DP Biology HL C4.2 Transfers of Energy and Matter

DP Biology HL C4.2 Transfers of Energy and Matter - Full notes

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DP Biology HL | C4.2 Transfers of Energy and Matter

DP BIOLOGY · THEME C: INTERACTION AND INTERDEPENDENCE

C4.2

Transfers of Energy and Matter

Full notes for HL · IB-style

Contents

1	Ecosystems as open systems
2	Food chains, food webs and trophic levels
3	Energy flow and losses between trophic levels
4	Pyramids of energy and productivity
5	Decomposers and the recycling of matter
6	The carbon cycle
7	Human impacts on the carbon cycle
8	Ecosystems as open systems
9	Food chains and food webs
10	Producers and photosynthesis
11	Energy losses between trophic levels
12	Ecological efficiency
13	Pyramids of energy
14	Primary productivity
15	Secondary productivity
16	The carbon cycle
17	Keeling curve
18	Decomposers and nutrient recycling
19	Peat and fossil fuel formation
20	Mixed review: worked problems
21	Additional exam-style practice
22	Energy flow diagrams
23	Biomass and dry mass
24	Common misconceptions and links
25	Exam-style question with markscheme
26	Check your understanding
27	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain ecosystems as open systems with energy flow and nutrient cycling
construct food chains and webs and assign trophic levels
explain why energy is lost between trophic levels and calculate efficiency
construct pyramids of energy and explain primary and secondary production
explain the role of decomposers
describe the carbon cycle, including photosynthesis, respiration, combustion and fossilization
explain how human activities affect the carbon cycle and atmospheric CO₂

There is no additional HL content in this subtopic. SL and HL students study the same material.

Guiding questions

How does energy flow through ecosystems, and how is matter recycled? How are humans changing the carbon cycle?

How this topic is examined

C4.2 is examined through energy flow calculations and diagrams, pyramids of energy, carbon cycle diagrams and CO₂ data such as the Keeling curve.

Remember the central principle: energy flows (it is not recycled), while matter (carbon, nitrogen) is recycled.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Ecosystems as open systems

Energy flows through; matter cycles round.

Think about it

A sealed glass jar with a plant, water and snails can survive for years if it is kept in the light. What enters and leaves the jar, and what stays inside?

An ecosystem is a community of organisms interacting with each other and with their abiotic environment. Ecosystems are open systems: energy enters (mainly as sunlight) and leaves (as heat), and matter can also enter and leave (for example, nutrients washed in by rivers).

Almost all ecosystems depend on sunlight as their initial energy source, captured by producers in photosynthesis. Exceptions include deep-sea hydrothermal vent ecosystems, which depend on chemical energy captured by chemoautotrophs (B4.2).

Energy flows through ecosystems in one direction and is eventually lost as heat; it cannot be recycled. Matter (chemical elements such as carbon and nitrogen) is recycled between organisms and the environment.

Key point

Energy flows; matter cycles. This single idea explains food chains, pyramids and nutrient cycles.

2 Food chains, food webs and trophic levels

Who eats whom.

A food chain shows a sequence of feeding relationships, with arrows showing the direction of energy flow (from food to feeder). Producers (autotrophs) form the first trophic level. Primary consumers (herbivores) eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers. An organism's trophic level is its position in the chain.

In real ecosystems, most organisms eat several kinds of food and are eaten by several predators, so feeding relationships form a food web. An organism can occupy different trophic levels in different chains: a human eating vegetables is a primary consumer, but eating fish is a tertiary or higher consumer.

Figure 1. A marine food chain. Arrows show the direction of energy flow.

Common mistake

Drawing arrows from the eater to the food. Arrows show energy flow, from the organism eaten to the one that eats it.

3 Energy flow and losses between trophic levels

Only about 10% passes to the next level.

Producers convert light energy into chemical energy in organic compounds. When a consumer eats, only part of the energy in its food becomes part of its own biomass. Energy is lost at each step:

Respiration: much of the chemical energy is released in respiration to power life processes and is ultimately lost as heat. Uneaten parts: not all of an organism is eaten (roots, bones). Undigested material: some food passes out in faeces and goes to decomposers. Excretion: some energy is lost in urea and other wastes. Death: organisms that die without being eaten pass energy to decomposers.

On average, only about 10% of the energy in one trophic level is transferred to biomass in the next. This is why food chains rarely have more than four or five trophic levels: there is not enough energy left to support another level.

Efficiency of energy transfer = energy in next trophic level ÷ energy in previous trophic level × 100

WORKED EXAMPLE 1 Energy transfer efficiency

Producers in an ecosystem fix 25 000 kJ m⁻² yr⁻¹. Herbivores gain 2 200 kJ m⁻² yr⁻¹ and carnivores 180 kJ m⁻² yr⁻¹. Calculate the efficiency of each transfer.

Solution

Producers to herbivores: 2 200 ÷ 25 000 × 100 = 8.8%A1

Herbivores to carnivores: 180 ÷ 2 200 × 100 = 8.2%A1

Most energy is lost as heat from respiration, and in faeces and uneaten partsR1

Quick check

1. Why is energy lost between trophic levels?

2. Why are food chains short?

Answers: 1. respiration (heat), uneaten parts, faeces, excretion 2. too little energy remains to support higher levels

4 Pyramids of energy and productivity

Each level is smaller than the one below.

A pyramid of energy shows the energy flowing through each trophic level per unit area per unit time (kJ m⁻² yr⁻¹). Each bar is drawn to scale. Because energy is lost at each step, pyramids of energy are always pyramid-shaped, with the widest bar at the bottom.

Gross primary production is the total energy fixed by producers in photosynthesis. Net primary production is what remains after producers' own respiration: it is the energy available to primary consumers. Secondary production is the biomass produced by consumers. Productivity is highest in tropical rainforests and coral reefs, and lowest in deserts and open ocean.

Figure 2. Energy at each trophic level (a pyramid of energy drawn as bars).

Quick check

1. Why are pyramids of energy always pyramid-shaped?

2. Distinguish gross and net primary production.

Answers: 1. energy is lost at each trophic level 2. gross: total fixed; net: after producer respiration

5 Decomposers and the recycling of matter

Without decomposers, nutrients would stay locked in dead matter.

Decomposers, mainly saprotrophic bacteria and fungi, break down dead organisms, faeces and other organic waste by secreting enzymes and absorbing the products. In doing so they release inorganic nutrients (such as nitrate, phosphate and CO₂) back into the soil, water and air, where producers can use them again. Detritivores such as earthworms and woodlice help by breaking dead matter into smaller pieces.

Decomposition is fastest in warm, moist, well-aerated conditions. In waterlogged, acidic or cold conditions (bogs, peatlands), decomposition is slow, and organic matter accumulates as peat.

Quick check

1. What do decomposers release?

2. Why does peat form?

Answers: 1. inorganic nutrients and CO₂ 2. waterlogged, acidic, anoxic conditions slow decomposition

6 The carbon cycle

Carbon moves between the atmosphere, living things, oceans and rocks.

Carbon is the basis of all organic molecules. The carbon cycle describes how carbon moves between stores (also called sinks when they take in more than they release, or sources when they release more): the atmosphere (CO₂), living organisms (biomass), oceans (dissolved CO₂ and carbonates), soils, fossil fuels and limestone rocks.

Photosynthesis removes CO₂ from the air and converts it to organic compounds in producers. Feeding passes carbon along food chains. Respiration by producers, consumers and decomposers returns CO₂ to the atmosphere. Combustion of biomass and fossil fuels releases CO₂. Fossilization: when decomposition is incomplete, carbon compounds are buried and over millions of years form coal, oil and natural gas, or limestone from the shells of marine organisms. CO₂ also dissolves in the oceans, forming carbonic acid and hydrogen carbonate ions; some is used by marine organisms to build calcium carbonate shells and reefs.

Methane (CH₄) is produced by methanogenic archaea in anaerobic conditions (wetlands, rice paddies, cattle guts) and is oxidized to CO₂ in the atmosphere.

Figure 3. The carbon cycle as a flow of processes.

Process	Effect on atmospheric CO₂
Photosynthesis	removes CO₂
Respiration	releases CO₂
Combustion	releases CO₂
Fossilization	locks carbon away for millions of years
Dissolving in oceans	removes CO₂ (ocean acidification)

Quick check

1. Name two processes that remove CO₂ from the air.

2. How are fossil fuels formed?

Answers: 1. photosynthesis; dissolving in oceans 2. incomplete decomposition of buried organic matter over millions of years

7 Human impacts on the carbon cycle

We are moving carbon from rocks to the air.

Since the Industrial Revolution, the burning of fossil fuels has released carbon that was stored underground for millions of years, faster than natural processes can absorb it. Deforestation reduces photosynthesis and often involves burning. As a result, atmospheric CO₂ has risen from about 280 ppm before 1800 to over 420 ppm today.

Measurements at the Mauna Loa Observatory in Hawaii since 1958 (the Keeling curve) show a steady rise, with an annual oscillation: CO₂ falls in the Northern Hemisphere summer, when photosynthesis exceeds respiration, and rises in winter.

Rising CO₂ is the main cause of the enhanced greenhouse effect and climate change (D4.3), and of ocean acidification, which makes it harder for corals and shellfish to build calcium carbonate structures.

Figure 4. The Keeling curve: rising CO₂ with annual oscillations.

Quick check

1. Why does CO₂ rise and fall each year?

2. Give two human activities that increase atmospheric CO₂.

Answers: 1. seasonal photosynthesis in the Northern Hemisphere 2. burning fossil fuels; deforestation

8 Ecosystems as open systems

Energy and matter flows.

Ecosystems are open systems: energy enters (mainly as sunlight) and leaves (as heat); matter can enter and leave (nutrients carried by water, migrating animals). Energy flows through ecosystems; matter (chemical elements) is recycled.

WORKED EXAMPLE 2 Energy versus matter (standard)

Explain why energy flows but nutrients cycle.

Solution

Energy is lost as heat at each stage and cannot be reusedR1

Nutrients are returned to the environment by decomposers and reusedR1

9 Food chains and food webs

Feeding relationships.

Food chains show the transfer of energy from producers to primary, secondary and tertiary consumers. Food webs show interconnected chains. Arrows point in the direction of energy flow (from food to feeder).

WORKED EXAMPLE 3 Trophic levels (basic)

In grass → rabbit → fox, identify the trophic level of each organism.

Solution

Grass: producer; rabbit: primary consumerA1

Fox: secondary consumerA1

10 Producers and photosynthesis

Entry of energy.

Producers (autotrophs) capture light energy (or chemical energy for chemoautotrophs) and convert it into chemical energy in organic compounds. Only about 1–2% of light reaching plants is fixed in photosynthesis.

Quick check

1. Give two reasons why most light is not used in photosynthesis.

Answers: 1. reflected or transmitted; wrong wavelength; other limiting factors

11 Energy losses between trophic levels

Why chains are short.

Typically only about 10% of the energy at one trophic level is passed to the next. Losses: respiration (released as heat), material not eaten, indigestible material (faeces), and death without consumption. This limits food chains to about 4–5 levels.

WORKED EXAMPLE 4 Calculation (standard)

Producers fix 50 000 kJ m−2 yr−1. Estimate the energy available to tertiary consumers with 10% efficiency.

Solution

50 000 × 0.1 × 0.1 × 0.1M1

= 50 kJ m−2 yr−1A1

12 Ecological efficiency

Calculating transfer.

Efficiency of transfer = (energy at higher level/energy at lower level) × 100. It varies: herbivores often have low efficiency (plants are hard to digest), endotherms lose more energy as heat than ectotherms.

WORKED EXAMPLE 5 Efficiency (standard)

Herbivores receive 2400 kJ m−2 yr−1; carnivores 180. Calculate the efficiency.

Solution

180/2400 × 100M1

= 7.5%A1

Quick check

1. Why are endotherms less efficient at converting food to biomass than ectotherms?

Answers: 1. more energy used in respiration to maintain body temperature

13 Pyramids of energy

Always pyramid-shaped.

A pyramid of energy shows the energy at each trophic level per unit area per year. It is always pyramid-shaped because energy is lost at each transfer. Pyramids of numbers and biomass can be inverted in some ecosystems.

WORKED EXAMPLE 6 Inverted pyramid (standard)

Explain why a pyramid of numbers for an oak tree food chain may be inverted.

Solution

One oak tree supports many herbivorous insectsR1

Large producer, numerous small consumersR1

14 Primary productivity

Gross and net.

Gross primary productivity (GPP) is the total energy fixed by producers. Net primary productivity (NPP) = GPP − respiration; NPP is the energy available to consumers. Tropical rainforests and coral reefs have high NPP; deserts and tundra low.

WORKED EXAMPLE 7 NPP (standard)

GPP is 30 000 kJ m−2 yr−1 and plant respiration 18 000. Calculate NPP.

Solution

30 000 − 18 000M1

= 12 000 kJ m−2 yr−1A1

15 Secondary productivity

Consumers' growth.

Secondary productivity is the rate of biomass gain by consumers: energy ingested minus losses in faeces and respiration. Livestock farming is less efficient than eating plants directly because energy is lost at an extra trophic level.

Quick check

1. Explain why a vegetarian diet can feed more people per hectare.

Answers: 1. avoids the energy loss (about 90%) from an extra trophic level

16 The carbon cycle

Fluxes and stores.

Carbon stores (sinks) include the atmosphere, oceans, biomass, soil and fossil fuels. Fluxes: photosynthesis removes CO2; respiration, decomposition and combustion release it; oceans dissolve CO2; sedimentation forms limestone. Human activities have increased atmospheric CO2.

WORKED EXAMPLE 8 Processes (exam level)

Outline three processes that release CO2 into the atmosphere.

Solution

Cell respirationR1

Combustion of fossil fuels and biomassR1

Decomposition by saprotrophsR1

17 Keeling curve

Rising CO2 with seasonal cycles.

Measurements at Mauna Loa since 1958 show atmospheric CO2 rising from about 315 ppm to over 420 ppm, with annual oscillations caused by seasonal photosynthesis in the Northern Hemisphere.

Figure 5. Approximate annual mean CO2 at Mauna Loa.

WORKED EXAMPLE 9 Rate (standard)

Calculate the mean rate of increase between 1960 and 2020.

Solution

(414 − 317)/60M1

= 1.6 ppm per yearA1

18 Decomposers and nutrient recycling

Returning elements.

Saprotrophs (fungi and bacteria) secrete enzymes onto dead organic matter and absorb the products, releasing CO2, ammonium and other nutrients. Without decomposers, nutrients would remain locked in dead material.

Quick check

1. What conditions slow decomposition?

Answers: 1. low temperature, waterlogging (low oxygen), acidity

19 Peat and fossil fuel formation

Incomplete decomposition.

In waterlogged, acidic, anaerobic conditions, decomposition is incomplete and organic matter accumulates as peat. Over geological time, peat forms coal; marine organisms form oil and gas. These are long-term carbon stores.

Quick check

1. Why does draining peatlands increase CO2 emissions?

Answers: 1. aerobic decomposition of stored peat releases CO2

20 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 10 Mixed problem (exam level)

Explain why there are rarely more than five trophic levels.

Solution

About 90% of energy lost at each transferR1

Too little energy remains to support further levelsR1

21 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 11 Paper 1 question (standard)

Which pyramid can never be inverted? A. numbers B. biomass C. energy D. both biomass and numbers

Solution

CA1

WORKED EXAMPLE 12 Paper 2 question (exam level)

Outline the role of saprotrophs in ecosystems.

Solution

Decompose dead organic matter by external digestionR1

Release nutrients and CO2 for reuseR1

22 Energy flow diagrams

Quantifying an ecosystem.

Energy flow diagrams show inputs and outputs at each trophic level, with arrow widths proportional to energy. Calculations involve subtracting losses (respiration, faeces) to find what is passed on.

WORKED EXAMPLE 13 Flow (exam level)

A herbivore population ingests 1000 kJ; 600 kJ is lost in faeces; 300 kJ in respiration. Find the energy available to the next trophic level.

Solution

1000 − 600 − 300M1

= 100 kJ (10%)A1

23 Biomass and dry mass

Measuring stored energy.

Biomass is the total mass of organic matter. Dry mass (water removed by heating at about 100 °C to constant mass) is used because water content varies. The energy content of biomass can be found by burning dried samples in a calorimeter.

Quick check

1. Why is the sample heated to constant mass?

Answers: 1. to ensure all water is removed

24 Common misconceptions and links

Students often think…	Actually…
energy is recycled in ecosystems	energy flows and is lost as heat; matter is recycled
food chain arrows show who eats whom	arrows show energy flow from food to feeder
most energy goes into growth	most is lost in respiration as heat

How C4.2 connects to the rest of the course

Topic	Connection
C1.2 and C1.3	respiration and photosynthesis
B4.2 Niches	modes of nutrition
C4.1 Communities	food webs
D4.3 Climate change	carbon and the greenhouse effect
D4.2 Sustainability	nutrient cycling

25 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 6. Energy at each trophic level in a grassland.

(a) Calculate the efficiency of energy transfer from primary to secondary consumers. [2]

(b) Explain why energy is lost between trophic levels. [3]

(d) Describe the role of decomposers in the carbon cycle. [2]

(e) Explain the annual fluctuations in the Keeling curve. [2]

Markscheme

Part	Answer	Marks
(a)	180 ÷ 2 200 × 100	M1
	= 8.2%	A1
(b)	respiration releases heat	R1
	uneaten parts; faeces	R1
	excretion; death	R1
(c)	about 10% transferred each step	R1
	too little energy for further levels	R1
(d)	break down dead organic matter	R1
	respiration releases CO₂	R1
(e)	more photosynthesis in Northern Hemisphere summer lowers CO₂	R1
	respiration exceeds photosynthesis in winter	R1

Examiner's comments. In (b), answers that said energy is 'used up' rather than lost as heat scored less.

26 Check your understanding

Section A: multiple choice

1. Energy in ecosystems

A. flows and is lost as heat B. is recycled C. increases at each level D. stays constant

2. About what percentage of energy passes to the next trophic level?

A. 50% B. 90% C. 10% D. 1%

3. Decomposers are mainly

A. plants B. bacteria and fungi C. herbivores D. carnivores

4. Which process removes CO₂ from the atmosphere?

A. respiration B. combustion C. decomposition D. photosynthesis

5. Peat forms because

A. photosynthesis is fast B. decomposition is slowed in waterlogged acidic conditions C. temperatures are high D. oxygen is abundant

6. Pyramids of energy

A. are always pyramid-shaped B. can be inverted C. show numbers of organisms D. show biomass only

Section B: structured

7. Construct a food chain of four organisms and label trophic levels. [3]

8. Explain how carbon moves through the carbon cycle. [5]

9. Evaluate the use of pyramids of energy compared with pyramids of numbers. [3]

Answers

1. A 2. C 3. B 4. D 5. B 6. A

7. Producer, primary, secondary, tertiary consumers with arrows of energy flow.

8. Photosynthesis; feeding; respiration; decomposition; combustion; fossilization; oceans.

9. Energy pyramids always pyramid-shaped and comparable; numbers pyramids can be inverted (one tree, many insects); energy data harder to collect.

27 Summary and topic map

Idea	Remember
Ecosystem	open system; energy flows, matter cycles
Trophic levels	producers, primary, secondary, tertiary consumers
Energy loss	respiration (heat), faeces, uneaten parts; about 10% passed on
Pyramids	energy always pyramid-shaped
Decomposers	recycle nutrients; peat when slowed
Carbon cycle	photosynthesis, respiration, combustion, fossilization, oceans
Human impacts	fossil fuels, deforestation; Keeling curve

Before the exam, make sure you…

•draw energy-flow arrows correctly
•calculate efficiencies with units
•remember energy flows, matter cycles
•explain Keeling curve oscillations

Transfers of Energy and Matter on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 7. Topic map for Transfers of Energy and Matter.

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