school resources - bioenergetics

• Plants make use of the Sun’s energy to make food (glucose)

  This process is called photosynthesis.
• The plant manufactures glucose from

  carbon dioxide and water using energy

  transferred from the environment to the

  chloroplasts by light.

  This is an endothermic reaction because

  photosynthesis needs an input of energy

  from the environment.
• carbon dioxide + water -- glucose + oxygen
• Carbon dioxide is one of the reactants needed for plants to make

  glucose. The rate of photosynthesis will increase when a plant is

  given higher concentrations of carbon dioxide up to a point.
• Another factor is now preventing

  the rate of photosynthesis from

  increasing. This is called a limiting

  factor.

  Possible limiting factors could be

  light intensity, temperature or

  amount of chlorophyll.
• Temperature affects the rate of all chemical reactions including

  photosynthesis. As the environment warms up, chemical reactions

  speed up. Photosynthesis is an enzyme controlled reaction. If the

  temperature increases too much, then the enzymes become

  denatured and the rate of reaction will decrease and stop.

  Temperature is a limiting factor of photosynthesis.
• The amount of light a plant receives

  affects the rate of photosynthesis.

  Plants found in areas of lower light do

  not tend to grow as tall.

  Light intensity decreases as the

  distance between the plant and the

  light source increases.
• The graph shows that as light

  intensity increases so does the rate

  of photosynthesis up to a point. At

  point X another factor is limiting

  the rate of photosynthesis. This

  could be carbon dioxide

  concentration, temperature or

  amount of chlorophyll.

  Light intensity is a limiting factor.
• Investigating the effect of light intensity on

  photosynthesis in pondweed.

  Fill a boiling tube with 0.2% sodium

  hydrogen carbonate solution.

  Freshly cut a 10 cm piece of pondweed and

  place it in the boiling tube with the cut end at

  the top.

  Set up an LED lamp at a distance of 10 cm to

  the boiling tube and leave to settle for 5

  minutes.

  Start the stopwatch and count the number of

  bubbles released in one minute.

  Repeat twice and calculate the mean number

  of bubbles.

  6. Repeat steps 1-6, altering distance of the

  lamp so it is 30 cm, 40 cm and 50cm away

  from the boiling tube.
• Why do we use sodium hydrogen

  carbonate solution?

  This provides excess dissolved carbon

  dioxide for the plant to use in

  photosynthesis so it is not a limiting

  factor.
• Why is an LED lamp used?

  LED lamps produce less heat and this

  reduces the effect of temperature on

  the experiment.
• Light is a limiting factor when the light intensity is too

  low, but very high light intensities may slow the rate of

  photosynthesis too. This may be caused by:

  saturation of the active sites in the enzymes

  catalysing the reactions,

  b) bleaching of chlorophyll.
• Light intensity obeys the

  inverse square law.

  This means if you

  double the distance

  between the plant and

  the light source you

  quarter the light

  intensity.
• If sunlight shines onto water and carbon

  dioxide, a reaction will not occur.

  The energy must be transferred from the

  environment, to the chlorophyll, by light.
• This energy is used to convert carbon

  dioxide and water into glucose and oxygen.

  Chlorophyll is essential to the process of

  photosynthesis.
• If there is a reduction in the amount of

  chlorophyll available to the plant then the

  amount of glucose made by

  photosynthesis will reduce. The plant will

  not grow as well.
• In laboratory investigations, plants experience variation in only one

  environmental factor.

  Normally in nature, more than one environmental factor will vary and

  the rate of photosynthesis is due to the interaction of these factors.

  Any one of the environmental factors may limit the rate of

  photosynthesis.
• In this experiment temperature is

  controlled.

  At low light intensity the photosynthetic

  reaction becomes limited at point X.

  If the light intensity is increased the

  reaction rate also increases.

  Light intensity is therefore the limiting

  factor at point X.

  A different factor is now limiting the rate

  of photosynthesis at Y.

  This could be environmental temperature

  or the amount of chlorophyll.
• Farmers apply their

  understanding of limiting factors

  to improve crop yields.

  They can control conditions

  inside greenhouses more easily

  than in the fields.

  Heating can be used to provide

  optimum temperatures for

  maximum plant growth.

  Artificial lighting enhances the

  natural sunlight especially

  overnight and on cloudy days.

  Extra carbon dioxide gas can be

  pumped into the air inside the

  greenhouses.
• In commercial greenhouses the

  environmental factors are often controlled

  by computerised systems to minimise cost.

  The farmer must balance the economics of

  additional costs of heating, lighting and

  computer systems to achieve maximum

  photosynthesis whilst still making a profit.
• Uses of glucose made from photosynthesis

  To produce cellulose which strengthens plant cell walls.

  To be converted into amino acids for protein synthesis. Glucose is combined with nitrate ions absorbed from the soil. Specific amino acids join in long chains to make a named protein.

  To produce fat or oil for storage.

  Seeds and nuts contain lots of fat or oil as an energy store.

  To be converted into insoluble starch for storage inside cells or special areas like roots or bulbs.
• Respiration is also sometimes called cellular respiration. This is

  because the reactions of respiration occur inside cells.

  Every living cell needs energy and this energy is released from food

  (glucose) by a series of chemical reactions called respiration.
• Respiration is an

  exothermic reaction

  which means energy is

  transferred to the

  environment. Some of

  the energy is used

  inside cells and the rest

  will be transferred out

  of the cell.
• The reactions of

  respiration occur

  24 hours a day,

  continuously, in

  all living cells.
• Uses for energy

  • keeping warm - keep a steady body temperature in a cold environment
  • to enable muscles to contract in animals
  • for chemical reactions - to build larger molecules from smaller new ones
• Respiration can transfer energy in cells anaerobically (without oxygen).

  During hard exercise, muscles cells are respiring so fast that the blood cannot

  transport enough oxygen to meet their needs.

  The muscle cells switch to use ANAEROBIC RESPIRATION to transfer energy.

  Glucose is not completely broken down to carbon dioxide and water, so less

  energy is transferred. An end product called lactic acid is formed. This builds

  up in the muscle cells.
• aerobic respiration


  glucose + oxygen --> carbon dioxide + water
• anaerobic respiration


  glucose ---> lactic acid
• After exercise the lactic acid must be

  combined with oxygen to convert it to

  carbon dioxide. The amount of oxygen

  which must be taken in to convert all

  the lactic acid to carbon dioxide is

  called the oxygen debt.
• Anaerobic respiration in yeast

  cells is called fermentation.

  This process is economically

  important in the manufacture of

  alcoholic drinks and bread.
• Anaerobic respiration also occurs in plant and yeast cells.

  The end products are ethanol and carbon dioxide.
• During exercise the human body reacts to the increased

  demand for energy. If insufficient oxygen is supplied to the

  muscle cells then anaerobic respiration occurs.

  • The heart rate increases to pump oxygenated blood faster through the muscle cells.
  • Breathing rate and breath volume increase. This increases the amount of oxygen entering the bloodstream.
• response to exercise

  • Lactic acid builds up in the muscle cells during exercise.
  • Blood flows through the muscle cells and transports the lactic acid to the liver.
  • The liver oxidises the lactic acid and converts it back to glucose.


  Glucose is used in aerobic respiration or it is converted to glycogen and stored in the liver for later use.
• Organisms use the energy

  transferred by respiration for the

  continual enzyme controlled

  processes of metabolism.
• The conversion of glucose to

  starch is a metabolic reaction.
• Glucose and nitrate ions

  from the soil form amino

  acids. Amino acids are

  used to synthesise

  proteins. These are

  metabolic reactions.
• Metabolism = the sum of all reactions in

  a cell or body
• Other examples of metabolic reactions that you need to

  know are:

  The conversion of glucose to cellulose in plants to

  strengthen cell walls;

  The conversion of glucose to glycogen in animal cells

  for storage;

  The formation of lipids from a molecule of glycerol and

  three molecules of fatty acids;

  The breakdown of proteins to form urea for excretion

  Respiration.