AP Biology Unit 2: Cells Notes

Introduction

Membranes create selective boundaries, and cells use geometry, proteins, energy, and compartments to control exchange and organize reactions.

Start with the membrane as a physical boundary rather than a list of parts. Its nonpolar interior blocks some substances more strongly than others; proteins provide selective routes; pumps use energy to maintain gradients; and vesicles move cargo too large for a transporter.

At the larger scale, cell size and compartment geometry influence exchange. Internal membranes create distinct reaction spaces and expand working surface, but every compartment still depends on regulated transport and conserved matter.

Cell geometry

surface available per unit volume

Bilayer chemistry

selective permeability

Gradient plus transport mechanism

net movement

Internal membranes

localized reactions and controlled exchange

Current exam guidance: Aligned to the current 2026–27 AP Biology course framework, including scientific practices, data analysis, and model-based reasoning.

2.1

Cell Structure and Function

How do subcellular structures cooperate to carry out cellular work?

Short answer

Organelles provide specialized surfaces and chemical environments, but cellular function emerges from material moving through connected structures rather than from isolated organelles acting alone.

Know firstProtein structure · Membrane organization · Nucleic-acid information

Trace the product through the system

A secreted protein gives a clear structure–function path. A ribosome associated with rough endoplasmic reticulum translates the mRNA, and the growing product enters the endomembrane pathway. Transport vesicles move cargo to the Golgi apparatus, where products are modified and sorted. A later vesicle fuses with the plasma membrane and releases cargo. Blocking one transfer changes the location of the product even if earlier synthesis continues.

Other organelles solve different physical problems. Smooth ER supports lipid synthesis and detoxification, while lysosomes confine hydrolytic enzymes used in digestion and participate in programmed cell death. A plant central vacuole stores water and solutes and contributes to turgor; smaller animal-cell vacuoles store cellular materials. Mitochondrial inner-membrane folds increase surface for reactions involved in aerobic ATP production, and chloroplast membranes establish compartments used in photosynthesis. Form follows the reaction conditions and transport requirements.

Prokaryotic cells have ribosomes and organized biochemical regions but lack the eukaryotic endomembrane organelle set. This is a difference in organization, not a ranking of organisms. Microscopy can show colocalization and protein-tag experiments can track movement, but a static image alone may not establish the direction or mechanism of transport.

Detailed public-domain cutaway diagram of a plant cell and its organelles — A plant cell organizes work into compartments
Use the cutaway to locate the nucleus, endomembrane system, chloroplasts, mitochondria, vacuole, cell membrane, and wall in one cellular context.
Use this visual to see
Orient major organelles spatially before assigning each a function.
What it cannot show
The illustration is a generalized composite, not a microscope image or a scale model of one plant cell.
Mariana Ruiz Villarreal (LadyofHats) · Public domain · Wikimedia Commons item record

Public-domain diagram of a prokaryotic cell with nucleoid, ribosomes, membrane, wall, pili, and flagellum — Prokaryotic organization does not mean no organization
A prokaryotic cell lacks a membrane-bound nucleus yet still contains DNA, ribosomes, a plasma membrane, and organized molecular systems.
Use this visual to see
Identify shared cellular machinery while distinguishing membrane-bound compartmentalization.
What it cannot show
The drawing combines common bacterial features; not every prokaryote has every labeled structure or this shape.
Mariana Ruiz Villarreal (LadyofHats) · Public domain · Wikimedia Commons item record

A conserved cargo pathway

Membrane budding and fusion transfer cargo between compartments while preserving the cargo ledger; pathway interruption redistributes rather than destroys it.

A conserved cargo pathwayIn the normal state, most of 24 units have reached late compartments. With ER-to-Golgi transfer blocked, 18 accumulate in the ER and only 2 are extracellular.

normal

ER: 2
Golgi: 4
secretoryVesicles: 6
extracellular: 12

ER to Golgi blocked

ER: 18
Golgi: 2
secretoryVesicles: 2
extracellular: 2

Entities

rough ER
transport vesicle
Golgi apparatus
secretory vesicle
extracellular space
24 labeled cargo units

Rules

Cargo count is conserved during transfer.
A blocked step causes accumulation before the block.
The pathway does not imply every protein is secreted.

A conserved cargo pathway states
State	id	ER	Golgi	secretoryVesicles	extracellular	total
1	normal	2	4	6	12	24
2	ER-to-Golgi-blocked	18	2	2	2	24

Limitations: The counts and compartments are synthetic and omit synthesis, degradation, recycling, and protein-specific signals.
Accessible summary: In the normal state, most of 24 units have reached late compartments. With ER-to-Golgi transfer blocked, 18 accumulate in the ER and only 2 are extracellular.

Match structure to physical job

Structure	Feature	Job supported
ribosome	rRNA–protein complex	translation
rough ER and Golgi	connected trafficking pathway	synthesis, modification, and sorting of selected products
smooth ER	membranous synthesis surface	lipid synthesis and detoxification
lysosome	bounded hydrolytic compartment	controlled digestion and programmed cell death
vacuole	membrane-bound storage space	water, solute, or material storage; plant turgor
mitochondrion	folded inner membrane	surface and gradient for aerobic ATP production
chloroplast	thylakoid and stroma compartments	light reactions and carbon fixation

Keep these three ideas

Structures create specialized reaction conditions.
Cellular pathways connect organelles through material transfer.
Perturbations often change product location before changing total matter.

Cells

Introduction

Cell Structure and Function

Trace the product through the system

A plant cell organizes work into compartments

Prokaryotic organization does not mean no organization