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

Condensed recall for the Pearson Edexcel International A Level Biology specification (YBI11), Issue 2, Topic 4, statements 4.1 to 4.7 including Core Practical 7. This is International AS (IAS) content. For full explanations use the study guide, then test yourself with the practice questions. Also see the Edexcel A Level Biology hub, the checklist and the free diagnostics.

4.1 and 4.2: structures and what they do

Structure	Key features	Function
Cell wall	Cellulose microfibrils in a pectin and hemicellulose matrix	Support, prevents bursting, allows turgor
Middle lamella	Pectin-rich layer between two cells	Cements neighbouring cells together
Pits	Thin areas of wall, no secondary thickening	Movement of water and solutes between cells
Plasmodesmata	Channels through walls lined by the plasma membrane, with cytoplasm	Communication and transport between cells
Vacuole	Large, filled with cell sap	Stores solutes, keeps cell turgid
Tonoplast	Single membrane around the vacuole	Controls movement into and out of the vacuole
Chloroplast	Double membrane, grana, stroma, starch grains	Photosynthesis
Amyloplast	Double membrane, starch grains, no chlorophyll	Stores starch

Plant cell versus animal cell. Plant: wall, large vacuole, chloroplasts, starch, plasmodesmata. Animal: no wall, small vacuoles, glycogen, no chloroplasts.

Shared: nucleus, mitochondria, ribosomes, rough ER, Golgi, plasma membrane.

Method: recognising organelles in an EM image

1. Look for the boundary: double membrane (chloroplast, amyloplast) or single (vacuole).
2. Look inside: stacked discs mean grana; large pale blobs mean starch grains.
3. Wall features: the middle lamella is the central line between two cell walls; gaps through the wall are plasmodesmata.
4. Use the scale bar to check the size.

Magnification reminders

magnification = image size ÷ actual size. Put both sizes in one unit before dividing (1 mm is 1000 µm).

4.3: starch and cellulose

	Starch	Cellulose
Monomer	α-glucose	β-glucose
Bonds	α-1,4 glycosidic (amylose); α-1,4 and α-1,6 (amylopectin)	β-1,4 glycosidic
Shape	Amylose coiled; amylopectin branched	Straight, unbranched
Holding structure	Hydrogen bonds stabilise the coil	Hydrogen bonds between parallel chains make microfibrils
Role	Energy store	Structural, cell wall strength
Why suited	Compact, insoluble, no effect on water potential, many ends for quick hydrolysis	Many hydrogen bonds give high tensile strength; insoluble

Why cellulose chains are straight: every second β-glucose is turned upside down so the –OH groups line up for β-1,4 bonds.

Glycosidic bonds hold residues in a chain. Hydrogen bonds hold different chains together.

Number of water molecules released in forming a chain = number of residues − 1.

4.4 and 4.5: wall structure, xylem, sclerenchyma, phloem

• Primary wall: loose mesh of microfibrils, allows growth.
• Secondary wall: layers of microfibrils at different angles, and lignin. Stronger, waterproof, rigid.
• Lignin deposits are in rings, spirals or networks in xylem, which leaves gaps between the thickened parts, where water can move sideways, and keeps flexibility.
• Strength comes from cellulose microfibrils plus lignin: strong in tension and compression.

Feature	Sclerenchyma fibre	Xylem vessel	Phloem sieve tube
Function	Support	Support plus water and mineral ion transport	Translocation of organic solutes
Living?	No	No	Yes
Wall	Thick, lignified	Lignified, with pits	Thin, unlignified
Lumen	Narrow, empty	Wide, empty, no end walls	Cytoplasm present, sieve plates
Position in stem	Outer part of vascular bundles and cortex	Inner side of vascular bundle	Outer side of vascular bundle
Helper cells	None	None	Companion cells

Plant fibres (flax, hemp, jute) are sclerenchyma fibres. Their high tensile strength and flexibility let people spin them into rope, cloth and paper and use them in composites.

4.6 Core Practical 7

Task	Rules
Plan diagram	Low power, outlines of tissues only, no cells, no shading, single sharp lines, correct proportions, title, magnification, ruled label lines
Cell drawing	High power, a few cells, clear wall outlines, label the features you can see
Identify	Lignin stains help find xylem and sclerenchyma; sieve plates are easier to see in longitudinal section

Typical stem order, from the outside: epidermis, cortex (with sclerenchyma), vascular bundles in a ring (phloem outermost, xylem innermost), pith.

Typical root order: epidermis, cortex, endodermis, vascular tissue in the centre.

Typical leaf order: upper epidermis, palisade mesophyll, spongy mesophyll with vascular bundles, lower epidermis with stomata.

4.7: sustainability

Aspect	Oil-based plastic	Plant-based alternative
Source	Non-renewable crude oil	Renewable crops
Carbon dioxide	Released from fossil carbon	Taken up by the crop as it grows
End of life	Persists for a long time	Can often be composted or biodegraded
Drawbacks	Pollution, waste	Land, water and fertiliser use; energy for processing; some need industrial composting

Plant-based products: starch-based plastics, fibre composites, paper packaging, natural rope and textiles.

Worked reminders

Scale bar. A bar of 24 mm on an image stands for 2 µm. Magnification = 24 000 µm ÷ 2 µm = ×12 000. A structure that measures 18 mm on the image is 18 000 ÷ 12 000 = 1.5 µm long.

Plan diagram. A root section is 2.5 mm across and your drawing is 75 mm wide. Magnification = 75 ÷ 2.5 = ×30. A tissue 6 mm wide on the drawing is 6 ÷ 30 = 0.2 mm in the section.

Mass of a polymer. A chain of 500 glucose residues (Mr of glucose 180, water 18): 500 × 180 − 499 × 18 = 90 000 − 8 982 = 81 018.

Describing and explaining: sentence frames

• Structure to function: “Cellulose chains are straight, so many hydrogen bonds form between neighbouring chains, producing microfibrils with high tensile strength.”
• Xylem: “Lignin in the thickened wall prevents the vessel collapsing under the tension in the transpiration stream, and the empty lumen with no end walls gives an unobstructed tube.”
• Phloem: “Sieve plates let the solution flow between sieve tube elements, and the companion cells supply the living functions that the sieve tube elements lack.”
• Sclerenchyma: “Thick lignified walls and tapering, overlapping ends make the fibres strong and flexible, giving support to the stem.”
• Starch: “Amylopectin has many branches, so many enzymes can act at once and glucose is released quickly.”

Linking tissues to their job

Question to ask	Xylem	Sclerenchyma	Phloem
Does it need to be hollow?	Yes, for water flow	Almost, narrow lumen only	No, cytoplasm carries the solutes
Does it need to be waterproof?	Yes, lignin	Lignin gives resistance to decay	No, solutes move through the cell
Does it need living contents?	No	No	Yes, with the companion cell
Does it need to be strong?	Yes, resists collapse	Yes, supports and resists pulling	Moderate

Comparing electron microscope features

Organelle	What you see	Common confusion
Chloroplast	Double membrane, grana stacks, stroma, starch grains	Mitochondrion has cristae, not stacked discs
Amyloplast	Double membrane, large pale starch grains, no grana	Looks like a chloroplast but has no thylakoids
Vacuole	One boundary line, plain interior	Often wrongly labelled as a vesicle; it is far larger
Cell wall	Thick, fibrous band outside the plasma membrane	Plasma membrane sits inside the wall
Plasmodesma	Narrow channel crossing the wall	Pit is a thin area, not a channel

Quick self-test

1. A nucleus image measures 28 mm and its actual diameter is 2.0 µm. What is the magnification?
2. A drawing of a cell is 45 mm long at ×600. What is the actual length in µm?
3. How many water molecules are released when a cellulose chain of 250 glucose residues forms?
4. A starch-based film loses mass in compost, from 62 g to 40 g. What is the percentage decrease?
5. Name the membrane around the vacuole.
6. Which plant tissue transports sucrose and is living?
7. What is the function of the middle lamella?
8. State two ways cellulose differs from starch.
9. Why do xylem vessels not collapse under tension?
10. State two features that identify a plan diagram as well drawn.

Answers

1. 28 000 µm ÷ 2.0 µm = ×14 000
2. 45 000 µm ÷ 600 = 75 µm
3. 250 − 1 = 249
4. (62 − 40) ÷ 62 × 100 = 35.5%
5. The tonoplast.
6. Phloem (sieve tubes).
7. It cements neighbouring cells together.
8. Cellulose is made of β-glucose and is straight with hydrogen bonds between chains; starch is made of α-glucose and is coiled or branched.
9. Their walls are thickened and strengthened with lignin.
10. Any two: no individual cells, no shading, sharp lines, correct proportions, labelled, title and magnification.

Where marks are usually lost

• Writing “cellulose is a branched polymer” or “starch is made of β-glucose”.
• Saying hydrogen bonds hold glucose residues together in a chain.
• Failing to state that many hydrogen bonds together, not one, give strength.
• Mixing up the tonoplast (vacuole membrane) and the plasma membrane.
• Calling chloroplasts and amyloplasts the same, and forgetting amyloplasts store starch.
• Not converting mm and µm before working out magnification.
• Drawing cells or shading in a plan diagram.
• Putting phloem inside the xylem in a stem diagram.
• Writing “strong” without saying why (lignin, microfibrils, hydrogen bonds).
• Evaluating bioplastics one-sidedly and ignoring land and energy use.

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.