12.1 Cells and Organelles

Biology on the USTET Science subtest draws heavily from Grade 11 Earth and Life Science. One of the highest-yield topics is the cell: what it is, how its parts work, and how it divides. If you can explain organelle jobs in one sentence each and clearly separate mitosis from meiosis, you will handle most cell questions under time pressure.

Cell Theory and Why Cells Matter

Modern cell theory has three core claims:

1. All living organisms are composed of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. New cells arise only from pre-existing cells by cell division.

A unicellular organism (such as an amoeba or many bacteria) is a single cell that performs all life functions. A multicellular organism (plants, animals, fungi) has specialized cells organized into tissues, organs, and systems. On the exam, "basic unit of life" almost always points back to the cell—not the atom, tissue, or organ.

Prokaryotic vs. Eukaryotic Cells

Cells fall into two major structural groups.

Prokaryotes are still fully alive: they metabolize, grow, and reproduce. They simply package their genetic material without a nucleus and lack mitochondria, chloroplasts, endoplasmic reticulum, and Golgi bodies. Eukaryotes compartmentalize work into organelles so complex processes can run efficiently in parallel.

Exam tip: If a question mentions "no nucleus" or "circular DNA," think prokaryote (bacterium). If it mentions mitochondria, chloroplasts, or a nuclear membrane, think eukaryote.

Major Organelles and Their Functions

Memorize function first; structure details matter less on a timed entrance exam.

Nucleus and Genetic Control

The nucleus houses chromosomes made of DNA and proteins (chromatin). When a cell prepares to divide, chromatin condenses into visible chromosomes. The nucleus does not "make energy"; that is the mitochondrion's job. Confusing nucleus with mitochondrion is a common trap.

Energy Organelles

Mitochondria convert chemical energy in food molecules into ATP through cellular respiration. Nearly all eukaryotic cells have mitochondria. Chloroplasts capture light energy and build sugars via photosynthesis; they appear in plant cells and some algae, not in typical animal cells. Both mitochondria and chloroplasts have double membranes and their own DNA—evidence linked to endosymbiotic origin—but USTET usually tests function more than evolutionary history.

Protein Production Line

Think of protein synthesis as a pipeline:

1. DNA in the nucleus is transcribed to mRNA.
2. mRNA exits to the cytoplasm.
3. Ribosomes translate mRNA into polypeptide chains.
4. Proteins destined for secretion or membranes often enter the rough ER, then move to the Golgi apparatus for finishing and packaging into vesicles.

If a question asks which organelle synthesizes proteins, the answer is ribosomes—not the Golgi, even though Golgi packages them.

Plant Cells vs. Animal Cells

Both are eukaryotic, but several structures differ.

Plant cells still have mitochondria. Photosynthesis makes sugar; respiration (in mitochondria) releases usable ATP from that sugar. Do not assume plants "only use chloroplasts for energy."

Plasma Membrane and Transport (Brief)

The plasma membrane is a phospholipid bilayer with embedded proteins. It is selectively permeable.

• Passive transport (diffusion, osmosis, facilitated diffusion) moves substances down their concentration gradient without ATP.
• Active transport moves substances against a gradient and requires ATP.
• In osmosis, water moves toward the side with higher solute concentration (lower water potential).

In hypotonic surroundings, animal cells may swell and burst; plant cells become turgid because the wall resists over-expansion. In hypertonic surroundings, plant cells plasmolyze (membrane pulls away from the wall).

Mitosis vs. Meiosis

Cell division questions on USTET are usually comparative, not stage-by-stage lists of every phase name.

Mitosis keeps chromosome number constant and produces clones for tissue growth. Meiosis halves the chromosome number so fertilization restores the diploid count. Without meiosis, chromosome number would double every generation.

Phases at a Glance (Mitosis)

A useful order mnemonic is PMAT:

1. Prophase — chromosomes condense; nuclear envelope breaks down; spindle forms.
2. Metaphase — chromosomes line up at the metaphase (equatorial) plate.
3. Anaphase — sister chromatids separate and move to opposite poles.
4. Telophase — nuclei re-form; chromosomes decondense.
5. Cytokinesis — cytoplasm divides, producing two cells.

Interphase (G1, S, G2) is not a mitotic phase, but DNA replication in S phase is essential before mitosis or meiosis begins.

Why Meiosis Creates Variation

Two major mechanisms increase genetic diversity:

• Crossing over exchanges segments between homologous chromosomes in prophase I.
• Independent assortment randomly orients homologous pairs at metaphase I.

Together with random fertilization, these processes explain why siblings (except identical twins) are not genetic copies of each other.

Putting It Together for USTET

When a Science item describes a structure, ask: Is this about identity (prokaryote/eukaryote, plant/animal), function (which organelle?), or division (mitosis/meiosis counts and purpose)? Match the question to one of those three frames before reading every option. Clear organelle-function pairs and the mitosis–meiosis table above cover the majority of Grade 11 cell items you are likely to see.