4.4 - Bioenergetics

4.4.1 Photosynthesis

4.4.1.1 Photosynthetic reaction

• Photosynthesis uses energy to change carbon dioxide and water into glucose and oxygen

• Photosynthesis takes place in chloroplasts - they contain a green pigment called chlorophyll that absorbs light

• Photosynthesis is endothermic:

  • Means that energy is transferred from the environment to the chloroplasts by light

• Word equation for photosynthesis:

  • carbon dioxide + water →light  glucose + oxygen

• Chemical symbol equation for photosynthesis:

  • 6CO₂ + 6H₂O →light C₆ H₁₂ O₆ + 6O₂

4.4.1.2 Rate of photosynthesis

The limiting factors of photosynthesis:

• Intensity of light - at night light can be the limiting factor

• Temperature - in winter temp can be the limiting factor

• Concentration of CO2 - if it’s warm enough and bright enough it usually is the limiting factor

• Amount of chlorophyll - can be affected by a disease or a lack of nutrients

  • Means that  rate of photosynthesis is reduced because they can’t absorb as much light

Light intensity:

• Light provides the energy needed for photosynthesis

• As the light level is raised the rate of photosynthesis increases steadily - but only until a certain point

• Beyond that, it won’t make a difference as it is no longer the limiting factor

  • Temp or concentration of CO2 is now the limiting factor

Concentration of Carbon dioxide:

• Affects photosynthesis as it is one of the raw materials needed for photosynthesis

• Concentration of CO2 will only increase the rate of photosynthesis only up to a certain point

  • CO2 is no longer the limiting factor

Temperature:

• Temp affects the rate of photosynthesis because it affects the enzymes involved

• If the plant is too cold then the enzymes needed work slowly

• If the plant gets too hot the enzymes needed get denatured (active site shape changes) so no enzyme-substrate complexes can be formed

  • Happens at about 45 degrees Celsius

Graph example that shows more than one limiting factor:

REVISE REQUIRED PRACTICAL 6 FROM PMT

Inverse square law:

• Distance and light intensity are inversely proportional

  • As the distance increases the light intensity decreases

• Equation for inverse square law - 

OR

• Light intensity is measured in arbitrary units (a.u.)

How greenhouses can be used to gain the maximum rate of photosynthesis while still maintaining profit:

• Temperature - 

  • Greenhouse gases help to trap the sun’s heat so that the temp doesn’t become the limiting factor

  • In winter (no sun) - farmers may use a heater to keep the temp at ideal levels

  • In summer - farmers may use shades and ventilation to reduce the temperature

• Light intensity - 

  • Farmers may supply artificial light at night give plants the energy they need to carry out photosynthesis

• Concentration of carbon dioxide - 

  • Farmers may use a paraffin heater - as it burns it releases carbon dioxide as a by-product

• Keeping plants enclosed makes it easier to keep them free from pests and diseases

• Farmer can easily add fertilisers to the soil to provide minerals needed for healthy growth

  • Adding magnesium ions also help with the production of  chlorophyll so more light can be absorbed for photosynthesis

• However - 

  • Sorting all this out costs money

  • Therefore farmers must supply the right amount of heat, light and CO2 and nothing more so that you don’t waste money

  • Must provide just the right conditions tho so that the farmers can harvest plants more often

    • These crops can then be sold

4.4.1.3 Uses of glucose from photosynthesis

C - Cellulose: glucose is converted into cellulose for making strong plant cell walls

R - Respiration: releases energy from glucose which is used for many things

O - Oils or fats: glucose is turned into lipids for storing in seeds

P  - Proteins: glucose is combined with nitrate ions (absorbed from the soil) to make amino acids which are then made into proteins

S - Stored as starch: starch is insoluble which makes it better for storing than glucose as a cell with lots of glucose may draw loads of water and swell up

4.4.2 Respiration

4.4.2.1 Aerobic and anaerobic respiration

Respiration = the process of transferring energy from glucose, which goes on in every cell:

• Respiration is exothermic - transfers energy to the environment

• Transfers energy used for all living process - 

  • Energy is used to build up larger molecules from smaller ones 

    • Like proteins from amino acids

  • Energy is used in animals for movement (muscle contraction)

  • Energy is used to keep body temp steady in a cold environment in animal and birds

Respiration in cells can take place aerobically (using oxygen):

• Is the most efficient way to transfer energy from glucose

• Takes place in the mitochondria

• Goes on all the time in plants and animals

• Word equation:

  • glucose + oxygen → carbon dioxide + water

• Symbol equation:

  • C₆ H₁₂ O₆ + 6O₂ → 6CO₂ + H₂ O

Respiration in humans can take place anaerobically (without oxygen):

• Happens in the cytoplasm of muscle cells

• Happen when your body can’t supply enough oxygen to your muscles

  • Start doing both types of respiration

• It is the incomplete breakdown of glucose making lactic acid -

  • Word equation = glucose → lactic acid

• Does not transfer as much energy as aerobic respiration because the glucose isn’t fully oxidised 

  • Glucose needs oxygen to be fully broken down and release all of its energy

Anaerobic respiration in plants and yeast:

• Happens in the cytoplasm of plant cells and yeast cells

• Anaerobic respiration in yeast cells is called fermentation

  • In bread making - carbon dioxide from fermentation makes the bread rise

  • In beer and wine-making - the fermentation process produces alcohol (ethanol)

• Word equation for anaerobic respiration in plants and yeast cells:

  • glucose →  ethanol⁢  +  carbon dioxide

4.4.2.2 Response to exercise

Muscles need energy from respiration to contract - more muscle contraction means more energy and increased respiration:

• Means you need to get more oxygen into cells

  • Breathing rate and breath volume increases and heart rate increases

    • To get oxygenated blood around the body faster and to supply the muscles with more oxygenated blood

• If insufficient oxygen is supplied anaerobic oxygen is supplied - 

  • Anaerobic respiration takes place in the muscle cells

  • The incomplete oxidation causes a build up of lactic acid and creates an oxygen debt

• During long periods of vigorous exercise -

  • Muscles become fatigued and stop contracting efficiently

Oxygen debt:

• Oxygen debt = the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells

• Oxygen reacts with the lactic acid to from CO2 and water

• Means that you have to keep breathing hard for a while even after you stop exercising to get more oxygen in your blood

  • Heart rate and breathing rate stays high whilst there are high levels of lactic acid and CO2

• Also, blood that enters your muscles transports the lactic acid to the liver where it is converted back into glucose

4.4.2.3 Metabolism

Metabolism = the sum of all the reactions in a cell or the body:

• All of these metabolic reactions are controlled by enzymes

• Energy transferred by respiration in cells is used by the organism for the continual enzyme controlled processes of metabolism that synthesize new molecules

Metabolism includes:

• Conversion of glucose to starch, glycogen and cellulose

  • Starch - an insoluble storage molecule in plant cells

  • Glycogen - an insoluble storage molecule in animal cells

• Formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids

• Glucose is combined with nitrate ions to make amino acids - which are then made into proteins

• Glucose is broken down in respiration

• The breakdown of excess proteins to from urea for excretion in urine