Plant cells and cell walls: exam questions

Answer: B – amyloplast (stores starch, colourless, no thylakoids)

• similarity: both have a nucleus / cell surface membrane / mitochondria / ribosomes / RER / Golgi apparatus (any one)
• plant cell has a cellulose cell wall, animal cell does not
• plant cell has chloroplasts, animal cell does not
• plant cell has a large permanent vacuole (with a tonoplast), animal cell has none or small temporary vacuoles
• plant cell stores starch, animal cell stores glycogen
• Max 4. Do not accept a statement about one cell type only, as compare needs both.

• narrow channels / gaps that pass through the cell walls between adjacent cells
• lined with cell surface membrane and containing a strand of cytoplasm
• allow movement of substances / chemical signals / water between cytoplasm of neighbouring cells

• (a) chloroplast
• (b) double membrane / envelope
• (b) stacks of thylakoids / grana
• (b) starch grains (accept stroma as a feature)
• (c) site of photosynthesis
• Max 4. In (b) any two features, one mark each.

• cellulose is made of β-glucose and alternate glucose units are rotated through 180°
• this forms long, straight, unbranched chains
• many hydrogen bonds form between the –OH groups on adjacent chains
• each hydrogen bond is weak but there are very many, so together they are strong
• many chains bundled together form microfibrils, which are laid in layers / embedded in a matrix in the wall
• Max 4

• insoluble, so does not affect the water potential / does not move out of the cell by osmosis or diffusion
• compact (amylose coiled into a helix) so stores a lot of glucose in a small space
• amylopectin is branched, giving many ends where enzymes can hydrolyse it quickly to release α-glucose

• secondary thickening with cellulose microfibrils gives a strong, rigid wall that supports the plant / resists compression
• lignin is waterproof, so water cannot leak out of the vessel
• wall does not collapse inwards under tension / negative pressure from transpiration pull
• pits (thin areas of wall without thickening) allow water to move sideways into neighbouring cells
• Max 4

• fibres are sclerenchyma: long cells with thick lignified secondary walls
• cellulose microfibrils in layers / parallel to the length give high tensile strength, so the rope is difficult to break
• cells overlap / are long and narrow, so they can be twisted together and remain flexible