Lecture notes
Transport in Plants 3.1.3
Detailed notes on transport in plants. Notes written using OCR Biology A textbooks, past papers and more. Written by a student with all A*s at GCSE with an offer for Natural Sciences at Cambridge.
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3.1.3 Transport in plants(a) the need for transport systems in multicellular plants
Transport systems
The need for an exchange system is affected by:
▪ size
▪ surface area to volume ratio
▪ activity levels
In small plants, the cytoplasm of their cells is very close to the environment, so diffusion will
supply enough oxygen and nutrients to keep them alive. Large plants have multiple layers of
cells, so the diffusion pathway is too long – diffusion is too slow to enable a sufficient supply to
inner cells.
Small plants have a large S.A: vol ratio so have a larger surface over which exchange can take
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎
place. Surface area: vol ratios are always expressed as a ratio of x:1. 𝑅𝑎𝑡𝑖𝑜 = 𝑉𝑜𝑙𝑢𝑚𝑒
Plants are not very active and have a low respiration rate, so the low demand of oxygen can be
met by diffusion, but the plant still needs water and sugars. A transport system is needed to
transport water and minerals from the roots up to the leaves and sugars from the leaves to the
rest of the plant.
(b) (i) the structure and function of the vascular system in the roots, stems
and leaves of herbaceous dicotyledonous plants
Vascular tissue
Dicotyledonous plants: plants with two seed leaves and a branching pattern of veins in the leaf.
In dicotyledonous plants, the xylem and phloem are found together in vascular bundles, which
may also contain other types of tissue e.g. collenchyma and sclerenchyma, which give the bundle
strength and help to support the plant.
Phloem
The phloem carries assimilate, such as sugars and amino acids, up or down from the leaves.
The phloem consists of sieve tubes made of:
▪ Sieve tube elements
▪ Companion cells
Sieve tube elements
Sieve tubes are formed from sieve tube elements lined up end to end.
They have no nucleus and little cytoplasm to allow space for mass flow
to occur. At the ends of the sieve tube elements are perforated cross-
walls called sieve plates which allow movement of sap from one
element to the next. The sieve tubes have very thin walls and are
usually five or six sided.
Companion cells
Companion cells are between sieve tubes and are small cells with a
large nucleus and a dense cytoplasm. They have many mitochondria to
produce ATP needed for active processes. The companion cells carry
out the metabolic processes needed to load assimilates actively into the
sieve tubes.
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