Summary
Highlights
Proteins have four levels of structure. Primary: the linear sequence of amino acids (coded by DNA). Secondary: local folding into alpha helices or beta-pleated sheets, stabilized by hydrogen bonds between backbone atoms. Tertiary: the overall 3D shape, determined by interactions between R-groups (ionic, hydrogen, disulfide bonds, hydrophobic interactions). Quaternary: multiple polypeptide chains combining (e.g., collagen, hemoglobin).
Amino acids in cell membrane proteins interact with their environment; hydrophilic amino acids face the aqueous regions, while hydrophobic ones face the lipid tails. Channel proteins have polar amino acids lining their pores for polar molecules. Denaturation is the permanent change in a protein's 3D shape, caused by heat or extreme pH, making it lose its function (e.g., cooking an egg).
Carbon forms the backbone of all organic molecules due to its ability to form four bonds, allowing it to create complex structures. Methane (CH4) is a simple example where carbon makes four stable bonds with hydrogen atoms.
Carbohydrates are sugars, typically ending in '-ose' (e.g., glucose, maltose). Monomers are individual subunits (monosaccharides like glucose), dimers are two subunits (disaccharides like maltose or sucrose), and polymers are multiple subunits (polysaccharides like glycogen, starch, cellulose). These are formed by condensation reactions and broken down by hydrolysis. The bond formed between carbohydrates is a glycosidic bond.
Glucose is a hexose sugar and a vital energy source, being soluble and chemically stable. Alpha and beta glucose differ in the orientation of their hydroxyl group at carbon 1. Alpha glucose forms starches (amylose and amylopectin) and glycogen (animal storage), which are helical and compact. Beta glucose forms cellulose, which has inverted alternating molecules, allowing for hydrogen bonding and creating strong, sheet-like structures for plant cell walls, indigestible by humans.
Glycoproteins, with oligosaccharide chains attached to proteins, are important for cell recognition (e.g., blood types). Lipids are non-polar, hydrophobic biomolecules including oils, fats, waxes, and steroids. They are distinguished from carbohydrates by a lower oxygen-to-carbon ratio. Triglycerides (glycerol + three fatty acids) are classic lipids, while phospholipids (glycerol + two fatty acids + phosphate group) are amphipathic, forming cell membranes.
Fatty acids have a carboxyl group at one end and a methyl group at the other. Saturated fatty acids are 'saturated' with hydrogens, having no double bonds. Unsaturated fatty acids have at least one double bond, which creates a bend in the molecule. Cis fatty acids have hydrogens on the same side of the double bond, creating a healthy bend. Trans fatty acids have hydrogens on opposite sides, making them straight and considered unhealthy due to tighter packing.
Triglycerides are for long-term energy storage, releasing twice as much energy per gram as carbohydrates, crucial for mobility. They also act as thermal insulators (e.g., blubber) and shock absorbers. Steroids are characterized by four rings and are also lipids. Phospholipids spontaneously form bilayers, making up cell membranes. (HL content: cholesterol reduces membrane fluidity but prevents stiffening at low temperatures).
Proteins are chains of amino acids, whose order is coded by DNA. Amino acids consist of an alpha carbon, hydrogen, R group (variable), carboxyl group, and amino group. Twenty different amino acids exist, with varying chemical properties. They link via condensation reactions, forming peptide bonds. Essential amino acids must be obtained from diet, while non-essential ones can be synthesized.
Cell membranes are phospholipid bilayers with a non-polar core. Small, non-polar molecules (like O2, CO2) freely diffuse. Polar molecules and ions have low permeability. Four types of membrane transport: simple diffusion (high to low concentration, directly through membrane), facilitated diffusion (high to low concentration, through channel proteins), osmosis (net movement of water from low to high solute concentration), and active transport (low to high concentration, via protein pumps using ATP).
The composition of fatty acid tails affects membrane fluidity. Saturated tails are straight, leading to tightly packed, viscous membranes with higher melting points. Unsaturated tails (especially cis) have bends, making membranes more fluid and flexible at lower temperatures. Cholesterol decreases membrane fluidity but prevents crystallization and fracturing at very low temperatures.
The endomembrane system involves continuous membrane flow. Vesicles (membrane-bound sacs) bud off from organelles like the ER, transport contents (e.g., proteins) to the Golgi apparatus for modification and sorting, and then fuse with the plasma membrane for secretion (exocytosis). Cells also take in substances via endocytosis, forming vesicles to bring external particles into the cell.
Gated ion channels (voltage-gated or neurotransmitter-gated) regulate ion flow across membranes, crucial for nerve impulses. The sodium-potassium pump is a key active transport example, using ATP to pump three sodium ions out and two potassium ions in, maintaining cell potential. Co-transport can also occur, where the gradient created by one pump drives the movement of another molecule (e.g., glucose with sodium).
Cell adhesion molecules (CAMs) help cells form tissues. Organelles are discrete structures within cells; their membrane count (0, 1, or 2) is important. Cell walls, cytoplasm, and cytoskeleton are not considered organelles. Compartmentalization allows for concentrated enzymes and substrates, protection from harmful substances (e.g., lysosomes), and maintenance of specific pH conditions for processes.
Mitochondria have deep infoldings of their inner membrane (cristae) to increase surface area for electron transport. A small intermembrane space allows for quick proton gradient buildup. The matrix contains concentrated enzymes for the Krebs cycle. Chloroplasts have thylakoid stacks (grana) for light absorption and electron transport. A small thylakoid space builds a proton gradient, and the stroma concentrates Calvin cycle enzymes.
The nucleus has a double membrane with nuclear pores for mRNA, ribosomes, and proteins to pass. This double membrane also facilitates its breakdown and reformation during mitosis. Ribosomes on the RER produce proteins for secretion or transport, while free ribosomes produce proteins for internal use. The Golgi apparatus modifies, sorts, and packages proteins received from the RER in vesicles. Clathrin, a three-legged protein, aids vesicle formation and budding.
All human cells originate from a single fertilized egg but differentiate into 220 distinct cell types due to gradients of signaling chemicals that activate specific genes (DNA expression). Stem cells are undifferentiated cells that can divide endlessly and differentiate. Totipotent stem cells (early embryo) can become any cell type; pluripotent (later embryo) can differentiate into many but not all; multipotent (adult) can form a few types. Research focuses on inducing multipotent cells back to pluripotent states for therapy.
Cell size and shape are adapted for function. Sperm are tiny, narrow, and motile for egg reaching. Eggs are large with food reserves for early development. Red blood cells are small and flattened to navigate capillaries. Neurons vary greatly in length for communication. Striated muscle fibers are larger for force generation. Cells must remain small to maintain a large surface area-to-volume ratio and a short diffusion path for nutrient uptake and waste removal.
Red blood cells have flattened indentations to increase surface area-to-volume ratio and reduce diffusion distance. Microvilli and invaginations increase surface area for absorption (intestines). Alveoli in the lungs form grape-like clusters to maximize surface area for gas exchange. Type I alveolar cells are thin for gas diffusion, while Type II cells produce surfactant to prevent alveolar collapse.
Striated skeletal muscle cells are long, multinucleated, and unbranched, attached to bones for movement. Cardiac muscle cells are shorter, branched, and mostly uninucleated, allowing for rapid electrical signal propagation and synchronized heart contraction. Gametes (sperm and egg) are sex cells with haploid nuclei. Sperm are adapted for rapid movement, with a tail, mitochondria-rich midpiece, and an acrosome for egg penetration. Egg cells are large, contain significant food reserves, and have mechanisms (zona pellucida, cortical granules) to prevent polyspermy.