Specialised cells are organised into tissues, tissues into organs and…

The idea

A tissue is a group of similar cells working together to carry out one function: squamous epithelium lining the alveoli, ciliated epithelium lining the trachea, cardiac muscle in the heart wall, xylem and phloem in a plant. An organ is made of several different tissues working together to perform an overall function; the stomach combines glandular epithelium that secretes enzymes, smooth muscle that churns, connective tissue and blood vessels, and a leaf combines palisade mesophyll, spongy mesophyll, epidermis and vascular tissue. An organ system is a set of organs working together, such as the digestive system of mouth, oesophagus, stomach, ileum, pancreas and liver, or the gas exchange and circulatory systems. This hierarchy allows large organisms to overcome the limits of diffusion, since a small surface area to volume ratio means diffusion alone cannot supply all cells.

What earns the marks

• Award credit for defining a tissue as a group of similar or specialised cells with a shared function.
• Award credit for defining an organ as different tissues working together to perform a particular function.
• Award credit for defining an organ system as a group of organs working together.
• Award credit for a correct worked example running through all the levels, such as epithelial cell, epithelium, ileum, digestive system.
• Award credit for linking the level of organisation to body size and a small surface area to volume ratio, so that diffusion alone is insufficient.

Common mistakes and how to put them right

• Calling blood, or xylem, an organ when each is a tissue.

• Defining a tissue as 'lots of cells' without saying that they are similar and share a function.

• Assigning organs to only one system, for example placing the pancreas in the endocrine system alone when it is also a digestive organ.

• Stating that an organ is made of one type of tissue.

• Mixing plant and animal examples within the same chain, so the hierarchy no longer holds together.

Exam tips

• Give one worked example at every level in the same answer; examiners credit the chain, not the definitions alone.
• Use plant examples as readily as animal ones, since leaf and root questions are common alongside gut questions.
• If asked why large organisms need organ systems, bring in surface area to volume ratio explicitly.