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Q9 Theories require evidence to support them. List all the evidence that led to the oceans forming, nitrogen being produced and oxygen being produced. Here are some other questions to practice.

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Q8 List all the ways CO2 was removed from the early atmosphere, and all the ways CO2 is put into the atmosphere now

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The level of carbon dioxide in the atmosphere is maintained by several processes, including photosynthesis [ [ photosynthesis: The chemical change that occurs in the leaves of green plants. It uses light energy to convert carbon dioxide and water into glucose. Oxygen is produced as a by-product of photosynthesis. ] , respiration respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration ] and combustion combustion: Combustion is the process of burning by fire..

Green plants remove carbon dioxide from the atmosphere by photosynthesis. Living organisms - including all plants and animals - release energy from their food using respiration. Respiration and combustion - burning - both release carbon dioxide into the atmosphere.

These processes form a carbon cycle in which the proportion of carbon dioxide in the atmosphere remains about the same. The animation should help you to understand how the cycle works. Note that you do not need to know about decomposition and fossilisation.

Human influences

Carbon dioxide is produced by burning fossil fuels. Increased energy consumption is leading to a rise in the use of fossil fuels, which in turn increases the release of carbon dioxide into the atmosphere.

The rising human population is adding to atmospheric carbon dioxide in other ways, too. When land is cleared for timber and farms, there are fewer trees to remove carbon dioxide from the atmosphere by photosynthesis. If the fallen trees are burned or left to rot, additional carbon dioxide is released into the atmosphere. This is particularly important when forests are cleared to make way for farms. Not only are there then fewer trees to absorb carbon dioxide, but the burning of the trees releases carbon dioxide.

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The level of carbon dioxide in the atmosphere is maintained by several processes, including photosynthesis [ [ photosynthesis: The chemical change that occurs in the leaves of green plants. It uses light energy to convert carbon dioxide and water into glucose. Oxygen is produced as a by-product of photosynthesis.

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Q5 Look carefully at the diagram and make sure you kow what happens when the switch is closed. Explain the difference between a permanent and an electromagnet.

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When an electric flows in a wire it creates a magnetic field around the wire. By winding the wire into a coil we can strengthen the magnetic field. Electromagnets are made from coils like this.

We can make an electromagnet stronger by:

The magnetic field around an electromagnet is just the same as the one around a bar magnet. It can, however, be reversed by turning the battery around. Unlike bar magnets, which are permanent magnets, the magnetism of electromagnets can be turned on and off just by closing or opening the switch.

Relays are electrically operated switches. They use an electromagnet, often to control a high-power circuit using a low-power signal or where multiple circuits need to be controlled by one circuit.

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BBC Bitesize - GCSE Physics (Single Science) - Electromagnets and motors - Revision 1

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When an electric flows in a wire it creates a magnetic field around the wire. By winding the wire into a coil we can strengthen the magnetic field. Electromagnets are made from coils like this. We can make an electromagnet stronger by: The magnetic field around an electromagnet is just the same as the one around a bar magnet.

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Q7 Look up the composition of our atmosphere and compare it that of Mars. Explain all the ways out atmosphere changed over time.

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BBC - GCSE Bitesize: Composition of the Earth's atmosphere

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A secondary school revision resource for Edexcel GCSE Science about the Earth's sea and atmosphere

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Q4 Look back through your notes about parallel circuits and make sure you can explain the rules about current in parallel circuits.

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You need to know how to measure the current that flows in a component and the potential difference [ potential difference: The voltage between two points that makes an electric current flow between them. ] across it.

Current

A current flows when an electric charge moves around a circuit. No charge can flow if the circuit is broken, for example, when a switch is open. Click on the animation to see what happens to the charge when the switch is opened or closed.

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Measuring current:

Current is measured in amperes

Amperes is often abbreviated to amps or A

Current flowing in a component in a circuit is measured using an ammeter

The ammeter must be connected in series with the component

Potential difference

A 'potential difference' across an electrical component is needed to make a current flow in it. Cells or batteries often provide the potential difference needed.

'Potential difference' is often called 'voltage'.

Measuring potential difference:

Potential difference is measured in volts, V

Potential difference across a component in a circuit is measured using a voltmeter

The voltmeter must be connected in parallel with the component

Energy in circuits

The potential difference between any two points in a circuit is the energy transferred to, or from, a given amount of charge as it passes between those points. In the circuit above, the charges gain energy in the cell, and then transfer that same amount of energy into light and heat in the lamp. That is why the potential difference across the cell is the same as the potential difference across the lamp; it is the same amount of energy.

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how do parallel and series circuits work

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You need to know how to measure the current that flows in a component and the potential difference [ potential difference: The voltage between two points that makes an electric current flow between them. ] across it. A current flows when an electric charge moves around a circuit.

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Q2 Make sure you read the table carefully. When a plant carries out photosynthesis what products are produced and how do these products contribute to the mass of the plant increasing?

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Where Does a Plant’s Mass Come From? Have you ever wondered where plants get their mass? All those leaves and branches have to come from somewhere, but where? It turns out that the main ingredients

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Q1 Think back to your photosynthesis assessment. Make sure you can explain how this experiment would be carried out.

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Without photosynthesis life as we know it would not exist. It's worth a moment's reflection...

There would be no biology without photosynthesis. Plant biomass is the food and fuel for all animals. Plants are the primary producers. These amazing organisms are capable of capturing the energy of sunlight and fixing it in the form of potential chemical energy in organic compounds. The organic compounds are constructed from two principle raw materials; carbon dioxide and water (which is a source of hydrogen). These compounds are stable and can be stored until required for life processes. Hence animals, fungi and non-photosynthetic bacteria depend on these for the maintenance of life.

But how can we measure the rates at which photosynthesis takes place?

The quantities are mind boggling. A hectare (e.g. a field 100 m by 100 m) of wheat can convert as much as 10,000 kg of carbon from carbon dioxide into the carbon of sugar in a year, giving a total yield of 25,000 kg of sugar per year.

There is a total of 7000 x 10 ⁹ tonnes of carbon dioxide in the atmosphere and photosynthesis fixes 100 x 10 ⁹ tonnes per year. So 15% of the total carbon dioxide in the atmosphere moves into photosynthetic organisms each year.

What are the different methods of measuring the rate of photosynthesis?

There are a few key methods to calculate the rate of photosynthesis. These include:

1) Measuring the uptake of CO ₂

2) Measuring the production of O ₂

3) Measuring the production of carbohydrates

4) Measuring the increase in dry mass

As the equation for respiration is almost the reverse of the one for photosynthesis, you will need to think whether these methods measure photosynthesis alone or whether they are measuring the balance between photosynthesis and respiration.

Measuring photosynthesis via the uptake of carbon dioxide

Using 'immobilised algae' - It's easy and accurate to measure the rate of photosynthesis and respiration using immobilised algae in hydrogen carbonate indicator solution - known as the 'algal balls' technique. Read the full protocol on using immobilsed algae to measure photosynthesis.

Using an IRGA - Uptake of CO ₂ can be measured with the means of an IRGA (Infra-Red Gas Analyser) which can compare the CO ₂ concentration in gas passing into a chamber surrounding a leaf/plant and the CO ₂ leaving the chamber.

Using a CO2 monitor - More simply, you could put a plant in a plastic bag and monitor the CO ₂ concentration in the bag using a CO 2 monitor. Naturally, the soil and roots must NOT be in the bag (as they respire). Alternatively, you could place some Bicarbonate Indicator Solution in the bag with the plant and watch the colour change. This would best be done with a reference colour chart to try to make the end-point less subjective. This could give a comparison between several plants. There are difficulties with this method, as I'm sure you can appreciate. The leaf area of the plants should be measured so you can compensate for plant size. Atmospheric air is only 400ppm CO ₂, so there is not much CO ₂ to monitor and the plant will soon run out of CO ₂ to fix.

Measuring photosynthesis via the production of oxygen

Oxygen can be measured by counting bubbles evolved from pondweed, or by using the Audus apparatus to measure the amount of gas evolved over a period of time. To do this, place Cabomba pondweed in an upside down syringe in a water bath connected to a capillary tube (you can also use Elodea, but we find Cabomba more reliable). Put the weed in a solution of NaHCO ₃ solution. You can then investigate the amount of gas produced at different distances from a lamp. Read a full protocol on how to investigate photosynthesis using pondweed.

Measuring photosynthesis via the production of carbohydrates

There is a crude method where a disc is cut out of one side of a leaf (using a cork borer against a rubber bung) and weighed after drying. Some days (or even weeks later), a disk is cut out of the other half of the leaf, dried and weighed. Increase in mass of the disc is an indication of the extra mass that has been stored in the leaf. This is very simple to do and enables you to investigate plants growing in the wild. However, you can probably think of several inaccuracies in this method.

Measuring photosynthesis via the increase in dry mass

Dry mass is often monitored by the technique of 'serial harvests' where several plants are harvested, dried to constant weight and weighed - this is repeated over the duration of the experiment. If you harvest several plants and record how much mass they have accumulated you will have an accurate measure of the surplus photosynthesis over and above the respiration that has taken place. As with most methods, you need several plants so you have replicate measurements and you can find an average and a standard deviation if necessary.

Investigating the light-dependent reaction in photosynthesis

The rate of decolourisation of DCPIP in the Hill Reaction is a measure of the rate of the light-requiring stages of photosynthesis

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A brief outline of various ways to measure the rate of photosynthesis - including measuring photosynthesis via the uptake of carbon dioxide (e.g. using immobilised algae), via the production of oxygen in pondweed, via the increase in dry mass, and via the production of carbohydrates.