Cell Structure and Function — AP Biology

AP Biology · CED Unit 2: Cell Structure and Function · 18 min read

1. Cell Organelles: Structure and Function ★★☆☆☆ ⏱ 5 min

All cells share four core features: a cell membrane, cytosol, ribosomes, and genetic material, but eukaryotic cells (plant, animal, fungi, protist) have specialized membrane-bound organelles that prokaryotes (bacteria, archaea) lack. Below are the high-yield organelles tested most frequently on the AP exam:

**Nucleus**: Structure: Double lipid bilayer (nuclear envelope) with selective nuclear pores, internal nucleolus, and chromatin (DNA wrapped around histone proteins). Function: Stores genomic DNA, regulates gene expression, and assembles ribosomal subunits.
**Ribosomes**: Structure: Two subunits made of rRNA and protein, free in cytosol or bound to rough ER. Function: Site of protein synthesis; free ribosomes make proteins for internal use, bound ribosomes make secreted/membrane proteins.
**Rough Endoplasmic Reticulum (RER)**: Structure: Folded membrane network contiguous with the nuclear envelope, studded with bound ribosomes. Function: Folds and modifies proteins for transport to the Golgi.
**Smooth Endoplasmic Reticulum (SER)**: Structure: Tubular membrane network with no ribosomes. Function: Synthesizes lipids, detoxifies drugs, stores calcium ions.
**Golgi Apparatus**: Structure: Stacked flattened membrane sacs (cisternae), with cis (receiving) and trans (shipping) faces. Function: Modifies, sorts, packages proteins/lipids for transport.
**Lysosomes**: Structure: Membrane-bound sacs of hydrolytic enzymes maintained at pH ~4.5. Function: Breaks down damaged organelles, pathogens, and macromolecules.
**Mitochondria**: Structure: Double membrane, smooth outer membrane, folded inner membrane (cristae), matrix with own circular DNA/ribosomes. Function: Site of aerobic cellular respiration, produces ATP.
**Chloroplasts (plants/algae only)**: Structure: Double membrane, stacked thylakoids (grana), stroma with own circular DNA/ribosomes. Function: Site of photosynthesis.

Exam tip: Examiners frequently ask to predict organelle abundance in specialized cell types. Always link organelle function to the cell's specific role to earn full marks.

2. Cell Membrane: The Fluid Mosaic Model ★★★☆☆ ⏱ 4 min

Proposed by Singer and Nicolson in 1972, the fluid mosaic model describes the cell membrane as a dynamic, flexible structure made of a phospholipid bilayer with a mosaic of embedded proteins.

Three primary factors control membrane fluidity, a very common FRQ test topic:

**Temperature**: Higher temperatures increase phospholipid movement, raising fluidity; lower temperatures reduce movement, decreasing fluidity.
**Fatty acid saturation**: Unsaturated fatty acid tails have double bonds that create kinks, preventing tight packing and increasing fluidity. Saturated tails pack tightly, reducing fluidity.
**Cholesterol (animal cells only)**: Acts as a fluidity buffer: at high temperatures, it restricts phospholipid movement to reduce fluidity; at low temperatures, it prevents tight packing to maintain fluidity.
**Mosaic components**: Integral proteins span the membrane for transport/signaling; peripheral proteins attach to the surface for support/signaling; extracellular carbohydrates enable cell-cell recognition.

Exam tip: Always explicitly link changes in membrane composition to changes in fluidity, then to changes in cell function to earn all FRQ points.

3. Membrane Transport: Passive and Active ★★★☆☆ ⏱ 5 min

Membrane transport describes the movement of molecules across the selectively permeable cell membrane, divided into two categories based on energy requirements and concentration gradient direction.

**Simple diffusion**: Small nonpolar uncharged molecules (O₂, CO₂) pass directly through the bilayer, no protein required.
**Facilitated diffusion**: Large/polar/charged molecules (glucose, ions) move down gradient via specific transport proteins.
**Osmosis**: Diffusion of water across a selectively permeable membrane from higher to lower water potential.

\Psi = \Psi_S + \Psi_P

Where $\Psi_S$ = solute potential (always negative; more solute = more negative $\Psi_S$) and $\Psi_P$ = pressure potential (positive in turgid plant cells, 0 in open containers/animal cells).

**Protein pumps**: Transmembrane carriers that use ATP to move molecules against gradient. The Na⁺/K⁺ pump pumps 3 Na⁺ out and 2 K⁺ in per ATP, creating an electrochemical gradient.
**Bulk transport**: Active transport of large quantities of material via vesicles: endocytosis (cell takes material in) and exocytosis (vesicles fuse with membrane to release contents outside the cell).

Exam tip: For full marks on transport questions, always specify if transport is passive/active, what subtype it is, and if a transport protein is required. Most water transport occurs via aquaporins (facilitated diffusion), not simple diffusion, so explicitly mention this.

4. Cell Compartmentalization and Metabolic Efficiency ★★★★☆ ⏱ 3 min

Compartmentalization refers to the separation of the eukaryotic cell into distinct, membrane-bound organelles, each with an internal environment optimized for a specific function. This adaptation drastically improves metabolic efficiency via four key mechanisms:

**Isolation of incompatible reactions**: Separates reactive or pH-dependent processes from the rest of the cell, preventing damage and cross-interference.
**Increased surface area for enzymatic reactions**: Folded membranes provide more space for embedded enzyme complexes, increasing total reaction rate.
**Concentration of reactants and enzymes**: Organelles accumulate high concentrations of pathway-specific enzymes and substrates, increasing reaction rate without toxic side effects.
**Supports larger cell size**: Prokaryotes lack organelles, so they are limited to smaller size and lower metabolic rates compared to eukaryotes.

Exam tip: Always link changes in membrane surface area to changes in reaction rate when answering FRQs on compartmentalization to earn full credit.

5. AP Style Concept Check ★★★☆☆ ⏱ 3 min

Common Pitfalls

Why: Students memorize isolated fact lists instead of the structure-function relationship that the AP exam exclusively tests.

Why: Students incorrectly assume water moves only via simple diffusion, and mix up concentration gradient direction.

Why: Students forget solute potential is always negative, and higher solute concentration means lower (more negative) water potential.

Why: Students learn the "standard" animal/plant cell model and ignore the relationship between organelle abundance and specialized cell function.

Why: Students only remember cholesterol affects fluidity, not its opposing effects at high and low temperatures.