Cell Structure and the Plasma Membrane: Foundations of Anatomy & Physiology

Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, forming the basic living, structural, and functional unit of the human body. The human body contains more than 100 trillion cells, each specialized for particular functions that contribute to the organism's survival and homeostasis.

• Cell Biology (Cytology): The scientific study of cells, including their structure, function, and life processes.

• Hierarchy: Cells → Tissues → Organs → Systems → Organism

Main Parts of a Cell

• Plasma Membrane: The flexible outer surface that separates the cell's internal environment from the external environment.

• Cytoplasm: The cellular contents between the plasma membrane and the nucleus, including cytosol (fluid) and organelles.

• Nucleus: The large organelle that houses most of the cell's DNA and controls cellular activities.

Checkpoint: The three principal parts of a cell are the plasma membrane, cytoplasm, and nucleus.

The Plasma Membrane

Structure and Function

The plasma membrane is a selectively permeable barrier that surrounds the cell and contains the cytoplasm. It is best described by the fluid mosaic model, which depicts the membrane as a dynamic structure with a mosaic of proteins floating in or on a fluid lipid bilayer.

• Selective Permeability: The membrane allows some substances to pass while restricting others, maintaining the internal environment.

• Fluid Mosaic Model: The membrane is composed of a double layer of lipids (phospholipids, cholesterol, glycolipids) with proteins interspersed throughout.

Structure of the Plasma Membrane

The Lipid Bilayer

• Phospholipids: Make up about 75% of the membrane lipids. Each molecule has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails, forming a bilayer with heads facing outward and tails inward.

• Cholesterol: About 20% of membrane lipids; stabilizes the membrane and affects fluidity.

• Glycolipids: About 5%; lipids with attached carbohydrate groups, contributing to cell recognition.

Arrangement of Lipids

• The amphipathic nature of phospholipids (having both hydrophilic and hydrophobic regions) causes them to arrange in a bilayer.

• Cholesterol molecules are weakly amphipathic and interspersed among phospholipids, affecting membrane fluidity.

• Glycolipids appear only in the membrane layer facing the extracellular fluid, contributing to the glycocalyx.

Arrangement of Membrane Proteins

• Integral (Transmembrane) Proteins: Firmly embedded in the membrane, often spanning the bilayer. Function as channels, carriers, receptors, enzymes, or cell-identity markers.

• Peripheral Proteins: Loosely attached to the membrane surface; function as enzymes or linkers.

The Glycocalyx

• A carbohydrate-rich area on the cell surface formed by glycolipids and glycoproteins.

• Functions in cell recognition, adhesion, and protection.

Functions of Membrane Proteins

• Channels: Allow specific ions to move through water-filled pores (e.g., potassium channels).

• Carriers (Transporters): Selectively move substances across the membrane by changing shape.

• Receptors: Recognize and bind specific molecules (ligands), triggering cellular responses (e.g., insulin receptor).

• Enzymes: Catalyze specific chemical reactions at the inside or outside surface of the cell.

• Linkers: Anchor the plasma membrane to neighboring cells or protein filaments inside and outside the cell.

• Cell-Identity Markers: Enable cells to recognize other cells of the same kind (e.g., blood type antigens).

Example: When insulin binds to its receptor (a membrane protein), it triggers the cell to take up glucose from the blood, illustrating the receptor function of membrane proteins.

Membrane Fluidity

• Membrane lipids and proteins can move laterally within the layer, contributing to membrane flexibility and self-sealing properties.

• Cholesterol modulates fluidity: more cholesterol = less fluidity at high temperatures, more fluidity at low temperatures.

• Fluidity is essential for cell movement, growth, division, and formation of cellular junctions.

Membrane Permeability

• Selective Permeability: The plasma membrane allows some substances to cross more easily than others.

• Lipid-soluble molecules (e.g., oxygen, carbon dioxide, steroids) pass easily; charged or polar molecules (e.g., ions, glucose) require transport proteins.

Equation for Selective Permeability:

$P = \frac{D \cdot K}{d}$

• P: Permeability coefficient

• D: Diffusion coefficient

• K: Partition coefficient (solubility in membrane)

• d: Membrane thickness

Additional info: This equation shows that permeability increases with higher diffusion and partition coefficients and decreases with greater membrane thickness.

Summary Table: Major Components and Functions of the Plasma Membrane

Component	Structure	Main Function(s)
Phospholipids	Bilayer with hydrophilic heads and hydrophobic tails	Barrier to water-soluble substances; structural framework
Cholesterol	Interspersed among phospholipids	Modulates membrane fluidity and stability
Glycolipids	Lipids with attached carbohydrates	Cell recognition, protection (glycocalyx)
Integral Proteins	Span the membrane	Transport, receptors, enzymes, cell-identity markers
Peripheral Proteins	Attached to membrane surface	Enzymes, linkers

Key Terms

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions (e.g., phospholipids).

• Selective Permeability: Property of membranes to allow some substances to pass while blocking others.

• Glycocalyx: Carbohydrate-rich area on the cell surface important for cell recognition and protection.

• Integral Protein: Protein embedded within the membrane, often spanning its entire width.

• Peripheral Protein: Protein attached to the membrane surface.