• Multicellular organisms are able to undertake functions that unicellular organisms cannot, e.g. move over vast distances and digest large macromolecules
• • Scientists sometimes summarise this with the phrase "The whole is greater than the sum of its parts"This is a result of properties emerging when?individual cells organise and interact to produce living organisms
• Traditionally, scientists have approached the study of biology from a reductionist perspective, looking at the individual cells, however, due to emergent properties there is an argument that the?systems approach?should be used

The organisation of multicellular organisms

• In multicellular organisms,?specialised cells?of the?same type?group?together?to form?tissues
• • Epithelial cells group together to form epithelial tissue (the function of which, in the small intestine, is to absorb food)
  • Muscle cells (another type of specialised cell) group together to form muscle tissue (the function of which is to contract in order to move parts of the body)A tissue is a group of cells that?work together?to perform a?particular function. For example:
• • The heart is made up of many different tissues (including cardiac muscle tissue, blood vessel tissues and connective tissue, as well as many others)Different tissues?work together?to form?organs. For example:
• Different organs?work together?to form?organ systems
• Organ systems?work together?to carry out the life functions of a complete?organism

The organisation of multicellular organisms

Levels of Organisation in Humans Table

• In complex?multicellular?organisms?eukaryotic?cells become?specialised?for?specific functions. This can also be referred to as?the division of labour
• Specialisation enables the cells in a tissue to function more efficiently as they develop specific adaptations for that role. The development of these distinct specialised cells occurs by?differentiation
• These specialised eukaryotic cells have?specific adaptations?to help them carry out their functions
• For example, the?structure of a cell?is adapted to help it carry out its function (this is why specialised eukaryotic cells can look extremely?different?from each other)
• • The?shape?of the cell
  • The?organelles?the cell contains (or doesn’t contain)Structural adaptations include:
• • Cells that make large amounts of?proteins?will be adapted for this function by containing?many ribosomes?(the organelle responsible for protein production)For example:

The biconcave shape of erythrocytes increases the surface area available for oxygen absorption

Gene Expression

• Every nucleus within the cells of a multicellular organism contains the same genes, that is, all cells of an organism have the identical?genome
• Despite cells having the same genome, they have a diverse range of functions because during?differentiation?certain genes are?expressed?('switched' on)
• Controlling gene expression is the key to development as the cells differentiate due to the different genes being expressed
• Once certain genes are expressed the specialisation of the cell is usually fixed so the cell cannot adapt to a new function

Expression of genes resulting in cell differentiation