Life Science 8-19-26

Life Science, Grade 7

The Chemistry of Living Things, Lesson 2 of 3

Carbohydrates

35 Minutes  |  Room 212  |  Mr. Gibbs

At a Glance

• 0:00 to 0:12  Review and connect, back to the family tree

• 0:12 to 0:21  Carbohydrates, the fast fuel

• 0:21 to 0:31  Activity, Build a Carbohydrate, then Reset

• 0:31 to 0:35  Close

Prep Before Class

• Paper strips, about 25 per pair. Cut colored paper or cardstock into strips roughly 1 inch by 6 inches. One color is fine, two is better. This is the only real prep and it can be done with a paper cutter in ten minutes.

• Tape, one roll per pair, or glue sticks.

• A stapler, just one, for you. You will use it in the last round and the students do not get it.

• The Build a Carbohydrate handout, one per student.

No chemicals, no food, no allergy concerns, and nothing to clean up beyond paper scraps. If you want a quick optional demonstration, the saltine chew test works well, but it requires eating a sample, so check for gluten sensitivity first and treat it as separate from lab procedure.

1. Review and Connect

12 minutes. Redraw the family tree from scratch. Do not point at yesterday’s board.

• Recall first, cold call, no hands: name the four families of organic compounds. What do all organic compounds have in common? Carbon.

• Redraw the tree on the board: ORGANIC COMPOUNDS at the top, and four branches. Lipids. Carbohydrates. Proteins. Nucleic acids. Have students supply the names rather than writing them yourself.

• Review lipids properly, since it is only one day old: what did the grease spot test show? What does it mean that lipids do not dissolve in water? Name two jobs lipids do.

• Ask for the one that surprised them yesterday: where is there a lipid in every single cell? The membrane. Make them say it.

• Now build the bridge to today, and make it a question: lipids store a lot of energy in a small space. So why does your body bother with anything else? Why not run entirely on fat?

• Take answers, then give the frame: because storing energy and using energy quickly are two different jobs. Fat is excellent storage and slow to get at. Your body needs something it can spend right now.

• Use the money comparison, which lands well at this age: carbohydrates are the cash in your pocket. Lipids are the savings account. You can spend cash instantly, but you cannot carry much. Savings holds far more and takes work to reach.

• Circle carbohydrates on the tree. Tell them today they will test for one, and remind them nucleic acids are still waiting for the heredity unit later in the year.

The comparison question is the point of this review block. Students who can say why the body needs both fats and carbohydrates understand the chapter. Students who can only list the four families do not.

2. Carbohydrates, the Fast Fuel

9 minutes. Concrete, one example each. Stop at nine minutes.

• Start with the name, because it gives away the answer: carbo for carbon, hydrate for water. Carbohydrates are made of carbon, hydrogen, and oxygen. Ask which family they are in and why. Carbon, so organic.

• What they are: sugars and starches. If a food tastes sweet, or if it is bread, rice, pasta, or potato, you are looking at carbohydrates.

• The small ones, simple sugars: glucose is the important one. It is the fuel your cells actually burn. Every carbohydrate you eat gets broken down into glucose before your cells can use it.

• The big ones, and this is the key idea: link many sugars together in a chain and you get something different. Starch is a chain of sugars that plants use to store energy. Glycogen is the same idea in animals, stored in your liver and muscles. Cellulose is a chain of sugars too, but it is built for strength instead of storage.

• Draw it simply on the board: circles in a row. One circle is a sugar. A long row of circles is a starch. Same building blocks, different arrangement, different job.

• Job 1, quick energy: the main job. This is why an athlete eats pasta the night before and why you feel better after eating when you are shaky.

• Job 2, short term storage: starch in a potato or a seed. Ask what a seed is storing food for, and let them get to the sprout that has not reached sunlight yet.

• Job 3, structure, and connect it straight back to yesterday: cellulose is what plant cell walls are made of. Yesterday you learned every cell membrane is made of lipids. A plant cell has both, a lipid membrane and a cellulose wall outside it. That wall is why a tree can stand up and you cannot.

• The fact they will repeat at dinner: humans cannot digest cellulose. It passes straight through, and we call it fiber. A cow can, but only because of microorganisms living in its stomach doing the work. Ask why a cow can live on grass and a person cannot.

• Christian integration, one sentence: the same simple sugar unit, arranged one way, feeds you, and arranged another way, holds up a hundred foot tree. That is economy in design, not accident.

3. Activity, Build a Carbohydrate

10 minutes. Pairs. Paper strips and tape out, handout out. Everyone stays at their desk.

• Set it up in one sentence: every paper strip in front of you is one sugar molecule. That is the only building block you get today. Everything you make will be made out of these.

• Round 1, the simple sugar: have each student hold up one single loop, taped into a ring. That is a simple sugar. Ask what a cell can do with it. Burn it immediately. This is the cash in your pocket.

• Round 2, build a starch: link ten loops into a chain, each one through the last. Give them three minutes and let them work. Tell them this is starch, and that a potato plant spent a whole summer making these underground.

• Ask while they are holding it: "Can your cell burn this chain the way it burned the single loop?" No. It has to be taken apart first.

• Round 3, digestion, and let them do it: tell them to break the chain back into single loops with their hands. That is what their digestive system does to every piece of bread they eat. Ask the real question: if the body just has to take it apart again, why did the plant bother making a chain at all? Push until someone says storage. A chain packs a lot of sugar into a small space and does not wash away.

• Round 4, cellulose, and this is the payoff: take five loops from each pair and staple them into a chain yourself, up at the front, where they can see it. Hand it back. Tell them it is made of the exact same sugar loops, just fastened differently, and that this one is cellulose.

• Then give the instruction: "Take it apart with your hands. No tools." They cannot. Ask why not. They do not have the right tool for staples.

• Land it: that is exactly your situation with cellulose. Same sugar, joined a different way, and your body does not have the tool to open it. It passes straight through you, and we call it fiber. A cow gets help from microorganisms that do have the tool.

• At nine minutes: "Thirty seconds. Paper scraps in the trash, tape back on the desk, chains stay on your desk."

RESET.  "Reset in three, two, one." Wait in silence until every student has finished before you say anything else.

• Handback question: "Voices off. Every single thing you built today was made of the same one piece. Name two different jobs those pieces were doing." Take three or four answers.

4. Close

4 minutes.

• Point at the three things on their desks: a single loop, a broken pile of loops, and a stapled chain. Same building block, three different results.

• Name them one more time: one loop is a simple sugar and it is fast energy. A taped chain is starch and it is storage. A stapled chain is cellulose and it is structure, and it is the fiber you cannot digest.

• Connect straight back to yesterday: a plant cell has a lipid membrane, which you learned yesterday, and a cellulose wall outside it, which you built today. Two days, two molecules, one cell. That wall is why a tree stands up and you do not.

• Ask the connecting question: why does your body need both carbohydrates and fats instead of just one? Push back to cash and savings.

• Preview: point at the tree on the board. Tomorrow we circle proteins, the builders and the workers. Nucleic acids still wait for the heredity unit.

• Collect handouts. Name the reading and the due date. Dismiss by procedure, not by the bell.

Objectives

• Students will state that carbohydrates are made of carbon, hydrogen, and oxygen, and identify them as organic compounds.

• Students will explain the difference between a simple sugar and a starch using the chain model.

• Students will name quick energy, storage, and structure as the three jobs of carbohydrates.

• Students will explain why the body needs both carbohydrates and lipids rather than only one.

• Students will build a physical model showing that simple sugars, starch, and cellulose are made from the same building block arranged differently.