1.2: Temas y Conceptos de Biología

Habilidades para Desarrollar

• Identificar y describir las propiedades de la vida
• Describir los niveles de organización entre los seres vivos
• Reconocer e interpretar un árbol filogenético
• Enumerar ejemplos de diferentes subdisciplinas en biología

La biología es la ciencia que estudia la vida, pero ¿qué es exactamente la vida? Esto puede sonar como una pregunta tonta con una respuesta obvia, pero no siempre es fácil definir la vida. Por ejemplo, una rama de la biología llamada virología estudia virus, que exhiben algunas de las características de las entidades vivientes pero carecen de otras. Resulta que aunque los virus pueden atacar a los organismos vivos, causar enfermedades, e incluso reproducirse, no cumplen con los criterios que utilizan los biólogos para definir la vida. En consecuencia, los virólogos no son biólogos, estrictamente hablando. De igual manera, algunos biólogos estudian la evolución molecular temprana que dio origen a la vida; dado que los eventos que precedieron a la vida no son eventos biológicos, estos científicos también están excluidos de la biología en el sentido estricto del término.

Desde sus inicios, la biología ha lucido con tres preguntas: ¿Cuáles son las propiedades compartidas que hacen que algo “viva”? Y una vez que sabemos que algo está vivo, ¿cómo encontramos niveles significativos de organización en su estructura? Y, finalmente, ante la notable diversidad de la vida, ¿cómo organizamos los diferentes tipos de organismos para que podamos entenderlos mejor? A medida que cada día se descubren nuevos organismos, los biólogos continúan buscando respuestas a estas y otras preguntas.

Propiedades de la vida

Todos los organismos vivos comparten varias características o funciones clave: orden, sensibilidad o respuesta al ambiente, reproducción, adaptación, crecimiento y desarrollo, regulación, homeostasis, procesamiento energético y evolución. Cuando se ven juntas, estas nueve características sirven para definir la vida.

Orden

Los organismos son estructuras altamente organizadas y coordinadas que constan de una o más células. Incluso los organismos unicelulares muy simples son notablemente complejos: dentro de cada célula, los átomos forman moléculas; estos a su vez forman orgánulos celulares y otras inclusiones celulares. En organismos multicelulares (Figura\(\PageIndex{1}\)), células similares forman tejidos. Los tejidos, a su vez, colaboran para crear órganos (estructuras corporales con una función distinta). Los órganos trabajan juntos para formar sistemas de órganos.

Figura\(\PageIndex{1}\): Un sapo representa una estructura altamente organizada que consiste en células, tejidos, órganos y sistemas de órganos. (crédito: “Ivengo” /Wikimedia Commons)

Sensibilidad o Respuesta a los Estímulos

Los organismos responden a diversos estímulos. Por ejemplo, las plantas pueden doblarse hacia una fuente de luz, trepar sobre cercas y paredes, o responder al tacto (Figura\(\PageIndex{2}\)). Incluso las bacterias diminutas pueden moverse hacia o alejarse de los productos químicos (un proceso llamado quimiotaxis) o la luz (fototaxis). El movimiento hacia un estímulo se considera una respuesta positiva, mientras que el alejamiento de un estímulo se considera una respuesta negativa.

Figura\(\PageIndex{2}\): Las hojas de esta planta sensible (Mimosa pudica) se inclinarán instantáneamente y se doblarán cuando se toquen. Después de unos minutos, la planta vuelve a la normalidad. (crédito: Alex Lomas)

Enlace al aprendizaje

Video: Mira este video para ver cómo las plantas responden a un estímulo, desde la apertura hasta la luz, pasando por envolver un zarcillo alrededor de una rama, hasta capturar presas.

Reproducción

Los organismos unicelulares se reproducen duplicando primero su ADN y luego dividiéndolo por igual a medida que la célula se prepara para dividirse para formar dos nuevas células. Los organismos multicelulares a menudo producen células germinales reproductivas especializadas que formarán nuevos individuos. Cuando ocurre la reproducción, los genes que contienen ADN se pasan a la descendencia de un organismo. Estos genes aseguran que la descendencia pertenecerá a la misma especie y tendrá características similares, como tamaño y forma.

Crecimiento y Desarrollo

Los organismos crecen y se desarrollan siguiendo instrucciones específicas codificadas por sus genes. Estos genes proporcionan instrucciones que dirigirán el crecimiento y desarrollo celular, asegurando que la cría de una especie (Figura\(\PageIndex{3}\)) crecerá para exhibir muchas de las mismas características que sus padres.

Figura\(\PageIndex{3}\): Aunque no hay dos iguales, estos gatitos han heredado genes de ambos padres y comparten muchas de las mismas características. (crédito: Rocky Mountain Feline Rescue)

Regulación

Incluso los organismos más pequeños son complejos y requieren múltiples mecanismos reguladores para coordinar las funciones internas, responder a estímulos y hacer frente a las tensiones ambientales. Dos ejemplos de funciones internas reguladas en un organismo son el transporte de nutrientes y el flujo sanguíneo. Los órganos (grupos de tejidos que trabajan juntos) realizan funciones específicas, como transportar oxígeno por todo el cuerpo, eliminar desechos, entregar nutrientes a cada célula y enfriar el cuerpo.

Homeostasis

Para funcionar correctamente, las células necesitan tener condiciones adecuadas como temperatura, pH y concentración adecuada de diversos químicos. Estas condiciones pueden, sin embargo, cambiar de un momento a otro. Los organismos son capaces de mantener condiciones internas dentro de un rango estrecho casi constantemente, a pesar de los cambios ambientales, a través de la homeostasis (literalmente, “estado estacionario”) —la capacidad de un organismo para mantener condiciones internas constantes. Por ejemplo, un organismo necesita regular la temperatura corporal a través de un proceso conocido como termorregulación. Los organismos que viven en climas fríos, como el oso polar (Figura\(\PageIndex{4}\)), tienen estructuras corporales que les ayudan a soportar bajas temperaturas y conservar el calor corporal. Las estructuras que ayudan en este tipo de aislamiento incluyen pelaje, plumas, grasa y grasa. En climas cálidos, los organismos tienen métodos (como la transpiración en humanos o el jadeo en los perros) que les ayudan a arrojar el exceso de calor corporal.

Figura\(\PageIndex{4}\): Los osos polares (Ursus maritimus) y otros mamíferos que viven en regiones cubiertas de hielo mantienen su temperatura corporal generando calor y reduciendo la pérdida de calor a través del pelaje grueso y una densa capa de grasa debajo de la piel. (crédito: “longhorndave” /Flickr)

Procesamiento de Energía

Todos los organismos utilizan una fuente de energía para sus actividades metabólicas. Algunos organismos capturan energía del sol y la convierten en energía química en los alimentos; otros utilizan energía química en moléculas que toman como alimento (Figura\(\PageIndex{5}\)).

Figura\(\PageIndex{5}\): El cóndor de California (Gymnogyps californianus) utiliza energía química derivada de los alimentos para impulsar el vuelo. Los cóndores de California son una especie en peligro de extinción; esta ave tiene una etiqueta de ala que ayuda a los biólogos a identificar al individuo. (crédito: Pacific Southwest Region U.S. Fish and Wildlife Service)

Niveles de organización de los seres vivos

Los seres vivos están altamente organizados y estructurados, siguiendo una jerarquía que puede ser examinada en una escala de pequeña a grande. El átomo es la unidad más pequeña y fundamental de la materia. Consiste en un núcleo rodeado de electrones. Los átomos forman moléculas. Una molécula es una estructura química que consiste en al menos dos átomos unidos por uno o más enlaces químicos. Muchas moléculas que son biológicamente importantes son macromoléculas, moléculas grandes que normalmente se forman por polimerización (un polímero es una molécula grande que se elabora combinando unidades más pequeñas llamadas monómeros, que son más simples que las macromoléculas). Un ejemplo de una macromolécula es el ácido desoxirribonucleico (ADN) (Figura\(\PageIndex{6}\)), que contiene las instrucciones para la estructura y funcionamiento de todos los organismos vivos.

Figura\(\PageIndex{6}\): Todas las moléculas, incluyendo esta molécula de ADN, están compuestas por átomos. (crédito: “brian0918” /Wikimedia Commons)

Enlace al aprendizaje

Video: Mira este video que anima la estructura tridimensional de la molécula de ADN mostrada en la Figura\(\PageIndex{6}\).

Some cells contain aggregates of macromolecules surrounded by membranes; these are called organelles. Organelles are small structures that exist within cells. Examples of organelles include mitochondria and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars. All living things are made of cells; the cell itself is the smallest fundamental unit of structure and function in living organisms. (This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack the reproductive mechanism of a living cell; only then can they obtain the materials they need to reproduce.) Some organisms consist of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled or colonial organisms that do not have membrane-bound nuclei; in contrast, the cells of eukaryotes do have membrane-bound organelles and a membrane-bound nucleus.

In larger organisms, cells combine to make tissues, which are groups of similar cells carrying out similar or related functions. Organs are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs; it includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms.

All the individuals of a species living within a specific area are collectively called a population. For example, a forest may include many pine trees. All of these pine trees represent the population of pine trees in this forest. Different populations may live in the same specific area. For example, the forest with the pine trees includes populations of flowering plants and also insects and microbial populations. A community is the sum of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest’s community. The forest itself is an ecosystem. An ecosystem consists of all the living things in a particular area together with the abiotic, non-living parts of that environment such as nitrogen in the soil or rain water. At the highest level of organization (Figure \(\PageIndex{7}\)), the biosphere is the collection of all ecosystems, and it represents the zones of life on earth. It includes land, water, and even the atmosphere to a certain extent.

Art Connection

Figure \(\PageIndex{7}\): The biological levels of organization of living things are shown. From a single organelle to the entire biosphere, living organisms are parts of a highly structured hierarchy. (credit “organelles”: modification of work by Umberto Salvagnin; credit “cells”: modification of work by Bruce Wetzel, Harry Schaefer/ National Cancer Institute; credit “tissues”: modification of work by Kilbad; Fama Clamosa; Mikael Häggström; credit “organs”: modification of work by Mariana Ruiz Villareal; credit “organisms”: modification of work by "Crystal"/Flickr; credit “ecosystems”: modification of work by US Fish and Wildlife Service Headquarters; credit “biosphere”: modification of work by NASA)

Which of the following statements is false?

1. Tissues exist within organs which exist within organ systems.
2. Communities exist within populations which exist within ecosystems.
3. Organelles exist within cells which exist within tissues.
4. Communities exist within ecosystems which exist in the biosphere.

The Diversity of Life

The fact that biology, as a science, has such a broad scope has to do with the tremendous diversity of life on earth. The source of this diversity is evolution, the process of gradual change during which new species arise from older species. Evolutionary biologists study the evolution of living things in everything from the microscopic world to ecosystems.

The evolution of various life forms on Earth can be summarized in a phylogenetic tree (Figure \(\PageIndex{8}\)). A phylogenetic tree is a diagram showing the evolutionary relationships among biological species based on similarities and differences in genetic or physical traits or both. A phylogenetic tree is composed of nodes and branches. The internal nodes represent ancestors and are points in evolution when, based on scientific evidence, an ancestor is thought to have diverged to form two new species. The length of each branch is proportional to the time elapsed since the split.

Figure \(\PageIndex{8}\): This phylogenetic tree was constructed by microbiologist Carl Woese using data obtained from sequencing ribosomal RNA genes. The tree shows the separation of living organisms into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotes, single-celled organisms lacking intracellular organelles. (credit: Eric Gaba; NASA Astrobiology Institute)

Evolution Connection: Carl Woese and the Phylogenetic Tree

In the past, biologists grouped living organisms into five kingdoms: animals, plants, fungi, protists, and bacteria. The organizational scheme was based mainly on physical features, as opposed to physiology, biochemistry, or molecular biology, all of which are used by modern systematics. The pioneering work of American microbiologist Carl Woese in the early 1970s has shown, however, that life on Earth has evolved along three lineages, now called domains—Bacteria, Archaea, and Eukarya. The first two are prokaryotic cells with microbes that lack membrane-enclosed nuclei and organelles. The third domain contains the eukaryotes and includes unicellular microorganisms together with the four original kingdoms (excluding bacteria). Woese defined Archaea as a new domain, and this resulted in a new taxonomic tree (Figure \(\PageIndex{8}\)). Many organisms belonging to the Archaea domain live under extreme conditions and are called extremophiles. To construct his tree, Woese used genetic relationships rather than similarities based on morphology (shape).

Woese’s tree was constructed from comparative sequencing of the genes that are universally distributed, present in every organism, and conserved (meaning that these genes have remained essentially unchanged throughout evolution). Woese’s approach was revolutionary because comparisons of physical features are insufficient to differentiate between the prokaryotes that appear fairly similar in spite of their tremendous biochemical diversity and genetic variability (Figure \(\PageIndex{9}\)). The comparison of homologous DNA and RNA sequences provided Woese with a sensitive device that revealed the extensive variability of prokaryotes, and which justified the separation of the prokaryotes into two domains: bacteria and archaea.

Figure \(\PageIndex{9}\): These images represent different domains. The (a) bacteria in this micrograph belong to Domain Bacteria, while the (b) extremophiles (not visible) living in this hot vent belong to Domain Archaea. Both the (c) sunflower and (d) lion are part of Domain Eukarya. (credit a: modification of work by Drew March; credit b: modification of work by Steve Jurvetson; credit c: modification of work by Michael Arrighi; credit d: modification of work by Leszek Leszcynski)

Branches of Biological Study

The scope of biology is broad and therefore contains many branches and subdisciplines. Biologists may pursue one of those subdisciplines and work in a more focused field. For instance, molecular biology and biochemistry study biological processes at the molecular and chemical level, including interactions among molecules such as DNA, RNA, and proteins, as well as the way they are regulated. Microbiology, the study of microorganisms, is the study of the structure and function of single-celled organisms. It is quite a broad branch itself, and depending on the subject of study, there are also microbial physiologists, ecologists, and geneticists, among others.

Career Connection: Forensic Scientist

Forensic science is the application of science to answer questions related to the law. Biologists as well as chemists and biochemists can be forensic scientists. Forensic scientists provide scientific evidence for use in courts, and their job involves examining trace materials associated with crimes. Interest in forensic science has increased in the last few years, possibly because of popular television shows that feature forensic scientists on the job. Also, the development of molecular techniques and the establishment of DNA databases have expanded the types of work that forensic scientists can do. Their job activities are primarily related to crimes against people such as murder, rape, and assault. Their work involves analyzing samples such as hair, blood, and other body fluids and also processing DNA (Figure \(\PageIndex{10}\)) found in many different environments and materials. Forensic scientists also analyze other biological evidence left at crime scenes, such as insect larvae or pollen grains. Students who want to pursue careers in forensic science will most likely be required to take chemistry and biology courses as well as some intensive math courses.

Figure \(\PageIndex{10}\): This forensic scientist works in a DNA extraction room at the U.S. Army Criminal Investigation Laboratory at Fort Gillem, GA. (credit: United States Army CID Command Public Affairs)

Another field of biological study, neurobiology, studies the biology of the nervous system, and although it is considered a branch of biology, it is also recognized as an interdisciplinary field of study known as neuroscience. Because of its interdisciplinary nature, this subdiscipline studies different functions of the nervous system using molecular, cellular, developmental, medical, and computational approaches.

Figure \(\PageIndex{11}\): Researchers work on excavating dinosaur fossils at a site in Castellón, Spain. (credit: Mario Modesto)

Paleontology, another branch of biology, uses fossils to study life’s history (Figure \(\PageIndex{11}\)). Zoology and botany are the study of animals and plants, respectively. Biologists can also specialize as biotechnologists, ecologists, or physiologists, to name just a few areas. This is just a small sample of the many fields that biologists can pursue.

Biology is the culmination of the achievements of the natural sciences from their inception to today. Excitingly, it is the cradle of emerging sciences, such as the biology of brain activity, genetic engineering of custom organisms, and the biology of evolution that uses the laboratory tools of molecular biology to retrace the earliest stages of life on earth. A scan of news headlines—whether reporting on immunizations, a newly discovered species, sports doping, or a genetically-modified food—demonstrates the way biology is active in and important to our everyday world.

Summary

Biology is the science of life. All living organisms share several key properties such as order, sensitivity or response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing. Living things are highly organized parts of a hierarchy that includes atoms, molecules, organelles, cells, tissues, organs, and organ systems. Organisms, in turn, are grouped as populations, communities, ecosystems, and the biosphere. The great diversity of life today evolved from less-diverse ancestral organisms over billions of years. A diagram called a phylogenetic tree can be used to show evolutionary relationships among organisms.

Biology is very broad and includes many branches and subdisciplines. Examples include molecular biology, microbiology, neurobiology, zoology, and botany, among others.

Art Connections

Figure \(\PageIndex{7}\): Which of the following statements is false?

1. Tissues exist within organs which exist within organ systems.
2. Communities exist within populations which exist within ecosystems.
3. Organelles exist within cells which exist within tissues.
4. Communities exist within ecosystems which exist in the biosphere.

Answer

Communities exist within populations which exist within ecosystems.

Glossary

atom

smallest and most fundamental unit of matter

biochemistry

study of the chemistry of biological organisms

biosphere

collection of all the ecosystems on Earth

botany

study of plants

cell

smallest fundamental unit of structure and function in living things

community

set of populations inhabiting a particular area

ecosystem

all the living things in a particular area together with the abiotic, nonliving parts of that environment

eukaryote

organism with cells that have nuclei and membrane-bound organelles

evolution

process of gradual change during which new species arise from older species and some species become extinct

homeostasis

ability of an organism to maintain constant internal conditions

macromolecule

large molecule, typically formed by the joining of smaller molecules

microbiology

study of the structure and function of microorganisms

molecule

chemical structure consisting of at least two atoms held together by one or more chemical bonds

molecular biology

study of biological processes and their regulation at the molecular level, including interactions among molecules such as DNA, RNA, and proteins

neurobiology

study of the biology of the nervous system

organ

collection of related tissues grouped together performing a common function

organ system

level of organization that consists of functionally related interacting organs

organelle

small structures that exist within cells and carry out cellular functions

organism

individual living entity

paleontology

study of life’s history by means of fossils

phylogenetic tree

diagram showing the evolutionary relationships among various biological species based on similarities and differences in genetic or physical traits or both; in essence, a hypothesis concerning evolutionary connections

population

all of the individuals of a species living within a specific area

prokaryote

single-celled organism that lacks organelles and does not have nuclei surrounded by a nuclear membrane

tissue

group of similar cells carrying out related functions

zoology

study of animals