Practical 23
Marine Biology
CAPS block 46 · Term 2 · 7 + 2 + 2 hours
Seaweeds, Green and Red — Pigments, Zones and a Packet of Nori
Separate the pigments of a green, a brown and a red seaweed — and find the one pigment that chromatography cannot catch. Walk a rocky shore from the top of the tide to the bottom and map who grows where. Then use a packet of sushi nori to test how purple laver survives a day in the sun.
What CAPS asks for
Investigations column, block 46: "Visit to rocky shore or aquarium to see other seaweeds in situ. Plants and structures selected for discussion should be recognisable from diagrams and photographs. Ideally they should be highlighted for study during field trip. Paper chromatography of crushed samples: a comparison to observe different pigments in the three groups." Resources: "Pestle and mortar · Suitable solvent · Filter paper · Beakers · Fresh samples of algae." For the green and red algae: "Live samples of the prescribed examples should be obtained for examination" — sea lettuce and purple laver.
The chromatography is Investigation 1. The rocky-shore visit is Investigation 2. Examining purple laver — through its supermarket relative, nori — is Investigation 3. Investigation 4 covers the life cycles CAPS asks you to illustrate, and the booklet's "Test your knowledge" questions.
What you need
- 🌊 shore A small handful each of washed-up green, brown and red seaweed from the beach — never pulled off the rocks — plus a few spinach leaves to compare
- 🟢 kitchen A pestle and mortar (or a small bowl and the back of a spoon), a pinch of clean sand, white coffee filters cut into strips 2 cm × 15 cm, tall glasses, clothes pegs, a pencil, cling film, toothpicks and a ruler
- 🟡 pharmacy Nail-polish remover that contains acetone (or surgical spirit), and optionally a small UV (black-light) torch
- 🟢 kitchen A packet of nori (the dried seaweed sheets used for sushi), scissors, a kitchen scale and a plate
- 🌊 shore For the shore walk: a coat-hanger quadrat (Practical 27), a tape measure or a 15 m rope knotted every 2 m, a phone camera and a notebook
- 🔴 school lab Chromatography paper and a petroleum-ether solvent give sharper bands — you will not have these at home. That's fine: a lab result is supplied.
No seaweed, no shore, or the wrong tide? That's fine.
Investigation 1 supplies four lab chromatograms and a water test. Investigation 2 supplies a full transect from a rocky shore near Cape Recife. Investigation 3 needs only a packet of nori from the supermarket, and its readings are supplied too. Investigation 4 is a desk task.
Safety — solvents, UV light and the shore
Acetone and surgical spirit are flammable: no flames or heaters nearby, work by an open window, keep the glasses covered, and do not breathe the vapour. Wear gloves if you have them and wash your hands afterwards. Never shine a UV torch into anyone's eyes. On the shore: go at spring low tide on a falling tide, never alone, never turn your back on the sea, and watch for slippery seaweed-covered rock. Take nothing living — photograph instead; in a marine protected area even washed-up seaweed may not be removed.
1
Paper Chromatography of the Three Groups
CAPS: "Paper chromatography of crushed samples: a comparison to observe different pigments in the three groups"
Aim
To separate the photosynthetic pigments of a green, a brown and a red seaweed, and compare them with a land plant.
Method
- Rinse each seaweed in fresh water and pat it dry. Cut about two teaspoons of one kind into small pieces.
- Grind the pieces in the mortar with a pinch of sand and one teaspoon of acetone until the liquid is dark.
- Draw a pencil line across a filter strip 2 cm from the bottom. With a toothpick, dab the extract onto the middle of the line; let it dry; repeat 10–15 times to make a small, dark spot.
- Pour acetone into a tall glass to a depth of about 1 cm — below the pencil line. Hang the strip from a peg across the top of the glass so the bottom just dips in. Cover with cling film.
- When the solvent has climbed nearly to the top (15–30 minutes), take the strip out, mark the solvent front in pencil at once, and let it dry. Measure from the pencil line to the solvent front, and to the middle of each coloured band.
- Repeat with the other seaweeds and with spinach.
- The water test. Put some red seaweed in a jar of tap water in the freezer overnight. Let it thaw, and look at the water — then shine the UV torch on it in a dark room.
Rf value = distance moved by the pigment ÷ distance moved by the solvent front. It is always less than 1. Under the same conditions each pigment has its own Rf, so it can be used to identify the pigment — together with its colour.
Supplied results — four lab chromatograms, and the water test
⚠️ About these numbers
These chromatograms are a typical example of a school-lab run (chromatography paper, petroleum ether and acetone), drawn up for this exercise. Rf values change with the paper, the solvent and the temperature, so real runs differ — use these to practise; identify pigments by colour as well as by Rf.
| Sample | Band 1 | Band 2 | Band 3 | Band 4 |
|---|---|---|---|---|
| spinach | 9.4 yellow-orange | 5.6 blue-green | 4.3 yellow | 3.5 yellow-green |
| green — sea lettuce | 9.4 yellow-orange | 5.6 blue-green | 4.3 yellow | 3.5 yellow-green |
| brown — kelp | 9.4 yellow-orange | 5.6 blue-green | 2.9 orange-brown (wide) | 0.6 pale green |
| red — a red seaweed | 9.3 yellow-orange (faint) | 5.6 blue-green | 4.2 yellow (faint) | — |
| Pigment | Colour | Found in |
|---|---|---|
| beta-carotene | yellow-orange | all three seaweed groups and land plants |
| chlorophyll a | blue-green | all three seaweed groups and land plants |
| xanthophylls (e.g. lutein) | yellow | green algae and land plants; some in red algae |
| chlorophyll b | yellow-green | green algae and land plants only |
| fucoxanthin | orange-brown | brown algae (and diatoms) |
| chlorophyll c | pale green | brown algae (and diatoms) |
| phycoerythrin | red-pink | red algae (and some cyanobacteria) — a protein that dissolves in water, not in acetone |
Observations. The red seaweed's acetone extract was green-brown, not red, and the ground-up pieces left in the mortar were still pink-red. In the water test, the thawed water around the red seaweed had turned pink; under the UV torch it glowed bright orange. The water around a piece of sea lettuce treated the same way stayed clear. On home coffee-filter strips with nail-polish remover the bands were wider and closer together — a yellow band near the top, a dark green band, and a greenish smear below — but in the same order.
Results
| Sample | Band 1 | Band 2 | Band 3 | Band 4 |
|---|---|---|---|---|
| spinach | ||||
| green | ||||
| brown | ||||
| red |
Analysis
- Why is the seaweed ground with sand and solvent, and why must the spot be small and dark? 2
- Why is the start line drawn in pencil, not pen, and why must it sit above the solvent in the glass? 2
- Calculate the Rf value of every band in Data set 1.1 (or your own), showing the formula once. 4
- Use the colours and the reference table to name the pigment in each band of the four chromatograms. 4
- Which pigments appeared in all three seaweed groups? What does this suggest about the seaweeds? 2
- Which pigment appeared only in the green seaweed and the spinach? Explain how this supports CAPS's statement that green algae "are the most closely related to land plants". 2
- Name the two pigments found only in the brown seaweed. Kelp contains chlorophyll, yet it looks brown — explain. 3
- The red seaweed's extract was not red, and its red colour stayed behind in the mortar. Use the water test to explain why, and why this chromatography could never show the red group's most important pigment. 3
- The pink water glowed orange under UV light. What does that tell you about the pigment and the light energy it absorbs? 2
- CAPS says phycoerythrin "absorbs blue light"; your booklet says it absorbs "violet-to-blue". A red seaweed looks red because of the light it reflects. Explain why a red pigment cannot be absorbing much red light, and suggest which colours phycoerythrin absorbs most. 3
- The booklet's question 5.3: why are the very red forms of the red seaweed group normally found growing in deeper water? Use your chromatography in your answer. 3
2
A Rocky-Shore Seaweed Transect
CAPS: "Visit to rocky shore or aquarium to see other seaweeds in situ"
Aim
To find out how the cover of green, brown and red seaweeds changes from the top of the shore to the bottom, and to explain the pattern.
Method
- At spring low tide, lay the rope straight down the shore from the spring high-tide line (the top of the barnacles) to the lowest rocks you can safely reach.
- Every 2 m, lay the quadrat beside the rope. Estimate the percentage of the quadrat covered by green, brown and red seaweed, and by bare rock or animals. Photograph it. Note any seaweed you can name.
- Look closely at three places: the highest seaweed on the shore; the high-shore rock pools; and the stalks (stipes) of any kelp at the bottom.
- Take nothing, and put back any rock you lift the way you found it.
Supplied results — a transect near Cape Recife at spring low tide in July
⚠️ About these numbers
This transect is a composite, built from the pattern typical of a south-coast rocky shore in winter. It is not one real survey. Use it to practise; do not quote it as a measurement.
| Distance down the shore (m) | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 |
|---|---|---|---|---|---|---|---|---|
| green | 0 | 5 | 15 | 30 | 10 | 5 | 0 | 0 |
| brown | 0 | 0 | 5 | 10 | 20 | 25 | 40 | 50 |
| red | 25 | 30 | 10 | 15 | 35 | 50 | 45 | 40 |
| bare rock or animals | 75 | 65 | 70 | 45 | 35 | 20 | 15 | 10 |
Field notes. (a) At 0–2 m the red cover was all purple laver: the top fronds were dark, dry and crinkled like cellophane, but the fronds underneath were still soft and moist. (b) A high-shore pool at 4–6 m was packed with bright green sea lettuce; it was thickest where a stormwater pipe ran into the pool. (c) Lower down, the red cover was mostly tongue weed, pink coralline crusts and feather corallines, which felt hard and gritty. (d) At 12–14 m, small red seaweeds were growing on the stipes of the kelp. (e) Some of the "brown-looking" seaweeds at 8–10 m keyed out as red algae.
Analysis
- Draw a kite diagram or a stacked bar chart of the cover of green, brown and red seaweed against distance down the shore. 5
- Describe where on the shore each of the three groups was most common. 3
- Your booklet says red seaweeds grow deepest and greens near the surface — yet the highest seaweed on this shore was a red one. Explain how both can be true. 3
- Give three adaptations that let purple laver survive at the very top of the shore. 3
- Explain why sea lettuce does well in high-shore pools, and why it was thickest beside the stormwater pipe. 3
- What is the term for the small red seaweeds growing on the kelp stipes? Explain why they are not parasites. 2
- Give one reason for photographing rather than collecting, and one reason for using a quadrat rather than simply looking. 2
- The booklet's question 3: draw a table comparing seaweeds on the warmer east coast and the colder west coast for size, abundance and species diversity, using "larger" or "smaller" in each box. 3
3
Purple Laver's Drying Trick — with a Packet of Nori
CAPS purple laver concepts 5 and 6 · "Live samples of the prescribed examples should be obtained for examination"
Aim
To measure how quickly laver takes up water, and to test the booklet's claim that "the dry top layer insulates those below".
The nori used for sushi is made from close relatives of our purple laver — until recently they were all placed in the same genus, Porphyra. A dried sheet is a pressed mat of laver fronds, so it behaves the way laver does on a hot afternoon.
Method
- Rehydration. Cut four 5 cm squares of nori and weigh them together. Dip them in water for 30 seconds, blot them lightly on paper towel, and weigh them. Repeat after 1, 2 and 5 minutes in total.
- Drying. Soak ten fresh squares for 5 minutes and blot them. Lay five side by side on a plate. Stack the other five neatly on top of each other on a second plate. Leave both in a warm, airy place.
- After 30, 60 and 90 minutes, weigh one of the separate squares, and the bottom square of the stack (lift the top four off quickly and put them back). Note how each one feels.
Supplied results — soaking and drying nori
| Time in water | dry | 30 s | 1 min | 2 min | 5 min |
|---|---|---|---|---|---|
| Mass (g) | 1.2 | 3.9 | 4.6 | 4.9 | 5.0 |
| Time (min) | 0 | 30 | 60 | 90 |
|---|---|---|---|---|
| a separate square | 1.25 | 0.78 | 0.42 | 0.33 |
| the bottom square of the stack | 1.25 | 1.18 | 1.09 | 0.98 |
At 90 minutes the separate squares were crisp and brittle. The top square of the stack was crisp too, but the bottom square was still soft and floppy.
A research card — the woman who saved nori. Until 1949, nori farmers in Japan could not grow laver from seed: nobody knew where it went in summer. That year the British phycologist Kathleen Drew-Baker showed that tiny pink threads boring into old shells were not a separate seaweed at all — they were laver's microscopic sporophyte. Farmers learned to seed shells with it and release spores onto their nets, and the industry recovered. She is still honoured with a festival in Japan.
Analysis
- Calculate the mass of water the four squares took up after 5 minutes, and what percentage of that had been taken up within the first minute. 2
- Plot both drying curves on one graph. 4
- Compare the two curves and explain the difference, using the booklet's words "the dry top layer insulates those below". 3
- The booklet says laver survives drying by losing fluid "from a gel-like material between the cells, rather than from cells themselves". Explain why that matters for staying alive. 2
- Use the research card to explain the booklet's statement that laver's "sporophyte is microscopic and bores into mollusc shells", and why understanding a seaweed's life cycle mattered to people. 3
4
Life Cycles and the Booklet's Questions
CAPS: "Illustrate this with a diagram to show typical alternation" · "Test your knowledge" questions 1, 2, 4 and 5
Aim
To draw and label the alternation of generations, compare sea lettuce with purple laver, and answer the booklet's own questions.
The booklet's question 4 diagram
A cycle of boxes, starting at the left and going clockwise: sporophyte (2n) → meiosis → box 4.1.1 (n) → box 4.1.2 (n) → which produces eggs (n) and box 4.1.3 (n) → fertilisation → zygote (4.1.4) → back to the sporophyte. The diagram's title is box 4.1.5.
Analysis
- The booklet's question 4.1: give the labels for boxes 4.1.1 to 4.1.5. 5
- The booklet's question 4.2: what is the advantage of this type of life cycle? 2
- Draw the life cycle of sea lettuce as a labelled cycle, marking each stage n or 2n and showing where meiosis and fertilisation happen. Use it to explain the terms isomorphic and isogametes. 5
- In purple laver, which generation is the visible seaweed and which is microscopic? How does this differ from sea lettuce? 3
- The booklet's question 5: (a) what does "highly calcified" mean? (b) Give two reasons why this adaptation would be useful. 3
- The booklet's question 1: give the term for (a) being attached to the seabed; (b) a multicellular plant structure which exhibits little or no cell differentiation; (c) seaweeds which incorporate calcium carbonate to make them coral-like and resistant to wave action and herbivores; (d) a branching stem which grows horizontally, on or below the substrate; (e) plants attached to or growing on another plant, without being parasitic. 5
- The booklet's question 2: define (a) holdfast; (b) macroalgae; (c) isomorphic; (d) carposporophyte. 4
- Explain why scientists prefer "seaweed" or "macroalgae" to "sea plants", naming the kingdoms CAPS places the three groups in. 2
Memo — worked answers and mark allocation (does not print)
Total: 97 marks — Investigation 1: 30 · Investigation 2: 24 · Investigation 3: 14 · Investigation 4: 29.
Investigation 1 — paper chromatography
- Q1 (2): Grinding with sand breaks open the cell walls and chloroplasts so the solvent can dissolve the pigments out (1). A small, concentrated spot gives sharp bands that do not overlap; a big or weak spot gives smeared, faint bands (1).
- Q2 (2): Pencil (graphite) does not dissolve in the solvent; pen ink would run and add bands of its own (1). If the line were below the solvent surface, the pigments would wash off into the solvent instead of travelling up the paper (1).
- Q3 (4): Rf = distance moved by pigment ÷ distance moved by solvent front (1). Spinach and green: 0.94, 0.56, 0.43, 0.35. Brown: 0.94, 0.56, 0.29, 0.06. Red: 0.93, 0.56, 0.42 (3 for all correct; deduct 1 per error).
- Q4 (4): Spinach and green: beta-carotene, chlorophyll a, xanthophyll, chlorophyll b. Brown: beta-carotene, chlorophyll a, fucoxanthin, chlorophyll c. Red: beta-carotene, chlorophyll a, xanthophyll (1 per chromatogram).
- Q5 (2): Beta-carotene and chlorophyll a (1). All three groups photosynthesise using chlorophyll a — they share the same basic photosynthetic machinery (and probably a common ancestry for their chloroplasts) (1).
- Q6 (2): Chlorophyll b (1). Green algae have the same pigment set as land plants — chlorophylls a and b with carotene and xanthophylls — evidence that they are the closest relatives of land plants (1).
- Q7 (3): Fucoxanthin and chlorophyll c (1). Fucoxanthin is orange-brown and present in large amounts (the wide band) (1), so it masks the green of the chlorophyll and the kelp looks brown (1).
- Q8 (3): The red pigment, phycoerythrin, is a protein that dissolves in water but not in acetone (1) — the water test proves it: it came out into the water and turned it pink (1). An acetone-based chromatogram only carries pigments that dissolve in acetone, so phycoerythrin stays behind in the mortar and can never appear on the strip (1).
- Q9 (2): The pigment absorbs light energy — including UV and blue-green light (1) — and gives some of it out again as orange light (it fluoresces); in the living seaweed that absorbed energy is passed on to chlorophyll a instead (1).
- Q10 (3): A pigment's colour is the light it reflects, not the light it absorbs (1); if phycoerythrin absorbed red light it would not look red (1). It absorbs most strongly in the green and blue-green (roughly 495–570 nm) — the colours that remain at depth, especially in green coastal water (1). In an exam on this topic, CAPS's "absorbs blue light" is the expected wording.
- Q11 (3): Deeper water receives only the short blue-green wavelengths; the red and orange light chlorophyll uses best has been absorbed (1). Red seaweeds have phycoerythrin, which traps the blue-green light that reaches them and passes the energy to chlorophyll a (1); the chromatography shows green seaweeds have only chlorophylls a and b and carotenoids — the land-plant set — which suit the full light near the surface (1).
Investigation 2 — a rocky-shore transect
- Q12 (5): A kite diagram (each group's band width proportional to its cover) or stacked bars (1); distance down the shore on the horizontal axis (1); cover (%) labelled and scaled (1); all three groups plotted accurately (1); a key and title (1).
- Q13 (3): Green — most common in the middle of the shore, peaking at 6 m in the high pools (1). Brown — absent at the top, increasing steadily to a maximum at the bottom (14 m) (1). Red — present at the very top (laver), less in the mid-shore, and most common on the lower shore at 10 m (1).
- Q14 (3): The booklet's depth rule is about light in the subtidal: below the surface, reds can use the blue-green light that reaches deepest (1). Some red seaweeds have evolved to live near or above the surface and have "lost their true colours" (1); on the intertidal shore what decides who grows where is mainly drying out, wave action and grazing, not the colour of the light — and laver is adapted to drying (1).
- Q15 (3): Any three: it loses water from the gel between its cells rather than from the cells; the dry top fronds insulate the moist ones below; it rehydrates very quickly when the tide returns; its thin sheets can dry like cellophane without dying; it grows in the cooler months and spends summer as a microscopic sporophyte in shells; grazers rarely reach so high.
- Q16 (3): Sea lettuce tolerates a wide range of temperatures, salinities and chemical concentrations, so it survives a pool cut off from the sea for days (1). All its cells photosynthesise and divide, so it grows very fast (1). The stormwater carries nutrients (fertiliser, sewage), so it grows thickest there — it is a sign of nutrient pollution (1).
- Q17 (2): Epiphytic seaweeds (epiphytes) (1). They use the kelp only for attachment and do not penetrate its tissue or take food from it (1).
- Q18 (2): Photographing: collecting living seaweed may be illegal (permits, marine protected areas) and removes habitat; a photo is a record that can be checked later (1). Quadrat: it samples the same area at each point, so cover estimates are fair and comparable rather than impressions (1).
- Q19 (3): East (warmer): size smaller, abundance smaller, diversity larger. West (colder): size larger, abundance larger, diversity smaller (1 per row correct).
Investigation 3 — purple laver's drying trick
- Q20 (2): 5.0 − 1.2 = 3.8 g of water (1). After 1 minute 4.6 − 1.2 = 3.4 g: 3.4 ÷ 3.8 × 100 ≈ 89 % (1).
- Q21 (4): Axes labelled with units (1); both lines plotted accurately (2); a key (1).
- Q22 (3): The separate square lost almost three-quarters of its mass in 90 minutes and went crisp; the bottom of the stack lost only about a fifth and stayed soft (1). The squares above it shielded it from moving air and warmth, and once the top square was dry it formed a barrier that slowed evaporation from those beneath (1). On the shore, the dry top fronds of a laver clump protect the moist fronds underneath until the tide returns (1).
- Q23 (2): Water is lost from outside the cells first, so the cells' own contents — cytoplasm and organelles — stay hydrated and are not damaged (1); when the sea returns the gel takes up water again quickly and the seaweed carries on living (1).
- Q24 (3): Laver's sporophyte generation is a tiny pink thread that lives inside old shells, so it is microscopic and easily overlooked (1). Before 1949 nobody knew that this was part of laver's life cycle, so farmers could not collect spores to seed their nets (1). Once the life cycle was understood, the sporophyte could be grown on shells and its spores used to farm nori, saving the industry (1).
Investigation 4 — life cycles and the booklet's questions
- Q25 (5): 4.1.1 spores · 4.1.2 gametophyte · 4.1.3 sperm (male gametes) · 4.1.4 2n · 4.1.5 "Alternation of generations (a typical seaweed life cycle)".
- Q26 (2): It combines sexual reproduction, which produces variation and helps the species adapt (1), with spores that spread the seaweed to new places — two chances to reproduce and disperse in one cycle (1).
- Q27 (5): A cycle: sporophyte (2n) → meiosis → spores (n) → gametophytes (n) → gametes (n) → fertilisation → zygote (2n) → sporophyte (3 for a correct, labelled cycle with n/2n). Isomorphic: the sporophyte and gametophyte look the same (1). Isogametes: the male and female gametes look the same (1).
- Q28 (3): In laver the visible leafy fronds are the gametophytes (1); the sporophyte is microscopic and lives bored into shells (1). In sea lettuce both generations are visible and look alike (isomorphic) (1).
- Q29 (3): (a) It has a great deal of calcium carbonate laid down in its cell walls, making it hard and stony (1). (b) Any two: it resists wave action and abrasion; it is hard and unpleasant for grazers to eat and digest; it can build up as thick encrustations and reefs (2).
- Q30 (5): (a) benthic · (b) thallus · (c) coralline · (d) rhizome · (e) epiphytic.
- Q31 (4): (a) Holdfast — the structure that anchors a seaweed firmly to the substrate (1). (b) Macroalgae — all marine photosynthetic organisms easily visible to the naked eye (1). (c) Isomorphic — different structures (generations) that are visibly identical (1). (d) Carposporophyte — a third generation in the life cycle of red seaweeds (1).
- Q32 (2): "Plants" strictly means members of the Kingdom Plantae, but not all seaweeds belong to it (1): green and red algae are placed in Plantae, brown algae in Chromista (1).
Marking note. Q8 is the discovery the sheet is built around: CAPS asks for chromatography to compare the three groups, yet the red group's defining pigment cannot appear on an acetone chromatogram at all. The freezer-and-water test shows where it went. Q10 flags that phycoerythrin absorbs mostly green and blue-green, not simply "blue" — CAPS's wording remains the expected exam answer. Q14 resolves the apparent clash between the depth rule and a red seaweed at the top of the shore.