Characteristics of Living Organisms and Human Survival Requirements

Characteristics of All Living Organisms

• Criteria for Life: All living organisms, regardless of their unique anatomical structures or physiological functions, must satisfy a specific set of seven basic criteria to be considered living.

• Cells:     * Definition: Cells are the smallest unit considered living by science and serve as the basic unit of all living things.     * Unicellular Organisms: Organisms composed of only one cell. Examples include bacteria such as E. coli, and organisms like yeast.     * Multicellular Organisms: Organisms composed of more than one ($1+$) cell.         * The tardigrade (commonly known as a "water bear") is a very small multicellular animal known for its resilience and ability to survive extreme conditions; it can be viewed under a low-power microscope.         * The African elephant is a large-scale example, which can reach heights of upal to $11\,ft$ and weigh nearly $6\,\text{tons}$.

• Complexity in Organization:     * Living things possess significantly higher organizational complexity compared to inanimate objects.     * The Human Example: The adult human body consists of approximately $37$ trillion ($37 \times 10^{12}$) cells. These are organized into specific locations and groups, operating at specific times.     * Internal Compartmentalization: Each cell contains internal compartments and structures serving shared and individual functions. These structures are made of various chemicals with unique properties.     * Functional Variation: Some cells function independently, while others collaborate on collective tasks. These tasks fluctuate based on the cell's location in the body, the state of the internal environment, or the time of day.     * Inanimate Comparison: Simple non-living objects like a pen, desk, computer, or smartphone can typically be disassembled and reassembled by humans. Conversely, taking a single-celled or multicellular organism apart and putting it back together is immensely more complex and is a unique challenge of biology.

• Growth and Development:     * Growth: Defined as an increase in size ($\text{Growth} = \Delta \text{Size}$). In humans and other multicellular organisms, this occurs by:         1. Increasing the number of existing cells.         2. Increasing the amount of non-cellular material surrounding cells (e.g., mineral deposits in bone tissue).         3. Increasing the size of existing cells, though this happens only within very narrow limits.     * Development: Refers to all the changes the body undergoes during its lifespan.     * Differentiation: A key part of development where unspecialized cells become specialized in structure and function to perform specific tasks within the body.     * Repair: Development also encompasses the processes of repair for damaged tissues.

• Energy Consumption and Utilization:     * Organisms must intake and utilize energy to power life functions such as growth and development.     * Mechanism: Energy is obtained by consuming molecules (food and beverages) and converting them into energy used for movement and the building or maintenance of body structures.

• Stable Internal Environment (Homeostasis):     * Living organisms must maintain stable internal conditions regardless of external changes.     * Ranges: Survival depends on maintaining specific ranges for factors such as temperature and chemical concentrations. If these conditions deviate from the normal range for long periods, diseases, disorders, or death can occur.     * Thermoregulation Examples:         * Hot environments: The body sweats to facilitate cooling.         * Cold environments: The body shivers to generate heat.

• Responsiveness:     * Definition: The ability of an organism to adjust to changes in internal or external environments.     * External Stimuli: Moving toward food or water sources; moving away from perceived danger.     * Internal Stimuli: Adjustment to light levels. In bright light, the pupils constrict (become small) to reduce light entry. In low light, pupils expand (dilate) to allow more light in.

• Reproduction using DNA:     * Definition: The formation of a new organism from parent organisms. In humans, this involves the male and female reproductive systems.     * DNA (Deoxyribonucleic Acid): The genetic material found in almost every cell containing the instructions for life.     * Function: DNA is passed to the new organism during reproduction, allowing for inheritance of genetic traits and evolutionary processes.

Human Requirements for Life

• Atmospheric Context: Humans have acclimated to Earth's atmosphere for at least the past $200,000$ years.

• Oxygen:     * Role: Required for chemical reactions that produce energy.     * Concentration: Atmospheric air is approximately $20\%$ oxygen.     * Brain Sensitivity: Brain cells are highly sensitive to oxygen deprivation due to their high, steady energy demand.         * $5\,\text{minutes}$ without oxygen: Likely brain damage.         * $10\,\text{minutes}$ without oxygen: Likely death.     * Clinical Interference: Oxygen delivery can be cut off via restricted breathing or broken blood vessels (e.g., a stroke).     * Carbon Dioxide ($CO_2$): A byproduct of oxygen use. High levels of $CO_2$ trigger the urge to breathe.     * Breath-holding statistics:         * Average duration: $30\text{--}90\,\text{seconds}$.         * Male World Record: Budimir Šobat (Croatia) at $24\,\text{minutes}$ and $37.36\,\text{seconds}$.         * Female World Record: Karoline Mayer (Brazil) at $18\,\text{minutes}$ and $32.59\,\text{seconds}$.

• Nutrients:     * Definition: Essential substances found in food and beverages.     * Macronutrients: Required in large amounts.         * Water: The most critical nutrient. Humans can only survive a few days without it. It constitutes roughly $70\%$ of an adult's body mass. It transports chemicals, facilitates reactions, regulates temperature, and lubricates joints.         * Carbohydrates, Lipids (Fats), and Proteins: Large molecules that provide energy and building blocks for tissues. Humans can survive without these for at least several weeks.     * Micronutrients: Required in small amounts.         * Vitamins and Minerals: Examples include calcium (structure) and elements needed for nerve impulses.         * Storage: Some vitamins can be stored in tissues. Water-soluble nutrients like Vitamin C and most B Vitamins cannot be stored and must be consumed every $1\text{--}2$ days.

• Narrow Range of Temperature:     * Critical Average: Normal body temperature is approximately $37^\circ\text{C}$ ($98.6^\circ\text{F}$).     * Responses to Cold: Shivering (random muscle movement) and increased breakdown of stored energy. If reserves deplete and core temperature drops, the brain loses oxygen/energy, leading to confusion, lethargy, loss of consciousness, and death.     * Controlled Hypothermia: A clinical procedure lowering body temperature to reduce metabolic rate.         * Used in open-heart surgery to decrease metabolic needs of the heart and brain.         * Patient is cooled to $25\text{--}32^\circ\text{C}$ ($79\text{--}89^\circ\text{F}$).         * The heart may be cooled to below $15^\circ\text{C}$ ($60^\circ\text{F}$) during surgery.         * Emergency medicine uses it for cardiac arrest patients, lowering temperature to $91^\circ\text{F}$ for $24\,\text{hours}$ to reduce heart workload.

• Narrow Range of Atmospheric Pressure:     * Definition: Force exerted by the mixture of gases (mostly nitrogen and oxygen) in the atmosphere.     * Gas Solubility: Pressure keeps gases like nitrogen dissolved in body fluids.     * Ejection Simulation: Moving from normal to very low pressure (like space) causes blood nitrogen to expand and form bubbles, potentially bursting cells.     * Altitudes: Lower pressure at high altitudes (e.g., Denver, Colorado at $1592\,m$ or $5223\,ft$) makes breathing harder.     * Altitude Sickness: Symptoms include shortness of breath, confusion, headache, lethargy, and nausea.

• Decompression Sickness (DCS):     * Common Term: "The Bends."     * Cause: Rapid reduction of pressure causes dissolved gases (primarily nitrogen) to form bubbles in blood and tissues.     * Analogy: Opening a carbonated drink; lowering the seal pressure causes $CO_2$ to bubble out.     * Occurrences: Underwater divers surfacing too fast or pilots in unpressurized cabins.     * Symptoms: Joint pain (most common), headache, and vision disturbances ($10\%$ to $15\%$ of cases). Severe cases lead to death.     * Treatment: Immediate pure oxygen and a hyperbaric chamber (a closed chamber pressurized higher than the atmosphere to repressurize the body gradually).", "title": "Characteristics of Living Organisms and Human Survival Requirements"}