Bioenergetics

Photosynthesis

Photosynthesis is the process by which plants make glucose from sunlight.

It is an endothermic reaction in which energy is transferred from the environment to the chloroplasts by light - light taken in from the surroundings

carbon dioxide + water → glucose + oxygen

Carbon dioxide: CO2

Water: H2O

Oxygen: O2

Glucose: C6H12O6

Glucose is used in respiration to produce energy, converted to starch as a store of energy in the plant and lipids in seeds for germination, converted to cellulose to build/strengthen cell walls, combined with nitrates to make proteins (essential for growth)

humans eat to get the glucose needed for respiration and breathe to get the oxygen.

Factors affecting the rate of photosynthesis

A limiting factor is any factor that slows down the rate of photosynthesis if there is not enough of it, regardless of if you increase other factors.

temperature: enzymes used to meet the activation energy of photosynthesis have an optimum temperature. prior to reaching the optimum temperature, their kinetic energy is low, so the enzyme and substrate won’t collide very often to form complexes. past the optimum, the enzymes have denatured, thus they won’t fit the substrate → limiting everywhere but optimum.

light intensity: light strength and even colour can affect rate - green light is reflected, for example. increasing past the optimum amount for a plant will have no affect → limiting until optimum

carbon dioxide: more CO2 = higher rate of photosynthesis until optimum → limiting until optimum

amount of chloroplasts/chlorophyll: photosynthesis takes place due to this pigment, so if they’re reduced due to mineral deficiencies (e.g magnesium), they may be limiting

for example in this graph, light isn’t limiting as it’s constant, and carbon dioxide is increasing yet the rate of reaction isn’t, so temperature is limiting.

CO2 is almost always limiting due to its low atmospheric concentration on land or, if underwater, due to respiration taking place in animals. Temperature is an unlikely limiting factor underwater too, due to the high specific heat capacity of water.

Farming and photosynthesis

farmers can use knowledge of limiting factors to enhance greenhouse condition to increase the rate of photosynthesis to produce more crops and gain more profit.

hydroponics involves suspending plants in a mineral solution to reduce the chances of mineral deficiencies that affect growth (nitrate) or reduce chlorophyll (magnesium)

temperautre and CO2 can only be controlled in greenhouses, but irrigation and fertilizers are commonly used in fields to increase rate of photosynthesis.

Pondweed required practical

By measuring oxygen production, we can calculate the rate of photosynthesis.

Equipment:

• lamp/light source: to be moved further away to change light intensity

• ruler: to help placing the light at certain intervals

• water bath: to keep temperature of the plant constant and avoid having temperature be limiting

• aquatic plant: easier to regulate CO2 and easier to observe oxygen bubbles

• bicarbonate solution: breaks down into CO2 in water, increasing levels and avoiding it as a limiting factor

• boiling tube: to hold the plant and some water immediately around it

Start with the light 1 metre away from the plant and move it towards it in increments of 10 or 20 cm each time. count the number of bubbles of O2 given off by the plant in a set amount of time, and add to a table. When you plot this as a graph, it should exponentially decrease in accordance with the inverse square law:

Light intensity ∝1/distance²

because light intensity is inversely proportional to distance.

• more data = more representative of a situation = more accurate average = easier to identify anomalies

Respiration

respiration is the RELEASE of energy (not production, as this energy was just taken in via photosynthesis). it is exothermic and can be measured with respirometers.

the release of energy enables a host of living processes:

• active transport

• movement (e.g muscle contractions)

• building of polymers/molecules

• maintaining a constant body temperature (in warm blooded animals - cold blooded animals have their body temps determined by their environments)

Aerobic respiration

respiration that uses oxygen. yields the most energy, but is slow. complete breakdown of glucose. most of the reactions that make up aerobic respiration occur in the mitochondria.

oxygen + glucose → carbon dioxide + water

Anaerobic Respiration

respiration that doesn’t use oxygen. doesn’t yield much energy due to the partial breakdown of glucose - bond breaking releases energy, but releases it fast. changes between animals and plants due to having different microbes present:

glucose → lactic acid in humans

lactic acid: C3H6O3 → half a glucose molecule

glucose → ethanol + carbon dioxide in plants/bacteria/yeast - aka fermentation

ethanol: C2H6O

Response to exercise

more energy is needed for increasing muscle contractions, so respiration must occur faster - oxygen to be delivered and CO2 to be taken away faster. the body tries to do this via aerobic respiration, as it releases more energy (it also uses these processes, to a lesser extent, to pay back oxygen debt) in three ways:

• increasing heart rate → heart pumps oxygen rich blood faster and removes CO2 faster too

• increases breath rate → more oxygen + breathing out CO2 more frequently

• increasing breath volume/depth → more oxygen + breathing out more CO2 with every breath

if there’s not enough oxygen though, cells will begin to aerobically respire. the acidic nature of lactic acid causes muscle fatigue as it builds up, as it can only be broken down by oxygen. this creates an 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. blood flowing through the muscles transports the lactic acid to the liver where it is converted back to glucose.

Metabolism

metabolism: the sum of all chemical reactions involved in maintaining the living state of the cells of an organism. regulated by hormones.

• anabolism: synthesis of compounds needed by cells (e.g. photosynthesis)

• catabolism: breakdown of molecules to release energy (e.g. respiration)