ما هي معادلة البناء الضوئي الكيميائية الموزونة؟ What is the balanced equation for photosynthesis?

المعادلة الموزونة هي: 6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂ (ستة جزيئات ثاني أكسيد الكربون + ستة جزيئات ماء في وجود طاقة الضوء وصبغة الكلوروفيل تعطي جزيء جلوكوز وستة جزيئات أكسجين). وتتطلب العملية تفاعلات ضوئية لإنتاج ATP و NADPH يعقبها تفاعلات لاضوئية لتثبيت الكربون. [الصيغة الرمزية الموزونة: 6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂]

4.2.6Explaining Change

Biodiversity

5 min read 9 parts Lesson 16 of 28 1 quiz

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Section 1 of 9What biodiversity means

Biodiversity is the variety of all the different species of organisms on Earth, or within a single ecosystem. A habitat such as an ancient woodland has high biodiversity because it supports many species of plants, insects, birds and mammals. A field of one crop has low biodiversity because almost everything in it is the same species.

High biodiversity also means a healthy ecosystem. Species in an ecosystem depend on each other for food, shelter and pollination, and they depend on the non-living parts of the environment such as soil, water and light. A stable ecosystem is one where the populations of species and the abiotic factors stay in balance over time.

Section 2 of 9Why biodiversity matters

A large number of species makes an ecosystem more stable. If one species declines, other organisms may be able to take over its role, so food webs are less likely to collapse. A diverse ecosystem also provides many resources that humans use.

Why biodiversity matters

Food and materials

Food and materials crops, timber, fibres and fish all come from living organisms.

Medicines

Medicines many drugs were first discovered in plants and microorganisms; unstudied species may hold new ones.

Ecosystem services

Ecosystem services pollination of crops, recycling of nutrients, flood control by wetlands and soil formation.

Genetic variety

Genetic variety wild relatives of crops carry alleles for disease resistance that can be used in breeding.

Humans also have an ethical duty to avoid causing extinction, and many people value nature for its beauty and for recreation.

Section 3 of 9How humans reduce biodiversity

The growing human population uses more resources and produces more waste. Many human activities cause habitats to shrink, become polluted or disappear, and species that lose their habitat may become extinct.

Building houses, roads and quarries on natural land (see Land Use).

Clearing forests for farming or timber (see Deforestation).

Pollution of water, land and air (see Waste Management).

Releasing greenhouse gases that contribute to global warming, changing the range in which species can live.

Monoculture farming, where a single crop is grown over huge areas, and the removal of hedgerows.

Section 4 of 9Measuring and comparing biodiversity

To compare habitats, scientists sample the species present.

Measuring and comparing biodiversity

Number of different species

The number of different species (species richness) and the number of individuals of each are both important.

Quadrats are used to sample plants, and pitfall traps or sweep nets are used for small animals.

Samples must be random and large enough to be representative, otherwise the data may not reflect the whole area.

If one habitat has 12 species in a sample and another has 3, the first has the higher biodiversity. Repeating sampling over several years shows whether biodiversity is rising or falling.

Section 5 of 9Positive human action

Humans can also protect biodiversity.

Positive human action

Governments, charities and individuals can create nature reserves, regulate hunting and fishing, restore hedgerows and replant woodlands,

and run breeding programmes for endangered species.

These ideas are developed in the lesson on maintaining biodiversity.

Protecting biodiversity usually has to be balanced against the needs of people for food, jobs and housing,

so decisions often involve compromise between economic, social and environmental factors.

Data from sampling can be presented in a table of species and numbers. Calculating the mean number of each species per quadrat and then multiplying up for the total area gives a population estimate, and comparing estimates from different years shows trends, for example whether a rare orchid is increasing under a conservation scheme.

Section 6 of 9Biodiversity in food webs and over time

Biodiversity is not fixed. Over geological time new species arise by evolution and others become extinct. What is different today is the speed: the present rate of extinction is far higher than the natural background rate, and most of the cause is human activity. Once a species has gone, its unique combination of alleles is lost for ever.

Consider a pond food web.

Algae and pondweed are producers; water fleas, snails and tadpoles feed on them; sticklebacks and dragonfly nymphs feed on those; a heron feeds at the top.

If pollution kills the snails, the pond still contains other primary consumers that can pass energy upwards.

If the pond contained only one primary consumer, its loss would remove the energy supply for every higher trophic level.

This is the biological reason why variety gives stability: there are alternative pathways for energy and nutrients.

Keystone species have an effect out of proportion to their numbers.

Biodiversity in food webs and over time

Removing a predator can let its prey multiply and overgraze the producers, which changes the whole community.

Pollinators such as bees are another example: crop plants and wild flowers set seed only if they are visited,

and loss of the pollinators threatens both food supply and the animals that eat seeds and fruit.

Section 7 of 9Exam-style questions

Explain why a species-rich woodland is more stable than a field growing only wheat. [3]3 marks
Exam-style questions
1. The woodland has many different producers and consumers, so there are many food chains linked in a web [1].
2. If one species declines, consumers can feed on alternative species, so the ecosystem is not destroyed [1].
3.
The wheat field has one producer,
so a disease or pest affecting wheat removes the whole food source; it is only maintained by human input such as fertiliser and pesticide [1].
A student sampled two habitats with quadrats. Habitat A had 9 species; habitat B had 4 species. Both had a similar total number of individuals. Which has the higher biodiversity and why? [2]2 marks
Give three reasons why it is important to maintain biodiversity. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Biodiversity	The variety of living species in an area or on Earth.
Ecosystem	All the living organisms in an area and the non-living parts of the environment they interact with.
Extinction	The permanent loss of all members of a species.
Habitat	The place where an organism lives.
Monoculture	Growing a single crop species over a large area.
Species richness	The number of different species present in a sample or area.
Population	All the organisms of one species living in an area.

Section 9 of 9Exam technique

Saying biodiversity is just the number of animals. It is the variety of species, including plants, fungi and microorganisms, not the number of individuals.

Thinking extinction only matters for large, attractive animals. Loss of small species such as pollinating insects can damage whole food webs.

Assuming a large number of individuals means high biodiversity. A huge population of one species is low biodiversity.

Writing that humans only damage biodiversity. Conservation action by humans can protect and increase it.