Cell Structure and Function: Comprehensive Study Guide
Overview of Cell Types: Prokaryotic vs. Eukaryotic Cells
The biological world comprises two primary categories of cells: prokaryotic cells and eukaryotic cells.
Shared Characteristics between Prokaryotic and Eukaryotic Cells:
Presence of Deoxyribonucleic Acid (DNA): Both cell types utilize a double-helix DNA molecule constructed from the nitrogenous bases adenine (), thymine (), cytosine (), and guanine (). Note that prokaryotic DNA is circular, whereas eukaryotic DNA is typically linear.
Plasma Membrane: Both possess an enclosing cell membrane (plasma membrane) that regulates the movement of substances.
Ribosomes: Both utilize ribosomes to synthesize proteins.
Cytoplasm: Both contain an internal fluid-filled matrix termed cytoplasm.
Cytoskeleton: Both maintain internal structural support and transport networks via a cytoskeleton.
Prokaryotic Cell Characteristics:
Lack membrane-bound organelles (such as a true nucleus, mitochondria, or endoplasmic reticulum).
Display smaller physical dimensions compared to eukaryotic cells.
Represent the earliest evolved cellular life forms on Earth.
Exhibit lower structural and functional complexity.
Exclusively single-celled (unicellular) organisms.
Representative examples include bacteria such as Escherichia coli (E. coli) and Salmonella.
Eukaryotic Cell Characteristics:
Contain distinct membrane-bound organelles (including a nucleus, chloroplasts, and mitochondria).
Evolved historically from ancestral prokaryotic cells.
Display larger physical dimensions.
Exhibit high structural complexity.
Can exist as single-celled organisms or form complex multicellular organisms.
Representative examples include plant cells, animal cells (including human cells), protists (such as Paramecium, Amoeba, and single-celled algae), and fungi (such as mushrooms).
Structure and Function of the Plasma Membrane
Lipid Bilayer Organization:
Constructed primarily as a lipid bilayer composed of phospholipids and embedded proteins.
Phospholipid orientation: Hydrophilic (water-attracting) heads face outward toward the aqueous external and internal cellular environments, while hydrophobic (water-repelling) tails face inward toward each other, isolated from water.
Fluid Mosaic Model:
Fluid Quality: The individual phospholipids and embedded proteins are constantly in motion, continuously shifting and moving past one another according to the cell's physiological demands.
Mosaic Quality: The membrane is composed of numerous small, distinct molecular pieces (lipids, carbohydrates, and varied proteins) assembled together to form a cohesive, larger structure.
Specialized Membrane Components:
Protein Pumps: Active transport structures embedded in the bilayer to move molecules against gradients.
Recognition Proteins: Located on the outer surface of the membrane to identify cellular identity and recognize external molecules.
Receptor Proteins: Situated on the external surface to bind specific signaling molecules (such as hormones) and initiate intracellular biochemical reactions.
Protein Channels / Transmembrane Proteins: Proteins that span across the entire thickness of the lipid bilayer to form channels for molecule passage.
Primary Function of the Plasma Membrane:
Controls and regulates all transport into and out of the cell.
Is semi-permeable (selectively permeable): "semi" means partially and "permeable" means allowing substances to pass through. It permits specific molecules or ions to cross while preventing others.
Prokaryotic Cell Organization and Diversity
Domains and Genetic Diversity:
Prokaryotes are divided into two main domains: Archaea and Eubacteria.
Display immense biological diversity: DNA sequence analysis demonstrates that Archaea and Eubacteria are as genetically distinct from one another as humans are from bacteria. For example, common eubacteria like E. coli are as molecularly distinct from extremophilic archaea living in deep-sea hydrothermal vents as human cells are from bacteria.
Internal and External Structural Features:
Genetic Material: DNA is circular and localized within an un-enclosed cellular region called the nucleoid. Prokaryotes lack a membrane-enclosed nucleus.
Ribosomes: Present in the cytoplasm to execute protein synthesis using ribosomal RNA ().
Cell Wall: A rigid structural layer located outside the plasma membrane.
Plasmids: Small, extra-chromosomal circular DNA molecules residing in the cytoplasm.
Flagella: Hair-like surface extensions that rotate to provide active cellular motility.
Eukaryotic Organelles and Internal Architecture
Comparative Organelle Content: Plant Cells vs. Animal Cells:
Unique Plant Cell Features:
Cell Wall: Rigid outer structural barrier outside the plasma membrane.
Chloroplasts: Specialized organelles for light energy conversion.
Central Vacuole: A massive vacuole occupying to of mature plant cell volume, primary for storing water and maintaining turgor pressure.
Unique Animal Cell Features:
Centrioles: Barrel-shaped organelles resembling churros that organize spindle fibers during cell division (mitosis) to separate chromosomes.
Multiple smaller vacuoles instead of a single central vacuole.
Shared Eukaryotic Organelles: Nucleus, cytoplasm, plasma membrane, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and cytoskeleton.
The Nucleus and Nucleolus:
Structure: Surrounded by a double-membrane nuclear envelope containing regulated nuclear pores (small holes) that strictly control molecular transport to protect nuclear DNA from damaging cytoplasmic chemicals.
Function: Stores genomic DNA and acts as the master control center ("brains of the cell") directing intracellular chemical reactions.
Chromatin vs. Chromosome terminology:
Chromatin: The uncoiled, comprehensive mass of DNA and associated proteins filling the nucleus.
Chromosome: A single, distinct, condensed molecule of DNA. Human somatic cells contain chromosomes.
Nucleolus: A specialized sub-region within the nucleus responsible for synthesizing ribosomal RNA () and assembling ribosomal subunits.
Ribosomes:
Composition: Synthesized in the nucleolus from ribosomal RNA () and proteins.
Structure: Composed of two distinct parts—a large ribosomal subunit and a small ribosomal subunit.
Function: Synthesize proteins via translation. Messenger RNA () passes between the small and large subunits, directing the step-by-step synthesis of a polypeptide chain.
Endoplasmic Reticulum (ER):
Rough Endoplasmic Reticulum (Rough ER):
Structure: Studded with ribosomes across its outer cytoplasmic surface, giving it a rough texture.
Function: Takes synthesized polypeptide chains from attached ribosomes and folds them into secondary, tertiary, and quaternary protein structures; packages proteins for intracellular transport or extracellular secretion.
Smooth Endoplasmic Reticulum (Smooth ER):
Structure: Lacks ribosomes on its surface.
Function: Synthesizes lipids and detoxifies harmful chemicals and drugs.
Golgi Apparatus (Golgi Body / Golgi Complex):
Structure: Stacked, flattened membrane-bound sacs resembling a pile of pancakes.
Function: Serves as the cellular sorting, packaging, and shipping facility (analogous to UPS). It chemically modifies proteins received from the ER, then "bags and tags" them into membrane-bound vesicles marked for specific intracellular destinations or extracellular secretion.
The Endomembrane System and Secretory Pathway:
Interconnected network comprising the nucleus, rough ER, smooth ER, Golgi apparatus, transport vesicles, and plasma membrane.
Sequential Steps of Protein Secretion:
DNA in the nucleus is transcribed into messenger RNA ().
exits the nucleus via nuclear pores into the cytoplasm and docks at ribosomes on the Rough ER.
Ribosomes translate the code into a polypeptide chain.
Polypeptides enter the Rough ER lumen and fold into specific functional conformations.
Proteins transition through the Smooth ER and transport vesicles to the Golgi apparatus for chemical modification and tagging.
The Golgi packages the proteins into secretory vesicles.
Secretory vesicles move to and fuse with the plasma membrane, releasing the protein outside the cell via exocytosis (e.g., insulin produced in pancreatic cells is secreted into the bloodstream to act on target tissues).
Lysosomes, Peroxisomes, and Apoptosis:
Lysosomes:
Structure: Membrane-bound vesicles filled with hydrolytic enzymes.
Function: Serves as the intracellular digestion center ("garbage disposal" of the cell), using hydrolytic enzymes to perform hydrolysis (breaking down chemical bonds in macromolecules or worn-out organelles using water).
Apoptosis (Programmed Cell Death): Referred to as "suicide sacs" due to their ability to intentionally lyse and cause controlled cell destruction. For example, during human fetal development in the uterus, apoptosis breaks down the webbed tissue between digits to form separated fingers.
Peroxisomes:
Structure: Membrane-bound enzymatic vesicles.
Function: Neutralize and decompose toxic hydrogen peroxide () generated as a byproduct of normal metabolic processes into harmless substances; detoxify alcohol in liver and kidney cells (which accordingly contain high concentrations of peroxisomes).
Mitochondria:
Structure: Enclosed by a double membrane. The inner membrane features extensive folds (cristae) that dramatically increase surface area, accommodating high densities of membrane-bound enzymes.
Function: Functions as the "powerhouse of the cell" by generating adenosine triphosphate () through cellular respiration (breaking down glucose). provides the energy required for motor proteins and cellular work.
Vacuoles and Turgor Pressure:
Structure: Phospholipid bilayer membrane-bound sacs that store water, macromolecules, and nutrients.
Central Vacuole in Plant Cells: Takes up to of internal cell volume in mature plant cells.
Turgor Pressure Mechanism:
When filled with water, the central vacuole expands and pushes against the rigid cell wall, exerting turgor pressure that keeps the plant upright.
When dehydrated, the central vacuole loses water and shrinks away from the cell wall, causing a loss of turgor pressure and plant wilting.
Analogy: A fully inflated balloon inside a cardboard box pushes firmly against the box walls, giving the container strength to withstand external weight; a deflated balloon inside the box provides no support, leaving the cardboard easy to collapse.
Chloroplasts:
Structure: Double-membrane organelles containing internal, pancake-like stacked thylakoid membranes surrounded by a fluid matrix called the stroma.
Compartment Functions:
Thylakoid membranes: Site of light-dependent reactions of photosynthesis.
Stroma: Site of glucose production (Calvin cycle / light-independent reactions).
Primary Function: Captures solar light energy and converts it into chemical energy stored in glucose molecules.
Evolutionary Origins: The Endosymbiotic Theory
Structural and Functional Similarities Between Mitochondria, Chloroplasts, and Free-Living Bacteria:
Double Membrane Structure: Possess both an inner and outer membrane, consistent with endocytic engulfment.
Autonomous DNA: Contain their own circular DNA independent of nuclear DNA (e.g., a eukaryotic plant cell contains nuclear DNA, chloroplast DNA, and mitochondrial DNA).
Autonomous Ribosomes: Contain their own prokaryotic-like ribosomes to synthesize independent proteins.
Division via Binary Fission: Divide independently within the eukaryotic cell using binary fission, the exact mechanism bacteria use to divide.
Physical Size: Measure approximately the same physical size as typical single-celled bacteria.
Variable Cellular Abundance: Can replicate independently based on metabolic demand (e.g., heavily exercised quadriceps muscle cells contain high concentrations of mitochondria compared to skin cells).
Core Premise of Endosymbiotic Theory:
Explains the evolutionary origin of eukaryotic cells from prokaryotic ancestors.
Proposes that ancient mitochondria-like and chloroplast-like prokaryotic microbes were engulfed (ingested) by larger host microbes.
Instead of digesting the engulfed microbes, the host cell established a mutually beneficial symbiotic relationship (endosymbiosis) with them. Over time, these engulfed microbes lost independence and evolved into specialized eukaryotic organelles.
Cell Junctions, Cellular Extensions, and Motility
Plant Cell Junctions:
Plasmodesmata: Membrane-lined channels crossing adjacent plant cell walls, allowing direct cytoplasmic exchange of water and nutrients.
Animal Cell Junctions:
Tight Junctions: Create waterproof, leak-proof seals between adjacent cells, preventing fluid movement across cell layers (e.g., present in skin cells and stomach lining cells to contain acidic fluids).
Adhering Junctions / Desmosomes: Function like structural mechanical rivets that permit cell layers to stretch without tearing (e.g., present in skin, stomach, urinary bladder, and heart tissues).
Gap Junctions: Open protein channels connecting adjacent cell cytoplasms to permit rapid intercellular chemical and ionic communication (e.g., present in heart tissue to synchronize muscle contractions).
Specialized Cellular Extensions and Motility Structures:
Pseudopods ("false feet"): Dynamic cytoplasmic extensions used for cellular movement and engulfing food particles (e.g., an Amoeba extending pseudopods to capture bacteria).
Cilia: Numerous short, hair-like surface structures that beat in coordinated waves to move fluids across tissue surfaces (e.g., cilia lining the human respiratory tract moving mucus up or down).
Flagella: Long, whip-like tail structures used for cell propulsion. The human sperm cell is the only human cell equipped with a flagellum to swim and fertilize an egg.
Surface Area-to-Volume Ratio and Constraints on Cell Size
Metabolic Determinants of Cell Size:
Volume: Determines the cell's total internal metabolic rate. Larger volume increases metabolic activity, requiring more nutrient input and generating more waste output.
Surface Area: Determines the total plasma membrane area available for nutrient uptake (, glucose) and waste elimination ().
The Surface Area-to-Volume Ratio ():
Formula:
Mathematical Inverse Relationship: As a cell increases in physical size, its volume increases cubically while its surface area increases quadratically. Consequently, as cell size increases, its surface area-to-volume ratio () decreases.
Physiological Implications of Cell Size:
Small cell size provides a large ratio, ensuring fast, efficient diffusion of nutrients inward and waste products outward across the entire cell.
Above a maximum size threshold, the plasma membrane surface area becomes insufficient to supply metabolic demand or clear waste fast enough, resulting in cell death.
Scale Comparison: Individual cells in large animals (e.g., an elephant) are not larger than individual cells in small animals (e.g., a mouse); larger organisms simply consist of a vastly greater number of small cells, preserving optimal ratios across all tissues.