GCSE Biology - Photosynthesis

Cell organelle important for photosynthesis

Chloroplasts - contain chlorophyll, which absorbs light energy for photosynthesis

Photosynthesis word equation

Carbon dioxide + water —> glucose + oxygen

Photosynthesis symbol equation

6CO2 + 6H2O —> C6H12O6 + 6O2

Why is photosynthesis described as an endothermic reaction?

It needs an input of energy from the envirnonment, since the energy used to break the bonds holding the CO2 and H2O is greater than the energy given back when new bonds are formed in glucose and oxygen. The extra energy required is transferred from the environment by light.

What do leaves need to be effective at photosynthesis?

• Large SA to absorb light

• Many chloroplasts containing chlorophyll

• A supply of water and carbon dioxide

• A system for carrying away the products of photosynthesis to other parts of the plant

Function of the upper epidermis

Produces the waxy cuticle

Function of the waxy cuticle

Reduces the water loss from the surface of the leaf

Function of the palisade layer

It is the main photosynthetic layer with many chloroplasts

Function of the spongy mesophyll

Has air spaces to allow CO2 to diffuse through

Function of the veins in a plant

Bring plenty of water in the xylem to the cells of the leaves and remove the products of photosynthesis in the phloem

Function of the guard cells

Open and close the stomata to regulate gas exchange

Function of the stomata

Opening in the leaf’s surface to allow gaseous exchange, oxygen and water out, carbon dioxide in

Function of the air spaces

Allow carbon dioxide to get to the cells and oxygen to leave by diffusion

Diffusion

The movement of/spreading out of particles from a high to low concentration

How do the stomata open and close?

Ions increase the concentration of the solution which decreases the water potential inside the guard cells, so water moves into the cell by osmosis down a concentration gradient and the cell swells unevenly as the inner wall is less flexible than the thicker outer wall.

How would you test for the presence of oxygen?

By inserting a glowing splint into a test tube into which a gas has been released. If the gas is oxygen, the splint will relight.

Explain the method of testing a leaf for starch

Place the leaf in boiling water for about 1 minute, in order to burst the cells and stop the chemical reactions.

Then, turn off the bunsen burner and place the leaf in a test tube half full with ethanol, before placing the test tube into a bath of hot water for 10 minutes. This is to dissolve the chlorophyll, removing the green colour from the leaf.

Return the leaf to the hot water for about a minute to soften the leaf and then spread the leaf out on a petri dish and cover it with iodine solution to test for the presence of starch.

Explain how you would investigate if light is needed for photosynthesis

Would place a palnt in the dark for 48 hours to de starch it, and I would place a strip of foil over part of the leaf to exclude light on both the upper and lower surface, before watering the plant and placing it in bright light for 24 hours, then testing it for the presence of starch. When the iodine solution is applied, the uncovered areas will turn blue black, indicating a presence of starch.

Why does light lead to the production of starch by plants?

Light is needed to produce glucose via photosynthesis, and the glucose is then converted into starch.

Variegated leaf

Some parts of the leaf have chlorophyll while others do not

Explain how you would investigate if chlorophyll is needed for photosynthesis

I would first de-starch a variegated plant by placing it in the dark for 48 hours, and then water it and put it in bright light for 24 hours, before testing the leaf for the presence of starch. The starch will only be found in the areas where the leaf had chlorophyll.

De-starching

A process that ensures that any starch found at the end of the test was produced during the investigation, and so that the plant was actively photosynthesising during it. Plants are left in the dark for 48 hours, which leads to any stored starch being converted to glucose to be used up in respiration since photosynthesis cannot take place in the dark.

Why is starch only located in the areas of a leaf that contain chlorophyll?

Because light is absorbed by the chlorophyll to provide energy for photosynthesis which produces glucose and is then converted into starch for storage

How would you investigate if carbon dioxide is needed for photosynthesis?

I would destarch 2 plants and place them each in a polythene bag.

In one, I would place sodium hydroxide (soda lime), which would absorb carbon dioxide from the air, and in the other I would place saturated sodium hydrogen carbonate, which gives out carbon dioxide.

I would then leave the plants out in the light for 24 hours, and then test a leaf from each plant for the presence of starch.

Factors that can limit the rate of photosynthesis

• Temperature

• CO2 concentration

• Light intensity and wavelength

• Chlorophyll levels

How does temperature affect photosynthesis?

Photosynthesis is controlled by enzymes and they are destroyed at a temperature of around 45 degrees, so photosynthesis will increase as temperature does until the point where the enzymes are denatured.

How does the concentration of CO2 affect photosynthesis?

As CO2 concentration increases, the rate of photosynthesis proportionally increases, until it reaches a point where another factor becomes limiting.

How does light intensity affect photosynthesis?

As light intensity increases, the rate of photosynthesis proportionally increases, until it reaches a point where another factor becomes limiting.

How does the wavelength of light affect photosynthesis?

Wavelength affects the colour of the light, and plants reflect green light (looking green), but they absorb and require sufficient levels of red and blue light.

How can the rate of photosynthesis be measured?

By measuring the volume of oxygen produced through a gas syringe in a set time, or by producing the number of bubbles produced per minute, or by measuring the volume of carbon dioxide lost in a set amount of time.

Hydroponics

The practice of growing plants in either a bath or flow or highly oxygenated, nutrient enriched water, without any soil. The water is kept flowing to circulate minerals to the roots, and the water is well oxygenated so that the roots can respire producing the energy needed to take up the minerals by active transport.

Positives of using hydroponic system in greenhouses

• Can change levels of limiting factors

• Turnover is fast, meaning that profits are high

• Crops are clean and soil free

• No need to prep or plough land

• Crops can be grown in areas where the land is poor

Disadvantages to using hydroponics in a commercial greenhouse

• Cost of using gas and electricity to maintain lighting, temp and CO2 levels

• Cost of expensive monitoring equipment

• Size of business and space required

Why is the crop yield always higher when they are grown hydroponically?

Eliminates limiting factors so more photosynthesis takes place

Why would you control the length of the pondweed during the RP?

More chlorophyll available from a larger sample so more photosynthesis if it is longer, control by measuring to 10cm

Why would you control the temperature during the RP?

A change in temperature will change the rate of the enzyme controlled reactions

Why would you control the concentration of sodium hydrogen carbonate solution in the RP?

Affects the concentration of CO2 in water which will change the rate of photosynthesis, control by using a 0.2% solution of NaHCO3.

How was the rate of photosynthesis (dependent variable) measured in the RP?

By counting the number of oxygen bubbles produced per minute

How could you improve the accuracy of measuring the dependent variable in the RP?

Oxygen bubbles vary in size so they don’t have a constant volume, could improve by using a gas syringe

What was the function of the NaHCO3 that was added to the water in the RP?

Breaks down to release CO2 which will ensure a constant concentration is being maintained.

How to find light intensity?

1000/distance²

How does respiration use glucose?

Plant and algal cells use some of the glucose produced during photosynthesis to respire and the glucose is broken down using oxygen to provide energy to the cells.

How is cellulose linked to starch?

Energy from respiration converts glucose into cellulose, which strengthens cell walls, and plants convert some glucose into starch. Unlike glucose, starch is insoluble in water, and so does not affect the water balance, so starch is stored in the leaves, tubers and bulbs.

How do nitrates and proteins link to carnivorous plants?

Plants combine glucose with nitrates and other mineral ions to make amino acids, which then form proteins and enzymes using energy from respiration. Algae also does this by taking nitrate ions from the water. Plants need a sufficient supply of minerals to survive, and carnivorous plants adapt to live in mineral deficient areas by digesting insects in order to supply the needed nutrients.