exchange gases directly with the environment and transports the carbon dioxide waste products out to where it can be removed from the body. For example, earthworms exchange gases directly through their skin, where it then diffuses into tiny blood vessels just below the skin surface. Fish use gills for gas exchange between their environment and blood, and terrestrial vertebrates such as amphibians, reptiles, birds, and mammals have well-developed respiratory systems with lungs to facilitate and regulate gas exchange. Several homeostatic mechanisms work together to maintain proper balances of molecules in the cells and bloodstreams of these complex organisms.

Terrestrial species that do not have an aqueous environment evolved methods to

maintain internal water levels and avoid dehydration. Extreme examples of such adaptations can be seen in harsh desert environments, where plants often lack leaves and have thick, waxy coatings to help prevent water loss. Desert animals are often nocturnal and have adaptations that allow them to survive for long periods of time without drinking water.

Cells of plants, prokaryotes, fungi, and some protists are surrounded by

rigid cell walls. When the cell starts to swell from excess water intake, it eventually presses against the cell wall. The pressure exerted by the rigid cell wall against the expanding cell, called turgor pressure, counteracts further water uptake.

The mechanisms and adaptations used to maintain osmoregularity vary depending on..

the habitats in which the organisms live. In general, the adaptations that help each type of organism maintain osmoregulation reflect the species' evolutionary history (phylogeny) and its environment. Some organisms, such as marine invertebrates, have adapted to maintain an internal osmolarity equal to that of their surroundings. These organisms are called osmoconformers. This means the marine invertebrates do not have to work to constantly counteract the effects of osmosis, because the concentrations are the same inside and outside of the body's cells. Most organisms, whether osmoregulators or osmoconformers, cannot tolerate large changes in the osmolarity of their environment. However, some organisms have adapted to tolerate greater changes in concentration, allowing them to inhabit environments or niches others cannot. For example, many barnacles live on sea shores where they are covered and uncovered by ocean water throughout the day as tides change. Their ability to tolerate these changes allows them to occupy a niche that other organisms cannot.

Other impacts on ecosystem balance include:

-fires, hurricanes, tornadoes, and floods -water limitations and drought -salinization, the buildup of salts in the soil

Dehydration

Dehydration is a major homeostatic problem for terrestrial plants and animals, but some evolutionary adaptations help reduce the amount of water loss. Body coverings such as exoskeletons on insects, layers of skin on humans and other vertebrates, and waxy leaves of plants help seal in moisture and prevent excess evaporation. Some animals employ other adaptations to minimize water loss, such as being active at night (nocturnal) instead of during the hottest parts of the day. Even with these adaptations, many land-dwelling animals still lose significant amounts of water during gas exchange, in urine and feces necessary for waste removal, and across the skin. Humans and animals must drink water and eat moist foods to replenish the water lost each day and maintain internal water balance.

04.04 Homeostatic Mechanisms: Summary

Similarities between homeostatic mechanisms reflect common ancestry, while differences may reflect changes that occurred over time in response to specific environmental conditions. For example, aquatic plants experience gas exchange directly through cell membranes. This is true for the root cells of terrestrial plants, while stems and leaves must use stomata to balance gas exchange with protection from water loss. All vertebrates use bloodstreams to circulate gases to and from the cells throughout the body. However, these organisms employ different structures to facilitate the gas exchange between their bloodstream and the environment. These differences reflect adaptations to specific environments and structural needs. Disruptions to an organism's homeostasis have a negative impact on health, while disruptions to an ecosystem can negatively impact the dynamic balance of the ecosystem. Such ecosystem disruptions include the introduction of nonnative species, natural disasters, water limitations, and habitat loss. Many of the threats to biodiversity on a local, regional, and global scale stem from human impacts and over development.

Impact on Ecosystem Balance

The growth of the human population has had a negative impact on the dynamic homeostasis of local and global ecosystems due to increases in pollution and decreases in biodiversity and habitat. Habitat loss brought on by agriculture, urban development, forestry, and mining is one of the greatest threats to the diversity of species on this planet.