APBIO Modules 9-12 Study Guide

Module 9: Cell Membranes

9: Big ideas & overarching themes

• Cells must maintain boundaries and regulate internal conditions; the cell (plasma) membrane is the primary structural boundary for cells.

• The membrane is not just a static barrier but a dynamic, fluid structure that facilitates communication, transport, and homeostasis.

• Understanding membrane structure (lipids, proteins, carbohydrates) is essential to understanding how materials move, signals pass, and compartments function.

• Membrane properties (selective permeability, fluidity, asymmetry) underpin many of the cell’s interactions with its environment and internal organelles.

9: Key vocabulary

• Plasma membrane (cell membrane)

• Phospholipid bilayer – amphipathic molecules, hydrophilic head, hydrophobic tails.

• Fluid Mosaic Model – describes the membrane as fluid (lipids & proteins move laterally) and made of many parts.

• Integral (transmembrane) proteins & peripheral proteins - proteins that are permanently embedded within the lipid bilayer of a cell membrane

• Glycolipids/glycoproteins – carbohydrate attachments on the extracellular side for recognition, adhesion.

• Cholesterol – modulates fluidity in eukaryotic membranes.

• Selective (semi-) permeability - Some molecules can pass through

• Membrane asymmetry – different compositions of inner vs outer leaflet, orientation of proteins.

• Lipid rafts – microdomains enriched in cholesterol/sphingolipids that organize signaling.

• Membrane potential – voltage difference across a membrane due to ion distribution.

• Electrochemical gradient – combination of chemical (concentration) and electrical (charge) gradients across a membrane.

• Osmosis, diffusion, passive/active transport (though these are more in Modules 10/11)

9: Important concepts

1. Structure of the membrane

   • The phospholipid bilayer: hydrophilic heads face aqueous environments (cytosol/extracellular), hydrophobic tails form the interior barrier.

   • Proteins embedded in or bound to the membrane: functions include transport, signal transduction, cell-cell recognition, anchoring cytoskeleton/extracellular matrix.

   • Carbohydrate chains attached to lipids/proteins on the extracellular side contribute to cell recognition, adhesion, immune response.

   • Cholesterol (in animal cells) modulates membrane fluidity: at high temperature it stabilizes, at low temperature it prevents tight packing of tails → maintains fluidity.

2. Membrane fluidity & factors affecting it

   • Temperature: higher temps increase fluidity, lower temps decrease fluidity.

   • Lipid composition: unsaturated fatty acid tails (with cis‐double bonds) prevent tight packing → increased fluidity; saturated tails → less fluid.

   • Presence of cholesterol (in eukaryotes) moderates extremes.

   • Length of fatty acid tails: shorter tails → more fluid; longer tails → less fluid.

3. Selective permeability and what can cross

   • Hydrophobic (nonpolar) molecules (O₂, CO₂, N₂) cross easily.

   • Small uncharged polar molecules (H₂O, glycerol) cross somewhat, but slower.

   • Large polar molecules (glucose, amino acids) and charged ions (H⁺, Na⁺, K⁺, Cl⁻) do not cross easily without assistance.

   • The membrane thus acts as a barrier and a filter.

4. Electrochemical gradients and membrane potential

   • Differences in ion concentrations across the membrane (chemical gradient) combined with differences in charge (electrical gradient) generate the electrochemical gradient.

   • Membrane potential arises because the inside of many cells is more negative relative to their exterior (due to ion distribution).

   • The electrochemical gradient is a source of potential energy that can drive transport or signal transduction.

5. Membrane asymmetry & compartmentalization

   • In eukaryotic cells, membranes of organelles differ in composition (lipids/proteins) from the plasma membrane.

   • This asymmetry allows each compartment to specialize: e.g., mitochondrial inner membrane, lysosomal membrane.

   • Proper function depends on the correct orientation of proteins (extracellular vs cytosolic sides).

6. Membrane functions beyond barrier

   • Transport channels, carriers, pumps.

   • Signal transduction: receptors in the membrane detect external signals and initiate intracellular responses.

   • Cell adhesion and recognition: glycoproteins/glycolipids, integrins.

   • Maintaining homeostasis: controlling what gets in and out, preventing unwanted leakage.

9: Study/Exam Tips

• Be sure you can draw a phospholipid, label head vs tail, and explain why they self-assemble into a bilayer.

• Understand the relationship between structure and function: e.g., why unsaturated tails increase fluidity, how cholesterol stabilizes membranes.

• Practice comparing passive vs facilitated vs active transport (this leads into Module 10).

• Know the difference between the electrochemical gradient vs simple concentration gradient.

• Be ready to connect membrane structure & properties to bigger scale phenomena: how a cell senses its environment, why organelle membranes matter, how certain drugs work (by altering membrane fluidity or transport).

Module 10: Membrane Transport

10: Big ideas & overarching themes

• After establishing the membrane’s structure and nature in Module 9, Module 10 focusses on how substances move across membranes, how that movement is regulated, and the energy implications.

• Transport across membranes is critical for nutrient uptake, waste expulsion, maintenance of ion gradients (which power many cellular functions), and inter-cellular communication.

• The difference between passive (no energy required) and active (energy required) transport is foundational.

• Transport systems (proteins, channels, pumps) link back to membrane structure; thus Modules 9 and 10 must be integrated.

10: Key vocabulary

• Passive transport

  • Diffusion – movement of molecules from high to low concentration until equilibrium.

  • Facilitated diffusion – diffusion through a membrane protein (channel or carrier) down a gradient.

  • Osmosis – diffusion of water across a selectively permeable membrane.

• Active transport – movement of substances against their concentration or electrochemical gradient, requiring energy (often ATP).

• Channel proteins – provide hydrophilic passageways for ions or molecules.

• Carrier (transport) proteins – bind specific substances and change shape to shuttle them across.

• Pumps – a type of carrier protein driven by energy, e.g., Na⁺/K⁺-ATPase.

• Uniport, symport, antiport – transport modalities: one substance, two substances same direction, two substances opposite direction.

• Electrogenic pump – generates net charge movement across the membrane (thus contributing to membrane potential).

• Bulk transport – endocytosis & exocytosis (though strictly speaking may fall beyond just membrane transport modules).

• Selectivity / specificity – transport proteins are specific to particular molecules/ions.

• Saturation / transport maximum (Tₘₐₓ) – carriers can become saturated when all are occupied, limiting rate.

10: Important concepts

1. Passive transport details

   • Diffusion: driven by concentration gradients; stops at equilibrium (but note: molecules still move, just no net movement).

   • Facilitated diffusion: requires a protein; still energy-neutral (down gradient).

   • How channel characteristics (gating, specificity) affect rate, direction, and regulation.

2. Active transport details

   • Energy required → often in the form of ATP hydrolysis (direct) or indirect via coupling to other gradients.

   • Example: the Na⁺/K⁺-ATPase pump in animal cells: pumps 3 Na⁺ out, 2 K⁺ in per ATP → creates/maintains gradients.

   • Secondary active transport: uses energy stored in gradients (set up by primary pumps) to move other substances (e.g., glucose-Na⁺ symport).

3. Integration with membrane potential & electrochemical gradients

   • Transport not only depends on concentration gradients but also electrical gradients (especially for ions).

   • Example: moving a positively charged ion into a cell that is negative inside might be favourable chemically but unfavourable electrically (or vice versa).

   • Transport proteins may exploit combined gradients: e.g., H⁺ gradient driving ATP synthase or symporters.

4. Rate and regulation of transport

   • Factors affecting rate: number of transport proteins, gradient magnitude, permeability of membrane, gating/regulation of channels.

   • Saturation: for carriers, once all binding sites occupied, increasing substrate concentration doesn’t increase rate.

   • Inhibition/competition: e.g., molecules resembling substrate may compete.

   • Channel regulation: gating by voltage, ligand, mechanical stress, phosphorylation.

5. Bulk transport (if covered)

   • Endocytosis: phagocytosis, pinocytosis, receptor-mediated endocytosis.

   • Exocytosis: secretion of vesicles, membrane recycling.

   • Large scale transport allows transfer of big molecules, particles, or large fluid volumes across membranes despite selective permeability.

6. Connection to cell/organism function

   • Ion gradients maintained by pumps are used for nerve impulses, muscle contraction, nutrient uptake.

   • Transport defects cause disease (e.g., cystic fibrosis – defective Cl⁻ channel).

   • Transport regulation plays into homeostasis: e.g., kidney tubule reabsorption, water/ion balance in cells.

10: Study/Exam Tips

• Be sure you can compare and contrast passive vs active transport: differences in energy requirement, direction relative to gradient, type of protein, examples.

• Be able to sketch a transport scenario: e.g., a carrier moving glucose with Na⁺ (symport) – identify gradient driving force, where energy originates.

• Know a few key pumps/channels (e.g., Na⁺/K⁺-ATPase) and why they matter.

• Understand that transport is not isolated: membrane structure (from Module 9) influences transport; cell shape and organelle compartmentalization may affect transport distances and efficiency.

• Practice rate/limiting steps: what happens when gradient decreases, or carrier number changes.

• Relate transport to bigger scale: e.g., how kidney cells, neurons, or plants use transport mechanisms.

Module 11: Water Movement: Osmosis, Tonicity, and Osmoregulation

11: Big ideas & overarching themes

• While Module 10 covers transport in general, Module 11 zeroes in on water movement and how cells/organisms regulate water and solute balance.

• Water movement is critical because cells live in aqueous environments, osmotic imbalances can lead to cell swelling, shrinking, or bursting, and organisms must regulate their internal fluid environment.

• Tonicity (relationship between solute concentrations inside vs outside), osmosis, and osmoregulation (mechanisms organisms use to maintain water balance) are key.

11: Key vocabulary

• Osmosis – diffusion of water across a selectively permeable membrane from low solute concentration (high free water) to high solute concentration (low free water).

• Tonicity – relative concentration of solutes outside the cell compared to inside; how the extracellular solution will affect cell volume.

  • Isotonic – solute concentration outside equals inside; no net water movement.

  • Hypotonic – extracellular solute concentration lower than inside; water flows into the cell; cell may swell/lyse.

  • Hypertonic – extracellular solute higher than inside; water flows out; cell may shrink (crenate).

• Osmoregulation – the control of water balance and solute concentrations within an organism or cell.

• Aquaporins – channel proteins specialized for water transport.

• Plasmolysis – in plant cells, when the plasma membrane pulls away from the cell wall due to water loss in a hypertonic solution.

• Turgor pressure – in plant cells, the pressure of the cell contents against the cell wall when water intake keeps the cell firm.

• Osmotic pressure – the pressure required to stop the net movement of water via osmosis.

• Facultative/obligate osmoregulators, osmoconformers – organismal level strategies (if discussed).

• Bulk flow, cotransport (in water/ion regulation contexts).

11: Important concepts

1. Mechanism of osmosis

   • Water moves down its own gradient, which is often simply expressed as from high water potential / low solute concentration toward low water potential / high solute concentration.

   • The membrane is water-permeable (often via aquaporins) but less permeable to solutes; thus solute differences drive water movement.

   • The concept of water potential (Ψ) may be introduced: Ψ = Ψs (solute potential) + Ψp (pressure potential).

2. Tonicity and cell/organism consequences

   • In an isotonic environment: no net movement of water, cell volume stable.

   • In hypotonic: water enters cell → animal cell may lyse; plant cell becomes turgid (safe because of cell wall).

   • In hypertonic: water leaves cell → animal cell shrinks; plant cell plasmolyses (cell wall remains but membrane pulls away).

3. Osmoregulation strategies

   • At the cellular level: use of ion pumps/channels to regulate intracellular solute concentration; aquaporins to regulate water flow.

   • At the organismal level: in animals – kidneys, excretory systems; in plants – stomata, root uptake; in freshwater/saltwater organisms – osmoregulatory organs.

   • Example: marine fish drink seawater, excrete salts via gills/kidneys; freshwater fish gain water, lose salts, so excrete dilute urine and absorb salts.

4. Linking membrane transport to water movement

   • Many transport events involve solutes → changes in solute concentration alter water movement (osmosis).

   • Pumps/channels (from Module 10) indirectly adjust osmotic gradients.

   • Cells must integrate solute transport with water transport to maintain volume and function.

5. Applications & cell/organism survival

   • Understanding how red blood cells respond in different tonicity: hypotonic → hemolysis; hypertonic → crenation.

   • Plant cell turgor pressure is critical to structure/growth; wilting occurs when turgor lost.

   • Medical relevance: intravenous solutions must be isotonic to avoid damaging cells.

11: Study/Exam Tips

• Be able to predict what happens to a cell (animal or plant) placed in a hypotonic, isotonic or hypertonic solution.

• Know how aquaporins affect water movement — faster water movement compared to simple diffusion.

• Practice integrating transport of solutes (Module 10) with water movement (Module 11): e.g., pumping Na⁺ out reduces solute inside → water leaves → cell shrinks. Or vice versa.

• Understand organismal examples of osmoregulation (freshwater vs marine vs terrestrial).

• Be ready for questions asking to compare plant vs animal responses to tonicity (due to cell wall in plants).

Module 12: Origin of Compartmentalization and the Eukaryotic Cell

12: Big ideas & overarching themes

• Up to Module 11 we focused on membranes, transport, water movement. Module 12 shifts to how cells evolved compartments (organelles) and how eukaryotic cells arose — linking structure to evolution, function, and complexity.

• Compartmentalization increases efficiency of cellular processes: different environments, localized conditions, separation of incompatible reactions.

• Understanding the origin of eukaryotes (endosymbiotic theory), the structure and function of organelles, and how compartmentalization supports cell/organism complexity is crucial for AP Biology.

• It integrates cell biology with evolutionary biology: the origin of eukaryotes is an evolutionary story.

12: Key vocabulary

• Prokaryote – cell lacking a nucleus and membrane-bound organelles (e.g., Bacteria, Archaea)

• Eukaryote – cell with a true nucleus and membrane-bound organelles (e.g., plants, animals, fungi, protists)

• Endomembrane system – network of membranes in eukaryotic cells including the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles.

• Endosymbiotic theory – proposes that mitochondria and chloroplasts originated as symbiotic prokaryotes inside host cells.

• Organelles – specialized subunits within a cell that have specific functions (mitochondria, chloroplasts, nucleus, ER, Golgi, lysosomes, peroxisomes, etc).

• Compartmentalization – the division of the cell into different membrane‐bound compartments, which allows specialization.

• Cytoskeleton – network of protein fibers (microfilaments, microtubules, intermediate filaments) that provide structure, support, and transport routes within eukaryotic cells.

• Nuclear envelope – double membrane surrounding the nucleus, continuous with endoplasmic reticulum.

• Membrane trafficking – movement of substances between compartments (vesicle transport, exocytosis, endocytosis).

• Mitochondrial inner membrane, matrix, cristae – features relevant to energy transformation in eukaryotes.

• Chloroplast stroma, thylakoid membranes, granum (pl. grana) – relevant for photosynthetic eukaryotes.

• Organelle specialization – e.g., rough ER for protein synthesis, smooth ER for lipid synthesis/detoxification.

12: Important concepts

1. Why compartmentalize?

   • Differing internal conditions (pH, ion concentration, proteins) allow optimization of reactions.

   • Enables separation of reactive processes (e.g., digestion in lysosome vs synthesis in ER).

   • Increases surface area for membrane-based reactions (e.g., mitochondrial inner membrane).

   • Facilitates regulation, specialization, and higher complexity (cells can develop organelles with distinct functions).

2. Endomembrane system in eukaryotes

   • The nuclear envelope, ER, Golgi apparatus, lysosomes, vacuoles, vesicles are interconnected and exchange materials via vesicles.

   • Rough ER: has ribosomes, synthesises proteins; Smooth ER: lipid synthesis, detoxification.

   • Golgi: modifies, sorts, packages proteins/lipids for secretion or use.

   • Lysosomes (or vacuoles in plants) digest internalised material; peroxisomes detoxify and break down fatty acids.

   • Membrane trafficking: exocytosis (secretion), endocytosis (uptake of materials), phagocytosis, pinocytosis, receptor-mediated endocytosis.

3. Origin of eukaryotic cell & endosymbiosis

   • The endosymbiotic theory: e.g., mitochondria derived from aerobic prokaryote; chloroplasts from photosynthetic prokaryote. Evidence: their own DNA, double membranes, ribosomes similar to prokaryotes, replicate by binary fission.

   • Evolutionary advantage: host cell gets reliable energy source; symbiont gets protection and nutrients → mutualism.

   • Compartmentalization allowed eukaryotes to evolve large size, multicellularity, diverse organelles, internal specialization.

4. Cytoskeleton and cell architecture in eukaryotes

   • Microfilaments (actin), microtubules (tubulin), intermediate filaments: roles in cell shape, motility, intracellular transport.

   • Motor proteins (kinesin, dynein, myosin) walk along cytoskeletal fibers, carrying organelles/vesicles.

   • The cytoskeleton also interacts with the plasma membrane and extracellular matrix—important for cell movement, division, signalling.

5. Integration: membrane + transport + compartmentalization

   • Organelle membranes share the same chemistry as the plasma membrane but modified for specific function (e.g., mitochondrial inner membrane highly folded into cristae to increase surface area for ATP production).

   • Transport mechanisms originally discussed (Modules 9/10/11) are critical for moving molecules between compartments (e.g., nuclear pores, ER→Golgi vesicles, import/export of proteins into mitochondria).

   • Osmotic and fluid balances (Module 11) also matter inside organelles—cells must regulate internal pH, ion concentrations, etc., inside compartments.

6. Functional consequences and complexity

   • Eukaryotic cells can perform more complex functions (multicellularity, specialization, large size) because they can isolate processes, dedicate compartments, and manage internal logistics.

   • Organismal implications: complex multicellular life depends on eukaryotic cell structure. Malfunctions of organelles lead to diseases (mitochondrial diseases, lysosomal storage diseases).

12: Study/Exam Tips & Summary Table

• Be able to compare prokaryotic vs eukaryotic cells: cell size, presence/absence of nucleus/organelles, compartmentalization, cytoskeleton.

• Know the evidence for the endosymbiotic theory and the major organelles it pertains to (mitochondria, chloroplasts).

• Understand how compartmentalization enhances efficiency: pick an example (e.g., mitochondria’s inner vs outer membrane; plant vacuole vs cytosol).

• Be prepared to integrate Modules: e.g., how the plasma membrane’s structure (Module 9) enables nutrient uptake, which then is handed off to organelles (Module 12) for processing.

• Practice drawing a eukaryotic cell, labeling major organelles, and briefly stating their functions, especially in relation to membranes and transport.

• Pay attention to how water/ion regulation (Module 11) works not just at the plasma membrane but also inside cells and between compartments.