Plant Cell Wall
- 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.
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Plant Cell Wall (cont)
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.
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- 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.
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- 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.
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- 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.
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Secondary Cell Wall
- 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.
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- 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).
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Primary Plant Body
- 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.
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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.
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Growth of Primary Plant Body
- 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.
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- 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.
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- 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.
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Ground Tissue- Parenchyma
- 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.
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Ground Tissue- Collenchyma
- 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.
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Ground Tissue- Sclerenchyma
- 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.
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Ground Tissue- Sclerenchyma: Elongated Fibres
- 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.
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Ground Tissue- Sclerenchyma: Sclerids
- Sclereids are impregnated with lignin.
- They are packed together very tightly.
- They form very rigid support for plant stem.
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Cross-Section of Stem
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- Vascular bundles contain the transport tissues xylem and phloem.
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- 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.
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- 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.
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