XYLEM
The xylem transports water and minerals from the roots up the plant stem and into the leaves.
In a mature flowering plant or tree, most of the cells that make up the xylem are specialised cells called vessels.
Vessels:
• Lose their end walls so the xylem forms a continuous, hollow tube.
• Become strengthened by a chemical called lignin. The cells are no longer alive. Lignin gives strength and support to the plant. We call lignified cells wood. Transport in the xylem is a physical process. It does not require energy.
PHLOEM
The phloem moves food substances that the plant has produced by photosynthesis to where they are needed for processes such as:
• growing parts of the plant for immediate use
• storage organs such as bulbs and tubers
• developing seeds Transport in the phloem is therefore both up and down the stem. Transport of substances in the
phloem is called translocation. Phloem consists of living cells. The cells that make up the phloem are adapted to their function:
• Sieve tubes – specialised for transport and have no nuclei. Each sieve tube has a perforated end so its cytoplasm connects one cell to the next.
• Companion cells – transport of substances in the phloem requires energy. One or more companion cells attached to each sieve tube provide this energy. A sieve tube is completely dependent on its companion cell(s).
DIAGRAMS
DEFINITION OF THE DIFFERENT PARTS FOUND IT THE STEM
• These hydrogen bonds hold the layers of cellulose firmly together.
• Cellulose is very strong because of these bonds.
• Cellulose forms long straight chains rather than spirals like amylose.
• Cellulose forms microfibrils which are held together by glue like carbohydrates.
• This makes the material of the cell wall, which is flexible.
• When the cell is turgid, it supports the wall making the plant stand upright.
• When cells are flaccid the plant wilts. Microfibrils stuck together by a polysaccharide ‘glue’ made from short branched polysaccharides called hemicelluloses and pectins.
Hemicelluloses are polymers of other sugars (not glucose) e.g. xylose Pectins are polymers of a sugar acid (galacturonic acid) The strength of the cell wall comes form cellulose microfibrils Bundles of 60 – 70 straight unbranched cellulose molecules can lie parallel to form cellulose microfibrils
