Section 2 a) Summary

Living organisms are made up of different levels of organisation, each categorised as follows:

Organelle: found within a cell, it helps the cell to carry out different functions, for example mitochondria respires and therefore provide the cell with energy, and a nucleus contains DNA within chromosomes that direct the cell's actions.

Cell: contains organelles, can be an organism on its own (uni-cellular) or part of a larger organism (multi-cellular). Makes up tissues. Can carry out a specific function, e.g. neurones, or nerve cells, transmit electrical impulses around the body, and in plants the palisade cell contains lots of chloroplasts for photosynthesis.

Tissue: made up of similar cells working together to carry out a specific function. Tissues make up organs and carry out vital roles, e.g. muscle tissue allows animals to move as it contracts and relaxes, connective tissues hold organs together and support them in place

Organ: Made up of a group of different tissues, found within organ systems. Carries out one or more specific function, e.g. the skin protects the body and regulates heat, the eyes pick up light that allows us to see

Organ system: a group of organs that work in a system to carry out life processes, e.g. circulatory system pumps blood around the body so oxygen can reach everywhere for respiration, nervous system transmits electrical impulses so the body can react to stimuli