AP Biology Unit 2: Cellular Organelles and Membrane Transport

Endomembrane System and Cellular Organelles

• Nucleus:

  • Structure: Surrounded by a double membrane system known as the nuclear envelope, which contains specialized nuclear pores to regulate molecular movement.

  • Functions:

  • Serves as the primary repository for storing genetic information ($DNA$).

  • Directs the synthesis of $RNA$ through transcription.

  • Facilitates ribosome subunit assembly within the nucleolus.

• Rough Endoplasmic Reticulum (Rough ER):

  • Structure: Extensive membrane network studded with bound ribosomes directly attached to the outer membrane of the nuclear envelope.

  • Functions:

  • Serves as the site for synthesizing membrane-bound proteins and proteins destined for secretion outside the cell.

  • Contributes to intracellular compartmentalization by separating specialized enzymatic reactions from the cytosol.

  • Offers mechanical structural support to the cell.

  • Facilitates intracellular transport of synthesized molecules.

• Smooth Endoplasmic Reticulum (Smooth ER):

  • Structure: Interconnected system of folded, tube-like membranous structures termed cisternae, devoid of ribosomes.

  • Functions:

  • Functions in cellular detoxification of harmful metabolic byproducts, drugs, and toxins.

  • Acts as a storage site for calcium ions ($Ca^{2+}$).

  • Synthesizes lipids, including phospholipids and steroid hormones.

• Golgi Complex / Apparatus:

  • Structure: Membrane-bound cytoplasmic structure composed of a series of flattened, non-continuous sacs termed cisternae.

  • Functions:

  • Mediates the correct folding and chemical modification of newly synthesized proteins received from the Rough ER.

  • Sorts, packages, and tags protein traffic into membrane vesicles for targeted intracellular delivery or secretion.

• Ribosomes:

  • Structure: Non-membrane-bound molecular complexes formed from ribosomal $RNA$ ($rRNA$) and functional proteins, arranged into distinct large and small subunits.

  • Subtypes: Found either freely suspended in the cytoplasm (free ribosomes) or bound to the membranes of the Rough ER (bound ribosomes).

  • Functions: Serves as the universal site of translation (protein synthesis) in all cellular life.

Energy Transducing Organelles

• Mitochondria:

  • Structure: Double-membrane bounded organelle consisting of a smooth outer membrane and an extensively folded inner membrane. The internal folds are termed cristae, which enclose a fluid-filled internal space known as the matrix.

  • Functions:

  • Serves as the primary site of oxidative phosphorylation during cellular respiration, situated along the folded cristae and inner membrane to maximize functional surface area.

  • Functions as the site for the Krebs Cycle (Citric Acid Cycle) within the fluid matrix.

• Chloroplast:

  • Structure: Bounded by a double outer membrane. Internally features flattened, membranous sacs called thylakoids, stacked into structures called grana, all surrounded by an aqueous fluid termed the stroma.

  • Functions:

  • Functions as the site of photosynthesis in green plants and photosynthetic algae.

  • The thylakoid membranes contain light-absorbing pigments and host the light-dependent reactions.

  • The fluid stroma houses metabolic enzymes to carry out the light-independent reactions (Calvin-Benson Cycle).

Specialized Organelles for Digestion and Storage

• Lysosome:

  • Structure: Spherical, membrane-enclosed sacs packed with acidic hydrolytic enzymes.

  • Functions:

  • Executes intracellular digestion by breaking down engulfed food particles, foreign pathogens, and damaged cellular organelles (autophagy).

  • Plays an essential role in programmed cell death (apoptosis) via controlled enzymatic breakdown.

• Vacuole:

  • Structure: Variable-sized, single-membrane-bound fluid storage sacs.

  • Functions:

  • General role in the temporary storage and controlled release of macromolecules, essential nutrients, and cellular waste products.

  • Central Vacuole: Found in plant cells; plays a key role in water retention, exerting internal hydrostatic turgor pressure against the cell wall to maintain rigidity.

  • Contractile Vacuole: Present in freshwater protists (such as Paramecium); specialized for active osmoregulation by pumping excess cytosolic water out of the cell.

  • Food Vacuole: Formed during phagocytosis to hold engulfed nutritive material prior to fusion with a lysosome.

Cell Size and Surface Area-to-Volume Relationships

• Functional Significance of Cell Size:

  • Smaller cells typically maintain a significantly higher surface area-to-volume ratio ($SA/V$).

  • A higher surface area-to-volume ratio optimizes the efficiency of exchanging metabolic nutrients, dissolved gases, and waste products across the plasma membrane with the surrounding extracellular environment.

• Geometric Calculations for Surface Area and Volume:

  • Sphere:

  • Volume: V = \n\frac{4}{3}\n\pi r^3

  • Surface Area: $S = 4\pi r^2$

  • Cube:

  • Volume: $V = s^3$

  • Surface Area: $S = 6s^2$

  • Rectangular Prism:

  • Volume: $V = LWH$

  • Surface Area: $S = 2LH + 2LW + 2WH$

  • Cylinder:

  • Volume: $V = \pi r^2 h$ or $V = Bh$

  • Surface Area: $S = 2\pi r^2 + 2\pi rh$

Membrane Structure and Dynamics

• Phospholipid Bilayer: Formed by amphipathic phospholipids with hydrophilic phosphate heads facing aqueous environments and hydrophobic fatty acid tails pointing inward.

• Membrane Proteins:

  • Intrinsic (Integral) Proteins: Penetrate into or span across the hydrophobic core of the lipid bilayer (e.g., transmembrane transport proteins and channels).

  • Extrinsic (Peripheral) Proteins: Loosely bound to the outer or inner surface of the plasma membrane.

• Carbohydrate Components:

  • Glycolipids: Lipids covalently linked to carbohydrates, functioning in cell recognition.

  • Glycoproteins: Membrane proteins covalently bonded to carbohydrate chains, serving as surface receptors and cellular identification markers.

• Cholesterol: Steroid molecule embedded within the hydrophobic membrane core; buffers membrane fluidity across varying temperature ranges.

Mechanisms of Membrane Transport

• Simple Diffusion:

  • Energy Demand: Passive transport process requiring no chemical energy input ($ATP$).

  • Direction: Net movement proceeds down the concentration gradient (from high concentration to low concentration).

  • Permeability: Allows passage of small, nonpolar hydrophobic molecules directly across the lipid bilayer without transport proteins.

  • Examples: Carbon dioxide ($CO_2$), oxygen ($O_2$), nitrogen ($N_2$), and nonpolar steroid hormones. Small amounts of water ($H_2O$) can slowly leak through unassisted.

• Facilitated Diffusion:

  • Energy Demand: Passive transport process requiring no chemical energy input ($ATP$).

  • Direction: Net movement proceeds down the concentration gradient (from high concentration to low concentration).

  • Permeability: Transports small polar, hydrophilic, or charged substances across the membrane.

  • Protein Requirement: Requires specific transmembrane transport proteins, categorized into:

  • Channel Proteins: Hydrophilic corridors allowing rapid passage of ions or water (e.g., aquaporins).

  • Carrier Proteins: Proteins that undergo conformational changes to transfer specific solutes across the bilayer.

  • Examples: Water ($H_2O$), sodium ions ($Na^+$), potassium ions ($K^+$), and calcium ions ($Ca^{2+}$).

• Active Transport:

  • Energy Demand: Requires direct metabolic energy input, typically derived from $ATP$ hydrolysis.

  • Direction: Pumps molecules against their established concentration gradient (from low concentration to high concentration).

  • Protein Requirement: Demands specialized transmembrane carrier proteins or solute pumps.

  • Examples: Sodium ions ($Na^+$), potassium ions ($K^+$), calcium ions ($Ca^{2+}$), and hydrogen ions / protons ($H^+$).

• Endocytosis (Bulk Import):

  • Active transport mechanism that imports large extracellular materials into the cytoplasm via membrane-derived vesicles.

  • Phagocytosis: "Cellular eating"—engulfment of large particles, cellular debris, or intact microorganisms into large food vacuoles.

  • Pinocytosis: "Cellular drinking"—nonspecific uptake of extracellular fluid drops containing solute molecules via small invaginations.

  • Receptor-Mediated Endocytosis: Bulk uptake triggered by specific ligands binding to cell-surface protein receptors localized in coated pits, forming coated vesicles.

• Exocytosis (Bulk Export):

  • Active secretion mechanism where intracellular vesicles fuse with the plasma membrane to release contents outside the cell.

  • Secretion Pathway: Polypeptides synthesized on ribosomes of the Rough ER \n\rightarrow\n packaged into transport vesicles \n\rightarrow\n processed and modified in the Golgi complex \n\rightarrow\n packaged into secretory vesicles \n\rightarrow\n fused with the plasma membrane.

Osmosis, Water Potential, and Solution Tonicity

• Osmosis Fundamentals:

  • Osmosis is the passive diffusion of free water molecules across a selectively permeable membrane.

  • Water moves passively from regions of high water potential (low solute concentration / high free water concentration) to regions of low water potential (high solute concentration / low free water concentration).

• Tonicity Classifications:

  • Hypertonic Solution:

  • Contains a higher solute concentration and a lower free water concentration relative to an adjacent solution or cytoplasm.

  • Net water movement proceeds out of a cell placed in a hypertonic environment toward the solution.

  • Hypotonic Solution:

  • Contains a lower solute concentration and a higher free water concentration relative to an adjacent solution or cytoplasm.

  • Net water movement proceeds into a cell placed in a hypotonic environment.

  • Isotonic Solution:

  • Contains an equal solute concentration and equal free water concentration relative to another solution.

  • Results in dynamic equilibrium with zero net movement of water into or out of the cell.

• Tonicity Effects on Plant Cells:

  • In a hypertonic solution: Water leaves the vacuole and cytoplasm by osmosis; the protoplast contracts away from the rigid cell wall, causing plasmolysis.

  • In an isotonic solution: Water inflow and outflow are balanced; the cell becomes flaccid.

  • In a hypotonic solution: Water enters the central vacuole by osmosis; internal hydrostatic pressure pushes against the cell wall, causing the cell to become turgid.

• Tonicity Effects on Animal Cells:

  • In a hypertonic solution: Water exits the cell, leading to cellular shriveling (crenation).

  • In an isotonic solution: Equivalent water movement maintains stable cell volume.

  • In a hypotonic solution: Excessive water entry causes cell swelling and ultimate membrane rupture (lysis).

Practice Questions & Analytical Applications

• Multiple Choice Practice 1: Dialysis Bag Investigation:

  • Scenario: A solution containing starch and glucose is placed inside a dialysis tubing bag, which is submerged in a beaker filled with pure water to observe the effect of molecular size on membrane permeability.

  • Experimental Principles:

  • Dialysis tubing acts as a selectively permeable membrane with specific pore size limits.

  • Starch is a large complex polysaccharide and cannot pass through the pores, remaining trapped inside the bag.

  • Glucose is a small monosaccharide and diffuses down its concentration gradient out of the bag into the beaker.

  • Water moves into the dialysis bag by osmosis down its water potential gradient, increasing internal fluid volume.

  • Graphical Trend:

  • Starch: Remains constant at its initial relative level inside the dialysis bag over time.

  • Glucose: Exhibits a steady decline over time as it diffuses out into the beaker solution.

  • Water: Displays a gradual upward curve as water enters the dialysis bag via osmosis.

• Multiple Choice Practice 2: Paramecium Osmoregulation:

  • Scenario: Unicellular Paramecia are placed into salt solutions of progressively increasing osmolarity ($\text{mM}$), and the contraction rate of their osmoregulatory contractile vacuole is recorded.

  • Observed Trend: As external solution osmolarity increases, the rate of contractile vacuole contraction linearly decreases.

  • Explanation: In environments with lower osmolarity (hypotonic surroundings), the steep osmotic gradient causes water to rapidly enter the paramecium cytoplasm. The contractile vacuole must pump frequently to expel excess water and prevent cell lysis. As external solute concentration increases, the osmotic gradient diminishes, reducing water influx into the organism and decreasing the necessary contraction rate.

• Free Response Practice 1 (2018 #6): CFTR Gated Ion Channel:

  • Context: Cystic fibrosis is linked to defects in the CFTR protein, an $ATP$-binding gated ion channel that facilitates the diffusion of chloride ions ($Cl^-$) across the cell membrane.

  • (a) Pathway of Normal Production:

  • Gene transcription in the nucleus produces mRNA \n\rightarrow\n mRNA exits nuclear pores to bound ribosomes on the Rough ER for translation \n\rightarrow\n nascent CFTR protein enters the Rough ER lumen \n\rightarrow\n vesicle transport to the Golgi apparatus for processing \n\rightarrow\n secretory vesicle transport to and fusion with the plasma membrane.

  • (b) Cellular Location of Ribosomes:

  • Synthesized by ribosomes bound to the membrane of the Rough Endoplasmic Reticulum (Rough ER).

  • (c) Location of Mutant CFTR Protein:

  • A mutant CFTR protein with an amino acid substitution in the $ATP$-binding site is still targeted correctly and resides in the cellular/plasma membrane, though its gating function is impaired.

• Free Response Practice 2 (2019 #8): Morning Glory Flower Opening:

  • Context: During flower opening in the Mexican morning glory (Ipomoea tricolor), petal cells swell and change pigment color from red to blue due to pH shifts in the central vacuole containing heavenly blue anthocyanin.

  • Petal Cell Changes Summary:

  • Bud State: Vacuole pH = $6.6$; Flower Color = Red; Cell Volume = Small; $(K^+ / H^+)$ Transport Protein = Inactive.

  • Open Flower State: Vacuole pH = $7.7$; Flower Color = Blue; Cell Volume = Large; $(K^+ / H^+)$ Transport Protein = Active.

  • (a) Responsible Cellular Component & Role:

  • Component: $(K^+ / H^+)$ transport protein located in the vacuolar membrane.

  • Role: Active transport of protons ($H^+$) out of the vacuole into the cytoplasm reduces the internal hydrogen ion concentration, causing the vacuolar pH to rise from $6.6$ to $7.7$.

  • (b) Supporting Mechanism for Vacuolar Swelling:

  • The active $(K^+ / H^+)$ transport protein pumps potassium ions ($K^+$) into the central vacuole while moving $H^+$ out.

  • The accumulation of $K^+$ ions elevates the total solute concentration inside the vacuole, rendering the vacuole hypertonic (hyperosmotic) relative to the surrounding cytosol and lowering its internal water potential.

  • Water moves down its water potential gradient via osmosis from the cytosol into the vacuole, causing the vacuole and the petal cell to expand and swell.

Questions & Discussion

• Q: What happens to a cell's surface area-to-volume ratio as it grows larger?

  • A: As a cell increases in size, its volume grows proportionally to the cube of its linear dimension ($r^3$ or $s^3$), while its surface area increases only by the square ($r^2$ or $s^2$). Consequently, the overall surface area-to-volume ratio ($SA/V$) decreases, reducing the efficiency of material exchange across the plasma membrane.

• Q: How do nonpolar versus polar molecules differ in their passage across the plasma membrane?

  • A: Small nonpolar molecules (such as $O_2$ and $CO_2$) dissolve directly through the hydrophobic interior of the phospholipid bilayer via simple diffusion. Polar or charged molecules (such as $Na^+$ or glucose) are repelled by the hydrophobic lipid tails and require specific channel or carrier proteins to cross via facilitated diffusion or active transport.