AP Bio Penguins Unit 2 Review: Cell Structure and Function

Cellular Organelles and Their Functions

• Ribosomes:
       - Structure: Ribosomes are non-membrane-bound organelles comprised of a large subunit and a small subunit made up of ribosomal RNA ($rRNA$) and proteins. They are essential for the assembly of amino acids into proteins, a process termed translation.
       - Small Subunit: This subunit binds to the messenger RNA ($mRNA$), ensuring that the correct amino acids are added in sequence during protein synthesis.
       - Large Subunit: It attaches to transfer RNA ($tRNA$), which carries specific amino acids to the ribosome, enabling polypeptide chain formation.
       - Primary Function: The main role of ribosomes is protein synthesis, where they synthesize polypeptides according to the sequence of the mRNA, which ultimately dictates protein structure and function.
       - Evolutionary Significance: Ribosomes are present across all domains of life (bacteria, archaea, and eukarya), providing crucial evidence for the theory of evolution and common ancestry due to their structural and functional conservation throughout evolution.

• The Endomembrane System:
       - Components: The endomembrane system includes the Endoplasmic Reticulum ($ER$), Golgi apparatus, lysosomes, vacuoles, nuclear envelope, and plasma membrane, which interact in various ways to perform cell functions.
       - General Function: These organelles collaborate to package, transport, and modify biomaterials (polysaccharides, lipids, proteins), facilitating efficient cellular organization and function through specialization of roles.
       

• Endoplasmic Reticulum ($ER$):
       - General Roles: The $ER$ provides mechanical support, determines cell shape, and aids in the intracellular transport of materials.
       - Rough $ER$: Characterized by ribosomes on its cytoplasmic surface, the Rough $ER$ plays a vital role in synthesizing proteins destined for secretion, incorporation into membranes, or lysosomes.
       - Smooth $ER$: This structure, devoid of ribosomes, is involved in lipid synthesis, detoxifying harmful metabolic byproducts, storage of calcium ions, and synthesizing steroid hormones.
       

• Golgi Complex:
       - Analogy: Often known as the "mailman" of the cell, the Golgi apparatus is crucial for processing and packaging macromolecules for secretion or delivery to other organelles.
       - Structure: It is comprised of flattened, stacked sacs called cisternae, featuring a distinct organization with a cis face (receiving side) and a trans face (shipping side) where vesicles bud off for transport.
       - Function: It modifies proteins and lipids received from the $ER$, sorts them, and packages them into vesicles for transport to their intended destination, either inside or outside the cell.
       

• Mitochondria:
       - Function: These organelles are often referred to as the powerhouse of the cell, where cellular respiration occurs, transforming nutrients into usable energy in the form of ATP through the Krebs cycle and oxidative phosphorylation.
       - Structure: Mitochondria feature a double membrane; the smooth outer membrane encloses the organelle, while the highly folded inner membrane (containing cristae) increases surface area for metabolic reactions.
       - Surface Area: The folding creates a large surface area to accommodate a high volume of enzymes needed for ATP synthesis.
       - Compartments: The inner chamber contains the matrix, where metabolic processes, such as the Krebs cycle, take place, while the intermembrane space is involved in the electron transport chain's function.
       

• Lysosomes:
       - Function: These membrane-enclosed sacs are rich in hydrolytic enzymes necessary for the breakdown of macromolecules, recycling cellular components, and digesting foreign materials through phagocytosis.
       - Programmed Cell Death: Lysosomes also play a significant role in apoptosis, initiating pathways that lead to cell death, thus eliminating damaged or unnecessary cells, effectively maintaining tissue homeostasis.
       

• Vacuoles:
       - General: Membrane-bound vacuoles act as storage compartments, helping to store various substances including nutrients, waste products, and other materials.
       - Central Vacuole (in Plants): This large organelle helps maintain turgor pressure necessary for structural integrity and stores organic compounds and ions, playing a critical role in plant cell function.
       - Contractile Vacuole: Found in certain freshwater organisms, this vacuole helps regulate osmotic pressure by expelling excess water from the cell, preventing cell lysis due to overfilling.
       - Food Vacuole: These vacuoles are formed during phagocytosis, merging with lysosomes to assist in digestion of engulfed food particles.
       

• Chloroplasts:
       - Occurrence: Chloroplasts are fundamental to photosynthetic organisms, including plants and algae, facilitating the process of converting light energy into chemical energy via photosynthesis.
       - Structure: They have a double outer membrane and an internal system of thylakoids organized into stacks called grana (which are the sites of light absorption) and surrounded by a fluid called stroma (where the Calvin cycle occurs).
       - Function: Chlorophyll, located in the thylakoid membranes, captures light energy to convert carbon dioxide and water into glucose and oxygen, forming the foundation of the food web in terrestrial ecosystems.

Evolutionary Theories and Biological Principles

• Endosymbiotic Theory:
       - Proposes that mitochondria and chloroplasts originate from free-living prokaryotic organisms that were engulfed by ancestral eukaryotic cells, leading to a symbiotic relationship.
       - Evidence: This theory is supported by the presence of double membranes, the structure of their own circular DNA, composition of ribosomes resembling those of bacteria, and their ability to replicate independently through a binary fission-like process.
       

• Compartmentalization:
       - The advantages of compartmentalization within membrane-bound organelles include the separation and organization of biochemical processes, like metabolic reactions, providing an increased efficiency that enables eukaryotic cells to grow larger and more complex than prokaryotic cells.
  

Cell Size and Surface Area to Volume Ratio ($SA:V$)

• Core Principle: Cells must optimize their size to maintain a large surface area relative to their volume ($SA:V$) for efficient function, particularly affecting resource exchange and signal transduction. A larger $SA:V$ ratio allows for more efficient transport of materials and communication between the cell and its environment.

• Functions Influenced by $SA:V$:
       - Enhanced ability to obtain nutrients and eliminate waste across the plasma membrane, crucial for cellular health.
       - Increased efficiency in thermal energy dissipation to maintain homeostasis, critical for metabolic reactions.
       - Effective exchange of chemical signals and energy, which is essential for cell communication and response mechanisms to environmental changes.
  

• Efficiency: Smaller cells exhibit a higher $SA:V$ ratio, facilitating more effective transport processes essential for metabolic activities and overall cell viability. A decrease in the $SA:V$ ratio in larger cells can lead to inefficiencies in nutrient uptake and waste removal.

• Formula Sheet Equations:
       - Sphere: SA = 4eta imes r^2; V = rac{4}{3}eta imes r^3
       - Rectangular Solid: $SA = 2lw + 2lh + 2wh$; $V = lwh$
       - Cylinder: SA = 2eta imes rh + 2eta imes r^2; V = eta imes r^2h
       - Cube: $SA = 6s^2$; $V = s^3$
  

• Metabolic Rate: Typically, smaller organisms exhibit higher metabolic rates relative to their body mass, as their larger $SA:V$ ratios facilitate rapid heat loss and thus demand higher energy requirements for sustaining cellular activities and functions.

Plasma Membrane Structure

• Phospholipid Bilayer:
       - Hydrophilic Heads: Comprised of polar phosphate groups, these heads orient towards the aqueous environment both externally and internally, allowing interactions with surrounding fluids.
       - Hydrophobic Tails: The non-polar fatty acid chains face inward, away from water, contributing to the membrane's integrity and fluid nature that changes with environmental conditions.
  

• Fluid Mosaic Model:
       - This model comprehensively describes the plasma membrane as a dynamic structure where diverse components such as phospholipids, proteins, sterols, and carbohydrates move flexibly within the bilayer, contributing to various functions.
  

• Membrane Components:
       - Cholesterol: A crucial lipid that modulates membrane fluidity; it serves as a temperature buffer, enhancing stability across temperature fluctuations.
       - Glycolipids/Glycoproteins: These molecules, featuring carbohydrate attachments, play vital roles in cell recognition, signaling, and formation of tissue structures—important for immune responses.
       - Proteins: Integral proteins cross the lipid bilayer, facilitating transport and communication, while peripheral proteins associate with the membrane surface, contributing to structural and regulatory functions within cellular reactions.
  

• Cell Wall:
       - Present in plants (composed of cellulose), fungi (constructed from chitin), and bacteria (constructed from peptidoglycan), the cell wall provides structural support and protection against osmotic pressure, which is critical for maintaining cellular integrity during environmental changes.

Membrane Transport Mechanisms

• Passive Transport:
       - The movement of substances across membranes without the use of energy, occurring down their concentration gradient (from high to low concentration).
       - Simple Diffusion: Small, non-polar molecules such as nitrogen ($N_2$), oxygen ($O_2$), carbon dioxide ($CO_2$), and lipid-soluble hormones diffuse unassisted across the plasma membrane.
       - Facilitated Diffusion: This involves specific transmembrane proteins (channels or carriers) aiding the transport of polar or charged molecules (e.g., water through aquaporins, ions like $Na^+$, $K^+$, and $Ca^{2+}$).
  

• Active Transport:
       - Requires cellular energy (ATP) to transport substances against their concentration gradient (from low to high concentration), vital for maintaining ionic gradients essential for cellular processes.
       - Mechanism: Involves carrier proteins which undergo conformational changes to move substances across membranes; for example, the sodium-potassium pump, which actively transports $3 ext{Na}^+$ ions out of the cell and $2 ext{K}^+$ ions into the cell, crucial for maintaining resting membrane potential.
  

• Bulk Transport:
       - Exocytosis: Involves the fusion of internal vesicles with the plasma membrane to release large quantities of substances, such as the secretion of neurotransmitters or hormones like insulin.
       - Endocytosis: The process by which the plasma membrane engulfs external materials, bringing them into the cell; methods include phagocytosis (cellular eating of solids), pinocytosis (cellular drinking of fluids), and receptor-mediated endocytosis, where specific ligands bind to receptors to enrich the uptake process.
  

Water Potential and Tonicity

• Water Potential ($Ψ$): A measure of the potential energy in water, predicting the net movement of water molecules; water naturally moves from areas of high water potential to regions of lower water potential.
       - Formula: $Ψ = Ψ_p + Ψ_s$ where $Ψ_p$ is the pressure potential and $Ψ_s$ is the solute potential.
       - Solute Potential ($Ψ_s$): Calculated using the Van't Hoff equation where $Ψ_s = -iCRT$:
        - i: Ionization constant (e.g., sucrose/glucose = 1; sodium chloride = 2).
        - C: Molar concentration of the solute.
        - R: Pressure constant ($0.0831 ext{ L} ext{ bar} / ext{ mol K}$)
        - T: Temperature in Kelvin ($^ ext{o}C + 273$).

• Tonicity Comparisons:
       - Hypertonic: A solution with a higher concentration of solutes outside the cell compared to inside; cells in a hypertonic solution will lose water and may shrivel (crenation).
       - Hypotonic: A solution containing a lower solute concentration than inside the cell, leading to water influx and possible cell lysis (bursting).
       - Isotonic: Solutions having equivalent solute concentrations, resulting in no net movement of water across the membrane, allowing the cell to maintain stable conditions.

• Biological Effects:
       - In hypertonic environments, animal cells undergo crenation, while in hypotonic environments, they risk lysis. Conversely, plant cells prefer a hypotonic state, leading to turgidity due to water influx, while hypertonic conditions can cause plasmolysis, where the cell membrane pulls away from the cell wall, compromising structure and function.