Ecosystem interdependence

An ecosystem is a natural system in which a community of living organisms interacts with its non-living environment. The living parts are called biotic components. The non-living parts, or abiotic components, include water, light, temperature, air, soil and mineral nutrients.

These components are interdependent: organisms depend on environmental conditions and on one another, while their activities can also change those conditions. A change in one part of an ecosystem can therefore affect other parts.

Ecosystem scale and boundaries

Ecosystems can be studied at different scales, from a small pond to an extensive forest. A boundary identifies the system being studied, but it does not completely close that system off. Water, organisms, nutrients and energy can cross ecosystem boundaries.

Interdependence in a freshwater pond

In a small UK freshwater pond, aquatic plants depend on light, water and dissolved nutrients to grow. Animals depend on suitable water conditions and on other organisms for food. Plants, in turn, provide habitats and release oxygen during photosynthesis, influencing conditions for other pond organisms.

Dynamic balance and change

Ecosystem balance is dynamic, not an unchanging state. Populations and environmental conditions fluctuate, but their interactions can maintain a broadly functioning system.

For example, if pond water becomes more turbid, or cloudy, less light reaches submerged plants. Their growth may decline, reducing the food and shelter available to animals. Changes to those animal populations can then produce further changes across the pond community. The initial change in water conditions spreads through the ecosystem because its components depend on one another.

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.

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.

Populations and communities

A population is all the organisms of one species living in the same area at the same time. For example, all the oak trees in a woodland form an oak population.

A community consists of the populations of different species living and interacting in an area. The oak trees, squirrels and fungi in a woodland are part of the same community. Their populations interact: squirrels feed on acorns produced by oak trees, while fungi break down dead tree material.

A community therefore contains multiple populations, with each population made up of one species. The species are connected through their interactions with one another.

Humans also interact with populations and communities. For example, people depend on trees for timber, but clearing woodland can remove habitats and reduce the populations of other organisms living there.

Photosynthesis

Green plants and algae capture light energy and use it to make organic compounds from carbon dioxide and water. This process is called photosynthesis. Organic compounds contain carbon; glucose is one example.

During photosynthesis, carbon from carbon dioxide becomes part of organic molecules. This is called carbon fixation. Water supplies hydrogen for making these molecules, and oxygen is released as a product. The light energy needed for the process is captured by the green pigment chlorophyll.

The overall production of glucose can be represented by the equation:

The organic compounds made by photosynthesis provide material for growth and a source of energy for organisms. They form a foundation for life on Earth because other organisms obtain these compounds directly or indirectly from plants and algae.

Cellular respiration

Cells need a supply of energy to carry out the processes that keep them alive. Cellular respiration is a sequence of chemical reactions in cells that releases energy from organic compounds used as fuels, such as glucose.

During respiration, the fuel is changed through a series of reactions rather than in a single step. The energy released is made available for other reactions and processes in the cell, such as building larger molecules and moving substances across cell membranes. Some energy is also transferred to the surroundings as heat.

Chemical cycling in ecosystems

The substances that organisms need do not remain in one place. They move between organisms and the non-living environment, which includes the air, water and soil. As they move, the matter in these substances is reused.

Organisms take substances in from their surroundings. For example, plants take up water and mineral substances from the soil. Chemicals can then pass from one organism to another when one feeds on the other.

Organisms also release waste, and eventually they die. Decomposers break down waste and dead material, returning substances to the non-living environment. These substances become available for organisms to take up again.

This repeated movement between living organisms and their surroundings is chemical cycling. The substances may change form as they move, but their matter is reused rather than being used only once.

Genome and environment in characteristics

A characteristic is a feature of an organism, such as its height or eye colour. Characteristics are influenced by both the organism’s genetic material and the conditions in which it lives.

An organism’s genome is its complete set of genetic material. It includes genes, which contain information that contributes to characteristics. For example, many genes contain instructions for making proteins that affect how the body develops and functions.

The environment includes the conditions an organism experiences, such as its diet. These conditions can affect how a characteristic develops or is expressed. For example, genes influence a person’s potential height, but inadequate nutrition during growth can limit the height they reach.

The relative influence of genes and the environment varies between characteristics. Eye colour is mainly influenced by genes, whereas height is influenced by both genes and environmental conditions. A characteristic is therefore not necessarily determined by genes alone.

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.