Biology: Concentration Gradient and Aquaporins

Concentration Gradient

• Concentration refers to the amount of a substance in a given area
• Concentration gradient is the difference in concentration between two areas
• Areas with high concentration are considered concentrated
• Areas with low concentration are considered dilute
• Substances move from areas of high concentration to areas of low concentration
• The greater the difference in concentration, the steeper the concentration gradient
• The steeper the gradient, the faster the movement of substances

Aquaporin

• Aquaporins are transmembrane proteins
• They form channels that allow water molecules to pass through the cell membrane
• Aquaporins are highly specific for water molecules
• They have three features that contribute to their specificity:
  • Size restriction: Aquaporin channels are narrow, allowing only water molecules to pass through
  • Electrostatic forces: Arginine 195 residue in the channel repels positively charged ions, such as H3O+
  • Water dipole: The shape of the aquaporin channel causes water molecules to reorient, improving size and charge specificity

Active Transport

• Active transport is the movement of molecules across a cell membrane against their concentration gradient
• Requires energy input to move molecules against their concentration gradient
• Two main types:
  • Primary active transport: Directly uses ATP for energy
  • Secondary active transport: Uses the electrochemical gradient of another molecule to drive transport

ATPases

• Active transport relies on ATPases
• ATPases are enzymes that can hydrolyze ATP to release energy
• The energy released from ATP hydrolysis drives the movement of molecules across the cell membrane

Cell Form vs. Cell Function

• The shape and structure of a cell are directly related to its function
• Different cell types have different shapes and structures that allow them to perform specific tasks

Major Cell Types

• There are over 200 different types of cells in the human body, estimated to be 35 trillion cells
• Four major classes:
  • Neurons: Responsible for communication within the nervous system
  • Muscle cells: Responsible for movement
  • Epithelial cells: Line and protect organs and cavities
  • Connective cells: Support and connect different tissues

Neuron Cell Body

• Also known as the soma or perikaryon
• Biosynthetic center of the neuron, responsible for protein synthesis, membrane production, and chemical production
• Contains the nucleus, nucleolus, and rough endoplasmic reticulum (chromatophilic substance or Nissl bodies)
• The plasma membrane acts as a receptive region, receiving input from other neurons
• Most neuron cell bodies are located in the central nervous system (CNS)
• Clusters of neuron cell bodies in the CNS are called nuclei

Cellular Products Need to Move in Neurons

• Neurons can have lengths ranging from 1 mm to over 1 meter
• Energy, waste products, and other cellular components need to be transported between the cell body and axon terminals
• Two main types of axonal transport:
  • Anterograde transport: Movement from the cell body to the axon terminal
  • Retrograde transport: Movement from the axon terminal to the cell body
• Slow axonal transport: Moves materials relatively slowly, typically for structural components of the axon
• Fast axonal transport: Moves materials rapidly, typically for neurotransmitters and other cellular components
• Both rely on a microtubule network

Neuroglia

• Support cells of the nervous system
• Outnumber neurons in the nervous system
• Play crucial roles in maintaining the chemical environment of neurons, supporting neuron development, and providing physical support
• Types:
  • Astrocytes: Most numerous glial cells, involved in maintaining the blood-brain barrier, regulating the extracellular chemical environment, guiding neuron development, and synapse formation
  • Oligodendrocytes: Form and maintain myelin sheaths in the central nervous system (CNS)
  • Microglia: Phagocytes that remove cellular debris, damaged cells, and pathogens
  • Ependymal cells: Produce and circulate cerebrospinal fluid (CSF)

Neuroglia: Peripheral Nervous System (PNS)

• Schwann cells: Form and maintain myelin sheaths in the peripheral nervous system (PNS), also participate in PNS axon regeneration

Myelination and Functions of the Myelin Sheath

• Myelin sheath is a fatty, insulating layer that wraps around axons
• Increases the speed of action potential conduction by decreasing ion leakage
• Found in both the central nervous system (CNS) and peripheral nervous system (PNS)
• Formed by:
  • Schwann cells: in the PNS
  • Oligodendrocytes: in the CNS
• The myelin sheath is interrupted at regular intervals by gaps called Nodes of Ranvier
• These gaps allow for the rapid transmission of electrical signals
• The presence of myelin gives white matter (the regions rich in myelinated axons) its characteristic color

Overview of Major Cell Types in Body

• There are approximately 35 trillion cells in the human body
• There are over 200 different cell types
• There are four major classes of cells: neurons, muscle cells, epithelial cells, and connective cells
• Neurons are responsible for transmitting information throughout the body
• Muscle cells are responsible for movement
• Epithelial cells form the lining of organs and cavities
• Connective cells provide support and structure to tissues

Cell Junctions

• Some cells are free-floating and not bound to any other cells, such as blood cells and sperm cells
• Most cells are bound together to form tissues and organs
• Cell junctions are the three main ways cells can be bound to one another: tight junctions, desmosomes, and gap junctions
• Hemidesmosomes and adherens junctions also contribute to cell binding

Cellular Responses to Stress - Adaptation

• Normal cells have a narrow range of functionality
• Physiological adaptations can be metabolic or structural; however, they are reversible

Principles of Bioelectricity

• Cells have an uneven distribution of positive and negative charges along their membranes
• The difference in electrical charges between two regions, such as across the plasma membrane, is known as the electrical gradient
• Excitable cells, like neurons and muscle cells, utilize this electrical gradient to function
• Other cells, like epithelial cells, erythrocytes, hepatocytes, renal, and splanchnic cells, maintain the electrical gradient but do not utilize it directly

Neuron Processes - Axon

• The axon is the conducting region of the neuron
• It generates nerve impulses and transmits them along the axolemma to the axon terminal
• The axon terminal is the region that secretes neurotransmitters into the extracellular space
• Neurotransmitters can either excite or inhibit the neurons they come into contact with
• Axons can carry on many conversations with different neurons simultaneously
• Axons rely on the cell body to renew proteins and maintain function throughout the axon

Cellular Products Need to Move in Neurons

• The distance between the start and end of a single neuron can vary from 1 millimeter to over 1 meter.
• This requires the cell to maintain energy and eliminate waste products throughout the neuron's structure.
• Anterograde and retrograde transport are processes that move materials throughout the cell, using a network of microtubule transport.

Neuroglia

• Astrocytes are the most numerous type of neuroglia
• They help maintain the blood-brain barrier, regulate the extracellular chemical environment, guide neurons during development, and are involved in synapse formation
• Oligodendrocytes form and maintain the myelin sheath in the CNS
• Microglia are phagocytes that remove debris, damaged cells, and pathogens from the CNS
• Ependymal cells produce and assist in the circulation of cerebrospinal fluid (CSF)

Neuroglia: PNS

• Schwann cells form and maintain the myelin sheath in the PNS
• Schwann cells also play a role in PNS axon regeneration
• Myelin sheath increases the speed of conduction of action potentials by acting as electrical insulation.

Myelination and Functions of the Myelin Sheath

• Myelin is found in both the CNS and PNS.
• Oligodendrocytes are responsible for myelination in the CNS, while Schwann cells are responsible for myelination in the PNS.
• Gaps in the myelin sheath are called Nodes of Ranvier.
• The presence of myelin is a characteristic of white matter.

Test your understanding of concentration gradients and aquaporins with this quiz. Explore how substances move across membranes and the specific properties of aquaporins that allow water to pass through cell membranes. Challenge yourself and enhance your knowledge of cellular processes!