Pearson Edexcel International A Level Biology: Plant Cells, Starch, Cellulose and Fibres (YBI11)

This guide is written for the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2. It covers statements 4.1 to 4.7 of Topic 4, including Core Practical 7. Topics 1 to 4 belong to the International AS (IAS) and Topics 5 to 8 to the International A2 (IA2), so this unit is IAS content. There are no tiers.

Use it with the revision notes and the practice questions. Hub: Edexcel A Level Biology. Printable checklist. To find weak spots first, try the free diagnostics.

What this unit covers

Statement	What you must be able to do
4.1	Know the structure and ultrastructure of plant cells (cell wall, chloroplast, amyloplast, vacuole, tonoplast, plasmodesmata, pits, middle lamella), compare with animal cells, and explain what each structure does
4.2	Recognise these organelles in electron microscope (EM) images
4.3	Explain the structure and function of starch and cellulose, including hydrogen bonds between β-glucose molecules in microfibrils
4.4	Link microfibril arrangement and secondary thickening to the properties of xylem vessels and sclerenchyma fibres, and to human uses of plant fibres
4.5	Compare sclerenchyma fibres, xylem vessels and phloem: structure, position in the stem and function
4.6	Core Practical 7: use a light microscope to draw plan diagrams and cells, and identify tissues
4.7	Explain how plant fibres and starch can contribute to sustainability, including replacing oil-based plastics

4.1 and 4.2: plant cell ultrastructure

Plant and animal cells are both eukaryotic. Both have a nucleus, rough and smooth endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes and a plasma membrane. A plant cell also has the structures below.

Cell wall. A layer outside the plasma membrane made of cellulose microfibrils embedded in a matrix of other polysaccharides such as pectin and hemicellulose. It is fully permeable to water and solutes. It resists expansion, so when the cell takes in water the protoplast pushes against the wall and the cell becomes turgid. The wall supports the cell and stops it bursting.

Middle lamella. The layer shared by two neighbouring cells, rich in pectin, that sticks the cells together.

Pits. Regions where the wall is thin, because no secondary wall is laid down there. Pits in neighbouring cells usually line up, so water and dissolved substances can pass between them without crossing a thick wall.

Plasmodesmata. Narrow channels through the cell walls, lined by the plasma membrane and carrying a thread of cytoplasm that links neighbouring cells. They allow movement of substances between cells (the symplast pathway) and communication.

Large permanent vacuole and tonoplast. The vacuole is filled with cell sap (water, sugars, mineral ions, pigments, waste). It is bounded by a single membrane, the tonoplast. It stores solutes, helps keep the cell turgid by osmosis, and the tonoplast controls what enters and leaves.

Chloroplast. Surrounded by a double membrane (the envelope). Inside is the stroma, which contains starch grains, circular DNA and small ribosomes, and stacks of thylakoid membranes called grana. Chloroplasts carry out photosynthesis.

Amyloplast. A plastid that stores starch as grains. It has a double membrane, no thylakoids and no chlorophyll. Amyloplasts are common in storage tissues such as potato tubers.

Feature	Plant cell	Animal cell
Outer wall	Cellulose wall present	None
Shape	Fixed, regular	Variable, often rounded
Vacuole	Large and permanent	Small and temporary, if present
Chloroplasts	Present in photosynthetic cells	Absent
Carbohydrate store	Starch	Glycogen
Links between cells	Plasmodesmata	Gap junctions

Recognising organelles in EM images (4.2)

Use size, outline and internal pattern.

• Chloroplast: oval, double membrane, internal stacks of discs (grana) and often pale oval starch grains.
• Amyloplast: similar double membrane but filled with large pale starch grains and no grana.
• Vacuole: large, clear, single boundary line (the tonoplast), often pushing the nucleus to the edge.
• Cell wall and middle lamella: a thick grey band between cells. The middle lamella is the thin line in the centre, often a different shade.
• Plasmodesmata and pits: narrow gaps crossing the wall (plasmodesmata) or thin areas of wall (pits).

Worked example 1: magnification from an EM image

A chloroplast is 4.5 µm long. Its image on an EM photograph measures 54 mm.

magnification = image size ÷ actual size Convert to the same unit: 54 mm = 54 000 µm. magnification = 54 000 ÷ 4.5 = ×12 000

Worked example 2: using a scale bar

A scale bar on an EM image is 30 mm long and represents 1.5 µm. The cell wall measures 33 mm across the image.

magnification = 30 mm ÷ 1.5 µm = 30 000 µm ÷ 1.5 µm = ×20 000 actual thickness = 33 000 µm ÷ 20 000 = 1.65 µm

4.3: starch and cellulose

Both are polysaccharides made of glucose, joined by glycosidic bonds formed in condensation reactions. They differ because they use different isomers.

Starch is made from α-glucose. It is the plant’s energy store and has two parts.

• Amylose: an unbranched chain with α-1,4 glycosidic bonds. It coils into a helix, which makes it compact.
• Amylopectin: α-1,4 chains with α-1,6 branches. The branches give many ends where enzymes can release glucose quickly.

Starch is insoluble, so it has no effect on water potential and does not leave the cell. Because it is large it cannot diffuse out, and it is easily hydrolysed to glucose for respiration. It is stored as grains in chloroplasts and amyloplasts.

Cellulose is made from β-glucose. In β-glucose the –OH on carbon 1 is above the ring. To make a β-1,4 link, each residue must be inverted relative to its neighbour, so the chain is straight and unbranched, not coiled.

Each glucose residue has –OH groups that stick out sideways. These form many hydrogen bonds with –OH groups on parallel chains. One bond is weak, but thousands together hold the chains side by side in bundles called microfibrils, which group into larger fibres. This gives very high tensile strength.

Property	Starch	Cellulose
Monomer	α-glucose	β-glucose
Links	α-1,4 (and α-1,6)	β-1,4
Shape	Helical or branched	Straight chains
Hydrogen bonding	Within the helix	Between chains, forming microfibrils
Function	Energy store	Structural strength in cell walls

Worked example 3: condensation in a chain

An amylose molecule has 1000 glucose residues. A glucose molecule has Mr 180 and water has Mr 18.

Number of glycosidic bonds = 1000 − 1 = 999, so 999 water molecules are released. Mr of chain = 1000 × 180 − 999 × 18 = 180 000 − 17 982 = 162 018

4.4 and 4.5: fibres, xylem and phloem

Microfibril arrangement. In a primary wall the microfibrils lie in a loose mesh, so the wall can still stretch as the cell grows. In a secondary wall they are laid in several layers, with the direction changing between layers. The layers resist forces from different directions. In a fibre, microfibrils at a small angle to the long axis give high strength when the fibre is pulled.

Secondary thickening. Extra wall material, including lignin, is deposited inside the primary wall. Lignin is a rigid, waterproof polymer that bonds with the cellulose and matrix. It makes the wall stronger and resistant to compression and decay. In xylem and sclerenchyma the cell contents die, leaving only the wall.

Comparing the three tissues

Feature	Sclerenchyma fibres	Xylem vessels	Phloem (sieve tubes)
Function	Support	Support and transport of water and mineral ions	Translocation of organic solutes
Living?	Dead at maturity	Dead at maturity	Living
Cell shape	Long, narrow, tapered ends	Long tubes of joined cells	Long tubes of cells joined end to end
Walls	Thick, lignified	Thick, lignified in rings, spirals or networks, with pits	Thin cellulose walls
Ends	Overlap, no open ends	End walls broken down, forming a continuous tube	Sieve plates with pores, cytoplasm continuous
Contents	Empty lumen	Empty lumen, no cross walls	Little cytoplasm, no nucleus; companion cells beside them
Position in a stem	Outer side of vascular bundles and in the cortex or ring beneath the epidermis	Inner side of each vascular bundle	Outer side of each vascular bundle, outside the xylem

Xylem vessels are strong because the lignin prevents them collapsing inwards when water is under tension in the transpiration stream. The pits allow lateral movement of water. Sclerenchyma fibres have high tensile strength and are flexible but not elastic, which is why they can be exploited by humans.

Human uses. Fibres from plants such as flax, hemp and jute are spun for textiles, rope and string, and used in paper and in composite materials. Starch is used in foods, as a thickener and as a raw material for plastics.

4.6 Core Practical 7: microscopy of plant tissues

You must be able to use a light microscope to observe, draw and label plan diagrams of transverse sections of roots, stems and leaves, and also cells of plant tissues, and to identify sclerenchyma fibres, phloem, sieve tubes and xylem vessels.

Method for a plan diagram

1. Start on low power and focus on the section.
2. Draw outlines of tissue regions only, with sharp single lines and no shading. Do not draw individual cells.
3. Keep the proportions of tissues the same as on the slide.
4. Label each tissue with ruled label lines that do not cross, and add a title and the magnification.

Method for drawing cells. Switch to high power. Draw a few representative cells, drawing the cell walls with clean lines. Show distinguishing details such as thick walls and sieve plates, and label them.

Stains. Stains such as toluidine blue or phloroglucinol highlight lignified walls, helping you find xylem and sclerenchyma. Longitudinal sections show end walls and sieve plates more easily than transverse ones.

Worked example 4: magnification of a plan diagram

A stem section has an actual diameter of 3.2 mm. Your plan diagram is 96 mm across. magnification = 96 ÷ 3.2 = ×30 The xylem region measures 9 mm on the drawing, so its actual width = 9 ÷ 30 = 0.3 mm.

4.7: sustainability

Oil-based plastics come from a non-renewable resource, take a very long time to break down and add to pollution. Plant-based alternatives include:

• bioplastics made from starch, for example from maize or potato, or from sugars made from starch by fermentation
• fibre composites in packaging, insulation and building materials, using hemp or flax fibres
• paper and card in place of plastic wrap

Advantages: the plants are renewable, they take up carbon dioxide as they grow, and many products can be composted or biodegraded. Limits to weigh up: crops need land, water and fertiliser, land used for materials may compete with food, processing uses energy, and some bioplastics break down only in industrial composting.

Common errors

• Writing that cellulose has branches or a helix. It is straight, unbranched chains.
• Saying hydrogen bonds join glucose residues in a chain. Glycosidic bonds do that; hydrogen bonds join separate chains.
• Describing xylem as living, or phloem as dead.
• Drawing cells in a plan diagram, or shading.
• Forgetting to convert µm and mm before finding magnification.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification, Issue 2, February 2021, Pearson Education Limited: Topic 4, statements 4.1 to 4.7.