Photosynthesis is usually summed up as turning carbon dioxide, water and light into glucose, but very little of that sugar stays plain glucose for long. The plant treats it as both fuel and raw material, burning some for energy and rebuilding the rest into nearly everything it is made of. Some even leaves the plant entirely, pushed out through the roots to feed soil microbes and fungi.1PubMed. Sugars en route to the roots. Transport, metabolism and storage within plant roots and towards microorganisms of the rhizosphere
The First Fork Starch Or Sucrose
The first products of photosynthesis are small three-carbon sugars, and the leaf quickly builds them into larger ones. From there the sugar goes one of two ways. Some is turned into starch, a stored chain of glucose units, kept inside the chloroplasts where photosynthesis happens.2PubMed Central. Daylength and Circadian Effects on Starch Degradation and Maltose Metabolism The rest is exported into the surrounding leaf cell and made into sucrose, the same molecule as table sugar.3PubMed Central. Starch metabolism in potato Solanum tuberosum L. Sucrose is the main form in which plants move sugar over long distances.4PubMed Central. Evidence for an essential role of the sucrose transporter in phloem loading and assimilate partitioning
The two routes cover for each other. When a plant cannot make starch as fast, it makes more sucrose instead, and photosynthesis carries on normally.5PubMed. Control analysis of photosynthate partitioning: Impact of reduced activity of ADP-glucose pyrophosphorylase or plastid phosphoglucomutase on the fluxes to starch and sucrose inArabidopsis thaliana (L.) Heynh.6PubMed. Control of carbon partitioning and photosynthesis by the triose phosphate/phosphate translocator in transgenic tobacco plants (Nicotiana tabacum L.). I. Comparative physiological analysis of tobacco plants with antisense repression and overexpression of the triose phosphate/phosphate translocator
The split is not fixed. It shifts with how much sucrose the rest of the plant is pulling out of the leaf. When sucrose piles up because the users are not drawing it off, more of the new sugar tends to be diverted into starch.
Plants convert glucose rather than keep it for practical reasons. Starch packs many glucose units into a compact form that does not dissolve or pull in water. Sucrose is chemically steadier than glucose, which makes it a good form for travel. Textbooks write “glucose” as shorthand for all the sugar photosynthesis produces, so the equation’s glucose is better thought of as a pool of carbon the plant reshapes.
Daytime Starch That Feeds The Plant At Night
Leaf starch works like a short-term battery. Leaves build it up during the day and break it down at night.7PubMed Central. Daylength and Circadian Effects on Starch Degradation and Maltose Metabolism The grains suit this job because they are tightly packed, insoluble granules made of branched chains of glucose.8PubMed. The diurnal metabolism of leaf starch After dark the starch is broken into maltose and glucose, and these sugars leave the chloroplast to make sucrose and to fuel respiration and growth through the dark hours.9PubMed. The diurnal metabolism of leaf starch Maltose is the main form in which carbon leaves the chloroplast at night.10PubMed Central. Daylength and Circadian Effects on Starch Degradation and Maltose Metabolism
A leaf holds its largest starch reserve by evening. Overnight the plant draws it down steadily, so that by dawn the reserve is nearly used, just as light returns. Leaf starch is cycled daily and never builds up for long. Storage starch in tubers and seeds is laid down to last weeks or months.
Not all plants handle leaf starch the same way. In mature corn leaves, the outer leaf cells stop storing starch unless sugar export is blocked.11PubMed. The role of transitory starch in C(3), CAM, and C(4) metabolism and opportunities for engineering leaf starch accumulation Desert plants such as cacti, which open their pores at night, break down leaf starch by a different route than ordinary plants.12PubMed. The role of transitory starch in C(3), CAM, and C(4) metabolism and opportunities for engineering leaf starch accumulation
How Sugar Travels From Leaves To Roots And Fruit
Plants have sugar sources and sugar users. Mature leaves act as sources, while roots, stems, fruits and seeds are users that import sugar.13PubMed. Phloem Loading and Unloading of Sucrose: What a Long, Strange Trip from Source to Sink Sucrose travels between them in the phloem, a tissue running through the plant’s veins.14PubMed. Phloem Loading and Unloading of Sucrose: What a Long, Strange Trip from Source to Sink
A single part can switch roles. Young leaves import sugar until they mature and start exporting. A potato tuber stores sugar in summer but feeds sprouts the next spring.
Loading takes some work. In many crops, sucrose is first released outside the leaf cells and then pumped into the phloem by special carriers.15PubMed. Phloem Loading and Unloading of Sucrose: What a Long, Strange Trip from Source to Sink Some plants instead turn sucrose into larger sugars that get trapped inside the phloem.16PubMed Central. Source-to-sink transport of sugar and regulation by environmental factors Either way, the heavy concentration of sugar near the leaves builds pressure that pushes sap toward the roots and fruit, where pressure is lower.17PubMed Central. Source-to-sink transport of sugar and regulation by environmental factors
The phloem is a separate system from the plant’s water pipes. Xylem carries water and dissolved minerals up from the roots, through empty tubes of dead cells. Phloem is made of living cells, which is part of why loading sugar into it takes energy. Because the two run side by side, a plant can move water up and sugar to wherever it is needed at the same time.
Blocking this transport shows how vital it is. Potato plants with a weakened sucrose carrier built up more than 20 times the normal sugar and 5 times the starch in their leaves.18PubMed Central. Evidence for an essential role of the sucrose transporter in phloem loading and assimilate partitioning Their leaves bleached and curled, and root growth and tuber harvest fell sharply.19PubMed Central. Evidence for an essential role of the sucrose transporter in phloem loading and assimilate partitioning
Burning Sugar For Energy
Plants respire too. Their cells break down sucrose or starch to make ATP, the energy currency of cells.20Annals of Botany. ATP yield of plant respiration: potential, actual and unknown Respiration uses oxygen and releases carbon dioxide, roughly reversing photosynthesis. Roots, stems and flowers cannot photosynthesize, so they rely entirely on sugar delivered to them for energy.
Plants respire all the time, including during the day. In daylight photosynthesis outpaces respiration, so leaves take in more carbon dioxide than they release. At night only respiration runs, so plants give off carbon dioxide. A sizeable share of the sugar a plant makes is burned this way rather than built into its body. The energy pays for a lot of work: pumping sucrose into the phloem, building cell walls, proteins and oils, and taking up nutrients from the soil through the roots.
Building Cell Walls And Wood
Cellulose, the fiber in plant cell walls, is made of long chains of glucose. Protein complexes in the cell membrane spin it out from a glucose building block that can be made from sucrose right on the spot.21PubMed. Cellulose microfibrils in plants: biosynthesis, deposition, and integration into the cell wall Plants and algae put a major share of their carbohydrate into these walls.22PubMed Central. Engineering cyanobacteria as photosynthetic feedstock factories
Each chain is long and straight, and many chains line up side by side and bond to each other into tough fibers. These are laid down around the cell like reinforcing cables, which is why plant cells hold a firm shape. A cell that is still growing has a thinner, more flexible wall, and it stiffens as more cellulose and other material is added.
Wood adds lignin. Sugar-derived building blocks pass through a series of steps that produce it.23PubMed Central. Glycosylation Is a Major Regulator of Phenylpropanoid Availability and Biological Activity in Plants Lignin is the second most abundant plant material on Earth after cellulose.24PubMed Central. Glycosylation Is a Major Regulator of Phenylpropanoid Availability and Biological Activity in Plants In trees, most long-term carbon above ground is stored as wood.25Board of Water and Soil Resources. What Happens to the Glucose in Photosynthesis? This is also why most of a tree’s mass comes from air. The carbon in wood comes from carbon dioxide pulled from the air, while soil contributes water and minerals but little of the plant’s dry weight.
Turning Sugar Into Oils And Proteins
Seeds work like factories that convert sugar from the leaves into storage oils.26PubMed. Seeds as oil factories Breaking down glucose produces a small molecule that is the starting point for fatty acids.27PubMed Central. Regulatory mechanism of carbohydrate metabolism pathways on oil biosynthesis of oil plant Symplocos paniculata Oil-rich fruits ramp up their sugar-breakdown machinery during the weeks they store oil,28PubMed Central. Regulatory mechanism of carbohydrate metabolism pathways on oil biosynthesis of oil plant Symplocos paniculata and the fatty acids are then assembled into the oils stored in seeds.29PubMed Central. Enhancing Plant Seed Oils for Human Nutrition
Protein needs nitrogen too. Plants combine carbon from sugars with nitrogen to make amino acids, the building blocks of protein.30Oxford Academic (Molecular Plant). Metabolic and signaling aspects underpinning the regulation of plant carbon nitrogen interactions Nitrogen is also needed for chlorophyll and DNA building blocks.31Oxford Academic (Molecular Plant). Metabolic and signaling aspects underpinning the regulation of plant carbon nitrogen interactions A plant short on nitrogen cannot use its sugar fully for growth, which is why fertilizer often contains nitrogen. Beans and lentils store a lot of protein built partly from sugar carbon, while rice and wheat store mostly starch.
Before sugar becomes an amino acid, it supplies the carbon skeletons, the backbone of each one, and nitrogen is attached to those skeletons. The nitrogen usually comes from the soil, taken up by roots as dissolved nitrogen compounds, and some of it is handled in the roots before moving up. Amino acids are then linked in long chains to form proteins, such as the enzymes that run all of the plant’s chemistry.
Storing Sugar In Potatoes Seeds And Bulbs
A potato shows how storage works. Sucrose delivered to the tuber is split into simpler sugars and rebuilt into starch.32PubMed Central. Starch metabolism in potato Solanum tuberosum L. Sucrose cannot be turned directly into starch, so the tuber first cuts it into glucose and fructose, or glucose-based building blocks. Those pieces are reassembled into long glucose chains inside special storage structures in the cells. The splitting also keeps sucrose concentration low inside the tuber, which helps draw more sucrose in from the phloem.
Early in growth, when cells are dividing quickly, the tuber mainly uses one route to split sucrose, and it switches to another once it starts piling up starch.33PubMed Central. Starch metabolism in potato Solanum tuberosum L. Weakening that second route lowers the starch content of mature tubers.34PubMed Central. Starch metabolism in potato Solanum tuberosum L. Starch is the main ingredient of a potato and largely decides its yield, cooking quality and nutrition.35PubMed. Understanding starch biosynthesis in potatoes for metabolic engineering to improve starch quality: A detailed review Potato starch has unusually large oval grains, with 20 to 33% of it in a straighter chain form.36PubMed. Understanding starch biosynthesis in potatoes for metabolic engineering to improve starch quality: A detailed review
The reserve is not permanent. When a potato sprouts it breaks starch back into sugar to feed the new shoots, and it turns soft and wrinkled as the reserve is used. Seeds do the same when they germinate, before their leaves can photosynthesize.
Sugar That Protects Plants From Cold And Pests
Some plants, including many grasses, store sugar as fructan, a chain of fructose units. Fructan builds up as plants harden off in cool weather, and higher levels go along with better frost tolerance.37PubMed Central. Fructan and its relationship to abiotic stress tolerance in plants Fructans dissolve in water and may help protect cell membranes during freezing.38PubMed. Plant fructans in stress environments: emerging concepts and future prospects
Many plants raise their sugar levels as temperatures drop, which is why kale, carrots and parsnips taste sweeter after frost. Dissolved sugar slightly lowers the freezing point inside cells, one of several ways sugar helps protect them. Sugar also helps with defense, since chemicals built from sugar-derived compounds help plants respond to disease-causing microbes.39PubMed Central. Glycosylation Is a Major Regulator of Phenylpropanoid Availability and Biological Activity in Plants
Sugar acts as a signal as well. Plants sense their own sugar levels to decide whether to grow or conserve: plenty of sugar switches on growth, and scarce sugar slows it down. These signals guide timing from seed sprouting to leaf aging.
Sugar That Leaves The Plant
Part of the carbon sent to roots is released into the soil around them.40PubMed. Sugars en route to the roots. Transport, metabolism and storage within plant roots and towards microorganisms of the rhizosphere It feeds helpful microbes, though some is also taken by harmful ones.41PubMed. Sugars en route to the roots. Transport, metabolism and storage within plant roots and towards microorganisms of the rhizosphere Fungi living on roots receive sugar and help the plant reach water and nutrients in return. In northern forests, about 50 to 70% of soil carbon storage comes from roots and the fungi and microbes tied to them.42Board of Water and Soil Resources. What Happens to the Glucose in Photosynthesis?
Plant sugar also ends up as soil and climate storage. Fallen leaves, branches and dead roots decay into soil organic matter. Carbon locked in soil and wood stays out of the atmosphere for years, which is why forests and soil matter in discussions of climate.