4.2 Energy Flow Flashcards
What do most ecosystems rely on?
Most ecosystems rely on a supply of energy from sunlight. This is because it is the initial source of energy. Living organisms can harvest this energy by photosynthesis. Three groups of autotroph can out photosynthesis; plants, eukaryotic algae including seaweeds that grow on rocky shores, and cyanobacteria. These organisms are often referred to by ecologists as producers.
Heterotrophs do not use light energy directly but they are indirectly dependent on it. There are several groups of heterotrophs in ecosystems; consumers, detritivores and saprotrophs. All of them use carbon compounds as their energy source. In most ecosystems all or almost all energy in the carbon compounds will originally have been harvested by photosynthesis in producers.
What are the three main producers?
Plants, algae and cyanobacteria (bacteria capable of photosynthesis).
What are the three main heterotrophs?
Consumers, detritivores and saprotrophs
How is the energy from light converted to chemical energy?
Light energy is converted to chemical energy in carbon compounds by photosynthesis. Producers absorb sunlight using chlorophyll and other photosynthetic pigments. Producers then release energy from their carbon compounds by cell respiration and then use it from cell activities. Energy released in this way is eventually lost to the environment as waste heat. However, only some of the carbon compounds in producers are used in this way and the largest part remains in the cells and tissues of producers. The energy in these carbon compounds is available to heterotrophs.
How does energy transfer to heterotrophs?
Chemical energy in carbon compounds flows through food chains by means of feeding.
How is energy used in organisms?
- Synthesising large molecules like DNA, RNA and proteins
- Pumping molecules or ions across membrane by active transport.
- Moving things around inside the cell, such as chromosomes or vesicles, or in muscle cells the protein fibres that cause muscle contraction.
ATP supplies energy for these activities. Each cell produces its own ATP.
How is energy stored and released in cells?
All cells can produce ATP by cell respiration. In this process carbon compounds such as carbohydrates and lipids are oxidised. These oxidation reactions are exothermic and the energy released is used in endothermic reactions to make ATP. So cell respiration transfers chemical energy from glucose and other carbon compounds to ATP. The reason for doing this is that the chemical energy in carbon compounds such as glucose is not immediately usable by the cell, but the chemical energy in ATP can be used directly for many different activities.
The second law of thermodynamics states that energy transformations are never 100% efficient. Not all the energy from the oxidation of carbon compounds in cell respiration is transferred to ATP. The remainder is converted to heat. Some heat is also produced when ATP is used in cell activities. Muscles warm up when they contract for example. Energy from ATP may reside for a long time in large molecules when they have been synthesised, such as DNA and proteins, but when these molecules are eventually digested the energy is released as heat.
What energy conversion can living organisms NOT perform?
Living organisms cannot convert heat to other forms of energy. When ATP is used to contract muscle for example some of the energy is converted to movement by making the muscle fibres contract, and a bit is lost as heat. This heat energy cannot be converted back and will eventually be lost.
Why do living organisms make ATP?
Because glucose is not immediately useable, and ATP releases energy for many processes and is.
What energy conversions can living organisms perform?
They can:
- Turn light into chemical energy in photosynthesis
- Chemical energy to kinetic energy in muscle contraction
- Chemical energy to electrical energy in nerve cells
- Chemical energy to heat energy in heat-generating adipose tissue
BUT THEY CANNOT CONVERT HEAT ENERGY INTO ANYTHING.
How is energy lost in ecosystems?
Organisms are unable to convert heat energy into any other type of energy, and according to the laws of thermodynamics in physics as heat passes from hotter to cooler bodies, heat produced in living organisms will all eventually be lost to the abiotic environment.
Explain the length of food chains?
There are rarely more than 4 or 5 stages in a food chain, we might expect them to be limitless with one species being eaten by another to infinity but this does not happen. In ecology, as in all branches of science, we try to explain natural phenomena such as the restricted length of food chains using scientific theories. In this case it is the concept of energy flow along food chains and the energy losses that occur between trophic levels that can provide an explanation.
Why are food chains normally only 4 or 5 organisms long?
Energy losses between trophic levels restrict the length of food chains and the biomass of higher trophic levels. The energy added to biomass by each successive trophic level is less. This means too much energy is lost to sustain it past 4 or 5, because the final consumer would have to eat so much of the consumer below it, and this would take effort and it would probably not survive.
This is because:
- Most of the energy in food that is digested and absorbed by organisms in a trophic level is released by them in respiration for use in cell activities. It is therefore lost as heat. The only energy available to organisms in the next trophic level is chemical energy in carbohydrates and other carbon compounds that have not been used up in cell respiration.
- The organisms in a trophic level are not usually entirely consumed by organisms in the next trophic level. For example bones and hair are not eaten. Energy in uneaten material passes to saprotrophs or detritivores rather than passing to organisms in the next trophic level.
- Not all parts of food ingested by the organisms is digested and absorbed. Some material is indigestible and is egested in feces. Energy in feces does not pass along the food chain and instead passes to saprotrophs and detritivores.
Because of these losses, only a small proportion of the energy in biomass of organisms in one trophic level will ever become part of the biomass of organisms in the next trophic level. The figure of 10% is often quoted, but the level of energy loss between trophic levels is variable. As the losses occur at each stage in a food chain, there is less and less energy available to each successive trophic level. After only a few stages in a food chain the amount of energy remaining would not be enough to support another trophic level. For this reason the number of trophic levels in food chains is restricted.
Biomass measured in grams also diminishes along food chains, due to loss of carbon dioxide and water from respiration and loss from the food chain of uneaten or undigested parts of organisms. The biomass of higher trophic levels is therefore usually smaller than that of lower levels. There is generally a higher biomass of producers, the lowest trophic level of them all.
What is biomass?
Biomass is the total mass of a group of organisms. It consists of the cells and tissues of those organisms, including the carbohydrates and other carbon compounds that they contain. Because carbon compounds have chemical energy biomass has energy.
How can biomass tell us about food chains?
Biomass is the total mass of a group of organisms. It consists of the cells and tissues of those organisms, including the carbohydrates and other carbon compounds that they contain. Because carbon compounds have chemical energy, biomass has energy. Ecologist can measure how much energy is added per year by groups of organisms to their biomass. The results are calculated per square metre of the ecosystem so that different trophic levels can be compared. When this is done the same trend is always found, that the energy added to biomass by each successive trophic level is less. In secondary consumers, for example, the amount of energy is always less per year per square metre of ecosystem than in primary consumers.
