Cell Structure & Organelles: A Tour Inside the Cell

Every Texas Biology CBE has at least one organelle-identification question. Master the eight structures every cell biology question is built around — what each one looks like, what it does, and how plant cells differ from animal cells.

9 min TEKS 4A,4B,5D Biology

Why organelles dominate this exam

Roughly 15% of the Texas Biology CBE tests organelle identification — either by name (“Which organelle produces ATP?”) or by picture (“The organelle labeled X is the…”). The good news: you only need to know eight structures, and most have one signature feature you can spot in any diagram.

The eight you must know

Nucleus — large round structure, dark center (nucleolus). Stores DNA.
Mitochondrion — bean-shaped with inner folds (cristae). Makes ATP.
Endoplasmic reticulum (ER) — winding membrane network. Rough ER has ribosomes.
Golgi apparatus — stack of flat curved sacs. Packages proteins.
Ribosomes — tiny dots on ER or floating free. Synthesize proteins.
Lysosomes — small round vesicles. Break down waste.
Cell membrane — outer thin boundary of every cell. Controls entry/exit.
Cell wall + chloroplast + vacuole — plant cells only. Wall = rigid outer; chloroplasts = green ovals; vacuole = giant central bubble.

The standard animal-cell diagram

Almost every cell-identification question on the CBE uses the same textbook illustration style. Here’s the canonical layout you should be able to recognize:

Animal cell with the seven core organelles labeled. Practice identifying each by its visual signature, not by position.

Visual signatures — how to spot each one

On the exam you’ll see a single diagram and four answer choices. Don’t memorize positions; memorize what each organelle uniquely looks like:

Nucleus: largest, dark, central. The only structure with a darker dot inside (nucleolus).

Mitochondrion: oval/bean shape with parallel folds inside. Always orange-pink in textbooks.

ER: extensive winding sheets adjacent to the nucleus, often dotted with ribosomes.

Golgi: stacked “pancakes” — 4–6 flat curved sacs, often with vesicles budding off.

Ribosomes: tiny black dots, either on ER or scattered in the cytoplasm.

Lysosomes: small uniform round vesicles, much smaller than mitochondria.

Plant cells: three structures animal cells lack

If the diagram is rectangular with a thick green-brown outer border, it’s a plant cell. Three structures are only in plant cells:

Cell wall — the rigid green-brown outer layer (outside the membrane).
Chloroplasts — green ovals with stacked discs (thylakoids) inside. Site of photosynthesis.
Central vacuole — one giant pale-blue bubble taking up 30–50% of the cell. Stores water/keeps the cell turgid.

Side-by-side comparison. The student should be able to name what's in only the right cell — without any labels.

Three-step strategy for any cell-ID question

Identify the cell type first — square with green wall = plant; round irregular = animal.
Match the visual signature to the function asked — “makes ATP” → bean shape with folds; “packages proteins” → stacked sacs; “stores DNA” → biggest dark structure.
Eliminate distractors — if the question asks for a plant-only structure (e.g., “chloroplast”), distractors will include animal-only structures.

Exam tip

When the question shows an animal cell and the answer choices include “chloroplast” or “cell wall,” eliminate them immediately — those structures don’t exist in animal cells.

Check yourself

Quick check #1

Which organelle has a bean shape with internal folds called cristae and produces ATP?

Quick check #2

A textbook diagram shows a rectangular cell with a thick green outer layer, a large central blue space, and several oval green structures. Which organelle is found ONLY in this kind of cell, never in animal cells?

Apply it to real CBE questions

Every concept above shows up on the actual exam. Try these questions to lock it in: