Cells, tissues, and organs

Cells

All living organisms are made up of cells, and in animals, they are usually categorized into eukaryotes.
Eukaryotic cells have a clear, membrane-bound nucleus which contains genetic material.
Animals cells also contain various other organelles, such as mitochondria, ribosomes, the endoplasmic reticulum, and the golgi apparatus.
The cell membrane controls the movement of substances in and out of the cell.
Cells divide through a process called mitosis where one cell divides to produce two identical cells.

A tissue is a group of similar cells that work together to carry out a particular function.
In animals, there are four primary types of tissues: epithelial, connective, muscle, and nervous tissues.
Epithelial tissue covers the whole surface of the body and provides a protective layer.
Connective tissue connects or separates other tissues or organs, often containing fibres such as collagen.
Muscle tissue is classified into three types: skeletal (which is voluntary), cardiac, and smooth muscle (both of which are involuntary).
Nervous tissue is found in the brain, spinal cord, and nerves, and is responsible for transmitting signals and communications.

An organ is a structure that contains at least two different types of tissue functioning together for a common purpose.
Examples of organs include the heart, lungs, and liver.
For instance, the heart is an organ that contains muscle tissue (to pump blood), connective tissue (to provide structure and support), and nervous tissue (to regulate heartbeat).
Similarly, other organs have different tissues working together to perform a particular function.
Organ systems are formed when various organs work together to perform major bodily functions, such as the circulatory system or the respiratory system.