A2 ESSENTIAL GUIDE TO PHOTOSYNTHESIS

A2 ESSENTIAL GUIDE TO PHOTOSYNTHESIS

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B io Factsheet
September 1997 Number 2
The essential guide to photosynthesis
Photosynthesis is the use of light energy from the sun to fix carbon dioxide i.e. turn it into sugars. These sugars can then be converted
into other essential substances - fats and proteins etc. - which plants need to live and grow. At GCSE level the process of photosynthesis
is represented by the following equation:
6CO2 + 6H2O Light energy C6H12O6 + 6O2
Carbon dioxide, a Root hairs absorb water passively i.e. Glucose, which can be used Oxygen, which may be
colourless gas which no energy is required. Water is needed The reaction is to make other substances used by the plant in
makes up 0.04% (by to: catalysed by light such as fats and proteins respiration or which may
volume) of the 1. Keep plant tissues turgid eg. to energy absorbed by etc. diffuse out of the leaves
atmosphere. Enters maximise leaf surface area for light chlorophyll via the stomata.
through microscopic absorption. contained in
pores (stomata) on 2. As a source of electrons in non- chloroplasts in
leaves. cyclic photophosphorylation leaves and green
(Fig 2). stems.
3. As a solvent (all chemical
reactions must occur in solution).
The equation is an over-simplification. 1. There is clear evidence that photosynthesis occurs in two stages - one which is light dependant (the LDS)
and one which is light independent (the LIS). 2. Glucose is not the first or only useful product. 3. Only visible light - a small part of the
electromagnetic spectrum - is used. 4. Certain wavelengths are much more important than others
Structure to function: chloroplast
Chlorophyll and light absorption Fig 1. The absorption and action spectra 1. Internal compartmentalisation. LDS and LIS
Chlorophyll absorbs light from the effectively separated, thus allowing rate-
visible part of the electromagnetic Absorption spectrum determining factors such as pH and enzyme
spectrum. Chlorophyll is made up concentrations to be optimized.
Chlorophyll a
of a number of different pigments: Chlorophyll b 2. DNA and ribosomes means chloroplast can code
chlorophyll a, chlorophyll b, for and produce its own proteins eg. enzymes
chlorophyll c along with other such as RuBPC.
Absorbance
pigments such as carotenoids. Each 3. Double membrane provides control of substances
of these absorb different wavelengths entering/ leaving the organelle.
of light so that the total amount of 4. Thylakoid membranes provide large
light absorbed is greater than if a surface area for light absorption.
single pigment were involved.
Not all wavelengths of light are
Structure to function: the leaf
absorbed equally. An absorption 1. Thin, therefore rapid light penetration.
Wavelength (nm)
spectrum is a graph showing the 2. Waxy cuticle reduces water loss.
percentage absorption plotted 3. Upper epidermis transparent to light.
Action spectrum 4. Palisade mesophyll arranged at 90o to surface of
against wavelength of light (Fig 1).
leaf minimising amount of light absorbed by cell
Rate of Photosynthesis
An action spectrum is a graph walls before it reaches chloroplasts.
showing the rate of photosynthesis
plotted against wavelength of light
(Fig 1).
The similarity between the 5. Chloroplasts in palisade mesophyll are capable
absorption spectrum and the action of being moved to optimise light absorption.
spectrum shows that red (650- 6. Spongy mesophyll has many air spaces therefore
700nm) and blue (400-450nm) rapid gaseous diffusion.
wavelengths, which are absorbed 400 550 700
most strongly, are also the Exam Hint - Structure: function is a key concept on
Wavelength (nm) all syllabuses. The strongest candidates will
wavelengths which stimulate
demonstrate a clear understanding of the
photosynthesis the most. Green light Typical Exam Question adaptations of photosynthetic organs, tissues, cells
(550mm) is mostly reflected. Why do leaves appear green? and organelles.
1

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The essential guide to photosynthesis Bio Factsheet
Figure 2. The Biochemistry of Photosynthesis 1. Light energy is absorbed by chlorophyll molecules in PSI and
6 (b) PSII.
Key
6 (a) e- carrier PSI: Photosystem 1 2. The electrons in the chlorophyll molecules are boosted to a
4e- PSII: Photosystem 2 higher energy level and are emitted.
e-: Electrons 3. The loss of electrons from PSII stimulates the loss of
e- carrier electrons from water i.e.…read more

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The essential guide to photosynthesis Bio Factsheet
Photosynthesis investigations
Figure 3. Using Elodea Separating Chlorophyll Pigments
Thermometer 1. Cut up green leaves and macerate
with a solvent eg. acetone.
Single wavelength Water Calibrated scale (allows Maceration breaks cell walls,
of light (to volume of released O2 to
absorb releasing chlorophyll, which
be calculated by r2h dissolves.
heat
from
where r = radius of tube
light) and h = length of bubble). 2.…read more

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The essential guide to photosynthesis Bio Factsheet
Practice Questions Answers
(semicolons indicate marking points)
1. Define the term action spectrum
(2 marks) 7. The diagram below shows the absorption 1. A graph;
and action spectra for some chloroplast which shows the effectiveness of
2. Distinguish between cyclic and non-cyclic pigments. different wavelengths in stimulating
photophosphorylation (4 marks) photosynthesis;
Absorption spectrum
3. Outline the role of each of the following in 2.…read more

Comments

rac1

Great resource!

Sumpthink

Thanks, this is very useful.

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