Comprehensive AP Biology Unit 2 Review: Cell Structure & Function

Buckle up, we're about to shrink down and go full fantastic voyage into the microscopic world of the cell. From

power generating mitochondria to the selective fortress of the plasma membrane, every structure has a story.

And spoiler alert, it all matters for your AP bio score. So whether you're the nucleus of your study group or just

floating around like cytoplasm, this unit 2 recap has you covered. Hey, I'm Melanie King from the Absolute

Recap and I make AP Bio easier with podcasts, study guides, videos, the ultimate review packet, and the ultimate

exam slayer. This updated unit 2 review video all about cell structure and function follows the AP Biology course

and exam description released by the College Board for fall 2025. But it's more than just prep for your next quiz.

Think of it as your go-to tool for your midterm, your final, and of course, the AP Bio exam in May. Before we get

started, don't forget to download your free copy of the AP Bio Ultimate Review Packet. The links in the description.

It's packed with a video align study guide with an answer key, multiplechoice practice, skillbuilding worksheets with

video walkthroughs, and links to even more videos and podcasts. Not currently in AP Bio? No problem. These resources

are great for any high school or college level biology course, too. And if you're just here for a quick review, then check

out the Ultimate Exam Slayer. Focused 100% on exam prep. You'll get access to unit tests, fulllength practice tests,

one pages for every unit, math and graphing review sheets, and exclusive unit and exam tip videos. Okay, let's

zoom in. 2.1 cell structure and function. First up, ribosomes. These small but mighty structures are made of

two key ingredients. Ribosomal RNA, RRNA, and protein. Ribosomes are not membrane bound, which is an important

distinction. You'll find them floating freely in the cytoplasm or attached to the rough endopplasmic reticulum. More

on that in a bit. Why do they matter? Because ribosomes synthesize proteins based on instructions carried by

messenger RNA. This process is called translation and it's the central dogma of molecular biology. The fact that

ribosomes are found in all forms of life from bacteria to humans supports the theory of common ancestry. Every living

thing needs to make proteins and ribosomes make that possible. Next, let's look at the endommembrane system.

A group of membranebound organels that work together like a cellular assembly line to modify, package, and transport

within the cell and beyond. Here's the breakdown. The endopplasmic reticulum, ER, comes in two varieties. Rough ER has

ribosomes attached. It helps compartmentalize the cell and plays a major role in protein synthesis and

processing. Smoothie R on the other hand has no ribosomes. It handles lipid synthesis and helps detoxify substances

in the cell. Think of it as the cellular version of a detox spa. The Golgi complex consists of flattened sacks that

fold, chemically modify, and package proteins for their final destination. This could be within the cell or out to

the extracellular environment. The system also includes the nuclear envelope, plasma membrane, losomes,

transport vesicles, and vacules, all working together to keep the traffic of materials flowing smoothly. Let's talk

about waste and storage because even cells need a cleanup crew. Losomes are membrane bound sacks filled with

hydraytic enzymes that break down damaged cell parts, waste, and even pathogens. They also play a key role in

apoptosis or programmed cell death, which is essential for development and disease prevention. Vacules are membrane

enclosed storage units with different functions depending on the type of cell. In plant cells, the central vacule is

huge and crucial. It stores water and nutrients and maintains trigger pressure, helping the plant stay

upright. In animal cells, vacules are smaller and more numerous, storing ions, nutrients, and waste products. Now for

some serious energy talk. The mitochondria. These double membraned organels are the site of aerobic

cellular respiration. The outer membrane is smooth but the inner membrane is highly folded into structures called

christe. This folding increases surface area allowing for more ATP production the energy currency of the cell. The

compartments created by these membranes allow different metabolic reactions to occur in an organized efficient way. So

more reactions equals more energy and more function. Last but not least chloroplasts. These are found only in

plants and photosynthetic algae and are the site of photosynthesis. Like mitochondria, chloroplast have a double

membrane and inside they contain stacks of membranes called philoids where light energy gets converted into chemical

energy glucose. Just like mitochondria, chloroplast create compartments to increase efficiency. Got all that? I

know there are a lot of organels to understand. I highly recommend trying the unit 2 practice sheet and video

inside the ultimate review packet to sharpen your organel identification skills by structure and function. 2.2.

Cell size. What does surface area and volume actually mean for cells? Surface area refers to the total area of the

cell's outer membrane, the interface between the inside of the cell and the environment. Volume is the amount of

space inside the cell. Basically, the cell's stuff or cytoplasm and organels. For cells to survive, the surface area

has to be big enough to support the needs of volume inside, like supplying nutrients and removing waste. Here's the

catch. As cells get bigger, its volume grows much faster than its surface area. This means the surface area to volume

ratio actually decreases, and that's a problem. A smaller ratio means less membrane surface to exchange materials

per unit of volume inside the cell. So, the cell can't get what it needs or get rid of waste efficiently. That's why

cells are small, easier to keep a high surface area to volume ratio and stay efficient. If cells get too big, they'd

starve internally or get poisoned by waste buildup because their membranes just can't keep up. Some cells and

organels solve this by having lots of folds or extensions in their membranes, increasing surface area without making

the structure huge. But the surface area to volume ratio doesn't just affect single cells, it affects whole

organisms, too. As animals get bigger, their surface area to volume ratio decreases, which changes how quickly

they gain or lose heat. Smaller animals lose heat quickly and have higher metabolic rates to stay warm, while

bigger animals hold heat longer and tend to have slower metabolisms. This explains why a tiny shrews heart beats

faster and it eats constantly, while an elephant's metabolism is much slower. Surface area and volume calculations

help biologists predict how cell size affects function and efficiency. All of the equations you need are on the

formula sheet. And if you need help understanding the math, check out the math review sheet in the ultimate exam

slayer. 2.3 plasma membrane. At the heart of the cell membrane is the phospholipid billayer, a double layer of

molecules that have split personalities. Each phospholipid has a hydrophilic head, meaning it loves water and

hydrophobic tails that avoid water at all costs. These molecules arrange themselves with their heads facing the

watery environments inside and outside the cell while the fatty acid tails huddle together in the middle away from

water. This setup forms a flexible but protective barrier that's selective about what crosses through. But the

membrane isn't just made of lipids. Scattered throughout are proteins. Some float, some anchor, and each one plays a

unique role. These proteins can be hydrophilic, hydrophobic or a combination of both depending on the

side chains of their amino acids. These proteins are involved in transport, signaling and maintaining structure. So

why is it called a fluid mosaic model? Fluid refers to the fact that the membrane components aren't locked in

place. They move and shift within the layer. Mosaic refers to the variety of components embedded in the membrane.

phospholipids, proteins, cholesterol, glyoproteins, and glyolippids forming a complex colorful patchwork. 2.4 Membrane

permeability. The plasma membrane's job is to separate the internal environment of the cell from the outside world, and

it does so with selective permeability. So, what actually makes it pass? Small non-polar molecules like nitrogen gas,

oxygen, and carbon dioxide slip right through, no problem. But larger polar molecules or ions, they need assistance.

And that's where embedded channels and transport proteins come in. Now, the plasma membrane isn't working alone.

Some cells have an extra layer of security. The cell wall found in bacteria, archa, fungi, some proise, and

plants. The cell wall provides a sturdy structural boundary. Importantly, it protects against osmotic lis. That

dangerous situation when too much water rushes in and the cell could literally burst. 2.5 Membrane transport. Selective

permeability is essential because it allows cells to create concentration gradients, differences in the amount of

a substance inside versus outside the cell. Without those gradients, cells couldn't power many of the processes

that keep life going. Now, let's talk about how molecules actually move across the selective barrier. The first method

is passive transport. Think of it like rolling a ball downhill. molecules naturally move from areas of high

concentration to areas of low concentration and the cell doesn't have to spend any energy. Diffusion,

facilitated diffusion, and osmosis all fall into this category. But sometimes cells need to move molecules against the

flow, like pushing a ball uphill. That's where active transport comes in. Active transport requires energy, usually in

the form of ATP, because it's moving substances from low concentration to high concentration, which doesn't happen

naturally. So what about molecules that are simply too big to squeeze through the membrane no matter what? That's when

cells rely on bulk transport, endoccytosis and exocytosis. In endoccytosis, the cell wraps its plasma

membrane around the material from the outside, creating little vesicles to pull it in. In exocytosis, it's the

opposite. Vesicles inside the cell fuse with the plasma membrane and release their contents out into the environment.

2.6 Facilitated diffusion. Facilitated diffusion is like a special entrance that helps certain molecules passively

get across the membrane. Think of it like a toll-free bridge. You're still going downhill, but you need a structure

to cross safely. For charged ions, the membrane is a total barrier without channel proteins. These ions rely on

proteins that act like tunnels carved straight through the lipid blayer. As they move, they can change the

electrical balance across the membrane. We call that polarization. If you've ever rubbed a balloon on your hair and

stuck it to the wall, you've seen static electricity in action. Cells use a similar trick with ions to create

charges across their membranes. Now, what about water? It may seem small, but moving lots of water at once isn't so

simple. That's why cells use aquaporns, protein channels dedicated to transporting huge amounts of water

quickly and efficiently. 2.7. Tenicity and osma regulation. When it comes to water movement in and out of cells, the

external environment makes all the difference. A solution can be hypotonic with less solute concentration,

hypertonic with greater solute concentration, or isotonic compared to the inside of the cell. The rule of

thumb here is simple. Water always moves from hypotonic to hypertonic regions. Or if you like to think of it in terms of

potential, water flows downhill from areas of high water potential to areas of low water potential. This process,

osmosis, is passive and doesn't require energy. But living organisms aren't just passive bystanders. They've got to

regulate water and solute balance to survive. That's where osmo regulation comes in. The body's way of making sure

cells don't shrivel up like raisins or burst like overfilled balloons. Osmo regulation helps maintain growth and

homeostasis by carefully controlling solute composition and water potential inside cells. The equations for water

potential and solute potential are on the equations and formula sheet provided on the exam. A higher solute

concentration means lower solute potential. Basically, the saltier it gets, the less water potential there is.

And calculated negative values for solute potential totally make sense because adding solutes makes water

potential drop. If you need more help with these calculations, you'll find it in the unit 2 practice sheet in the URP

and math practice sheet in the UEES. 2.8 Mechanisms of transport. When it comes to moving molecules and ions across the

cell membrane, sometimes simple diffusion isn't enough. And that's where active transport comes in. Without it,

larger charged molecules couldn't get where they need to go, and the whole system would stall. Membrane proteins

decide what gets transported, and they use energy from ATP to push molecules against their concentration gradient.

One of the most famous of these transport proteins is the sodium potassium pump. For every cycle, three

sodium ions are pushed out of the cell and two potassium ions are brought in. This uneven exchange creates and

maintains what we call a membrane potential, an electrical difference across the membrane. You can picture

this like charging a battery. The pump sets up an electrochemical gradient storing potential energy that the cell

can use later, just like your phone stores charge until you need it. In ATP, that's the currency funding the whole

operation. 2.9 Cell compartmentalization. If you tried to chop vegetables, boil pasta, and bake

cookies all on the same counter at the same time, things would get messy fast. But when you use separate cutting

boards, pots, and ovens, each task happens in its own space without interfering with the others. And that's

exactly what compartmentalization does in cells. It keeps reactions organized and efficient. Internal membranes also

solve another big problem, space. By folding and stretching out into complex shapes, they massively increase surface

area. More surface area means more places for enzymes to work and reactions to happen. Mitochondria, for example,

pack their inner membranes with folds of Christ, giving them the surface area needed to churn out lots of ATP. 2.10

origins of cell compartmentalization. When we look at the big differences between proaryotic and ukareotic cells,

one of the biggest gamech changers is the presence of membranebound organels. Ukareotic cells have these separate

compartments. Well, proarotes like bacteria generally do not. But here's the twist. Some of those ukareotic

organels actually started out as their own free-living proaryotic cells based on their own DNA, ribosomes, and double

membranes. The leading explanation is called the endo symbiotic theory. Imagine one cell swallowing another not

to digest it but to form a partnership. Over time, the engulfed procarot became a permanent resident evolving into

organels like mitochondria and chloroplast. Meanwhile, proarotic cells might not have these separate membrane

rooms, but they're not just a shapeless blob. They still have organized internal regions that carry out specialized

functions like the nucleoid where DNA hangs out or thyloin membranes and cyanobacteria which help capture light

energy. To recap, all living things are made of cells. Proarotic or ukareotic organels like the

nucleus, mitochondria, and chloroplast carry out essential cell functions. The endommembrane system coordinates protein

production and transport. The plasma membrane is selectively permeable thanks to phospholipids and proteins.

Diffusion, osmosis, and active transport move materials across membranes. Surface area to volume ratio affects how

efficient a cell can be. Compartmentalization lets ukarotic cells multitask efficiently. Mitochondria and

chloroplast have their own DNA, supporting the endo symbiotic theory. Overall, understanding cell structure

helps explain how life functions on every level. Need more support? Sign up for the ultimate review packet. The unit

2 free preview includes a study guide and practice multiple choice questions. And if you're ready to shift into full

exam mode, the ultimate exam slayer has practice tests, one pagers, and tips to help you dominate in May. All links are

in the description. Thanks for watching, and I'll see you next recap for unit 3, cellular energetics.