Molecular structure and function

A biological molecule’s structure helps determine what it can do. The types of atoms it contains and the way they are bonded and arranged give the molecule its three-dimensional shape and chemical properties.

These features affect how the molecule interacts with other molecules. Its shape can determine whether it fits against another molecule, while its chemical properties affect whether the two molecules attract or repel each other. Such interactions allow biological molecules to carry out their functions in living organisms.

For example, an enzyme has a region called an active site, where a substrate binds. The active site’s shape and chemical properties help determine which substrate can bind, so they affect which reaction the enzyme can catalyse.

If a molecule’s structure changes, its shape or chemical properties may change too. This can alter its interactions with other molecules and therefore its function. For example, a change in an enzyme’s active site may make a substrate less able to bind, reducing the enzyme’s activity. Not every structural change has an effect on function.

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.

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.

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 can have different inherited characteristics. If a characteristic helps an individual survive or reproduce in its environment, that individual may leave more offspring than others. Over generations, the inherited characteristic can become more common in the population. This process is called natural selection.

A phenotype is an individual's observable characteristics. Some differences in phenotype are heritable: they are influenced by alleles that can be passed from parents to offspring. Natural selection acts when individuals with different heritable phenotypes differ in their survival or reproductive success.

Selection pressures

A selection pressure is a feature of the environment that affects survival or reproduction. Predation, disease and competition can all be selection pressures. For example, if predators more easily spot some individuals than others, individuals with inherited colouring that makes them harder to see may be more likely to survive and reproduce.

An advantageous phenotype is one that improves survival or reproductive success under the prevailing conditions. If conditions change, the same phenotype may no longer provide an advantage.

Natural selection and evolution

Individuals with an advantageous phenotype are more likely to reproduce and pass the alleles contributing to that phenotype to their offspring. Not every individual with the advantage will reproduce, but if they leave more offspring on average, those alleles can become more common over generations.

The gene pool is the collection of alleles in a population. Evolution is a change in the frequencies of alleles in a population over time. Natural selection is one process that can produce this change: individuals are selected through differences in survival and reproduction, while allele frequencies change in the population across generations.

Evolution and common ancestry

Populations can change over many generations as inherited characteristics become more or less common. This change is called evolution. It occurs in populations over generations; an individual organism does not evolve during its lifetime.

Natural selection can contribute to evolution. Individuals in a population have inherited differences. If a characteristic helps individuals survive and reproduce in their environment, they may pass it on to more offspring. Over generations, that characteristic can become more common in the population.

Evolution and shared ancestry

Populations descended from the same ancestral population can gradually become different from one another. This divergence can eventually give rise to different species. The formation of different species contributes to biodiversity, the variety of living organisms.

Living organisms are related through common ancestry: their ancestral lineages connect at some point in the past. Organisms that share a more recent common ancestor are more closely related than organisms whose most recent common ancestor lived further in the past.

Levels of ecological organisation

Organisms can be considered at different levels, from a group of one species to all the living and non-living parts of an area.

A population is a group of organisms of the same species living in the same area at the same time. For example, all the frogs of one species in a pond form a population.

A community consists of populations of different species living and interacting in an area. In a pond, the frog population is part of a community that also includes populations of other species, such as insects and plants.

An ecosystem includes the community and the non-living environment with which its organisms interact. In the pond ecosystem, organisms interact with non-living conditions such as water, light and temperature.

Humans are part of ecosystems too. They can influence organisms and environmental conditions, and they can also be affected by them. For example, pollution entering a pond can harm its organisms and affect people who use the water.

Photosynthesis

Green plants and algae use energy from sunlight to make organic compounds, such as sugars. They take in carbon dioxide and water, then use carbon from the carbon dioxide and hydrogen derived from the water to build these compounds. This incorporation of carbon into organic compounds is called carbon fixation. Oxygen is released during the process.

Photosynthesis transfers light energy into chemical energy stored in the organic compounds. Plants and algae use these compounds to support their growth and other life processes.

Photosynthesis and other organisms

The organic material made by plants and algae also supports many other organisms. Some obtain it directly by feeding on plants or algae; others obtain it indirectly by feeding on organisms that have eaten them.

Photosynthesis also supplies much of the oxygen used in aerobic respiration, a process that releases energy from food.

Cellular respiration

Living cells need a usable supply of energy to carry out the processes that keep them alive. Cellular respiration is a series of chemical reactions in cells that breaks down organic fuel molecules and transfers energy to forms the cell can use.

One such form is an energy-carrying molecule called ATP. Cells use energy transferred to ATP to support other chemical reactions and processes, such as building molecules and moving substances across cell membranes.

Organic fuels include molecules such as glucose. Photosynthetic organisms can make these fuels by photosynthesis, while organisms can also obtain them as food. Photosynthetic organisms use some of the fuel they make in their own cellular respiration.

Cycling of matter in ecosystems

The matter that makes up living organisms moves between organisms and the non-living environment: the air, water and soil. Organisms take in chemical substances, use them to build their bodies and carry out life processes, and return substances to their surroundings. The matter can then be taken up again.

Movement through organisms

Organisms obtain substances either directly from their surroundings or by feeding on other organisms. For example, plants take up water and mineral substances from the soil. When an animal eats a plant, some of the substances in the plant pass to the animal.

Return to the environment

Several processes return substances to the non-living environment:

• Respiration releases carbon dioxide into the air or water.
• Excretion removes waste substances produced by organisms.
• Decomposition breaks down dead organisms and waste materials, returning substances to the surroundings, including the soil.

These returned substances can be taken up by organisms again. Different chemicals follow different routes, and they may be changed into other substances along the way. However, their constituent matter is reused, rather than permanently consumed by living processes.

Genetic and environmental influences on phenotype

An organism’s phenotype is its observable characteristics, such as its height or appearance. These characteristics develop under the influence of both the organism’s genetic information and the conditions in which it lives.

An organism’s genome is its complete set of genetic material. It contains genes that carry information influencing how the organism develops and functions. For example, genes can affect how tall a person is likely to grow.

Environmental conditions can also affect how characteristics develop or are expressed. Nutrition, for example, influences growth: a person who does not receive sufficient nutrients may not reach the height they could have reached with adequate nutrition. People with different genetic information may also grow to different heights even if their nutrition is similar.

Phenotype therefore reflects the interaction of genetic and environmental influences. Genes provide information that influences characteristics, while environmental conditions can affect how those characteristics develop. The relative influence of each can vary between characteristics.