Green Plants as Organisms

Contents Photosynthesis Photosynthesis vs. Respiration Plant Hormones Commercial Uses of Plant Hormones Transport and Water Relations

Photosynthesis Photosynthesis is the chemical change which happens in the leaves of green plants. It is the first step towards making food. During this reaction, carbon dioxide and water are converted into glucose and oxygen. The reaction requires energy in the form of sunlight, and chlorophyll must also be present. Plants are producers – they produce their own food Plants produce their food via photosynthesis

carbon dioxide + water glucose + oxygen Photosynthesis Occurs in the leaf Needs: Chlorophyll – not used up CO2 (from the air) Water (from the soil) Sunlight energy (any light except green light) – not used up Produces: Glucose Oxygen (waste product) carbon dioxide + water glucose + oxygen 6CO2 + 6H2O C6H12O6 + 6O2 chlorophyll + light energy

Factors Affecting Photosynthesis Three factors limit photosynthesis from going any faster: Light level Without enough light a plant cannot photosynthesise very fast, even if there is plenty of water and carbon dioxide. Increasing the light intensity will make photosynthesis faster. 2. Carbon dioxide level Photosynthesis can be limited by the level of carbon dioxide. Even if there is plenty of light a plant cannot photosynthesise if it has run out of carbon dioxide. 3. Temperature. If it gets too cold the rate of photosynthesis will slow right down. If it gets too hot then plants cease to be able to photosynthesise.

Night-time vs. Daytime Daytime: Photosynthesis & Respiration Night-time: (no sunlight therefore no photosynthesis) Only respiration

Plant Hormones Stem  always grows towards light Roots  always grow away from light Auxins are produced in the growing tips of shoots and roots Auxins respond to light, gravity and moisture Auxins cause some cells to elongate or grow at a faster rate Tropism = plant’s response to light, gravity or moisture Positive tropism = response towards stimulus Negative tropism = response away from stimulus controlled by auxins

Plant Hormones Geotropism = plant’s response to gravity Positive geotropism = roots grow towards direction of gravity Negative geotropism = shoots grow away from direction of gravity Hydrotropism = plant’s response to water Positive hydrotropism = shoots always follow direction of water Phototropism = plant’s response to light Light destroys auxins Auxins = plant hormones controlling its growth Auxins on sunlit side of plant are destroyed, growth slows… Growth of plant on shaded side continues  plant growth leans towards light

Commercial Uses of Plant Hormones Growing cuttings: cuttings are dipped into powder containing growth hormones. Plants are cloned quickly and cheaply Producing fruit without seeds: Growth hormones sprayed on unpollinated flowers  grow fruit without fertilisation Ripening fruit: Plant hormone ethene ripens fruit. Sprayed onto unripened fruit during transportation to supermarkets Increasing size of fruit: e.g. grapes

Transport & Water Relations Plants require water for growth, temperature regulation and to hold themselves up Transpiration is the loss of water from the leaves by evaporation Transpiration system: Water absorbed into roots through root hair cells Water carried to leaves through xylem vessels Water evaporates from underside of leaves Water escapes through holes called stomata As water evaporates, more is sucked up xylem Stomata (tiny holes under a leaf allowing it to breathe) open and close to control water loss Open – O2 and CO2 enter and exit Closed – Reduce water loss

Xylem and Phloem Vessels Xylem are vein like tissues that transport water and minerals up a plant Phloem are vein like structures through which food is transported around a plant Xylem Phloem Made of Dead cells Living cells Cell wall thickness Thick Thin Cell wall material Lignin (rigid) Cellulose Permeability Impermeable Permeable Cytoplasm? None Cytoplasm lining Transports… Water & minerals Food Carried to… Leaves Growing parts & storage organs Direction of flow Upwards Up and down Tissue also has… Fibres Companion cells

Water Provides Support Green plants rely on cell turgor which is the stiffness given to cell walls to hold themselves up Cell walls become turgid with water In woody plants, they do not need cell turgor, but instead rely on the support of the strong, impermeable xylem vessels

Summary Photosynthesis: carbon dioxide + water  glucose + oxygen Limiting factors: temperature, CO2, light intensity Daytime: respiration & photosynthesis; night-time: respiration Plant hormone, auxin, responds to light, gravity and moisture Light destroys auxin Auxin speeds up plant growth Water travels around the plant via the transpiration system Xylem and phloem vessels transport water, minerals and food Water is needed in green plants for structure and support