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Cambridge IGCSE0610

Photosynthesis

Biology 0610 Chapter Notes

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In this chapter

  1. 1Photosynthesis and chlorophyll
  2. 2Uses and storage of carbohydrates
  3. 3Nitrate and magnesium ions
  4. 4Chlorophyll, light and carbon dioxide
  5. 5Gas exchange in an aquatic plant
  6. 6Factors affecting the rate
  7. 7The balanced equation
  8. 8Limiting factors
  9. 9Large and thin
  10. 10The layers of a leaf, top to bottom
  11. 11Palisade mesophyll
  12. 12Spongy mesophyll and air spaces
  13. 13Stomata and guard cells
  14. 14Vascular bundles: xylem and phloem

1. Photosynthesis and chlorophyll

Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. The raw materials are carbon dioxide and water.

Chlorophyll is a green pigment that is found in chloroplasts. It transfers energy from light into energy in chemicals, for the synthesis of carbohydrates.

carbon dioxide + water xrightarrow[chlorophyll]light glucose + oxygen

Examiner insight

“Many candidates found completing the word equation for photosynthesis challenging. A common error was to give the word equation for respiration.” “Many stated it was used to trap light instead of light energy and others described light in terms of sunlight or the Sun which was not acceptable.”

Worked example 12 marks

Plants need chlorophyll to photosynthesise.

Describe the role of chlorophyll in photosynthesis.

  1. 1

    transfers energy from light into energy in chemicals [1]

  2. 2

    for the synthesis of carbohydrates / glucose [1]

2. Uses and storage of carbohydrates

The carbohydrates made in photosynthesis are used or stored in five ways.

Starch is an energy store. Cellulose is used to build cell walls. Glucose is used in respiration to provide energy.

Sucrose is for transport in the phloem. Nectar attracts insects for pollination.

Examiner insight

“General ideas such as ‘for food’ and ‘growth’ were insufficiently detailed.” “Many candidates gave inaccurate functions of starch, sometimes referring to starch as a store of food rather than energy.”

Worked example 13 marks

The carbohydrate made during photosynthesis is made into larger carbohydrates with specific functions.

Complete Table 2.1.

carbohydratefunction in a plant
starch
cellulose
translocated in the phloem
  1. 1

    starch: energy store [1]

  2. 2

    cellulose: builds cell walls [1]

  3. 3

    sucrose: translocated in the phloem [1]

3. Nitrate and magnesium ions

Plants take in mineral ions to make the substances they need.

Nitrate ions are needed to make amino acids. Amino acids are used to make proteins, which are needed for growth.

Magnesium ions are needed to make chlorophyll. A specific function earns the mark. ‘Growth’ alone does not.

Examiner insight

“Many candidates stated that magnesium ions are used to make chloroplasts, this answer was not accepted.” “Some statements, such as ‘for good growth’ are too vague to gain any credit.”

Worked example 12 marks

Explain why plants need magnesium ions to photosynthesise.

  1. 1

    make chlorophyll [1]

  2. 2

    transfers light (energy) into chemical energy [1]

4. Chlorophyll, light and carbon dioxide

Glucose made in photosynthesis is converted into starch for storage, so a leaf that has photosynthesised contains starch. Starch gives a blue-black colour with iodine solution. With no starch, the iodine solution stays yellow-brown.

Each investigation changes one thing and keeps a control for comparison. For chlorophyll, a variegated leaf has green parts with chlorophyll and white parts without. For light, one part of a leaf is kept in the dark while the rest is lit. For carbon dioxide, soda lime absorbs it from the air around one leaf, and a control leaf keeps it.

Examiner insight

“The most common error was for candidates to state the colour of a positive starch test as blue, which is too vague. The colour of a positive starch test is blue-black.” “Candidates should also be reminded that iodine solution, rather than iodine (which is a solid at room temperature) is used for this test.”

Worked example 14 marks

A leaf with green and white areas is left in bright light. It is then tested for starch with iodine solution.

State the colour of each area after the test and explain the result.

  1. 1

    green area: blue-black, starch is present [1]

  2. 2

    white area: stays yellow-brown, the colour of the iodine solution [1]

  3. 3

    green area has chlorophyll, so it photosynthesises and stores the glucose as starch [1]

  4. 4

    white area has no chlorophyll, so no photosynthesis and no starch [1]

5. Gas exchange in an aquatic plant

Hydrogencarbonate indicator solution changes colour as the carbon dioxide concentration changes. When carbon dioxide decreases it turns purple. When carbon dioxide increases it turns yellow.

In the light, photosynthesis takes in carbon dioxide and releases oxygen. Photosynthesis and respiration both occur in the light, but the rate of photosynthesis is higher.

In the dark, only respiration occurs. Carbon dioxide is released and oxygen is used.

Examiner insight

“The reduction in carbon dioxide in the indicator solution will turn it purple.” “A significant number of candidates incorrectly believed that photosynthesis occurs in the light, but respiration does not, or that respiration can only occur when photosynthesis is not taking place.”

Worked example 14 marks

An aquatic plant is sealed in a test-tube of hydrogencarbonate indicator solution. The test-tube is wrapped in aluminium foil for several hours.

State the colour of the indicator after this time and explain the change.

  1. 1

    yellow [1]

  2. 2

    no light, so no photosynthesis [1]

  3. 3

    the plant respires and releases carbon dioxide [1]

  4. 4

    the carbon dioxide concentration increases [1]

6. Factors affecting the rate

The rate of photosynthesis is measured as bubbles of oxygen per minute, or as the volume of oxygen produced in a set time. Change one factor and keep the others constant.

A higher light intensity or a higher carbon dioxide concentration raises the rate, up to a maximum. With temperature, the rate increases then decreases. The highest rate is at the optimum temperature.

Examiner insight

“Rather than referring to the rate of photosynthesis increasing or decreasing, a significant number of candidates referred to ‘the line’ going up or down.” “Temperature affects the rate of photosynthesis so it must be controlled to ensure that the carbon dioxide concentration is the only variable that changes in the investigation.”

Worked example 13 marks

A student investigates the effect of temperature on the rate of photosynthesis in a leaf.

Figure 2.1 shows the results of the investigation.

Use Figure 2.1 to describe the effect of temperature on the rate of photosynthesis.

Use data from Figure 2.1 in your answer.

  1. 1

    the rate increases then decreases [1]

  2. 2

    between 10 ^(°)C and 25 ^(°)C the rate increases from 2 to 5 arbitrary units [1]

  3. 3

    after 35 ^(°)C the rate decreases, reaching zero at 55 ^(°)C [1]

7. The balanced equation

The balanced chemical equation uses formulae. Carbon dioxide and water are on the left. Glucose and oxygen are on the right.

There are six molecules of carbon dioxide, six of water and six of oxygen for each molecule of glucose.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Examiner insight

“Some candidates included carbon dioxide in the products. Some candidates did not balance the equation.” “Errors included giving the word equation, writing an equation that was not balanced, for example by including a ‘6’ before glucose, and giving the equation for aerobic respiration.”

Worked example 12 marks

State the balanced chemical equation for photosynthesis.

  1. 1

    6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ with the correct formulae on each side [1]

  2. 2

    balanced: 6 carbon, 12 hydrogen and 18 oxygen atoms on each side [1]

8. Limiting factors

A limiting factor is something in the environment that is in short supply, restricting life processes.

On a graph of rate against light intensity, light intensity is the limiting factor where the rate increases. Where the rate is constant, light intensity is no longer limiting. Another factor is limiting, such as carbon dioxide concentration or temperature.

Examiner insight

“Few candidates were familiar with the term limiting factor to describe something in the environment that is in short supply, restricting life processes.” “This suggests that candidates did not understand that the plateau region of the graph is where light intensity is no longer a limiting factor.”

Worked example 11 mark

A student investigated the rate of photosynthesis in tomato plants.

The graph shows the results.

Which factor does the graph indicate is limiting the rate of photosynthesis at X?

A carbon dioxide concentration · B light intensity

C oxygen concentration · D temperature

  1. 1

    B: light intensity. At X the rate is still increasing as light intensity increases. [1]

Recap

  • Photosynthesis makes carbohydrates from carbon dioxide and water, using energy from light.
  • Word equation: carbon dioxide + water, in light and chlorophyll, gives glucose + oxygen.
  • Chlorophyll, a green pigment in chloroplasts, transfers energy from light into energy in chemicals.
  • Carbohydrates: starch stores energy, cellulose builds cell walls, sucrose is transported in the phloem.
  • Nitrate ions make amino acids. Magnesium ions make chlorophyll.
  • Starch is blue-black with iodine solution. Hydrogencarbonate indicator is purple at low carbon dioxide, yellow at high.
  • More light or carbon dioxide: rate rises, then levels off. Higher temperature: rate rises, then falls.
  • Extended: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
  • Extended: where the rate is constant, light intensity is no longer the limiting factor.

9. Large and thin

Most leaves have a large surface area and are thin. Both features are adaptations for photosynthesis.

With a large surface area, a leaf can absorb light for more photosynthesis. A large surface area also increases gas exchange.

Examiner insight

“In general, candidates knew that the large surface area of a leaf was connected to the absorption of light for increased photosynthesis or the production of glucose.” “Fewer candidates linked the larger surface area to increased gas exchange, though this was equally valid.”

Worked example 11 mark

How are most leaves adapted for photosynthesis?

A They have a large surface area and are thick. · B They have a large surface area and are thin. · C They have a small surface area and are thick. · D They have a small surface area and are thin.

  1. 1

    B: a large surface area and thin [1]

10. The layers of a leaf, top to bottom

A section through a leaf has its layers in a fixed order. From the top surface: the cuticle, the upper epidermis, the palisade mesophyll, the spongy mesophyll and the lower epidermis.

The cuticle lies above the epidermis. It is not a tissue. The cells of the epidermis are transparent, so light can pass through them.

Vascular bundles lie within the spongy mesophyll.

Examiner insight

“Many thought that the epidermis is above the cuticle or that the spongy mesophyll layer is above the palisade mesophyll layer.” “The majority of candidates were able to identify the epidermis, but the palisade mesophyll layer was frequently identified as the spongy mesophyll, or just mesophyll which was too ambiguous.”

Worked example 13 marks

A section is cut through a leaf and examined from the top surface downwards. Name the layer directly below the cuticle. Then name the two mesophyll layers in the order they are met.

  1. 1

    upper epidermis [1]

  2. 2

    palisade mesophyll (not 'mesophyll' alone) [1]

  3. 3

    spongy mesophyll [1]

11. Palisade mesophyll

The palisade mesophyll is near the top of the leaf, directly below the upper epidermis. It absorbs light for photosynthesis.

Its cells contain many chloroplasts, more than other cells in the plant. The chloroplasts are arranged around the edge of the cell.

The cells are closely packed. Say that the cells are closely packed, not the tissue.

Examiner insight

“There was sometimes confusion between the adaptations of the leaf and the adaptations of the palisade layer.” “A common misconception was to refer to the tissue being tightly packed rather than the cells within the palisade layer being tightly packed.”

Worked example 11 mark

Which structure in a leaf is adapted for photosynthesis by being near the upper surface and containing a large number of chloroplasts?

A epidermis cell · B guard cell · C palisade mesophyll cell · D xylem vessel

  1. 1

    C: the palisade mesophyll cell is near the top of the leaf and has many chloroplasts [1]

12. Spongy mesophyll and air spaces

The spongy mesophyll lies below the palisade mesophyll. Like palisade mesophyll cells, spongy mesophyll cells contain chlorophyll.

There are air spaces between the spongy mesophyll cells. Air spaces allow gases to diffuse through the leaf for photosynthesis.

Carbon dioxide enters the leaf through the stomata. It then diffuses through the air spaces to the spongy mesophyll cells.

Examiner insight

“Instead of mesophyll cells, common answers provided were plant cells or leaf cells, and instead of air spaces, candidates stated tiny spaces or empty spaces.” “Candidates should be encouraged to use the terminology given in the syllabus. The term ‘air gap’ was ignored.”

Worked example 13 marks

Describe the pathway carbon dioxide takes from the air outside a leaf to a spongy mesophyll cell.

  1. 1

    carbon dioxide enters the leaf through the stomata [1]

  2. 2

    it diffuses (not ‘is absorbed’) through the air spaces [1]

  3. 3

    to the spongy mesophyll cells [1]

13. Stomata and guard cells

Stomata (one is a stoma) are in the epidermis. Stomata are not cells. Most stomata are on the underside of the leaf, with very few on the upper surface.

Stomata are for gas exchange. Carbon dioxide diffuses into the leaf through the stomata. Oxygen, a product of photosynthesis, moves out through them.

Each stoma has two guard cells around it. Guard cells contain chlorophyll. The other cells of the epidermis have no chloroplasts.

Examiner insight

“Most candidates incorrectly selected option B, indicating that there are three guard cells in the diagram. There are in fact six; two guard cells around each stoma.” “Frequent incorrect responses were stoma, epidermal cell and leaf cell.”

Worked example 12 marks

A piece of lower epidermis has 5 stomata. State the number of guard cells. Name the structure found in a guard cell that is absent from the other cells of the epidermis.

  1. 1

    5 × 2 = 10 guard cells [1]

  2. 2

    chloroplasts [1]

14. Vascular bundles: xylem and phloem

A vascular bundle contains xylem and phloem. The vascular bundle is the whole group of tissues, not the xylem alone.

Xylem transports water and mineral ions to the leaf. It also gives support.

Examiner insight

“Candidates had more difficulty identifying the vascular bundle with several incorrect references to xylem or mesophyll.” “The function that was commonly missed was the structural support function of xylem.”

Worked example 14 marks

Name the two tissues found in a vascular bundle of a leaf. State two functions of the xylem.

  1. 1

    xylem [1]

  2. 2

    phloem [1]

  3. 3

    xylem transports water and mineral ions [1]

  4. 4

    xylem gives support [1]

Recap

  • Most leaves have a large surface area and are thin. A large surface area absorbs more light and increases gas exchange.
  • Layers from the top: cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis.
  • The cuticle lies above the epidermis and is not a tissue.
  • Palisade mesophyll: near the top of the leaf, many chloroplasts, closely packed cells.
  • Spongy mesophyll cells have air spaces between them, so gases diffuse through the leaf.
  • Stomata are not cells. Each stoma has two guard cells, and guard cells contain chlorophyll.
  • A vascular bundle contains xylem and phloem. Xylem transports water and mineral ions and gives support.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State the word equation for photosynthesis.2 marks
  2. Name the green pigment in a leaf and the structure that contains it. State what the pigment does.3 marks
  3. State the function in a plant of (a) starch, (b) cellulose, (c) sucrose.3 marks
  4. Explain the importance of (a) nitrate ions and (b) magnesium ions to a plant.3 marks
  5. The leaves of a plant contain no starch. A strip of black paper is clipped across one leaf. The plant is kept in bright light. The leaf is then tested with iodine solution. State the colour of the covered strip and of the rest of the leaf, and explain the difference.3 marks
  6. An aquatic plant is sealed in a test-tube of hydrogencarbonate indicator solution and left in bright light for several hours. The indicator turns purple. Explain why.3 marks
  7. A student collects the oxygen given off by pondweed for 5 minutes at four temperatures. (a) Calculate the rate of photosynthesis at 30 °C in cm³ / min. (b) State the effect on the rate of raising the temperature from 30 °C to 40 °C. (c) State one factor that must be kept constant. | temperature / °C | volume of oxygen in 5 minutes / cm³ | | --- | --- | | 10 | 2.0 | | 20 | 5.0 | | 30 | 9.5 | | 40 | 3.0 |4 marks
  8. A student writes the equation for photosynthesis as 6CO₂ + 6H₂O → 6C₆H₁₂O₆ + 6O₂. (a) Identify the mistake and use the number of carbon atoms on each side to show that the equation is not balanced. (b) State the balanced chemical equation for photosynthesis.4 marks
  9. A scientist measures the rate of photosynthesis of lettuce plants in a glasshouse as the light intensity increases. She does this in three sets of conditions. The table shows the conditions for lines P, Q and R. The graph shows the results. (a) State the limiting factor for all three lines at a light intensity of 2 arbitrary units. Explain how the graph shows this. (b) Identify the limiting factor for line P at a light intensity of 9 arbitrary units. Use lines P and Q to explain your answer. (c) Identify the limiting factor for line Q at a light intensity of 9 arbitrary units. Use lines Q and R to explain your answer. | line | carbon dioxide concentration / % | temperature / °C | | --- | --- | --- | | P | 0.04 | 15 | | Q | 0.12 | 15 | | R | 0.12 | 25 |6 marks
  10. A grower measures the rate of photosynthesis of strawberry plants in a glasshouse in six tests. The table shows the conditions and the results. (a) The grower says, ‘The plants in test 3 are in the brightest light, so light intensity limits the rate in test 3.’ Use tests 2, 3 and 4 to explain why the grower is wrong, and identify the limiting factor in test 3. (b) Identify the limiting factor in test 5. Use the results to explain your answer. (c) The grower runs a new test at a light intensity of 20 arbitrary units, 0.08% carbon dioxide and 20 °C. Use the idea of limiting factors to predict the rate. Explain your answer. | test | light intensity / arbitrary units | carbon dioxide concentration / % | temperature / °C | rate of photosynthesis / arbitrary units | | --- | --- | --- | --- | --- | | 1 | 20 | 0.04 | 20 | 24 | | 2 | 40 | 0.04 | 20 | 30 | | 3 | 80 | 0.04 | 20 | 30 | | 4 | 80 | 0.08 | 20 | 45 | | 5 | 80 | 0.16 | 20 | 45 | | 6 | 80 | 0.16 | 30 | 60 |7 marks
  11. State two features of most leaves that adapt them for photosynthesis.2 marks
  12. Name the structure inside a palisade cell where photosynthesis takes place.1 mark
  13. Put these in the order they are met from the top of a leaf downwards: palisade mesophyll, cuticle, spongy mesophyll, upper epidermis.3 marks
  14. A student writes ‘stomata are cells in the epidermis’. State what is wrong with this. Name the cells found around each stoma.2 marks
  15. Explain how a large surface area helps a leaf to photosynthesise.2 marks
  16. A student says ‘the vascular bundle is the xylem’. State what is wrong with this. State two functions of xylem.3 marks
  17. Describe how palisade mesophyll is adapted for photosynthesis.3 marks
  18. Oxygen is made in a spongy mesophyll cell in the light. Describe how it leaves the leaf.3 marks
  19. A student peels the lower epidermis from a leaf and looks at it under a microscope. In an area of 0.25 mm² she counts 45 stomata. (a) Calculate the number of stomata per mm². (b) Calculate the number of stomata per cm². (c) Calculate the number of guard cells in 1 cm² of this epidermis. (d) The same area of upper epidermis from this leaf has 2 stomata. State what this tells you about where most stomata are. State why a leaf needs stomata for photosynthesis.5 marks
  20. A student looks at a section through a leaf under a microscope. She numbers the five layers from the top surface downwards. Her notes are in the table. (a) Name layers 1, 3 and 4. (b) Calculate how many times more chloroplasts a cell in layer 3 has than a cell in layer 4. (c) Use the table to state two ways layer 3 is adapted for photosynthesis. (d) Layer 5 is the lower epidermis. Name its paired cells and the gap between each pair. | layer | what the student sees | chloroplasts in one cell | | --- | --- | --- | | 1 | a thin layer covering the top of the leaf | 0 | | 2 | one row of cells | 0 | | 3 | long cells, closely packed | 48 | | 4 | cells with large air spaces between them | 16 | | 5 | one row of cells, some in pairs with a gap between each pair | 0, except 6 in each paired cell |8 marks

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