Cell Biology: The Structure and Function of Life's Building Block

The cell is the smallest unit of life, and like a tiny factory, it's organized into specialized parts — organelles — that each handle a specific job. The nucleus stores the instructions; the mitochondria produce energy; ribosomes build proteins. You don't need to memorize every detail for the ACT, but a solid grasp of the major structures and their functions makes biology passages much faster to read and is the foundation of any biology course.

This guide covers cell types, the major organelles, and plant-versus-animal differences, with worked and practice questions matched to the level seen in ACT Science and high-school biology at Northside Tutoring.

Why Cell Biology Matters

The ACT Science section often draws on biology passages, and while the answers come from the figures, background knowledge lets you read faster and reason more confidently. Cell biology is also the foundation of high-school and AP Biology. (Note: the SAT has no science section.)

Two Types of Cells

All cells fall into two categories. Prokaryotic cells (bacteria) are small and simple, with no nucleus and no membrane-bound organelles — their DNA floats freely. Eukaryotic cells (plants, animals, fungi, protists) are larger and contain a true nucleus and many specialized organelles.

The Major Organelles

Organelle	Function
Nucleus	stores DNA; controls the cell
Mitochondria	produce energy (ATP) — the "powerhouse"
Ribosomes	build proteins
Endoplasmic reticulum (ER)	processes/transports proteins and lipids
Golgi apparatus	packages and ships molecules
Chloroplast (plants)	photosynthesis — makes food from light

The Cell Membrane

The cell membrane surrounds every cell and is selectively permeable — it controls what enters and leaves. This gatekeeping keeps the cell's internal environment stable, a property called homeostasis. Small molecules can pass; larger ones need help through special channels.

Plant vs. Animal Cells

Both are eukaryotic, but plant cells have three structures animal cells lack:

• A rigid cell wall outside the membrane (support and shape).
• Chloroplasts for photosynthesis.
• A large central vacuole for storage and water balance.

Connecting Structure to Function

The theme to remember: structure fits function. Mitochondria have folded inner membranes to maximize energy-producing surface area; the small intestine's cells have finger-like projections to absorb more nutrients. On ACT Science and in biology class, expect questions that link a structure to the job it performs.

Where You'll See This — Test by Test

Cell biology is background that supports ACT Science biology passages; the SAT has no science section and the SSAT doesn't test it. It's core high-school and AP Biology content.

Watch the Lesson

Sometimes a diagram needs a voice. In the short video below, one of our Northside tutors walks through the core idea and works through test-style problems in real time.

Worked Example Problems

These problems are calibrated to the difficulty you'll actually see on test day. Try each one before opening the solution.

Common Mistakes to Avoid

Practice Problems — You Try

Three problems below. Work each before checking the solution.

The Northside Method — How We Teach This 1-on-1

Reading a blog is a great starting point. But there's a meaningful gap between understanding a concept and reflexively applying it under timed conditions. That gap is exactly what our tutors close.

Every Northside student works through a four-step framework:

1. Assessment. We diagnose which specific skills are slowing your student down — not just whether they "get it" in the abstract.
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