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Summary AS and A level Biology
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AS and A level Biology
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• Unlike animals, plants make their own organic molecules, using the process ofphotosynthesis.
• Carbon dioxide gas is the source of carbon and light is the source of energy.
• The main photosynthetic organs are the leaves, which have evolved a large surface area:
volume ratio for efficient capture of carbon dioxide and light.
• As a result, most plants do not have compact bodies like animals, but have extensive
branching bodies with leaves above ground.
• In order to obtain the water and mineral salts also needed for nutrition, plants have
extensive root systems below ground.
• The plant body therefore spreads out to obtain the carbon dioxide, light energy, water and
inorganic mineral ions it needs from its environment to make organic molecules like sugars
and amino acids.
• Transport systems are therefore needed for the following reasons:
1. To move substances from where they are absorbed to where they are needed.
2. To move substances from where they are produced to where they are needed for
metabolism.
3. To move substances to a different part of the plant for storage.
4. Carbon dioxide.
Photosynthetic plant cells require a supply of carbon dioxide during daylight.
Most photosynthetic tissue is in leaves, and most plants have evolved thin, flat leaves with a large
surface area ideal for absorbing as much carbon dioxide as possible.
They obtain this by diffusion from the air.
5. Oxygen.
All plant cells require a supply of oxygen for respiration, but cells which are actively
photosynthesising produce more than enough oxygen for their own needs because oxygen is a
waste product of photosynthesis.
Cells which are not photosynthesising need to take in oxygen from their environment.
Plants have much lower energy demands than animals, so they respire at much lower rates.
They therefore do not need such a rapid supply of oxygen.
The branching shape of plants and a network of air spaces in the plant body provide a large enough
surface area for effective absorption of oxygen by diffusion.
,• It is relatively easy for carbon dioxide and oxygen to diffuse into and out of the bodies of
plants, reaching and leaving every cell quickly enough down diffusion gradient.
• Consequently, there is no need for a transport system for these gases.
• However, transport systems are needed for distribution of water, inorganic and organic
nutrients, as well as other substances such as plant hormones.
• The design of a plant’s transport system is quite different from that of a mammal.
• In fact, plants have two transport systems, xylem and phloem.
• Xylem carries mainly water and inorganic ions (mineral salts) from roots to the parts above
ground.
• The xylem sap contained in the xylem can move in only one direction, from roots to the rest
of the plant.
• The second system is phloem.
• This carries substances made by photosynthesis from the leaves to other areas of the plant.
• At any one time, phloem sap can be moving in different directions in different parts of the
phloem.
• In neither of these systems do fluids move as rapidly as blood does in a mammal, nor is
there an obvious pump such as the heart.
• Neither plant transport system carries oxygen or carbon dioxide, which travel to and from
cells and their environment by diffusion alone.
• Stems, roots and leaves are the main organs involved in transport within plants.
• Organs are composed of more than one tissue.
• Tissues are collections of cells specialised for a particular function.
• The cells may be of the same type, such as parenchyma, or of different types, as in xylem
and phloem.
• The tissues found in stems, roots and leaves are most easily studied using prepared slides of
transverse sections of these organs.
• Structure is closely linked with function.
• Flowering plants (angiosperms) may be monocotyledons (monocots) or dicotyledons
(dicots).
• Each type has its own characteristics.
• Monocotyledonous plants, such as grasses, typically have long, narrow leaves.
• Dicotyledonous plants typically have leaves with blades and stalks (petioles).
• The mechanisms of transport through both types of plant are the same, but there are
differences in the distribution of xylem and phloem in their roots, stems and leaves.
• Only dicotyledonous plants are described in this book.
, • This is a continuous layer on the outside of the plant, one cell thick, that provides protection.
• In stems and leaves it is covered with a waxy cuticle which is waterproof and helps to
protect the organ from drying out and from infection.
• In leaves, it also has pores called stomata which allow entry of carbon dioxide for
photosynthesis.
• In roots, it may have extensions called root hairs to increase the surface area for absorption
of water and mineral salts.
• Parenchyma is made up of thin-walled cells used as packing tissue.
• The cells are very metabolically active and may be used for many functions.
• They may be used for storage of foods like starch.
• When they are turgid (fully inflated with water) they help to support the plant, preventing
wilting.
• Air spaces between the cells allow gas exchange.
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