Cells: the Basic Units of Life: a full term of science

Ten ready-to-teach lessons for Year 8 Biological sciences. Print this pack and the term is prepared: every lesson comes with a step-by-step plan, the questions to ask, a student worksheet, cut-out cards, an evidence sheet or exit tickets, an assessment kit and worked answers.

Start here: five minutes to Monday

1. Skim the term at a glance on the next page.
2. Print the lesson you need. Each lesson is three A4 sheets: plan, worksheet, and cards, an evidence sheet or tickets.
3. Gather the few items under “You need” on the plan. Most come from the science prep room, the kitchen or the garden.
4. Open the free interactive unit on your board or projector. Every plan tells you which picture to show and when.
5. Teach straight from the plan. Timings, talk prompts, misconceptions and worked answers are all on the one page.

For any Year 8 science teacher

The pack suits a biology specialist and a teacher working outside their own subject alike: each plan explains the idea in plain words, lists the misconceptions students bring, and gives worked answers. The big idea of the term: every living thing is built from cells, the smallest units that are still alive. Plant and animal cells share a core plan of membrane, nucleus, cytoplasm and mitochondria; plant cells add a wall, chloroplasts and a large vacuole. Each organelle has a job, and a cell that specialises in one job builds up the parts that job needs. Students meet the idea through their own microscope work, measuring as well as looking.

Two ways to run each lesson

Every lesson works as one 60-minute block, or as two 30-minute sessions; the split point is marked in every plan. Look ahead: Lesson 5 needs prepared slides of cheek cells, Lesson 6 two leaves you have starch-tested beforehand, and Lesson 8 a tray of cabbage jelly made the day before.

Safety and care in one look

Microscopes are carried with two hands and never focused with the coarse knob at high power. Iodine solution stains, so gloves and eye protection go on for Lesson 4. Cheek cells come from prepared slides unless your school’s procedure allows swabs. The ethanol for the starch tests is warmed only in hot water from a kettle, with no flame nearby, and nothing in the science room is tasted, not even the jelly.

The term at a glance

One lesson a week for a term. Each lesson stands on the ones before it, so run them in order where you can.

How the sequence builds

Lessons 1 and 2 establish that living things are made of cells and how small cells are. Lesson 3 teaches the microscope, and Lessons 4 and 5 use it on plant and animal cells, measuring each. Lesson 6 looks closely at the chloroplast, Lesson 7 at all the organelles, and Lesson 8 at why cells stay small. Lesson 9 shows how cells specialise, and Lesson 10 brings it together.

Curriculum links (Australian Curriculum V9)

The whole term teaches the Science Understanding descriptor AC9S8U01 quoted on the cover. The lessons also work these Science Inquiry and Human Endeavour descriptors:

Assessment in this pack

• Every plan ends with “Answers and look-fors”: what meeting the idea sounds like in a Year 8 voice, with working.
• The assessment sheet near the front has a class observation checklist and a three-level rubric.
• Lesson 10 is the summative pair: the three views task, plus the “Show what we know” check.

Materials for the whole term

One gathering session covers all ten lessons. Most of the kit is in a science prep room: microscopes, slides, coverslips and iodine solution. The rest comes from the supermarket, the garden and the school. Three things are worth doing ahead: order prepared cheek cell slides for Lesson 5, run the two starch tests before Lesson 6, and make the cabbage jelly the day before Lesson 8.

The one-trip list

• From the prep room: light microscopes, slides, coverslips, droppers, forceps, mounted needles, iodine solution, prepared slides of human cheek cells (and of blood, if you have them), ethanol and beakers for your starch tests, gloves and eye protection.
• From the supermarket: an onion, dried yeast, plain yoghurt, a red cabbage, gelatine, bicarbonate of soda, white vinegar, foil and a candle.
• From home or the garden: moss from a shady wall or pot, a pot plant with green-and-white leaves, newspaper, a tray, plastic knives and cups.
• From the school: clear rulers marked in millimetres, interlocking cubes, large paper, a kettle, a fridge and timers.

Safety and care in one look

• Carry a microscope with two hands, one under the base; never use the coarse focus at high power; never point a mirror microscope at the Sun.
• Iodine solution stains skin and clothes: gloves and eye protection on, drips wiped at once. Chipped slides go in the glass bin.
• Cheek cells come from prepared slides unless your school’s procedure allows swabs.
• Ethanol is flammable: the teacher warms it only in hot water from a kettle, with no flame nearby. Nothing is tasted, and hands are washed after handling moss.

Assessment without extra work

The term assesses itself. Every lesson plan ends with worked answers and look-fors, and Lesson 10 is the summative pair: the three views task, in which students name three unlabelled views down a microscope and work out the size of their cells, and the final check. This sheet is the place to note what you see along the way.

The three levels

Class observation checklist

A tick a lesson is plenty; the Lesson 10 task and check fill the gaps.

Word wall cards

Cut out the cards and build the wall as the words arrive: cell, cell theory, unicellular and multicellular in Lesson 1; micrometre in Lesson 2; magnification and field of view in Lesson 3; chloroplast and photosynthesis in Lesson 6; organelle, nucleus, cytoplasm, cell membrane, cell wall, mitochondrion and vacuole in Lesson 7; diffusion in Lesson 8; specialised cell in Lesson 9.

Dear families

This term in science, Year 8 studies cells, the tiny living units that every plant, animal and fungus is built from. Students learn that some living things are a single cell, that plant and animal cells share a core plan, and that each part of a cell has a job.

Along the way your student will learn to use a microscope safely, look at onion skin, cheek cells and moss, measure cells in micrometres (thousandths of a millimetre), find out why cells stay so small with jelly cubes and vinegar, and design a cell for a job of their own.

Try this at home

• Look at the skin of an onion against a window, or through a magnifying glass. Can you see any pattern of tiny boxes?
• Mix a spoon of dried yeast and a spoon of sugar in a cup of warm water. What happens after ten minutes, and why does it mean the yeast is alive?
• Cut a potato into a 1 cm cube and a 3 cm cube. Which has more outside for the amount of potato inside?
• Find three living things in the garden that are made of many cells, and guess what job one of their cells might do.

What to ask your scientist

• Why is a cell called the basic unit of life?
• What does a plant cell have that your cells do not?
• Why do muscle cells have so many mitochondria?
• Why can a cell not just keep growing bigger?

A small note on care: students handle microscopes and stains with care, any heating is done by the teacher, nothing in the science room is tasted, and everyone washes their hands after handling plants and soil.

Warm regards,

The Year 8 science team

Printed from the free seegongsik Cells teaching pack · seegongsik.com/au/y8/biological/AC9S8U01/pack

Made of cells

Every living thing is made of cells, and a cell is the smallest part of a living thing that is still alive. Some living things are a single cell that feeds, grows, responds and reproduces on its own; most of the living things we can see are built from many cells that share the work. Lifelike things show why the test is the cell: a crystal grows, a flame uses oxygen and a robot moves and senses, but none is made of cells. Pairs sort fifteen cards into three piles, then meet the three ideas of cell theory.

We are learning to

• explain why the cell is called the basic unit of life,
• sort living things into one-celled and many-celled, and tell them from lifelike things,
• state the three ideas of cell theory.

Success criteria

• I can sort the cards and give a reason for the three hardest.
• I can explain why a flame or a virus is not a cell, though it does some things living things do.

You need

• the organism cards (third sheet), one set per pair, and the worksheet (next sheet), one per student,
• a packet of dried yeast and a pot of plain yoghurt to show (not to taste), and a candle in a holder for the opening.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the candle, the board and the cards in Session A, and cell theory and Part C in Session B.

Watch for these ideas

• “Anything that moves or grows is alive.” A crystal grows and a robot moves; living things are made of cells.
• “A single cell is too small to be a real living thing.” A one-celled organism feeds, grows, responds and reproduces on its own.
• “A virus is a very small cell.” A virus has no cell at all; it can copy itself only inside a living cell.

Make it easier, make it bigger

• Easier: sort into two piles first, made of cells and not made of cells, then split the first pile in two.
• Bigger: a virus copies itself and changes over many generations, yet it is not made of cells. Should it be called alive? Give one reason each way.

Answers and look-fors

• Part A: one cell A, B, G, I, J, O; many cells D, F, H, L, N; not made of cells C, E, K, M.
• Part B: 1. made of cells, for example the gum tree (H) and the sea sponge (N); 2. smallest unit of life, for example the yoghurt bacterium (A) or the amoeba (G), each a whole organism in one cell; 3. cells from cells: the yeast buds, each new cell growing out of an existing one.
• Part C: 1. It feeds, grows, responds and reproduces on its own, so one cell is the whole organism. 2. It is not made of cells: it cannot grow by itself from food, respond or reproduce; it only burns while the wax lasts. 3. It is built from many cells, which feed, grow and make new cells, so it belongs with the many-celled living things, brain or no brain.
• Bigger: for, it copies itself and its offspring change over time; against, it has no cell, takes in no food and can copy itself only by using a living cell.

Made of cells

A. Three piles

B. Three ideas of cell theory

A clue for idea 3: yeast makes new cells by budding, a small new cell growing out of an existing one.

C. Questions

One cell, many cells, or none?

Cut out the cards. Lay down the three grey heading cards, read each card, and sort it.

Teacher note: 6 cards are one cell, 5 are many cells and 4 are not made of cells.

How small is a cell?

Cells are measured in micrometres: one micrometre (1 µm) is a thousandth of a millimetre. The eye can just make out something about 100 µm across if it stands out from its background, and a light microscope shows details down to about 0.2 µm. Most cells sit between the two, which is why nobody saw a cell until microscopes were made, and why each new tool changed what scientists knew. Students convert nine sizes, place them on a size line, and put the discoveries behind cell theory in order.

We are learning to

• convert between millimetres and micrometres, and compare sizes,
• decide which tool can show a thing of a given size,
• explain how new tools and new evidence built cell theory.

Success criteria

• I can convert a size in millimetres to micrometres and place it on the size line.
• I can name a discovery that needed a new tool, and the idea of cell theory it supported.

You need

• the worksheet (next sheet), one per student, and the history cards (third sheet), one set per pair,
• a clear ruler marked in millimetres for each pair, and a printed page with full stops on it.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the millimetre, the sizes and the size line in Session A, and the board and the history cards in Session B.

Watch for these ideas

• Multiplying by 100 instead of 1000: 0.15 mm is 150 µm, not 15 µm.
• “Big living things have big cells.” An elephant’s cells are about the size of a mouse’s; it has far more of them.
• “Hooke saw living cells.” Cork is dead bark: he saw the empty walls the cells had left.

Make it easier, make it bigger

• Easier: give a conversion strip (mm × 1000 = µm) with two worked examples before Part A.
• Bigger: which things in the table can a light microscope never show clearly, and why could the electron microscope?

Answers and look-fors

• Part A, smallest to largest: 3 (0.1 µm), 9 (2 µm), 5 (5 µm), 7 (7 µm), 1 (60 µm), 4 (120 µm), 2 (150 µm), 8 (180 µm), 6 (250 µm). Tools: 3 electron microscope; 9 light microscope; 5 light microscope; 7 light microscope; 1 light microscope; 4 eye (just); 2 eye (just); 8 eye (just); 6 eye (just).
• Part B: each thing marked at its size; the virus is left of the light microscope band, the egg, sand grain, paramecium and onion cell just past 100 µm.
• Part B questions: 1. 1000 ÷ 7 ≈ 143 red blood cells. 2. 400 ÷ 5 = 80 yeast cells across a 0.4 mm full stop.
• Part C: order E, B, G, D, A, H, F, C (1665, 1674, 1831, 1838, 1839, 1855, 1882, 1931). Idea 1: E, D, A; idea 2: B; idea 3: H, F. New tools: E, B, F, C (Hooke’s and van Leeuwenhoek’s microscopes, Flemming’s dyes, the electron microscope).
• Bigger: the flu virus (0.1 µm) is smaller than the 0.2 µm limit of light; electrons let a microscope show far smaller details.

How small is a cell?

A. Nine sizes

1 mm = 1000 µm. Convert each size to micrometres, then decide which tool shows it.

B. The size line

Write each number above the line at its size. Each mark is ten times the one before.

C. Who saw what, and how

How we came to know about cells

Cut out the cards and lay them in time order. Then ask of each card: was it a new tool, new evidence, or a new idea?

Teacher note: the cards are printed out of order on purpose; the answer is on the plan.

The light microscope

A light microscope magnifies twice: the objective lens makes an enlarged image and the eyepiece enlarges it again, so the total magnification is the two multiplied (10 × 4 = 40). The more it magnifies, the smaller the part of the slide you see: four times the magnification shows a field of view a quarter as wide. Measure the field once at the lowest power and every cell can be measured. Pairs put the focusing steps in order, follow them to view a letter e, and measure the field of view with a clear ruler.

We are learning to

• name the parts of a light microscope and what each does,
• focus safely, starting with the lowest power,
• work out the total magnification and the field of view at each power.

Success criteria

• I can focus on a specimen at low power and then at a higher power without crashing the lens.
• I can work out the field of view at a higher power from the one I measured.

You need

• for each pair: a light microscope, two slides and coverslips, a dropper of water, a lowercase letter e cut from newspaper, and a clear ruler marked in millimetres that fits on the stage,
• the worksheet (next sheet), one per student, and the focusing step cards (third sheet), one set per pair.

Care: carry the microscope with two hands, one under the base. Never use the coarse focus at the highest power.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the parts, the steps and the letter e in Session A, and the field of view and the board in Session B.

Watch for these ideas

• “Higher power shows more.” It shows less of the slide, in more detail.
• Adding the magnifications (10 + 40). They multiply: 10 × 40 = 400.
• Surprise that the e looks upside down and back to front, and moves the opposite way to the slide: the lenses turn the image round.

Make it easier, make it bigger

• Easier: tape a number tag beside each part of one class microscope, matching the worksheet drawing.
• Bigger: another microscope has a 15× eyepiece and the same objectives, and a field of 3.0 mm at its lowest power. Find its magnifications and its field at the highest.

Answers and look-fors

• Part A: 1 eyepiece, 2 objective lenses, 3 stage, 4 stage clips, 5 lamp, 6 coarse focus knob, 7 fine focus knob, 8 arm.
• Part B: total magnifications 40×, 100×, 400×. Class mean at 40×: 31.4 ÷ 7 ≈ 4.5 mm = 4500 µm. Field at 100×: 4500 × 40 ÷ 100 = 1800 µm; at 400×: 450 µm.
• Part C: the e appears upside down and back to front; moving the slide left moves the image right. Low power first: the widest view, so the specimen is easy to find, and the most space between lens and slide.
• Cards: C, G, E, A, H, B, F, D.
• Bigger: 60×, 150×, 600×; field at 600× = 3.0 mm × 60 ÷ 600 = 300 µm.

The light microscope

A. The parts

B. Magnification and field of view

Class results, the width of the view at 40× (mm): 4.5, 4.4, 4.6, 4.5, 4.3, 4.6, 4.5. The mean, to one decimal place: ________ mm = ________ µm.

C. The letter e

Focusing, step by step

Cut out the cards and put them in order. Each step needs the one before it to be done first. Keep the cards by your microscope all term.

Teacher note: the cards are printed out of order on purpose; the answer is on the plan.

Plant cells: onion skin

The skin on the inside of an onion scale is a single layer of cells, thin enough for light to pass through. Iodine solution stains the nucleus and makes the walls stand out. The cells are box-shaped and fit together like bricks; each has a wall, a membrane pressed against it, cytoplasm and a nucleus. There are no chloroplasts: the bulb grows underground, in the dark. Pairs make a wet mount, draw a few cells as a scientific drawing, and work out the size of a cell from how many fit across the field of view.

We are learning to

• prepare a stained wet mount of plant cells,
• make a scientific drawing of what we see,
• estimate the size of a cell from the field of view.

Success criteria

• My drawing shows a few cells with single clear lines, ruled labels, a title and the magnification.
• I can work out the length and width of an onion cell, and show my working.

You need

• for each pair: a microscope, a slide and coverslip, a quarter of an onion, forceps, a mounted needle, a dropper of water, iodine solution in a dropper bottle and paper towel,
• the worksheet (next sheet) and the drawing check (third sheet), one each per student.

Safety: iodine solution stains skin and clothes; wear gloves and eye protection, and wipe up drips at once.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the drawing check and the slide in Session A, and the drawing, the counts and the board in Session B.

Watch for these ideas

• Drawing a textbook cell instead of the cells in view. Draw what you see, even if a part is missing.
• Taking air bubbles for cells: bubbles are perfectly round with a thick black rim, and they move when the coverslip is pressed.
• “The wall and the membrane are the same thing.” The membrane is pressed so close to the wall that we cannot tell them apart here.

Make it easier, make it bigger

• Easier: start from a slide you have made, so the pair only focuses, draws and counts.
• Bigger: put a drop of salty water at one edge of the coverslip and draw it under with paper towel at the other. After five minutes, what has happened to the cytoplasm, and why?

Answers and look-fors

• Part C, example class: mean along the field at 100× = 42 ÷ 6 = 7, so a cell is about 1800 ÷ 7 ≈ 257 µm long. Across at 400× = 43 ÷ 6 ≈ 7.2, so about 450 ÷ 7.2 ≈ 63 µm wide.
• Part D: 1. The bulb grows underground with no light, so its cells have no chloroplasts; the green leaves above ground do. 2. It stains the nucleus and the walls so they stand out against the clear cytoplasm. 3. Light has to pass through the cells; a thick piece piles cells on top of each other and blocks the light.
• Drawing check: X has shading, sketchy doubled lines, arrows that cross, labels scattered, and no title or magnification. Y has single clean lines, ruled label lines that do not cross, labels in a column, a title and the magnification.
• Bigger: water leaves the cells into the saltier liquid, so the cytoplasm and membrane shrink away from the wall; the stiff wall keeps its shape.

Plant cells: onion skin

A. The method

1. Snap a piece of onion scale and peel the thin skin from its inner side with forceps.
2. Lay a small piece flat in a drop of water on a slide, without folds.
3. Add one drop of iodine solution.
4. Lower a coverslip at an angle with a mounted needle, so no air is trapped.
5. Blot the extra liquid with paper towel, then focus at low power and move up to 100 times.

B. My scientific drawing

Pencil only; three or four cells; single clear lines, no shading; label lines drawn with a ruler that do not cross; a title and the magnification.

C. How big is an onion cell?

D. Questions

What makes a good scientific drawing?

Two students drew the same onion cells. Compare their drawings.

Five faults in Drawing X

Our class rules for a scientific drawing

Animal cells, and comparing

Animal cells share the core parts of plant cells, a membrane, a nucleus, cytoplasm and mitochondria, but have no wall, no chloroplasts and no large vacuole, so they are rounder and can change shape. Cheek cells are flat cells from the lining of the mouth; on a prepared slide stained with methylene blue each nucleus shows as a dark spot. Pairs look at cheek cells, work out their size, compare them with the onion cells of Lesson 4, and sort twelve features into plant, animal or both.

We are learning to

• observe and describe animal cells,
• compare plant and animal cells, part by part,
• explain the differences in shape using the cell wall.

Success criteria

• I can name three parts both cells share and three that only plant cells have.
• I can explain why a cheek cell is round and floppy but an onion cell keeps a box shape.

You need

• for each pair: a microscope and a prepared slide of human cheek cells (or another animal tissue your school has), and their onion drawings from Lesson 4,
• the worksheet (next sheet), one per student, and the feature cards (third sheet), one set per pair.

Prepared slides avoid taking swabs from students’ mouths; follow your school’s procedure if it allows swabs.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the prediction, the cheek cells and the board in Session A, and the cards and Part D in Session B.

Watch for these ideas

• “Plant cells have a wall instead of a membrane.” They have both: the membrane sits just inside the wall.
• “All plant cells have chloroplasts.” Only cells in green parts do; onion bulb and root cells have none.
• “Plant cells are bigger.” Onion cells happen to be large, but size does not tell plant from animal cells.

Make it easier, make it bigger

• Easier: fill in the shape row of the comparison table together before pairs start.
• Bigger: Euglena is a single cell with chloroplasts, no cell wall, and a tail it swims with. Is it more like a plant cell or an animal cell? Explain why it is hard to sort.

Answers and look-fors

• Part A, example class: mean = 37 ÷ 5 = 7.4, so a cheek cell is about 450 ÷ 7.4 ≈ 61 µm across.
• Part B: onion, box-shaped and regular, wall yes, chloroplasts no, large vacuole yes (hard to see), nucleus yes, about 257 by 63 µm; cheek, flat and irregular, no wall, no chloroplasts, no large vacuole, nucleus yes, about 61 µm.
• Part C: both B, D, F, J, K; plant cells only E, H, I, L; animal cells only A, C, G.
• Part D: 1. The stiff cell wall holds a plant cell’s shape; an animal cell has only a thin membrane, so it can change shape. 2. No: plant cells come in many sizes, some smaller than a cheek cell; the parts, such as a wall or chloroplasts, are the evidence.
• Bigger: chloroplasts are plant-like, but no wall and swimming are animal-like. Euglena does not fit either group neatly, which is why scientists place it among the protists, neither plant nor animal.

Animal cells, and comparing

A. Cheek cells

B. Onion cell and cheek cell

C. Twelve features

D. Questions

Plant cells, animal cells, or both?

Cut out the cards. Lay down the three grey heading cards, read each card, and sort it.

Teacher note: the piles are not the same size; the answer is on the plan.

Chloroplasts make food

Chloroplasts make a plant’s food. Their green pigment, chlorophyll, absorbs light energy, which the chloroplast uses to join carbon dioxide and water into sugar, giving off oxygen: photosynthesis. Much of the sugar is stored as starch, which iodine turns blue-black. Pairs look at moss leaves, where most of the leaf is one cell thick and the chloroplasts show as green discs, count the chloroplasts in five cells, and use the results of two starch tests you have run to decide what a leaf needs to make starch.

We are learning to

• observe chloroplasts and describe what they do,
• write the word equation for photosynthesis,
• use starch test results as evidence that a leaf needs light and chlorophyll.

Success criteria

• I can find the mean and range of a set of chloroplast counts.
• I can explain both starch test results using the word chloroplast.

You need

• for each pair: a microscope, a slide and coverslip, a dropper of water, forceps and a few strands of moss from a shady wall or pot,
• the worksheet (next sheet) and the starch test evidence sheet (third sheet), one each per student; your two tested leaves to show, if you ran the tests.

Safety for the starch tests (a teacher demonstration, done beforehand): ethanol is flammable; warm it only in a beaker of hot water from a kettle, with no flame nearby.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the tree, the moss and the board in Session A, and the starch tests and Part C in Session B.

Watch for these ideas

• “Plants get their food from the soil.” Most of a plant’s mass is built from carbon dioxide from the air, joined with water.
• “Plants make oxygen instead of breathing.” Plant cells also use oxygen in their mitochondria, day and night.
• “Chlorophyll and chloroplasts are the same.” Chlorophyll is the green pigment inside the chloroplast.

Make it easier, make it bigger

• Easier: show a labelled drawing of moss cells beside the microscope, so pairs know what to look for.
• Bigger: the plant was kept in the dark for two days before the test. What could leaf 2 show if that step were skipped, and why would it spoil the conclusion?

Answers and look-fors

• Part A, example counts: mean = 165 ÷ 5 = 33; range = 38 − 29 = 9. Counts differ because cells differ in size, some chloroplasts hide behind others, and a moving disc can be counted twice.
• Part B: carbon dioxide + water → sugar + oxygen, using light energy absorbed by chlorophyll. Carbon dioxide comes from the air through tiny holes in the leaf, water from the soil through the roots, light from the Sun.
• Part C: 1. Roots grow in the dark soil, where chloroplasts could not capture light; leaves are in the light. 2. Sugar made in the leaves is carried to the other cells through the plant. 3. Mostly from the air: carbon dioxide joined with water, using light.
• Evidence sheet: 1. Leaf 1, the middle turned blue-black and the white edges stayed orange-brown: only the green parts, which have chlorophyll in chloroplasts, made starch. 2. Leaf 2, the strip that was under the foil stayed orange-brown: without light, no starch was made. 3. The dark days used up the starch already stored, so any starch found was made during the test. 4. Boiling kills the cells and softens the leaf; the ethanol takes out the green chlorophyll, so the iodine colour can be seen.
• Bigger: the strip under the foil might still hold starch made before the test and turn blue-black, so the test could not show that light is needed.

Chloroplasts make food

A. Moss cells

B. Photosynthesis

C. Questions

What does a leaf need to make starch?

How the test was done (by the teacher)

1. A potted plant with green-and-white leaves was kept in a dark cupboard for two days.
2. A strip of foil was folded over one leaf, and the plant stood in bright light for a day.
3. Two leaves were picked: one with white edges (leaf 1), and the leaf with the foil (leaf 2, foil removed).
4. Each leaf was dipped in boiling water for a minute, then left in warm ethanol in a hot-water bath until pale.
5. The pale leaves were rinsed and covered with iodine solution.

Iodine solution is orange-brown. It turns blue-black where there is starch.

Questions

Organelles and their jobs

A cell is not an empty bag: it is packed with organelles, each built for a job. Naming the parts matters less than knowing what each one does. An analogy, such as a cell as a school, helps us remember the jobs, but every analogy breaks somewhere, and spotting where it breaks is good science. Pairs match seven part cards to seven job cards, label a plant cell, and judge the school analogy part by part.

We are learning to

• name the organelles of a plant cell and describe the job of each,
• use an analogy to explain the jobs,
• judge where an analogy fits and where it breaks.

Success criteria

• I can label seven parts of a plant cell and give the job of each.
• I can say where the school analogy breaks for at least three parts.

You need

• the part and job cards (third sheet), one set per pair, and the worksheet (next sheet), one per student,
• a large sheet of paper per group for the analogy discussion.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the cards, the board and the labelling in Session A, and the analogy and Part C in Session B.

Watch for these ideas

• “The nucleus is the cell’s brain.” It holds the instructions; it does not think.
• “Mitochondria make energy.” Energy cannot be made; they release the energy stored in sugar.
• Mixing up the cell wall and the cell membrane, which is just inside it.

Make it easier, make it bigger

• Easier: give the seven jobs as a word bank for Part A.
• Bigger: invent a better analogy for one organelle, such as a city, a factory or a sports team, and say where it breaks.

Answers and look-fors

• Cards: A nucleus with K, B mitochondrion with N, C cell membrane with I, D chloroplast with H, E cytoplasm with M, F vacuole with J, G cell wall with L.
• Part A: 1 cell wall, 2 cell membrane, 3 nucleus, 4 vacuole, 5 chloroplast, 6 mitochondrion, 7 cytoplasm; jobs as on the cards.
• Part B, where the analogy breaks: nucleus, the instructions are a chemical code copied into every new cell, not a person making choices; cell membrane, the membrane is the whole border, and substances cross it everywhere, not at one gate; cytoplasm, it is a jelly full of reactions, not empty space to walk through; mitochondrion, it releases energy from sugar inside itself, using oxygen; nobody hands out the energy; cell wall, water and dissolved substances pass through it; brick walls are solid; chloroplast, it makes sugar, a store of energy, from carbon dioxide and water, not electricity; vacuole, it also presses outwards on the wall, which keeps the plant cell firm.
• Part C: the nucleus holds the instructions for building a new cell; with no nucleus, a red blood cell has no instructions to copy, so it cannot divide (new red blood cells are made in the bone marrow).

Organelles and their jobs

A. Label the cell

B. A cell is like a school

C. A question

Which part does which job?

Cut out the cards. Match each part card to its job card.

Part cards

Job cards

Teacher note: the job cards are printed in a different order from the part cards; the matches are on the plan.

Why are cells so small?

Everything a cell needs comes in through its membrane, and its wastes go out the same way, mostly by diffusion: particles spreading from where they are crowded to where there are fewer. As a cube grows, its volume grows faster than its surface: double the side and the surface is four times bigger but the volume eight times. A large cell would have too little membrane for all the cytoplasm it must supply, and its middle would be too far from the edge. Students work out the surface area to volume ratio of cubes, and measure how far vinegar diffuses into jelly cubes of three sizes in 15 minutes.

We are learning to

• describe diffusion through the cell membrane,
• calculate the surface area to volume ratio of a cube,
• use the ratio and our results to explain why cells are small.

Success criteria

• I can work out the surface area, volume and ratio of a cube, with units.
• I can use the jelly results to explain why a very large cell could not survive.

You need

• for each group: a block of blue-green cabbage jelly (see below), a plastic knife, a ruler marked in millimetres, a cup of white vinegar, a spoon, paper towel and a timer; a bag of interlocking cubes,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

The day before: pour boiling water from a kettle over chopped red cabbage, strain it after ten minutes, dissolve gelatine in the purple liquid, stir in a pinch of bicarbonate of soda until it turns blue-green, and set it 3 cm deep in a tray in the fridge. Vinegar turns it pink.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Make the jelly cubes and the surface area table in Session A (soak and measure the cubes then), and the board, Part B and Part C in Session B.

Watch for these ideas

• “A bigger cube has a bigger ratio.” It has more surface, but less surface for each cubic centimetre inside.
• Mixing square and cubic units: surface area in cm², volume in cm³.
• “The vinegar went in faster on the small cube.” It went in at the same rate (look at the depths); there was simply less cube to fill.

Make it easier, make it bigger

• Easier: count the faces and blocks of the interlocking cubes instead of using formulas.
• Bigger: a flat box 1 × 2 × 4 cm has the same volume as the 2 cm cube. Which has more surface? What does that suggest about long or flat cells?

Answers and look-fors

• Part A: side 1: 6 cm², 1 cm³, ratio 6 : 1; side 2: 24 cm², 8 cm³, ratio 3 : 1; side 3: 54 cm², 27 cm³, ratio 2 : 1; side 4: 96 cm², 64 cm³, ratio 1.5 : 1.
• Part B, example class: mean depths 1 cm cube 2.5 mm, 2 cm cube 2.4 mm, 3 cm cube 2.5 mm. Reached: 1 cm 88%, 2 cm 56%, 3 cm 42%. Worked for the 1 cm cube: the unreached middle is 5 mm across, 5³ = 125 of 1000 mm³, so 88% was reached.
• Part C: 1. The 1 cm cube had the largest ratio (6 : 1) and was reached most. 2. A cell the size of a tennis ball would have far too little membrane for its volume, and diffusion would take far too long to bring oxygen and food to its middle. 3. The outgrowth adds a lot of membrane for very little extra volume, so more water and minerals can enter.
• Tickets: side 5 = 150 cm², 125 cm³, ratio 1.2 : 1. Bigger: the box has 28 cm² against the cube’s 24 cm², so long, thin or flat cells have more surface for their volume.

Why are cells so small?

A. Surface area and volume

Surface area of a cube = 6 × side × side. Volume = side × side × side. Ratio = surface area ÷ volume.

B. Jelly cubes in vinegar

Example class: depth of the pink layer after 15 minutes (mm), five groups.

Worked example for the 1 cm cube (10 mm): the pink went in 2.5 mm from each face, so the unreached middle is a cube 10 − 2 × 2.5 = 5 mm across. Its volume is 125 mm³ of 1000 mm³, so the share reached is 88%.

C. Questions

Exit tickets

Cut out one ticket for each student.

Specialised cells

In a many-celled living thing, cells take on different jobs, and each is shaped and stocked for its job. A muscle cell is packed with mitochondria, a palisade cell with chloroplasts; a red blood cell has no nucleus, which leaves room for the substance that carries oxygen; a nerve cell is very long, and a root hair cell has a long outgrowth for more surface. Specialised cells mostly have the same parts as other cells, in different numbers and shapes. Pairs match eighteen cards in sets of three: the cell, its special feature and its job.

We are learning to

• describe how the structure of a specialised cell suits its job,
• link a special feature to an organelle or to surface area,
• design a cell for a job, and justify each feature.

Success criteria

• I can match each cell to its feature and job, and explain how the feature helps.
• I can design a cell for a new job and give a reason for each feature.

You need

• the cell, feature and job cards (third sheet), one set per pair, and the worksheet (next sheet), one per student,
• a prepared slide of blood, if your school has one, to look at red blood cells at 400 times.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the cards in Session A, and the design, the gallery and Part B in Session B.

Watch for these ideas

• “Specialised cells have different parts.” They mostly have the same parts, in different numbers and shapes.
• “A red blood cell is not a real cell because it has no nucleus.” It is made with a nucleus in the bone marrow and loses it as it matures.
• “Any cell can do any job.” In a many-celled body each cell does one job well and depends on the others.

Make it easier, make it bigger

• Easier: match only the cell and job cards first, then add the feature cards.
• Bigger: a white blood cell changes shape to squeeze out of blood vessels and swallow bacteria. Which feature of animal cells makes that possible, and why could no plant cell do it?

Answers and look-fors

• Part A: red blood cell A, J, O; nerve cell B, K, R; muscle cell C, G, Q; root hair cell D, L, N; palisade cell E, I, M; guard cells F, H, P.
• How each helps, at standard: room for more oxygen-carrier and a shape that squeezes through tiny vessels; length that carries a message far in one cell; mitochondria to release the energy for moving; more surface for taking in water; more chloroplasts to capture light; shape changes that open and close the hole.
• Part B: 1. The athlete’s leg muscle: it does far more work, so it needs more energy and more mitochondria. 2. It grows underground in the dark, where chloroplasts could capture no light. 3. Heart muscle works without rest, so its cells are packed with mitochondria.
• Part C, at standard: many tiny folds on the surface facing the food, for more membrane to absorb through; many mitochondria for the energy to take food in; a thin shape close to a blood vessel. Each feature has its reason.
• Bigger: no cell wall, only a flexible membrane, so the cell can change shape; a plant cell’s stiff wall holds its shape.

Specialised cells

A. Sets of three

B. Questions

C. Design a cell

Job: line the small intestine and take in digested food as it passes. Draw your cell and label each feature with its reason.

Which feature, which job?

Cut out the cards. Make six sets of three: a cell, its special feature and its job.

Teacher note: the three kinds of card are printed in different orders; the sets are on the plan.

Show what we know

The last lesson brings the term together. A technician viewed three slides, a leaf, a cheek scraping and a drop of pond water, and lost the labels. Students use what they can see to decide which view is which, back each choice with evidence, and work out the size of the cells from the field of view and the magnification. Then each student sits the final check on their own, and the class finishes with the unit quiz on the board.

We are learning to

• identify plant cells, animal cells and single-celled organisms from what we see,
• work out the size of a cell from the field of view,
• show what we know about cells on our own.

Success criteria

• I can name each view and give two pieces of evidence for it.
• I can back each answer on the final check with a part, a number or a reason.

You need

• the three views task (next sheet) and the final check (third sheet), one each per student,
• the interactive unit open for the closing quiz.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the task and the partner check in Session A, and the final check and the class quiz in Session B.

Watch for these ideas

• Using the 400 times field for the 100 times view. Each view has its own field, printed under it.
• Calling the pond organism an animal cell. It is a whole organism in one cell; look for it living on its own.
• “The biggest-looking cells are the biggest.” How big a cell looks depends on the magnification.

Make it easier, make it bigger

• Easier: give the three names (plant cells, animal cells, a single-celled organism) and ask students to match and justify.
• Bigger: the pond water also held tiny dots too small to make out at 400 times. What might they be, and how could a scientist see them?

Answers and marking guide

• Task: View A, plant cells (box shapes in rows, cell walls and green chloroplasts); 450 ÷ 5 ≈ 90 µm. View B, a single-celled organism (one whole cell on its own, with hair-like edges for swimming); 1800 ÷ 8 ≈ 225 µm. View C, animal cells (flat, irregular shapes, a nucleus in each, no wall and no chloroplasts); 450 ÷ 7 ≈ 64 µm.
• Last questions: View B was at the lowest magnification (100×), so its cells look smaller than they are; the working shows they are the largest. A drawing without its magnification cannot be used to work out real sizes.
• Check: 1. a moss leaf. 2. All living things are made of cells; the cell is the smallest unit of life; every cell comes from an existing cell. 3. 400 times. 4. Any two: cell wall (supports and protects), chloroplasts (capture light to make sugar), large vacuole (stores sap and keeps the cell firm).
• Check: 5. mitochondrion. 6. 1800 ÷ 6 = 300 µm. 7. As a cell grows, its volume grows faster than its surface, so a large cell has too little membrane to take in what it needs, and substances take too long to diffuse to its middle. 8. No chloroplasts: it is underground, where there is no light to capture; the outgrowth gives more membrane surface for taking in water and minerals.
• Bigger: bacteria, about 2 µm long; a light microscope at higher power with an oil lens shows them as dots, and an electron microscope shows their shape.
• Scoring: 1 mark each for 1, 3 and 5; 2 marks each for 2, 4, 6, 7 and 8. The rubric near the front of the pack turns the task and the check into a level.

Three views, no labels

A technician viewed three slides, a leaf, a cheek scraping and a drop of pond water, and lost the labels. The circle is the whole field of view; its width is printed under each view.

A. Which is which?

Cells across the field: View A about 5, View B about 8, View C about 7.

B. Two questions

Show what we know

Show what you know about cells. Read each item, then circle or write.

Teacher note: the marking guide is on this lesson’s plan; the rubric near the front of the pack turns the score into a level.