Lesson 6 Photosynthesis Introduction

Question 1

Chloroplast Structure

Answer 1

Variable in structure but share
some features:

Chloroplast envelope –
double membrane enclosing
the chloroplast.

Thylakoids – an extensive
system of membranes with
a fluid filled space inside

Stroma – colourless fluid
containing enzymes that
surrounds the thylakoids

If a chloroplast has
been photosynthesizing
rapidly there may be
starch grains or lipid
droplets in the stroma.

Grana – stacks of thylakoids.

Question 2

Photosynthesis

Answer 2

Photosynthesis uses light energy to combine simple inorganic compounds to
produce carbon containing energy storage molecules, such as glucose.

The green colour of plants is due to the chloroplasts in plant cells which contain
a pigment called chlorophyll.

Chlorophyll cannot absorb green light and therefore reflects it, giving the cell
a green appearance.

Chlorophyll absorbs red an blue light particularly well and are primarily used
to provide the energy needed for photosynthesis.

Question 3

Photosynthesis

Answer 3

Photosynthesis is a complex series of reactions catalysed by enzymes. We
consider photosynthesis as occurring in two stages:

Light dependent reactions

Light independent reactions

The light dependent reactions occur as the first stage and can only occur in the
presence of light.

Chlorophyll absorbs light energy to produce ATP and split water molecules
into hydrogen and oxygen (photolysis).

The light independent reactions use the hydrogen ions, electrons and ATP
produced in the light dependent reactions.

Carbon dioxide is taken in from the environment and is combined with
hydrogen and ATP to form a range of organic molecules in a process known
as carbon fixation.

Question 4

Limits to Photosynthesis

Answer 4

The rate of photosynthesis of a plant depends on several factors in its
environment.

Some of the key factors are:

Light intensity

Temperature

Concentration of carbon dioxide

Question 5

Light Intensity

Answer 5

An increase in light intensity will
produce an increase in the rate of
photosynthesis that is directly
proportional to the increase in light
intensity.

This increase stops as the enzymes
are working at heir maximum
capacity, limited by temperature
and the availability of CO2.

Question 6

Temperature

Answer 6

Increasing temperature
results in in an increase in
the rate of photosynthesis
due to the increased
frequency of collisions of
the molecules involved.

Since photosynthesis is
enzyme controlled there is
a maximum temperature,
after which the rate of
photosynthesis decreases
dramatically as the
enzymes denature.

Question 7

Concentration of Carbon Dioxide

Answer 7

Increased concentration of carbon
dioxide causes the rate of
photosynthesis to increase since CO2
is a raw material in the process.

This increase has a maximum limited
by the amount of enzyme present.
Once all the enzymes are being used
at maximum capacity there can be no
further increase in the rate of
photosynthesis, regardless of any
further increases in [CO2]
All three of these variables
may be changing in nature
or be manipulated by
horticulturalists for
maximum crop yield when
using a greenhouse.