Bio module 2
Cellular Foundations and Microscopy
Microscopic Visualization of Cells:
When examining specimens under a light microscope, individual cellular components and organelles are often translucent and difficult to distinguish.
To resolve internal structural details, scientists apply special stains that selectively bind to specific biological molecules, increasing contrast.
Fundamental Unit of Life:
The cell is established as the basic structural, functional, and biological unit of all living organisms.
Prokaryotic Structure and Physiology:
Prokaryotic organisms, such as Escherichia coli (E. coli), are single-celled organisms that lack a membrane-bound nucleus and membrane-bound organelles.
Nutrient and Waste Exchange: Prokaryotes rely primarily on diffusion to obtain necessary metabolic materials from their environment and to expel cellular wastes across their plasma membrane.
Bacterial Fimbriae:
Fimbriae are small, thin, hair-like protein structures located on the external surface of bacterial cells.
Their primary function is to facilitate attachment to external surfaces, environmental substrates, and host cell membranes.
Bacteria that lack fimbriae are significantly less likely to adhere to cell surfaces and host tissues, severely impacting their colonization ability.
Conserved Cellular Structures Across Domains:
Both prokaryotic and eukaryotic cells share essential cellular structures, most notably ribosomes, which are responsible for protein synthesis across all domains of life.
Organelles and the Endomembrane System
Nuclear Envelope and Nucleoplasm:
The nucleoplasm is contained within the nuclear envelope.
The nuclear envelope is composed of a double-membrane system made up of two distinct phospholipid bilayers (comprising four layers of phospholipids in total).
Peroxisome Function and Enzymatic Reactions:
Peroxisomes carry out specialized oxidation reactions that break down long-chain fatty acids and amino acids.
These organelles derived their name because toxic hydrogen peroxide () is produced during their oxidation reactions, which is subsequently converted into water and oxygen by internal enzymes like catalase.
Plant Central Vacuoles vs. Animal Lysosomes:
Plant cells generally lack distinct, standalone lysosomes.
In plant cells, the hydrolytic degradation of macromolecules is performed by the large central vacuole.
The central vacuole contains specialized digestive enzymes that execute hydrolytic breakdown, effectively performing the exact functional role as lysosomes in animal cells.
Lysosomal Storage Disorders:
Tay-Sachs Disease is a severe genetic disorder characterized by the progressive destruction of neurons in the central nervous system.
This destruction is caused by a accumulation of sphingolipids within the cells.
The organelle malfunctioning in Tay-Sachs disease is the lysosome, due to a deficiency in hydrolytic enzymes required to break down sphingolipids.
Endomembrane System Components:
The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and plasma membrane.
The mitochondrion is not a component of the endomembrane system; it is a semi-autonomous organelle involved in cellular respiration.
Smooth Endoplasmic Reticulum (SER):
The smooth endoplasmic reticulum is primarily involved in lipid synthesis, steroid hormone synthesis, carbohydrate metabolism, and detoxification of chemicals.
Tissues specialized in endocrine function—specifically cells that produce steroid hormones—contain the highest cellular concentration of smooth endoplasmic reticulum.
Protein Synthesis, Modification, and Transport Pathway:
The sequential order of steps involved in incorporating a proteinaceous molecule within or outside a cell proceeds through the endomembrane pathway:
Ribosomes on the Rough ER: Protein synthesis begins on ribosomes bound to the rough endoplasmic reticulum.
Endoplasmic Reticulum: The synthesized protein enters the ER lumen for proper protein folding and initial structural modifications.
Transport Vesicle: The protein is packaged into a transport vesicle that buds off the ER and travels to the Golgi apparatus.
Golgi Apparatus: The Golgi apparatus receives the protein, carries out further carbohydrate modification (glycosylation), and tags/labels the molecule for targeted delivery.
Secretory Vesicle: A secretory vesicle containing the tagged protein buds off from the Golgi apparatus to distribute the final protein to its target organelle or to the plasma membrane for exocytosis.
Glycosylation and Golgi Disorders:
Glycosylation is the enzymatic process of attaching complex carbohydrate/sugar chains to proteins or lipids to form glycoproteins or glycolipids.
Major processing and maturation of glycoproteins occurs within the Golgi apparatus.
Consequently, congenital disorders of glycosylation directly involve malfunctioning within the Golgi apparatus.
Cytoskeleton and Cellular Motility
Dynamic Cytoskeletal Polymerization:
Microfilaments (actin filaments) and microtubules possess highly dynamic structures.
Both microfilaments and microtubules have the ability to rapidly assemble (polymerize) and disassemble (depolymerize) in response to cellular signals, allowing cells to change shape, divide, and move.
Structural Role of Intermediate Filaments:
Unlike microfilaments and microtubules, intermediate filaments serve a purely structural role in maintaining cell shape, bearing mechanical tension, and anchoring organelles.
Intermediate filaments do not play a direct role in intracellular movement or active transport pathways.
Cilia and Flagella Function in Humans:
In humans, flagella are used to move an entire cell through its liquid environment (e.g., sperm cell motility).
In contrast, cilia are used to move fluid, mucus, and foreign particles across the extracellular surface relative to a stationary tissue layer (e.g., ciliated epithelial cells of the respiratory tract).
Cell Junctions and Intercellular Communication
Plant-Specific Intercellular Junctions:
Plasmodesmata are specialized membrane-lined channels traversing plant cell walls.
Plasmodesmata connect the cytoplasm of adjacent plant cells, facilitating direct chemical exchange; they are found only in plant cells.
Desmosome Architecture:
Desmosomes are intercellular anchoring junctions that provide strong mechanical attachment between neighboring animal cells in tissues subject to stretching.
The key protein components of desmosomes are transmembrane cell-adhesion proteins called cadherins, which anchor internally to intermediate filaments.
Gap Junctions and Cell Death Propagation:
Gap junctions in animal tissues form open intercellular channels that allow ions, nutrients, and small signaling molecules to pass directly between adjacent cytoplasm.
When diseased or damaged animal cells initiate controlled apoptosis (programmed cell death), apoptotic signaling factors ("death molecules") pass directly through gap junctions into healthy neighboring cells, causing adjacent cells to die as well.
Tight Junctions:
Tight junctions create watertight seals between adjacent animal epithelial cells, preventing extracellular fluids and solutes from leaking between cells.
Plasma Membrane Structure and Dynamics
Membrane Composition and Arrangement:
Proteins: Membrane proteins can be peripherally attached to the membrane surface or integrally embedded within the hydrophobic core of the phospholipid bilayer.
Carbohydrates: Carbohydrates bound to extracellular proteins (glycoproteins) or lipids (glycolipids) function primarily in cell identification, cellular recognition, and immune targeting.
Membrane Fluidity Regulation:
The presence of double bonds in the fatty acid tails (unsaturated fatty acids) creates kinks that prevent tight molecular packing, thereby increasing membrane fluidity.
Thermal Adaptation in Membranes:
Organisms adapted to high-temperature environments (e.g., animals living in the Mojave Desert) face potential destabilization and excessive fluidity of their cell membranes.
To maintain membrane structural integrity, cells from desert-adapted animals exhibit a higher cholesterol concentration in their plasma membranes to dampen fluidity at high temperatures.
Organisms adapted to temperate environments (e.g., Mediterranean coast animals) maintain plasma membranes suited for cooler conditions.
Membrane Transport Mechanisms
Diffusion Dynamics:
The principal driving force behind net molecular movement in simple and facilitated diffusion is the concentration gradient.
Simple Passive Diffusion:
Small, nonpolar molecules diffuse directly across the lipid bilayer down their concentration gradient without requiring transport proteins.
For example, oxygen moving into a cell after oxygen deprivation occurs via simple passive diffusion without the aid of a channel or carrier protein.
Osmosis and Osmoregulation:
Osmosis is defined as the net diffusion of water molecules across a selectively permeable membrane from an area with a high concentration of water (low solute concentration) to an area of lower water concentration (high solute concentration).
Freshwater Challenges:
Freshwater environments are hypotonic relative to intracellular cytoplasm.
As a result, organisms living in freshwater face the constant physiological problem that their bodies tend to take in too much water through osmosis.
Active Transport and Electrochemical Gradients:
Active transport mechanisms must function continuously using ATP because passive diffusion is constantly moving solutes in opposite directions down their concentration gradients.
Sodium-Potassium Pump (-ATPase):
The sodium-potassium pump actively transports cations out of the cell for every cations brought in.
This pump maintains a net negative charge inside the cell by expelling more cations than are taken in.
Electrochemical Gradient:
The combined influence of a chemical concentration gradient and an electrical charge gradient across a biological membrane is termed an electrochemical gradient.
Bulk Transport: Phagocytosis and Exocytosis:
Phagocytosis: Known as "cell eating," this is a form of endocytosis in which a cell's plasma membrane wraps around and engulfs large foreign particles, debris, or whole microorganisms.
Exocytosis: During exocytosis, internal membrane-bound vesicles transport cargo to the cell perimeter. The membrane of the vesicle fuses with and becomes part of the plasma membrane, delivering its internal contents to the extracellular environment.