Photosynthesis

Question 1

What is the equation for photosynthesis

Answer 1

6CO2+6H20 C6H12O6 + 6O2

Carbon dioxide diffuses into the leaf through the stomata

Water transported via xylem tissues of roots, steps and leaf to the photosynthesizing cells

Glucose is made in photosynthesisng cells

oxygen ‘6O2’ is the waste product

Question 2

What is the function of the chloroplast envelop

Answer 2

Permeable to small molecules
and forms a border to the
stroma respectively.

Question 3

What is the function of the thylakoid

Answer 3

Thylakoid- fluid-filled sacs
that are suspended in the
stroma. This is the site of
light- dependent reactions of
photosynthesis.

Grana- stacks of thylakoids

Question 4

What is the function of the stroma

Answer 4

Stroma - aqueous fluid that
contains appropriate enzymes
and a suitable pH for the
Calvin cycle to occur. This is
the site of light independent
reactions.

Question 4

What is the function of the lamellae

Answer 4

Lamellae - connects and
separates thylakoid stacks
(grana), maximising
photosynthetic efficiency

Question 5

What is a pigment?

Answer 5

A pigment is a substance whose molecules absorb some wavelength
(colours) of light but not others.

NB: We see unabsorbed wavelengths

Question 5

What is Chemiosmosis

Answer 5

Process in which the diffusion of protons (H+) down a proton gradient,
produced across the inner membrane of the mitochondria, provides
energy for ATP synthesis.

Question 6

Explain the steps involved in cyclic photophosphorylation

Answer 6

Steps:

Photosystem I traps light energy. This causes the electrons to
be raised to a high energy level.

The electrons are passed through a series of electron carriers
(plastoquinone and plastocyanin).

As the electrons are transported, ATP is synthesized by
chemiosmosis.

The electron has lost its extra energy and eventually returns
to PSI

NB: It involves only PSI and
results in the synthesis of ATP

Question 6

What are the 3 main steps of the calvin cycle?

Answer 6

Carboxylation of ribulose bisphosphate

Reduction of glycerate-3-phosphate

Regeneration of ribulose bisphosphate

Question 7

Explain Step 1 of the Calvin Cycle : Carbon Fixation

Answer 7

The Calvin cycle begins with a 5C compound called
ribulose bisphosphate (or RuBP)

An enzyme, RuBP carboxylase (or Rubisco),
catalyses the attachment of a CO2 molecule to
RuBP

The resulting 6C compound is unstable, and breaks
down into two 3C compounds – called
glycerate-3-phosphate (GP)

A single cycle involves three molecules of RuBP
combining with three molecules of CO2 to make six
molecules of GP

Question 8

Explain Step 2 of the calvin cycle: Reduction of Glycerate-3-Phosphate

Answer 8

Glycerate-3-phosphate (GP) is converted into
triose phosphate (TP) using NADPH and ATP

Reduction by NADPH transfers hydrogen atoms
to the compound, while the hydrolysis of ATP
provides energy

Each GP requires one NADPH and one ATP to
form a triose phosphate – so a single cycle
requires six of each molecule

Question 9

Explain Step 3 of the calvin cycle :

Answer 9

Of the six molecules of TP produced per cycle,
one TP molecule may be used to form half a sugar
molecule

Hence two cycles are required to produce a single
glucose monomer, and more to produce
polysaccharides like starch

The remaining five TP molecules are recombined
to regenerate stocks of RuBP (5 × 3C = 3 × 5C)

The regeneration of RuBP requires energy derived
from the hydrolysis of ATP

Question 10

List Atleast 3 factors that influence the rate of photosynthesis

Answer 10

Carbon dioxide

Water

Temperature

Light intensity

Inorganic ions

Question 11

Explain how Light Intensity Impacts photosynthesis

Answer 11

As light intensity increases, the rate of
photosynthesis increases. The rate
increases until LIGHT SATURATION
is reached. Beyond this point, a further
increase in light intensity does not
increase the rate of photosynthesis.
Some other factor is limiting the rate of
photosynthesis. (most likely carbon
dioxide concentration)

Question 12

Explain how Carbon dioxide concentration impacts photosynthesis

Answer 12

Carbon dioxide concentration in the
atmosphere is approximately 0.04%.
The amount is consistent and is
maintained by a thin balance
between photosynthesis and
respiration.

More carbon dioxide = faster
photosynthesis.

Question 13

Explain How temperature impacts the rate of photosynthesis

Answer 13

Phytochemical reactions are not affected by
temperature and so the light dependent stage
of photosynthesis is not affected by
temperature. The Calvin cycle, however, is
affected.

Some plants can photosynthesize at sub-zero
temperatures. The minimum temperature
however is 0C.

The rate of photosynthesis doubles for every
10C rise in temperature. Most plants reach
their maximum photosynthesis rate at 25C.

Question 14

List and Explain at least two adaptations of palisade cells

Answer 14

NB:

The palisade mesophyll is the main site of
photosynthesis, as there are more chloroplasts per
cell than in the spongy mesophyll.

Palisade cells show several adaptations for light
absorption.
* They are long cylinders arranged at right-angles
to the upper epidermis. This reduces the number
of light-absorbing cross walls in the upper part
of the leaf so that as much light as possible can
reach the chloroplasts.
* The cells have a large vacuole with a thin
peripheral layer of cytoplasm. This restricts the
chloroplasts to a layer near the outside of the
cell where light can reach them most easily.