Unit 2: Cell Structure and Function

19 terms. Say what each one means, then open the card to check.

• Ribosome

  A complex of RNA and protein that links amino acids into a polypeptide. It is not wrapped in a membrane and is found in every kind of cell.

  Example. A bacterium has no nucleus or organelles bound by membranes, yet thousands of ribosomes in its cytoplasm build its proteins.

• Rough endoplasmic reticulum

  A network of membrane sacs studded with ribosomes. Proteins made there enter the sacs to be folded and sent on toward the Golgi complex.

  Example. Cells in a fictional frog's skin gland that pour out a mucus protein are packed with rough endoplasmic reticulum.

• Golgi complex

  A stack of flattened membrane sacs that modifies, sorts and packages proteins and lipids into vesicles bound for the surface or other organelles.

  Example. A protein that arrives from the rough ER gets sugar groups added in the Golgi complex before a vesicle carries it to the plasma membrane.

• Mitochondrion

  A double-membrane organelle where cellular respiration makes most of a eukaryotic cell's ATP. Its folded inner membrane gives a large surface for that work.

  Example. Flight muscle cells of a hummingbird hold many more mitochondria than the fat cells under its skin.

• Lysosome

  A membrane sac in animal cells that holds digestive enzymes working at acidic pH. It breaks down food particles, worn-out organelles and invaders.

  Example. After a white blood cell engulfs a bacterium, a lysosome fuses with the food vacuole and its enzymes digest the bacterium.

• Surface-area-to-volume ratio

  A cell's membrane area divided by its volume. As a cell grows, volume rises faster than surface area, so the ratio falls and exchange per unit volume slows.

  Example. A cube 1 cm on a side has 6 square cm of surface and 1 cubic cm of volume, a ratio of 6 to 1; a 3 cm cube has 54 to 27, or 2 to 1.

• Fluid mosaic model

  The picture of a membrane as a phospholipid bilayer in which lipids and many proteins drift sideways, with proteins scattered through it like tiles.

  Example. A student bleaches a small spot of glowing tagged proteins on a cell membrane, and within a minute unbleached proteins drift in and the spot glows again.

• Selective permeability

  A membrane lets some substances cross easily and holds others back. Small nonpolar molecules pass the bilayer; ions and large polar molecules need proteins.

  Example. Oxygen gas slips straight through a red blood cell's membrane, while potassium ions cross only through channel or pump proteins.

• Cell wall

  A rigid layer outside the plasma membrane of plants, fungi and most bacteria. It gives shape and resists bursting but is not the selective barrier.

  Example. A lettuce leaf cell in fresh water swells against its cellulose wall, and the wall pushes back so the cell stays firm instead of bursting.

• Facilitated diffusion

  Passive movement of a substance down its concentration gradient through a channel or carrier protein. It uses no energy from the cell.

  Example. Water rushes into a kidney cell through aquaporin channels when the fluid outside it has a lower solute concentration.

• Active transport

  Movement of a substance against its concentration gradient by a protein pump that uses energy, usually from ATP.

  Example. The sodium-potassium pump uses one ATP to push 3 sodium ions out of a nerve cell and bring 2 potassium ions in.

• Cotransport

  A carrier protein uses one ion moving down its gradient to drive a second substance across against that substance's gradient.

  Example. In a gut lining cell, sodium flowing in through a carrier pulls glucose in with it, even when glucose is already more concentrated inside.

• Exocytosis and endocytosis

  Bulk transport by vesicles. In exocytosis a vesicle fuses with the plasma membrane and spills its contents out; in endocytosis the membrane folds in to take material in.

  Example. A gland cell releases stored hormone by exocytosis, while an amoeba takes in a yeast cell by endocytosis.

• Tonicity

  How a solution compares with a cell in solutes that cannot cross the membrane: hypertonic has more, hypotonic has less, isotonic has the same.

  Example. Sea water is hypertonic to the cells of a freshwater fish, so a fish moved into it would lose water from its cells.

• Water potential

  A measure of water's tendency to move, written as pressure potential plus solute potential. Water moves from higher water potential to lower water potential.

  Example. Water moves from soil at -2 bar into a root cell at -5 bar, because -2 bar is the higher value.

• Solute potential

  The part of water potential set by dissolved solutes: -iCRT. It is 0 for pure water and grows more negative as solute concentration rises.

  Example. A 0.2 M sucrose solution at 300 K has a solute potential of -1 x 0.2 x 0.0831 x 300 = -4.986 bar.

• Plasmolysis

  In a hypertonic solution, a walled cell loses water so its plasma membrane shrinks and pulls away from the cell wall.

  Example. Onion skin cells soaked in strong salt water show their purple contents shrunk into a ball in the middle of each rigid wall.

• Compartmentalization

  Internal membranes split a eukaryotic cell into separate spaces, so different reactions can run at once under their own conditions without interfering.

  Example. Acid-loving digestive enzymes work inside lysosomes at about pH 5 while the surrounding cytosol stays near pH 7.

• Endosymbiotic theory

  The idea that mitochondria and chloroplasts began as free-living prokaryotes that were engulfed by an ancestral cell and came to live inside it.

  Example. Chloroplasts carry their own circular DNA and bacteria-like ribosomes and divide by splitting in two, much as a cyanobacterium does.