Substance Exchange

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