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B io Factsheet
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Number 258
Biology of cereals 1: Sorghum, a C4 plant
These adaptations are reflected in the water use efficiency of
sorghum (Table 2)
This Factsheet:
• Summarises the importance of cereals worldwide
• Describes and explains the adaptations of sorghum, a C4 plant
• Reviews the very many types of exam questions that are set on
sorghum in particular and cereals in general
Table 2. Water use efficiency of sorghum
Cereal
Cereal crops such as sorghum are a major source of nutrients all
over the world. Cereals grow well in many different environments,
their grains are easy to store, contain a wide variety of nutrients
and are often high in particularly important ones such as vitamin B.
They also have high energy and fibre content.
Water use efficiency/ kg water used per kg
dry mass produced
Sorghum
300
Wheat
350
Maize
550
Typical exam Question
Scientists investigated the effect of the water-holding capacity
(WHC) of soil on the yield of sorghum and maize. Seeds of both
plants were planted in plots containing high WHC soil and in
plots containing low WHC soil. The yield of sorghum and maize
from all the plots was measured at the end of the investigation.
The graph shows the results.
Sorghum
6000
Tracey Slotta @ USDA-NRCS PLANTS Database
5000
Sorghum is a cereal that is well adapted to survive in hot, dry
conditions (Table.1).
yield/kg
per hectare
Table.1 Adaptations of sorghum
Feature
Explanation
Extensive, deep root system
Maximises water absorption
Curled, small leaves
Reduces water loss by
evaporation/transpiration
3000
2000
1000
Reduces surface area over
which transpiration can occur
and traps layer of moist air
0
Reduces water loss by
evaporation/transpiration
Increased sclerenchyma and
silica
Reduces wilting
Tolerant of high temperatures and
heat shock proteins (HSPs)
activate at very high temperatures
Can grow in tropical
conditions HSPs help prevent
damage to proteins
wheat
LWC
wheat
HWC
Markscheme
Reduced stomatal numbers and
stomata in pits
sorghum
HWC
(a) No. of seeds sown;
Planting pattern/density;
Planting depth;
Watering regime;
Aspect;
Wind/shelter;
Soil pH;
(b) (i) yield for sorghum greater than yield for wheat;
WHC had little effect on sorghum yield;
WHC had much greater effect on wheat yield;
(ii) sorghum tolerant of dry soils/ adapted to dry conditions;
wheat not well adapted to dry conditions;
yield for wheat is better in HWC soil / little difference in
yield for sorghum;
Traps layer of moist air,
reducing the diffusion
gradient
sorghum
LWC
(a) Suggest three factors that would have to be controlled during
this investigation to ensure that the results were valid (3).
(b) (i) Describe the results shown (2)
(ii) Suggest an explanation for the results (2)
Waxy leaves composed of esters Wax doesn’t melt and shiny
and fatty acids with a melting surface reflects heat
point much higher than nontropical plants
Leaves have rows of motor cells
along the midrib of the upper
surface of the leaf, enabling the
leaves to roll up.
4000
1
Bio Factsheet
258. Biology of cereals 1: Sorghum, a C4 plant
www.curriculum-press.co.uk
Photorespiration
Photorespiration isn’t on many specifications but that doesn’t stop the Examiners from setting questions on it – they often don’t mention
the term but give you a diagram of it that tests your ability to apply biological principles that are on the spec to a topic that isn’t. In other
words, they are testing your ability to think on your feet. Nice people these Examiners!
In the light-independent stage of photosynthesis, the enzyme ribulose bisphosphate carboxylase (Rubisco) catalyses the combination of
carbon dioxide with ribulose bisphosphate (RuBP).
The 6C compound formed immediately breaks down into two 3C compounds ( GP) which are then converted into triose phosphate ( (TP)
using ATP and NADPH from the light dependent stage (Fig. 1)
Fig. 1 The Calvin cycle (C3 pathway)
CO2
Light Dependent Stage
occurs in thylakoids
CO2 + RuBP
Rubisco catalyst
carbon fixation
6C Unstable &
breaks down
2GP
(Glycerate-3-phosphate)
Regeneration of RuBP
ATP
(from Light
dependent stage)
NADPH
(from Light
dependent stage)
ADP
NADP
2TP
(Triose phosphate)
Light Independent Stage
(Celvin Cycle) occurs in stroma
Synthesis of polysaccharides,
amino acids, lipids and nucleic
acids
However, Rubisco can also bind with oxygen. In hot, dry conditions plants close their stomata to prevent excess water loss. But this also
stops CO2 entering the leaf. When CO2 levels inside the leaf fall to low levels Rubisco combines with O2 instead of CO2. This process is
called photorespiration. It is called photorespiration because:
•
•
•
It occurs in light
It requires oxygen, like aerobic respiration
It produces CO2 and water
However, it does not produce ATP and it reduces carbon dioxide assimilation because some of the intermediates in the Calvin cycle are
degraded. Photorespiration therefore reduces crop yields.
•
GP is the first stable product of the light-independent stage. It is a three- carbon compound and for this reason the Calvin cycle is also
called the C3 pathway.
•
Plants such as sorghum are C4 plants. They have evolved an additional way of initially fixing the carbon dioxide and preventing
photorespiration.
2
Bio Factsheet
258. Biology of cereals 1: Sorghum, a C4 plant
www.curriculum-press.co.uk
Fig. 2 The C4 pathway
CO2
1
(3C) Phosphoenol
pyruvate
Mesophyll cell
1. Within the leaf mesophyll
cells, instead of CO2 being
fixed by the enzyme
Rubisco and combined
with ribulose
bisphosphate, it is fixed
by the enzyme
phosphoenol pyruvate
carboxylase (PEPC) and
combined with
phosphoenol pyruvate
(PEP) to form the fourcarbon compound
oxaloacetate (hence, C4
pathway).
NADPH
AMP
2
NADP
ATP
5
Pyruvate
(3C)
(4C)
Malate
3. The malate then moves out
of the mesophyll cells into
the bundle sheath cells that
are tightly packed around
the veins of the leaves.
3
5. The pyruvate re-enters
the mesophyll cells and
recats with ATP,
regenerating PEP.
Bundle sheath cell
(4C)
Olalocetate
2. Oxaloacetate is
reduced by NADPH
into malate.
Pyruvate
(3C)
4
CO2
Calvin
Cycle
Polysaccharides,
amino acids,
lipids and
nucleic acids
4. Inside the bundle sheath cells a
decarboxylase enzyme converts
malate into pyruvate and
releases CO2. The levels of CO2
NADP
inside the bundle sheath cells
become extremely high: 10 – 60
NADPH
times the CO 2 concentration
found in mesophyll cells. This
then enables the normal Calvin
(C3) pathway to begin i.e.C4
plants exploit both a C4 and the
normal C3 pathway.
(4C) Malate
Vein
So what is the point of all this?
Fig. 3 Transverse section through the leaf of a C4 plant
PEPC has an extremely high affinity for carbon dioxide, binding it
even when carbon dioxide levels are very low, and it has no affinity
at all for oxygen. It also has a high optimum temperature – it
works best in hot conditions (45oC), precisely when the plant
may have closed its stomata, thus limiting the amount of CO2 that
can enter. In fact the enzyme is so efficient that C4 plants can
afford to keep their stomata closed (preventing water loss) much
longer than C3 plants can.
epidermis/epidermal cell
The tightly-packed mesophyll cells surrounding the bundle sheath
cells don’t let oxygen into them so Rubisco can’t bind to it when
CO2 levels are low.
Thus, C4 crops such as sorghum, maize and sugar cane are able
to withstand higher temperatures and light intensities, lose less
water via transpiration and hence provide greater yields than C3
crops
bundle sheath (cell)
mesophyll (cell)
Extract from Chief Examiner’s Report:- Many candidates could
not explain why the C4 pathway enables plants to grow well in
hot conditions. Many candidates seemed to believe that the C4
pathway was a modified form of respiration.
3
Bio Factsheet
258. Biology of cereals 1: Sorghum, a C4 plant
www.curriculum-press.co.uk
Typical Exam Question
(d) Species X has evolved an altered form of photosynthesis that
helps it to concentrate carbon dioxide in some cells. The diagram
shows the interconversion and transfer of some organic
compounds, which contain either three carbon atoms (C3) or
four carbon atoms (C4), in a leaf of species X.
(a) The diagram shows sections through a typical leaf of three
different cereal plants.
Cereal B
Cereal C
Cereal A
cuticle
CO2
Cell A
Cell B
C3
compound
stomata
C4
compound
Which cereal is most likely to be grown in hot, dry conditions?
Explain your answer (2).
(b) The diagram shows a section of the lower leaf epidermis of
sorghum.
Y
2
1
0
10
20
30
40
50
60
Light intensity/arbitary units
70
Markscheme
0
(a)
3
(b)
4
(c)
X
(d)
(c) The graphs show the effects of light intensity and temperature
on the rate of photosynthesis in two crop species of plant, X
and Y.
Rate of
photosynthesis
/arbitary units
CO2
Leaf A; Thick cuticle; Few stomata; Sunken stomata;
Reduce water loss;
grows in hot, dry /arid conditions;
reduces transpiration/water loss; ref to C4;
Species X;
Higher rate of photosynthesis at high light intensity/ light intensity
is still limiting at high intensity;
Higher rate of photosynthesis at high temperature / Y slows above
30oC / temperature is still limiting at high temperature;
(i) C3 reaction rapidly fixes the CO2 / keeps CO2 concentration
low;
Maintains the concentration gradient;
Allowing more CO2 to diffuse in;
(ii) Decarboxylation of the C4 compound is rapid/ CO2 is released
rapidly in B;
Movement of the C3 compound from B to A is rapid and
accelerates decarboxylation/ ref to feedback control;
Ref to use of ATP energy to transfer C3 compound into cell A;
(iii) High temperatures may result in high transpiration/ water loss;
Stomatal closure reduces this;
But also stops CO2 diffusion into the leaf;
Decarboxylation in cell B maintains high CO2 concentration;
Maintaining high rate of photosynthesis;
Sorghum has relatively few stomata. Explain the significance
of this (2)
5
C4
compound
Use information in the diagram to explain:
(i) how reactions in cell A result in a large amount of carbon
dioxide to enter cell B (2)
(ii) how very high concentrations of carbon dioxide are built
up in cell B (2)
(iii) the significance of the fact that the stomata of species X
are only open for approximately 65% of the time that those
of species Y are open (3).
0.1 mm
6
AT P
C4
compound
C3
compound
6
X
5
4
Rate of
photosynthesis
/arbitary units
3
Extract from Chief Examiner’s Report
(d) This part of the question was simply about diffusion but
many candidates appeared to “give up” apparently because
they were intimidated by the unfamiliar diagram. Most
candidates did not even mention diffusion, nor the maintenance
of concentration gradients; candidates need more experience
of applying simple biological principles to unfamiliar
material.
2
Y
1
0
0
10
20
30
Temperature /0C
40
One of the crops is well adapted to growing in tropical
conditions. Suggest, with reasons whether this is species X or
Y (2)
4
Bio Factsheet
258. Biology of cereals 1: Sorghum, a C4 plant
www.curriculum-press.co.uk
Scientists are collaborating worldwide to try to understand how
sorghum can tolerate the high temperatures that would kill many
other important crop plants. One suggestion is that their heat
tolerance is due to a substance known as GB.
(a) Explain how this leaf anatomy helps the plant achieve high
rates of carbon fixation at high temperatures (6).
(b) Germinating seeds use the enzyme alpha amylase to break
down starch. Scientists investigated the effect of temperature
on alpha amylase in seeds of sorghum and rice that were
germinating. The graph shows the results.
Scientists investigated this by growing two varieties of sorghum,
one containing high levels of GB and one containing low levels of
GB, in both distilled water and in sodium chloride solution (sorghum
is often grown in irrigated water that contains sodium chloride).
They measured the rate of photosynthesis of all the plants. The
table shows the results.
Variety of sorghum
4
3
Rate of photosynthesis / arbitrary units
Distilled water
NaCl solution
High GB
31
18
enzyme
activity
/arbitrary 2
units
Low GB
33
8
1
sorghum
What does this data tell us?
• The scientists wanted to find out whether GB levels affected
photosynthesis, hence yield
• So they selected varieties with high and low GB levels and then
grew them in distilled water (no NaCl ) and in water that was
similar to the irrigation water (with NaCl)
• Simple Biology tells us that, normally, plant roots growing in
salty water will have water pulled out of them by osmosis. Worse,
in hot conditions, irrigation water will evaporate, making the
remaining soil even saltier
• In the NaCl solution the variety with high GB achieved more
than twice the rate of photosynthesis of the variety containing
low GB
• This suggests that the GB is lowering the water potential of the
sorghum roots, stopping water being withdrawn osmotically,
enabling the plant to absorb water, survive and grow
0
10
rice
20
30
40
50
temperature / 0C
60
70
80
(i) Describe the results (3)
(ii) What do these result suggest about the tertiary structure
of alpha amylase in sorghum and rice? (3)
(b) (i) in both, enzyme activity increases and then falls;
alpha amylase in sorghum has higher activity at all temperatures;
alpha amylase in sorghum has higher optimum temperature;
alpha amylase in sorghum has higher maximum activity;
(ii) tertiary structure/active site of alpha amylase of sorghum more
stable;
able to form more enzyme-substrate complexes;
high temperatures affect H bonds more than other types of bond;
so alpha amylase in sorghum may have less H bonds;
2. (a) tightly packed mesophyll cells;
prevent O2 cannot reaching bundle sheath cells;
light independent stage/Calvin cycle occur in bundle sheath cells;
malate moves from mesophyll cell to bundle sheath cell;
decarboxylation of malate maintains high CO2 concentration in bundle
sheath cells;
PEP carboxylase has high optimum temperature / greater affinity for
/ no affinity for O2;
PEP carboxylase not denatured;
photorespiration is avoided;
photorespiration prevents carbon dioxide entering the Calvin cycle;
from which biomass is produced;
photorespiration also destroys compounds in the Calvin cycle
Heat shock proteins
Despite sorghum being able to tolerate and take advantage of high
temperatures, yields of sorghum can be severely reduced if it is
exposed to very high temperatures during germination or at the
young seedling stage.
When sorghum seedlings are exposed to temperatures above 37°C,
they produce heat shock proteins ( HSPs ). These intracellular
proteins help protect enzymes against denaturation and help prevent
damage to proteins as they are being synthesised. However, very
young sorghum seedlings are unable to activate sufficient HSPs
and this is why they are particularly vulnerable.
1. Thick waxy cuticle;
Sunken stomata;
Reduces water loss;
Adult/embryo plants able to tolerate high temperatures;
C4 pathway;
Allows photosynthesis when stomata closed/more efficient at high
temperatures;
Dense/wide/deep root system;
Allows water to be collected from large area;
Rolling of leaves;
Reduces area from which water can evaporate;
Practice Questions
1. Outline how sorghum is adapted to growing in hot, dry
conditions (6)
2. The diagram shows a transverse section through the leaf of a
C4 plant.
epidermis/epidermal cell
Markschemes
bundle sheath (cell)
Acknowledgements:
This Factsheet was researched and written by Kevin Byrne.
Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.
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mesophyll (cell)
5