Models
Scientists use modelAn equation, diagram or analogy that helps explain a scientific idea. 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 ethicalRelating to right and wrong, eg ethical companies are often ones that act in a responsible way. or practical difficulties.
However, a model cannot explain everything. Models have limitations.
The table shows the different types of model.
| Type of model | Description |
| Representational model | Uses shapes or analogies to describe a system |
| Descriptive model | Uses words to describe the features of a system and how they interact |
| Mathematical model | Uses patterns of data of past events, known scientific relationships and calculations to make predictions |
| Computational model | A mathematical model that needs a computer to carry out complex calculations |
| Spatial model | A computational model used to show how predicted data appears in three dimensions |
| Type of model | Representational model |
|---|---|
| Description | Uses shapes or analogies to describe a system |
| Type of model | Descriptive model |
|---|---|
| Description | Uses words to describe the features of a system and how they interact |
| Type of model | Mathematical model |
|---|---|
| Description | Uses patterns of data of past events, known scientific relationships and calculations to make predictions |
| Type of model | Computational model |
|---|---|
| Description | A mathematical model that needs a computer to carry out complex calculations |
| Type of model | Spatial model |
|---|---|
| Description | A computational model used to show how predicted data appears in three dimensions |
Particle model
The particle model The scientific theory used to explain the properties of solids, liquids and gases. It involves the arrangement and movement of the particles in a substance. 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 pointThe temperature at which a solid changes into a liquid as it is heated. vary between substances.
The lock and key theory
The lock and key hypothesis Model which compares the specificity of enzymes with a key and its lock. is a model that explains how enzymeA protein which catalyses or speeds up a chemical reaction. are specific for their substrateA substance on which enzymes act.. 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 siteThe part of the enzyme to which a specific substrate can attach or fit on to. 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 mitosisA type of cell division which produces daughter cells identical to the parent. and meiosisReduction division in a cell in which the chromosome number is halved from diploid to haploid. 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:
- kingdom
- phylumA taxonomic rank between kingdom and class (the plural is phyla). The arthropods, for example, are a phylum.
- class
- order
- family
- genus
- speciesA type of organism that is the basic unit of classification. Individuals of different species are not able to interbreed successfully.
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 eukaryoteAn organism whose cells contain DNA which is surrounded by a membrane-bound nucleus. organisms (those with cells with a nucleus). This was based upon analysis of DNA. This technique didn't exist in Linnaeus' lifetime.
Question
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.
Question
What analysis caused scientists to propose the three-domain model?
DNA analysis.