AP Biology Unit 2 Notes: Cell Structure & Organization (Structure–Function, Size, and Compartmentalization)

Plasma membrane

A selectively permeable boundary (phospholipid bilayer with embedded proteins) that controls what enters/leaves the cell and enables communication/signaling.

Phospholipid bilayer

Two layers of phospholipids formed in water with hydrophobic tails facing inward and hydrophilic heads facing outward; the basic structure of cellular membranes.

Amphipathic

Having both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions, as in phospholipids.

Selectively permeable

Describes a membrane that allows some substances to cross more easily than others, helping maintain cellular homeostasis.

Fluid mosaic model

Model describing membranes as dynamic structures where lipids and proteins move laterally, creating a “mosaic” of components rather than a rigid barrier.

Transport protein

A membrane protein (channel or pump) that helps ions and many polar/charged molecules cross the hydrophobic core of the lipid bilayer.

Cytoplasm

All contents inside the plasma membrane (in eukaryotes, excluding the nucleus), including cytosol and organelles.

Cytosol

The fluid portion of the cytoplasm where many metabolic pathways (e.g., glycolysis) occur.

Ribosome

An rRNA–protein complex (large and small subunits) that translates mRNA into a polypeptide (protein).

Free ribosomes

Ribosomes in the cytosol that typically synthesize proteins used in the cytosol.

Bound ribosomes

Ribosomes attached to rough ER that typically synthesize proteins destined for secretion, membranes, or specific organelles.

Nucleus

Membrane-bound organelle that stores most cellular DNA and regulates gene expression; separates transcription-related processes from the cytosol.

Nuclear envelope

Double membrane surrounding the nucleus, containing nuclear pores that regulate traffic between nucleus and cytosol.

Nucleolus

Region inside the nucleus where rRNA is produced and ribosome subunits begin assembling.

Rough endoplasmic reticulum (rough ER)

ER studded with ribosomes; modifies and folds proteins and contributes to membrane production (often includes carbohydrate additions).

Smooth endoplasmic reticulum (smooth ER)

ER lacking ribosomes; functions in lipid synthesis, detoxification, carbohydrate metabolism, and calcium storage (notably in muscle cells).

Golgi apparatus

Organelle that receives proteins/lipids in vesicles, modifies them, sorts them, and packages them for delivery or secretion.

Lysosome

Acidic digestive organelle (common in animal cells) containing hydrolytic enzymes for recycling macromolecules/organelles and digesting pathogens.

Central vacuole

Large plant-cell vacuole involved in storage, waste breakdown, and maintaining turgor pressure.

Mitochondrion

Double-membraned organelle where cellular respiration produces ATP; key for energy conversion and metabolism.

Cristae

Folds of the inner mitochondrial membrane that increase surface area for electron transport and ATP synthase.

Chloroplast

Double-membraned organelle in plants/algae that performs photosynthesis, converting light energy into chemical energy (sugars).

Thylakoid

Membrane sac in chloroplasts (stacked into grana) containing photosystems and electron transport proteins for the light reactions of photosynthesis.

Surface area-to-volume ratio (SA:V)

Comparison of membrane surface area available for exchange to internal volume demand; decreases as a cell grows, limiting efficient diffusion and transport.

Endosymbiotic theory

Explanation that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by an ancestral host cell; supported by double membranes, their own DNA, prokaryote-like ribosomes, and binary fission-like division.