Life Science
Biology
Biology is the study of living things, including plants, animals, and single-celled organisms. In this section, we’ll explore the basic building blocks of life and how they function.
Life
Something that is living undergoes self-sustaining physical processes such as growth, homeostasis (keeping stable internal conditions like temperature and water balance), and reproduction. Humans, other animals, plants, and bacteria are all examples of living organisms.
Cells
A cell is the smallest unit of life that can carry out biological functions. It is contained within a protective membrane. Single-celled organisms like bacteria are made of just one cell.
Multicellular organisms like plants and animals are made up of many cells, each of which has its own specialization.
Anatomy of a cell
Cells contain smaller components called organelles that have specialized purposes. There are more than organelles within an animal cell, but here are some of the most important ones to know.
Nucleus: holds the genetic information (DNA) of the organism. It is usually the largest organelle in an animal cell.
Mitochondria: produce energy. Also called the “powerhouse” of the cell.
Ribosome: creates proteins.
Cell membrane: acts as a barrier that separates the cell from the surrounding environment.
Animal vs. plant cells
Both animal and plant cells have a nucleus, mitochondria, ribosomes, and cell membranes, but plant cells also have a few additional organelles.
Cell wall: gives a plant cell a rigid structure.
Chloroplasts: allow a plant to photosynthesize (convert sunlight into energy). Photosynthesis turns carbon dioxide, water, and sunlight into glucose and oxygen; cellular respiration runs the reverse reaction, turning glucose and oxygen back into carbon dioxide, water, and usable energy.
Central vacuole: a large sac that stores water and helps keep the plant cell firm; it’s often the largest organelle in a plant cell.
Because animal cells lack a cell wall, they are more rounded and flexible than plant cells.
Genetics
Genetics is the study of gene expression, gene variation, and heredity in organisms.
DNA: the molecule that holds all of an organism’s genetic instructions
Genes: segments of DNA that produce traits. They are passed from organism to offspring during reproduction.
Heredity: how organisms pass down genes to their offspring.
Dominant and recessive traits: each gene comes in different versions called alleles, and an organism inherits one allele from each parent. A dominant allele determines the trait whenever it’s present, while a recessive trait only shows up when the organism inherits two recessive alleles. For example, two brown-eyed parents who each carry a hidden blue-eye allele can have a blue-eyed child if that child happens to inherit the recessive allele from both parents.
Chromosomes
DNA is packaged into structures called chromosomes. In humans, chromosomes are arranged into pairs, so most people have chromosomes total; other species have different numbers of chromosomes.
Reproduction
Reproduction is the creation of offspring.
Some species use asexual reproduction, where one parent produces offspring that are genetically identical to itself (though mutations can occasionally introduce genetic differences).
Most species of plants and animals use sexual reproduction, where offspring are created with genetic information from two parents. This produces offspring with unique DNA, roughly half from each parent. As egg and sperm cells form, each parent’s DNA is also reshuffled into new combinations, a process called genetic recombination.
Sexual reproduction allows for more genetic diversity, which increases a species’ chance of survival. Asexual reproduction grows a population faster, as only one member of the species is needed to complete the process, and all members of the species can do it.
Mutations
When DNA is replicated (either from parent to offspring or within one organism), it is sometimes copied incorrectly. This is called a mutation and is a naturally occurring part of genetics and inheritance.
Most mutations are neutral: they neither help nor harm the organism that carries them.
Some mutations are harmful and can cause problems for the organism or even lead to death.
Occasionally, a mutation will be beneficial to the organism. When that happens, it is most likely to be passed to the next generation.
There are four main types of genetic mutations:
Substitution: one nucleotide (a single building block of DNA) is replaced with another
Deletion: one or more nucleotides are deleted
Insertion: one or more nucleotides are added
Frameshift: an insertion or deletion that shifts the reading frame, changing how the DNA is translated into proteins
Evolution
Evolution is the process by which a population’s inherited traits change over generations, often driven by natural selection acting on genetic mutations.
If a mutation is particularly beneficial for a population, the organism with the mutation is more likely to live and pass the mutation to their offspring. These offspring are then more likely to live and pass the mutation to their offspring as well. Over millions of years, the mutations that benefit the group the most end up shaping the population as a whole.
Human body systems
The human body relies on several organ systems working together:
- Circulatory system: moves blood, oxygen, and nutrients through the body using the heart and blood vessels.
- Respiratory system: brings oxygen into the body and removes carbon dioxide, mainly through the lungs.
- Digestive system: breaks down food into nutrients the body can absorb and use.
- Nervous system: carries electrical signals between the brain, spinal cord, and the rest of the body to control actions and responses.
Ecology and natural systems
Ecology is the study of how organisms interact with their environment. Within one environment, there will be many different types of living organisms, all with their own particular role in keeping that environment balanced.
Trophic levels
Organisms are often grouped by the role they play in a food chain, a single feeding path from producer to top predator. In a real ecosystem, many food chains overlap to form a food web. The main categories that an organism can fit into are producers, consumers, and decomposers.
Level 1 - Primary producers
At the first level are the producers. These are organisms that can produce their own food using nutrients from the earth and energy from the sun, such as plants and algae.
Level 2 - Primary consumers
At the second level are the herbivores. These are organisms that cannot make their own food and primarily consume producers to survive. These include insects like butterflies and grasshoppers as well as mammals like rabbits, deer, and giraffes.
Level 3 - Secondary consumers
The third level is for organisms that eat primary consumers. Many animals at this level are omnivorous (meaning they eat both plants and animals), so they often eat both primary producers and primary consumers. Secondary consumers include rats, foxes, snakes, owls, and coyotes.
Level 4 - Tertiary consumers and apex predators
Tertiary consumers feed on secondary consumers (though, again, they may occasionally eat primary consumers or even primary producers as well). If the tertiary consumer has no natural predators, they are considered apex predators and sit at the “top” of the food chain. Tertiary consumers include hawks, lions, sharks, and tigers.
Decomposers
Decomposers act on every level of the food chain. All living organisms will eventually die, and decomposers break down dead organisms and organic waste back into nutrients. These nutrients go back into the soil to sustain the primary producers, which is how the food web continues to sustain itself. Decomposers include bacteria and fungi.
This system is often drawn as an energy pyramid (also called an ecological pyramid). Only about of the energy available at one level passes up to the next; the rest is used for life processes or lost as heat. That means each higher level has less energy to work with, so it usually supports fewer organisms than the level below it. This is why a tertiary consumer population collapses if there are not enough secondary consumers for them to feed on.