Summary
Highlights
The cell membrane acts as a selectively permeable boundary, controlling passage of substances. It's composed of a phospholipid bilayer, with hydrophobic tails and hydrophilic heads, stabilized by weak bonds. The fluid mosaic model describes the membrane as a dynamic structure of phospholipids, proteins, and cholesterol in constant motion. Proteins are embedded in the membrane as transmembrane, integral, or peripheral proteins, depending on their hydrophobic and hydrophilic regions.
Diffusion is the spontaneous movement of molecules from high to low concentration. Passive transport (no cellular energy required) includes simple diffusion for small nonpolar molecules and facilitated diffusion for polar molecules and ions via protein channels. Active transport moves molecules against their concentration gradient, requiring cellular energy (ATP or electron flow). Bulk transport methods like endocytosis (membrane pinches in) and exocytosis (vesicle fuses with membrane) also require energy and cytoskeletal involvement. Membrane potential, an electrical charge across the membrane, is created by actively pumping ions and is crucial for processes like ATP synthesis in mitochondria and nerve impulse transmission.
Osmosis is the diffusion of water from higher to lower concentration (hypotonic to hypertonic solutions). Water flows from hypotonic (more water, less solute) to hypertonic (less water, more solute) regions. This principle explains the expansion of a gummy bear in water, plasmolysis (wilting) in plant cells in hypertonic environments, maintenance of turgor pressure in hypotonic environments, and lysis or crenation of animal cells. Organisms like Paramecium use contractile vacuoles for osmoregulation in freshwater. Plant stomata, regulated by guard cells, use osmosis to open and close, controlling gas exchange and water loss.
Water potential (Ψ) is a quantitative measure of water's tendency to move. It's determined by solute potential (Ψs) and pressure potential (Ψp). Adding solute decreases water potential, while adding pressure increases it. Water moves from areas of higher water potential to lower water potential. This concept provides a more precise way to understand water movement than just tonicity, especially in situations involving pressure, such as within plant cells or experimental setups.
Cells are the fundamental units of life, characterized by a membrane separating cytoplasm from the exterior, genetic information in DNA, and systems for maintenance and replication. Protein synthesis involves DNA transcription to mRNA and translation by ribosomes into proteins, which act as enzymes, membrane components, or exported substances.
Prokaryotic cells are small and simple, lacking a nucleus and having circular chromosomes with plasmids. They are found in Archaea and Bacteria. Eukaryotic cells are larger and more complex, possessing a nucleus, multiple linear chromosomes with associated proteins, mitochondria, and numerous membrane-bound organelles. Eukaryotes arose from a mutualistic endosymbiosis event approximately 1.8 billion years ago.
Cell size is limited by the need for sufficient surface area to volume ratio to support diffusion of substances in and out. Smaller cells have a much higher surface area to volume ratio, allowing for efficient exchange. As an object grows, its surface area to volume ratio decreases, making diffusion less efficient. This principle explains adaptations like thin tissue sheets (fish gills) and highly folded surfaces (mitochondrial membranes, intestinal villi) to maximize surface area, and why large marine mammals better conserve heat in cold environments.
Cellular compartmentalization, the internal division of space, offers advantages like creating distinct chemical environments (e.g., lysosomes with hydrolytic enzymes) and providing abundant internal surface area for membrane-bound enzymes and ribosomes. Eukaryotic cells are highly compartmentalized with many internal membranes, forming the endomembrane system. This dynamic system includes the nuclear membrane, ER, Golgi, lysosomes, and vesicles, with continuous flow and exchange of membrane and material.
Mitochondria and chloroplasts originated through mutualistic endosymbiosis. An ancestral archaeal cell engulfed a bacterial cell, which evolved into the mitochondrion, leading to eukaryotic cells. Later, a second endosymbiotic event involved a eukaryotic cell engulfing a cyanobacterium, which became the chloroplast, giving rise to algae and plants. Evidence for this includes their circular DNA, binary fission replication, unique ribosomes, and double membranes.
The nucleus stores and protects DNA, organized into chromatin or chromosomes, and contains the nucleolus (ribosome assembly). Ribosomes (free or bound) translate mRNA into proteins. Mitochondria convert food energy to ATP. The ER (rough and smooth) synthesizes proteins (rough ER) and lipids, detoxifies, and metabolizes carbohydrates (smooth ER). The Golgi complex modifies and packages proteins into vesicles. Lysosomes (animal cells) contain hydrolytic enzymes for intracellular digestion and apoptosis. The cytoskeleton provides structure and enables cell movement. Centrosomes (animal cells) organize spindle fibers for cell division. Plant cells feature a large central vacuole for water storage, waste sequestration, and maintaining turgor pressure, chloroplasts for photosynthesis, and a cell wall composed primarily of cellulose for structural support and preventing overexpansion.