Photosynthesis Revision

Photosynthesis Revision

Y11 GCSE

Recall

• For producers to "make their own food" they need to make the sugar glucose

• To do that they need to do photosynthesis

• Photosynthesis is a chemical reaction that uses light energy to drive the reaction forward

• Plants absorb carbon dioxide from the air and water through their roots and react them together in their leaves to produce oxygen and glucose

Recall

• Plants use the glucose from photosynthesis

• to respire

• to grow

• to store in roots or put in seeds

Multiple Select

What substances are the reactants in the photosynthesis equation?

• light

• sugar (glucose)

• carbon dioxide

• water

Multiple Select

What substances are the products of the photosynthesis reaction?

• sugar (glucose)

• carbon dioxide

• oxygen

• water

The rate of photosynthesis

• The "rate" of anything is a measure of how fast something happens

• For example how fast a plant grows over time

• The chemical reaction of photosynthesis (like all chemical reactions) can have a fast rate or a slow rate

• Let's look at what affects how fast or slow photosynthesis works

Factors that affect the rate of photosynthesis

• How long a plant gets light

• The brightness of the light (known as light intensity)

• The temperature of the air around the plant

• The amount of carbon dioxide around

• The amount of water in the soil

A quick note on light intensity received by an object

• It is affected by 3 things:

• How bright the light source is

• How far away the light source is

• If anything is between the light source and the object it is lighting

Multiple Choice

Will the rate of photosynthesis be higher on a cloudy day or a sunny day?

Multiple Choice

Will the rate of photosynthesis be faster on a cold day or a warm day?

Multiple Choice

Will the rate of photosynthesis be faster when there's lots of carbon dioxide or little carbon dioxide?

• Lots of carbon dioxide

• Little carbon dioxide

• no difference

Multiple Choice

Wil a plant that lives close to a tree have the same rate of photosynthesis than a plant that lives further from the tree?

• It will be faster

• It will be slower

• There will be no difference

Gas exchange in plants

The gases of photosynthesis enter and exit on the underside of leaves though small holes called stomata through diffusion. Water is also lost this way through evaporation.

Stomata

Stomata can open and close depending on the two cells that surround it. When they are full of water they bulge and stay open. When they are lacking water they shrink and shut the stomata, so no more water is lost.

Multiple Choice

What are the names of the 2 cells that surround the stomata?

• Sentry cells

• Stoma cells

• Guard cells

• Sausage cells

Multiple Choice

Why is it important that the plant close the stomata when there is not enough water in the soil?

• So it doesn't overheat

• It lowers the amount of water lost

• So that oxygen can't escape

Measuring the rate of photosynthesis

• Set up apparatus as shown

• Elodea is an underwater plant (pondweed) which releases oxygen as bubbles

• Using a stopwatch, you can count how many bubbles are released over time. This is the rate of photosynthesis.

Distance of lamp vs. no. of bubbles

• As the distance increases the number of bubbles produced decreases

• Notice it is not a straight line

• As the lamp gets closer there is less of an effect on the number of bubbles produced

Multiple Select

What variables should be controlled in the experiment?

• Distance of lamp from pondweed

• Temperature of water

• Amount of carbon dioxide available

• The colour of the light

• The amount of time you count the bubbles for

Well done!