How do plants make their food, why does that matter for life on Earth, and what are annual, biennial and perennial life cycles?
The word equation for photosynthesis and how to investigate the effect of light and chlorophyll on it, the importance of photosynthesis and rainforests for life on Earth, and the annual, biennial and perennial life cycles with examples.
A focused CCEA GCSE Agriculture and Land Use answer on photosynthesis and plant life cycles, covering the word equation for photosynthesis, investigating light and chlorophyll, the importance of photosynthesis and rainforests, and annual, biennial and perennial life cycles with examples.
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What this dot point is asking
CCEA wants you to give the word equation for photosynthesis, describe how to investigate the effect of light and chlorophyll, explain why photosynthesis (and rainforests) matter for life on Earth, and define annual, biennial and perennial life cycles with examples.
Photosynthesis
Plants make glucose (their food) using light energy, which is absorbed by the green pigment chlorophyll. The glucose is used in respiration for energy, stored as starch, or used to build the plant. Oxygen is released as a by-product.
Investigating light and chlorophyll
You test a leaf for starch as the sign that photosynthesis has happened. The method is: boil the leaf in water to kill it, boil it in ethanol to remove the chlorophyll (using a water bath, not a flame, because ethanol is flammable), soften it in hot water, then add iodine solution. A blue-black colour shows starch is present.
- To show light is needed: cover part of a leaf with foil, leave the plant in the light, then test the whole leaf. Only the exposed part turns blue-black.
- To show chlorophyll is needed: use a variegated leaf (part green, part white). Only the green parts turn blue-black, proving chlorophyll is needed.
Why photosynthesis matters for life on Earth
Rainforests are especially important: their huge mass of plants are major oxygen producers and carbon dioxide absorbers, helping to balance gases in the atmosphere and slow climate change, which is why their loss is such a concern.
Plant life cycles
Plants are grouped by how long their life cycle takes:
- Annual. Completes its whole life cycle in one year (germinate, grow, flower, set seed, die). Examples: wheat, many bedding plants.
- Biennial. Takes two years: grows leaves and stores food in year one, then flowers, sets seed and dies in year two. Examples: carrot, foxglove.
- Perennial. Lives for many years, flowering and setting seed year after year. Examples: fruit trees, grass, shrubs.
Growers choose the type to suit the purpose: annuals for a fast single crop, perennials such as grass or fruit for harvests over many years.
Examples in context
Example 1. A cereal crop is an annual. A wheat crop germinates in autumn or spring, grows, flowers, ripens its grain and dies all within one year, so the farmer must sow it fresh each season. Knowing it is an annual shapes the whole management plan, from sowing to a single harvest.
Example 2. Grass as a perennial. Grass in a grazing field is a perennial, so it regrows after grazing or cutting and lasts for several years before reseeding is needed. This is why grassland can support livestock year after year and is the backbone of Northern Ireland farming.
Try this
Q1. Write the word equation for photosynthesis. [2 marks]
- Cue. Carbon dioxide + water, using light energy, gives glucose + oxygen.
Q2. Give one reason photosynthesis is important for life on Earth. [1 mark]
- Cue. It produces oxygen for respiration, or it makes the food at the base of food chains, or it removes carbon dioxide.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA Unit 1 style4 marksState the word equation for photosynthesis and describe how you could show that chlorophyll is needed for it.Show worked answer →
Two marks for the equation and two for the chlorophyll method.
Word equation: carbon dioxide plus water, using light energy, produces glucose plus oxygen. Chlorophyll absorbs the light energy.
To show chlorophyll is needed, use a variegated leaf (one that is partly green and partly white). Leave the plant in the light, then test the whole leaf for starch: boil it in water to kill it, boil it in ethanol to remove the chlorophyll (using a water bath, not a flame, as ethanol is flammable), soften it in hot water and add iodine solution.
Only the parts that were green turn blue-black, showing starch is present there, while the white parts stay brown. This proves photosynthesis, which makes the starch, only happened where chlorophyll was present. Markers reward the correct equation, the variegated-leaf starch test, and the safe use of ethanol.
CCEA Unit 1 style3 marksExplain the difference between annual, biennial and perennial plants, giving an example of each.Show worked answer →
Three marks, one for each life cycle correctly described with an example.
An annual completes its whole life cycle, from germination to producing seeds and dying, within one year or growing season, for example a cereal such as wheat or a bedding plant.
A biennial takes two years: in the first year it grows leaves and stores food, and in the second year it flowers, sets seed and dies, for example a carrot or a foxglove.
A perennial lives for many years, flowering and producing seeds year after year, for example a fruit tree, grass or a shrub.
Markers reward the correct time span for each (one year, two years, many years) plus a suitable example for each type.
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