Course: Introduction to Biology — General Biology I (BIOL 101) · Silver Oak University (fictional sample) · Prof. Castellano
Module: Week 4 of 16 · Fall 2026 · in-person, two 75-minute lectures + one weekly lab
Objective covered: Objective 3 — Describe cell structure, the organelles and their functions, the plasma membrane, and how materials cross it — and explain why cells stay small using the surface-area-to-volume relationship.

This file holds two pieces: (A) the Module 4 Overview page ("Start Here") and (B) the Welcome Announcement that drips out when the module opens. Dates below assume a Tuesday/Thursday lecture pattern with Week 4 meeting Tue Sep 22 and Thu Sep 24, a lab that same week, and end-of-week work due Sunday Sep 27, 11:59 p.m. Adjust the day-of-week and times to match your section.

(A) Module 4 Overview — Start Here

Welcome to Week 4: Cell Structure & Function

This is your home base for the week. Read it first, then work the checklist below from top to bottom. Everything you need is linked inside the module.

For three weeks we've been zooming in — from "what is life?" to atoms, bonds, and water, to the four families of macromolecules. This week those molecules finally come together into the smallest thing that is unambiguously alive: the cell. We'll tour the organelles the way you'd tour a factory — every part has a job, and its structure fits its function. Then we'll open the plasma membrane (the cell's "skin") and watch how things get in and out — some for free, some only when the cell spends energy. And we'll answer a question you've probably never thought to ask: why are your cells microscopic instead of one giant blob? The answer is a clean piece of arithmetic — surface-area-to-volume — and it's this week's quantitative skill.

The week's big question

"Why is the cell the basic unit of life — and why does a cell have to stay small?"

By Friday you'll be able to tell a prokaryote from a eukaryote, match each organelle to its function, explain how the membrane controls traffic (diffusion, osmosis, active transport), and compute surface-area-to-volume to show why cells can't just keep growing.

By the end of this week, you can…

Use this as a checklist. If you can do all four out loud, you're ready for the quiz.

• [ ] Tell a prokaryotic cell from a eukaryotic cell — no nucleus vs. a true (membrane-bound) nucleus, smaller vs. larger — and name the four things all cells share (plasma membrane, cytoplasm, DNA, ribosomes).
• [ ] Match each organelle to its function — nucleus, ribosome, rough & smooth ER, Golgi, mitochondrion, chloroplast, lysosome, vacuole, cell membrane, cell wall — as structure → function.
• [ ] Explain membrane transport — the phospholipid bilayer / fluid mosaic; passive transport (diffusion, osmosis, facilitated) needs no energy; active transport needs ATP; and use hypotonic / hypertonic / isotonic correctly.
• [ ] Compute surface-area-to-volume for a cube "cell" (SA = 6s², V = s³, ratio = 6/s) and explain why SA:V drops as a cell grows — so cells stay small or use folds/microvilli.

What's due this week, and when

Work these in order — each one gets you ready for the next.

Heads-up on the AI tools: you'll use a chatbot to draft and explain, and then you judge its work against what we cover in class. Chatbots routinely claim plant cells lack mitochondria (they don't), say osmosis moves the solute (it moves water), reverse hypertonic and hypotonic, or mis-divide a surface-area-to-volume ratio. Catching the model is the point — in the tutorial, the assignment, and the lab.

Late policy reminder: 10% off per day late. If life happens, reach out before the deadline — I'd much rather hear from you early.

How to succeed this week

• Lead with the idea, not the jargon. Every term this week is a plain-English idea first (a mitochondrion is just the cell's power plant; osmosis is just water moving toward saltier; the membrane is just a smart gate). The vocabulary comes after the idea clicks.
• Memorize two tiny hooks. "Structure fits function" (every organelle's shape matches its job). And "Osmosis moves WATER, not the salt" (water flows toward the saltier side).
• Do the surface-area-to-volume move once by hand. For a cube of side s: SA = 6s², V = s³, ratio = 6 ÷ s. Compute side 1 (6:1), side 2 (3:1), side 3 (2:1), side 4 (1.5) and watch the ratio fall as the cell grows. Doing it once makes the lab and the quiz easy.
• Remember the headline lesson: small is not a flaw — it's a requirement. A cell can't keep growing because its surface (which feeds and cleans it) can't keep up with its volume. That single ratio explains why cells divide, fold their membranes, and grow microvilli.
• Treat the chatbot as a smart intern, not an oracle. It drafts; you check — especially its biology (plant-cell mitochondria, osmosis direction) and its arithmetic (the SA:V division).

You don't need anything special for this week — just the idea that a cell is a tiny, busy factory with rules about size and traffic. Come to class ready to argue about why an elephant isn't made of one giant cell. See you Tuesday.

(B) Welcome Announcement — Module 4

Release setting: post on the module's start day (offset = 0 days), i.e., Tue Sep 22, 2026 — not before. If your platform won't preserve the scheduled date on import, post this as a draft labeled "Release: Tue Sep 22."

Subject: Welcome to Week 4 — why aren't you made of ONE giant cell? 🔬

Hi everyone,

Quick warm-up before we start: why are your cells microscopic? You're made of trillions of tiny cells instead of a few big ones — or one enormous one. That's not an accident, and it's not because evolution couldn't "figure out" big cells. There's a hard physical rule that stops a cell from growing past a certain size, and this week you'll be able to prove it with a little arithmetic.

This week — Cell Structure & Function — we tackle the big question: Why is the cell the basic unit of life, and why does a cell have to stay small? We'll tour the organelles (every part has a job — structure fits function), open up the plasma membrane to see how things get in and out (diffusion, osmosis, active transport), and use surface-area-to-volume to explain why cells stay tiny.

Three things not to miss:
1. Lecture Tutorial 4 — work through prokaryote vs. eukaryote, the organelles, transport, and the surface-area-to-volume calculation with one approved chatbot (Gemini, Claude, or ChatGPT) and submit the share link. You'll catch the model's mistakes — it loves to claim plant cells lack mitochondria, or to flip osmosis backwards. Due Sun Sep 27.
2. Lab 4 ("How Big Can a Cell Get?"), Quiz 4, Discussion 4, and Assignment 4 also close Sun Sep 27 — the lab has you compute the surface-area-to-volume of model cells, so bring your arithmetic.
3. Open the Start Here page first — it lays out everything in order with due dates.

One promise: by Friday you'll never look at the word "cell" the same way. You'll know why a cell is shaped the way it is, why salt wilts lettuce and kills slugs, and why — physically — you have to be made of trillions of microscopic cells instead of one big one.

Bring your curiosity (and a guess about why an elephant isn't one giant cell) to class on Tuesday.

See you soon,
Prof. Castellano