In multicellular organisms, cells are organised into levels of increasing complexity:specialised cells → tissues → organs → organ systems → organism Specialised cells
All the cells of an organism come from one zygote and contain the same genes. During development, cells differentiate: different genes are switched on and off, so each cell type makes different proteins and develops a structure suited to one function. Examples are red blood cells (no nucleus, packed with haemoglobin), sperm cells, palisade cells and root hair cells.Tissues
A tissue is a group of similar cells, together with any substances they secrete (such as the matrix of bone), that work together to carry out a particular function. Animal examples are muscle tissue, nervous tissue and epithelium; plant examples are xylem, phloem and palisade mesophyll. Some tissues, such as xylem and phloem, contain more than one cell type.Organs
An organ is a structure made of several different tissues working together to perform one or more functions. For example, the stomach contains epithelium, muscle, connective tissue and nervous tissue; a leaf contains epidermis, mesophyll, xylem and phloem.Organ systems
Groups of organs working together form organ systems, such as the digestive system.This organisation allows a division of labour: each cell type becomes very efficient at one job, while depending on other cells for everything else. The cost is that specialised cells cannot survive on their own.
All the cells of an organism come from one zygote and contain the same genes. During development, cells differentiate: different genes are switched on and off, so each cell type makes different proteins and develops a structure suited to one function. Examples are red blood cells (no nucleus, packed with haemoglobin), sperm cells, palisade cells and root hair cells.Tissues
A tissue is a group of similar cells, together with any substances they secrete (such as the matrix of bone), that work together to carry out a particular function. Animal examples are muscle tissue, nervous tissue and epithelium; plant examples are xylem, phloem and palisade mesophyll. Some tissues, such as xylem and phloem, contain more than one cell type.Organs
An organ is a structure made of several different tissues working together to perform one or more functions. For example, the stomach contains epithelium, muscle, connective tissue and nervous tissue; a leaf contains epidermis, mesophyll, xylem and phloem.Organ systems
Groups of organs working together form organ systems, such as the digestive system.This organisation allows a division of labour: each cell type becomes very efficient at one job, while depending on other cells for everything else. The cost is that specialised cells cannot survive on their own.