Cell Membrane Transport, Membrane Potential, and Neuron Structure: Study Notes for Anatomy & Physiology

Cell Membrane Transport

Overview of Cell Membrane Transport

Cell membrane transport refers to the mechanisms by which substances move across the selectively permeable cell membrane. These processes are essential for maintaining cellular homeostasis and function.

• Passive transport: Movement of substances without energy input from the cell (no ATP required).

• Active transport: Movement of substances requiring cellular energy, usually in the form of ATP.

• Cytosol: Fluid portion inside the cell.

• Extracellular Fluid (ECF): Fluid outside the cell.

• Concentration gradient: Difference in concentration of a substance between two areas.

Diffusion

Diffusion is the random movement of molecules from an area of higher concentration to an area of lower concentration, driven by kinetic energy.

• Simple diffusion: Passive movement down a concentration gradient.

• Equilibrium: Movement continues until concentrations are equal, but molecular movement persists.

Rate of Diffusion

The rate at which diffusion occurs depends on several factors:

• Concentration gradient: As gradient increases, rate increases.

• Molecular size: As size increases, rate decreases.

• Distance: As distance increases, rate decreases.

• Temperature: As temperature increases, rate increases.

Membrane Proteins

Membrane proteins facilitate transport across the cell membrane.

• Channel proteins: Form tubes for molecules to pass through.

• Carrier proteins: Bind and move molecules across the membrane.

• Pumps: Use energy (ATP) to move solutes against their concentration gradient (e.g., Na+/K+ ATPase pump).

Types of Transport

Transport mechanisms can be classified based on energy requirement, direction, and type of molecules moved.

Type of Transport	Plasma Membrane Protein	Energy Source	Direction	Types of Molecules Moved
Simple Diffusion (passive)	No	Solute kinetic energy	Down concentration gradient	Lipid soluble (non-polar), oxygen, carbon dioxide
Facilitated Diffusion (passive)	Yes	Solute kinetic energy	Down concentration gradient	Polar: Na+, K+, Ca2+, glucose, amino acids
Primary Active Transport	Yes	ATP	Against concentration gradient	Na+ in, K+ out
Secondary Active Transport	Yes	ATP (indirect), concentration gradient of another substance	One substance down, another up its gradient	Glucose, Cl-, bicarbonate ions, H+
Carrier + symport (cotransport)	Yes	Concentration gradient	Both molecules move in same direction	Ex: Na+ in and glucose in
Carrier + antiport (counter-transport)	Yes	Concentration gradient	Molecules move in opposite directions	Ex: Na+ in and H+ out

Endocytosis and Exocytosis

Bulk Transport Mechanisms

Cells use endocytosis and exocytosis to move large molecules or particles across the membrane.

• Phagocytosis: "Cell eating"; large particles (e.g., bacteria, debris) are engulfed.

• Pinocytosis: "Cell drinking"; small molecules are taken in via vesicles.

• Receptor-mediated endocytosis: Specific molecules are taken in using membrane receptors (e.g., cholesterol, iron, hormones).

• Exocytosis: Release of substances (e.g., hormones, neurotransmitters, enzymes) out of the cell.

Osmosis and Tonicity

Osmosis

Osmosis is the movement of water across a selectively permeable membrane from an area of lower solute concentration to higher solute concentration.

• Water crosses, but solute cannot.

• Water follows solute: Water moves toward higher solute concentration.

Tonicity and Water Movement

Tonicity describes the effect of a solution on cell volume, based on osmolarity.

• Isotonic: Solution/ECF has same concentration as cell (300 mOsm); no net water movement.

• Hypotonic: Solution/ECF has lower concentration than cell (<300 mOsm); water moves into cell, cell swells.

• Hypertonic: Solution/ECF has higher concentration than cell (>300 mOsm); water moves out of cell, cell shrinks.

Solution/ECF	Tonicity of solution/ECF	Water movement
300 mOsm	Isotonic	No net movement
500 mOsm	Hypertonic	Water moves out of cell
200 mOsm	Hypotonic	Water moves into cell

Osmolarity and Solute

Osmolarity is the concentration of solute particles per liter of solution (Osm).

• 1 mole of substance always has the same number of molecules ($6.02 \times 10^{23}$).

• Molarity: moles/liter.

• Osmolarity: concentration of solute particles/liter.

• Some substances split in solution (e.g., NaCl splits into Na+ and Cl-).

• 1 mole NaCl = 2 Osm (Na+ + Cl-).

• 1 mole glucose = 1 Osm (does not split).

Membrane Potential and Gradients

Membrane Potential

Membrane potential is the electrical gradient caused by separation of charges across the cell membrane.

• Cell membrane allows separation of electrical charge.

• Inside cell is more negative; outside is more positive.

• Most of cell and ECF is neutral.

Ion Concentrations and Gradients

Ion concentrations are unequal across the membrane, creating gradients.

• Inside cell: high K+, low Na+

• Outside cell: high Na+, low K+

• Role of Na+/K+ pump: maintains these gradients by pumping Na+ out and K+ in.

Ion	Concentration In Cell	Concentration Out of Cell	Chemical Gradient	Electrical Gradient	Electrochemical Gradient	Equilibrium Potential (mV)
Na+	Low	High	Inward	Inward	Inward	+60
K+	High	Low	Outward	Inward	Outward (net)	-94

Additional info: Equilibrium potentials are inferred based on standard values for Na+ and K+ in neurons.

Gradients

• Electrical gradient: Charge inside cell is negative; opposite charges attract, ions move into cell.

• Chemical gradient: Ions move from high to low concentration.

• Electrochemical gradient: Sum of electrical and chemical gradients for an ion.

Gated Channels

Gated channels regulate ion movement across the membrane.

• Leak channels: Always open, allow ions to move with their gradients.

• Ligand-gated channels: Open in response to binding of a molecule (ligand).

• Voltage-gated channels: Open in response to changes in membrane potential.

Equilibrium Potential

Equilibrium potential is the membrane potential at which the electrical gradient equals and opposes the chemical gradient for a specific ion.

• For K+: $E_K \approx -94$ mV

• For Na+: $E_{Na} \approx +60$ mV

Neuron Structure

Components of a Neuron

Neurons are specialized cells for communication in the nervous system. Their structure supports the generation and conduction of electrical signals.

• Cell body (soma): Carries out metabolic functions.

• Axon: Generates and conducts action potentials; usually carries information away from cell body.

• Dendrite: Receives information from other neurons and carries it to cell body.

• Myelin sheath: Insulating membrane that speeds up conduction of action potentials.

• Myelin sheath gap (Node of Ranvier): Gaps in myelin sheath surrounding axon.

• Axon terminal: Contains vesicles with neurotransmitters.

• Axon hillock: Cone-shaped region of cell body where axon originates.

• Initial segment (trigger zone): Generates action potential.

Example: Neuron Function

When a neuron receives a signal, the dendrites transmit it to the cell body. If the signal is strong enough, the axon hillock initiates an action potential, which travels down the axon to the axon terminals, where neurotransmitters are released to communicate with other cells.

Additional info: Standard values for equilibrium potentials and neuron structure terminology have been added for completeness.