Biology: Study of Life

Introduction

Biology is the scientific study of life and living organisms. It encompasses a wide range of topics, from the molecular mechanisms within cells to the interactions of organisms within ecosystems. Understanding what defines life and how scientists study living things is foundational to all biological sciences.

Criteria of Life

Overview

All living things share a set of characteristics that distinguish them from non-living matter. These criteria are essential for defining life and are used by biologists to classify organisms.

• Metabolism: The set of chemical reactions that allow organisms to obtain and use energy and materials from their environment, transforming them into different forms necessary for life.

• Motility: The ability to move, either as a whole organism or through movement of internal structures.

• Responsiveness: The capacity to sense and respond to stimuli from the environment.

• Growth and Development: Increase in size and number of cells (growth) and the process by which cells become specialized for specific functions (development).

• Reproduction: The ability to produce new individuals, either sexually or asexually.

• Organization: Living things are highly organized, composed of one or more cells, which are the basic units of life.

• Homeostasis: The maintenance of stable internal conditions necessary for survival.

• Evolution: Populations of living organisms change over time through genetic variation and natural selection.

Note: All living things must exhibit all eight criteria of life. All living things share a common ancestor.

Levels of Biological Organization

From Atoms to Ecosystems

Life is organized in a hierarchical structure, from the smallest chemical building blocks to the complexity of entire ecosystems.

• Subatomic Particles: Protons, neutrons, and electrons are the fundamental components of atoms.

• Atoms: The basic units of matter, formed by combining subatomic particles.

• Emergent Properties: New properties that arise with increasing complexity, such as when atoms combine to form molecules.

• Molecules: Combinations of atoms. For example, two hydrogen atoms and one oxygen atom form a water molecule.

• Cells: The basic unit of life, formed by combining molecules. Types of important biological molecules include:

  • Protein molecules

  • Lipid molecules

  • Nucleic acid molecules

  • Carbohydrate molecules

  • Water molecules

• Tissues: Groups of similar cells that perform a specific function.

• Organs: Structures composed of different tissues working together (e.g., heart, veins, arteries).

• Organism: An individual living entity (e.g., a penguin).

• Population: A group of organisms of the same species living in a particular area.

• Community: Interacting groups of various species in a common location.

• Ecosystem: A community of organisms and their physical environment.

Metabolism

Definition and Types

Metabolism refers to all the chemical processes that occur within a living organism to maintain life. These processes allow organisms to obtain and use energy and materials from their environment.

• Autotrophs: Organisms that can produce their own food from simple inorganic molecules (e.g., plants using CO2 and H2O to make sugar via photosynthesis; cyanobacteria).

• Heterotrophs: Organisms that cannot make their own food and must obtain nutrition by consuming other organisms (e.g., a caterpillar eating a leaf, a fungus digesting a strawberry).

Example: Photosynthesis is a metabolic process where light energy is used to make sugar:

Homeostasis

Maintaining Internal Stability

Homeostasis is the process by which living organisms maintain stable internal conditions necessary for survival, despite changes in the external environment.

• Examples of Homeostasis:

  • Humans: Sweating to cool down body temperature, thirst to maintain water balance.

  • Dogs: Panting to regulate body temperature.

Motility

Movement in Living Organisms

Motility is the ability of an organism or its parts to move. This can include whole-body movement or movement of internal structures.

• Locomotion: Movement from one place to another (e.g., animals walking, swimming, or flying).

• Plants: Do not exhibit locomotion, but do show motility at the cellular level (e.g., cytoplasmic streaming).

Examples:

• Cockroach hissing when touched (response to stimulus)

• Plant leaves moving when touched

• Flowers opening or closing in response to temperature

• Animals migrating to warmer climates

Responsiveness

Reacting to Environmental Stimuli

Responsiveness is the ability of an organism to sense and respond to changes in its environment.

• Examples:

  • Cockroach hissing when touched

  • Plant leaves moving in response to touch

  • Flowers responding to temperature changes

  • Animals migrating in response to seasonal changes

Reproduction

Producing New Individuals

Reproduction is the process by which living organisms produce new individuals, ensuring the continuation of their species.

• Asexual Reproduction: Involves a single parent and produces offspring that are genetically identical to the parent (e.g., sea anemone).

• Sexual Reproduction: Involves two parents and results in offspring with genetic variation (e.g., ladybugs with different color patterns).

Growth and Development

Increase in Size and Specialization

Growth refers to an increase in size and number of cells, while development involves the specialization of cells for specific functions.

• Growth: Increase in size and cell number.

• Development: Differentiation of cells to perform specialized tasks.

Example: A caterpillar entering a chrysalis and transforming into a butterfly.

Evolution

Change in Populations Over Time

Evolution is the process by which populations of organisms change over generations through genetic variation and natural selection.

• Key Point: Populations evolve, not individual organisms.

• Example: All living things share a common ancestor.

Methods for Studying Biology

Scientific Approaches

Biologists use systematic methods to study living things, including descriptive and hypothesis-based approaches.

• Descriptive (Discovery) Science: Begins with observations of what is not known.

• Hypothesis-Based Science: Begins with what is known and uses the scientific method to test explanations.

The Scientific Method

1. Make an observation: Notice something in the natural world.

2. Form a hypothesis: Propose a testable and falsifiable explanation based on prior knowledge.

3. Make a prediction: State what should happen if the hypothesis is correct.

4. Test the prediction: Conduct experiments or further observations.

5. Draw a conclusion: Analyze results to support or refute the hypothesis.

Example:

• Observation: The lamp does not turn on when the switch is flipped.

• Hypotheses:

  • Light bulb burned out

  • Outlet is dead

  • Not fully plugged into outlet

  • Light bulb not fully screwed in

• Predictions and Experiments:

  • If the switch is on, the light will go on (test by turning on the switch).

  • If the bulb is not fully screwed in, it will light up when tightened (test by screwing in the bulb).

  • If the bulb is burned out, replacing it will make the lamp work (test by changing the bulb).

Key Terms:

• Testable: A hypothesis must be able to be tested by experiment or observation.

• Falsifiable: A hypothesis must be able to be proven false.

Summary Table: Criteria of Life