TOPIC 2 UNIT 5 - duncanrig.s-lanark.sch.uk · Web viewNational 5 BiologyPhotosynthesis. ... Write...

Unit 3: Life on EarthSub-Topic 3.3: Photosynthesis

On completion of this sub-topic I will be able to: State that photosynthesis is the chemical process by which green plants

make sugar, a carbohydrate. Identify the raw materials for photosynthesis as carbon dioxide and

water. Identify the requirements for photosynthesis as light and chlorophyll. Identify the end products as sugar and oxygen. Write the word equation for photosynthesis. State that photosynthesis takes place in the chloroplast. State that photosynthesis has two stages called the light reactions and

carbon fixation. Identify the first stage of photosynthesis as the light reaction. State that during the light reaction, water is split to produce hydrogen and

oxygen. Oxygen is a waste product and diffuses out of the cell. This process is called photolysis.

State that during the light reaction, light energy from the sun is trapped by chlorophyll in the chloroplasts and is converted into chemical energy which is used to generate ATP.

Identify the second stage of photosynthesis as the carbon-fixation stage. State that carbon fixation is a series of enzyme controlled reactions. State that during carbon fixation, the hydrogen from stage one is combined

with carbon dioxide from the air to form sugar. State that ATP (from stage one) provides the energy used in the production

of this sugar. The chemical energy stored in sugar is available for respiration. Any sugar not used in respiration can be converted into other plant products

such as starch (a storage carbohydrate) and cellulose (a structural component of the plant cell wall).

State that a limiting factor is a variable which, when in short supply, reduces or limits the rate of photosynthesis taking place.

State that the rate of photosynthesis and therefore plant growth can be limited by carbon dioxide concentration, light intensity and temperature. These affect photosynthesis and therefore plant growth.

Analyse limiting factor graphs.

National 5 Biology Photosynthesis

The Structure of the Leaf and Photosynthesis

Photosynthesis is the process by which green plants make their own food.

Carbon dioxide from the air and water from the soil are combined in the presence of light energy to produce sugar and oxygen.

The light energy is absorbed by the green pigment chlorophyll, which is found in the chloroplasts in green leaf cells.

Chloroplast structure

A chloroplast is a disc like structure containing a mixture of pigments including chlorophyll which absorb light energy during the first stage of photosynthesis.Label the parts indicated on the diagram.

Chloroplast

Duncanrig Secondary School 2018 of 21

chloroplast


The leaf is adapted to its role in photosynthesis.

Complete the table below using the class resources.

Structure Function Adaptation

Upper epidermis

Protects leaf and allows light to pass through to the layer below

Transparent

Palisade mesophyll

Absorb light energy for photosynthesis

Column shaped and tightly packed to maximise the number present. Densely packed with chloroplasts for maximum light absorption

Spongy mesophyll Gas exchange

Irregular shape creates spaces filled with moist air between cells

Guard cells Open and close stomataSausage shaped cells. Lose water, become flaccid and close stoma, gain water, become turgid and open stoma

StomaAllows the carbon dioxide to enter the leaf and oxygen to exit the leaf

Pore which is opened during the day when light is available for photosynthesis and closed at night when gas exchange is no longer required

Veins

1. Xylem

2. Phloem

Transport system of leaf

Transports water from root to leaf

Transports dissolved sugar from the leaf

Narrow, hollow tube reinforced with spirals of lignin

Continuous strands of cytoplasm flow from one phloem cell to the next phloem cell

Photosynthesis word equation


raw materials

products

light energy

chlorophyll

essential requirements

carbon dioxide

sugar oxygen

water ++


Can photosynthesis take place if one of the raw materials or essential requirements is missing? To investigate this, the following experiment was set up. All the plants below were initially de-starched, then put into the light for 24 hours under the following conditions and tested for the presence of starch using iodine solution.

Is light necessary?Set up Presence of starch Conclusion

Is chlorophyll necessary?Set up Presence of starch Conclusion


Green (chlorophyll)

Variegated plant

Clear plastic bag

Black plastic bag

Clear plastic bag

White (no chlorophyll)

PLANT B

PLANT A

PLANT E

Is carbon dioxide necessary?Set up Presence of starch Conclusion

Clear plastic bag

Clear plastic bag

Sodium hydroxide solution

Water

PLANT C

PLANT D


The Chemistry of Photosynthesis

Photosynthesis can be divided into two stages.

1. The light reactions2. Carbon- fixation

Stage One: The light reaction

The light reaction occur in the chloroplasts.

During stage one, water is split into hydrogen and oxygen using light energy trapped by chlorophyll in the chloroplasts of the plant cells. This is called photolysis.

The oxygen which results from this process is not required and diffuses out of the cell, out of the leaf and into the atmosphere.

The hydrogen produced plays an essential role in the second stage to produce sugar.

Some of the light energy trapped by the chlorophyll in the chloroplasts is used to join a molecule of ADP (adenosine diphosphate) to a molecule of Pi (inorganic phosphate) to form ATP (adenosine triphosphate). This is called photophosphorylation.

The ATP stores energy until the energy is required in stage two of photosynthesis.

Complete the summary diagram below.


ATPADP + Pi

oxygen hydrogenenergy

water

light energy absorbed by _________________


Stage 2: Carbon Fixation

This second stage of photosynthesis also occurs in the chloroplasts.

It is a series of enzyme-controlled reactions, which use hydrogen and ATP (produced by the light reactions) with carbon dioxide to produce sugar.

The energy for carbon fixation is supplied by the ATP from the light reaction.

+ = +

Complete the diagram on the following page which summarises the carbon- fixation stage of photosynthesis.


Pi

Pi

Pi

Adenosine Pi

Pi

Adenosine Pi

high energy bond breaks releasing energy


Summary of photosynthesisComplete the following diagram:


ATP

sugar

_______ from the airl________ e_______ from the sun

CO2

w_______ taken up by roots

o__________ released into the

air

trapped by the pigment c____________

w______

CO2

ADP + PiUsed to form cellulose

Light Reaction Carbon

Fixation

o_________ h___________rogen

s______

2ADP +2Pi

2ATP

fates of sugar:1.2.3.

chloroplast


The fate of the sugar produced by photosynthesis

The sugar formed during photosynthesis can be used immediately to provide energy for cells by the process of respiration.

sugar + oxygen carbon dioxide + water + energy

If the sugar is not required for energy it can be converted into other carbohydrates.Some of the remaining sugar molecules are converted into starch and stored in the leaf.Other sugar molecules are built into long chains of cellulose for the construction of cell walls.

Formation of storage and structural carbohydrate

Complete the following diagram


cellulose (structural) glucose (energy source)

starch (storage)

formation of cellulose for b_________

long chains of c___________

start here

g_________ molecules

single unit of g___________

formation of starch for s__________

potato tuber cell

c____ w_____

sugar produced during photosynthesis

single unit of g___________

s________ g______


Key Questions: Answer the questions below.

1 State the site of photosynthesis.

___________________________________________________________________

2 (a) Write the word equation for photosynthesis.

_____________ + _____________ _____________ + ______________

3 (a) Name the type of energy absorbed by plants during photosynthesis.

_________________________________________________________________

(b) Give the colour and name of the chemical in a plant cell that captures this

source of energy.

Colour: _____________________ Name: ________________________

(c) State the exact location of this energy absorbing chemical.

____________________________________________________________________

4 (a) Name the first stage of photosynthesis.

___________________________________________________________________

(b) In this reaction light energy is converted to chemical energy. Name the energy

storing molecule formed during this reaction

___________________________________________________________________

(c) Write the equation for the formation of this molecule.

_________ + _________ + _________ __________



(d) During this first stage of photosynthesis water molecules are split. State what happens to the oxygen and hydrogen produced.

Oxygen ___________________________________________________________

Hydrogen __________________________________________________________

5 (a) Name the second stage of photosynthesis.

____________________________________________

(b) Name the products from the first stage that are required during in the second stage.

_______________________________ and _______________________________

(c) State the role of carbon dioxide in the second stage of photosynthesis.

__________________________________________________________________

6. The sugar produced in photosynthesis is used by the plant in other processes. State 3 ways in which it may be used by the plant once it has been synthesised and the products it is converted into.

1. Use ______________________ name ___________________________________

2. Use ______________________ name ___________________________________

3. Use ______________________ name ___________________________________



Factors affecting the rate of photosynthesis: Limiting factors

Several environmental factors affect the rate of photosynthesis.

These include light intensity, carbon dioxide concentration and the temperature of the surroundings.

If any of these factors is insufficient it will limit the rate of photosynthesis. If the rate of photosynthesis is affected then the growth of the plant will be affected.

These factors are important in crop production. Growers need to provide the optimum conditions for maximum growth.

Measuring the rate of photosynthesis

We can estimate the rate of photosynthesis by measuring the rate of decrease in raw materials or rate of increase in products i.e.

1. Volume of oxygen given off per unit of time2. Uptake of carbon dioxide per unit of time3. Production of carbohydrate (as increase in dry weight) per unit of time



Investigating the effect of varying light intensity

Elodea bubbler experiment

At first the lamp is placed exactly 100cm from the plant and the number of oxygen bubbles released per minute counted. The lamp is then moved to a new position (80cm from the plant) and the rate of bubbling noted (once the plant has had a short time to become acclimatised to this new higher light intensity).

The process is repeated by placing the lamp closer to the plant e.g. 60cm then 40cm etc.

Results

Distance from plant (cm)

Light intensity (units)

Rate of photosynthesis(Number of oxygen bubbles released per

minute)

100 4 480 11 1060 25 1940 45 2230 64 2520 100 25


The number of oxygen bubbles released per minute by the cut end of an Elodea stem indicates the rate at which photosynthesis is taking place.


On the grid below plot a line graph to show the rate of photosynthesis at the various light intensities.

Label the position on the graph with the letter X to show where light intensity is the limiting factor.

Label the position on the graph with the letter Y to show where light intensity is no longer limiting and other factors are now limiting the rate of photosynthesis.

List the factors which are limiting at point Y.

___________________________________________________________________

Describe the relationship between light intensity and the rate of photosynthesis.____________________________________________________________________________

____________________________________________________________________________

________________________________________________________________


0 10 20 30 40 50 60 70 80 90 100

Light intensity (units)

Rate of photosynthesis

(number of oxygen bubbles

released per minute)


Answer the following questions:

1. What would the rate of photosynthesis be when there is no light?

_______________________________________________________________

2. Describe what happens to the rate of photosynthesis as the light intensity increases to from 4 to 64 units?

_______________________________________________________________

3. Describe what happens to the rate as the light intensity increases from 64 to 100 units?

_______________________________________________________________

4. Between which light intensities is light the limiting factor?

_______________________________________________________________

5. What evidence is there that light intensity is no longer a limiting factor when it is above 64 units?

_______________________________________________________________

_______________________________________________________________



Investigating the effect of varying carbon dioxide concentration

Results:

On the grid below plot a line graph to show the rate of photosynthesis at the various concentrations of carbon dioxide.

On the graph paper, plot the results of the investigation as a line graph. Label the position on the graph with the letter X to show where carbon

dioxide concentration is the limiting factor. Label the position on the graph with the letter Y to show where carbon

dioxide concentration is no longer limiting and other factors are now limiting the rate of photosynthesis.


The number of oxygen bubbles released is counted, as before. The lamp is kept in one position to give uniform light of high intensity. This ensures that only one variable, the concentration of carbon dioxide, is being changed.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 concentration of carbon dioxide (%)

Rate of photosynthesis(number of oxygen

bubbles released per minute)

differing concentrations of sodium hydrogen carbonate solution

Concentration of carbon dioxide (%)

Rate of photosynthesis(Number of oxygen bubbles released per

minute)0 0

0.1 20.2 40.3 80.4 80.5 8


List which factors are limiting at point Y.

___________________________________________________________

Describe the relationship between carbon dioxide concentration and the rate of photosynthesis.



Investigating the effect of varying temperature

The plant is given constantly high light intensity and a rich supply of carbon dioxide to ensure that neither of these factors limits the process.

Results

The rate of photosynthesis rises to an optimum at around 35OC. Beyond the optimum, the rate drops rapidly. This is because photosynthesis consists of many reactions controlled by enzymes and these are denatured at high temperatures, making the reactions stop.(The optimum temperature varies from species to species and is often lower than 35OC).


The apparatus is placed in a large water bath whose temperature is under thermostatic control. Plastic bags containing ice cubes are used to create low temperatures.The lamp is kept in one position to give uniform light of high intensity.

high concentration of sodium hydrogen carbonate solution

Rate of photosynthesis(oxygen bubbles



Removing Limiting Factors

Horticulture is the cultivation of crops in gardens and greenhouses.By overcoming limiting factors in horticultural situations, photosynthesis in greenhouse crops e.g. tomatoes, cucumbers and peppers can be maximised, giving earlier crops and increased yields.In greenhouses, supplementary lighting i.e. additional to normal daylight can be used to increase light intensity as well as the duration of the light period in a day, especially during winter.It is also possible to increase the carbon dioxide available in a greenhouse and at the same time increase the temperature by heating the greenhouse using paraffin heaters or propane heaters as these two fuels release carbon dioxide on burning.

Key Questions1. Explain what is meant by the term limiting factor.

_______________________________________________________________

2. Name THREE possible limiting factors in the process of photosynthesis

1. _____________________

2. _____________________

3. _____________________

3. Describe THREE ways in which rate of photosynthesis can be measured.

1. ___________________________________________________________

2. ___________________________________________________________

3. ___________________________________________________________

4. As the distance between a lamp and an Elodea plant is gradually decreased, state the effect this has on:

(i) The intensity of the light reaching the plant.

______________________________________________________

(ii) The rate of photosynthesis.

___________________________________________________



5. The graph below shows the effect of various combinations of carbon dioxide concentration and light intensity on the rate of photosynthesis.

(a) Name the factor limiting the rate of photosynthesis between A and B.

____________________________________________________________

(b) Name the factor limiting the rate of photosynthesis between B and C.

_________________________________________________________________

6. Explain the benefit horticulturalists gain by providing plants in a glass house

with supplementary heating and lighting during the winter?

____________________________________________________________________

_______________________________________________________________


Experiment in higher light intensity

Experiment in lower light intensity

Rate of photosynthesi

s(oxygen bubbles


Concentration of CO2 (units)

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