Summary
Highlights
Cells are the fundamental units of life, possessing a membrane, genetic information (DNA), and systems for maintaining and replicating themselves, including the transcription of DNA into mRNA and its translation into proteins by ribosomes. Proteins, especially enzymes, regulate cell metabolism. Cells are categorized into prokaryotic and eukaryotic types. Prokaryotic cells are smaller, simpler, lack a nucleus, have circular chromosomes, and plasmids, found in archaea and bacteria. Eukaryotic cells are larger, more complex, possess a nucleus, multiple linear chromosomes, and many membrane-bound organelles like mitochondria.
Cell size is critical because cells need sufficient membrane surface area for substance diffusion. Smaller cells have a much higher surface area-to-volume ratio than larger cells, enabling efficient nutrient intake and waste expulsion. This principle also explains phenomena like how certain tissues (e.g., fish gills, elephant ears, mitochondrial membranes, intestinal villi) increase surface area for enhanced diffusion or heat regulation. The surface area-to-volume ratio also explains why large marine mammals (like whales) maintain body heat more effectively than small ones, and why there are no mouse-sized marine mammals due to excessive heat loss.
Metabolism is the sum of chemical processes in an organism, and metabolic rate is the energy expended over time. Basal metabolic rate is measured at rest. Endotherms (like mammals and birds) generate internal body heat through metabolism and regulate body temperature around a set point, requiring more food than ectotherms. As endothermic animals grow larger, their overall metabolic rate increases. However, their relative metabolic rate (per unit of body mass) decreases, meaning smaller mammals have a much higher mass-specific metabolic rate to compensate for greater heat loss due to their larger surface area-to-volume ratio.
Cellular compartmentalization is the internal division of space into sections, offering advantages like specialized internal chemistry (e.g., lysosomes with hydrolytic enzymes) and increased internal surface area for membrane-bound enzymes. Eukaryotic cells are highly compartmentalized with extensive internal membranes, including the endoplasmic reticulum, Golgi complex, and vacuoles. The endomembrane system is a dynamic, interconnected network of internal membranes (nuclear membrane, ER, Golgi, lysosomes, vesicles) where membranes and materials flow between compartments. This constant exchange maintains the dynamic nature of the cell.
Mitochondria and chloroplasts originated about 1.8 billion years ago through mutualistic endosymbiosis. An archaeal cell took up a bacterial cell, which evolved into a mitochondrion. A second endosymbiotic event, involving free-living cyanobacteria, led to chloroplasts, setting the stage for algae and plants. Evidence for this theory includes: both organelles have their own circular DNA, replicate via binary fission like prokaryotes, use their own ribosomes to synthesize proteins (which resemble bacterial ribosomes), and possess two membranes (the outer being a vestige of an endocytotic vesicle).
The nucleus stores and protects DNA, which is organized into chromosomes or diffused as chromatin. The nucleolus within the nucleus assembles ribosomes. The nuclear membrane has pores for molecular exchange (e.g., mRNA and transcription factors). Ribosomes (free or bound to the rough ER) translate mRNA into proteins. Mitochondria convert food energy into ATP, featuring a folded inner membrane maximizing surface area for ATP synthesis. The endoplasmic reticulum (ER) has two forms: rough ER (with ribosomes) synthesizes proteins for organelles, membranes, or export, while smooth ER (without ribosomes) synthesizes lipids, detoxifies, and metabolizes carbohydrates. The Golgi complex modifies, packages, and sends proteins via vesicles. Lysosomes (in animal cells) contain hydrolytic enzymes for intracellular digestion, recycling, and programmed cell death. The cytoskeleton provides internal support and facilitates cell movement and internal transport. Centrosomes (with centrioles) create spindle fibers for chromosome separation. Plant cells uniquely feature a large central vacuole for water storage, macromolecule management, waste sequestration, and maintaining turgor pressure. Chloroplasts, with their own DNA and ribosomes, conduct photosynthesis. The plant cell wall, primarily composed of cellulose, acts as a pressure vessel to prevent overexpansion from water inflow, maintaining cell structure and plant rigidity.
The cell membrane acts as a selectively permeable boundary, allowing only specific substances to pass, which is vital for cell regulation. Phospholipids, the primary components, have a hydrophobic tail and a hydrophilic head. In water, they form a bilayer, which is the basic framework of the membrane. The fluid mosaic model describes the membrane as a dynamic structure composed of phospholipids, proteins, and cholesterol in motion. Proteins are integrated into the membrane in three main ways: transmembrane proteins span the entire bilayer, integral proteins embed partially, and peripheral proteins attach to the surface.
Diffusion is the passive movement of molecules from high to low concentration. Passive transport, requiring no cellular energy, includes simple diffusion (for small, nonpolar molecules like oxygen and steroids) and facilitated diffusion (for polar molecules and ions using protein channels). Active transport, however, pumps molecules against their concentration gradient, requiring cellular energy (e.g., ATP or electron flow to create ATP). Bulk transport mechanisms, endocytosis (membrane engulfs particles) and exocytosis (vesicles release contents), also require energy and cytoskeletal involvement. Membrane potential is an electrical charge across a membrane created by cells actively pumping ions, forming a voltage difference (e.g., in mitochondria and nerve cells), which is crucial for processes like ATP synthesis and nerve impulses.
Osmosis is the diffusion of water from a hypotonic (higher water, lower solute concentration) to a hypertonic (lower water, higher solute concentration) solution. This principle can be illustrated by a gummy bear expanding in water as water flows into its sugar-rich interior. In plants, a hypertonic environment causes water to leave the cell, leading to plasmolysis and wilting, while a hypotonic environment causes water to enter, creating turgor pressure that keeps the cell firm. Animal cells in a hypertonic environment shrivel, in an isotonic environment are stable, and in a hypotonic environment can burst due to lacking a cell wall. Freshwater protists like Paramecium use contractile vacuoles to expel excess water entering by osmosis, maintaining osmoregulation. Plant stomata, regulated by guard cells, open when water is available and potassium ions are pumped in, making guard cells hypertonic, causing water to follow and the cells to buckle, enabling gas exchange and water vapor release. Conversely, stomata close when water is scarce.
Water potential (Ψ) is a quantitative measurement of water's tendency to move. It's calculated as solute potential (Ψs) plus pressure potential (Ψp). Adding solute decreases water potential, while adding pressure increases it. Water always flows from areas of higher water potential to lower water potential. For example, adding solute to one side of a U-tube lowers its water potential, causing water to move to that side. Similarly, water moves from pure water (higher water potential) into a potato cell (lower water potential), causing the cell to expand. This concept is used in lab settings to quantify water movement based on solute and pressure changes.