Cambridge International AS and A Level Biology Revision Guide
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Handwritten and Digital Notes Cambridge International AS and A Level Biology Students Coursebook
Handwritten and Digital Notes Cambridge International AS and A Level Biology Students Coursebook
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9700 CIE As Level biology
9700 CIE Alevel biology (9700)
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9700 CIE biology A Level
1
,9700 CIE biology A Level
Unit 07: Transport in Plants
A copy of the syllabus content is shown below:
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,9700 CIE biology A Level
7.1 Structure of transport tissues
Necessity of a transport system
* Why do unicellular organisms do not need a transport system?
(3 marks)
- Unicellular organisms like Amoeba have only one cell.
Therefore all the surfaces of its cells are in contact with the
external environment. In other words, their surface area to
volume ratio is very high. Therefore, substances necessary for
metabolic activities and substances which are waste products
can be efficiently diffused in an out of the cell by simple
diffusion. So a transport system is not necessary.
*Why do multicellular organisms require a transport system?
(3 marks)
- Multicellular organisms have many cells so all the cells will
not be in contact with the external environment. Therefore,
its surface area to volume ratio is very low and this decreases
the efficiency of particles diffusing in an out of the cells. The
organism cannot survive with such a low rate of substances
moving so it requires a well-organized and a complex
transport system.
MCQ hint: Higher the surface area, higher the rate of diffusion. Always say surface area to
volume ratio and not just surface area.
Theory hint:
A flatworm does not require a transport system for respiratory
gases. Explain?
The flatworm has a high surface area to volume ratio (show
Calculations if values are given). Therefore, most of its cells
are expose to the external environment so it has a high
rate of simple diffusion of respiratory gases which is
sufficient for its metabolic activities.
The descriptions in the paper is very useful when answering questions as it gives hints on
what your answer should include so read it carefully.
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,9700 CIE biology A Level
How does a transport system help a multicellular organism?
*Transport systems help to increase the surface area to volume ratio for efficient exchange
of substances. E.g. Lungs have many alveoli, kidneys have many nephrons, roots have many
root hairs, and leaves have many stomas.
* So, in a plant there is a transport system for water and mineral ions and also a transport
system for assimilates but, no transport system for gases.
* Plants do not require a transport system or gases because they have certain adaptations
within themselves to help exchange gases efficiently. They are branched and further
branched with many leaves so they have many stoma to increase the efficiency of gas
exchange by increasing the surface area to volume ratio. Plants release oxygen as a
byproduct of photosynthesis so they can use this oxygen for respiration too.
* Therefore, plants have only 2 transport systems:
1. Xylem: for water and mineral ions. (Transpiration)
2. Phloem: for assimilates like sucrose an amino acids. (Translocation)
* There are two types of plants known as dicots and
monocots, in our syllabus we mainly focus on dicot plants.
So, we discuss the structures of a dicot plant ut touch
upon only certain areas of a monocot leaf.
Monocot leaf Dicot leaf
Parallel veins Network of veins
Narrow, long leaves broad leaf
Vascular bundles scattered all over. Vascular bundles arranged in a ring.
One cotyledon Two cotyledons
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,9700 CIE biology A Level
Cross-section of stem in a dicot plant (transverse section) Phloem Cambium Xylem
The phloem and the xylem is collectively called a vascular bundle. In between the xylem
and phloem, there is a strip of meristem cells called the cambium.
In the stem the vascular bundles are arranged in a ring.
Bundle cap is not require in the syllabus but, it is important to know it as in some
pictures, students confuse it with the phloem.
The pith and the cortex are the cells surrounding the vascular bundles.
Epidermis forms the outermost layer of the stem.
Functions:
1. Xylem - Transport of water and minerals; provide support.
2. Phloem - Transport of sucrose an amino acids.
3. Pith and cortex – Packing cells that provide support to the vascular bundles and keep
them in position.
4. Epidermis – Prevent water loss.
5. Cambium- produces new xylem cells to the side of the xylem and new phloem cells to
the side of the phloem.
*In a monocot stem vascular bundles are scattered.
HINT: xylem tissues are shown with dark and
thickened circles in any leaf.
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,9700 CIE biology A Level
Cross-section of a root in a dicot plant (transverse secction)
Xylem
Phloem
Casparian strip
Root hairs
*In the root, the xylem and phloem does not exists in several vascular bundles.
* Xylem is at the center in an X-shape.
*Additional features include the endodermis and pericycle and root hairs.
* Endodermis contains a water impermeable substance in their cell walls called suberin.
*The endodermis and suberin is called the casparian strip.
* There is another ring of cells inner to the endodermis called the pericycle.
*root hairs increase the rate of absorption.
Function:
Endodermis- creates root pressure by blocking the apoplast pathway.
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Monocot root has air spaces in the cortex.
Air spaces
*MCQ HINT: you must know the positions of the xylem and phloem in a root,
stem and leaf.
1, 4, 6 – Xylem
2, 3, 5 - phloem
Comparing the cross sections of a root and stem of a dicot plant.
stem root
Many vascular bundles. Only one vascular bundles.
Pith is present. Pith is absent.
No suberin Have suberin
No pericycle or endodermis. Has pericycle or endodermis.
No root hairs. Has root hairs.
MCQ HINT: The girth of a tree increases in the night due to the reduced tension
but in the day it decrease as tension develops during transpiration.
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Cross section of a dicot leaf
Cuticle
Upper epidermis
Palisade mesophyll
cells.
Spongy mesophyll
cells.
Intercellular air spaces
Lower epidermis
Phloem Xylem
Stomata
Features and functions:
1. Cuticle: thin, waxy, non-cellular layer which prevents water loss.
2. Upper epidermis: single layer of transparent cells which allow light to enter.
3. Palisade mesophyll: largest number of chloroplast, present closest to the epidermis to
receive maximum light, tightly packed cells arrange perpendicular to the upper
epidermis so more are present in the surface. It has a thin film of moist layer around it
to dissolve gases. It carries out photosynthesis.
4. Spongy mesophyll: has chloroplast and is loosely packed with many air spaces in
between them for gas exchange. Also has a layer of moisture to dissolve gases.
*chloroplast can move to the outer most area to receive maximum light for
photosynthesis when it is dark.
5. Lower epidermis: Has guard cells to control the stomata. Has stoma for gas exchange. It
forms the boundary to the cells.
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Xylem and phloem in detail
Xylem
*Xylem takes part in transpiration which is a passive process.
* Made of xylem vessel elements and has no nucleus.
*Its function is to transport water and mineral ions from the roots towards the leaves (upward)
and also to provide support to the plant and prevent it wilting. Also carries plant hormones.
* It transports only upwards and are dead vessels.
Adaptations of xylem to carry out its functions (very important)
1. Thick cellulose cell wall lining helps to create adhesive forces with water and prevents
it collapsing.
2. Cell wall impregnate with a water impermeable substance called Lignin which provides
support and prevents it collapsing. It also prevents water losses (water proof).
3. Has pits which allow lateral movement of water and mineral ions so surrounding cells
can get the needed substances.
4. Long continuous tubes with no cross walls so less resistance to movement of
substances.
5. Wide lumen so a larger volume of water is transported.
6. Hollow tubes (no cytoplasm, do not say it’s dead) so less interruption to water
movement.
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