뒤로Anatomy & Physiology: Cells and Membrane Transport
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Cell theory states that a cell is the structural and functional unit of life, and all cells arise from preexisting cells. The organism's function depends on the activities of its cells.
The three basic parts are: plasma membrane (flexible outer boundary), cytoplasm (intracellular fluid with organelles), and nucleus (DNA-containing control center).
The plasma membrane is a phospholipid bilayer with embedded proteins arranged as a fluid mosaic, controlling entry and exit of substances.
Passive transport types are simple diffusion, facilitated diffusion, and osmosis. They require no energy and move substances down their concentration gradient.
Active transport moves solutes against their concentration gradient using ATP. It is needed when solutes are too large, not lipid soluble, or moving against the gradient.
The Na+/K+ pump uses ATP to move 3 Na+ ions out and 2 K+ ions into the cell, maintaining concentration gradients essential for cell function.
Secondary active transport uses energy stored in ion gradients created by primary active transport to move other solutes, often via symporters.
Vesicular transport includes endocytosis (phagocytosis, pinocytosis, receptor-mediated), exocytosis, transcytosis, and vesicular trafficking within the cell.
The cytoplasm includes cytosol (gel-like fluid), inclusions (insoluble molecules), and organelles (membranous and nonmembranous structures).
Mitochondria produce most of the cell's ATP via aerobic respiration and have their own DNA and ribosomes.
Rough ER has ribosomes and synthesizes proteins for secretion; smooth ER lacks ribosomes and functions in lipid metabolism, detoxification, and steroid synthesis.
The Golgi apparatus modifies, sorts, and packages proteins and lipids from the ER for secretion or use in the cell.
Lysosomes are membranous sacs containing digestive enzymes that break down bacteria, toxins, and nonfunctional organelles.
The cytoskeleton includes microfilaments (actin), intermediate filaments (tough fibers), and microtubules (tubulin tubes) for support and transport.
The centrosome is the microtubule organizing center near the nucleus, containing centrioles that form the basis of cilia and flagella.
Cilia are short, motile extensions that move substances across cell surfaces; flagella are longer and propel the entire cell.
The nuclear envelope is a double membrane with pores that regulate transport between the nucleus and cytoplasm.
Chromatin is DNA and protein in a loose form; it condenses into chromosomes during cell division to protect DNA.
Messenger RNA (mRNA) carries DNA code, ribosomal RNA (rRNA) forms ribosomes, and transfer RNA (tRNA) brings amino acids for protein synthesis.
Protein synthesis occurs in transcription (DNA to mRNA) and translation (mRNA decoded to assemble polypeptides).