Cells are the basic units of life. All organisms are made up of cells. There are two main types of cells:
- Prokaryotic cells: do not contain a nucleus; lack structure; usually found in single-cell organisms
- Eukaryotic cells: contain a nucleus as well as other parts that work together; usually compose multi-cellular organisms
For the rest of this review, assume we are talking about eukaryotic cells unless otherwise specified.
Parts of a Cell
Each cell has a nucleus, which contains its DNA— the genetic information for the organism that the cell is a part of. DNA is contained in units called chromosomes.
Besides the nucleus, most cells have other parts called organelles that perform different functions within the cell. Which organelles a cell has depends on what type of organism it is part of. We’re going to focus specifically on the parts of animal and plant cells.
Animal and plant cells have these major parts in common:
- Nucleus: control center of the cell; contains genetic code (DNA)
- Cytoplasm: liquid between contained within the cell membrane in which all the other parts of the cell float
- Cell membrane: permeable barrier on the outside of the cell
- Mitochondria: powerhouse of the cell; makes energy (ATP)
Image by domdomegg
You’ll notice in the diagram that plant cells have a few extra parts that animal cells do not possess. These parts are unique to plant cells:
- Cell wall: rigid outer layer of the cell
- Permanent vacuole: large, central vacuole used for storing food and water; filled with cell sap to keep the cell turgid (note: animal cells can also contain vacuoles but they are much smaller and do not contain sap)
- Chloroplast: site of photosynthesis (the process by which plants make their own food)
There are two types of cellular reproduction: mitosis and meiosis.
Mitosis occurs when a cell reproduces by splitting itself into two new ones. Each new cell is an exact copy of the original.
Meiosis occurs when cells divide and each one contains only half the genetic information of the original cell. Two of these half-cells must join to create a complete new cell/organism. This is how the reproductive cells of animals work. A male sperm and a female egg each contain half the genetic code needed to form a new organism. They unite to create a complete set of chromosomes.
Organisms are classified according to a system called the Linnaean Taxonomy. The example below shows the taxonomic categories, as well as the example of the classification of a red fox.
Image by Annina Breen
The most common way to refer to organisms in the scientific community is to use their “scientific name,” which is made up of an organism’s genus and species. Humans, for example, are homo sapiens.
Plants are made up of several basic parts:
- Roots: anchor the plant into the soil and carry water and nutrients from the soil into the rest of the plant
- Stem: transports water between the roots and the rest of the plants
- Leaves: site of photosynthesis, the process by which plants make their own food by using their chlorophyll to convert sunlight, water, and carbon dioxide into glucose and oxygen
- Seeds: contain genetic information for new plants
- Flower: site of reproduction
Image by Siyavula Education
Animals’ bodies contain body systems in which various organs work together to carry out necessary life functions. The chart below lists the major body systems. The organs involved in each system vary by species.
|Circulatory||Carries blood throughout the body|
|Digestive||Consumes and processes food|
|Endocrine||Regulates body processes through hormones|
|Excretory||Removes waste from the body|
|Immune||Protects the body against infection|
|Musculoskeletal||Gives the body its structure and allows it to move|
|Nervous||Sends electrical signals from the brain to the rest of the body|
|Respiratory||Takes in oxygen and releases carbon dioxide|
Genetics and Heredity
Genetics refers to how parents pass on traits to their offspring through genes. Two terms you should know:
- Genotype: the genetic code for a particular trait
- Phenotype: the expression of the trait
We talked earlier about how meiosis means that an organism gets half its genetic code from each parent cell. Let’s apply that to human reproduction.
A human being’s DNA has 46 chromosomes, in pairs of 23. You get 23 chromosomes from your mother’s egg and 23 from your father’s sperm. These chromosomes combine so that you will likely have some genetic traits that you share with your father and some that you share with your mother.
Because you have genes from two parents, you have some traits in your DNA that you may not actually display. For example, if your parents have different eye colors from one another, you will have genetic information for both possibilities, but your eyes will actually show up as one color or the other. Some traits are more likely to show up as the visible phenotype than others. We call these dominant traits because they overpower the recessive, or hidden, traits.
One common way of looking at how traits are inherited is through a Punnett square. These charts show the possible combinations for the traits of offspring based on the parents’ genes.
The example below refers to brown and blue eyes. Brown eyes (B) is dominant, so any combination that contains a B will present as brown (BB or Bb). Blue (b) is recessive, so only bb will present as blue. In this chart, you can see two brown-eyed parents that each have a recessive blue gene (Bb) have a 75% chance of a brown-eyed offspring and a 25% chance of a blue-eyed offspring.
Image by Purpy Pupple
The theory of evolution, first published by Charles Darwin, holds that species change over time in response to their environment. These changes are the result of natural selection. Those members of a species that have traits that are best suited to the environment are most likely to survive (“survival of the fittest”) and, therefore, reproduce and pass on those favorable traits. Over time, this can lead to large-scale changes in the species.
Organisms and the Environment
An ecosystem is an area where many different types of organisms live. The organisms in an ecosystem all rely on one another. If the population of one species increases or decreases for any reason, all the other species in an ecosystem can be affected. Likewise, environmental changes like pollution, deforestation, and natural disasters can disrupt ecosystems.
The Food Chain
Energy is transferred through an ecosystem through the food organisms eat.
Producers (plants) make their own food using photosynthesis.
Consumers (animals) get their energy by eating other organisms.
- Herbivores: eat plants
- Carnivores: eat other animals
- Omnivores: eat both plants and meat
Decomposers eat dead organisms and return those nutrients back to the soil. Plants then use them, completing the cycle. Decomposers include organisms such as fungi and earthworms.
The food chain is a way to describe the transfer of energy through an ecosystem. It shows the producers, various levels of consumers, and decomposers.
Adapted from image by LadyofHats
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