UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 21/05/2023 [SEARCH]

Photosynthesis: 4. What does the plant do with the glucose produced by photosynthesis?

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(4) What does the plant do with the glucose produced by photosynthesis using sunlight energy?

  • Glucose provides energy and can be converted into, and help to synthesise, a wide range of molecules in plant cell chemistry (plant biochemistry). This means plants make their own food!

  • The glucose produced in photosynthesis may be converted into insoluble starch for storage in leaves, roots and stems.

    • The insoluble nature of starch makes it a very useful concentrated chemical store of energy - if it was soluble, it would dissolve and diffuse all over the place.

      • Starch is a natural polymer made from linking many glucose molecules together and is the main chemical energy store in a plant.

      • A plant can't photosynthesise at night, so it needs energy from somewhere to stay alive at night!

      • When needed, starch is hydrolysed (broken down) into the useful sugar glucose for respiration, so the process of starch formation is reversed.

      • Glucose sugar is soluble and easily transported around a plant and fuels respiration in the mitochondria of plant cells - which in turn provides the energy for all the cellular processes needed by a plant.

      • If a plant tried to store the soluble glucose, the cells would absorb water by osmosis, swell up and burst!

      • See demonstrating presence of starch in plant leaves.

    • Plants need energy from sugars (from photosynthesis) to power their own life supporting systems just as we do.

    • Plant cells use some of the glucose produced during photosynthesis for immediate respiration - release of energy to power the cell functions and particularly at night when no light can shine on the leaves.

      • Plant respiration in principle is the reverse of photosynthesis.

      • glucose + oxygen ==> products + chemical energy (to power the plant cell chemistry)

    • The energy released enables the plant to convert glucose plus other elements/ions like nitrogen/nitrate into other essential useful chemical substances - some are listed below.

    • At night there be a net loss of glucose/starch in respiration, but in daylight the rate of photosynthesis will exceed that of respiration in a growing plant so excess glucose can be converted into starch for storage.

    • ... noting that starch and glucose are chemical energy stores.

    • Glucose is consumed in plant respiration, e.g. in aerobic respiration, plants use oxygen to oxidise glucose to carbon dioxide and water.

      • The released chemical energy to power all the cell chemistry including the conversion of glucose into starch and making protein.

      • Don't forget that plants respire all the time, just like us!

    • Glucose can be converted into starch that can be stored in roots (e.g. potato), stems and leaves, this provides energy at night and in winter.

      • Starch has the advantage of being insoluble in water, so won't dissolve away unnecessarily from vital energy reserve storage areas.

      • It can be used when sunlight is low e.g. winter, and of course at night when photosynthesis stops completely.

      • Also, by being insoluble, it won't affect the water concentration in cells by osmosis.

      • A cell with a high concentration of glucose would swell up by water absorption interfering with its function.

  • The chemical energy from glucose is needed to build larger more complex molecules.

    • Through growth and accumulation of these larger molecules biomass is built up in plants and algae.

    • Biomass means the mass of living material.

    • The energy built up in a plant's or algal organism's biomass enters the food chain so animals can now feed on it (herbivores) and themselves be fed on by other animals (carnivores).

    • This is why at the start of this page it was emphasised that photosynthesising organisms are the main producers of food for most of life on Earth.

  • Examples of the larger molecules in the biomass of plants and algae

    • Glucose is used to produce fats or oils (lipids) for storage - provides sources of energy via aerobic respiration, seeds contain food stores based on oils and fats (think of cooking oil from olives or sunflower oil for margarine) and waxes.

    • Glucose is used to make cellulose, which makes up and strengthens the cell walls eg of the xylem and phloem and is particularly needed in larger quantities in rapidly growing plants.

    • Amino acids are first synthesised from glucose and nitrate ions (absorbed from soil through the roots) and other minerals before conversion to proteins for tissue cell growth and repair.

  • Ultimately, some of the carbohydrates like starch and cellulose, vegetable oils and plant protein end up being consumed in the food chains from microorganisms to large animals of  biomass and energy transfer systems.

  • AND note oxygen for aerobic respiration of plants and animals is a by-product of photosynthesis!

  • Note that to produce proteins, plants also use nitrate ions that are absorbed from the soil.


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