Brown Algae — a Kelp on the Kitchen Floor

Total: 95 marks — Investigation 1: 24 · Investigation 2: 19 · Investigation 3: 14 · Investigation 4: 12 · Investigation 5: 26.

Investigation 1 — examine a sea bamboo

• Q1 (5): A neat outline drawing in proportion (1); holdfast, stipe, pneumatocyst and fronds each correctly labelled with ruled lines (1 each, 4). (Deduct 1 if no scale line.)
• Q2 (3): Holdfast 2.6 ÷ 10.1 = 26 %; stipe 4.1 ÷ 10.1 = 41 %; fronds 3.4 ÷ 10.1 = 34 % (1 each; the float is under 1 %).
• Q3 (3): Volume displaced 0.48 L (1); mass of seawater displaced = 0.48 × 1.025 (1) = about 0.49 kg — the float can lift about half a kilogram (1).
• Q4 (4): In seawater the fronds weigh 30 ÷ 500 = 6 % of their weight in air (1): 3.4 kg × 0.06 ≈ 0.2 kg (1). The float lifts about 0.49 kg, so yes, it can hold them up (1). The seawater itself supports most of the fronds' weight, because kelp tissue is only a little denser than seawater — the float only has to make up the small difference (1).
• Q5 (2): The west coast has heavy Atlantic swell and strong surge (1); a kelp 7 m tall with a float pulling upward is dragged hard by every wave, so it needs a large, branching grip on the rock or it would be torn off — which is how this one was washed up (1).
• Q6 (3): A circle drawn with three labelled regions: outer dark skin (meristoderm) (1); thick pale middle layer (cortex) (1); central core of tangled threads (medulla) (1).
• Q7 (2): Kelps have no xylem or phloem — no true vascular tissue like a land plant (1); but the trumpet hyphae in the core are elongated cells, joined end to end, that carry dissolved sugars from the fronds down to the stipe and holdfast, so food is moved all the same (1).
• Q8 (2): Similar: it anchors the kelp, as roots anchor a plant (1). Different: it does not absorb water or nutrients — the kelp takes those in through all its surfaces from the seawater (1).

Investigation 2 — brown seaweeds and look-alikes

• Q9 (9): One mark each: 1 brown — sea bamboo, kelp (stipe with float and straps); 2 brown — split-fan kelp (stipe and split blade); 3 brown — spined kelp (stipe and spiny fronds); 4 brown — long-leafed sargassum (branched, with blades and floats); 5 brown — dead man's fingers (hollow fingers / cylinders); 6 red seaweed — tongue weed (flat blade); 7 vascular plant — eelgrass, a seagrass (ribbons with roots); 8 microalgae — a diatom film (single cells); 9 brown — a branched brown seaweed with cylindrical stems (no common name needed).
• Q10 (3): 6 — a red seaweed: its colour comes out in water, not acetone, so its pigment is phycoerythrin (1). 7 — a vascular plant (seagrass): roots, an underground stem and flowers (1). 8 — microalgae (diatoms): single cells in glass cases, not a seaweed (1).
• Q11 (3): Any three forms with an example: strap-shaped fronds — sea bamboo; flat blade split into fingers — split-fan kelp; finger-like hollow tubes — dead man's fingers; cylindrical branches — hanging wrack; branched with blades and floats — sargassum.
• Q12 (2): Sea bamboo's float lifts its fronds to the surface from up to 15 m down (1). Split-fan kelp has a stiff, solid stipe that holds its blade up in deeper, calmer water, where it uses the light that reaches it — it does not need to reach the surface (1).
• Q13 (2): It is endemic (1). It has no alternation of generations: the visible plant (the sporophyte) makes gametes directly in its berry-like branches, and the fertilised eggs grow straight into new sporophytes (1).

Investigation 3 — alginate

• Q14 (2): The fresh water washes out the sea salt, whose calcium would gel the alginate too early, inside the kelp (1). Washing soda (sodium carbonate) turns the insoluble alginic acid into soluble sodium alginate, which dissolves out into the water (1).
• Q15 (2): 1.4 ÷ 50 × 100 = 2.8 % of the fresh mass (1); 1.4 ÷ 7.5 × 100 ≈ 19 % of the dry mass (1).
• Q16 (3): Calcium ions link alginate chains together, turning the liquid into a gel (1). The outside of the drop meets the calcium first, so it gels into a skin at once (1); calcium then diffuses slowly inwards through the gel, so the jelly spreads towards the centre over time (1).
• Q17 (3): Tap water: with too little calcium, no gel forms — so the gel is caused by the calcium, not by the water or the dripping (1). Seawater: it contains enough calcium to make a soft gel (1). So in the sea, a kelp's alginate is always partly gelled: its walls are firm yet flexible and hold water, letting it bend in the waves without breaking and resist drying out (1).
• Q18 (2): Any two, with the property: thickening ice cream, sauces or toothpaste (it makes liquids thick and smooth); gel beads and "caviar" in cooking (it gels with calcium); wound dressings (it holds water and forms a soft gel); dental impressions (it sets into a firm gel).
• Q19 (2): Agar and carrageenan (1 each).

Investigation 4 — how fast does a kelp grow?

• Q20 (3): Growth = (distance on day 14 − 10.0 cm) ÷ 14 days: 8.9, 10.8, 7.0, 12.4, 9.3 and 10.4 mm per day (2). Mean ≈ 9.8 mm per day (1).
• Q21 (2): New tissue is made at the base of the frond (the growing zone, just above the float) (1); a hole near the base is pushed further away as new tissue forms beneath it, whereas the tip only wears away, so a hole there would measure nothing useful (1).
• Q22 (2): The fronds grow at the base but erode at the tip at about the same rate, like a conveyor belt, so the length stays about the same (1). The worn-off fragments become drifting food — detritus — for filter-feeders such as mussels and for other animals in the kelp forest (1).
• Q23 (2): The mean (about 10 mm per day) is below CAPS's 13 mm per day (1); 13 mm is the rate "in ideal conditions" — these fronds may have had less light, fewer nutrients, colder or warmer water, or it was not the peak growing season (1).
• Q24 (3): Moso bamboo: 1 000 ÷ 13 ≈ 77 times faster (1). Giant kelp: 500 ÷ 13 ≈ 38 times faster (1). A correct sentence, e.g. "Sea bamboo grows up to about 13 mm a day — fast for a seaweed, but far slower than bamboo or the giant kelp of California" (1).

Investigation 5 — a cell inside a cell, and the booklet's questions

• Q25 (3): Endosymbiosis: a relationship in which one organism lives inside the cells of another (1). Evidence: the brown alga's chloroplasts are wrapped in four membranes, not two (1) — two from the original chloroplast and extra ones from a swallowed red alga and its host — and the alga's DNA shows ancestry from two different groups (1).
• Q26 (4): Innermost two: the chloroplast's own double membrane — originally a free-living cyanobacterium (2). Third: the cell membrane of the engulfed red alga (1). Fourth (outermost): the host cell's membrane that wrapped around the red alga when it was engulfed (now joined to the host's endoplasmic reticulum) (1).
• Q27 (1): Zooxanthellae living inside the cells of coral polyps (accept any correct marine endosymbiosis, such as the bacteria inside a vent tube worm).
• Q28 (4): Any two reasons, each explained (2 each): a free-living gametophyte is small and fragile and many die before releasing gametes — keeping it on or inside the large sporophyte protects it; the gametes are released close together, so fertilisation is more likely; the plant does not spend energy and time growing a whole separate generation; the large sporophyte can photosynthesise and store food for the next generation, so new plants are better supplied.
• Q29 (5): A — meiosis; B — spores; C — gametophytes; D — fertilisation; E — zygote.
• Q30 (2): (a) pneumatocysts · (b) alginate (alginic acid).
• Q31 (7): One mark per heading correctly completed for all three groups. Kingdom: green Plantae, red Plantae, brown Chromista. Membranes: green 2, red 2, brown 4. Chlorophylls: green a and b, red a, brown a and c. Other pigments: green beta-carotene and xanthophylls; red phycoerythrin; brown fucoxanthin. Food store: green starch, fats and oils; red floridean starch; brown laminarin. Characteristic chemicals: green cellulose walls; red agar and carrageenan; brown alginate. Life cycles: green — isomorphic alternation, isogametes; red — often a third, carposporophyte generation; brown — large sporophyte with microscopic gametophytes, or (sargassum) no alternation.

Marking note. Q4 is the reasoning the sheet is built around: a float that displaces half a litre holds up 3.4 kg of fronds because the sea carries almost all of their weight. Q7 reconciles "no vascular tissue" with the trumpet hyphae that move food, and Q24 turns a corrected claim into arithmetic. In Q26, accept answers that say "the outer two membranes are associated with the endoplasmic reticulum", as the booklet does, provided the third membrane is credited to the red alga.