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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.
Plants also exchange substances with the environment, mainly taking part in gas exchange and mass flow of solutes.
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.
Diagram of leaf cross-section, showing guard cells and stomata. Source: researchgate.com
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.
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.
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. Source: microbenotes.com
The main difference between xylem and phloem is that xylem transports water, whereas phloem transports substances made by photosynthesis (assimilates).
Transports systems in animals also have a pump-like organ (heart) and more complex vessels. These adaptations satisfy the greater metabolic demand in animals.
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.
![Substance Exchange [+] Diagram showing a cross-section of a capillary [+] StudySmarter](https://studysmarter-mediafiles.s3.amazonaws.com/media/1865576/summary_images/image_HXSm5uQ.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIA4OLDUDE42UZHAIET%2F20220521%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20220521T234938Z&X-Amz-Expires=90000&X-Amz-SignedHeaders=host&X-Amz-Signature=2d470258b39f1f3c84bab8e3db0b1e5f28b6decfece3bf6cbe9b973246bcbd91)
Diagram showing a cross-section of a capillary. Source: getbodysmart.com
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.
![Substance Exchange [+] Diagram of a capillary bed surrounding tissues [+] StudySmarter](https://studysmarter-mediafiles.s3.amazonaws.com/media/1865576/summary_images/capillary_network.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIA4OLDUDE42UZHAIET%2F20220521%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20220521T234938Z&X-Amz-Expires=90000&X-Amz-SignedHeaders=host&X-Amz-Signature=f318f508ef58267bb047485d35722e679f2b7b24f8c1981fe707bffc709e4f2d)
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.
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.
![Substance Exchange [+] Diagram of a fish's gill [+] StudySmarter](https://studysmarter-mediafiles.s3.amazonaws.com/media/1865576/summary_images/gills.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIA4OLDUDE42UZHAIET%2F20220521%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20220521T234938Z&X-Amz-Expires=90000&X-Amz-SignedHeaders=host&X-Amz-Signature=380718810e4e71219e47ae50901476ccbdf8864a1390dd54a4b6847e7f3acdee)
Diagram of a section of a fish’s gill. Source: AQA A level biology coursebook
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.
![Substance Exchange [+] diagram of the counter-current system of gas exchange across a lamella in fish [+] StudySmarter](https://studysmarter-mediafiles.s3.amazonaws.com/media/1865576/summary_images/counter_current.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIA4OLDUDE42UZHAIET%2F20220521%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20220521T234938Z&X-Amz-Expires=90000&X-Amz-SignedHeaders=host&X-Amz-Signature=8debe259c61efe5fae97e9a55309ca9dac2861ddef15971027678437cd170ea2)
Diagram of the counter-current system of gas exchange across a lamella in fish. Source: AQA A level biology coursebook
In insects that lack capillaries, gas exchange occurs in the trachea.
![Substance Exchange [+] diagram of an insect’s trachea [+] StudySmarter](https://studysmarter-mediafiles.s3.amazonaws.com/media/1865576/summary_images/trachea.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIA4OLDUDE42UZHAIET%2F20220521%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20220521T234938Z&X-Amz-Expires=90000&X-Amz-SignedHeaders=host&X-Amz-Signature=31ad23b22a98be9f42200900eb5cbb9a13c4da5c50862ba333c6c68d0e5cf8bf)
Diagram of an insect’s trachea. Source: AQA A level biology coursebook
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.
![Substance Exchange [+] diagram of the human gas exchange system with the alveoli enlarged [+] StudySmarter](https://studysmarter-mediafiles.s3.amazonaws.com/media/1865576/summary_images/lungs_maS7V1T.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIA4OLDUDE42UZHAIET%2F20220521%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20220521T234938Z&X-Amz-Expires=90000&X-Amz-SignedHeaders=host&X-Amz-Signature=cdfafcea3c919c4ca7c67ffaa996a2f8c9bcafdbb16ed23a705579c33a824735)
Diagram of the human gas exchange system with the alveoli enlarged. Source: AQA A level biology coursebook
The alveoli are adapted for gas exchange via their thin (i.e. one cell thick) walls and close proximity with capillaries.
![Substance Exchange [+] Diagram showing the gaseous exchange between an alveolus and a capillary [+] StudySmarter](https://studysmarter-mediafiles.s3.amazonaws.com/media/1865576/summary_images/alveolus.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIA4OLDUDE42UZHAIET%2F20220521%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20220521T234938Z&X-Amz-Expires=90000&X-Amz-SignedHeaders=host&X-Amz-Signature=4fa79433d92565eaf0381419503a413270217330d093e57e4e6b44fa2743e772)
Diagram showing the gaseous exchange between an alveolus and a capillary. Source: AQA A level biology coursebook
As single-celled organisms have a high surface area to volume ratio, they simply rely on the cell membrane to exchange substances through processes such as diffusion, osmosis and active transport.
Solutes such as glucose dissolve in the plasma after being absorbed by the small intestines. As the plasma has a higher glucose concentration than body cells, glucose diffuses into body cells via capillaries to be respired to generate energy (ATP).
Capillaries are one cell thick to shorten the diffusion distance between the blood and neighbouring cells. They also form a network known as a capillary bed around neighbouring cells to increase the surface area needed for efficient diffusion.
As a result of their large sizes, the surface area to volume ratio of multicellular organisms is too small to enable diffusion alone for efficient substance exchange. Therefore, multicellular organisms have their own transport systems to achieve so. Plants have mass flow systems, whereas animals have circulatory systems.
The placenta is an organ that supplies nutrients to the fetus from the mother and removes metabolic wastes from the fetus into the mother’s blood. The placenta is surrounded by a rich network of maternal blood vessels. It comprises villi (finger-like projections similar to those of the small intestines) to increase the surface area and allow efficient substance exchange.
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