AP Biology - Big Ideas from Unit #2: Cell Structure & Function

Flashcard Set for AP Biology Unit #2, aligned to the 2020 CED (May 2025 exam) and compatible with the 2025 CED (May 2026+). 🎯 Built for review, this set covers key content, fixes common misconceptions, and includes scenario-based questions tied to AP Science Practices (1.C, 2.A, 4.B, 6.E). ✅ Covers organelles, membranes, transport, osmoregulation, and endosymbiosis (Topics 2.1–2.11). Great for daily prep toward a 4 or 5.

Nucleus

Houses genetic code of the cell, site of transcription

Ribosomes

Site of protein synthesis by translation. Can be located on Rough ER or in the cytosol. Composed of rRNA and protein.

T/F: Ribosomes are found in all forms of life.

TRUE! Ribosomes are found in all forms of life, reflecting the common ancestry of all known life.

Smooth ER

Lipid synthesis (e.g., phospholipids, steroids like hormones),

Detoxification of drugs and poisons (especially in liver cells),

Calcium ion storage, especially in muscle cells for signaling.

TL;DR: Lipid Synthesis, Detoxification, Calcium Ion Storage

EVALUATE: Smooth ER in Liver Cells

Consider... Justify why Smooth ER is more abundant in liver cells and explain how its structure supports this function.

🔍 Justify why Smooth ER is more abundant in liver cells and explain how its structure supports this function.

❌ Liver cells specialize in detoxifying drugs and toxins. To support this, they contain an abundance of Smooth ER, which provides increased membrane surface area for enzymes involved in detoxification. This structural adaptation enhances the cell’s ability to process harmful substances and maintain homeostasis.

📌 AP Insight: This is a classic example of the structure-function relationship tested in FRQs and MCQs—identify the functional need, explain the structural solution.

🧠 Science Practice: 6.E – Reasoning & Evidence

Rough ER

Compartmentalizes cell and is associated with membrane-bound ribosomes.

Golgi Body/Apparatus/Complex

Correct folding and chemical modification of newly synthesized proteins and packaging for protein trafficking

TL;DR: Folding, Chemical Modification, Packaging of Newly Synthesized Proteins

Mitochondria

Site of ATP synthesis by Oxidation Phosphorylation.

Mitochondria have a double membrane. The outer membrane is smooth, but the inner membrane is highly convoluted, forming folds. Mitochondrial double membrane provides compartments for different metabolic reactions. Folding of the inner membrane allows for increase in surface area, allowing for more ATP synthesis.

Location of Krebs Cycle (Mitochondrial Matrix), Oxidative Phosphorylation (Mitochondrial Cristae).

Lysosomes

Digestion of Macromolecules Initiates Apoptosis (Programmed Cell Death). Contains Hydrolytic Enzymes important in intracellular digestion, the recycling of a cell's organic materials, and programmed cell death (apoptosis).

Vacuoles

It stores and releases macromolecules and cellular waste products. In plants, it aids in the retention of water for turgor pressure.

Vesicles

Small membrane sacs that specialize in moving products into, out of, and within a cell

Chloroplasts

Transform Light Energy into ATP. Chloroplasts are specialized organelles found in photosynthetic algae and plants. They have a double outer membrane, which contains chlorophyll pigments and electron transport proteins that comprise the photosystems.

Where does the ETC and ATP Synthesis occur in Cellular Respiration?

Electron transport and ATP synthesis occur on the inner mitochondrial membrane.

Where does the Carbon Fixation (Calvin cycle) reactuons of photosynthesis occur?

The carbon fixation (Calvin-Benson cycle) reactions of photosynthesis occur in the STROMA.

Stroma

Stroma is the fluid within the inner chloroplast membrane and outside of the thylakoid.

Where do the Light-Dependent Reactions of Photosynthesis occur?

The light-dependent reactions of photosynthesis occur in the grana.

Grana - What is it?

The thylakoids are organized in stacks, called grana.

Cytoskeleton

A network of protein fibers in the cytoplasm that provides structural support, maintains cell shape, and aids in movement of the cell and its components.

Key Roles:

Cell shape and support

Intracellular transport

Cell division (spindle fibers)

Main Components:

a) Microtubules - largest, made of tubulin; help with chromosome movement, cilia/flagella, and organelle transport

b) Microfilaments (Actin) - thinnest; involved in cell movement, muscle contraction, and cytokinesis

c) Intermediate Filaments - medium-sized; provide mechanical strength and anchor organelles

3 similarites between prokaryotes and eukaryotes

1) They both have Ribosomes.

2) They both have Plasma Membranes

3) They both store genetic information in the form of DNA

3 differences between prokaryotes and eukaryotes

1) Prokaryotes do not have membrane-bound organelles, unlike Eukaryotes.

2) Eukaryotes generally have a bigger size than prokaryotes due to compartmentalization.

3) Transcription and Translation occur simultaneously in the cytosol of Prokaryotes, while in Eukaryotes, Transcription occurs in the Nucleus prior to Translation, which occurs in Ribosomes.

How's a prokaryotic cell compartmentalized?

Prokaryotes generally lack internal membrane-bound organelles but have internal regions with specialized structures and functions.

How's a eukaryotic cell compartmentalized?

Compartmentalized by membrane-bound organelles. Membrane-bound organelles allow for simultaneous biological processes and maintain different environments inside the membrane vs. in the cytoplasm.

Ex. Acidic environment in Lysosome due to Hydrolytic Enzymes.

From CB: Membranes and membrane-bound organelles in eukaryotic cells compartmentalize intracellular metabolic processes and specific enzymatic reactions.

Endosymbiotic Theory

The Mitochondria and Chloroplast were once their own prokaryotes but were engulfed by endocytosis into the cell. Mitochondria and Chloroplast were preserved due to the symbiotic relationship between them and the cell as they produced ATP (energy) for the cell.

Evidence for Endosymbiotic Theory

1) Both have their own Ribosomes

2) Both have circular DNA

3) Both reproduce via Binary Fission

Describe the relationship between the functions of endosymbiotic organelles and their free-living ancestral counterparts.

Membrane-bound organelles evolved from previously free-living prokaryotic cells via endosymbiosis.

Cell Size and the Significance of Being Small

Smaller cells are more efficient (in comparison to larger cells) as they have a higher SA/V ratio as they allow for more access to material exchange.

Describe the relatonship between SA & V

As a cell's volume increases, its surface area increases, but not as quickly. This results in a decrease in the surface area-to-volume (SA/V) ratio.

➤ A lower SA/V ratio means:

a) Less efficient exchange of materials with the environment

b) Increased demand for internal resources, like nutrients and energy

c) Slower removal of waste products

This is why smaller cells are more efficient — they have a higher SA/V ratio, which supports faster diffusion and nutrient/waste exchange.

Describe the relatonship between SA & Efficiency

⬆️ SA/V = ⬆️ Efficiency

As the Surface Area to Volume ratio increases, the level of efficiency increases. DIRECT RELATIONSHIP.

Describe the relationship between Cell Size and SA/V

⬆️ Cell Size = ⬇️ SA/V

INVERSE RELATIONSHIP.

This affects properties like the rate of heat exchange with the environment.

Explain the effect of surface area-to-volume ratios on the exchange of materials between cells or organisms and the environment

Surface area-to-volume ratios affect the ability of a biological system to obtain necessary resources, eliminate waste products, acquire or dissipate thermal energy, and otherwise exchange chemicals and energy with the environment.

Explain how specialized structures and strategies are used for the efficient exchange of molecules to the environment.

Organisms have evolved highly efficient strategies to obtain nutrients and eliminate waste. Cells and organisms use specialized exchange surfaces to obtain and release molecules from or into the surrounding environment.

Describe an adaptation plants have to increase their SA/V ratio

--Membrane Folding (like in the Mitochondria - present in all eukaryotes) increases SA/V.

--Root Hairs on the surface of plant roots increase the SA/V of the root, allowing for increased absorption of water and nutrients.

Describe an adaptation that the small intenstine has to increase its SA/V ratio

Villi!

Function of Cell Membrane

Boundary/Barrier between cell and external environment. Regulates what enters and leaves the cell.

Components of the Cell Membrane

Phospholipid bilayer, proteins, glycoproteins, glycolipids, and cholesterol (cholesterol only in eukaryotes!)

Function of Phospholipids

Compose most of the plasma membrane. Regulates what can and can't get into cell.

Function of Proteins on Membrane

Variety of functions including...

- Selective Transport Proteins (Channel & Carrier Proteins)

- Cell Identification

- Receptor Protein (Signal Transduction!)*

- Enzyme

- Cell Adhesion

- Attachment to Cytoskeleton (Aiding in maintaining cell structure)

Function of Selective Transport Proteins

Allow large, polar, charged molecules to enter membrane and cell.

Channel Protein

A membrane protein, specifically a transport protein, that has a hydrophilic channel that certain molecules or atomic ions use as a tunnel.

Carrier Protein

A membrane protein, specifically a transport protein, that holds onto molecules and changes their shapes in a way that shuttles them across the membrane.

Function of Enzyme Protein

Speed up biochemical reactions by lowering amount of activation energy required

Function of Cell Surface Receptor Protein

Receive Cell Signaling molecule (Ligand) and initiates Signal Transduction

*Connects to Unit #4 (4.2-4.3)

Function of Cell Surface Identity Marker Protein

Aids in identification of cell

Function of Cell Adhesion protein

Helps cells "stick" to each other

Function of Attachment to Cytoskeleton protein

Binds to cytoskeleton, reinforcing cell structure

T/F: Transport Proteins are specific to what they transport.

True! Transport proteins are specific to what they transport, like enzymes, which are specific to their substrate*.

Ex. Aquaporins are the transport proteins for water in large quantities. (From CED: SMALL amounts of water can passively diffuse through the cell membrane without the help of a protein).

*Connect to Unit #3

Fluid Mosaic Model

The currently accepted model of cell membrane structure envisions the cell membrane as a mosaic of individually inserted protein molecules drifting laterally in a fluid bilayer of phospholipids.

From the CED: "Cell membranes consist of a structural framework of phospholipid molecules that is embedded with proteins, steroids (such as cholesterol in eukaryotes), glycoproteins, and glycolipids that can flow around the surface of the cell within the membrane."

Function of Glycoproteins & Glycolipids on Membrane

Glycoproteins and Glycolipids aid in Cell Recognition and Communication.

Function of Cholesterol on Membrane

Regulation of Membrane Fluidity during changes in temperature

Structure of Phospholipid

The hydrophilic head (facing the exterior membrane) and hydrophobic tails face each other, avoiding the exterior.

Phospholipids are amphipathic.

Amphipathic

A molecule that has both a hydrophilic region and a hydrophobic region.

T/F: Selective permeability is a direct consequence, result of membrane structure.

True! The structure of cell membranes results in selective permeability.

Membrane Permeability

Degree to which a membrane allows a substance to pass through it

What type of molecules can diffuse through the membrane without any kind of help?

Small nonpolar molecules, very small polar uncharged molecules (small amounts of water)

General Rule of Selective Membrane Permeability

Nonpolar, small molecules = pass freely (diffusion)

Polar or charged molecules = need facilitated diffusion or active transport

Function of Cell Wall

Structural boundary and permeable barrier

How do molecules cross the cell wall?

Plasmodesmata*

Plasmodesmata are small holes between plant cells allowing transfer of nutrients, waste, and ions.

*Connects to Unit #4.

How are plant, fungi, and prokaryotic cell walls different?

Composed of different complex carbohydrates.

Plants - Cellulose (Polysaccharide)

Fungi - Chitin (Polysaccharide)

Prokaryotes - Peptidoglycan (Polymer consisting of Sugar and Amino Acids)

Forms of Transport

Diffusion, Facilitated Diffusion, Osmosis, Active Transport (ex. Endocytosis, Exocytosis)

It can be simply broken down into Passive Transport and Active Transport.

Passive Transport

Transport across the cell membrane that follows the concentration gradient (high --> low). NO ATP NEEDED!

(ex. Diffusion, Facilitated Diffusion)

Passive transport plays a primary role in the import of materials and the export of wastes.

Active Transport

Transport across the cell membrane that goes AGAINST the concentration gradient (low --> high). ATP REQUIRED! Needs Transport Protein.

(ex. Endocytosis, Exocytosis)

Diffusion

Movement of molecules from an area of higher concentration to an area of lower concentration. Passive Transport.

Small nonpolar molecules cross the cell membrane by diffusion.

Facilitated Diffusion

Movement of specific molecules across cell membranes following concentration gradient using transport proteins. Passive Transport.

THINK: Diffusion but with Transport Proteins involved.

Ex. of Facilitated Diffusion = Aquaporins for large quantities of water!

From CED:

Membrane proteins are required for facilitated diffusion of charged and large polar molecules through a membrane—

a) Large quantities of water pass through aquaporins.

b) Charged ions, including Na+ and K+, require channel proteins to move through the membrane.

c) Membranes may become polarized by movement of ions across membrane.

Osmosis

Diffusion of water through a selectively permeable membrane. No proteins involved. Passive Transport.

Endocytosis

To bring into cell using vesicles. In endocytosis, the cell takes in macromolecules and particulate matter by forming new vesicles derived from the plasma membrane. Active Transport.

Think: ENdocytosis = ENter

Forms of Endocytosis

pinocytosis, phagocytosis, receptor-mediated endocytosis

Phagocytosis

A type of endocytosis in which a cell engulfs large particles or whole cells. Active Transport (Endocytosis).

THINK: Cell Eating

Pinocytosis

A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes. Active Transport (Endocytosis).

THINK: Cell Drinking

Memory Trick: PI (sounds like PEE) - PEE is caused by DRINKING; THUS, PEENOCYTOSIS (actually spelled: PINOCYTOSIS) is CELL DRINKING!

Receptor-Mediated Endocytosis

The movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taken in; enables a cell to acquire bulk quantities of specific substances. Active Transport (Endocytosis).

THINK: Ligand-triggered Endocytosis*

*Connects to Unit #4

How are the food materials brought in by endocytosis digested?

Vesicles take food materials brought in via endocytosis to the LYSOSOME.

Recall the function of Lysosome is to digest food, macromolecules.

THINK: Food Brought In --> Lysosome

Exocytosis

To exit/kick out of the cell using vesicles. In exocytosis, internal vesicles fuse with the plasma membrane and secrete large macromolecules out of the cell. Active Transport.

THINK: EXocytosis = EXit

Examples of material that would require endocytosis

Secretory proteins, hormones, waste products

Membrane Potential

The difference in electrical charge across a cell's membrane, with the inside usually more negative than the outside. Created by the uneven distribution of ions like Na⁺ and K⁺.

How does the Na⁺/K⁺ ATPase contribute to membrane potential?

The Na⁺/K⁺ ATPase uses ATP to pump 3 Na⁺ out of the cell and 2 K⁺ in, creating a net negative charge inside. This establishes the membrane potential by maintaining an electrochemical gradient and is critical for nerve signaling, muscle contraction, and secondary transport.

From the CED: The Na+/K+ ATPase contributes to the maintenance of the membrane potential

Role of ATP in membrane potential

ATP powers the Na⁺/K⁺ pump, which actively transports ions to maintain the membrane potential.

Indirect Use of Ion Gradients in Active Transport

Sometimes, a gradient of ions (like H⁺ or Na⁺) created using ATP is used to help move other molecules into or out of a cell.Although ATP isn't used directly in the second step, it was needed to build the ion gradient. This allows substances like glucose or sucrose to move against their concentration gradient using a membrane protein.

TIP: You might see this on diagrams with two molecules moving together through a protein — if one is moving with its gradient and the other against, it's this concept.

Flow of Water in terms of Concentration

Water flows from areas of HIGH concentration to areas of LOW concentration.

THINK: HIGH --> LOW

Flow of Water in terms of Water Potential

Water flows from areas of HIGH water potential to areas of LOW water potential.

THINK: HIGH --> LOW

Flow of Water in terms of Osmolarity

Water flows from areas of LOW OSMOLARITY to areas of HIGH OSMOLARITY.

THINK (in terms of OSMOLARITY): LOW --> HIGH

Explain how osmoregulatory mechanisms contribute to the health and survival of organisms.

Growth and homeostasis are maintained by the constant movement of molecules across membranes.

Examples of Osmoregulatory Mechanisms: Contractile vacuole in protists, Central vacuoles in plant cells

Hypertonic

High Solute Concentration, Low Water Concentration

THINK: HYper = HIGH SOLUTE, LOW WATER!

Hypotonic

LOW Solute Concentration, HIGH Water Concentration

THINK: hyPOtonic = LOW (PO rhymes with LOW!) SOLUTE, HIGH WATER!

Isotonic

Equal concentration of both water and solute, water flows in and out, net movement of 0.

Describe the way water flows using the relative terms (Hypotonic, Hypertonic, Isotonic)

Knowing water always flows from areas of high concentration to areas of low concentration,

Water will always flow from a HYPOTONIC (High Water Concentration) to a HYPERTONIC (Low Water Concentration) environment/cell.

THINK: Hypotonic (HIGH water) --> Hypertonic (LOW water)

Water Potential

The physical property predicting the direction in which water will flow, governed by solute concentration and applied pressure.

THINK: Potential Energy of Water! Potential of Water to do Work!

Formula for Water Potential

Ψ= Ψp + Ψs

Water Potential = Pressure Potential + Solute Potential

Pressure Potential

A component of water potential consists of the physical pressure on a solution, which can be positive, zero, or negative.

In most biological open systems, there is equilibrium, so the pressure potential is 0 (ZERO!). In that case, the water potential is really just the solute potential.

Solute Potential

Ψs = -iCRT

Solute Potential = -(Ionization Constant)(Molar Concentration)(Pressure Constant)(Temperature in KELVIN).

You do not need to memorize the pressure constant, it's given to you on the formula sheet.

Ionization Constant

If it's a covalent molecule, it will be 1 as only one ion is generated when thrown into a solution. (ex. Glucose).

If it's a molecule like NaCl, it will be 2 as two ions will be generated when thrown into a solution.

Convert from Celsius to Kelvin

Tκ = °C + 273

Kelvin = Celsius + 273

Skill/Scenario Practice: A mutation prevents the folding of the inner mitochondrial membrane. What effect would this have on the cell?

It would reduce the surface area for oxidative phosphorylation, decreasing ATP production.

Science Practice: 6.E - Argumentation & Prediction

Why It Matters: Structure-function relationships appear in FRQs involving respiration efficiency and compartmentalization.

Skill/Scenario Practice: What happens to a eukaryotic cell's efficiency if its surface area-to-volume ratio decreases?

Efficiency decreases due to slower diffusion, reduced resource acquisition, and impaired waste elimination.

Science Practice: 6.E - Argumentation & Prediction

Why It Matters: Cell size and SA/V ratio appear in reasoning-based MCQs and experimental setups.

Skill/Scenario Practice: A plant cell lacks a central vacuole. Predict one effect on its ability to survive in dry environments.

It may not maintain turgor pressure effectively, leading to wilting or loss of structural support.

Science Practice: 6.E, 1.C - Prediction & Concept Explanation

Why It Matters: Ties osmoregulation to structure-function—a theme tested in application-based MCQs and FRQs.

Skill/Scenario Practice: A diagram shows a plasma membrane with a transport protein and a concentration gradient. What process is shown?

Facilitated diffusion of a polar or charged molecule moving with its gradient.

Science Practice: 2.A - Visual Representation

Why It Matters: Common in membrane transport visuals—you must ID passive vs. active processes accurately.

Skill/Scenario Practice: You see a mitochondrion diagram with folded inner membranes. What's the functional advantage of the folds?

Folds increase surface area for ATP-producing reactions, enhancing energy output.

Science Practice: 2.A - Visual Representation

Why It Matters: Organelle structure → function visuals are a frequent MCQ/FRQ item.

Skill/Scenario Practice: A chloroplast diagram shows the stroma and thylakoid. Where do the light-dependent reactions occur?

In the thylakoid membranes, which are stacked in the grana.*

Science Practice: 2.A - Visual Representation

Why It Matters: Expect to match photosynthesis stages to labeled parts of the chloroplast.

*This concept connects with Unit #3, when we're talking about Photosynthesis - 3.5 (2020 CED).

Skill/Scenario Practice: A water potential diagram shows Ψroot < Ψsoil. Which way will water flow?

Water moves from high Ψ (soil) to low Ψ (root) as water moves from areas of high water potential to areas of low water potential (high—> low).

Science Practice: 4.B - Data Interpretation

Why It Matters: Water movement logic is tested with diagrams and graphs regularly.

Skill/Scenario Practice: Experimental data show high [Na⁺] outside and high [K⁺] inside a cell. What maintains this distribution?

The Na⁺/K⁺ ATPase pump using active transport and ATP.

Science Practice: 4.B - Data Interpretation

Why It Matters: These data tables show up often in FRQs—must link to the correct transport process.

Skill/Scenario Practice: T/F - Facilitated diffusion requires ATP.

FALSE! It's a passive process relying on transport proteins for charged/polar molecules.

Science Practice: 1.A - Concept Identification

Why It Matters: Easy-to-miss trick question in MCQs—ATP is not used unless it's active transport.

Skill/Scenario Practice: T/F - All membrane proteins are used for transport.

FALSE! Some proteins act as receptors, enzymes, anchors, or identity markers.

Science Practice: 1.A - Concept Identification

Why It Matters: Fluid mosaic model includes various protein roles. Don't confuse function types.