Integration in Living Organisms (SL IB Biology)

• Complex living organisms have evolved to make use of living, or body, systems made up of component parts that collectively perform an overall function
• Coordination of these parts is required in order for the systems to fully integrate and work together for the whole organism
• Living systems are often made up of billions of cells and so require mechanisms of cell-cell communication within the system and with cells in a separate system in a different part of the organism
  • An example of this, found in both plants and animals, is the use of hormones; these are produced within one body system (the endocrine system) but may have an effect in a different body system (the reproductive system)

• Multicellular organisms have developed a hierarchy of organisation that allows for effective communication and interaction with their environment
• Specialised cells of the same type group together to form tissues
• A tissue is a group of cells that work together to perform a particular function. For example:
  • 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)
• Different tissues work together to form organs. 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 organs work together to form organ systems
• Organ systems work together to carry out the life functions of a complete organism
• At each hierarchical level there is great efficiency and complexity 

Example of the hierarchy of organisation diagram

Multicellular organisms have many levels of organisation

Emergent properties

• Multicellular organisms are able to undertake functions that unicellular organisms cannot, e.g. move over vast distances and digest large macromolecules
• This is a result of properties emerging when individual cells organise and interact to produce living organisms
  • Scientists sometimes summarise this with the phrase "The whole is greater than the sum of its parts" , this phrase describes the idea that the individual systems within the organism are more effective when they work together
• 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
  • For example a cheetah becomes an effective predator by integration of all its body systems

• Communication within the bodies of animals is primarily by the nervous system or the endocrine system
• Often these two systems are required to work together to maintain body processes such as digestion, maintaining heart rate, blood glucose levels and blood pressure
• These processes rely on transfer of energy and materials around the body of the organism
• Transport vessels within the blood system are at times required to move materials around the body to various tissues, for example:
  • Oxygen and glucose are transported to all cells of the body to facilitate respiration
  • Urea, produced by protein metabolism in the liver, is transported in the blood to be excreted by the kidneys
  • Hormones, such as FSH and LH, are transported via the blood from the pituitary gland in the brain to the ovaries during the menstrual cycle

The nervous system

• The human nervous system consists of:
  • Central nervous system (CNS) – the brain and spinal cord
  • Peripheral nervous system (PNS) – all of the nerves in the body
• It allows us to make sense of our surroundings and respond to them, and to coordinate and regulate body functions
• Information is sent through the nervous system in the form of electrical impulses – these are electrical signals that pass along nerve cells known as neurones
  • A bundle of neurones is known as a nerve
• The nerves spread out from the central nervous system to all other regions of the body and importantly, to all of the sense organs
• More information about the structure of the nervous system can be found here

The endocrine system

• A hormone is a chemical substance produced by an endocrine gland and carried by the blood
  • The endocrine glands that produce hormones in animals are known collectively as the endocrine system
  • A gland is a group of cells that produces and releases one or more substances (a process known as secretion)

Glands of the endocrine system diagram

Hormones are produced in the glands and travel round the body in the blood

• Hormones are chemicals which transmit information, via the blood, from one part of the organism to another and that bring about a change
• They alter the activity of one or more specific target organs
  • Hormones only affect cells with receptors that the hormone can bind to
  • These are either found on the cell surface membrane, or inside cells
  • Receptors have to be complementary to hormones for there to be an effect
  • Effects can be long lived, as long as hormones are bound to the receptors
• Hormones are used to control functions that do not need instant responses
  • They travel more slowly in the blood compared to a nervous impulse

Comparison of the nervous and endocrine systems table