B3.1.3: Transport in plants - OCR A Biology A level A* student notes
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Biology A Level
Year 1
Module 3: Exchange and Transport
3.1.3: Transport in plants
The need for transport systems in multicellular plants
Metabolic demands
● Unlike at the green parts of the plant, the cells in many internal and underground parts
of the plant do not photosynthesise and therefore cannot produce their own oxygen and
glucose. Oxygen and glucose are the reactants needed for respiration, in all cells.
Therefore, transport systems are needed to transport oxygen and glucose
● Removal of metabolic waste products
● Hormones made in one part of a plant need transporting to the areas where they are
required for different metabolic processes
● Mineral ions absorbed by the roots need to be transported to all cells, to make the
proteins required for enzymes and the structure of the cell
Size
● Plants require very effective transport systems to move substances both up and down,
from the tip of the roots to the topmost leaves and stems
● As plants continue to grow throughout their lives, many perennial plants (plants that live
for many years, flower reliably every year, reproduce year after year) are large, and some
of them are enormous. Some of the tallest trees eg. coastal redwood and giant redwood
in the USA, mountain ash in Australia
● Also, larger multicellular plants have a greater demand for substances due to their size
Surface area to volume ratio
● For simple organisms such as small plants (eg. mosses) substances diffuse through the
surface, into the leaves and simple roots. Don’t need to move far
once inside the plant and hence can move into and round the plant
by diffusion and osmosis
● However, larger organisms do not have sufficient absorbing area to
meet their needs, because as the size of plant increases, their
surface area in relation to their volume decreases (lower SA:V)
● Leaves have a relatively large SA:V as they are adapted to maximise
the exchange of gases, and the roots have root hairs that greatly increase their SA. Unlike
most animals, plants have a branching body shape that helps give even large plants a
higher SA:V
, ● But the size and complexity of multicellular plants means that when the stems, trunks
and roots are taken into account, they still have a relatively small SA:V
● Therefore plants cannot rely on diffusion alone to supply their cells with everything they
need, and require transport systems
The structure and function of the vascular system in the roots, stems and leaves of herbaceous
dicotyledonous plants
● Dicotyledonous plants, ‘dicots’, make seeds that contain 2 cotyledons (organs that act as
food stores for the developing embryo plant and form the first leaves when the seed
germinates)
- [Arborescent (woody) dicots: have hard, lignified tissues and a long life cycle]
- Herbaceous dicots: have soft tissues and a relatively short life cycle - leaves and
stems die down at the end of the growing season to the soil level. Herbaceous
plants, by definition, have no persistent woody stems above ground
Dicotyledonous plants are vascular plants; they have a series of transport vessels running
through the leaves, stems and roots called the vascular system
● In herbaceous dicots, this vascular system is composed of 2 main types of transport
vessels: the xylem and the phloem
● The xylem and phloem are transport tissues arranged together in vascular bundles in the
leaves, stems and roots. The pattern of the vascular tissue is easily recognised and shown
in transections (TS):
Stem - vascular bundles are situated around the edge to give strength and support
, Root - vascular bundles situated in the middle to help the plans withstand the tugging strains
that result as the stems and leaves are blown in
the wind
Leaf - midrib of a dicot leaf is the main vein carrying the vascular tissue through the organ.
Also helps to support the structure of the leaf. Many small, branching veins spread through the
leaf for both transport and support
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