Chapter 1 - Photosynthesis

• During photosynthesis, plants make their food in the presence of light using carbon dioxide and water as raw materials. The food they make is in form of glucose (a kind of simple sugar). Oxygen is given out during the process.

  
  

  It can be summarized using this word equation:

  
  

  Light

  Carbon dioxide + Water ----------> Glucose + Oxygen

  Chlorophyll
• Photosynthesis takes place in structures called chloroplasts found in leaves and other plant parts. They contain chlorophyll, a green pigment found in plant cells that captures sunlight. During photosynthesis, light energy is converted to chemical energy and stored in the plant.
• Plants do not use up all the glucose produced immediately, they store the excess glucose in the form of starch as it is insoluble in water, unlike glucose. Starch is a carbohydrate formed by many glucose molecules linked together to form a long chain. It is stored in the leaves or other parts of the plant.
• How to prepare to test a leaf for starch?

  1. Prepare the leaf by boiling it in water to break down the cell membranes and stop all the chemical reactions in the cells.
  2. Soak the boiled leaf in ethanol (a type of alcohol) in a test tube then put it in a beaker of hot water. This is to remove the chlorophyll so that change in the colour of the iodine solution will be visible.
  3. Wash the leaf in hot water to soften.
  4. Add a few drops of iodine solution on the leaf. If starch is present, the brown iodine solution will turn blue-black.
• How do we collect and test the gas produced during photosynthesis?

  1. Using a water plant, put the plant inside an upside-down filter funnel in a beaker.
  2. Fill the beaker with water until the plant is submerged.
  3. Put an inverted test tube above the opening of the filter funnel.
  4. Shine a bright light at the plant and watch what happens.
  5. Bubbles of gas will be seen coming out of the water plant rising into the inverted test tube, which water initially filled with water.
  6. The gas collected in the test tube can be identified using a glowing splint. If it relights, then the gas is oxygen.
• WHY DO PLANTS NEED WATER? (2) SUPPORT

  Plants have water in cells that keep the cells firm and turgid (swollen). When water enters the vacuole of a cell, the vacuole swells and pushes against the cell wall. The pressure is called turgor pressure which supports the plant so it doesn't wilt.

  
  

  a) Water enters the vacuole, causing it to swell and push against the cell wall. It becomes turgid and firm.

  b) Water is lost from the vacuole, causing it to shrink. It loses its shape and becomes flaccid (not firm).

  
• WHY DO PLANTS NEED WATER? (1) PHOTOSYNTHESIS

  Water is one of the reactants of photosynthesis. A proportion of the water is absorbed by the roots for photosynthesis. During photosynthesis, chlorophyll captures light energy. Some of this energy is used to split up water into oxygen and hydrogen. The hydrogen then combines with carbon dioxide to form carbohydrates in the form of glucose.
• WHY DO PLANTS NEED WATER? (3) CHEMICAL REACTIONS

  Many chemical reactions take place in plant cells. Water is a reactant or a product in some of these reactions.
• WHY DO PLANTS NEED WATER? (4) AS A SOLVENT

  Substances, such as the food made during photosynthesis and mineral salts, are dissolved in water before being transported to the different parts of the plant. Some reactants need to be dissolved in water before chemical reactions can take place.
• WHY DO PLANTS NEED WATER? (5) COOLING

  Only a small proportion of the water taken in by plant roots is used for growth and chemical reactions in the plant. Most of it is lost by a process called transpiration.

  During transpiration, water moves from the roots, through the plant, before evaporating from the stem, leaves, and flowers through the stomata. The water absorbs heat energy from the surrounding to change into water vapour. The surroundings, losing heat, become cooler.
• WHY DO PLANTS NEED WATER? (6) TRANSPORT

  Water helps to transport the dissolved mineral salts absorbed by the roots to the various parts of the plant through the xylem (vascular tissues of the plant). Water is drawn up the xylem during transpiration. Sugars and amino acids are dissolved in water and transported through vascular tissues called phloem to various parts of the plant.
• Mineral salts are nutrients for plants, necessary for healthy growth
• One way to provide plants with nutrients is by adding fertilizers to the soil
• There are 2 main types of fertilizers:

  • Organic fertilizers: come from living things like animal dung, fish and bone meals, decomposing plant matter, compost, and seaweed extracts; they gradually release mineral salts to the soil as decomposers act on them
  • Inorganic fertilizers: man-made fertilizers produced from chemicals, containing various types of mineral salts essential for plant growth; some are for leaf growth, while others are for healthy root growth and flower formation
• Mineral salts in fertilizers and their functions:

  1. Nitrate

  • To make proteins, DNA, and chlorophyll.
  • For Leaf development.
  • For growth and repair.
  • Prevent deficiency that can lead to leaves turning yellow, and stunted growth.

  2. Phosphorus

  • For development of roots, flowers, seeds, and fruit.

  3. Potassium

  • For development of roots.
  • For resistance against diseases.
  • For promoting flowering and fruiting.

  4. Magnesium

  • For making chlorophyll
  • To prevent deficiency that can lead to leaves turning yellow.