C4.2 Transfer of Energy & Matter

Define ecosystem

An ecological unit consisting of living organisms interacting with the natural environment 

What are the types of ecosystems? (2)

1) Open systems - A system that exchanges both energy and matter with its surroundings

2) Closed systems - A system that only exchanges energy (not matter) with its surroundings

Describe exceptions to sunlight being the primary energy source for ecosystems [3]

1) Caves - 

Much of the energy comes from organic matter that is carried by roots & floodwater into darkness

2) Aphotic zone - 

• Depths of water where sunlight is too weak to support photosynthesis 

• Marine snow & carcasses sink from illuminated surface waters, transferring organic matter to deeper communities 
  
  

3) Chemosynthetic sites -

At hydrothermal vents or other chemically rich sites, chemoautotrophs obtain energy from oxidation reactions & fix carbon dioxide

Explain why energy is transferred in different ways [3]

• Energy transfer is not 100% efficient so only a fraction passes onto next trophic level:

  • During respiration, while most chemical energy is converted into ATP, some is lost as heat  

  • Energy is also used for movement, growth reproduction, and lost in waste

Describe how energy is transferred through food chains [4]

Stage	Process
Producer	Plants convert sunlight (light energy) into chemical energy
Primary Consumer	Eats plants and obtains chemical energy from plant tissues
Secondary Consumer	Eats primary consumers and gains their stored energy
Tertiary Consumer	Eats secondary consumers, continuing energy transfer

Explain the difference between food chain and food web

• A food chain shows simple, linear feeding relationships between organisms

  • It represents the flow of energy & biomass, in which the direction is dictated by arrows 

• A food web is a more complex diagram showing multiple interconnected food chains in a community

  • It represents how organisms have varied diets → allowing energy & biomass to flow through multiple pathways

  • Better represents real ecosystems since organisms often eat more than one type of food 

Describe the nature of decomposers & the process they go through [5]

Nature: Decomposers obtain energy from carbon compounds found in faeces, dead parts from organisms (e.g. leaves, bark, hair), & dead whole organisms 

Process:

• They break down these carbon compounds (insoluble macromolecules) through respiration into soluble molecules/ions 

• This releases:

  • Energy for own growth & activity 

  • Carbon, nitrogen, & phosphorus are also returned back to soil & atmosphere

Describe the nature of autotrophs & the process they go through [5]

Nature: Autotrophs obtain energy from converting simple inorganic substances (e.g. CO₂ & water) into organic carbon compounds 

• They are also primary producers in ecosystems → providing energy & matter for heterotrophs & other life forms 

Process:

• Energy is acquired from sunlight (photoautotroph) & chemical reactions (chemoautotrophs) in order to:

  • Fix carbon dioxide into organic molecules (carbon fixation) 

  • Drive anabolic reactions that build complex macromolecules (e.g. carbohydrates, proteins, lipids)

Explain the difference between photoautotrophs and chemoautotrophs in energy use

• Photoautotrophs capture light energy from the sun, which powers photosynthesis 

• Light energy powers photosynthesis:

  • Converting CO₂ & water into organic compounds

  • Exciting electrons, which start a chain of reactions that produce energy-rich molecules (ATP, NADPH) 

• Chemoautotrophs gain energy by oxidising inorganic substances 

• Oxidation reactions release energy by transferring elections from molecules (e.g. iron, hydrogen sulphide, ammonia)

[Example] Chemoautotroph - Iron-oxidising bacteria & its energy generation process

1. Absorbs Fe²⁺ irons from the environment & removes an electron from it

2. The electron is excited & are accepted by electron chain carriers in the bacteria’s plasma membrane

3. The electrons flow & build a proton gradient for ATP production

4. Other excited electrons are passed to NAD → converting to NADH

• ATP & NADH are used to fix carbon dioxide to produce carbon compounds

How do heterotrophs synthesise required carbon compounds? [2]

• Heterotrophs digest complex molecules (e.g. proteins, nucleic acids, carbohydrates) both internally (mechanical/chemical by gut) & externally (by secreting enzymes)

• After digestion, smaller molecules (e.g. amino acids, sugars) are assimilated which are used to build carbon compounds the organism needs for growth, repair, & energy storage

Compare and contrast how autotrophs and heterotrophs release energy [4]

Similarities:

• Both autotrophs & heterotrophs release energy by oxidising carbon compounds (e.g. carbohydrates) through cellular respiration

• Energy released is stored as ATP which powers cellular processes

Difference:

• Autotrophs synthesise their own organic carbon compounds in photosynthesis & breaks them down for energy 

• Heterotrophs obtain carbon compounds by consuming other organisms & oxidise them to release energy