Biological Hierarchy

For multicellular organisms to function effectively and live successfully in order to reproduce, there needs to be a high level of organisation in the arrangement of their specialised cells.

\scriptsize \text{atoms} \rightarrow \text{molecules} \rightarrow \text{organelles} \rightarrow \text{cells} \rightarrow \text{tissues} \rightarrow \text{organ systems} \rightarrow \text{organisms}
Level of OrganisationExplanationExample
Atomic LevelAtoms are the smallest unit of an element that still maintains the property of that elementCarbon, Hydrogen, Oxygen
Molecular LevelAtoms form to combine into molecules which can have entirely different properties than the atoms they containWater, DNA, Carbohydrates
Cellular LevelCells are the smallest unit of life; they are enclosed by membrane or cell wall and often perform specific functionsMuscle cell, Skin cell, Neuron
Tissue LevelGroups of cells with similar functionsMuscle, Connective
Organ LevelOrgans are 2 types of tissue that work together to complete a taskHeart, Liver, Stomach
Organ System LevelGroup of organs that carry out a more generalised set of functionsDigestive system, Circulatory system
Organismal LevelSeveral organ systems which function togetherHuman

Human Body Systems

Body SystemMain Structural FeaturesFunction
Integumentary System– Hair
– Skin
– Nails
– Act as a barrier
– Retains body fluid
– Regulates body temp
Skeletal System– Bones
– Tendons
– Ligaments
– Cartilage
– Movement
– Calcium storage
– Support
– Organ protection
Lymphatic System– Lymph nodes
– Spleen
– Thymus
– Removal of excess fluids
– Absorption of fatty acids
– Protection of immune cells
Respiratory System– Airways
– Lungs
– Oesophagus
– Allows the intake and expel of oxygen and carbon dioxide
Muscular System– Muscle
– Muscle fibre
– Blood vessels
– Generation of force for movement
– Speech
– Eating
Nervous System– Spinal cord
– Brain
– Nerves
– Allows bodily communication
Digestive System– Stomach
– Liver
– Gallbladder
– Large intestine
– Small intestine
– Processes food for use in the body
– Removes waste from undigested food
Urinary System– Kidneys
– Urinary bladder
– Controls water balance in the body
– Removes waste from the blood and excretes them
Endocrine System– Pituitary gland
– Thyroid gland
– Pancreas
– Adrenaline gland
– Testes → male
– Ovaries → female
– Secretes hormones
– Regulates body temp
Circulatory System– Heart
– Blood vessels
– Delivers oxygen and nutrients to tissues
– Equalises body temp
Male Reproductive System– Prostate
– Epididymis
– Testes
– Produces sex hormones
– Delivers gametes to the female
Female Reproductive System– Mammary glands
– Ovaries
– Uterus
– Produces sex hormones and gametes
– Supports embryo and gametes until birth