Cells: The Living Units – Structure and Function of the Plasma Membrane

Cells: The Smallest Living Units

Cell Theory

The cell theory is a fundamental concept in biology, stating that cells are the structural and functional units of life. The performance of an organism depends on the activities of its cells, and the structure of each cell is closely related to its function. Biochemical functions are determined by cell shape and subcellular structures. Life continues through cellular reproduction, as cells arise only from preexisting cells.

• Structural and functional unit: All living things are composed of cells.

• Complementarity: Structure and function are interdependent.

• Cellular continuity: Cells reproduce to maintain life.

Cell Diversity

Human bodies contain over 250 different cell types, each varying in size, shape, and internal components. These differences enable specialized functions, such as movement, storage, defense, and communication.

• Examples: Muscle cells (movement), fat cells (storage), macrophages (defense), neurons (communication), sperm cells (reproduction).

Generalized Cell Structure

Despite diversity, all human cells share three basic components:

• Plasma membrane: Flexible boundary separating internal and external environments.

• Cytoplasm: Intracellular fluid containing organelles.

• Nucleus: Control center containing DNA.

Extracellular Materials

Types of Extracellular Materials

Substances outside cells include:

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

• Cellular secretions: Saliva, mucus.

• Extracellular matrix: Structural support and glue for cells.

Structure of the Plasma Membrane

Overview

The plasma membrane acts as a dynamic barrier, separating intracellular fluid (ICF) from extracellular fluid (ECF). It regulates the entry and exit of substances, maintaining cellular homeostasis.

Fluid Mosaic Model

The plasma membrane is composed of a flexible lipid bilayer with embedded proteins, forming a constantly changing mosaic. Surface carbohydrates form the glycocalyx, and cell junctions help bind cells together.

Membrane Lipids

The lipid bilayer consists of:

• Phospholipids (75%): Polar hydrophilic heads face water; nonpolar hydrophobic tails face inward.

• Glycolipids (5%): Lipids with sugar groups on the outer surface.

• Cholesterol (20%): Stabilizes membrane structure.

Membrane Proteins

Membrane proteins account for about half the mass of the plasma membrane and are essential for communication and transport. They are classified as:

• Integral proteins: Firmly embedded, often spanning the membrane. Functions include transport, enzymatic activity, and receptor roles.

• Peripheral proteins: Loosely attached, involved in support, enzymatic activity, and cell-to-cell connections.

Functions of Membrane Proteins

• Transport: Channels and carriers move substances across the membrane.

• Receptors: Bind chemical messengers and initiate cellular responses.

• Enzymatic activity: Catalyze reactions at the membrane surface.

• Cell recognition: Glycoproteins serve as identification tags.

• Attachment: Anchor to cytoskeleton and extracellular matrix.

• Cell joining: Form intercellular junctions.

Glycocalyx

The glycocalyx is a carbohydrate-rich area on the cell surface, composed of glycolipids and glycoproteins. It functions as a biological marker for cell recognition and helps the immune system distinguish self from nonself.

Cell Junctions

Types of Cell Junctions

Cells are bound together by three main types of junctions:

• Tight junctions: Impermeable barriers preventing passage of substances between cells.

• Desmosomes: Anchoring junctions providing mechanical strength.

• Gap junctions: Channels allowing communication and passage of ions and small molecules.

Membrane Transport

Selective Permeability

The plasma membrane is selectively permeable, allowing only certain molecules to cross. Transport occurs via:

• Passive transport: No energy required.

• Active transport: Requires ATP.

Passive Membrane Transport

Passive transport includes simple diffusion, facilitated diffusion, and osmosis. All involve movement down a concentration gradient.

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

• Facilitated diffusion: Polar or large molecules move via carrier or channel proteins.

• Osmosis: Movement of water across the membrane.

Factors Affecting Diffusion

• Concentration: Greater difference increases speed.

• Molecular size: Smaller molecules diffuse faster.

• Temperature: Higher temperature increases kinetic energy and diffusion rate.

Simple Diffusion

Nonpolar lipid-soluble substances (e.g., oxygen, carbon dioxide, steroid hormones, fatty acids) diffuse directly through the phospholipid bilayer.

Facilitated Diffusion

Facilitated diffusion is used by hydrophilic molecules (e.g., glucose, amino acids, ions) that cannot pass through the lipid bilayer. It occurs via:

• Carrier-mediated: Specific protein carriers transport molecules.

• Channel-mediated: Water-filled channels allow passage based on size and charge.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane, either through the lipid bilayer or aquaporins. Water moves from areas of low solute concentration to high solute concentration.

Osmolarity and Tonicity

Osmolarity measures the concentration of solute particles. Water moves to equalize osmolarity across membranes. Tonicity describes the effect of solutions on cell volume:

• Isotonic: No change in cell volume.

• Hypertonic: Cell shrinks (crenation).

• Hypotonic: Cell swells and may burst (lysis).

Equations

Osmolarity Calculation:

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

Example: $\text{NaCl} \rightarrow \text{Na}^+ + \text{Cl}^- \Rightarrow 1\,\text{M NaCl} = 2\,\text{Osm}$

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