What you'll learn

• How plant cells are built, including walls, vacuoles, chloroplasts, amyloplasts and plasmodesmata.
• Why starch is a good storage molecule in plants.
• Why cellulose is strong enough to support plant cell walls.
• How to connect plant cell structure to microscopy, staining and exam questions.

Plant cells are eukaryotic cells

A cell is the basic unit of life. Plant cells are eukaryotic, meaning they have internal compartments called organelles surrounded by membranes.

Plant cells share some structures with animal cells: a cell surface membrane, cytoplasm, nucleus, mitochondria and ribosomes. Their distinctive features include a cellulose cell wall, large permanent vacuole, chloroplasts, amyloplasts and connections between cells called plasmodesmata.

The diagram pulls together the main plant cell structures you should recognise in diagrams and photomicrographs.

Key plant cell structures

Cell wall, middle lamella and pits

The cell wall is a strong outer layer found outside the cell surface membrane. In plants, it is mainly made of cellulose, a structural polysaccharide. It gives support and prevents the cell from bursting when water enters by osmosis.

The middle lamella is a pectin-rich layer between neighbouring plant cells. It acts like a cement, holding adjacent cell walls together.

Pits are thinner regions of the cell wall, often where plasmodesmata pass between cells. They make cell-to-cell communication easier.

Vacuole and tonoplast

Most mature plant cells contain a large permanent vacuole filled with cell sap, a solution of water, ions, sugars, amino acids and other dissolved substances.

The membrane around the vacuole is the tonoplast. It helps control movement of substances between the cytoplasm and the vacuole.

Chloroplasts and amyloplasts

Chloroplasts are organelles where photosynthesis takes place. They contain the green pigment chlorophyll, which absorbs light energy.

Amyloplasts are colourless storage organelles found especially in storage tissues such as potato tubers and seeds. They store starch grains.

Both chloroplasts and amyloplasts are types of plastid, a group of plant organelles involved in making or storing useful molecules.

Microscopy link: measuring plant cells

In practical work, you may view plant tissue using a light microscope. A thin section or epidermal peel is placed on a slide, stained if needed, covered with a coverslip, and observed from low power to higher power.

If you measure cells from a micrograph, use the magnification equation:

$$M=\frac{image\; size}{actual\; size}$$

Here, magnification means how many times larger the image is than the real object. Magnification has no units.

Glucose monomers and polysaccharides

A monomer is a small molecule that can join to many similar molecules. A polymer is a large molecule made from repeating monomers.

Glucose is a monosaccharide, which means a single sugar unit. Plants use glucose to build larger carbohydrates, especially starch and cellulose.

A glycosidic bond is the covalent bond between two monosaccharides. It forms in a condensation reaction, where water is removed. It breaks in a hydrolysis reaction, where water is added.

Glucose exists in different forms called isomers. Alpha-glucose and beta-glucose have the same molecular formula, but the hydroxyl group on carbon 1 is arranged differently. This small difference has a huge effect on polymer shape and function.

The diagram compares how alpha-glucose forms starch and beta-glucose forms cellulose.

Starch: the plant storage polysaccharide

Plants make glucose during photosynthesis. They often convert glucose into starch for storage.

Starch is a mixture of two polysaccharides:

• Amylose: an unbranched chain of alpha-glucose joined by alpha-1,4 glycosidic bonds. The chain coils into a compact helix.
• Amylopectin: a branched chain of alpha-glucose with alpha-1,4 bonds along the chain and alpha-1,6 bonds at branch points.

Starch is useful for storage because it is:

• Insoluble, so it does not easily diffuse out of cells and has little effect on osmosis.
• Compact, so lots of glucose can be stored in a small space.
• Easily hydrolysed, so glucose can be released for respiration when needed.
• Branched in amylopectin, giving many ends where enzymes can work at once.

You can test plant material for starch using iodine solution. Iodine solution is orange-brown and turns blue-black if starch is present. For example, potato tissue often stains strongly because its amyloplasts contain many starch grains.

Cellulose: the plant structural polysaccharide

Cellulose is made from beta-glucose monomers joined by beta-1,4 glycosidic bonds. Each beta-glucose is rotated compared with the next one, producing long, straight chains.

Many cellulose chains run alongside each other. Hydrogen bonds form between chains. A hydrogen bond is a weak attraction between slightly charged parts of neighbouring molecules, but many hydrogen bonds together create great strength.

Groups of cellulose chains form microfibrils, and these microfibrils help build the cell wall. This gives plant cell walls high tensile strength, meaning they resist being stretched or pulled apart.

Cellulose is excellent for support because it is:

• Strong, due to many hydrogen bonds between parallel chains.
• Straight and unbranched, so chains can pack closely.
• Insoluble, so it remains in the wall.
• Difficult for many organisms to digest without the enzyme cellulase.

Humans produce amylase, an enzyme that digests starch, but we do not produce cellulase. That is why starch can be digested as an energy source, while cellulose acts as dietary fibre.

Comparing starch and cellulose

Practical skills to keep in mind

When preparing or analysing plant cell slides:

• Make the tissue thin so light can pass through and cells do not overlap too much.
• Lower the coverslip at an angle to reduce air bubbles.
• Use a stain only when it improves contrast or identifies a substance, such as iodine solution for starch.
• If measuring with an eyepiece graticule, calibrate it using a stage micrometer at the same objective lens.
• Draw what you see with clear continuous lines, labels and a stated magnification or scale bar.