Cell Membrane Structure and Function

Structural Components of the Cell Membrane

The cell membrane is a selectively permeable barrier that surrounds the cell, maintaining homeostasis and mediating communication with the environment.

• Phospholipid bilayer: Composed of hydrophilic heads and hydrophobic tails, forming the basic structure.

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids, involved in cell recognition.

• Cholesterol: Stabilizes membrane fluidity.

Example: Glycoproteins on the cell surface help immune cells recognize pathogens.

Membrane Transport Mechanisms

Types of Membrane Transport

Cells exchange substances with their environment through various transport mechanisms, which can be classified as passive or active.

• Passive Transport: Does not require energy. Includes diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires energy (ATP). Includes primary active transport and secondary active transport.

• Vesicular Transport: Includes endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis.

Definitions:

• Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Movement via membrane proteins.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Active Transport: Movement against a concentration gradient using energy.

Equation:

Where is the flux, is the diffusion coefficient, and is the concentration gradient.

Comparison of Passive and Active Transport

Passive and active transport differ in energy requirements and direction of movement.

Factors Affecting Rate of Diffusion

• Concentration gradient

• Temperature

• Surface area

• Membrane permeability

• Size of molecules

Example: Oxygen diffuses rapidly across alveolar membranes due to high concentration gradient and thin membrane.

Osmosis and Tonicity

Osmosis is the movement of water across a semipermeable membrane. Tonicity describes the effect of a solution on cell volume.

• Isotonic: No net water movement; cell volume remains constant.

• Hypotonic: Water enters cell; cell swells.

• Hypertonic: Water leaves cell; cell shrinks.

Example: Red blood cells in a hypertonic solution undergo crenation.

Vesicular Transport: Endocytosis and Exocytosis

Endocytosis

Process by which cells engulf substances into vesicles.

• Phagocytosis: "Cell eating"; uptake of large particles.

• Pinocytosis: "Cell drinking"; uptake of fluids.

• Receptor-mediated endocytosis: Specific uptake via receptors.

Exocytosis

Process by which cells expel substances via vesicles fusing with the membrane.

Example: Neurotransmitter release at synapses.

Cellular Organelles and Their Functions

Major Organelles

Organelles perform specialized functions within the cell.

• Nucleus: Contains genetic material; controls cell activities.

• Ribosomes: Protein synthesis.

• Mitochondria: ATP production.

• Endoplasmic Reticulum (ER): Protein and lipid synthesis.

• Golgi Apparatus: Modifies, sorts, and packages proteins.

• Lysosomes: Digestion of cellular waste.

Example: Muscle cells have abundant mitochondria for energy production.

Tissue Types and Characteristics

Classification of Tissues

Tissues are groups of cells with similar structure and function. Four primary types exist in the human body.

Connective Tissue: Structure and Function

Connective tissue provides structural support and connects other tissues. It consists of cells, fibers, and ground substance.

• Matrix: Extracellular material composed of fibers (collagen, elastin) and ground substance.

• Cells: Fibroblasts, chondrocytes, osteocytes, adipocytes.

• Types: Loose (areolar), dense, cartilage, bone, blood.

Example: Cartilage provides flexible support in joints.

Properties of Connective Tissue

• Matrix composition determines tissue properties.

• Cell type (-blast: immature, -cyte: mature).

• Vascularity varies (e.g., cartilage is avascular).

Example: Osteoblasts build bone matrix; osteocytes maintain it.

Additional info:

• Some content inferred from standard Anatomy & Physiology curriculum, such as the classification and function of tissues, and the details of membrane transport mechanisms.