UNIT 6 Plant Nutrition - Photosynthesis

Question 1

A student set up the apparatus shown in Fig. 3.1 to investigate the effect of light intensity on
the rate of photosynthesis of a pond plant.

The student maintained the temperature at 20 o
C and measured the distance travelled by the air bubble in the capillary tube for a period of five minutes on three occasions for each light intensity.

The student’s results are shown in Table 3.1 on page 8.

(a) (i) Explain why the student included the glass tank and the syringe in the apparatus.
glass tank?
syringe?

Answer 1

glass tank - absorbs heat from lamp

syringe - reposition the air bubble / return air bubble to top of tubing

Question 2

State three functions of water in plants.

Answer 2

maintaining cell turgidity ;

preventing wilting ;

transport of minerals / amino acids / sugars ;

medium for enzyme action ;

raw material for photosynthesis;

Question 3

(ii) Explain why the air bubble moves down the capillary tube.

Answer 3

THREE OF:

⭐ 1 photosynthesis produces oxygen/plant releases oxygen

⭐ 2 oxygen is, by-product (of photosynthesis) ;

3 from splitting of water / photolysis ;

4 oxygen comes out of solution / AW ;

⭐ 5 gas, collects / rises to the top ;

6 (gas) pushes water down the tube / displaces the water ;

Question 4

Interpolation - notice pattern. each mm distance = 0.2 mm/min rate of photosynthesis so 7mm distance, 0.2 x 7 = 1.4.

In graph, what should you make sure of?

Answer 4

all points plotted accurately ;

curved or straight line of best fit / straight lines between points ;

ignore if line continues beyond first and last points because of (c)(i)
R if line goes to 0

[allow a straight line of best fit that is close to the plotted
points]

Question 5

“Using the graph to help you,” should you extrapolate?

Answer 5

NO - just use graph given/what you drew

Question 6

(ii) Explain why the rate of photosynthesis decreases as the distance of the lamp
from the pond plant increases.

Answer 6

1 (increase distance gives) decrease light (intensity) ; ORA

2 ref. to light energy ;

3 absorbed by, chlorophyll / chloroplast ;

4 light (intensity) is limiting (factor) ;

So: “Increase in distance gives decrease in light intensity. Less light absorbed by chloroplast. Light intensity is the limiting factor.”

Question 7

Why upper epidermis is transparent

Answer 7

Allows light through

Light reaches chloroplasts

In the palisade cells

Chloroplasts need light for photosynthesis

Question 8

Why plants store starch

Answer 8

Use it as an energy store

When plant cannot photosynthesise (no sunlight)

Question 9

Why plants take up CO2 during photosynthesis

Answer 9

Carbon dioxide is a reactant of photosynthesis

Diffuses into leaf due to lower concentration inside of the leaf

Question 10

Why chlorophyll removed from leaf before testing for starch

Answer 10

Chlorophyll masks the colour change (shown with iodine)

Question 11

Explain the importance of the control plant in an investigation for the need of CO2 in photosynthesis

Answer 11

To show that the factor under test (CO2’s effect on photosynthesis) is responsible for the change observed

Question 12

Explain why plants were destarched

Answer 12

to be sure that starch is produced during the experiment

Question 13

Destarched plant. Why does the concentration of CO2 increase?

Answer 13

No photosynthesis
Plant respires
Carbon dioxide produced

Question 14

Plant GIVES OFF which gas?
[means PRODUCT]

Answer 14

Oxygen

Question 15

Raw materials? = ?

= ??

Answer 15

Raw materials = REACTANTS

in photosynthesis? Carbon dioxide and water

Question 16

Line for light intensity if the experiment was repeated with higher light intensity.

Phrase to rmbr?

Answer 16

Same trend/looking line, but higher/above it

Remember: “Rate of photosynthesis would increase then start to level off.”

Question 17

Even if chloroplasts are absent, does respiration occur?

Answer 17

Yes, it does

Question 18

3 uses of energy in organisms (not reproduction)

Answer 18

Growth

Movement

Active transport

Question 19

Phloem - transport sucrose & amino acids in plants

Uses of carbs & amino acids in plants?

Answer 19

MEMORISE:

Carbs -
⭐ For cell walls (glucose to cellulose)
⭐️ Converted to sucrose to be transported through phloem vessels
⭐ Starch storage
⭐ For energy/respiration
⭐ To attract insects to flowers

Amino acids -
⭐ To make hormones and enzymes
⭐ For growth & repair

Question 20

Predict & explain what happens to rate of photosynthesis if temperature exceeds 45 degrees Celsius.

Answer 20

Rate of photosynthesis decreases

Bc enzyme’s active sites are denatured

Question 21

For photosynthesis,

STATE:

⭐ Definition of photosynthesis

⭐ Word equation of pho. + “in the presence of…”

⭐ Definition of chlorophyll

⭐ What chlorophyll does

⭐ Chemical equation of pho.

Answer 21

⭐ Photosynthesis: the process by which plants synthesise carbohydrates ⁽ᵍˡᵘᶜᵒˢᵉ⁾ from raw
materials ⁽ᴴ²ᴼ & ᶜᴼ²⁾ using energy from light ⁽ᶜᵃᵗᵃˡʸˢᵗ⁾

⭐ Word equation for photosynthesis as:
carbon dioxide + water → glucose + oxygen
in the presence of light and chlorophyll

⭐ Chlorophyll is a green pigment that is
found in chloroplasts

⭐ Chlorophyll transfers energy from light into energy in chemicals, for the synthesis of carbohydrates

⭐ Balanced chemical equation for photosynthesis as:
6CO2 + 6H2O → C6H12O6 + 6O2

Question 22

⭐ Subsequent use and storage of the carbohydrates made in photosynthesis, limited
to:

Answer 22

(a) starch as an energy store [in plants]

(b) cellulose to build cell walls [from glucose]

(c) glucose used in respiration to provide energy

(d) sucrose for transport in the phloem (vessels) - [glucose converted to sucrose]

(e) nectar to attract insects for pollination

Question 23

Leaf adaptations -

Gases enter through special holes called stomata. These holes are opened & closed by guard cells. Once gases are in the leaf, they can diffuse easily bc there are air spaces in the spongey mesophyll layer. The cell membranes of the leaf cells allow gases & water to pass freely in & out of the cell.

Some leaf cells contain a pigment called chlorophyll which absorbs light energy. This pigment for photosynthesis are found in special organelles called chloroplasts. Most of these are found in the palisade layer at the top of the leaf. The cuticle and epidermal cells are transparent so that light can pass easily through to the chloroplasts.

Answer 23

Most leaves have a ⭐ large surface area to maximise exposure to sunlight for absorption of light & allows more diffusion of CO2

and are ⭐ thin, aiding in diffusion of oxygen and carbon dioxide, speeding up the exchange of the gases
+

“Thin - allows faster diffusion of CO2 to palisade mesophyll cells.”

Question 24

MEMORISE -

Leaf structures in a dicot plant

Answer 24

Refer to printed diagrams + class worksheet.

PARTS - Upper epidermis, chloroplasts, palisade mesophyll tissue, spongey mesophyll tissue, air space, vascular bundle (xylem & phloem), lower epidermis, guard cells, stoma

Question 25

Guard cells & stomata

Answer 25

Guard cells allow gas exchange and control water loss within the leaf

In the lower epidermis, guard cells open & close the stomata to allow carbon dioxide into the leaf and oxygen to diffuse out as well as water vapor

Stomata: allows oxygen out of cells & allows carbon dioxide in

[Site where water is evaporated from the leaf]

Question 26

(Waxy) cuticle

Answer 26

Allows light to pass through whilst protecting the leaf’s surface

Waxy but thin, so protects leaf from water loss without blocking sunlight

Question 27

Epidermal cells

Upper epidermis

Answer 27

Offers protection but thin to allow light through

Thin & transparent, so allows more light to reach the palisade cells

Question 28

Palisade cells

/ palisade mesophyll

Answer 28

⭐ Contains many chloroplasts, able to absorb more light from top layer

⭐ Packed tightly together

⭐ Maximises surface area

⭐ Large vacuole pushes chloroplasts to edges of cell

⭐ Thin cell walls to aid gaseous exchange

Question 29

Chloroplasts

Answer 29

Contains chlorophyll that absorbs light for photosynthesis

Chlorophyll transfers energy from light into energy in chemicals, for the synthesis of carbohydrates

Question 30

Spongey mesophyll

Answer 30

⭐ Irregular shaped

⭐ Fewer chloroplasts

⭐ Smaller than palisade cells

⭐ Spaced out with air pockets in between, allowing CO2 to easily diffuse through the leaf

Question 31

Air space

Answer 31

increase surface area for gas exchange and allow gases to move more easily.

Question 32

Vascular bundle - xylem & phloem

Answer 32

XYLEM vessels - transports water and mineral ions taken up from the soil by the roots to the stem and leaves

+ support

PHLOEM - transport sucrose & amino acids in plants AWAY from leaf to the rest of the plant

transport food materials (mainly sucrose and amino acids) made by the plant from photosynthesising leaves to non-photosynthesising regions in the roots and stem

Question 33

Explain the importance of:

(a) nitrate ions for making amino acids

(b) magnesium ions for making chlorophyll

Answer 33

a) ⭐ Nitrates needed for proteins - old leaves start to dry out bc not enough nitrogen to maintain proteins in all cells

⭐ Nitrate deficient plants -
➡ stunted growth
➡ leaves (usually near growing tip) start to turn yellow

b) 🌌 Magnesium required for chlorophyll

🌌 Lack of chlorophyll production will make yellowing occur between the veins of leaves (chlorosis), starting from the lower leaves

🌌 Slow down growth of plant

Question 34

Identify and explain the limiting factors of photosynthesis in different environmental
conditions

What is a limiting factor?

What’s it mean?

Main limiting factors of photosynthesis?

Answer 34

Limiting factor: something that is in short supply that restricts a process
+ “present in an environment” “life processes”

When a process depends on 2 or more factors (variables), the rate of that process is determined by the factor which is in shortest supply.

➡ ⭐ light intensity, ➡ ⭐ carbon dioxide concentration, ➡ ⭐ temperature

Question 35

Light intensity

Answer 35

• Rate of photosynthesis increases with light intensity
• When intensity of light increases to a certain point, photosynthesis cannot go any faster.

[Actual maximum varies.]

Question 36

Meaning of horizontal line on one of these graphs? Abt limiting factors

Answer 36

Means the limiting factor has changed

Question 37

Carbon dioxide concentration

Answer 37

• Increasing CO2 concentration increases rate of photosynthesis
• There’s a maximum rate where adding more CO2 will have no more effect/photosynthesis cannot go any faster

Question 38

Temperature

Answer 38

• Too much heat damages enzymes & causes them to denature
• Too much heat also evaporates water out of leaves & forces stomata to close
• Too little heat can reduce enzyme’s reaction rate by reducing number of collisions between the enzyme’s active site and substrate

Question 39

Ratio of photosynthesis to respiration

Answer 39

The ratio of photosynthesis to respiration in plants changes over the day cycle

Plants are constantly respiring & releasing carbon dioxide

Plants only photosynthesise during the day when sunlight is available, taking in carbon dioxide

Question 40

Investigations - Gas Exchange

Hydrogen carbonate indicator -

Hydrogen carbonate indicator is a pH indicator that changes colour depending on the pH of a solution.

CO2 is acidic. Higher CO2 concentration = more acidic

//

Investigate the effect of light on the net gas exchange in an aquatic plant using a hydrogen carbonate indicator.

Answer 40

Highest |CO₂| = yellow.
Higher |CO₂| = orange.

⬆ More respiration than photosynthesis. Lower pH, more acidic.

Atmospheric level = red.

⬆ Photosynthesis = respiration.

Lower |CO₂| = magenta.
Lowest |CO₂| = purple.

⬆ More photosynthesis than respiration. Higher pH, more alkaline.

Question 41

INVESTIGATIONS - Gas Exchange

⭐ Several leaves from the same plant are placed in closed/stoppered boiling tubes containing some hydrogen carbonate indicator

The effect of light can then be investigated over a period of a few hours.

Tube 1-3, contents, conditions, indicator colour, reason?

Answer 41

Tube 1: Leaf. In the light. Purple. The overall absorption of CO2 by leaf is high in light conditions.

Tube 2: Leaf. Dark. Yellow. The overall release of CO2 from the leaf is higher in dark conditions.

Tube 3: NOOO leaf. Light. Red. This is the CONTROL - since there’s no leaf, the level of CO2 is the same as that of the atmosphere.

Question 42

Net Carbon Dioxide - day vs night

Answer 42

During the day, plants photosynthesise faster than they respire, esp. when sun bright & light intensity high

➡ NET INTAKE of carbon dioxide in the DAY
[higher output of CO2]/if light bright enough, rate of CO2 absorption becomes greater than rate of CO2 release.

During the night, plants respire more than they photosynthesise,

➡ NET RELEASE of CO2 at NIGHT

Question 43

Factors needed for photosynthesis?

Answer 43

⭐ Chlorophyll - helps absorb the light required

⭐ Light - provides the energy to drive the reaction

⭐ Carbon dioxide - reactant in photosynthesis to be converted to sugars like glucose

Question 44

Investigate need for chlorophyll, light
& carbon dioxide [necessary factors] for photosynthesis, using appropriate controls

Investigate && describe the effects of varying
light intensity, carbon dioxide concentration and
temperature [limiting factors] on the rate of photosynthesis

Answer 44

• Removing starch from plant before investigation:
• Using iodine test for starch to see need for chlorophyll
• Removing chlorophyll using ethanol
• Boiling to soften

+ etc.!

Question 45

Why can’t leaves be tested for glucose?

NEED FOR CHLOROPHYLL

Answer 45

bc it’s quickly used, converted & transported/stored as starch.

Question 46

Why can the leaf be tested for starch instead?

NEED FOR CHLOROPHYLL

Answer 46

Starch is stored in chloroplasts. Testing a leaf for starch using iodine is a reliable indicator of which parts of the leaf are photosynthesising.

Question 47

NEED FOR CHLOROPHYLL (boil, ethanol, soften by boiling, white tile w/ iodine, indicates…?):

Leaves tested for starch

If it is orange-brown, starch is not present bc photosynthesis hasn’t occurred since there’s NO chlorophyll present.

If it is blue-black, starch is present & has been stored meaning photosynthesis occurred bc chlorophyll WAS present.

To prove chlorophyll IS needed for pho, we use a VARIEGATED leaf bc partially green, partially white, so only parts of leaf contain chlorophyll

Answer 47

Chlorophyll ⭐

➡ A leaf dropped in boiling water to kill the cells & break down the cell membranes so iodine can seep in && so it’s permeable

[Beaker, leaf in boiling water, Bunsen burner on]

➡ Leaf left for 5-10 mins in hot ethanol in a boiling tube, removing the chlorophyll so colour change from iodine can be seen more clearly, decolourising the leaf

[Boiling tube, ethanol, Bunsen burner OFF, ethanol turns green]

➡ Leaf dipped/rinsed in boiling water to soften it

➡ Leaf is spread out on a white tile & covered with iodine solution

[Reason: iodine will change colour in presence of starch]

⭐ Green leaf ➡ entirely blue-black, photosynthesis occurring in all areas of leaf

Test whether chlorophyll is needed for pho. by using a VARIEGATED leaf (one partially green & partially white)

🔗 White areas of leaf - no chlorophyll (so orange/brown/yellow)

= orange-brown = no photosynthesis occurring = no starch is stored

🔗 Only areas containing chlorophyll stain blue-black as starch will be present.

⭐ Testing a variegated leaf for starch: Take care; ethanol is extremely flammable, turn off Bunsen burner. To heat ethanol, electrical water bath INSTEAD OF beaker w/ Bunsen burner & an open flame.

Question 48

Investigating the need for LIGHT 💡🌞

SAME procedure

Destarched, aluminium foil, tested for starch w/ iodine

Answer 48

Light ⭐

➡ Plant needs to be destarched: before starting exper. - place plant in a dark cupboard for 48 hours

Ensures any starch already present in the leaves will be used up & won’t affect experiment’s results bc plant can’t photosynthesise, so use all starch for respiration

➡ After, a leaf of the plant can be partially covered with aluminium foil & the plant placed in sunlight for a day.

➡ A leaf dropped in boiling water to kill the cells & break down the cell membranes so iodine can seep in && so it’s permeable

➡ Leaf left for 5-10 mins in hot ethanol in a boiling tube, removing the chlorophyll so colour change from iodine can be seen more clearly, decolourising the leaf

➡ Leaf dipped/rinsed in boiling water to soften it

➡ Leaf then removed and tested for starch using iodine.

🔗 Area of leaf COVERED WITH ALUMINIUM FOIL will remain ORANGE-BROWN

bc it did not receive any sunlight & could not photosynthesise.

while…

🔗 Area EXPOSED TO SUNLIGHT will turn BLUE-BLACK.

This proves light is necessary for photosynthesise and production of starch.

Question 49

Investigating the need for CARBON DIOXIDE

Destarch, place 1 in bell jar w/ NaOH (absorb CO2) & 1 w/ water (control), place in light, test for starch using iodine, one w/ NaOH remains orange-brown

Exper. uses: potted plants, clear plastic bag tied over pots, LIGHT, glass containers, THEN one has sodium hydroxide solution to absorb CO2 + soda lime at top to absorb CO2. The other w/ beaker of water.

Answer 49

Carbon Dioxide ⭐

➡ Destarch 2 plants by placing in a dark place for a prolonged period of time (48 hours)

➡ Place 1 (potted) plant in a bell jar which contains a beaker of sodium hydroxide, which will absorb carbon dioxide from the surrounding air. Soda lime to absorb CO2.

➡ Place the other (potted) plant in a bell jar which contains a beaker of water (control experiment), which will NOT absorb CO2 from the surrounding air.

⬆ Clear plastic bag tied over the pot.

➡ A leaf dropped in boiling water to kill the cells & break down the cell membranes so iodine can seep in && so it’s permeable

➡ Leaf left for 5-10 mins in hot ethanol in a boiling tube, removing the chlorophyll so colour change from iodine can be seen more clearly, decolourising the leaf

➡ Leaf dipped/rinsed in boiling water to soften it

➡ Place both plants in bright light for several hours.

➡ Test both plants for starch using iodine.

➡ Leaf, from plant placed near:

• Sodium hydroxide (NO PHO.), will remain orange-brown bc it could not photosynthesise due to lack of carbon dioxide.
• Water (DOES PHO.), should turn blue-black as it had all necessary requirements for photosynthesis.

Question 50

INVESTIGATING THE RATE OF PHOTOSYNTHESIS

Investigate + desc. effects of varying light intensity, CO2 concentration &
temp. [limiting factors] on the rate of photosynthesis

Factors affecting rate of pho. ->

⭐ Light intensity
⭐ Carbon dioxide concentration
⭐ Temperature

Answer 50

To investigate how fast pho. is happening, use an aquatic plant that photosynthesises in water & count the number of oxygen bubbles it releases in a minute

As pho. occurs, oxygen gas is made & released

Since the plant is in water, the oxygen released can be seen as bubbles leaving the plant.

The number of bubbles produced over a minute can be counted to record the rate.

The more bubbles produced per minute the faster the rate of photosynthesis.

Question 51

Investigating Varying Light Intensity

Equipment: lamp, ruler, water [with sodium hydrogen carbonate], thermometer [to monitor temperature], [inverted] boiling tube, O2 bubbles, [inverted] funnel, aquatic plant

Answer 51

To change the light intensity, the lamp may be moved to different distances away from the beaker containing the plant. The number of bubbles produced per minute should be recorded at these different distances.

Graph is one with a positive slope then horizontal line.

Rate of pho. = y-axis
Light intensity (%) = x-axis.

Mid-point of positively sloped line = increasing light intensity/CO2 concentration increases the rate of photosynthesis.

At the corner between sloped & horizontal line, SOME OTHER FACTOR becomes LIMITING.

At end of horizontal line, the rate becomes constant.

Question 52

Investigating Varying CO2 Concentration

Equipment: lamp, 1% sodium hydrogen carbonate solution, thermometer [to monitor temperature], [inverted] boiling tube, O2 bubbles, [inverted] funnel, aquatic plant

Answer 52

To change the CO2 concentrations, different amounts of sodium hydrogen carbonate may be dissolved in the water in the beaker. The number of bubbles produced per minute should be recorded for these different CO2 concentrations. The resulting data will produce a graph like this.

Question 53

Investigating Varying Temperature

Equipment: lamp, water [with sodium hydrogen carbonate], hot plate [to change temperature], thermometer [to monitor temperature], [inverted] boiling tube, O2 bubbles, [inverted] funnel, aquatic plant

Answer 53

The temperature of the water in the beaker may be changed to different levels & the number of bubbles produced per minute may be recorded at these different temperatures.

The resulting data will produce a graph like this:

Lopsided curve leaning to the right.

Mid-point of first part of curve = Rate increases as number of collisions between enzyme’s active sites and substrates increases.

Top point = optimum temperature.

Mid-point of second negatively sloped half of curve = Rate DECREASES as enzymes start to denature.

Question 54

In which order does water pass through these structures in a plant?

A mesophyll → root hair → xylem
B mesophyll → xylem → root hair
C root hair → mesophyll → xylem
D root hair → xylem → mesophyll

Answer 54

D))

root hair → xylem → mesophyll

Question 55

formed FIRST in a leaf as a result of photosynthesis?

Answer 55

glucose

Question 56

which part of a leaf does most water evaporate during transpiration?

Answer 56

the spongy mesophyll cells

Question 57

Partially permeable in plants?

Answer 57

Vacuole

Cell membrane

Question 58

Which cell type absorbs the most carbon dioxide during the day?

Answer 58

Palisade mesophyll cell

Question 59

Suggest and explain the reasons for the shape of the graph in region Y.

Answer 59

“Rate of photosynthesis levels off as the limiting factor changes.

Carbon dioxide concentration is no longer limiting factor. Light intensity could be limiting, temperature could be limiting.

Temperature affects activity of enzymes as too high can denature and too low can reduce enzyme’s reaction rate by reducing number of collisions between the enzyme’s active site and substrate”

Question 60

Counting bubbles may not be the best way to measure the rate of photosynthesis. The volume of the bubbles is not always exactly the same.

Suggest and explain one alternative way of measuring the gas given off to solve this problem. [3]

Answer 60

measure volume (of oxygen/ gas) ;

use, inverted test-tube/ measuring cylinder/ syringe (barrel) ;

reference to, graduations /markings ; A ‘take readings from…’/ ‘record
results…’
filled with water ;

Question 61

Explain why the concentration of carbon dioxide has increased between 1959 and 2013.

Answer 61

use/ combustion/ burning, of fossil fuels ;
reason for increased demand for energy ;
carbon dioxide from, volcanic activity / volcanoes ; deforestation ;
burning of, forests / trees ;

Question 62

Global warming is largely due to this increase in atmospheric carbon dioxide.

Explain how increases in atmospheric carbon dioxide concentrations contribute to global warming.

Answer 62

carbon dioxide is a greenhouse gas ;
(enhanced) greenhouse effect (in context of carbon dioxide) ;
heat/infra-red/ long wavelength radiation, radiated/ emitted, from /
absorbed/ trapped/AW, by, carbon dioxide/ greenhouse gases ;
travels /AW, back to the surface ;
heat cannot, leave (from the atmosphere)/ pass into outer space ;

Question 63

With reference to the water potential gradient, explain why plants may die when
grown in salty soil. [3]

Answer 63

salt concentration in soil is higher than in roots AW ;

ref. to water potential is greater in root cells than in soil / w.p gradient
goes from cells to soil AW;

so water is drawn out of roots + by osmosis ;

cells become flaccid ;

plant wilts ;
plant lacks water ;

Question 64

An article in a school science magazine stated, ‘Many plants contain genes which
enable them to pump salts out of their roots. These genes can be made more active by
genetic engineering, enabling the plants to remove salts
before the plants are damaged.’

Explain whether you think that the process described in the article above is an example
of genetic engineering.

Answer 64

the removal of a gene from one species ;

and its insertion into another species ;

(in article) genes are modified, not transferred AW ;

Question 65

Explain the difference in growth between the plants watered with low concentrations and those watered with high concentrations of salt solution.

Answer 65

salt lowers the water potential ;

plants absorb less water ;

loss of turgidity ;

less water for chemical reactions;

less, water for photosynthesis ;

stomata close ;

Question 66

Plants grown in soils of pH 10 may show symptoms of deficiency. They are stunted and their leaves are yellow.

Explain how deficiencies of magnesium ions and nitrate ions lead to the symptoms
described.

magnesium ions
nitrate ions

Answer 66

magnesium ions

needed for making chlorophyll ;

without chlorophyll plant, not green / yellow ;

cannot absorb (much) light ;

little / no, (energy for) photosynthesis ;

nitrate ions

needed to make amino acids ;
amino acids to proteins ;
protein needed for growth ;