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  • Created on: 08-02-21 08:33

Introduction to Photosynthesis

Photosynthesis Produces Glucose Using Light

- Photosynthesis uses energy to change carbon dioxide and water into glucose and oxygen

- It takes place in chloroplasts in green plant cells - they contain pigments like chlorophyll that absorb light

- Energy is transferred to the chloroplasts from the environment by light

- Photosynthesis is endothermic - this means energy is transferred from the environment in the process

The word equation for photosynthesis is: 

carbon dioxide + water --(light)  glucose + oxygen

The symbol equation is:

6CO2 + 6H20 + (light)→ C6H12O6 + 6O2

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Uses of Glucose

Plants Use Glucose in Five Main Ways

For respiration - This transfers energy from glucose which enables the plants to convert the rest of the glucose into various other useful substances

Making cellulose - Glucose is converted into cellulose for making strong plant cell walls

Making amino acids - Glucose is combined with nitrate ions (absorbed from the soil) to make amino acids, which are then made into proteins

Stored as oils or fats - Glucose is turned into lipids (fats and oils) for storing in seeds

Stored as starch - Glucose is turned into starch and stored in roots, stems and leaves, ready for use when photosynthesis isn't happening, like in the winter. Starch is insoluble, which makes it much better for storing than glucose - a cell with lots of glucose would draw in loads of water and swell up

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Rate of Photosynthesis: Limiting factors

Limitng Factors Affect the Rate of Photosynthesis

- Any of these three factors can become the limitng factor - this just means that it's stopping photosynthesis from happening any faster

- These factors have a combined effect on the rate of photosynthesis, but which factor is limiting at a particular time depends on the environmental conditions

  • at night it's pretty obvious that light is the limiting factor
  • in winter it's often the temperature
  • if it's warm enough and bright enoug, the amount of CO2 is usually limiting

Chlorophyll can also be a limiting factor of photosynthesis

The amount of chlorophyll in a plant ca be affected by diseases (e.g. infection with the tobacoo mosaic virus) or environmental stress, such as a lack of nutrients. These factors can cause chloroplasts to become damanged or to not make enough chlorophyll. This menas the rate of photosynthesis is reduced because they can't absorb as much light

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Limiting Factors: Light Intensity

Not Enough Light Slows Down the Rate of Photosynthesis

Light provides the energy needed for photosynthesis

Image result for rate of photosynthesiscarbon dioxide  graph gcse annotated rate (

As the light level is raised, the rate of photosynthesis increases steadily - but only up to a certain point

Beyond that, it won't make any difference - as light intensity increase

s, the rate will no longer increase. This is because it'll be either the temperature or the CO2 level which is now the limiting factor, not light

In the lab, you can chnage the light intensity by moving a lamp closer to or further away from your plant xxxx

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Limiting Factors: Carbon Dioxide

Too Little Carbon Dioxide Also Slows it Down

CO2 is one of the raw materials needed for photosynthesis

Image result for rate of photosynthesiscarbon dioxide  graph gcse annotated rate (

As with light intensity, the amount of CO2 will only increase the rate of photosynthesis up to a point

After this, the graph flattens out - as the amount of CO2 increases, the rate no longer increases. This shows that CO2 is no longer the limiting factor

As long as light and CO2 are in plentiful supply then the factor limiting photosynthesis must be temperature

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Limiting Factors: Temperature

The Temperature has to be Just Right

Temperature affects the rate of photosynthesis because it affects the enxymes involved

Image result for rate of photosynthesiscarbon dioxide  graph gcse annotated rate (

Usually, if the temperature is the limiting factor it's because it's too low - the enzymes needed for photosynthesis work more slowly at low temperatures

But if the plant gets too hot, the enzymes it needs for photosynthesisi and its other reactions will be damaged

This happens at about 45oC (which is pretty hot for outdoors, although greenhouses can get that hot if you're not careful)

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Rate of Photosynthesis: Combined factor graphs - T

The graph shows how the rate of photosynthesis is affected by light intensity and temperature

Image result for graph rate of photosynthesis light intensity and temps at 25 and 15 degrees

As the start, both of the lines show that as light intensity increases, the rate of photosynthesis increases steadily

But the lines level off when light is no longer the limiting factor. The line at 25oC levels off at a higher point than the one at 15oC, showing that temperature must have been a limiting factor at 15oC

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Rate of Photosynthesis: Combined factor graphs - L

The graph shows how the rate of photosynthesis is affected by light intensity CO2 concentration

Image result for graph shows how the rate of photosynthesis affected by light intensity and temperature

Again, both lines level off when light is no longer the limiting factor 

The line at the higher CO2 concentration of 0.4% levels off at a higher point than the one at 0.04%. This means CO2 concentration must have been a limitng factor at 0.04% CO2. The limiting factor here isn't temperature because it's the same for both lines

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Measuring the Rate of Photosynthesis: Practical

Oxygen Production Shows the Rate of Photosynthesis

A source of white light is placed at a specific distance from the pondweed

The pondweed is left to photosynthesise for a set amount of time. As it photosynthesises, the oxygen released will collect in the capillary tube

At the end of the experiment, the syringe is used to draw the gas bubble in the tube up alongside a ruler and the length of the gas bubble is measured. This is proportional to the volume of O2 produced

For this exerment, any variables that could affect the results should be controlled, e.g. the temperature and time the pondweed is left to photosynthesise

The experiment is repeated twice with the light source at the same distance and the mean volume of O2 produced is calculated

Then the whole experiment is repeated with the light source at different distancee from the pondweed

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Measuring the Rate of Photosynthesis: Practical Di

Canadian pondweed can be used to measure the effect of light intensity on the rate of photosynthesis. The rate at which the pondweed produces oxygen corresponds to the rate at which it's photosynthesising - the faster the rate of oxygen production, the faster the rate of photosynthesis

Image result for measuring the rate of photosyntehsis practical canadian pond weed

The apparatus above can be altered to measure the effect of temperature or CO2 on photosynthesis. For example:

The test tube of pondweed can be put into a wather bath at a set temperature, or a measured amount of sodium hydrogencarbonate can be dissolved in the water (which gives off CO2)

The experiment can the be repeated with different temperatures of water/concentrations of sodium hydrogencarbonate

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Measuring the Rate of Photosynthesis: Inverse Law

The Inverse Square Law Links Light Intensity and Distnace

In the practical just described, when the lamp is moved away from the pondweed, the amount of light that reaches the pondweed decreases

You can say that as the distance increases, the light intensity decreases. In other words, distance and light intesntiy are inversely poportional to each other

However, it's not quite as simple as taht. It turns out that light intensity decreases in proportion to the square of the distance. This is called the inverse square law and is written out like this:

light intensity ∝ 1/distance2

The inverse law means that if you halve the distance, the light intesnity will be four times greatedr and if you divide the distance by three, the light intensity will be nine times greater

Likewise, if you double the distance, the light intensity will be four times smaller and if you treble the distance, the light intensity will be nine times smaller

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Measuring the Rate of Photosynthesis: Inverse Law

Use the inverse law to calculate the light intensity when the lamp is 10cm from the pondweed

light intensity ∝ 1/d2

= 1/102

= 1/100

= 0.01 a.u. (arbitary units)

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Ideal Conditions for Photosynthesis

You can Artificially Create the Ideal Conditions for Farming

1) The most common way to artificially create the ideal environment for plants is to grow them in a greenhouse

2) Greenhouses help to trap the Sun's heat, and make sure that the temperature doesn't becoming limiting. In winter a farmer or gardener might use a heater as well to keep the temperature at the ideal level. In summer it could get too hot, so they might use shades and ventilation to cool things down

3) Light is always needed for photosynthesis, so commerical farmers often supply artificial light after the Sun goes down to give their plants more quality photosynthesis time

4) Farmers and gardeners can also increase the level of carbon dioxide in the greenhouse. A fairly common way is to use a paraffin heater to heat the greenhouse. As the paraffin heater burns, it makes carbon dioxide as a by-product

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Ideal Conditions for Photosynthesis: Continued

5) Keeping plants enclosed in a greenhouse also makes it easier to keep them free from pests and diseases. The farmer can add fertilisers to the soil as well, to provide all the minerals needed for healthy growth

6) Sorting all this out costs money - but if the farmer can keep the conditions just right for photosynthesis, the plants will grow much faster and a decent crop can be harvested much more often, which can then be sold. It's important that a farmer supplies just the right amount of heat, light, et.c - enough to make the plants grow well, but not more than the plants need, as this would just be wasting money

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You need energy to keep your body going. Energy comes from food, and it's transferred by respiration

Respiration is Not 'Breathing In and Out'

Respiration involves many reactions. These are really important reactions, as respiration transfers the energy that the cell needs to do just about everything - this energy is used for all living processes

1) Respiration is the process of transferring energy from the breakdown glucose (sugar) - and it goes on in every cell in your body continuosly

2) It happens in plants too. All living things respire. It's how they transfer energy from their food to their cells

Respiration is the process of transferring energy from glucose, which goes on in every cell

3) Respiration is exothermic - it transfers energy to the environment

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Respiration: Continued

Respiration Transfers Energy for All Kinds of Things

1) To build up larger molecules from smaller ones (like proteins from amino acids)

2) In animals it's used to allow the muscles to contract (so they can move about)

3) In mammals and birds the energy is used to keep their body temperature steady in colder surroundings. (Unlike other animals, mammals and birds keep their bodies constantly warm)

Respiration releases energy from glucose - Cyanide is a toxin that stops respiration by stopping enzymes involved in the process from working - so it's pretty poisonous (it can kill you). Your brain, heart and liver are affected first becauase they have the highest energy demands

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Metabolism is All the Chemical Reactions in an Organism

1) In a cell there are lots of chemical reactions happening all the time, which are controlled by enzymes

2) Many of these reactions are linked together to form bigger reactions

3) In some of these reactions, larger molecules are made from smaller ones. For example:

(1) Lots of small glucose molecules are joined together in reactions to form starch (a storage molecule in plant ells), glycogen (a storage molecule in animal cells) and cellulose (a component of plant cell walls)

(2) Lipid molecules are made from one molecule of glycerol and three fatty acids

(3) Glucose is combined with nitrate ions to make amino acids, which are then made into proteins

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Metabolism: Continued

4) In other reactions, larger molecules are broken down into smaller ones. For example:

(1) Glucose is broken down in respiration. Respiration transfers energy to power all the reactions in the body that make moleules

(2) Excess protein is broken down in a reaction to produce urea. Urea is then excreted in urine

5) The sum of all of the reactions that happen in a cell or the body is called its metabolism

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Aerobic and Anaerobic Respiration

Aerobic Respiration Needs Plenty of Oxygen

1) Aerobic respiration is respiration using oxygen. It's the most efficient way to transfer energy from glucose

2) Aerobic respiration goes on all the time in plants and animals

3) Most of the reactions in aerobic respiration happen inside mitochondria

glucose + oxygen --> carbon dioxide + water

C6H12O6 + 6O2 --> 6CO2 + 6H2O

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Aerobic and Anaerobic Respiration: Continued 1

Anaerobic Respiration is Used if There's Not Enough Oxygen

When you do vigorous exercise and your boddy can't supply enough oxygen to your muscles, they start doing anaerobic respiration

1) 'Anaerobic' just means 'without oxygen'. It's the incomplete breakdown of glucose, making lactic acid

glucose --> lactic acid

3) Anaerobic respiration does not transfer nearly as much energy as aerobic respiration. This is because glucose isn't fully oxidised (because it doesn't combine with oxygen

4) So, anaerobic respiration is only useful in emergencies, e.g. during exercise when it allows you to keep on using your muscles for a while longer

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Aerobic and Anaerobic Respiration: Continued 2

Anaerobic Respiration in Plants and Yeast is Slightly Different

1) Plants and yeast cells can respire without oxygen too, but they produce ethanol (alcohol) and carbon dioxide instead of lactic acid

glucose --> ethanol + carbon dioxide

3) Anaerobic respiration in yeast cells is called fermentation

4) In the food and drinks industry, fermentation by yeast is of great value because it's used to make bread and alcoholic drinks, e.g. beer and wine

5) In bread-making, it's the carbon dioxide from fermentation that makes bread rise

6) In beer and wine-making, it's the fermentation process that produces alcohol

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1) Muscles need energy from respiration to contract. When you exercise, some of your muscles contract more frequently than normal so you need more energy. This energy comes from increased respiration

2) The increase in respiration in your cells mean you need to get more oxygen into them

3) Your breathing rate and breath volume increase to get more oxygen into the blood, and your heart rate increases to get this oxygenated blood around the body faster. This removes CO2 more quickly at the same time

4) When you do really vigorous exercise (like sprinting) your body can't supply oxygen to your muscles quickly enough, so they start respiring anaerobically

5) This is not the best way to transfer energy from glucose because lactic acid builds up in the muscles, which gets painful

6) Long periods of exercise also cause muscle fatigue - the muscles get tired and then stop contracting efficiently

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Exercise: Continued

Aerobic Respiration Leads to an Oxygen Debt

1) After resorting to anaerobic respiration, when you stop exercising you'll have an 'oxygen debt'

2) An oxygen debt is the amount of extra oxygen your body needs to react with the build up of lactic acid and remove it from the cells. Oxygen rects with the lactic acid to form harmless CO2 and water

3) In other words you have to 'repay' the oxygen that you didn’t get to your muscles in time, because your lungs, heart and blood couldn’t keep up with the demand earlier on

4) This means you have to keep breathing hard for a while after you stop, to get more oxygen into your blood, which is transported to the muscle cells

5) The pulse and breathing rate stay which whilst there are high levels of arctic acid and CO2

6) Your body also has another way of coping with the high level of lactic acid - the blood that enters your muscles transports the lactic acid to the liver. In the liver, the lactic acid is converted back to glucose

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Investigating The Effect of Exercise on Your Body

You Can Investigate The Effect of Exercise on The Body

1) You can measure breathing rate by counting breaths, and heart rate by taking the pulse

2) To take the pulse, you put two fingers on the inside of your wrist or your neck and count the number of pulses in one minute

3) E.g. you could take your pulse after: (you could plot your results in a bar chart)

  • Sitting down for 5 minutes
  • Then after 5 minutes of gentle walking
  • Then again after 5 minutes of slow jogging
  • Then again after running for 5 minutes

4) Your pulse rate will increase the more intense the exercise is, as your body needs to get more oxygen to the muscles and take more carbon dioxide away from your muscles

5) To reduce the effect of any random errors on your results, do it as a group and plot the average pulse for each exercise

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