Cell Structure & Function Study Guide

Cellular Organelles and Structures

Cell Membrane / Plasma Membrane:
- Serves as the outer barrier of all cells.
- Possesses semi-permeable properties, allowing selective entry where some substances can cross while others are restricted.
- Controls the directional movement of materials into and out of the cell.
Cell Wall:
- A rigid, strong outer layer located external to the cell membrane.
- Present in plant cells, fungi, bacteria, and certain protists.
- Primary functions include:
  - Providing structural support and maintaining cell shape.
  - Protecting cellular components from mechanical injury and pathogen invasion.
  - Withstanding internal turgor pressure to prevent cytolysis (cell bursting).
Cytoplasm and Cytosol:
- The internal gelatinous fluid found in all cells.
- Serves as the site where numerous metabolic reactions and cellular processes take place.
Cytoskeleton:
- Composed of a network of protein fibers.
- Anchors and facilitates the active movement of organelles throughout the cell interior.
- Assists mechanically during cell division.
Chloroplast:
- A specialized energy-transducing organelle located in photosynthetic cells.
- Enclosed by a double membrane with distinct regions of internal membrane folds and internal fluid compartments.
- Captures and converts solar radiant energy into chemical energy stored within glucose.
Golgi Apparatus:
- Composed of a series of flattened, membrane-bound stacks (cisternae).
- Acts as the cellular processing, sorting, and packaging center for proteins destined to leave the cell.
Lysosomes:
- Membrane-bound organelles containing specialized hydrolytic and digestive enzymes.
- Enzymes are strictly segregated inside a membrane-bound sac to prevent accidental degradation of the surrounding cellular contents.
- Deconstructs and recycles damaged or worn-out organelles and cell debris.
- Carries out programmed cell death (apoptosis).
Mitochondria:
- Specialized energy-converting organelles present in all eukaryotic cells.
- Structured with a double membrane system containing areas of inner folds (cristae) and internal fluid chambers (matrix).
- Breaks down glucose to generate usable cellular chemical energy in the form of adenosine triphosphate ( $\text{ATP}$ ).
Nucleus:
- Contains and protects the cell's genetic material ( $\text{DNA}$ ).
- Enclosed by a specialized double membrane known as the nuclear envelope.
Ribosomes:
- Non-membrane-bound complexes responsible for protein synthesis.
- Localized in two distinct regions:
  - Bound to the rough endoplasmic reticulum ( $\text{RER}$ ): synthesize proteins targeted for export outside of the cell or for membrane integration.
  - Free in the cytoplasm: synthesize functional proteins destined for use inside the cell.
Rough Endoplasmic Reticulum ( $\text{RER}$ ):
- A membrane-bound organelle studded with ribosomes on its outer cytoplasmic surface.
- Receives newly synthesized proteins from bound ribosomes, facilitates their folding and structural processing, and packages them into transport vesicles targeted for the Golgi apparatus.
Smooth Endoplasmic Reticulum ( $\text{SER}$ ):
- A membrane-bound organelle that lacks surface-bound ribosomes.
- Responsible for the detoxification of drugs and metabolic poisons.
- Synthesizes cellular lipids.
Vacuoles:
- Membrane-bound storage cavities that house water, essential nutrients, and chemical reservoirs.
- Plant cells contain a prominent large central vacuole that maintains internal turgor pressure against the cell wall.
Vesicles:
- Small, spherical, membrane-enclosed transport compartments.
- Mediate material exchange along the endomembrane pathway and drive cellular mass transport processes such as endocytosis and exocytosis.

Endomembrane Pathway and Compartmentalization

Endomembrane Secretory Pathway:
- Represents the sequential, coordinated movement through organelles to manufacture, modify, package, and release a protein outside the cell.
- Sequential execution steps:
  - Step 1: A specific polypeptide chain is generated by a ribosome docked on the Rough Endoplasmic Reticulum ( $\text{RER}$ ).
  - Step 2: The $\text{RER}$ folds and biochemically processes the protein, subsequently packaging it into an emerging transport vesicle.
  - Step 3: The transport vesicle detaches from the $\text{RER}$ and travels to the Golgi apparatus.
  - Step 4: The Golgi apparatus further modifies, sorts, and chemically tags the protein for its specific extracellular target.
  - Step 5: A transport vesicle buds off the trans face of the Golgi apparatus and moves toward the plasma membrane.
  - Step 6: The vesicle membrane fuses directly with the cell membrane, releasing the protein into the extracellular space via exocytosis.
Cell Compartmentalization:
- Membrane-bound internal compartments form isolated, specialized microenvironments within the cell.
- Isolates distinct and incompatible chemical reactions, preventing competing enzymatic processes.
- Concentrates necessary reactants and enzymes to optimize overall metabolic efficiency.
Endosymbiotic Theory:
- Proposes that mitochondria and chloroplasts originated as independent prokaryotic cells that were engulfed by ancestral, primitive eukaryotic cells.
- Multiple lines of structural and genetic evidence support this evolutionary mechanism:
  - Both mitochondria and chloroplasts contain their own circular, non-histone-bound $\text{DNA}$ , structurally identical to bacterial chromosomes.
  - Both organelles possess their own functional ribosomes that closely resemble bacterial ribosomes.
  - Both organelles are enclosed by double membranes; the inner membrane represents the original plasma membrane of the engulfed prokaryote, while the outer membrane represents the host cell's engulfing vacuolar membrane.
  - Both organelles undergo independent self-replication using binary fission-like processes similar to bacteria.

Cell Size and Surface Area-to-Volume Ratio

Dimensions and Cellular Homeostasis:
- Cellular dimensions dictate the efficiency of material exchange across the boundary:
  - Surface area represents the absolute amount of exterior exposure an object displays.
  - Volume represents the internal three-dimensional space requiring metabolic support.
- Cells require a high surface area relative to a low internal volume to maintain cellular homeostasis.
- High surface area combined with lower volume dramatically accelerates the rate at which nutrients enter and metabolic wastes exit.
Surface Area-to-Volume Ratio ( $\text{SA:V}$ ):
- A higher $\text{SA:V}$ numerical value indicates high efficiency in diffusion-based exchange with the external surroundings.
- As a cell expands uniformly in size, its volume increases much faster than its surface area, causing the $\text{SA:V}$ ratio to drop sharply.
- Cells evolve specialized, highly convoluted, or folded membranes to artificially maximize their effective $\text{SA:V}$ ratio.
- Membrane folding facilitates rapid nutrient assimilation and waste expulsion without exceeding cellular transport limits.

Cell Membrane Structure and the Fluid Mosaic Model

Phospholipid Bilayer Core:
- The structural backbone consists primarily of amphipathic phospholipids arranged end-to-end (analogized to individuals touching feet).
- Hydrophilic Head (Phosphate Group):
  - Possesses polar characteristics capable of forming electrostatic interactions and hydrogen bonds with water.
  - Oriented toward the aqueous exterior environment and the aqueous interior cytosol, forming both external and internal surface boundaries.
- Hydrophobic Tails (Fatty Acid Chains):
  - Composed of nonpolar hydrocarbon chains incapable of forming hydrogen bonds with water.
  - Face inwards toward each other, sequestered in the interior of the membrane away from aqueous fluids.
Associated Membrane Components:
- Peripheral Proteins: Located loosely attached to the exterior surfaces of the membrane on either the intracellular or extracellular faces.
- Integral Proteins: Penetrate completely through the hydrophobic core, spanning the entire breadth of the bilayer, and function prominently in transmembrane transport.
- Cholesterol:
  - A hydrophobic lipid steroid embedded within the nonpolar hydrocarbon interior of the lipid bilayer.
  - Acts as a bidirectional fluidity buffer, ensuring structural stability and maintaining membrane integrity across fluctuating temperatures.
- Glycoproteins and Glycolipids: Carbohydrate chains covalently attached to membrane proteins or lipids, facing the extracellular space; utilized primarily for cellular identification, tissue recognition, and intercellular communication.
The Fluid Mosaic Model:
- The components of the plasma membrane do not occupy rigid, fixed locations.
- Lipids and embedded proteins exhibit lateral movement, drifting within the two-dimensional plane of the bilayer behaving like a flexible fluid.

Cell membrane structure showing phospholipid bilayer, proteins, and cholesterol

Membrane Transport Mechanisms

Passive Transport:
- Movement of chemical species from regions of high concentration to regions of low concentration along their natural concentration gradient.
- Occurs spontaneously without the expenditure of metabolic cellular energy ( $\text{ATP}$ ).
Modes of Passive Transport:
- Simple Diffusion:
  - Unassisted net movement of solute molecules directly across the phospholipid bilayer.
  - Permeant molecules include small nonpolar compounds such as carbon dioxide ( $\text{CO}_2$ ), molecular oxygen ( $\text{O}_2$ ), and atmospheric nitrogen ( $\text{N}_2$ ).
  - Small, polar molecules like water ( $\text{H}_2\text{O}$ ) can traverse the core directly, but this uncatalyzed process occurs at an extremely slow rate.
- Osmosis:
  - The passive net movement of water molecules across a selectively permeable membrane mediated by aquaporins.
  - Aquaporins are specialized channel proteins dedicated to the rapid conduction of water molecules.
  - The rate of transmembrane water diffusion is directly proportional to the density of active aquaporins inserted into the lipid bilayer.
- Facilitated Diffusion:
  - Passive, channel- or carrier-mediated transport for substances physically incapable of penetrating the nonpolar lipid interior.
  - Utilized by large, uncharged polar molecules such as glucose.
  - Utilized by charged ions such as sodium ( $\text{Na}^+$ ), potassium ( $\text{K}^+$ ), and chloride ( $\text{Cl}^-$ ) utilizing specialized transmembrane channels.
Active Transport:
- Movement of molecular species against their established concentration gradient, driving substances from regions of low concentration to high concentration.
- Strictly requires the expenditure of cellular metabolic energy.
- Energy is liberated via $\text{ATP}$ hydrolysis, wherein the high-energy phosphoanhydride bond between the terminal phosphate groups of $\text{ATP}$ is enzymatically cleaved into adenosine diphosphate and inorganic phosphate:

$\text{ATP} \rightarrow \text{ADP} + \text{P}$

Passive and active membrane transport pathways and vesicle comparison

Vesicular (Bulk) Transport:
- Mechanisms employed for macromolecules that cannot cross via channels or diffusion.
- Exocytosis: Secretory vesicles within the cytoplasm migrate toward, contact, and fuse with the plasma membrane, expelling bulk macromolecules (such as peptide hormones) into the extracellular environment.
- Endocytosis: The plasma membrane invaginates and pinches inward, wrapping around extracellular molecules or fluids that cannot diffuse through the membrane to package them into internal vesicles.

Mechanisms of exocytosis and endocytosis

Tonicity, Osmoregulation, and Water Potential

Tonicity Dynamics:
- Hypertonic Solution:
  - The extracellular environment has a higher solute concentration than the cellular interior.
  - Water exits the cell along its concentration gradient (moving from an area of high water concentration to low water concentration; or low solute concentration to high solute concentration).
  - Results in net water loss, causing animal cells to shrivel (crenate) and plant cells to undergo plasmolysis.
- Hypotonic Solution:
  - The extracellular environment contains a lower solute concentration relative to the cellular interior.
  - Water moves from the solution into the cell (moving from high water concentration to low water concentration; or low solute concentration to high solute concentration).
  - Causes animal cells to swell and potentially lyse; generates internal turgor pressure in plant cells, pressing the membrane against the cell wall to maintain structural firmness.
- Isotonic Solution:
  - Equal solute concentrations exist inside and outside the cell.
  - Dynamic equilibrium is established; water enters and leaves the cell at identical rates, resulting in no net movement of water.

Effects of hypertonic, isotonic, and hypotonic solutions on animal and plant cells

Principles of Water Potential ( $\Psi$ ):
- Water potential quantifies the potential energy and likelihood of water molecules to move freely in a system.
- Water moves from regions of high water potential to regions of low water potential.
- High water potential correlates with a high likelihood of water moving away; this condition arises in solutions with low solute concentrations.
- The highest theoretical water potential value for pure, unbound water under atmospheric conditions is $0$ .
- Low water potential correlates with a reduced likelihood of water moving away; this condition arises in solutions with elevated solute concentrations.
- As solute concentrations increase, the water potential value becomes progressively more negative.
Components Defining Total Water Potential:
- Solute Potential ( $\Psi_s$ ):
  - Measures the binding effect of dissolved solute particles on free water molecules.
  - When solutes dissolve, water forms hydration shells around them, reducing the mobility of the water molecules.
  - Solute potential values are always $0$ (for pure water) or negative.
  - Adding solutes to a system invariably depresses the overall water potential.
- Pressure Potential ( $\Psi_p$ ):
  - Measures the physical hydrostatic push or mechanical compression exerted on water.
  - Applying physical pressure increases the free energy and raises the water potential.
  - Pressure potential values are always $0$ or positive.