Characteristics of Transport Across Cell Membranes

Fluid Mosaic Model

Cell membrane with a pattern-like structure and flexibility

Integral (intrinsic) membrane proteins

Held in the membrane by hydrophobic interactions, and detergents are needed to remove these. Some are transmembrane proteins

Peripheral membrane

Not embedded in the membrane (removed easily). Loose, non-covalent, hydrogen bonding. Attached to either side of membrane.

Molecular (non-vesicular) proceses

Transport molecules as well as ions can be either passive or active

Passive diffusion

Simple diffusion and facilitated diffusion

Active

Primary/Secondary active transport

Bulk (vesicular) processes

Move ions and small molecules and all are active (need energy)

Simple Diffusion

Unassisted small hydrophobic molecules move from areas of high concentration to areas of low concentration. Exchange of gases across cell membranes

Channel Mediated Diffusion

Channel proteins allowing specific ion or molecule to cross membrane

Molecular Membrane Transport

Small, lipophilic molecules move by simple diffusion across lipid membranes

Aquaporin (AQPs)

Main source of water movement in and out of cell. Can be regulated by changed in pH or the cell can increase/decrease the #.

Facilitated Diffusion

Concentration gradient may exist for molecules or ions but they cannot cross membrane due to hydrophobic core

Facilitated Transport

Proteins allow molecules to cross

Channel-mediated Diffusion

Channel proteins span the membrane, allowing a particular ion or molecule to cross the membrane. Can be regulated by changed in pH

Gated Transport

Diffusion across cell membrane only occurs under certain circumstances

Carrier-mediated diffusion

Proteins change shape to move the target molecule from one side to other of the membrane. Selective in moving only one or a few ions or molecules.

Voltage Gated

Responds to electrical potential across membrane (Na+ and K+ channels in action potentials)

Ligand Gated

Opens by binding another molecules e.g NTs

Gated Carriers

Ligand binding sites change affinity when protein conformation changes. Open one side and close the other side. Move ligand across membrane

Uniport Carriers

Moves single molecule one way

Symport (coupled) carriers

2 Molecules moving one way

Antiport (coupled) carriers

2 molecules moving opposite ways

Carrier Mediated Transport

Specificity, Saturation, Competition

Saturation

Transport can reach maximum rate when all carrier binding sites are filled with substrate

Competition

Ex. Glucose transport decreases with galactose presence

Types of Primary Active Transport

Uniport, Cotransport

Secondary Active Transport

Antiport, Symport

Primary Active Transport

Movement of substance against electrochemical gradient, requiring ATP and requires carrier proteins

Na+/K+ ATPase

3 Na+ bind the pump, stimulating phosphorylation by ATP, causing Na+ to be expelled outside. 2 Extracellular K+ binds protein triggering release of phosphate group restoring protein original conformation, releasing K+ intracellularly.

Ca2+ ATPase

Active transport of calcium out of cell for maintenance of Ca2+ electrochemical gradient across cell membrane

Secondary Active Transport

ATPase pumps establish an ionic gradient, which is the driving force for the second substance. Requires primary transport activation.

Types of Secondary Active Transport

Cotransport systems, counter transport

Cotransport System

Na+ and substance transported move in same direction

Counter Transport

Na+ and transported substance move in opposite directions

Sodium-Glucose Pump

Electrochemical gradient created by Na+/K+ ATPase allows for glucose to be brought into cell. Glucose moves from low to high concentration.

Endocytosis

Plasma membrane folds into cell, forming membrane bound vesicles that enclose substance/fluid into cell.

Exocytosis

Vesicles fuse with plasma memrane and release contents to extracellular fluid

Phagocytosis

Large particles are engulfed into cell creating phagosome then fuses with lysosome containing digestive enzyme

Pinocytosis

Cell takes in substances from the extracellular fluid. Solutes bind cell membrane and then pinched off producing pinosome.

Clatherin-dependent Endocytosis

Molecule binds receptor, triggering catherin coating receptor and plasma membrane. Dynamin cuts of vesicle from membrane and lysosome does not fuse due to coating.

Caveolae Endocytosis

Cholesterol rich membrane has ligand bind receptor causing caveol binding.

Viruses

Evolved to bind to different receptors, causing coating (caveolae), making lysosome move away from vesicle.

Golgi Apparatus

Protein building and molecule sorting

Exocytosis

Vesicles fuse with membrane allowing substances to exit cell

Epithelial Transport

Move from Apical to Basolateral to allow for absorption

Apical (mucosal) Membrane

Faces lumen

Basal Memrbane

Faces ECF

Paracellular transport

Through junctions between adjacent cells

Transcellular transport

Through cells themselves

Absorption

Various ions or molecules from lumen or tubule absorb into ECF (glucose, amino acids)

Secretion

Releasing substances

Transepithelial Absorption of Glucose

Placement of specific transport proteins at the apical and basolateral surface of intestinal epithelium allows unidirectional movement of glucose.

Ion Channels

Provide passive transport of ions across cell membrane using integral membrane proteins

Leak Channels

Channels constantly open

Voltage Gated

Stimulus change is a change in the membrane potential (electrical charge of membrane)

Ligand Gated

Stimulus is presence of chemical messenger, can be a hormone, NT, or other chemical stimulus.

Voltage Gated Ion Channels

Ionotropic channel

Controlled by signaling molecules, open membrane channels and allows ions to enter or leave cell changing electrochemical gradient of cell membrane

Metabotropic Channel

Activates secondary intracellular messenger system, creating metabolic changed within cell