Substance exchange in biology is the exchange of substances (e.g. glucose) between the internal and external environments of cells. Put simply, this is how organisms get oxygen and nutrients from the external environment and how they get rid of metabolic waste. Multicellular organisms require specialised organ systems for substance exchange because their surface area to volume ratio is too small to rely on cell membranes as an exchange surface.
Substance Exchange in plants
Plants also exchange substances with the environment, mainly taking part in gas exchange and mass flow of solutes.
Gas exchange in plants
Carbon dioxide is exchanged for oxygen in leaves to allow photosynthesis. The underside of leaves has microscopic pores called stomata (singular stoma) that are key in facilitating gas exchange.
The stomata are not permanent pores as they are regulated by specialised cells termed guard cells as per the diagram below. Apart from the gas exchange, water also diffuses out of the leaf via stomata. This is transpiration, an undesirable event in plant biology.
Fig. 1 - Diagram of leaf cross-section, showing guard cells and stomata
Plants also need oxygen for respiration. Hence plant parts adapt to maximising absorption of oxygen by diffusion - mainly via increasing the surfaces area through branching and air spaces in the plant body.
Mass flow
Knowing how plants absorb water from their roots and produce glucose via photosynthesis, you may question how these get transported to distant parts of plants such as flowers.
Plants have their own transport systems to fulfil this purpose. The transport system of plants, termed mass transport, is composed of two systems - the xylem and phloem. Both systems facilitate mass transport through a process called translocation.
Fig. 2 - Diagram of xylem and phloem with brief descriptions of their characteristics, as well as a cross-section of xylem and phloem under a light microscope
The main difference between xylem and phloem is that xylem transports water, whereas phloem transports substances made by photosynthesis (assimilates).
Substance Exchange in animals
Transports systems in animals also have a pump-like organ (heart) and more complex vessels. These adaptations satisfy the greater metabolic demand in animals.
Role of capillaries in Substance Exchange
Animals have many blood vessels making up their transport systems. Substance exchange directly occurs in the capillaries.
Capillaries are adapted for efficient diffusion by being thin (only one cell thick) and forming a large network called capillary beds.
Fig. 3 - Diagram showing a cross-section of a capillary
Substance exchange between blood and muscle
There is a very high metabolic rate in muscle cells because they generate a lot of energy for contraction. They require a lot of glucose and oxygen from the blood and release a lot of carbon dioxide. A rich network of capillaries surrounds muscle cells for that purpose.
Fig. 4 - Diagram of a capillary bed surrounding tissues
Another key role of capillaries is facilitating gas exchange. We will explore the role of capillaries in gas exchange in the following examples.
Gas exchange in animals
Oxygen and carbon dioxide are the two substances exchanged during gas exchange.
Gas exchange occurs over a gas exchange surface. The gas exchange surfaces differ across animals with low oxygen demand (e.g., insects and fish) and animals with high oxygen demand (e.g., humans).
The gas exchange surfaces in fish are composed of structures called gills. Gills are divided into filaments that contain many lamellae.
Fig. 5 - Diagram of a section of a fish’s gill
Gas exchange occurs in the lamellae, which follow the counter-current system. This means that blood and water in the lamellae flow in opposite directions.
Fig. 6 - Diagram of the counter-current system of gas exchange across a lamella in fish
In insects that lack capillaries, gas exchange occurs in the trachea.
Fig. 7 - Diagram of an insect’s trachea
As organisms with high oxygen demand, humans have more complex gas exchange systems and protein pigments that transport oxygen (e.g. haemoglobin).
The human gas exchange system consists of multiple organs. Gas exchange takes place in specialised, grape-like structures in the lungs known as the alveoli.
Fig. 8 - Diagram of the human gas exchange system with the alveoli enlarged
The alveoli are adapted for gas exchange via their thin (i.e. one cell thick) walls and close proximity with capillaries.
Fig. 9 - Diagram showing the gaseous exchange between an alveolus and a capillary
Substance Exchange - Key takeaways
- Substance exchange occurs in all organisms. Substance exchange in plants is called mass flow systems, whereas animals have circulatory systems.
- The mass flow systems of plants comprise xylem and phloem. Xylem transports water and mineral ions from the roots to aerial plant parts, whereas phloem transports products from photosynthesis from the leaves to the rest of the plant.
- Capillaries are the main vessels involved in substance exchange in animals. Substance exchange occurs in the capillaries via diffusion. Capillaries also play a significant role in gas exchange, especially in fish and humans.
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