Structure and Transport in Plants

• The plant cell wall is rigid, containing cellulose microfibrils running through a matrix of hemicelluloses and pectic substances.
• Cellulose is made of beta glucose (one inverted after the other), joined by 1,4 glycosidic bonds.
• Hydrogen bonding between cellulose molecules (-OH and O) creates cross-linking between cellulose chains, forming cellulose microfibrils.
• The hemicelluloses and pectic substances act as a glue, holding cellulose microfibrils in place. 

The plant cell wall:

• Provides mechanical support (when turgid) and protection.
• Prevents osmotic bursting of the cell.
• Provides a pathway for movement of water and mineral salts.
• Can become lignified.
• Matrix of hemicelluloses and cellulose microfibrils is fexible, so the plant can bend with the wind.  

• The middle lamela is a thin layer of pectic substances.
• It is located between the cell walls of 2 neighbouring plant cells.
• Middle lamela glues neighbouring cells together. 

• Plasmodesmata are fine threads linking the cytoplasms of two neighbouring cells together.
• The plasmodesmata run from pores in a cell well, through the middle lamela, and through the neighbouring cell wall, linking 2 cytoplasms.
• They enable a continuous system of cytoplasm, the symplast, to be formed between neighbouring cells for transport of substances. 

• The tonoplast is the large central vacuole in a plant cell.
• It is a single-membrane bound sac containing cell sap.
• Cell sap contains mineral salts, sugars and enzymes.
• Tonoplast also stores waste products, and can sometiimes function as a lysosome. 

• When a plant cell is finished growing, it can deposit excess material and form a secondary cell wall.
• The excess material may be lignin, which makes wood.
• It may also be suberin, which makes cork.
• The secondary cell wall serves supporting or waterproofing roles. 

• Amyloplasts are specialised plant organelles.
• They store amylopectin and are used to provide energy when the cell needs it.
• They are formed from leucoplasts, which are unspecialised plant cells.
• Amyloplasts are an example of plastids (storage organelles). 

• The body of a vascular plant is divided into 3 principle organs: the leaves, the stem and the roots.
• The shoot system of a plant consists of all the stems and leaves.
• The points where leaves attach to the stem are nodes.
• Regions between leaves (nodes) are called internodes. 

The 2 functions of the stem are support and transport:

• Support: to hold leaves in the best position for obtaining sunlight for photosynthesis.
• Support: to support flowers in a way that maximises the chance of pollination.
• Transport: transport of products from photosynthesis to other parts of the plant.
• Transport: transport water and other reactants of photosynthesis to photosynthesising leaves.

• Growth in plants is confined to certain regions called meristems.
• A meristem is a group of cells that can divide by mitosis.
• They produce daughter cells which grow and form the rest of the plant body. 
• The daughter cells are unspecialed and totipotent. 
• The three types of meristematic tissue are protoderm, procambium and ground meristem, which give rise to the epidermis, vascular tissues and ground tissues (parenchyma) respecitively. 

• The epidermis protects cells beneath it.
• Epidermal cells secrete cutin, a waxy substance that forms the waxy cuticle.
• The waxy cuticle helps prevent water loss from stem surface. It also prevents pathogens from entering.
• Epidermal cells may form hairs, which act as an insulating layer, be hooked to help climbing plants, or be irritant for protection against preditors. 

• Ground tissue is found benieth the dermal tissue in the cortex.
• The ground tissue system carries out photosynthesis, stores photosynthetic products, and helps support the plant. 
• There are 3 types of ground tissue: parenchyma, collenchyma and slerenchyma.

• Parenchyma are unspecialised living cells, with a primary cell wall, shared middle lamela and large central vacuole.
• It serves as packing material and supports the stem.
• They are modified to store various substances such as starch, or so that they can carry out photosynthesis.
• Some parenchyma are modified to form collenchyma or sclerenchyma. 

• Collenchyma are living cells that support the stem.
• They have thick primary cell walls and no secondary cell wall.
• They provide support for perioles and growing organs.
• They are quite flexible, allowing a stem to sway in the wind without snapping. 

• Sclerenchyma are dead cells.
• They have a primary cell wall, and a thickened secondary cell wall.
• Their main function is also support.
• There are 2 forms of schlerenchyma: elongated fibres and sclerids. 

• Elongated fibres (made of cells) are found in bundles.
• Lignin is deposited to form lingified secondary cell wall. This makes the fibres strong yet flexible.
• Once the fibre is lignified the cell contents die to form a hollow tube (lumen) in middle of fibre.
• Lignin is permeable to water, so water enters fibres and flows through hollow lumen. 

• Sclereids are impregnated with lignin.
• They are packed together very tightly.
• They form very rigid support for plant stem. 

• Vascular bundles contain the transport tissues xylem and phloem.

• The xylem is a water transport system.
• Protoxylem is living tissue.
• Protoxylem becomes lignified and impermeable to water, the contents of the cell die and hollow tubes are formed inside.
• This lignified tissue is called metaxylem.
• Water moves out of xylem into surrounding cells through non-lignified parts of the wall, or through pits in the wall. 

• The phloem is a long line of living cells that meet end to end, forming seive tubes,
• The phloem transport food from their sources to tissues that either consume the food or store it.
• The sieve tube elements (hollow end to end cells) have adjacent companion cells that maintain all the phloem's organalles and may regulate the preformance of the sieve tube elements.