Cell Membranes, Transport, and Cellular Energy: Study Notes for Anatomy & Physiology

Cell Chemistry & Cell Components

Fluid Compartments of the Body

The body is composed of various fluid compartments, primarily the intracellular fluid (ICF) and extracellular fluid (ECF). These compartments are separated by cell membranes, which regulate the movement of substances between them.

Chemical Bonds in Biology

• Ionic Bonds: Formed when electrons are transferred from one atom to another, resulting in charged ions (e.g., Na+ and Cl- in table salt).

• Covalent Bonds: Formed when atoms share electrons (e.g., H2 molecule).

• Polar vs. Nonpolar: Polar covalent bonds have unequal sharing of electrons (e.g., O-H bond in water), leading to partial charges. Nonpolar covalent bonds share electrons equally.

• Hydrophilic: Substances attracted to water.

• Hydrophobic: Substances that repel water.

Cell Membrane Structure

The Phospholipid Bilayer

The plasma membrane is a selectively permeable barrier between the ECF and cytosol. It is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Proteins: Serve as channels, receptors, and carriers, each with unique properties.

• Cholesterol: Stabilizes the membrane during temperature changes.

• Carbohydrates: Glycolipids and glycoproteins are involved in cell recognition.

The membrane acts as a 'fence' with gates (proteins) that regulate entry and exit of substances, making it selectively permeable.

Membrane Protein Functions

• Enzymes: Catalyze chemical reactions at the membrane surface.

• Structural Support: Anchor the membrane to the cytoskeleton and extracellular matrix.

• Linking Adjacent Cells: Connect cells to form tissues.

Energy & Cell Processes

Adenosine Triphosphate (ATP)

ATP is the energy currency of the cell. Its high-energy phosphate bonds store and release energy for cellular processes. ATP is constantly recycled in the body.

• ATP-ADP Cycle: Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate; energy is absorbed when ADP is converted back to ATP.

Transport Across the Plasma Membrane

Concentration Gradient

A concentration gradient exists when there is a difference in the concentration of a substance across a space or a membrane. Substances tend to move from areas of higher concentration to lower concentration.

Passive Transport

Passive transport does not require energy and relies on concentration gradients.

• Simple Diffusion: Nonpolar solutes move directly through the lipid bilayer.

• Facilitated Diffusion: Charged or polar solutes cross the membrane with the help of membrane proteins.

• Osmosis: The movement of water (solvent) across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration.

Active Transport

Active transport requires energy (usually from ATP) to move substances against their concentration gradients.

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., sodium-potassium ATPase pump).

• Secondary Active Transport: Uses the energy from the movement of one substance down its gradient to drive the transport of another substance against its gradient.

Vesicular Transport

• Phagocytosis: "Cell eating"; the cell engulfs large particles.

• Pinocytosis: "Cell drinking"; the cell takes in fluid droplets.

• Endocytosis: The cell engulfs substances into a vesicle.

• Exocytosis: Substances are released from the cell via vesicles (e.g., hormones, neurotransmitters).

• Transcytosis: Substances are transported across the cell, entering by endocytosis and exiting by exocytosis (e.g., in kidneys and intestines).

Electrophysiology

Membrane Potential

The movement of ions across the plasma membrane creates unequal concentrations of ions, generating an electrical potential. The value of the membrane potential when a cell is at rest is called the resting membrane potential.

Medicines and Membrane Receptors

Agonists and Antagonists

• Agonists: Bind to receptors and mimic the actions of natural ligands (e.g., morphine mimics endorphins).

• Antagonists: Bind to receptors and block the actions of natural ligands (e.g., antihistamines block histamine receptors).

Clinical Application: IV Solutions and Cell Volume

Effects of Intravenous Solutions

• If a patient is given intravenous sterile water, their red blood cells will swell due to osmosis (water enters the cells, which may burst).

• The normal intravenous solution is isotonic saline, which does not cause net movement of water into or out of cells.

Sports Drinks and Dehydration

Dehydration and Cellular Effects

• Dehydration: Loss of water from the ECF increases its osmolarity, causing water to leave cells and cells to shrink.

• Sports Drinks: Contain water, electrolytes, and carbohydrates to help restore fluid and electrolyte balance during dehydration.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions
Osmosis	No	Water down gradient	Water movement
Primary Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Indirect (ATP)	Against gradient	Na+-glucose symport
Vesicular Transport	Yes (ATP)	Bulk movement	Endocytosis, exocytosis

Additional info: Academic context and explanations have been expanded for clarity and completeness. Images included are only those directly relevant to the adjacent content.