Scientists use models to explain ideas and to test predictions.

A model:

  • is a simpler representation of something
  • includes the key features of the thing being represented
  • shows how these key features connect with each other
  • is used to explain things, solve problems or to make predictions

Models can help to investigate an idea without ethical or practical difficulties.

However, a model cannot explain everything. Models have limitations.

The table shows the different types of model.

Type of modelDescription
Representational modelUses shapes or analogies to describe a system
Descriptive modelUses words to describe the features of a system and how they interact
Mathematical modelUses patterns of data of past events, known scientific relationships and calculations to make predictions
Computational modelA mathematical model that needs a computer to carry out complex calculations
Spatial modelA computational model used to show how predicted data appears in three dimensions

Particle model

The particle model is an example of a representational model. It can be used to explain and predict the behaviour of substances in the solid, liquid and gas states.

It has limitations. For example, it cannot explain why melting points vary between substances.

The lock and key theory

The lock and key theory is a model that explains how enzymes are specific for their substrate. It states that an enzyme is specific for its substrate like a key is for its lock. Each substrate must fit specifically into the active site of the enzyme or the reaction will not occur.

Models of diffusion and osmosis

Diffusion is the net movement of particles from an area of high to lower concentration. Osmosis is the net movement of water from an area of high to lower water concentration across a partially or selectively permeable membrane.

Models of cell division

The behaviour of chromosomes during mitosis and meiosis is complicated.

Mitosis can be modelled by a dance:

A definition and description of mitosis through rap and a practical demonstration

Models of classification

There are millions of species of life on Earth. It is important that we classify these organisms. If we do not know what they are, we cannot study them further. Without further study, we are unable to properly protect them. Until recently we thought all giraffes were the same species. Now we know they are made from four separate species.

The first model of classification was devised by Carl Linnaeus in 1735. His model of classification had three major groups called 'kingdoms'. They were animal, vegetable and mineral. Linnaeus also placed organisms into ever smaller groups within these kingdoms. The groups, based on Linnaeus' work, in decreasing order of size now are:

Models change over time. In 1977, Carl Woese and colleagues proposed a new model that divides all life into three domains. It is called the 'three-domain' model. It divides all life into archaea (primitive bacteria), bacteria and eukaryotic organisms (those with cells with a nucleus). This was based upon analysis of DNA. This technique didn't exist in Linnaeus' lifetime.


What does the lock and key model explain?

That enzymes are specific for their substrates like locks are for their keys. That the substrate must fit into the enzyme like a key in a lock.


What analysis caused scientists to propose the three-domain model?

DNA analysis.

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