B1.4 (photosynthesis)

• what do plants have to take in in order to make their own food?

  Carbon dioxide and water
• how does carbon dioxide enter the plant?

  Diffuses from the air into the plant through the stomata
• how does water enter the plant?

  Enters the roots from the soil through the root hair cells by osmosis
• what is oxygen made from photosynthesis used for?

  Some used by the plant in respiration and the rest is a waste product released back into the environment
• what is the equation for photosynthesis?

  Carbon dioxide + water ---> glucose + oxygen
• where does photosynthesis take place?

  Inside the plant's chloroplasts, it means that photosynthesis usually takes place in the leaf but a small amount happens in the green stem
• why are leaves and stems green?

  the pigment chlorophyll inside their chloroplasts, light transfers energy from the sun to chlorophyll where carbon dioxide and water react to make glucose
• how does glucose store energy?

  Within its chemical bonds
• what is stage 1 of photosynthesis?

  It is light dependent, energy is transferred from light and splits water molecules into oxygen gas and hydrogen ions
• what is stage 2 of photosynthesis?

  It is light independent, carbon dioxide gas combines with the hydrogen ions to make glucose
• what type of reaction is photosynthesis?

  An endothermic reaction, energy must be transferred from the surroundings in order to keep it going
• what does photosynthesis mean?

  Using light to create new substances
• how do plants use glucose?

  Uses some of it immediately during respiration, other glucose molecules are converted into other sugar molecules such as fructose (fruit sugar) and sucrose (table sugar)which is made up of one molecule each of glucose and fructose and which is found in high quantities in stems such as sugar cane as it is the form in which glucose is transported around the plant's tissues
• what happens to glucose not needed straight away?

  It is converted to starch which provides a store of energy which can be used at night for respiration when the plant is not photosynthesising
• name 5 ways plants use glucose


  1, sucrose (stored in fruit)

  2, starch (food store)

  3, cellulose (forming cell walls)

  4, glucose + nitrogen= proteins (growth and repair)

  5, fats and oils (food store and growth)
• what does testing a leaf for the presence of starch prove?

  That light, chlorophyll and carbon dioxide are essential for photosynthesis
• how do you test for starch in a leaf?

  kill the leaf by placing it in a beaker of boiling water for a minute, then place the leaf into a tube of boiling ethanol to remove all chlorophyll. then wash the leaf to remove the ethanol and add a few drops of iodine solution onto the leaf. if starch is present the iodine will turn from yellow/brown to blue/black
• why do you need to remove chlorophyll from the leaf before testing for starch?

  The colour of the chlorophyll could prevent you from being able to see what colour the iodine has changed to
• how can you de-starch a plant?

  keeping it in the dark for a minimum of 24 hours
• what do variegated leaves have?

  Only have chlorophyll in some areas of the leaf which means that some areas of the leaf appear green and others appear white
• how do you prove that chlorophyll is needed in photosynthesis?

  Place a destarched variegated plant in the sunlight for several hours, then see how starch levels vary in different parts of the plant depending on the colour (green or white) as the green parts of the plant contain more chlorophyll
• why do you de-starch a plant?

  So that the experiment is fair and there wasnt any starch in the plant from before the experiment started
• how do you prove light is needed for photosynthesis?

  Take a destarched plant and cover part of one of its leaves with black card/tin foil so light cannot reach the covered area. place the plant in sunlight for several hours and then remove the card from the leaf and test the leaf for the presence of starch. you should see that the part where the card was is yellow/brown due to there being no starch present and the rest of the leaf is blue/black due to the starch being present
• how can you prove carbon dioxide is needed for photosynthesis?

  Place a destarched plant in a polythene bag and add a pot of soda lime (a chemical that absorbs carbon dioxide and water vapour) and seal the bag. place the plant in the sun for several hours before testing one of its leaves for the presence of starch
• explain why the iodine added the leaf in the soda lime experiment stayed yellow/brown?

  There was no starch present due to the plant not being able to photosynthesise, soda lime absorbs carbon dioxide and water vapour, both of which are needed for photosynthesis to happen so no starch is produced
• how do you prove that oxygen is given off during photosynthesis?

  Place an upturned test tube above an aquatic plant such as the pondweed elodea, then put the apparatus in the light for maximum photosynthesis. when you have collected a full tube of gas, place a glowing splint inside the tube and it will relight due to the oxygen being present
• what are the limiting factors for photosynthesis?

  Light intensity, carbon dioxide and temperature as they affect the rate of photosynthesis
• what happens if light or carbon dioxide is in short supply or temperature is too low?

  The rate of photosynthesis is limited
• how can you measure the rate of photosynthesis?

  Measuring how much oxygen or glucose a plant makes in a given time
• what is another way to measure the rate of photosynthesis?

  calculate the increase in biomass in a given time as glucose is used to make new cells
• what is the rate of photosynthesis?

  A measure of how much photosynthesis occurs in a given time
• how to calculate the rate of photosynthesis?

  Rate = 1/t

  where t is the time for the measurement to occur
• what units of time are used in rate of photosynthesis?

  S-1 (per second), min-1 (per minute) and h-1 (per hour)
• how does light intensity affect the rate of photosynthesis?

  The higher the light intensity, the faster the rate of photosynthesis. this continues until photosynthesis reaches its maximum rate and in very low light levels or no light , photosynthesis stops
• how does carbon dioxide affect the rate of photosynthesis?

  Carbon dioxide is one of the reactants for photosynthesis. the greater the carbon dioxide, the faster the reaction.
• why is carbon dioxide most commonly the limiting factor?

  The atmosphere only contains around 0.04% carbon dioxide. farmers artificially increase the levels of carbon dioxide in greenhouses which increases the rate of photosynthesis.
• how does temperature affect the rate of photosynthesis?

  Photosynthesis is a series of enzyme controlled reactions, therefore, the higher the temperature the faster the reactions occur. however if the temperature is too high then the enzymes will denature and reactions will stop
• Which 2 limiting factor graphs look similar and which one looks different?

  Similar= carbon dioxide and light intensity

  different= temperature
• what is the volume of oxygen given off?

  A measure of the rate of photosynthesis, you can measure this by counting the number of bubbles produced per minute
• what is the inverse square law?


  When you double the distance from a light source, the light intensity falls by a factor of 4