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AS Biology Unit 2 Topic 4 Revision notes

Plant Structure, properties and uses1) Compare the ultra-structure of plant cells (cell wall, chloroplasts,amyloplasts, vacuole, tonoplast, plasmodesmata, pits and middle lamella) with that of animal Plant cell structure cells.They also have a cell membrane cytoplasm nucleus mitochondria RER, SER Golgi apparatus ribosomes Like animals cells, plants cells are Eukaryotic. lysosomes

Cell wall layer of tough cellulose fibrils which surrounds all plant cells provides mechanical strength, rigidity and support for the cell prevents the cell from bursting Chloroplasts lens shaped many internal membranes (called lamellae) which contain chlorophyll (each lamella is called a thylakoid, and a stack of thylakoids forms a granum) The sites of photosynthesis which traps light energy and uses it to produce carbohydrates (sugar etc) from carbon dioxide and water. Vacuole large central organelle in most plant cells, usually nearly fills the cell surrounded by a membrane called the tonoplast full of fluid (= cell sap; solution of sugars, mineral salts etc) used for storage e.g. sucrose; produce turgor pressure (essential for support) Amyloplasts A type of membrane-bound organelle called a plastid containing starch (often called starch grains) Plamosdesmata Minute pores through the cell wall with cytoplasmic connections continuous with the cytoplasm in adjacent cells Middle lamella Layer between cell walls of adjacent plant cells Joins the cells together Made of a glue-like material containing polysaccharides called pectins

2) Compare the structure and function of the polysaccharides starch and

cellulose including the role of hydrogen bonds between -glucose molecules in the formation of cellulose microfibrils. Cellulose: -glucose molecules joined by 1,4 glycosidic bonds alternate -glucose molecules are rotated through 180o the chains can be very long with between 1000 10 000 glucose molecules this results in the cellulose molecule forming a very long straight unbranched chain the hydroxyl (-OH) groups project from both sides of the chain the strength of the cell wall comes from cellulose microfibrils (bundles of 60 70 straight unbranched cellulose molecules can lie parallel and held together by hydrogen bonds to form cellulose microfibrils) spaces between the microfibrils make cell wall freely permeable and can also hold water and allow water to travel by microcapillary action Starch Polymer of -glucose Composed of two aglucose polymers Unbranched amylose and branched amylopectin Starch molecules form a helix Compact 3D structure with lots of ends Acts as a store of glucose molecules from which glucose can be released when required Cellulose Polymer of -glucose Only one polymer Long straight unbranched chains Cellulose molecules are straight Cellulose chains held together by hydrogen bonds to form bundles Acts as a strong flexible structural component of cell walls

3) Explain how the arrangement of cellulose microfibrils in plant cellwalls and secondary thickening contribute to the physical properties of plant fibres, which can be exploited by humans. Location of cells for mechanical support Sclerenchyma and xylem are located in the vascular bundles around the outside edge of the stem This provides maximum resistance to compression forces (from weight of shoot system) and bending forces imposed by the wind How are tall trees supported? As plants get taller they need more strengthening to withstand compression from the increasing mass of the shoot bending forces from the effect of wind on the larger plant body

This is achieved by division of the cambium (in vascular bundles, between xylem and phloem) which produce more xylem, making wood, which produces the characteristic annual rings; one ring per year. The distinction between one ring and the next is due to the differing sizes of the xylem vessels produced: in spring the vessels are large but in summer they are smaller Xylem cells are produced by differentiation of the unspecialised cells produced by the cambium - xylem cell genes are switched on to produce the characteristics of a xylem vessel Water transport through xylem vessels Learn this wording: Water constantly evaporates from the surface of the cells in the substomatal cavity The water lost is replaced by water moving across the leaf by capillary action through the cell walls This, in turn, draws water out of the xylem vessels Hydrogen bonding between water molecules, called cohesion, allows continuous columns of water molecules to be pulled up the xylem by the transpiration stream to replace the water lost by the leaf. Hydrogen bonding between the water molecules and the cellulose and lignin of the xylem vessel walls, called adhesion, prevents the column falling due to gravity Water is constantly being removed from the roots into the xylem by diffusion to replace the water which has been transported upwards. Water moves into and across the root from the soil by diffusion and osmosis.

Movement of water from soil to atmosphere through the plant is called the transpiration stream 4) Compare the structures, position in the stem and function of sclerenchyma fibres (support) and xylem vessels (support and transport of water and mineral ions). Strengthening cells 2 major types of elongated cells with thick, strong lignified walls and no living cell contents: Sclerenchyma and Xylem vessels Characteristics: Cellulose microfibrils run lengthways cellulose cells walls are thickened by the addition of lignin which is impregnated between the cellulose microfibrils this makes cells stiffer so less flexible and increases the tensile strength reduces the permeablity (so xylem vessels become waterproof) lignified cells are dead Sclerenchyma cells: called fibres

Long, narrow cells with pointed ends to allow fibres to interlock closely (unlike xylem, they do not form tubes) Very thick lignified walls; strong and inflexible Dead cell (no living protoplast) Principal function is support

Xylem vessels Mature xylem tubes (called vessels have no living contents (i.e. no protoplast, so dead) form hollow tubes which consist of lignified wall only so water can flow freely in lumen Xylem vessels are elongated cells; these are joined end to end with no cross walls to form long continuous tubes Wall thickened with lignin which is waterproof (so prevents water leaking out) strong (so prevents vessel collapsing) Water can pass into and out of xylem via pits (perforated holes) in the wall. Lignified walls provide support, strength and flexibility. Hollow vessels forming continuous tubes allow for the transport of water and dissolved mineral ions

5) Describe how the uses of plant fibres and starch may contribute tosustainability, e.g. plant-based products to replace oil-based plastics. Plant fibres are useful to us: because fibres = bundles of sclerenchyma and xylem which can be extracted intact from plants plants from which fibres can be obtained are relatively easy to grow in quantity the fibres themselves are useful because they are long and thin, strong, flexible relatively easily and cheaply extracted, durable, resistant to decay (cellulose is difficult for decomposers to break down) Uses for plant fibres include: Clothing e.g. linen (from flax plants); Rope e.g. hemp and sisal; Sacking e.g. jute; Matting e.g. coir or coconut matting; Paper Sustainability: Advantages of growing for fibres (e.g. nettles) a sustainable resource i.e. re-grow after they are cut so no need to plant more or harvest seeds and plant those so remnants of one crop sustains the next one i.e. it is self-sustaining can be grown in poorer soils so wont use up valuable agricultural land make profitable use of marginal land use of fibres (and other plant products made from starch) replace need for oil-based plastics so reduce reliance of fossil fuels, and reduce CO 2 emissions to reduce global warming natural products so biodegradable by decomposers

Tissue Parenchyma

Structure and OccurrenceGreater part of the cortex and pith. May have chloroplasts. Unspecialised but can be modified.

RoleLittle structural specialisation but do consist of pectin, hemicellulose and cellulose resulting in uneven thickening. Site of physiological and biochemical processes. Turgid to help support. Storage. Elastic, flexible support in leaves and young stems. Thick primary cell walls, even thicker in corners. Dead due to cutting off of nutrients and water by thickening and impermeability. Impregnated with lignin to allow support in terms of tensile and compression strength. Secretes cutin a waxy substance. Reduces water loss; protection (eg hairs which may have chemcals); continuous barrier (invasion of pathogens); insulation New cells for secondary tissues of xylem and phloem


Modified parenchyma. Immediatley below the epidermis in young stems, in leaves and tip of mid-rib Modified parenchyma. Outer cortex in mature stems. In and around vascular bundle and xylem. Long narrow pointed cells known as fibres. Short and round scleroids. Can be groups or individual. Outermost layer , one cell thick. With or without stomata




Unspecialised cell between phloem and xylem

Sclerenchyma fibres and xylem vessels can be seen through a light microscope.

6) Describe how to determine the tensile strength of plant fibrespractically. Core Practical. The tensile strength of a fibre is a maximum load it can take before it breaks. Attach the fibre to a clamp stand and hang a weight from the other end. Keep adding weights, one at a time until the fibre breaks Record the mass needed to break the fibre, the higher the mass, the higher the tensile strength. Repeat the experiment with different samples of the same fibre