كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

C4.2Theme C — Interaction and interdependence

Transfers of energy and matter — IB Biology SL Revision Notes & Topical Questions

10 min read 11 parts Lesson 9 of 9 2 quizzes

📚 IB Biology SL Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

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Lesson summary

C4.2 Transfers of energy and matter — IB BIOLOGY SL notes: Explain energy flow, losses and pyramids in ecosystems, production, and the cycling of carbon and nutrients.Ecosystem = a group of organisms interacting with each other and with the non-living environmentIt has biotic and…

Open system
Closed system
Laws in science
Exceptions
Chemical energy
Food chain

Part 1 of 9Ecosystems as open systems

Ecosystems as open systems

Ecosystem

Ecosystem a group of organisms interacting with each other and with the non-living environment

It has biotic and abiotic components and varies in size from a pond to an ocean.
It varies in complexity, with deserts relatively simple and rainforests and coral reefs highly complex.

Open system

Open system energy and matter can enter and leave
Energy enters as sunlight and in the tissues of migrating organisms.
Matter enters when organisms arrive and leaves when they leave, or is washed or cut away.
Most organisms stay in the ecosystem, so it is largely self-contained and recycles matter when they die.

Closed system

Closed system matter is recycled within the system and cannot enter or leave, but energy can
The Earth is an example, and a sealed terrarium is a small one.

Laws in science

Laws in science they describe patterns and do not explain them
The first law of thermodynamics says energy is converted and never created or destroyed.
A theory goes further by explaining the phenomenon.

Part 2 of 9Sources of energy and food webs

Sunlight

Sunlight the initial energy source for most food chains
Producers convert light energy to chemical energy in photosynthesis.
The energy passes to primary and then secondary consumers when they eat.

Exceptions

Exceptions deep-sea vent and cave communities rely on bacteria that use chemical energy
The statement that the sun is the source is a useful generalisation.

Chemical energy

Chemical energy stored in carbohydrates, lipids and proteins, and passed on when organisms are eaten
When an organism dies its energy passes to detritivores and saprotrophs.

Food chain

Food chain the feeding relationships in an ecosystem, with arrows showing the transfer of energy and biomass towards the eater

Food web

Food web connected food chains, which are more realistic because many organisms eat several foods
An omnivore can occupy more than one trophic level.

Part 3 of 9Modes of nutrition

Feature	Photoautotroph	Chemoautotroph	Consumer	Saprotroph
Energy source	Light	Oxidation of inorganic chemicals	Food eaten	Dead organic matter
Carbon source	Carbon dioxide	Carbon dioxide	Organic compounds	Organic compounds
Digestion	None	None	Internal	External
Example	Grass	Iron-oxidising bacteria	Rabbit	Mushroom fungi
Role	Producer	Producer	Transfers energy	Recycles nutrients

Autotroph

Autotroph makes its own organic molecules from simple inorganic substances
Anabolic reactions build carbohydrates, lipids and proteins.
Photoautotrophs split water by photolysis, releasing electrons, hydrogen ions and oxygen, and the electrons help make ATP.
Chemoautotrophs oxidise inorganic chemicals such as Fe²⁺ to Fe³⁺ to get electrons for ATP.

Heterotroph

Heterotroph ingests the tissues of other organisms and digests them, then rebuilds the molecules into its own
Consumers, detritivores and saprotrophs are heterotrophs.
Digestion can occur inside or outside the body.

Respiration

Respiration all organisms release energy as ATP by oxidising carbon compounds, with glucose as the fuel, and heat is produced

Part 4 of 9Decomposers and nutrient cycling

1Decomposition

Decomposition the breakdown of dead organisms, dead parts and faeces
Detritivores break apart the tissues first.
Saprotrophs release enzymes that break down organic molecules into inorganic nutrients.
Saprotrophs absorb some, and the rest in the soil is available to producers.

2Finite supply

Finite supply nutrients must be released from dead organisms because the supply is limited

3Recycled elements

Recycled elements carbon, nitrogen, calcium, phosphorus, sulfur and potassium
Carbon is returned to the atmosphere and the minerals to the soil.

4Matter compared with energy

Matter compared with energy matter is recycled but energy flows one way and cannot be recycled

Part 5 of 9Trophic levels

Trophic level	Name	Description
1	Producers	Make their own carbon compounds
2	Primary consumers	Herbivores that feed on plant tissue
3	Secondary consumers	Carnivores that eat primary consumers
4	Tertiary consumers	Carnivores that eat secondary consumers
5	Quaternary consumers	Carnivores that eat tertiary consumers

Apex predators

Apex predators at the top and with no predators

Decomposers

Decomposers not given one level, as they receive energy from all levels

Several levels

Several levels one species can occupy different levels in different chains
A sparrowhawk is at levels 3, 4 and 5 in the example web, and a robin at levels 3 and 4.

Part 6 of 9Pyramids of energy and energy losses

Pyramid of energy

Pyramid of energy shows the energy in the biomass at each trophic level
Bars are drawn to scale and are widest at the base, with stepped sides.
Levels are labelled producer, primary consumer, secondary consumer and so on.
The units are energy per unit area per year, for example kJ m^-2 year^-1.

Energy transfer

Energy transfer only about 10% passes to the next trophic level, with about 90% lost to the environment

Efficiency

Efficiency energy at the next level / energy at the level × 100

Reasons for losses

Not every part is eaten, such as roots, bones and woody parts.
Food is not completely digested, such as cellulose and fur, and is egested as faeces.
Heat is lost by radiation when consumers respire and use ATP.
Waste products such as urea are excreted.
Some organisms die without being eaten.

Decomposers

Decomposers they break down uneaten parts, waste and dead bodies, and they also lose heat

Part 7 of 9Biomass and length of food chains

Chain length

Chain length food chains rarely have more than four or five trophic levels
Less energy remains at each level, so top predators cannot find enough prey.

Biomass

Biomass it decreases at each level because of fewer or smaller individuals
A pyramid of biomass has a pyramid shape because of the energy losses.
The energy stored per unit mass does not change.
Do not confuse pyramids of biomass with pyramids of energy.

Heat

Heat organisms cannot convert heat back into chemical energy

Part 8 of 9Primary and secondary production

1Primary production

Primary production the accumulation of carbon compounds in the biomass of autotrophs

It is faster in biomes with more sunlight, optimum temperatures and more rainfall.
Tropical forests have high production, whereas tundra and deserts have low.

2Secondary production

Secondary production the accumulation of biomass in heterotrophs

It is calculated by subtracting respiratory losses from the stored energy ingested.
It is always lower than primary production.
Carbon is lost as CO₂ and energy as excreted waste such as water and urea.

3Rate of production

Rate of production biomass per unit area per year, for example g m^-2 yr^-1
Volume, g m^-3 yr^-1, is used in aquatic habitats.

4Measuring primary production

Measuring primary production harvest plots, dry to constant mass and compare over time

Part 9 of 9The carbon cycle

Carbon cycle

Carbon cycle the processes that transfer carbon between stores

Organic carbon is in the biomass of organisms.
Inorganic carbon is in atmospheric CO₂ and in the oceans as hydrogencarbonate ions.
Diagrams show stores (pools or sinks) and the transfers (fluxes) between them.

Carbon sink

Carbon sink takes up and stores carbon
Plants fix CO₂ into compounds in their tissues.
Fossil fuels and peat store carbon over very long times.
The oceans dissolve CO₂.

Carbon source

Carbon source releases carbon, through respiration, decomposition and burning

Net exchange

Net exchange an organism with photosynthesis faster than respiration is a net carbon sink, and one with respiration faster is a net source

Combustion

Combustion burning fossil fuels, peat and biomass releases CO₂ and water
Coal, oil and gas formed over millions of years from dead organisms.
Peat forms when plant matter does not fully decompose in waterlogged, acidic conditions.
Biomass burning matters less than burning peat or fossil fuels, because its carbon was removed from the air recently.

Autotrophs and heterotrophs

Autotrophs and heterotrophs photosynthesis gives the oxygen for aerobic respiration, and respiration supplies the CO₂ for photosynthesis

Keeling curve

CO₂ measured at Mauna Loa, Hawaii, since 1958, a site remote from local pollution
The yearly dips occur because photosynthesis rises in spring and summer.
The rising trend is due to burning fossil fuels faster than photosynthesis removes the CO₂.

Carbon dioxide is in the atmosphere.

It is fixed by photosynthesis into glucose in a leaf.

A herbivore eats the leaf and builds the carbon into its tissues.

The herbivore dies and its remains reach the soil.

Saprotrophs digest the remains.

Saprotroph respiration releases carbon dioxide to the air.

Key terms

Term	Meaning
Open system	A system in which energy and matter can enter and leave.
Trophic level	The position of an organism in a food chain, showing how many organisms energy has passed through.
Pyramid of energy	A diagram of the energy in the biomass at each trophic level, drawn to scale.
Primary production	The accumulation of carbon compounds in the biomass of autotrophs.
Secondary production	The accumulation of biomass in heterotrophs.
Carbon sink	A part of the carbon cycle that takes up and stores carbon.
Combustion	The burning of fossil fuels or organic material, releasing carbon dioxide.
Keeling curve	The record of atmospheric CO₂ at Mauna Loa, showing seasonal cycles and a rising trend.

Avoid these mistakes

Saying energy is recycled in ecosystems — energy flows one way and is lost as heat, whereas matter is recycled

Saying 10% of the energy is lost at each level — about 10% is passed on and about 90% is lost

Mixing up pyramids of energy and biomass — energy is per unit area per year and biomass is the mass at each level

Saying decomposers are at the top of a food chain — they receive energy from all trophic levels

Last updated: 25 October 2026