1.1 Characteristics of Life
Life is Organized
Life is organized, exhibiting a hierarchical structure from the simplest to the most complex forms. This organization can be understood through distinct levels, each building upon the complexity of the one before it:
Chemical Level
Atoms: The fundamental units of matter, consisting of a nucleus (protons and neutrons) and orbiting electrons. Essential biological atoms include carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), and sulfur (S).
Molecules: Formed when two or more atoms are joined together by chemical bonds. These can range from simple inorganic molecules like water (H_2O) to complex organic macromolecules essential for life, such as:
Carbohydrates: Source of energy (e.g., glucose (C6H12O_6), starches, cellulose).
Lipids: Energy storage, cell membrane structure (e.g., fats, phospholipids).
Proteins: Structural support, enzymes, transport, defense (e.g., hemoglobin, enzymes, antibodies).
Nucleic Acids: Genetic information storage and transfer (e.g., DNA, RNA).
Cellular Level
Organelles: Specialized subunits within a cell that perform specific functions, analogous to organs within the body. Examples include:
Mitochondria: Responsible for cellular respiration and ATP production (the cell's energy currency).
Chloroplasts: Site of photosynthesis in plant cells.
Nucleus: Contains the cell's genetic material (DNA) and controls cell growth and reproduction.
Endoplasmic Reticulum/Golgi Apparatus: Involved in protein synthesis, modification, and transport.
Cells: The basic structural, functional, and biological unit of all known organisms. All living things are composed of one or more cells, which can be prokaryotic (lacking a nucleus) or eukaryotic (possessing a nucleus and other membrane-bound organelles).
Tissue Level
Tissues: Groups of similar cells that originate from the same embryonic layer and work together to perform a specific function. In animals, there are four primary types:
Muscle tissue: For movement.
Nervous tissue: For communication and control.
Connective tissue: For support, protection, and binding other tissues (e.g., bone, blood, fat).
Epithelial tissue: For covering surfaces, lining cavities, and forming glands.
Organ Level
Organs: Distinct structural units, each composed of two or more different types of tissues that work together to perform a complex function necessary for the organism's survival. Examples include the heart (pumping blood), lungs (gas exchange), stomach (digestion), brain (coordination), and skin (protection).
Organ System Level
Organ Systems: Groups of organs that cooperate and interact to perform major functions of the body, contributing to overall homeostasis. Major human organ systems include:
Digestive system: Breaks down food.
Circulatory system: Transports substances.
Nervous system: Controls body functions.
Respiratory system: Facilitates gas exchange.
Skeletal system: Provides support and protection.
Muscular system: Enables movement.
Organismal Level
Organism: A complete living being; an individual living entity, whether it is a single-celled bacterium or a complex multicellular animal. It is composed of one or more organ systems (or equivalent structures in simpler organisms) working together to maintain life and ensure survival.
Population Level
Population: A group of individuals of the same species living in the same defined geographic area at the same time, capable of interbreeding. The study of populations focuses on dynamics such as birth rates, death rates, and migration.
Community Level
Community: Consists of all the different populations of various species living and interacting within a particular area. Interactions can include competition, predation, mutualism, and commensalism, influencing the structure and diversity of the community.
Ecosystem Level
Ecosystem: Encompasses all the living organisms (biotic components - the community) in an area, as well as the non-living physical components (abiotic components) of the environment, such as air, water, soil, sunlight, and temperature. These components interact as a functional unit, with continuous flows of energy and cycles of matter.
Biosphere Level
Biosphere: The highest level of biological organization; it is the sum of all the ecosystems on Earth. It represents the global ecological system integrating all living beings and their relationships, including their interaction with the lithosphere, geosphere, hydrosphere, and atmosphere. Essentially, it is the zone of Earth where life exists.
Life requires material and energy. Living organisms constantly need an outside source of materials (nutrients) and energy to maintain their organization, grow, reproduce, and carry on other activities that characterize them as being alive. Plants, such as trees, are primary producers; they use carbon dioxide, water, and solar energy to synthesize their own food through photosynthesis. Humans and other animals, being consumers, acquire materials and energy by eating food composed of organic molecules.
The food we eat provides essential nutrientsāmolecules that an organism needs for metabolism, growth, and repair. Our cells then use these nutrients in metabolic processes for building complex molecules, constructing internal cellular structures, and creating new cells.
Metabolism: Is defined as all of the chemical reactions that occur within a cell or organism, encompassing both anabolic (building up) and catabolic (breaking down) processes. These reactions are essential for life, allowing organisms to grow, reproduce, maintain their structures, and respond to environmental changes.
Life requires a constant source of material and energy to maintain its organization and carry out vital activities. For the vast majority of living organisms on Earth, the ultimate source of this energy is the sun, captured through solar energy. Exceptions include some chemosynthetic organisms found in deep-sea hydrothermal vents or subterranean environments that derive energy from chemical reactions without sunlight.
Solar energy initiates the flow of energy within ecosystems through organisms known as producers. These are typically photosynthetic organisms, such as plants, algae, and some bacteria, which are also referred to as autotrophs (self-feeders) because they produce their own organic food molecules. Producers convert light energy from the sun into chemical energy, primarily in the form of glucose (C6H12O6), through the process of photosynthesis (6CO2 + 6H2O + ext{light energy} \rightarrow C6H12O6 + 6O_2).
This chemical energy, stored in organic molecules, forms the base of the food web. The consumers (heterotrophs) obtain energy by feeding on other organisms or their products. These can be categorized by their trophic levels:
Primary Consumers (Herbivores): Organisms that feed directly on producers (e.g., deer eating plants, rabbits eating grass, caterpillars munching leaves).
Secondary Consumers (Carnivores/Omnivores): Organisms that feed on primary consumers (e.g., wolves eating deer; humans eating plants and animals; birds eating insects).
Tertiary Consumers: Organisms that feed on secondary consumers (e.g., eagles preying on snakes which eat rodents).
When producers and consumers die, their stored energy and nutrients become available to decomposers. Decomposers, primarily bacteria and fungi, play a critical role in ecosystems by breaking down dead organic matter and waste products. This process, called decomposition, releases essential inorganic nutrients (e.g., nitrogen, phosphorus, carbon, potassium) back into the soil, water, and atmosphere. These released nutrients can then be reabsorbed by producers, thus completing the nutrient cycle within the ecosystem. The continuous cycling of matter is crucial for the sustainability of life.
Energy flow within an ecosystem is largely a one-way process. It starts with solar energy, is captured by producers, transferred through various trophic levels of consumers, and ultimately dissipates as heat into the environment, in accordance with the laws of thermodynamics. In contrast, matter (nutrients) cycles continuously within the ecosystem, being reused and recycled.
Life Comes From Life
All forms of life have the inherent ability to reproduce, producing offspring that are similar to themselves. This ensures the continuation of the species. Reproduction can occur through various mechanisms:
Asexual reproduction: A single parent produces genetically identical offspring (clones). Examples include bacteria and protists simply splitting in two (binary fission), budding in yeasts, or vegetative propagation in plants.
Sexual reproduction: Involves the fusion of gametes from two parents, leading to genetically diverse offspring.
When organisms reproduce, their genes or genetic code is passed down to the next generation. Genes are specific sequences of nucleotides in DNA that contain the instructions for building and maintaining an organism.
Genes are made of long DNA (deoxyribonucleic acid) molecules. DNA is a nucleic acid polymer produced from the covalent bonding of nucleotide monomers that contain the sugar deoxyribose and a phosphate group, linked by phosphodiester bonds, and nitrogenous bases (adenine, guanine, cytosine, thymine). It is the genetic material of living organisms, carrying hereditary information.
Variations in genetic code are called mutations. Mutations are inheritable changes in the nucleotide sequence structure of an organismās DNA. These changes can arise spontaneously during DNA replication or be induced by environmental factors (mutagens). Not all mutations are detrimental; some can be neutral, while others can provide beneficial traits that enhance an organism's survival and reproductive success in a changing environment. These beneficial mutations are the raw material for evolution.
Life Responds to Its Environment
All organisms have the capacity to detect and respond to information, or stimuli, from both their external and internal environments. A stimulus is any detectable change in the internal or external environment of an organism that elicits a response.
Examples of stimuli and responses:
External stimuli: Light, heat, sound, touch, chemical presence.
A plant growing towards a light source (phototropism).
An animal shivering when cold to generate heat.
A human withdrawing their hand from a hot surface.
Internal stimuli: Changes in nutrient levels, hormone concentrations, pH levels.
Regulation of blood glucose levels by insulin and glucagon.
The movement of an organism, whether self-directed (e.g., seeking food) or in response to a stimulus (e.g., fleeing a predator), constitutes a large part of its behavior. Behavior is largely directed toward minimizing injury, acquiring food, finding mates, and reproducing.
Behavior: Observable, coordinated responses of organisms to environmental stimuli. These behaviors can be influenced by a variety of factors including genetics (innate behaviors), learning (learned behaviors), and environmental conditions, showcasing the remarkable adaptability of organisms to their surroundings.
Life Maintains an Internal Environment
One of the most crucial characteristics of life is the ability of organisms to maintain a relatively stable internal environment, despite fluctuations in the external environment. This stable internal environment is called homeostasis.
Homeostasis: The maintenance of normal internal conditions in a cell or an organism by means of self-regulating mechanisms. These mechanisms involve complex physiological processes that keep internal variables (such as temperature, pH, blood glucose, water balance, and osmotic pressure) within narrow, optimal ranges.
Organisms have various feedback and control systems to help regulate their internal environments. These systems often operate without any conscious or voluntary activity being necessary. They can be controlled at the cellular level, tissue level, and in multicellular organisms, by specialized organ systems such as the nervous system (e.g., hypothalamus regulating temperature) and the endocrine system (e.g., hormones regulating blood sugar).
Examples of Homeostatic Regulation:
Temperature regulation in mammals: When body temperature rises, sweating and vasodilation help dissipate heat. When it drops, shivering and vasoconstriction generate and conserve heat, maintaining a narrow temperature range (e.g., 37^ ext{o}C or 98.6^ ext{o}F in humans).
Blood sugar regulation: After a meal, blood glucose levels rise, prompting the pancreas to release insulin, which helps cells absorb glucose. If blood glucose drops (e.g., during fasting), the pancreas releases glucagon, which stimulates the liver to release stored glucose.
Regulation of blood pressure: Baroreceptors detect changes in blood pressure, triggering cardiovascular reflexes involving the heart and blood vessels to adjust heart rate and vessel diameter, maintaining adequate perfusion to tissues.
Regulation of pH levels: Buffers in the blood (e.g., bicarbonate buffer system) help maintain acid-base balance (typically around pH 7.4) to ensure proper enzyme function and cellular processes, as even slight shifts can be detrimental.
Life Has the Ability to Adapt
Life is dynamic and constantly evolving. As the environment changes over time, some individuals of a species may possess or develop traits that enhance their survival and reproductive success in that specific environment. This differential survival and reproduction lead to evolutionāa change in the genetic makeup of a population over generations.
Individuals of a species: A group of organisms that are capable of interbreeding with one another in nature to produce fertile offspring. This definition underscores the genetic continuity and evolutionary potential within a species.
We call such advantageous features adaptations.
Adaptations: Inherited modifications in the structure, function, or behavior of an organism that make a species more suitable or better fitted to its specific environment. Adaptations are the result of natural selection acting on genetic variation within populations. Over long periods, these adaptations accumulate, leading to the diversification of life forms.