Mechanisms of Transport Across Cell Membranes

Categories of Plasma Membrane Transport

The plasma membrane regulates the movement of substances into and out of the cell through various transport mechanisms. These can be broadly classified based on energy requirements and the involvement of carrier proteins.

• Noncarrier-mediated transport:

  • Simple diffusion of lipid-soluble molecules

  • Simple diffusion of ions through nonspecific channels

  • Simple diffusion of water (osmosis)

• Carrier-mediated transport:

  • Facilitated diffusion

  • Active transport

Energy Involvement in Membrane Transport

• Passive transport: Molecules move from higher to lower concentration without using metabolic energy.

• Active transport: Molecules move from lower to higher concentration using ATP and specific carrier pumps (e.g., Na+/K+ and Ca2+ pumps).

Introduction to Carrier-Mediated Transport

Carrier proteins facilitate the movement of substances across the membrane by changing shape. Unlike channels, which provide open passageways, carriers bind specific molecules and undergo conformational changes to transport them.

• Examples: Amino acids, glucose, and other organic molecules.

• Carrier proteins are specific to the molecules they transport.

• Characteristics of carriers:

  • Specificity: Each carrier is specific to a given molecule.

  • Competition: Similar molecules may compete for the same carrier.

  • Saturation: The number of carriers is limited, leading to a maximum rate of transport.

Facilitated Diffusion

Facilitated diffusion is a passive process powered by the random movement of molecules, requiring no ATP. It involves the net movement of molecules from high to low concentration via specific carrier-mediated proteins.

• Transport proteins may be permanently present or inserted when needed.

• Example: Glucose Transport (GLUT proteins)

  • GLUT1: CNS

  • GLUT2: Pancreatic beta cells & hepatocytes

  • GLUT3: Neurons

  • GLUT4: Adipose tissue & skeletal muscles (insulin-regulated)

Active Transport

Active transport moves molecules from areas of low concentration to high concentration, requiring energy (ATP). Carrier proteins involved in active transport are called pumps.

• Primary active transport: Direct use of ATP to transport molecules (e.g., Na+/K+ pump, Ca2+ pump).

• Secondary active transport: Uses the energy from the movement of one molecule down its gradient to drive the movement of another molecule up its gradient.

Primary Active Transport: Na+/K+ Pump

• Found in all body cells.

• ATPase enzyme pumps 3 Na+ out of the cell and 2 K+ into the cell.

• Functions:

  1. Maintains ionic gradients necessary for electrochemical impulses in neurons and muscle cells.

  2. Maintains osmolality.

  3. Provides energy for coupled transport of other molecules.

Primary Active Transport: Ca2+ Pump

• Located on all cell membranes and in the endoplasmic reticulum of striated muscle cells.

• Removes Ca2+ from the cytoplasm by pumping it into the extracellular fluid or cisternae of the ER.

• Creates a strong concentration gradient (high outside, low inside) for rapid movement of Ca2+ back into the cell.

• Essential for neurotransmitter release at synapses and muscle contraction.

Bulk Transport

Bulk transport is required for the movement of large molecules across the plasma membrane.

• Endocytosis: Movement of large molecules (e.g., cholesterol) into the cell.

• Exocytosis: Movement of large molecules (e.g., neurotransmitters) out of the cell.

Introduction to the Nervous System

Terminology Pertaining to the Nervous System

Introduction to the Nervous System

• Divided into:

  • Central nervous system (CNS): Brain and spinal cord

  • Peripheral nervous system (PNS): Cranial and spinal nerves, and ganglia

• Tissue is composed of two types of cells:

  • Neurons: Conduct electrical activity (impulses); in adults, typically lack the ability to divide.

  • Glial cells (neuroglia): Support neurons, do not conduct electrical impulses, but retain the ability to divide.

Neurons: General Concepts

• Structural and functional units of the nervous system.

• Features and functions:

  • Generate and conduct electrical activity.

  • Release neurotransmitters for neuronal communication.

  • Depending on their role, neurons can sense external sensory information (sensory neuron), send motor inputs (motor neuron), or act as interneurons.

• Enable perception of sensory stimuli, learning, memory, and control of muscles and glands.

Neuron Structure

• Dendrites (input): Receive signals and conduct graded impulses toward the cell body.

• Cell body: Contains the nucleus and organelles; integrates graded impulses.

• Axon (output): Conducts action potentials away from the cell body; can be covered in myelin with nodes of Ranvier.

• Synapse: Junction where one neuron communicates with another.

Functional Classification of Neurons

• Sensory neurons (afferent): Conduct impulses from sensory receptors to the CNS.

• Motor neurons (efferent): Conduct impulses from the CNS to target organs.

  • Somatic motor neurons: Control voluntary movements.

  • Autonomic motor neurons: Regulate involuntary functions.

• Interneurons: Located entirely within the CNS; integrate nervous system functions.

Axons

• Conduct action potentials away from the cell body.

• Vary in length from millimeters to meters.

• Connected to the cell body by the axon hillock, where action potentials are generated.

• Can form branches called axon collaterals.

• Covered in myelin with nodes of Ranvier.

Classification of Bundles of Axons

• Nerves: Bundles of axons located in the PNS.

• Tracts: Bundles of axons in the CNS.

• Most nerves are mixed (sensory and motor); some cranial nerves have only sensory fibers.

Neuroglial Cells and Their Functions

Myelin Sheath

• Produced by oligodendrocytes in the CNS and Schwann cells in the PNS.

• One oligodendrocyte can myelinate several axons.

Demyelinating Diseases

• Guillain-Barre syndrome: Immune attack on PNS myelin sheaths, causing muscle weakness.

• Multiple sclerosis: Autoimmune attack on CNS myelin sheaths, leading to demyelination.

Neuroregeneration in the PNS

• When an axon in the PNS is cut, Schwann cells form a regeneration tube.

• Growth factors (neurotrophic factors) stimulate axon regrowth.

• New axon eventually reconnects to the target.

Neurotrophic Factors or Neurotrophins

• Secreted proteins that promote survival, differentiation, and growth of neurons.

• Examples:

  • Nerve growth factor (NGF)

  • Brain-derived neurotrophic factor (BDNF)

  • Glial-derived neurotrophic factor (GDNF)

  • Neurotrophin-3, neurotrophin-4/5

• In adults, maintain sympathetic ganglia (PNS) and support sensory neuron regeneration.

CNS Regeneration

• Limited regeneration in the mature CNS compared to the PNS.

• Proteins such as Nogo (produced by oligodendrocytes) inhibit axon regeneration.

• Glial scars from astrocytes also prevent regeneration.

Electrical Activity in Neurons

Resting Membrane Potential

• Neurons have a resting potential of approximately .

• Established by the imbalance of charged ions across the membrane and large negative molecules inside the cell.

• Maintained by Na+/K+ pumps:

  • 3 Na+ out, 2 K+ in

• At rest:

  • Electrical gradient: more negative inside

  • Concentration gradient: [K+] higher inside, [Na+] higher outside

Altering Membrane Potential

• Neurons and muscle cells can change their membrane potentials (excitability).

• Changes are caused by altered permeability to certain ions.

• Ions move according to their electrochemical gradient (concentration gradient + attraction to opposite charges).

• Ion currents occur where ion channels are located.

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