Ch 6: Biological membranes

Integral proteins

Proteins that are deeply embedded in the lipid bilayer, often spanning the entire membrane (transmembrane).

Peripheral proteins

Proteins located on the membrane surface that non-covalently interact with integral proteins or phospholipids.

Where carbohydrates are located on the plasma membrane

As glycolipids or glycoproteins on the outer plasma membrane surface.

Functions of a membrane

Two major functions include providing structural components and signaling.

Membrane fluidity

Affected by the composition of saturated or unsaturated lipids, cholesterol, and temperature.

Unsaturation in fatty acids

Unsaturation (presence of double bonds) in fatty acids tends to increase membrane fluidity.

Saturation in fatty acids

Absence of double bonds in fatty acids tends to decrease membrane fluidity.

Cholesterol's effect on fluidity

Cholesterol can increase or decrease fluidity depending on the membrane structure.

Temperature and fluidity

Higher temperatures generally increase fluidity, while lower temperatures decrease it.

Factors affecting transport rate

Factors include the size of the molecule, electric charge, lipid solubility, and concentration gradient.

Nonpolar small molecules

Molecules like O2 and CO2 that can easily diffuse through the lipid bilayer.

Polar molecules

Molecules like glucose, charged ions (e.g., Na+, Cl-), and macromolecules like nucleotides and amino acids that have a low rate of movement through the lipid bilayer.

Diffusion

Movement from higher to lower concentration until equilibrium is reached.

Osmosis

Diffusion of water through a membrane, limited by the lipid bilayer but greatly increased by aquaporins.

Water movement in osmosis

Water moves from solutions with high water concentration (low solute concentration) to solutions with low water concentration (high solute concentration).

Isotonic solution effects

Animal cells remain stable; plant cells are flaccid.

Hypertonic solution effects

Animal cells shrivel; plant cells are plasmolyzed.

Hypotonic solution effects

Animal cells undergo lysis; plant cells become turgid.

Passive transport

Transport that does not require energy, where molecules move from high concentration to low concentration.

Ion Channel Proteins

Proteins that create aqueous pores for ion movement, with selectivity depending on pore size and structure. They can be opened or closed and are found in all cells, especially nerve and muscle cells.

Facilitated transport proteins

Carrier proteins that involve conformational changes to move specific molecules (solutes). The rate of transport increases with solute concentration until the carrier proteins become saturated.

Active transport

Transport that uses energy (e.g., from ATP, light, or electrons) to move solutes against their concentration gradient.

Electrochemical gradient

Charge separation and voltage differences across a membrane, driven by chemical reactions, often created by transporting ions.

Na+/K+ pump protein

Uses 1 ATP to move 3 Na+ ions out of the cell and 2 K+ ions into the cell across the plasma membrane.

Na+/K+ pump cycle

Involves ATP binding to the cytoplasmic surface

ATP hydrolysis to ADP and Pi

phosphorylation of the protein causing conformational changes

Na+ release, K+ binding

Pi cleavage causing more conformational changes

K+ release.

Cellular communication

The Na+/K+ pump creates a negative charge inside the cell, useful for nerve communication, muscle contraction, and co-transport of other molecules.

H+ pumps

Proton pumps in bacteria, plants, and fungi that create an H+ electrochemical gradient across the plasma membrane. It sets up a negatively charged interior and a high H+ concentration in the extracellular fluid. ATP hydrolysis provides energy for this action.

Co-transporter

An active transport system where the transport of one active solute is coupled to the transport of another molecule.

Co-transporter movement

The energy for the co-transport comes from the electrochemical gradient of the actively transported ion (e.g., H+ moving down its concentration gradient).

The co-transporter can move the ion and solute in the same direction (symport) or in the opposite direction (antiport).

Exocytosis

The process where vesicles from the Golgi apparatus fuse with the plasma membrane to release contents outside the cell. Examples include secretion of pancreatic hormones, secretory proteins from gland cells, and viruses replicating in a host cell.

Endocytosis

The process where the plasma membrane pinches off to internalize materials from the external environment.

Phagocytosis

A type of endocytosis that involves engulfing large particles or whole bacteria.

Pinocytosis

A type of endocytosis that involves engulfing extracellular fluid by 'gulps'.

Receptor-mediated endocytosis

A process where specific receptor proteins recognize large molecules and internalize them into the cell. Many viruses use this mechanism to enter host cells.