Plant Structure

Three structures present in plant cells but absent from animal cells are:

• Cellulose cell wall — gives the cell support and shape and prevents it bursting when it takes up water.
• Chloroplast — the site of photosynthesis, where light energy is used to make glucose.
• Large permanent vacuole (bounded by the tonoplast) — stores cell sap and keeps the cell turgid, providing support.

(An amyloplast, which stores starch, or plasmodesmata, which connect adjacent cells, would also be acceptable answers.)

Cellulose is made from the monomer β-glucose. The monomers are joined together by β-1,4 glycosidic bonds, formed by condensation reactions between carbon 1 of one β-glucose and carbon 4 of the next.

A xylem vessel is made of dead, empty cells joined end to end to form a long, continuous, hollow tube. The end walls have broken down, so there are no cross-walls to interrupt the flow — this allows water to move up the plant as a continuous, uninterrupted column.

The walls are thickened with lignin, which is strong and waterproof. The lignin provides support and stops the vessel collapsing inward under the tension created as water is pulled up, while keeping the water within the vessel. Pits (unlignified gaps in the wall) allow water to move sideways between adjacent vessels.

Cellulose is a polysaccharide made of many β-glucose monomers joined by β-1,4 glycosidic bonds in condensation reactions. Because of the orientation of the —OH group on carbon 1 in β-glucose, every other monomer is rotated (flipped) through 180° so that the bonds can form. This produces long, straight, unbranched chains.

The straight chains lie side by side, and many hydrogen bonds form between the —OH groups of adjacent parallel chains. Although each individual hydrogen bond is weak, there are so many of them that, collectively, they hold the chains tightly together, bundling around 60–70 chains into a strong structure called a microfibril. Microfibrils in turn group into larger macrofibrils.

Within the cell wall, the microfibrils are laid down in layers running in different directions (a cross-ply or net-like arrangement) and are embedded in a matrix of other polysaccharides. This crossed arrangement gives the wall high tensile strength in every direction, so it resists stretching and prevents the cell from bursting as it takes up water by osmosis.

At the same time, the wall remains fully permeable: there are spaces between the microfibrils through which water and dissolved solutes can pass freely, so the wall does not control what enters or leaves the cell — that is the job of the cell surface membrane.

Phloem is a living tissue made up of two main cell types: sieve tube elements and companion cells.

Sieve tube elements are living cells joined end to end to form continuous tubes. Their end walls are perforated to form sieve plates, which allow the sap (a solution of sucrose and amino acids) to flow from one element to the next. At maturity the sieve tube elements lose most of their organelles, including the nucleus, leaving a clear channel that does not obstruct the flow of assimilates.

Because the sieve tube elements have so few organelles, each is supported by a companion cell. Companion cells have a nucleus and are packed with mitochondria, which provide the ATP needed for the active loading of sucrose into the phloem. The companion cell is connected to the sieve tube element by plasmodesmata, so it can carry out the metabolic functions on which the sieve tube depends. Together these adaptations allow assimilates to be translocated efficiently from sources (such as leaves) to sinks (such as roots).

A plan (low-power) diagram shows the distribution and arrangement of the tissues in the section — for example the positions of the epidermis, cortex, vascular bundles (xylem and phloem), sclerenchyma and pith. The tissues are drawn as blocks bounded by lines, and no individual cells are drawn. By contrast, a high-power drawing shows only a small number of individual cells in detail, including their cell walls and contents.

To gain full marks on the plan diagram, the student should:

• Use a sharp pencil and draw clear, single, continuous lines with no shading or sketching.
• Draw the tissues in the correct positions and proportions, so the relative sizes of cortex, vascular bundles and pith match the specimen.
• Show no individual cells — only the boundaries between the different tissues.
• Label each tissue with ruled label lines that touch the structure being labelled.
• Include a scale bar or magnification, give the drawing a title, and make it large enough to fill at least half of the available space.

Drawing individual cells, shading, or using sketchy double lines are the errors that most commonly lose marks on a plan diagram.

Sclerenchyma fibres are long, narrow, elongated cells that are dead at maturity, having lost their cytoplasm and other cell contents so that the cell is hollow and contributes nothing but its wall.

Their most important feature is that the cell walls are heavily thickened and impregnated with lignin. Lignin is a strong, rigid and waterproof material. Because it is deposited in large amounts all around the wall, it gives the fibre a very high tensile strength — the cell can withstand large pulling (tensile) and compressive forces without breaking or being crushed. This is exactly what is needed for mechanical support, holding stems and leaves upright.

The fibres are typically grouped together in bundles and run longitudinally through the stem and around the vascular tissue. Grouping many lignified cells together combines their individual strength so that the bundle resists bending and supports the weight of the plant. Each fibre also has tapered, overlapping ends that interlock with neighbouring fibres, transmitting forces between cells and adding to the strength of the bundle.

Because the cells are dead and lignin is waterproof and indigestible, sclerenchyma provides permanent, long-lasting support without needing to remain turgid (unlike support from turgid parenchyma). Its only role is support and strength — unlike xylem it does not transport water.

Both cellulose and starch are polymers of glucose, but they are built from different isomers joined by different bonds, and this is what suits each to its role.

Cellulose is made of β-glucose joined by β-1,4 glycosidic bonds. Because the —OH group on carbon 1 of β-glucose points upwards, alternate molecules must be flipped 180° for the bonds to form, producing long, straight, unbranched chains. Many of these chains lie in parallel and are cross-linked by large numbers of hydrogen bonds to form strong microfibrils. The microfibrils are arranged in different directions in the cell wall, giving it high tensile strength, which is exactly what is needed for a structural / support role — the wall resists stretching and stops the cell bursting.

Starch is made of α-glucose joined by α-1,4 glycosidic bonds (with α-1,6 branches in amylopectin). Because of the orientation of the —OH group in α-glucose, the chains coil into a helix (amylose) or become branched (amylopectin) rather than lying straight. This makes starch compact and insoluble, so it can be packed densely into amyloplasts without affecting water potential — ideal for energy storage. The branching of amylopectin also means many free ends are available for rapid hydrolysis when glucose is needed.

So the same monomer family, joined by different bonds and isomers, produces a straight, fibre-forming molecule for support (cellulose) and a coiled, compact molecule for storage (starch).

(a) Trend. The mean breaking force increases from nettle to flax to hemp: nettle requires only 8.2 N to break, flax requires 12.6 N, and hemp requires the greatest force, 19.4 N. So of the three, hemp fibres withstand the largest force before breaking.

(b) Evaluation. The data do show that hemp has the highest breaking force, but it is not valid to conclude simply that hemp is the "strongest material", for several reasons.

Firstly, breaking force depends strongly on the diameter (cross-sectional area) of the fibre. If the hemp fibres tested were thicker than the others, they would break under a larger force even if the material were no stronger. The fair comparison is tensile strength = breaking force ÷ cross-sectional area, so the student should measure each fibre's diameter (e.g. with a micrometer) and calculate this.

Secondly, the reliability is uncertain: we are not told how many fibres of each species were tested. The student should test many fibres, calculate a mean and identify anomalies, and could show the spread using a measure such as the range or standard deviation.

Finally, other variables — fibre length, how the fibre was clamped, the rate at which force was added, temperature and humidity — must be controlled so that only the species differs. With these improvements the conclusion would be far more secure; as it stands, the most that can be said is that, under these conditions, hemp fibres had the highest mean breaking force.

Plant fibres from sclerenchyma and xylem are well suited to making ropes, fabrics and similar materials because of their structure. Both tissues consist of cells with walls thickened by lignin (and, in sclerenchyma, very heavily so). Lignin makes the cells strong and rigid, giving the fibres a high tensile strength — they can withstand large pulling forces without breaking, which is exactly the property needed for rope and woven fabric. The cells are also long and elongated, so they can be spun or twisted together into long threads, and they are lightweight.

There are several advantages to using plant fibres instead of oil-based materials such as nylon or other plastics:

• Plant fibres come from a renewable resource — crops such as flax (which gives linen), hemp and jute can be regrown each season, whereas oil is a finite, non-renewable resource.
• Plant fibres are biodegradable, so they break down naturally and do not accumulate as persistent plastic pollution in the environment.
• Growing the plants removes carbon dioxide from the atmosphere by photosynthesis, and manufacturing the fibres can use less energy than producing synthetic polymers from oil, which lowers the overall carbon footprint.

However, a balanced discussion should note some drawbacks: plant fibres may be less durable or weaker than some synthetic fibres, their properties can vary naturally between plants, and large-scale fibre crops require land and water that might otherwise be used for food. On balance, though, the renewability and biodegradability of plant fibres make them an attractive, more sustainable alternative to oil-based materials.