Specialisation of eukaryotic cells

In a complex multicellular organism, different cells perform different functions. A specialised cell has features that help it carry out a particular role. These features may include its shape, the abundance of a cell structure, or the way structures are arranged.

For example, a root hair cell absorbs water from the soil. Its long, thin extension gives it a large surface area in contact with the soil, allowing more water to enter the cell.

A sperm cell has a tail that propels it towards an egg. It also has many mitochondria, which release energy needed for the tail to move. Both features are suited to the sperm cell’s role in reaching the egg.

To explain a cell’s adaptation, name a specific feature and link it to the function it helps the cell perform. For a root hair cell, stating that it has a long extension identifies the feature; explaining that the extension increases the surface area for water absorption shows how the feature helps.

Adaptation

An adaptation is an inherited feature that makes an organism better suited to its environment. It can help the organism survive or reproduce, increasing its reproductive success: the chance of producing offspring that themselves survive and reproduce.

An adaptation is useful in relation to a particular environment. For example, a feature that reduces water loss can be advantageous where water is scarce, but may offer little advantage where water is plentiful. If the environment changes, a feature that was once advantageous may become less useful.

Types of adaptation

Adaptations can be grouped according to the kind of feature involved:

• Anatomical adaptations are body structures. For example, a thick, waxy surface on a plant can reduce water loss.
• Physiological adaptations are internal processes. For example, producing concentrated urine allows some animals to conserve water.
• Behavioural adaptations are inherited patterns of action. For example, being active mainly at night can help some desert animals avoid the heat of the day.

Adaptation and natural selection

Individuals in a population can inherit different versions of genes, called alleles. Some of these differences affect features that influence reproductive success in the environment.

If an inherited feature gives its bearers an advantage, they are more likely to reproduce and pass on the alleles associated with it. Over generations, those alleles can become more common in the population. This process is natural selection. As a result, the population can become better adapted to its environment.

Adaptations do not appear because an individual needs them, and individuals do not change their inherited features simply by trying to adapt. Natural selection acts on inherited differences already present in a population.

Changes in inherited characteristics across generations are part of evolution. Because different environments favour different features, adaptation through natural selection contributes to the diversity of living organisms.

Levels of biological organisation

A cell is a basic living unit. New cells are produced when existing cells divide. In a multicellular organism, many cells work together and are organised into increasingly larger structures.

A tissue is a group of cells working together. Different tissues work together to form an organ, and organs work together to form an organ system. An organism is an individual living thing made up of these coordinated parts.

The levels can be followed from the smallest to the largest:

Cell → tissue → organ → organ system → organism

Organisation in the human respiratory system

Ciliated cells form part of the tissue lining the trachea, or windpipe. The trachea is an organ made of different tissues working together. It is part of the respiratory system, an organ system within a human organism.

Natural selection

Individuals in a population are not genetically identical. They carry different versions of genes, called alleles, which can produce differences in inherited characteristics. In a particular environment, some of these characteristics may help individuals survive and reproduce more successfully than others.

More offspring are produced than the environment can support. Individuals therefore face a struggle for survival, which includes competition for limited resources such as food, water or space. Individuals with advantageous inherited characteristics are generally more likely to survive and reproduce, although this is not guaranteed for every individual.

When better-adapted individuals reproduce, they can pass the alleles associated with those characteristics to their offspring. If the characteristics continue to be advantageous, this pattern can be repeated over many generations. The advantageous alleles can then become more common in the population. This change is called natural selection.

Natural selection changes the frequency of alleles in a population over generations; it does not mean that an individual develops a useful inherited characteristic simply because it needs one. Whether a characteristic is advantageous also depends on the environment.