Cell Structure and Membrane Transport: Study Notes for Anatomy & Physiology I

Cells: The Smallest Living Units

Cell Basics

Cells are the fundamental structural and functional units of life. The performance of an organism depends on the activities of its individual cells, which are shaped by their structure and gene expression. The continuity of life is maintained through cellular reproduction, as cells arise only from preexisting cells.

• Definition: A cell is the smallest unit capable of performing all life processes.

• Structure and Function: The shape and internal components of a cell determine its biochemical functions.

• Cellular Basis of Life: All living organisms are composed of cells, and new cells are produced from existing cells.

Cell Diversity

Human bodies contain over 250 different types of cells, each specialized in size, shape, and function. This diversity enables the wide range of physiological processes necessary for life.

• Examples: Cells that connect body parts (fibroblasts), cells that move organs (muscle cells), cells that store nutrients (fat cells), cells that fight disease (macrophages), cells that gather information (nerve cells), and cells of reproduction (sperm).

Generalized Eukaryote Cell Structure

All human cells share three basic structural components: the plasma membrane, cytoplasm, and nucleus. These components work together to maintain cellular function and integrity.

• Plasma membrane: Flexible boundary separating the cell from its environment.

• Cytoplasm: Intracellular fluid containing organelles.

• Nucleus: Control center containing DNA.

Extracellular Materials

Types of Extracellular Materials

Substances found outside cells include body fluids, cellular secretions, and the extracellular matrix. These materials support cell function and communication.

• Extracellular fluids: Interstitial fluid, blood plasma, cerebrospinal fluid.

• Cellular secretions: Saliva, mucus.

• Extracellular matrix: Acts as a glue to hold cells together.

Plasma (Cell) Membrane

Structure and Function

The plasma membrane is an active barrier that separates intracellular fluid from extracellular fluid. It regulates the entry and exit of substances and plays a dynamic role in cellular activity.

• Membrane lipids: Form a flexible lipid bilayer.

• Membrane proteins: Float within the lipid bilayer, creating a fluid mosaic pattern.

• Glycocalyx: Surface sugars that function as biological markers.

• Cell junctions: Structures that help hold cells together.

Membrane Lipids

The lipid bilayer is primarily composed of phospholipids, glycolipids, and cholesterol. The arrangement of these molecules creates a selectively permeable barrier.

• Phospholipids: 75% of membrane lipids; have polar hydrophilic heads and nonpolar hydrophobic tails.

• Glycolipids: 5%; lipids with sugar groups on the outer membrane surface.

• Cholesterol: 20%; increases membrane stability.

Membrane Proteins

Membrane proteins are essential for cell communication and function. They are classified as integral or peripheral proteins, each with specialized roles.

• Integral proteins: Firmly embedded in the membrane, often spanning it (transmembrane); function as transport proteins, enzymes, or receptors.

• Peripheral proteins: Loosely attached to integral proteins; function as enzymes, motor proteins, or in cell-to-cell connections.

Functions of Membrane Proteins

• Transport: Provide channels or pumps for movement of substances across the membrane.

• Receptors for signal transduction: Bind chemical messengers and initiate cellular responses.

• Enzymatic activity: Catalyze metabolic reactions.

• Cell-cell recognition: Serve as identification tags for cell recognition.

• Attachment to cytoskeleton and extracellular matrix: Maintain cell shape and stabilize membrane proteins.

• Cell-to-cell joining: Form intercellular junctions for tissue integrity.

Membrane Carbohydrates and Glycocalyx

The glycocalyx is a layer of carbohydrates on the cell surface, attached to lipids (glycolipids) or proteins (glycoproteins). It functions as a biological marker for cell recognition and immune response.

• Cell recognition: Allows immune system to distinguish self from nonself.

Cell Junctions

Types of Cell Junctions

Cells are often bound together to form tissues and organs. The three main types of cell junctions are tight junctions, desmosomes, and gap junctions.

• Tight junctions: Form impermeable seals to prevent molecules from passing between cells.

• Desmosomes: Anchoring junctions that bind cells together and provide mechanical strength.

• Gap junctions: Communicating junctions that allow ions and small molecules to pass between cells.

Transport Across the Plasma Membrane

Selective Permeability

The plasma membrane is selectively permeable, allowing only certain molecules to cross. Transport occurs via passive or active mechanisms.

• Passive transport: No energy required; includes simple diffusion, facilitated diffusion, and osmosis.

• Active transport: Requires energy (ATP); includes primary and secondary active transport, and vesicular transport.

Passive Membrane Transport

Passive transport relies on diffusion, the movement of molecules from high to low concentration. The speed of diffusion is influenced by concentration gradient, molecular size, and temperature.

• Simple diffusion: Nonpolar, lipid-soluble substances move directly through the phospholipid bilayer.

• Facilitated diffusion: Polar or larger molecules move via protein carriers or channels.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Osmolarity and Tonicity

Osmolarity measures the concentration of solute particles in a solution. Water moves by osmosis from areas of low solute concentration to high solute concentration. Tonicity describes the effect of a solution on cell shape by altering internal water volume.

• Isotonic: Same osmolarity as inside the cell; cell volume unchanged.

• Hypertonic: Higher osmolarity than inside the cell; cell shrinks (crenation).

• Hypotonic: Lower osmolarity than inside the cell; cell swells and may burst (lysis).

Osmolarity Calculation

Osmolarity is calculated as molarity times the number of particles formed by ionization. For example, NaCl dissociates into two particles (Na+ and Cl-), so a 1 M solution of NaCl equals a 2 Osm solution.

• Formula: $\text{Osmolarity} = \text{Molarity} \times \text{Number of particles}$

Active Membrane Transport

Types of Active Transport

Active transport requires ATP and carrier proteins to move solutes against their concentration gradient. It is essential for maintaining cellular homeostasis.

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

• Secondary active transport: Indirect use of energy from ionic gradients created by primary active transport.

• Antiporters: Transport one substance in and another out.

• Symporters: Transport two substances in the same direction.

Vesicular Transport

Vesicular transport moves large particles, macromolecules, and fluids across membranes in vesicles. It includes endocytosis, exocytosis, transcytosis, and vesicular trafficking.

• Endocytosis: Transport into the cell; includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

• Exocytosis: Transport out of the cell; used for secretion of hormones, neurotransmitters, and wastes.

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

• Vesicular trafficking: Transport within the cell.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required	Direction	Examples
Simple Diffusion	No	High to Low	O2, CO2, fatty acids
Facilitated Diffusion	No	High to Low	Glucose, amino acids, ions
Osmosis	No	High to Low (water)	Water
Primary Active Transport	Yes (ATP)	Low to High	Na+-K+ pump
Secondary Active Transport	Indirect (gradient)	Low to High	Na+-glucose symporter
Vesicular Transport	Yes (ATP)	Varies	Endocytosis, exocytosis

Key Equations

• Osmolarity: $\text{Osmolarity} = \text{Molarity} \times \text{Number of particles}$

Additional info:

• Cell junctions are critical for tissue integrity and communication, especially in epithelial and muscle tissues.

• Membrane transport mechanisms are fundamental for maintaining homeostasis, nerve impulse transmission, and muscle contraction.