Editor: Professor Tapani Vuorinen, Aalto University
Biomass chemistry and physiology
Everything you need to know about structure, chemistry and living functions of plants and their physiology
Green plants are amazing organisms! With their roots, the plants, often in symbiosis with fungi or bacteria, extract water and inorganic nutrients from the soil, transport the water to the leaves, sometimes at a height of over one hundred metres, and convert it to oxygen gas just with the energy of sunlight. Through tiny openings in their leaves, the plants release the oxygen and uptake atmospheric carbon dioxide that they then convert into organic form with the electrons from the water molecules. In subsequent steps, the plants synthetise every organic substance that they need for their growth, mechanical support, protection from the hostile organisms and environment, reproduction, etc. For a chemist, doing the same would be a nightmare as water and carbon dioxide are the most stable and unreactive forms of hydrogen and carbon elements.
The plants build all their physical structures through repeating divisions and growth of tiny cells. For example, the trunks of trees are mostly composed of barely visible fibres, ca. a million of them in a cubic centimetre of wood. Highly organised, very thin cellulose fibrils form the skeleton of the fibre walls. A single fibre-formed cell synthetises, on average, a kilometre of cellulose fibril and folds it to a well-organised structure before the cell dies at the age of a couple of weeks to become part of the physical structure of the plant. The biggest tree on the Earth, the General Sherman, a giant sequoia in California, has been synthetising cellulose fibril at an average rate of light already for the last ca. 2500 years!
The plants hide inside them as continuous organisations of hollow cells for transporting water and nutrients up and down. To do that, the neighbouring living cells are equipped with nanosized pumps that are located on thin phospholipid membrane walls that surround the cells. They fuel the pumps with the organic substances the green cells originally produced through photosynthesis. The cells literally burn the nutrients and, to do that, the plants respire as we do. Some of the nanomachines pump pure water while others are specific for single inorganic or organic nutrients. The living cells host also smaller compartments, so called plastids, isolated from the surrounding liquid with thin phospholipid membrane walls. The plastids are also equipped with different kinds of pumps, with the aid of which the plant can store the nutrients, such as sugars, starch, fats or proteins in separate compartments for its reserve.
Some plants, especially trees, may live very long, up to thousands of years. This is an amazing achievement as the soil is full of organisms that degrade all kinds of biological substances. Trees have extremely efficient systems for their biological defence. For example, many trees can close the water conducting channels in their older parts that they do no longer use for their water and nutrient transportation. Some tree species synthetise strong antioxidants or fungicides that kill the foreign organisms that try to penetrate inside the stem. Softwood stems hold three-dimensional networks of canals in which some biologically active molecules flow in an organic solvent as an additional protection.
Thin layers of airtight, hydrophobic substances cover all plants throughout, including their roots, stems, leaves and even seeds, to prevent uncontrolled transfer of liquids and gases in and out. Without this invisible skin, the plants could not control their living functions and, at least the smaller plants, would dehydrate quickly during dry seasons. Still, the plants can control their external mass transfer with the closable openings on their leaves but also in roots and stems. Trees form bark that gives additional protection for the climate and hostile organisms. The inner part of the bark is a major reserve for nutrients in trees.
Living trees need to withstand high mechanical stresses caused by wind and sometimes also by snow. The physical strength of wood originates from parallel fibres glued together with lignin, the second most abundant organic substance in the world after cellulose. Neither lignin nor cellulose gets soft in water under any natural environment, which makes wood a durable material. If living trees could not bend, they would break easily under a mechanical load. Hemicelluloses that are present in the cell wall besides cellulose and lignin absorb water easily, which softens the cell wall but not the joints between the fibres. This behaviour is the reason for the ductility of living trees. Extremely low temperatures may cause the hemicelluloses to become glassy, leading to easier rupture of branches and tops of trees under heavy snow load.
To be able to utilise woody biomass, or lignocellulose, for production of materials and chemicals, it is essential to understand plants’ physical structure, chemical composition and variations in them and, depending on the plant, its anatomical parts, the season, etc. The complexity in the structure and composition is easier to understand through the main functions of living plants. That is the reason why this Biomass chemistry and physiology theme does not only discuss the structure and chemistry of the plants but also their living functions, although not very deeply.
The cell wall structure is the most important factor affecting the properties and reactivity of lignocellulose, especially wood. Therefore, this theme discusses this issue, including the structure of the main cell wall polymers, i.e. cellulose, lignin and hemicelluloses, in detail. The plant tissue and cell structures are important when utilising lignocellulose as a solid material or pulp, which justifies the weight on this topic area. Depending on its origin, plant biomass may contain significant amounts of non-cell wall components. Some of these are biologically active or otherwise interesting substances that have or could have commercial interest. This theme covers also the main protocols and techniques for analysing the microscopic and chemical features in plants and plant biomass.
