Energy and ecosystems

Plants, photosynthesis, production, planet Earth

Plants form the basis for all the ecosystems on planet Earth, and in an ecological context they are collectively referred to as producers.

They take in the simple compound carbon dioxide and build it up to produce more complex compounds, using the process of photosynthesis which is powered by light.

The main product of photosynthesis is sugars (mostly glucose).

Carbon dioxide is a vital reactant used in the second stage (the light independent reactions) of photosynthesis.

Its concentration in the atmosphere is about 0.04%.

It is quite soluble in water so it is easily accessible to plants in aquatic environments.

Sugars can be used for several purposes by/within the plant:

• as a substrate for respiration
• for conversion to other organic compounds used by plants for growth.

Simple sugars - such as the monosaccharide glucose - can be converted by condensation to the polysaccharide cellulose, used for cell walls, and also to other carbohydrates such as the disaccharide sucrose or the polysaccharide starch which are used to relocate and store photosynthetic products.

Sugars can also be chemically converted by plants into different classes of organic compounds: lipids (oils), proteins, etc.

Most of these substances remain within the plant's cells. Collectively they make up the plant's biomass. Of course, plant cells also take up water (and minerals) which adds to the fresh weight of the plant. Their water content can vary as a result of environmental influences - notably rainfall and temperature.

Photosynthesis is carried out by a wide variety of organisms: green plants on the earth's surface, as well as simpler plants ranging from single celled protoctistans and algae to larger seaweeds of several colours in aquatic environments (ponds, lakes and seas).

It is also carried out by Cyanobacteria (older name blue-green algae). In fact over 2 billion years ago they changed the conditions on planet Earth by photosynthesing, and they also became incorporated into chloroplasts and passed on to all higher plants today. See endosymbiont theory - link below.


This type of nutrition is described as autotrophic, or photoautotrophic - acknowledging the role of light in the process.

This is distinct from chemoautotrophic nutrition, by which certain bacteria and archaea use carbon dioxide to synthesise organic molecules using chemical energy from hydrogen sulphide, elemental sulphur, or (ferrous) iron Fe²⁺, mostly in deep sea vents (hydrothermal vents). These support ecosystems of consumer organisms which can tolerate the extreme conditions prevailing there.

Measuring biomass and chemical energy

Dry biomass

In order to estimate how much organic matter has been built up by a plant (or a number of plants), it is necessary to take a sample, (weigh it to get the fresh weight), then drive off the water. It is normal to place it for several hours in an oven set at 105 °C. The sample is then weighed and returned to the oven again. This is repeated 'to constant mass'. This gives the dry mass.

Obviously the plant material is killed by this process.

It is usual to express this 'dry biomass' of plant tissue in terms of the unit area occupied by the plants, and the time scale involved. This means that the process is repeated, perhaps annually or at different stages of the growing season.

Why is the oven set at 105 °C?
>This is (just above) the boiling point of water. Higher temperatures could cause the dried plant material to burn.
What does 'drying to constant mass' mean?
>Checking that all the water has been driven off.

Mass of carbon in biomass

Organic compounds all contain the element carbon, in chemical combination with varying amounts of other elements - mostly hydrogen and oxygen, and (about 16%) nitrogen in proteins.

In theory, this could be related back to the carbon taken in as carbon dioxide in the light independent stage of photosynthesis, but it is not possible to directly measure the mass of carbon (only) in biomass. However the energy it contains can be measured - see below.

Using calorimetry to measure chemical energy

The chemical energy in dry biomass can be measured by burning it and calculating the amount of heat energy that is released.

1 calorie is the amount of heat needed to raise the temperature of 1 g of water by 1 °C.
1 kilocalorie (kcal) is the the amount of heat needed to raise the temperature of 1000 g of water by 1 °C, or to raise the temperature of 100 g of water by 10 °C ....

It is more usual to express energy in terms of Joules and Kilojoules (kJ). 4.18 joules = 1 calorie; 4.18 kJ = 1 kcal

1 joule is the amount of heat needed to raise the temperature of 0.24 g (1/4.18) of water by 1 °C.
1 kilojoule (kJ) is the the amount of heat needed to raise the temperature of 240 g of water by 1 °C,
or to raise the temperature of 24 g of water by 10 °C ....

It is very inefficient attempting to measure heat by simply burning it under a container of water.

The standard equipment used in this is a bomb calorimeter.

The biomass sample is burned in an atmosphere of oxygen inside a strong metal container, surrounded by an insulated water jacket.

As the name implies, the combustion process is quite powerful, and it is started by supplying electrical power to a heating element.

The heat absorbed can be calculated after measuring the temperature rise.

This needs careful calibration using known compounds.

A similar process is used to calculate the energy values for food items.


The value obtained for chemical energy from dry biomass of plants can be compared with the value of light energy (from the sun) over the growing period, and the efficiency of transfer of the products of the photosynthetic process can thus be calculated.

Anyone who has stood in front of a burning log fire wll be aware of the energy content of biomass!
And anyone who has tried to light one knows the importance of the dryness of the wood!

Biomass in power generation

Although much of the UK's electrical power continues to be generated from fossil fuels (oil 7.8%, gas 40.2%, coal 8.6%), in recent years there has been an increased contribution from wind (10.6%) and solar, via photovoltaic cells (2.8%) and so-called Bio-energy (8.4%). Nuclear power plants provide 20.1% and Hydroelectric plants supply 1.5%.

Some of the bio-energy comes from methane gas from sewage and landfill sites, but it may also be provided by plant crops specially grown for the process. Some carbohydrate-rich crops may be used to feed fermentation plants which produce ethanol for burning or road fuel, but there is concern that these could be better used in providing food for animal or human use.

Less controversial is the cultivation of fast-growing plant crops which can be burnt in power stations. There is a long history of cultivation of certain tree species, often coppiced on a 12-18 year cycle for fencing purposes or to produce charcoal, but short-rotation coppicing of willow and poplar on a yearly cycle has produced fuel for power generation.

Other crops have been developed specifically for use in power generation. Miscanthus - also colloquially known as Elephant Grass - is being increasingly grown for this purpose.

Miscanthus being harvested





Both fossil fuels and bio-energy crops have a biological origin.

Why are fossil fuels being phased out?
> They release large amounts of carbon dioxide - from carbon in plant/animal remains held underground for millions of years, and the extra CO₂ in the atmosphere is a greenhouse gas which causes global warming and climate change.

Why are bio-energy crops seen to be less of a problem?
> They are seen as carbon-neutral in that they absorb the same amount of CO₂ for photosynthesis as they give out when they are burned. BUT fossil fuels are probably used in the harvesting and drying process as well as for delivery to the power stations.

Primary production - by Plants

(Another term used in this context: productivity)

Gross primary production (GPP) is the chemical energy store in plant biomass. This may be interpreted in terms of the area of the (terrestrial) ecosystem, or the volume of an aquatic ecosystem, and the time scale involved.

Net primary production (NPP) is the chemical energy store in plant biomass after the plants' respiratory losses (R) to the environment have been taken into account.

NPP = GPP – R

Plants can use this energy for their own purposes: growth, and reproduction (production of flowers or cones and seeds).

It is effectively also available to other organisms in the environment: animals which eat plants (herbivores), and bacteria and fungi which infect or break down parts of the plant body (decomposers).

You should be able to give some reasons why very little of the sunlight energy falling on the leaves of a plant can actually be incorporated into the plant's primary production.

> It may be the wrong wavelength
- e.g. some is ultraviolet (UV)
- green light of wavelength 500-600 nm is not absorbed by chlorophyll
- some is heat (infra-red) - its energy used to evaporate water
>Some is reflected, (so leaves have green colour)
>Some misses chloroplasts and passes through leaf
>Limiting factors in (light-independent stages of) photosynthesis may be operating:
low CO₂ concentration, temperature too low/high , photorespiration

It is very difficult to measure the rate of plant respiration (R opposite). Animal respiration can be (easily?) calculated by taking the rate of production of carbon dioxide, or the rate of uptake of oxygen. But plants photosynthesise for most of the day, effectivly reversing the process.

Biomass transfer to consumers

Similarly, when animals eat plants, their biomass is increased but there are more substantial losses due to respiration, and energy is also lost in faeces and urine. [Movement requires energy, and animals lose body heat, but both of these are provided by respiration.]

The net 'production' N by consumers at this stage can be expressed in this form:

N = I - (F + R)

where
I = chemical energy store in Ingested food
F = chemical energy lost to the environment in Faeces and urine
R = Respiratory losses to the environment.

In a food chain, each interaction between trophic levels - producers (green plants) converting some of the energy they receive into plant biomass, some of which is consumed by primary consumers (herbivores), and then being consumed by secondary consumers (carnivores), followed by tertiary consumers (carnivores) - involves loss of energy.

Annual flow of energy through a terrestrial woodland ecosystem
/ kJ m^-2



You should be able to explain why a food chain rarely contains more than four trophic levels

>Energy losses at each trophic level, as shown by the reduction in numbers from left to right
>Particularly in animals, there are a variety of processes that use energy: e.g. excretion / egestion / movement / respiration / as heat - do not refer to growth at this stage
>Eventually there is too little energy left to sustain higher trophic levels

In a biodiverse natural ecosystem a number of different organisms compete to find food for their day-to-day activities and their numbers are limited by the amount of energy available via the food web.

This can also be described as secondary production. This term is generally applied to the production of new biomass by all the consumer organisms (herbivores as well as carnivores) in an ecosystem, but it can refer to primary and secondary consumers separately. Their nutrition is described as heterotrophic.

Farming practices and agricultural production

Within an environment managed to maximise the production of food for human consumption, it is necessary to maintain the efficiency of energy transfer. This is sometimes known as plant and animal husbandry.

Plant growth is usually enhanced by the addition of fertilisers to the soil, because the supply of minerals is a limiting factor in plant growth. These may be 'artificial' (inorganic salts) or 'natural' (based on products of biological origin - plant and animal remains and waste products), and there are economic considerations to be made. This is in fact independent of energy transfer.

Pest control by the use of chemical pesticides (insecticides, fungicides, herbicides) or biological agents, and 'integrated systems' reduces the loss of biomass or quality of the resulting crops.

Obviously basic agricultural practices such as enclosing fields with hedges or fences reduce the possible contact between crops and larger animals which would eat them, and land drainage as well as water supply can increase the efficiency of plant growth (up to a point).

Farmers generally attempt to prevent access to crops by wild animals in a number of ways - bird scaring etc. Growing plants in plastic tunnels and glasshouses have the same effect, as well as raising the temperature which increases their photosynthetic efficiency.

All of these practices have the effect of reducing the biodiversity within the area of farmed land compared with the 'natural' ecosystem supported when these practices are not applied. The timing of certain farming activities can have dramatic effects on populations of wild animals and plants. Ground-nesting birds can be disturbed by land management practices (ploughing, harrowing etc), hedge maintenance can affect nesting birds, and 'wild flower meadows' can be adversely affected if hay is cut before flowers have spread their seeds.

They can be seen as simplifying food webs and reducing energy losses to non-human food chains.

Livestock (animals reared to provide meat for human consumption) also rely on products produced by plants, so farmers need to manage their land and plants growing on it.

This production may take place on the farm land itself, perhaps by cattle grazing on grass in fields, or eating fodder such as hay, (straw and) silage, which are produced when growing conditions are most suitable and stored for use as animal feed when plants are not growing and weather is inclement. Alternatively, products high in carbohydrates, proteins or fats may be bought in from other producers, possibly in other countries. There are economic and political questions here.

Especially during winter, it is normal to confine livestock to buildings rather than allow them to go outdoors, which might damage the land and reduce plant productivity. This means less energy is lost in carrying out respiration to provide energy for their movement, and less body heat is lost into the environment.

Some animals, such as 'battery hens' - now replaced by animals reared in "enriched cages" which allow them to scratch - and some dairy cattle, are exclusively reared in limited spaces indoors in environments where feeding, lighting and heating are closely controlled in order to regulate their bodily activities, maximising their growth or production of secondary products - eggs or milk.

These procedures can be seen as reducing respiratory losses within a human food chain.

There are a number of ethical (and economic and environmmental) issues associated with the enhancement of productivity in an agricultural context.