Energy Homeostasis

What is the ultimate goal in life for all living organisms?

To maximize fitness

What is fitness?

Fitness = survival + reproduction

In order to survive, all organisms must meet some intrinsic challenges. These include:

• Obtaining energy

• Water

• Necessary nutrients and elements

• As well as obtaining gases needed for metabolic activities

Remember, keep in mind whether an organism is…as we proceed

• Unicellular or multicellular

• Autotrophic or heterotrophic

• Aquatic or terrestrial

• Stationary or motile

Unicellular organisms

Single-celled life forms that perform all necessary life functions within one cell, including metabolism, reproduction, and response to environmental stimuli.

Multicellular organisms

Composed of multiple specialized cells that work together to sustain life, unlike unicellular organisms which rely on a single cell.

Heterotrophic organisms

Organisms that rely on consuming other organisms—plants, animals, fungi, or organic detritus—for their energy and carbon needs. Unlike autotrophs, which synthesize their own food via photosynthesis or chemosynthesis, heterotrophs are dependent on external sources of organic compounds. They have enzymes and metabolic pathways that allow them to break down carbohydrates, proteins, and fats into usable energy, typically through processes like cellular respiration.

Autotroph organisms

Organisms capable of synthesizing their own organic compounds from carbon dioxide and other inorganic molecules. Unlike heterotrophs, which rely on consuming organic matter, autotrophs obtain energy either from sunlight (phototrophs) or from chemical reactions (chemotrophs). They play a fundamental role in ecosystems as primary producers, forming the base of food chains and supporting all other life forms.

TypesofAutotrophicOrganisms

Aquatic organisms

Organisms that are distinguished by their ability to live in water for most or all of their life cycle. They exhibit special adaptations that enable survival in water-based habitats, such as:

Terrestrial organisms

Species that are adapted to live on or within the Earth's land surfaces. They have evolved traits that allow them to survive in environments where gravity, temperature fluctuations, and water availability present challenges different from aquatic habitats. These adaptations may include specialized respiratory systems, supportive body structures, water retention mechanisms, and behaviors that help them cope with the variability of terrestrial environments.

Stationary organisms

Stationary organisms, also called sessile organisms, are typically anchored to a surface and are unable to move independently like motile organisms. They rely on external processes such as water currents, wind, or animal interactions to obtain food, reproduce, or disperse their offspring. These organisms often have adaptations that help them survive in a fixed location, such as protective coverings, specialized feeding structures, or symbiotic relationships for nutrition

Motile organisms

Motility in biology refers to the ability of an organism to move independently, often in response to environmental stimuli such as light, chemicals, or temperature. Organisms that exhibit this ability are called motile organisms. This movement is powered by various structures and mechanisms inherent to the organism, such as cilia, flagella, pseudopodia, muscular systems, or other specialized appendages.

How do unicellular organisms obtain the energy they need?

• For unicellular organisms, the most important factors influencing their acquisition and use of energy are

  • Cell size

  • Cell shape

• Another way to think of this - 2 key considerations:

  • Acquisition: how do unicellular organisms obtain their food?

    • Cell membrane = surface area

    • Surface area = opportunity to GAIN energy

  • Use: once they acquire energy, how far does it have to travel to get where it will be used?

    • Cell biomass requiring support = VOLUME

    • Volume = energy LOST (to cell support and distance travelled)

• The size of a cell is influenced by the surface-to-volume ratio

  • Larger cells require more energy and have proportionally less surface area to support their volume

  • Higher ratio SA:vol = higher gain:loss

  • Consequently, unicellular organisms may have adapted shapes that facilitate access to energy resources (high SA) and diffusion throughout their cytoplasm (low vol) (ex: elongated or irregular shape)

How do multicellular organisms obtain the energy they need?

• Fact: smaller individuals use less total energy than larger ones (uni- and multicellular)

  • Thus, large multicellular organisms are more limited in their occurrence due to overall energy availability than small ones

  • Ex: a teaspoon of healthy soil can contain 100 million and 1 billion soil bacteria

    • Their energy source? Organic molecules in the soil

    • Soil bacteria serve as the energy source for countless nematodes…and so on up the food chain

  • However, smaller multicellular organisms individually require more energy per unit body mass than larger ones (think SA:vol ratio again)

    • For example, one gram of an elephant’s body uses up 25 times less energy than does one gram of a shrew’s body

  • Another way to think of this:

    • Acquisition: how do multicellular organisms obtain their food? What role does SA play?

      • Through reduced openings (not whole body surface)

      • Opportunity to LOSE heat energy to outside environment

    • Use: once they acquire energy, how does it travel to get to where it will be used? And, what gives them the opportunity to “use before they lose”?

      • Efficient specialized body systems

      • Cells specialized for storage and insulation

SA:vol for bacterium vs. mammals

• For bacterium

  • SA (+) = gain opp

  • Vol (-) = loss

• For mammals

  • SA (-) = loss

  • Vol (±) storage, loss

So how do energy requirements differ between unicellular and multicellular organisms, cont?

• Energy use for total volume = similar pattern

• As total volume increases, total energy requirements increase accordingly

• Recent research by Bai-Lian (2005) suggests:

  • Rate of energy consumption per unit body mass declines with growing body size within groups of evolutionarily close organisms (ex. Mammals, angiosperms, etc.)

  • Whereas, a bacterium - not closely related evolutionarily to an elephant - consumes approximately the same energy per unit body mass