Photosynthesis
Photo = light
Synthesis = making
Photosynthesis means “making with light”
Tr. Khaing
Exothermic Describes a reaction that releases energy.
Endothermic Describes a reaction that takes in energy.
Chloroplast A green structure found inside plant cells where photosynthesis
occurs.
Phloem A tube that carries sugars from leaves to other parts of a plant.
Xylem A tube that carries water and minerals from a plant’s roots to its leaves.
Iodine A chemical used to detect starch. It turns starch dark blue.
Energy
❖ all plants need light
❖ use energy from light to make sugars such as glucose (stores energy)
❖ energy can be released using respiration when the plant’s cells need energy
❖ Plants also use glucose to build new cells
❖ The more light energy plants absorb, the more they photosynthesise and the more their
biomass increases
❖ Plant biomass keeps every animal alive – including us
❖ It supplies the energy and building materials that herbivores use for growth
❖ it gets passed along food chains to carnivores
Respiration and photosynthesis are opposite processes.
Respiration Photosynthesis
✓ Breaks down glucose ✓ Makes glucose
✓ In plants and animals use oxygen ✓ Returns oxygen to the atmosphere
✓ Exothermic reaction- it releases ✓ Endothermic reaction- it takes in
energy light energy
Chloroplasts
▪ Photosynthesis takes place in chloroplasts
(leaf cells)
▪ cells take carbon dioxide from the air
▪ most leaves are very thin so gases diffuse in
and out quickly
▪ The water needed for photosynthesis is
brought up from the roots in xylem vessels
Evidence
▪ Seedlings need light
▪ in the dark they produce tall, weak shoots and tiny yellow leaves
▪ uses up their energy supply, the plants die after a few days
▪ in the sunlight have short sturdy stems
▪ use their first leaves to photosynthesise and produce the glucose they need to keep
growing
▪ Some of the sugars made by leaf cells are moved to cells that have no chloroplasts
▪ sugars travel through the plant in phloem tubes and other sugars are converted to
starch and stored in leaves
▪ Iodine can show where photosynthesis has taken place (turn starch dark blue)
Q&A
1. Explain why animals depend on plants to keep them alive.
2. Describe how carbon dioxide and water reach the chloroplasts in leaves.
3. Describe two pieces of evidence that show that plants cannot make their own food
without light.
4. Starch tests show where photosynthesis has taken place. What can you conclude using
the evidence from test 1?
5. What does the evidence from test 3 show?
6. The amount of oxygen in a sample of air can be measured using an electronic sensor.
How would you expect it to change around a sugar cane plant on a sunny day?
