Cell Transport

Cell Transport

• Cell transport is the movement of substances through the cell membrane.
• The majority of biochemicals do not pass readily through a phospholipid bilayer, a semi-permeable barrier.

Passive Transport (Diffusion)

• Molecules move down their concentration gradient.
• No cellular energy is required; uses kinetic energy.
• Types of passive transport:
• Simple Diffusion
  • Through the lipid bilayer (e.g., osmosis, diffusion of water)
  • No specific protein is needed
  • Rate of diffusion is directly proportional to the lipid solubility of the substance
• Facilitated Diffusion
  • Through carriers or transporters
  • Specific proteins facilitate diffusion across the membrane
  • No cellular energy is required
  • Carrier/Channel only acts upon specific substrates
  • Rate of transport will reach a maximum based on the number of carriers available in the membrane

Osmosis

• The net movement of water caused by a concentration difference of water across a membrane.
• Water can diffuse through the lipid bilayer and through selective protein channels called aquaporins.
• Aquaporins are highly specialized, with at least 13 types in various cell types of mammals.
• Pressure can also influence water diffusion.
• The amount of pressure required to stop osmosis is called OSMOTIC PRESSURE.

Active Transport

• Molecules move against their concentration gradient.
• Uses cellular energy to move molecules.
• Types of active transport:
• Primary Active Transport
  • Energy is derived directly from the breakdown of ATP or similar.
  • Transport is against a concentration/electrochemical gradient.
• Secondary Active Transport
  • Energy is provided by the concentration gradient of the driving ion.
  • Transporter protein couples the movement of an ion (typically Na+ or H+) down its electrochemical gradient to the uphill movement of another molecule or ion against a concentration/electrochemical gradient.
  • CO-TRANSPORT or SYMPORT: The simultaneous transport of two substances across a membrane in the same direction.
  • COUNTER-TRANSPORT or ANTIPORT: The simultaneous transport of two substances across a membrane in opposite directions.

Transcellular or Epithelial Transport

• In some places of the body, substances must be transported across the cell from one ECF compartment to another.
• Basic mechanism: Active transport through the cell membrane on one side, either simple diffusion or facilitated diffusion through the membrane on the opposite side.

Endocytosis and Exocytosis

• Impermeable molecules can be transported across the membrane by means other than membrane proteins, using part of the membrane itself as a carrier.

• Endocytosis:

• PHAGOCYTOSIS: Ingestion of large particles.

• PINOCYTOSIS: Ingestion of small particles.

• RECEPTOR MEDIATED ENDOCYTOSIS (e.g., Cholesterol).

• Exocytosis:

• CONSTITUTIVE SECRETION: All cells, no signal sequence, proteins are incorporated into the plasma membrane, and extracellular matrix or are signaling proteins.

• REGULATED SECRETION: Specialized cells, need signal to stimulate fusion and release to the cell exterior.### Carrier Proteins

• Responsible for facilitated diffusion of sugars, amino acids, and nucleosides

• Bind to specific molecules, undergo conformational changes, and allow the molecule to pass

• Facilitated diffusion proteins

Channels

• Open pores in the membrane, allowing ions and small hydrophilic molecules to pass
• Many protein channels are highly selective, allowing passage of only one or a few specific ions or molecules
• Selectivity comes from characteristics like diameter, shape, and nature of electrical charges and bonds along the inside surface

Aquaporins

• Water channels through which water molecules can cross the membrane much more rapidly than they can diffuse through the phospholipid bilayer
• Increase speed of diffusion

Ion Channels

• Mediate the passage of ions across the plasma membrane
• The gate of the channel controls ion permeability
• Some are always open (leak channels), while others have gates
• Several kinds of stimuli can open ion-gated channels, including changes in voltage, ligand binding, and mechanical stimuli

Voltage-Gated Channels

• Present in the plasma membrane of all excitable cells (nerve, muscle, endocrine, and egg cells)
• Responsible for the neuron's ability to transmit information along its length and to release neurotransmitters
• There is a range of membrane potentials that cause them to open
• Channel's threshold is the minimum membrane potential that causes the opening of the channel

Ligand-Gated Channels

• Found in the membrane of skeletal muscle cells, some neurons of the autonomic nervous system, and brain
• Channel opens in response to ligand binding
• Example: Acetylcholine released from a neuron binds to Nicotinic Acetylcholine Receptor in skeletal muscle cells at the neuromuscular junction

Active Transport

• Primary active transport: energy is derived directly from the breakdown of ATP or similar
• Transport is against a concentration/electrochemical gradient

Secondary Active Transport

• Energy is provided by the concentration gradient of the driving ion
• Transporter protein couples the movement of an ion (typically Na+ or H+) down its electrochemical gradient to the uphill movement of another molecule or ion against a concentration/electrochemical gradient

Co-Transport and Counter-Transport

• Co-transport (or symport): the simultaneous transport of two substances across a membrane in the same direction
• Example: Na+-D-Glucose co-transporter (SGLT-1)
• Counter-transport (or antiport): the simultaneous transport of two substances across a membrane in opposite directions
• Example: Na+ H+ antiporter in the kidney

Transcellular or Epithelial Transport

• In some places of the body, substances must be transported across the cell from one ECF compartment to another
• Basic mechanism: active transport through the cell membrane on one side, and either simple diffusion or facilitated diffusion through the membrane on the opposite side

Endocytosis and Exocytosis

• Impermeable molecules can be transported across the membrane by means other than membrane proteins
• Using part of the membrane itself as a carrier

Endocytosis

• Phagocytosis: ingestion of large particles
• Pinocytosis: ingestion of small particles
• Receptor-mediated endocytosis: example, cholesterol

Exocytosis

• Constitutive secretion: all cells, no signal sequence, proteins are incorporated into the plasma membrane, and extracellular matrix or are signaling proteins
• Regulated secretion: specialized cells, need signal to stimulate fusion and release to the cell exterior