Cells are the basic unit of life. Everything about biology works at the cellular level, and understanding cell structure and function is essential to understanding how organisms work. Unit 2 takes you inside the cell and shows you how cells maintain themselves, exchange materials with their environment, and organize their internal processes. This is where cell biology starts.
🎯 What You Need to Know for the Exam
Unit 2 makes up about 10-13% of the AP Biology exam. Focus your energy on these priorities:
- Organelle structure and function, especially the endomembrane system (ER, Golgi, lysosomes, mitochondria)
- Surface area to volume ratio and why it limits cell size
- The plasma membrane structure and how it selectively controls what enters and exits the cell
- The different types of transport across membranes, passive and active
- How osmosis and tonicity affect cells
- Why cells have compartments and how they form (endosymbiosis)
What's in this review:
- Cell Structure, Organelles, and Compartmentalization
- Surface Area to Volume Ratio
- The Plasma Membrane
- Selective Permeability
- Membrane Transport and Diffusion
- Tonicity and Osmoregulation
- Mechanisms of Active Transport
- Origins of Cell Compartmentalization
- Study Tips for Unit 2
- Summary, Review Questions & Practice
Topics 2.1 & 2.9: Cell Structure, Organelles, and Compartmentalization
Eukaryotic cells are compartmentalized. This means different structures inside the cell handle different jobs. That's one of the biggest advantages eukaryotes have over prokaryotes. You need to know what major organelles do and why having them separated from each other matters.
Key concepts to know:
- Ribosomes: These are the cell's protein factories. They're not membrane-bound, which means they float freely in the cytoplasm. Ribosomes read messenger RNA and build proteins according to the genetic code. Ribosomes exist in all forms of life, prokaryotes and eukaryotes, which tells you they're ancient structures.
- The endomembrane system: This is a network of membrane-bound organelles that work together to modify, package, and transport polysaccharides, lipids, and proteins. It includes the ER, Golgi complex, lysosomes, vacuoles, transport vesicles, and the nuclear envelope. Think of it as the cell's postal service.
- Rough endoplasmic reticulum (rough ER): "Rough" because it has ribosomes attached to its surface. Rough ER makes proteins that need to be shipped out of the cell or embedded in membranes. It also compartmentalizes the cell, separating certain reactions from others.
- Smooth endoplasmic reticulum (smooth ER): No ribosomes here. Smooth ER makes lipids and helps detoxify harmful chemicals in the cell.
- Golgi complex: A stack of flattened membrane sacs. The Golgi takes newly made proteins and lipids from the ER, modifies them chemically, folds them correctly, and packages them into vesicles for transport to their final destinations.
- Mitochondria: The powerhouse of the cell. Mitochondria have two membranes. The inner membrane is highly folded, which increases surface area for the reactions of aerobic cellular respiration. These folds are called cristae. The compartments created by these two membranes allow different metabolic reactions to happen in different places.
- Lysosomes: Membrane-enclosed sacs full of digestive enzymes. Lysosomes break down waste materials, dead organelles, and bacteria that have been taken into the cell. They also play a role in programmed cell death (apoptosis), a normal and healthy part of development.
- Vacuoles: Membrane-bound storage sacs. In plant cells, a large central vacuole maintains turgor pressure (the pressure that keeps plant cells firm and upright) by storing water and nutrients. In animal cells, vacuoles are smaller and more numerous, and they store various materials the cell needs.
- Chloroplasts: Found in plants and photosynthetic algae. Like mitochondria, chloroplasts have a double membrane. They're the site of photosynthesis, where light energy is converted to chemical energy in sugars.
- Compartmentalization: Separating different processes into different spaces is incredibly important. It allows competing reactions to happen side by side without interfering with each other. It also increases surface area where enzymatic reactions can occur, making the cell more efficient.
⚠ Watch out for:
Students often forget that organelles aren't just hanging around doing nothing. They work together as a system. A protein made on rough ER might get modified in the Golgi, packaged into a vesicle, and shipped to the plasma membrane to be secreted. Or it might go to a lysosome or mitochondria to do its job. The exam tests whether you understand these connections. Also, don't confuse the roles of mitochondria (aerobic respiration) and chloroplasts (photosynthesis). Both have two membranes for good reason, but they do completely different jobs.
Topic 2.2: Surface Area to Volume Ratio
Cells are limited in size. This isn't random. It's physics and chemistry. As cells get bigger, their volume increases faster than their surface area. This becomes a problem because the cell's surface area is where it exchanges materials with the environment, but the volume is where the cell needs to use those materials. Too much volume and not enough surface area, and the cell can't get supplies in or waste out fast enough. Eventually, the cell dies.
Key concepts to know:
- The ratio matters: Smaller cells have a higher surface area to volume ratio than larger cells. Smaller cells are better at exchanging materials with their environment relative to their size.
- Exchange limitations: As a cell gets bigger, the volume grows faster than the surface area. This means the cell needs more nutrients and produces more waste relative to how much membrane it has available for exchange. Eventually, the surface area isn't large enough to support the volume.
- Surface area solutions: Some cells solve this problem by having folds or extensions that increase surface area. Intestinal cells have microvilli (tiny finger-like projections) that dramatically increase surface area for absorption. Mitochondria have cristae (folds in the inner membrane) for the same reason.
- Heat exchange: Surface area to volume ratio also affects how quickly organisms can exchange heat with their environment. Smaller organisms have a higher surface area to volume ratio, so they exchange heat faster. This is why small animals like mice lose heat quickly and need high metabolic rates to stay warm. Larger animals exchange heat more slowly relative to their mass.
- Metabolic rate: There's a relationship between body size and metabolic rate per unit of mass. Smaller organisms have higher metabolic rates per unit mass than larger organisms. This is partly because they need to generate more heat to stay warm.
⚠ Watch out for:
The relationship between surface area and volume is mathematical, and the AP exam might ask you to calculate ratios. Don't panic if you see numbers. Just remember: as an object gets bigger, volume increases as the cube of the radius (or side length), but surface area increases only as the square. This is why large cells are at a disadvantage. Also, don't confuse "smaller cells are more efficient" with "cells should be as small as possible." There's a trade-off. Cells need to be big enough to have all the machinery they need to survive.
Topic 2.3: The Plasma Membrane
The plasma membrane is the boundary between the cell and the external environment. It's not just a passive barrier. It's selective, dynamic, and actively involved in controlling what gets in and out of the cell. Understanding membrane structure is critical.
Key concepts to know:
- Phospholipids: The membrane's structural foundation. Each phospholipid has a hydrophilic (water-loving) phosphate head and hydrophobic (water-fearing) fatty acid tails. In aqueous environments, phospholipids spontaneously arrange into a double layer, called a bilayer, with heads facing out and tails facing in.
- Embedded proteins: Proteins are woven into the phospholipid bilayer. Some proteins are hydrophilic and sit on the surface or span the entire membrane. Others are hydrophobic and stay buried in the fatty acid interior. These proteins do most of the actual work: they transport molecules, receive signals, provide structure, and catalyze reactions.
- Cholesterol and glycoproteins: Cholesterol molecules sit in the bilayer and help maintain membrane fluidity and structural stability in animal cells. Glycoproteins (proteins with carbohydrate chains attached) and glycolipids (lipids with carbohydrate chains) extend from the surface of the membrane. These carbohydrate chains are recognition sites for cell-to-cell communication.
- Fluid mosaic model: The membrane is not a solid structure. Its components move around. Phospholipids can slide past each other, and proteins float within the bilayer. This fluidity is essential for membrane function. The "mosaic" part refers to the mix of different components all working together.
⚠ Watch out for:
The fluid mosaic model is heavily tested. Make sure you understand that the membrane is dynamic, not static. Also, remember that the hydrophobic interior of the bilayer is what gives the membrane its selective permeability. This interior blocks ions and large polar molecules, which is the whole point. The embedded proteins provide the channels and carriers that allow these molecules through when needed.
Topic 2.4: Selective Permeability
The plasma membrane doesn't let everything through equally. It's selectively permeable, which means some molecules pass easily while others are blocked. This selectivity is what allows cells to maintain different internal conditions from their external environment.
Key concepts to know:
- The hydrophobic barrier: Small nonpolar molecules like O2, CO2, and N2 can pass directly through the phospholipid bilayer. They dissolve in the fatty acid tails and slip right through. This is why these gases exchange easily across membranes without any protein help.
- Polar molecules and ions: Large polar molecules and ions are charged or have lots of hydrogen bonding groups. They can't dissolve in the hydrophobic interior of the bilayer, so they can't cross the membrane on their own. They need help from channel proteins or transport proteins.
- Small polar molecules: Water molecules and ammonia (NH3) are small and polar. In small amounts, they can squeeze through the bilayer or pass through small gaps, but not efficiently. Water and ammonia movement is enhanced by special channel proteins called aquaporins (water channels).
- The cell wall adds protection: In plants, fungi, bacteria, and archaea, the cell wall sits outside the plasma membrane. It's rigid and provides structural support and protection from osmotic lysis (bursting).
⚠ Watch out for:
Students sometimes think the membrane blocks everything except what it "wants" to let through. That's close, but not quite right. The membrane is more like a filter with different pore sizes and a hydrophobic interior. Small nonpolar molecules get through easily because they dissolve in fat. Ions get blocked unless there's a channel protein. Large polar molecules are blocked unless a carrier protein actively helps them across. The exam tests whether you know which molecules need which mechanism.
Topics 2.5 & 2.6: Membrane Transport and Diffusion
Molecules cross the plasma membrane in several ways. Some ways require the cell to spend energy (ATP). Some don't. You need to know the differences and when each type happens.
Key concepts to know:
- Passive transport: Movement without energy input. Molecules move from regions of high concentration to regions of low concentration, following the concentration gradient. This is the direction molecules naturally move because it's favored by entropy.
- Simple diffusion: Small nonpolar molecules like O2 and CO2 diffuse directly through the phospholipid bilayer without any protein help. It's passive and goes down the concentration gradient.
- Facilitated diffusion: Larger polar molecules or ions need help to cross the membrane, but they still move down their concentration gradient, so no energy is required. Transport proteins and channel proteins do the work. Ions like Na+ and K+ use channel proteins. Larger polar molecules like glucose can use carrier proteins in some cases.
- Aquaporins: Special water channel proteins that allow water to cross the membrane very quickly. Water doesn't need these channels to cross the membrane, but they dramatically speed up water movement.
- Active transport: Movement against the concentration gradient, from low to high concentration. This requires energy (ATP). Membrane proteins pump molecules across the membrane. The classic example is the Na+/K+-ATPase pump, which uses ATP to pump sodium ions out of the cell and potassium ions in, maintaining the electrical and chemical gradients that cells depend on.
- Endocytosis and exocytosis: For very large molecules or large quantities of material, cells use vesicles. Endocytosis is when the plasma membrane folds inward, surrounding material and bringing it into the cell. Exocytosis is when internal vesicles fuse with the plasma membrane and release their contents outside the cell. Both require energy.
⚠ Watch out for:
The key distinction is energy and direction. Passive transport goes down the gradient (high to low) and doesn't need energy. Active transport goes against the gradient (low to high) and requires ATP. Facilitated diffusion is passive and uses proteins. Don't confuse it with active transport just because proteins are involved. Also, endocytosis and exocytosis are used for large particles or large amounts of material, not individual small molecules.
Topic 2.7: Tonicity and Osmoregulation
Water moves across membranes constantly. Understanding how and why is critical because it directly affects whether cells survive or die.
Key concepts to know:
- Osmosis: Water moves across a selectively permeable membrane from regions of high water potential to regions of low water potential. You can also think of it as moving from regions of low solute concentration to regions of high solute concentration. Water is moving toward where there's more dissolved stuff.
- Hypertonic, hypotonic, and isotonic: These terms describe the solute concentration outside the cell compared to inside. A hypertonic solution has more solutes outside than inside the cell, so water leaves the cell. A hypotonic solution has fewer solutes outside than inside, so water enters the cell. An isotonic solution has the same solute concentration as inside the cell, so no net water movement happens.
- Plasmolysis and turgidity: In plant cells, water leaving the cell causes plasmolysis (the cytoplasm shrinks away from the cell wall). The cell loses turgidity and wilts. In hypotonic solutions, water enters plant cells, maintaining turgor pressure and keeping the plant firm and upright.
- Lysis and crenation: In animal cells, water leaving causes crenation (the cell membrane crinkles and the cell shrinks). Water entering causes lysis (the cell swells and bursts) because animal cells don't have a rigid cell wall to protect them.
- Osmoregulation: Organisms maintain water balance and control their internal solute composition through osmoregulation. This allows organisms to survive in different environments. Fish in freshwater are in a hypotonic environment and lose water, so they actively excrete dilute urine. Fish in saltwater are in a hypertonic environment and gain solutes, so they excrete small amounts of concentrated urine and actively take in water.
⚠ Watch out for:
Osmosis is water movement, not solute movement. The membrane is selectively permeable, so solutes can't move freely. Water moves toward the solutes. Also, remember that "hypertonic" and "hypotonic" describe the solution relative to the cell. A hypertonic solution is concentrated (more solutes), so water leaves the cell. A hypotonic solution is dilute (fewer solutes), so water enters the cell. It's easy to mix these up, so practice using them until they feel natural.
Topic 2.8: Mechanisms of Active Transport
Active transport uses the cell's energy (ATP) to move molecules against their concentration gradient. The most important example is the Na+/K+-ATPase pump.
Key concepts to know:
- The Na+/K+-ATPase pump: This pump uses one ATP molecule to pump three sodium ions out of the cell and two potassium ions into the cell. This creates and maintains concentration gradients for both ions. These gradients are critical for cell function. The pump also creates a charge difference across the membrane (the membrane potential), with the inside of the cell more negative than the outside.
- Electrochemical gradients: The combination of concentration and electrical gradients. Na+ is pumped out, creating a concentration gradient (more Na+ outside) and an electrical gradient (negative inside). This gradient is used to power other processes.
- Why it matters: The Na+/K+ gradients power secondary active transport (moving other molecules using the energy in the gradient), generate action potentials in neurons, and maintain osmotic balance.
⚠ Watch out for:
The Na+/K+ pump is an asymmetrical pump. It pumps three sodiums out for every two potassiums in. This is important because it creates a net outflow of positive charge, making the inside of the cell more negative. This membrane potential is essential for cell signaling and muscle contraction. Don't think of the pump as simply "balancing" sodium and potassium. It's maintaining specific gradients that the cell depends on.
Topic 2.10: Origins of Cell Compartmentalization
The theory of endosymbiosis explains where mitochondria and chloroplasts came from. It's one of the most important ideas in cell biology.
Key concepts to know:
- Endosymbiosis: The idea that eukaryotic cells obtained mitochondria and chloroplasts by engulfing prokaryotic cells and incorporating them. The word literally means "living together inside." Free-living prokaryotes were engulfed by larger cells and eventually became permanent organelles.
- Evidence for endosymbiosis: Mitochondria and chloroplasts have their own DNA, separate from nuclear DNA. They have their own ribosomes, which are prokaryotic-style ribosomes (70S), not eukaryotic ribosomes (80S). They have double membranes, which makes sense if a prokaryote was engulfed inside another cell's membrane. Genetic and molecular evidence shows that mitochondrial and chloroplast DNA is more closely related to bacterial DNA than to eukaryotic nuclear DNA.
- Prokaryotes vs. eukaryotes: Prokaryotic cells (bacteria and archaea) lack membrane-bound organelles, but they're not disorganized inside. They have internal regions with specialized structures and functions. Eukaryotic cells have internal membranes that partition the cell into specialized regions (compartments). This compartmentalization is what allows eukaryotes to be so much more complex.
⚠ Watch out for:
Endosymbiosis is well-established science, not just a hypothesis anymore. The AP exam expects you to know the evidence and understand how it supports the idea that mitochondria and chloroplasts were once free-living prokaryotes. Also, remember that the double membrane of these organelles comes from the original prokaryote's single membrane plus the eukaryotic cell's membrane that engulfed it.
Study Tips for Unit 2
Practice Prompts for StarSpark:
- "Create a diagram of the endomembrane system and explain how proteins are made, modified, and shipped to their destination"
- "Quiz me on the difference between hypertonic, hypotonic, and isotonic solutions and what happens to animal and plant cells in each"
- "Give me scenarios where I have to identify whether a molecule moves by simple diffusion, facilitated diffusion, or active transport and explain why"
The key to Unit 2 is understanding the plasma membrane as a dynamic, selective barrier and recognizing that all the organelles inside the cell work together as a system. Don't memorize organelles in isolation. Understand how they connect. A protein synthesized on rough ER is processed in the Golgi and shipped somewhere. The mitochondria provide the ATP that powers the Na+/K+ pump. Everything is connected.
Summary, Review Questions & Practice
You've covered all the topics in Unit 2. Before you move on, test yourself with these scenario-based questions. If you can answer them confidently, you're in great shape for this section of the exam.
Review Questions: Test Yourself
- A cell placed in a hypertonic solution begins to shrivel. Explain what's happening in terms of water potential and osmosis. Would this cell eventually lyse, or is it more likely in a plant or animal cell?
- A molecule requires transport protein to cross the plasma membrane but moves down its concentration gradient without using ATP. What type of transport is this? Provide an example of a molecule that might use this method.
- Mitochondria have two membranes and their own DNA. How does endosymbiotic theory explain these features? What other evidence supports the endosymbiotic origin of mitochondria?
- A cell has a large volume and a low surface area to volume ratio. How might this affect its ability to obtain nutrients and eliminate waste? What cellular structures might increase surface area to solve this problem?
- Rough ER is associated with ribosomes, while smooth ER is not. Explain how this structural difference relates to their different functions in the cell.
Want more practice? Paste these questions into StarSpark to generate a full quiz with explanations.
Explore the Full AP Biology Study Guide
Unit 2 lays the groundwork for everything that comes next. Cell structure and membrane dynamics are foundational to understanding how cells capture energy, communicate, divide, and express genes.
Check out the full AP Biology study plan to see how this unit connects to the rest of the course.
Other Unit Reviews:
- AP Biology Unit 1: Chemistry of Life
- AP Biology Unit 3: Cellular Energetics
- AP Biology Unit 4: Cell Communication and Cell Cycle
- AP Biology Unit 5: Heredity
- AP Biology Unit 6: Gene Expression and Regulation
- AP Biology Unit 7: Natural Selection
- AP Biology Unit 8: Ecology
For official AP Biology resources, visit apcentral.collegeboard.org.
This review is aligned with the AP Biology Course and Exam Description. AP is a registered trademark of the College Board, which was not involved in the production of this guide.
