| Structure | Function |
|---|---|
| Cell wall | Structural support; prevents excess water uptake; made of cellulose microfibrils (primary) Β± lignin (secondary) |
| Middle lamella | Cements adjacent cells together; made of calcium pectate (pectin) |
| Plasmodesmata | Channels through cell wall; connect cytoplasm of adjacent cells; allow transport and communication |
| Pits | Thin areas in wall where secondary thickening is absent; allow water movement between cells |
| Vacuole (large, central) | Contains cell sap; maintains turgor pressure; stores ions and pigments |
| Tonoplast | Single membrane surrounding the vacuole; controls movement of substances in/out |
| Chloroplast | Site of photosynthesis; contains chlorophyll; double membrane + grana + stroma |
| Amyloplast | Stores starch grains; found in storage cells (e.g. potato); double membrane, no thylakoids |
| Feature | Plant | Animal |
|---|---|---|
| Cell wall | β | β |
| Chloroplasts | β (photosynthetic cells) | β |
| Amyloplasts | β (storage cells) | β |
| Large vacuole | β permanent | β (small, temp.) |
| Tonoplast | β | β |
| Centrioles | β | β |
| Carbohydrate store | Starch (in amyloplasts) | Glycogen (cytoplasm) |
| Nucleus, mitochondria, ER, Golgi | β | β |
| Structure | EM Appearance | Key Identifier |
|---|---|---|
| Cell wall | Thick, moderately electron-dense layer outside the cell surface membrane | Located outside the plasma membrane |
| Middle lamella | Thin, electron-dense line between two adjacent cell walls | Only visible between two neighbouring cells |
| Plasmodesmata | Narrow, dark channels through the cell wall | Thread-like gaps in cell wall connecting adjacent cells |
| Vacuole | Large, electron-lucent (pale/white) central space | Largest pale region in cell; central location |
| Tonoplast | Single, electron-dense membrane surrounding the vacuole | Single line bordering the pale vacuole |
| Chloroplast | Oval; double outer membrane; grana (stacked parallel dark lines = thylakoids); pale stroma | Internal parallel lines (grana) are unique to chloroplasts |
| Amyloplast | Double outer membrane; very electron-dense starch granules filling interior; no internal membrane system | Dark, featureless blobs inside a double-membrane organelle β no grana |
| Feature | Cellulose | Starch |
|---|---|---|
| Monomer | Ξ²-glucose | Ξ±-glucose |
| Bond | 1,4-glycosidic | 1,4 (amylose); 1,4 + 1,6 (amylopectin) |
| Chain shape | Straight, unbranched | Helical (amylose); branched (amylopectin) |
| H-bonds | Between parallel chains β microfibrils | Within helix β compact granule |
| Solubility | Insoluble | Insoluble |
| Function | Structural (cell wall) | Energy storage (amyloplasts) |
Q1. Describe the structure and function of plasmodesmata in plant cells. [2]
β’ Plasmodesmata are narrow channels / pores that pass through the cell wall (1) β’ They connect the cytoplasm of adjacent cells (1) β’ [Function:] They allow transport of water / small molecules / signals between cells (1) β award 1 mark for any valid function.
Q2. State TWO differences between the way plant cells and animal cells store carbohydrate. [2]
β’ Plant cells store starch; animal cells store glycogen (1) β’ Plant cells store carbohydrate inside amyloplasts / plastids; animal cells store glycogen granules in the cytoplasm (1)
Q3. A student is looking at a transmission electron micrograph of a plant leaf cell. They can see an organelle with a double outer membrane and stacked, parallel electron-dense internal membranes. Identify the organelle and give TWO pieces of evidence for your answer. [3]
β’ Organelle: Chloroplast (1) β’ Evidence 1: Double outer membrane / double (envelope) membrane visible (1) β’ Evidence 2: Stacked internal membranes = grana / thylakoids (1)
Q4. Explain how the structure of cellulose makes it suitable for its role in the plant cell wall. [4]
Award 1 mark each for any four of: β’ Made of Ξ²-glucose monomers (1) β’ Monomers joined by 1,4-glycosidic bonds; alternate monomers rotated 180Β° (1) β’ This produces a long, straight, unbranched chain (1) β’ Parallel chains held together by hydrogen bonds between βOH groups (1) β’ Chains form microfibrils (1) β’ Microfibrils arranged in layers at different angles / criss-cross (1) β’ Gives the cell wall high tensile strength / resists stretching (1)
Q5. State TWO structural differences between cellulose and starch (amylose). [2]
Any two of: β’ Cellulose contains Ξ²-glucose; starch contains Ξ±-glucose (1) β’ Cellulose chains are straight; starch (amylose) chains are helical / coiled (1) β’ In cellulose, hydrogen bonds form between parallel chains; in starch, hydrogen bonds form within the helix (1) β’ Cellulose is unbranched; starch (amylopectin) is branched β accept if amylopectin specified (1)