One of the most important carbohydrates in our diet is the potato. Simple sugars bond to form complex carbohydrates that are stored in the fattened tubers we bake, scallop, hash and mash. This cool-season vegetable is one of the most widely grown staples in the world. Although we enjoy and need it, the potato stores starch for its own use.
Starch in plants, called amylum, is a complex carbohydrate made up of sugar molecules bound together in long chains. All green plants make this starch during photosynthesis and store it to provide energy. Thin-walled cells called parenchyma in the potato's underground tubers store the bulk of its starch. Cells in the leaves called chloroplasts store the rest.
Potato Storage Systems
Potato plants are divided into two systems, a root system and a shoot system. Potatoes have parenchyma occurring throughout the plant. Shoot tissues (pith), roots, rhizomes and underground tubers are all parenchyma tissues. The potato shoot system consists of the stem, leaves and flowers above ground. The potato root system is made up of roots, rhizomes and tubers. The potato shoot system uses photosynthesis to produce sugars and starch. It stores starches in the stroma inside its leaf chloroplasts. The potato root system uses parenchyma cells in the tubers at the end of its rhizomes as starch storage depots. Grains of starch, called amyloplasts, are clustered inside these tuber cells.
Starch for Energy at Night
During the day potatoes photosynthesise, using chloroplasts in their leaves to make compounds they need for energy - sucrose and starch. Photosynthesis happens at such a fast pace that compounds form more quickly than the potato uses them. Potatoes store excess starch temporarily in their leaf chloroplasts. When photosynthesis stops at night the stored up starch from the leaves is converted to simple sugars by enzymes in the leaf cells. The potato uses these sugars to provide the energy to keep itself alive till the sun comes up, when it can start photosynthesis again.
Starch for Future Generations
The swollen root of the potato plant is a carbohydrate store for future potato generations. Potato plants produce glucose molecules during photosynthesis. The molecules become bound together in long chains, forming complex carbohydrates (starch). These complex carbohydrates, called polysaccharides, are stored in the potato tubers. As more starch is stored, the tubers enlarge.
Using Tuber Starch
When seed potatoes or pieces of larger potatoes containing an eye are planted the growth points respond to warmth and moisture in the soil and begin to grow. Until the sprouts are able to get their own nutrients from the soil or through photosynthesis, they use the stored starch in the potato for growth energy. As the plant grows, the original tuber shrinks, but other tubers (our edible potatoes) are formed at the ends of underground rhizomes. The potato continues to grow and make food with its leaves, storing starch in the potatoes. When fall comes, the upper part of the plant begins to die back. By winter the entire upper plant dies, but the potatoes hold enough energy to ensure that new plants will develop in spring.
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- National Science Foundation, Visionlearning: carbohydrates
- University of Illinois Extension: Watch Your Garden Grow, Potato
- Palomar College, Wayne's World Online Textbook of Natural History: Vegetables From Underground, Nightshade Family (Solanaceae): Potatoes
- Estrella Mountain Community College: Plant Anatomy II, Plants and Their Structure
- University of Hamburg Department of Biology: Ground Tissue or Parenchyma, Storage Functions
- Lafayette High School Science Department: Advanced Placement Biology, Chapter 7 - Energy-Acquiring Pathways