IndietroAnatomy & Physiology: Cell Structure and Membrane Transport
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Cell theory states that the cell is the smallest unit of life, all organisms are made of one or more cells, and cells arise only from other cells.
The three basic parts are: plasma membrane (flexible outer boundary), cytoplasm (intracellular fluid containing organelles), and nucleus (DNA-containing control center).
Substances outside cells including extracellular fluids (interstitial fluid, blood plasma, cerebrospinal fluid), cellular secretions (saliva, mucus), and extracellular matrix that holds cells together.
The plasma membrane is a phospholipid bilayer with embedded proteins, cholesterol for stiffness, and surface sugars forming the glycocalyx.
Cholesterol is located between phospholipid tails and stabilizes the membrane while decreasing its fluidity.
Integral proteins span the membrane and function as transporters, enzymes, or receptors. Peripheral proteins are loosely attached and function as enzymes, motor proteins, or in cell-to-cell connections.
Functions include: (a) transport, (b) receptors for signal transduction, (c) enzymatic activity, (d) cell-cell recognition, (e) cell-to-cell joining, and (f) attachment to cytoskeleton and extracellular matrix.
The glycocalyx is a carbohydrate-rich coating on the cell surface that acts as a biological marker for cell recognition and immune system identification of self vs. nonself.
Tight junctions form impermeable seals; desmosomes are anchoring junctions that prevent tearing; gap junctions allow communication via tunnels for small molecules.
Passive transport requires no energy and includes diffusion (movement down concentration gradient) and osmosis (water movement across membrane).
Speed is influenced by concentration gradient, molecular size, and temperature.
The membrane allows some substances to pass while blocking others, permitting nutrients in, wastes out, and maintaining concentration gradients.
Simple diffusion is unassisted movement of lipid-soluble or small molecules through the bilayer. Facilitated diffusion uses carrier or channel proteins to transport water-soluble or large molecules.
Osmosis is the movement of water across a selectively permeable membrane from low solute (high water) concentration to high solute (low water) concentration, often through aquaporins.
Tonicity is a solution's ability to change cell shape by altering water volume. Isotonic solutions cause no change, hypertonic cause cell shrinkage (crenation), and hypotonic cause swelling and possible bursting (lysis).
Active transport moves solutes against their concentration gradient using energy (ATP) because solutes may be too large, not lipid soluble, or moving from low to high concentration.
The pump uses ATP to move 3 Na+ ions out and 2 K+ ions into the cell, maintaining electrochemical gradients essential for muscle and nerve function.
Secondary active transport uses energy stored in ion gradients created by primary active transport to move other substances via cotransport proteins (symporters or antiporters).
Vesicular transport moves large substances in vesicles using energy. Types include endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis.
RMP is the voltage across the plasma membrane due to separation of charges, typically -50 to -90 mV, with the inside of the cell more negative than outside.
K+ diffuses out through leakage channels down its concentration gradient, making the inside more negative, but is pulled back by the electrical gradient, balancing RMP.
CAMs anchor cells to each other and the extracellular matrix, assist cell movement, attract white blood cells to injury, regulate junctions, and transmit intracellular signals.
They serve as binding sites for chemical signals, enabling contact signaling and chemical signaling that alter cellular activities.
Cytoplasm includes cytosol (gel-like fluid), inclusions (insoluble molecules), and organelles (specialized structures, membranous or nonmembranous).
Mitochondria produce most of the cell's ATP via aerobic respiration and contain their own DNA and ribosomes.
Ribosomes are protein synthesis sites; free ribosomes make soluble proteins, membrane-bound ribosomes make proteins for membranes or export.
Rough ER has ribosomes and synthesizes proteins; smooth ER synthesizes lipids, detoxifies chemicals, stores calcium, and metabolizes carbohydrates.
Golgi apparatus modifies, concentrates, and packages proteins and lipids from rough ER for transport.
Lysosomes contain digestive enzymes that break down bacteria, toxins, nonfunctional organelles, and glycogen; they can cause autolysis if released inside the cell.