Biology is the science that studies life, but what exactly is life? This may sound like a silly question with an obvious response, but it is not always easy to define life. For example, a branch of biology called virology studies viruses, which exhibit some of the characteristics of living entities but lack others. It turns out that although viruses can attack living organisms, cause diseases, and even reproduce, they do not meet the criteria that biologists use to define life.
Consequently, virologists are not biologists, strictly speaking. Similarly, some biologists study the early molecular evolution that gave rise to life; since the events that preceded life are not biological events, these scientists are also excluded from biology in the strict sense of the term. And once we know something is alive, how do we find meaningful levels of organization in its structure? All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, growth and development, regulation, homeostasis, and energy processing.
When viewed together, these characteristics serve to define life. Figure 1. A toad represents a highly organized structure consisting of cells, tissues, organs, and organ systems. Organisms are highly organized, coordinated structures that consist of one or more cells. Even very simple, single-celled organisms are remarkably complex: inside each cell, atoms make up molecules; these in turn make up cell organelles and other cellular inclusions.
In multicellular organisms Figure 1 , similar cells form tissues. Tissues, in turn, collaborate to create organs body structures with a distinct function. Organs work together to form organ systems. Organisms respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch Figure 2. Figure 2. The leaves of this sensitive plant Mimosa pudica will instantly droop and fold when touched.
After a few minutes, the plant returns to normal. Even tiny bacteria can move toward or away from chemicals a process called chemotaxis or light phototaxis. Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response.
Watch this video to see how plants respond to a stimulus—from opening to light, to wrapping a tendril around a branch, to capturing prey. Single-celled organisms reproduce by first duplicating their DNA, and then dividing it equally as the cell prepares to divide to form two new cells. Multicellular organisms often produce specialized reproductive germline cells that will form new individuals.
These genes ensure that the offspring will belong to the same species and will have similar characteristics, such as size and shape. Figure 3. Although no two look alike, these puppies have inherited genes from both parents and share many of the same characteristics. Organisms grow and develop following specific instructions coded for by their genes. Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, respond to stimuli, and cope with environmental stresses.
Two examples of internal functions regulated in an organism are nutrient transport and blood flow. Other questions to explore could be whether a computer which is turned on is alive; whether a hibernating bear is alive; and whether a deciduous tree in winter is alive. Consider listing key questions that arise in relation to this issue or review these ideas over time with students.
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Skip to content. Page Content. This focus idea is explored through: Contrasting student and scientific views Critical teaching ideas Teaching activities. Science continuum archive. Organisms can also be classified according to their subcellular structures. Those with a well-defined nucleus are referred to as eukaryotes whereas those without are called prokaryotes. Both of them possess a genetic material but the location differs. In eukaryotes, the genetic material is found inside the nucleus whereas, in prokaryotes, it is located in a special region called a nucleoid.
A modern system of classification groups living things into three distinct domains: 1 Archaea archaebacteria , 2 bacteria eubacteria , and 3 Eucarya eukaryotes.
Both archaea and bacteria are prokaryotic organisms while Eucarya, as the name suggests, it includes all the eukaryotes. The scientific study of all organisms is called biology.
Biology is a field in science that aims to study the structure, function, distribution, and evolution of living things. An organism is defined as an entity with life. Both living things and non-living things are basically made up of molecules. However, a living thing can be identified from an inanimate object by its distinctive characteristics. For example, an organism is made up of one or more cells. This structure is made up of molecules that are biologically produced and naturally occurring.
Such molecules are termed biomolecules. Examples are proteins , nucleic acids , lipids , and carbohydrates. These biomolecules can organize into complex particles, which in turn, can form subcellular structures. These subcellular structures are contained within a cell. The cell is regarded as the fundamental biological unit as every living thing is made up of at least one cell. One of the most important subcellular components of a cell is the chromosome.
The chromosome bears the genetic material. In bacteria and archaea, the chromosome is a circular strand of DNA. In humans and other higher forms of organisms, it is a threadlike, linear strand of DNA. The part of the DNA that is responsible for the physical and heritable characteristics of an organism is called a gene. The genes code for amino acids, proteins, and RNA molecules. Proteins are one of the most ubiquitous groups of biomolecules. Many of them are enzymes that catalyze many biological processes.
Changes involving a gene may lead to mutations. As a result, novel features could arise. While some mutations can be lethal or can cause detrimental effects, there are also certain mutations that can lead to beneficial outcomes. Mutations can drive evolution and natural selection. The acquisition of new traits from these mutations may be beneficial to the survival of a species. For example, a strain of bacteria that initially were susceptible to antibiotics could transform and become resistant to antibiotics when they acquire new genes.
In this regard, an organism is, therefore, capable of change by mutation and adaptation. Aside from enzymes, many biological reactions require energy.
The most common form of energy utilized by a living thing is ATP , i. In plants and other photosynthetic organisms, light energy is converted into chemical energy via the process of photosynthesis. Another way of producing energy is by cellular respiration. Cellular respiration is a cellular process wherein carbohydrates are processed to produce chemical energy. Organisms metabolize. This means that they carry out processes that keep them alive. Metabolic processes include growth, response to stimuli, reproduction, waste elimination, and biosynthesis.
Two forms of metabolism are anabolism and catabolism. Anabolism includes the energy-requiring reactions that lead to the building up of biomolecules. Conversely, catabolism includes processes of breaking down particles into simpler molecules. Living things carry out these metabolic processes in an orchestrated, systematized manner. They have diverse regulatory mechanisms to ensure that homeostatic conditions are kept and sustained.
Organisms are capable of detecting and responding to stimuli. They can detect changes in their environment. Humans and other animals have senses to detect stimuli.
The five fundamental senses are sight, smell, touch, taste, and hearing. The response is crucial to survival. For instance, an individual organism might move away from the source of the stimuli. Others might move towards it. Organisms can reproduce. They can give rise to another of the same kind species. There are essentially two ways to do this: 1 by sexual reproduction , i. In asexual reproduction, the offspring is a clone of the parent.
In sexual reproduction, the offspring is a new individual formed by the union of the sex cells. Organisms go through life stages. The offspring will grow to adulthood, meaning the phase at which it is also capable of reproducing. At the cellular level, growth entails an increase in size or an increase in number.
An increase in cell size is one in which the cell increases in girth as it synthesizes and stores biomolecules. An increase in the number entails an increase in the cell number through cellular division. A great activity for learning about the four major groups of biomolecules: fats, carbohydrates, nucleic acids, and proteins. The nucleus is an organelle that has a membrane called the nuclear envelope perforated with holes called nuclear pores. Inside the nucleus are genetic material and nuclear bodies suspended in the nucleoplasm.
Nucleoplasm is the protoplast material inside the nucleus. These nuclear structures are absent in a prokaryotic cell. The nucleus of a eukaryotic cell is where DNA replication the process in which a DNA segment is duplicated and transcription a process where mRNA transcript is produced occurs.
Conversely, these processes occur in the cytoplasm of a prokaryotic cell. The presence of a nucleus compartmentalizes the genetic material and these processes.
The nuclear envelope prevents the easy entry of molecules and thereby regulates the passage of molecules into and out of the nucleus. There is an instance though when the nucleus apparently disappears. During cellular division, the nuclear envelope disintegrates to allow the chromosomes to separate and move to opposite poles, and then reforms to compartmentalize the genetic material in each of the two new cells.
Apart from the nucleus, other membrane-bound organelles found in a eukaryotic cell that are not present in a prokaryotic cell are mitochondria , plastids , endoplasmic reticulum , Golgi apparatus , lysosomes , and endosomes. Because of the presence of larger cytoplasmic structures, a eukaryotic cell is notably larger than a prokaryotic cell. What is common between a prokaryotic cell and a eukaryotic cell is the presence of genes that store genetic information.
Ribosomes cytoplasmic structures that serve as the site of protein synthesis are also present in both cell types. Nevertheless, the prokaryotic ribosomes are 70S made up of 50S and 30S whereas the eukaryotic ribosomes are 80S made up of 60S and 40S.
And while the ribosomes of the prokaryotes are made in the cytoplasm the process of ribosome synthesis involves both the cytoplasm and the nucleolus of the nucleus in eukaryotes. Examples of prokaryotes are bacteria and archaea whereas eukaryotes include protists, fungi, plants, and animals. Organisms may be described as single-celled unicellular or multicellular.
Unicellular organisms are those that are made up of only one cell. Conversely, multicellular organisms are comprised of many cells that act as a unit performing a particular function. Examples of unicellular prokaryotes are bacteria and archaea and unicellular eukaryotes are protists and certain fungi.
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