Food chains and energy transfer

Question 1

What is a producer

Answer 1

A photosynthetic organism that manufactures organic substances using light energy, water, carbon dioxide and mineral ions.

Question 2

What are consumers

Answer 2

Organisms that obtain their energy by feeding on other organisms rather than using sunlight directly.

Question 3

What are saprobionts

Answer 3

• Decomposers
• They are a group of organisms that break down the complex materials in dead organisms into simple ones.
• In doing so, they release valuable minerals and elements in a form that can be absorbed by plants and so contribute to recycling.
• The majority of this work is carried out by fungi and bacteria.

Question 4

What is a food chain

Answer 4

• A feeding relationship in which the producers are eaten by primary consumers.
• These are then eaten by secondary consumers etc…
• Each stage in this chain is referred to as a trophic level.
• The arrows on food chain diagrams represent the direction of energy flow.

Question 5

Describe what a food web is and what the problem with them is

Answer 5

• Most animals do not rely on a single food source-within a single habitat many food chains link together to form a food web.
• The problem with food webs is their complexity
• In practice, it is likely that all organisms within an ecosystem will be linked to others in a good web.

Question 6

What is biomass

Answer 6

The total mass of living material in a specific area at a given time.

Question 7

What are the two methods of measuring biomass and what are their advantages and disadvantages

Answer 7

• Fresh mass is quite easy to assess, but the presence of varying amounts of water makes it unreliable.
• Measuring the mass of carbon or dry mass overcomes this problem but because the organisms must be killed, it is usually only made on a small sample and this sample may not be representative.

Question 8

How is biomass usually measured

Answer 8

• Biomass is usually measured using dry mass per given area on a given time.
• It is measured in grams per square metre (gm^-2)
• Where a volume is being sampled it is measured in grams per cubic metre (gm^-3).

Question 9

How do we measure the energy store in dry mass

Answer 9

Calorimetry

Question 10

Describe how bomb calorimetry can be used to estimate the chemical energy store in dry mass

Answer 10

• A sample of dry material is weighed and is the burnt in pure oxygen within a sealed chamber called a bomb.
• The bomb is surrounded by a water bath and the heat of combustion causes a small temperature rise in this water.
• As we know how much heat energy is required to raise the temperature of 1g of water by 1 degree Celsius and we know the volume of water and temperature rise, you can calculate the energy released from the mass of burnt biomass in units such as kJkg^-1

Question 11

What percentage of the suns energy do plants convert into organic matter

Answer 11

1% - 3%

Question 12

Why is most of the suns energy not converted to organic matter by photosynthesis

Answer 12

• Over 90% of the suns energy is reflected back into space by clouds and dust or absorbed by the atmosphere.
• Not all wavelengths of light can be absorbed and used for photosynthesis.
• Light may not fall on a chlorophyll molecule.
• A factor, such as low carbon dioxide levels, may limit the rate of photosynthesis.

Question 13

What is gross primary production (GPP)

Answer 13

The total quantity of the chemical energy store in plant biomass, in a given area or volume, in a given time.

Question 14

What is Net primary productivity

Answer 14

The chemical energy store that is left after energy has been used for respiration.

Question 15

What is the equation for net primary production (NPP) in plants

Answer 15

NPP= gross primary production (GPP) - respiratory losses R

Question 16

What is net primary production NPP available for

Answer 16

• Plant growth and reproduction.
• Other trophic levels of the ecosystem such as consumers and decomposers.

Question 17

What percentage of the net primary production in plants can be used by primary consumers for growth

Answer 17

Less than 10%?

Question 18

What percentage of energy from their prey do secondary and tertiary consumers transfer to their own body

Answer 18

About 20%

Question 19

Describe the reasons why primary, secondary and tertiary consumers only transfer a low percentage of the energy available from their prey to their body.

Answer 19

• Some of the organism is not consumed.
• Some parts are consumed but cannot be digested and are therefore lost in faeces.
• Some of the energy is lost in excretory materials such as urine.
• Some energy losses occur as heat from respiration and lost to the environment.
• These losses are high in mammals and birds because of their high body temperature.
• Much energy is needed to maintain their body temperature when heat is constantly being lost to the environment.

Question 20

How do we calculate the net production of consumers

Answer 20

• Equation: N= I— (F+R)
• Where N= the net production, I=the chemical energy store of ingested food, F=the energy lost in faeces and urine, R=the energy lost in respiration.

Question 21

What three things does the inefficiency of energy transfer between trophic levels explain:

Answer 21

1) Most food chains only have four or five trophic levels because insufficient energy is available to support a large enough breeding population at trophic levels higher than these.
2) The total mass of organisms in a particular place (biomass) is less at higher trophic levels.
3) The total amount of energy available is less at each level as one moves up a food chain.