Membrane Transport and Cellular Structures in Anatomy & Physiology

Membrane Transport

Overview of Membrane Transport Processes

Membrane transport refers to the movement of substances across the plasma membrane, which is essential for maintaining cellular homeostasis. There are several mechanisms by which solutes can cross the membrane, depending on their size, solubility, and the direction of movement relative to concentration gradients.

• Passive Transport: Movement of substances down their concentration gradient without energy input.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

Transport is necessary when:

• Solutes are too large for channels

• Solutes are not lipid-soluble

• Solutes must move against a concentration gradient

Types of Active Transport

Active transport uses carrier proteins (also called solute pumps) to move substances across the membrane. These proteins bind specifically and reversibly to the substances being transported.

• Uniporters: Transport one substance in one direction.

• Symporters: Transport two different substances in the same direction.

• Antiporters: Transport two different substances in opposite directions.

Active transport can be classified as:

• Primary Active Transport: Energy comes directly from ATP hydrolysis.

• Secondary Active Transport: Energy is obtained indirectly from ionic gradients created by primary active transport.

Primary Active Transport: Sodium-Potassium Pump

The sodium-potassium pump (Na+-K+ ATPase) is the most studied example of primary active transport. It is an enzyme that pumps three sodium ions out of the cell and two potassium ions into the cell for each molecule of ATP hydrolyzed.

• Present in all plasma membranes, especially active in excitable cells (e.g., nerve and muscle cells).

• Maintains electrochemical gradients essential for muscle and nerve function.

• Acts as an antiporter, moving Na+ and K+ in opposite directions.

Equation:

$3\,\mathrm{Na}^+_{\text{(in)}} + 2\,\mathrm{K}^+_{\text{(out)}} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na}^+_{\text{(out)}} + 2\,\mathrm{K}^+_{\text{(in)}} + \mathrm{ADP} + \mathrm{P_i}$

Example: The sodium-potassium pump helps maintain the resting membrane potential in neurons.

Secondary Active Transport

Secondary active transport depends on the ion gradients established by primary active transport. The energy stored in these gradients is used to drive the transport of other solutes.

• As sodium moves back into the cell down its concentration gradient, it can "drag" other molecules (such as glucose or amino acids) with it via symporters.

• This process is common in the absorption of nutrients in the intestines and kidneys.

Example: The sodium-glucose symporter uses the sodium gradient to transport glucose into cells against its concentration gradient.

Vesicular Transport

Overview

Vesicular transport is the movement of large particles, macromolecules, and fluids across the plasma membrane in vesicles. This process requires energy, usually in the form of ATP.

• Endocytosis: Transport into the cell.

• Exocytosis: Transport out of the cell.

• Transcytosis: Transport into, across, and then out of the cell.

• Vesicular Trafficking: Movement of substances from one area or organelle in the cell to another.

Types of Endocytosis

• Phagocytosis: "Cell eating"; the cell engulfs large particles or cells using pseudopods, forming a phagosome.

• Pinocytosis: "Cell drinking"; the cell engulfs extracellular fluid and dissolved solutes via membrane infolding.

• Receptor-Mediated Endocytosis: Highly selective; receptors on the cell surface bind specific molecules, which are then internalized.

Example: Macrophages use phagocytosis to engulf bacteria; cells use receptor-mediated endocytosis to take in cholesterol via low-density lipoprotein (LDL) particles.

Exocytosis

Exocytosis is the process by which cells expel materials in vesicles that fuse with the plasma membrane. This is important for the secretion of hormones, neurotransmitters, and waste products.

• Triggered by cell-surface signals or changes in membrane voltage.

• Vesicles use proteins (v-SNAREs and t-SNAREs) to dock and fuse with the membrane.

Example: Neurons release neurotransmitters into the synaptic cleft via exocytosis.

Resting Membrane Potential (RMP)

Definition and Establishment

The resting membrane potential (RMP) is the electrical potential difference across the plasma membrane of a cell in its resting state. It is produced by the separation of oppositely charged particles (ions) across the membrane.

• Typical RMP values range from -50 to -100 mV, with the inside of the cell being more negative relative to the outside.

• Primarily established by the diffusion of potassium (K+) out of the cell through leakage channels.

• Maintained by the sodium-potassium pump, which counteracts the leakage of Na+ and K+.

Equation:

$\text{RMP} \approx -90\,\mathrm{mV}$

Example: The RMP is essential for the generation of action potentials in nerve and muscle cells.

Cell-Environment Interactions

Glycocalyx and Cell Adhesion Molecules (CAMs)

Cells interact with their environment through direct contact and chemical signaling. The glycocalyx, a carbohydrate-rich area on the cell surface, plays a key role in these interactions.

• Cell Adhesion Molecules (CAMs): Glycoproteins that help cells adhere to each other and to the extracellular matrix.

• CAMs are involved in cell movement, immune responses, and tissue repair.

Example: White blood cells use CAMs to move toward sites of infection.

Plasma Membrane Receptors

Plasma membrane receptors are proteins that bind ligands (chemical messengers) and initiate cellular responses. These are crucial for communication between cells and their environment.

• Ligand binding can trigger enzyme activation, open ion channels, or activate second messenger pathways.

• Examples of ligands include neurotransmitters, hormones, and paracrines.

Example: Insulin binds to its receptor to trigger glucose uptake in cells.

Cytoplasm and Organelles

Cytoplasm

The cytoplasm is the cellular material between the plasma membrane and the nucleus. It consists of cytosol (fluid), inclusions (stored nutrients), and organelles (metabolic machinery).

• Cytosol: Gel-like solution containing water, proteins, salts, and other solutes.

• Inclusions: Non-living substances such as glycogen granules and pigments.

• Organelles: Specialized structures with specific functions.

Major Organelles

• Mitochondria: The "powerhouse" of the cell; site of aerobic ATP production. Contains its own DNA and RNA.

• Ribosomes: Sites of protein synthesis; can be free in cytosol or bound to rough ER.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER synthesizes proteins, smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, packages, and ships proteins and lipids.

• Peroxisomes: Contain enzymes for detoxifying harmful substances and breaking down fatty acids.

• Lysosomes: Contain digestive enzymes for breaking down waste and cellular debris.

Example: Liver cells have abundant smooth ER for detoxification.

Cytoskeleton and Cellular Extensions

Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, aids in cell movement, and organizes organelles.

• Microfilaments: Thin filaments of actin; involved in cell movement and shape.

• Intermediate Filaments: Rope-like fibers; provide tensile strength.

• Microtubules: Hollow tubes of tubulin; maintain cell shape and serve as tracks for organelle movement.

Cellular Extensions

• Cilia: Short, motile extensions that move substances across the cell surface (e.g., mucus in respiratory tract).

• Flagella: Longer extensions that propel the entire cell (e.g., sperm).

• Microvilli: Fingerlike projections that increase surface area for absorption (e.g., intestinal cells).

Nucleus and Genetic Material

Nucleus

The nucleus is the control center of the cell, containing the genetic material (DNA) and directing protein synthesis.

• Most cells are uninucleate, but some (e.g., skeletal muscle) are multinucleate, and some (e.g., red blood cells) are anucleate.

• Three main structures: nuclear envelope, nucleolus, and chromatin.

Nuclear Envelope

The nuclear envelope is a double membrane that encloses the nucleoplasm and is continuous with the rough ER. Nuclear pores regulate the passage of substances in and out of the nucleus.

Nucleolus

The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome subunits are assembled.

Chromatin

Chromatin consists of DNA wrapped around histone proteins. It condenses to form chromosomes during cell division.

• Histone modifications help regulate gene expression.

• Condensed chromatin protects DNA during cell division.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Passive Transport	No	Down gradient	Oxygen diffusion
Primary Active Transport	Yes (direct ATP)	Against gradient	Sodium-potassium pump
Secondary Active Transport	Yes (indirect ATP)	Against gradient (coupled)	Sodium-glucose symporter
Vesicular Transport	Yes (ATP)	Bulk movement	Phagocytosis, exocytosis