Cells: The Living Units

Introduction

Cells are the fundamental units of life in all living organisms. Understanding their structure and function is essential for the study of anatomy and physiology. This guide covers the plasma membrane, membrane transport, cytoplasm, and major organelles.

Plasma Membrane

Structure and Function

• Phospholipid bilayer and proteins form a dynamic, constantly changing fluid mosaic.

• Separates intracellular fluid (ICF) from extracellular fluid (ECF).

• Interstitial fluid (IF) is the ECF that surrounds cells.

Key Terms:

• Fluid mosaic model: Describes the flexible, dynamic nature of the plasma membrane.

• Phospholipid bilayer: Double layer of phospholipids with hydrophilic heads and hydrophobic tails.

Membrane Lipids

• 75% phospholipids (form the lipid bilayer)

• Phospholipid heads: polar and hydrophilic

• Phospholipid tails: nonpolar and hydrophobic

• 5% glycolipids: Lipids with sugar groups on the outer membrane surface

• 20% cholesterol: Increases membrane stability

Membrane Proteins

• Integral proteins: Firmly inserted into the membrane (most are transmembrane). Have hydrophobic and hydrophilic regions. Function as transport proteins (channels and carriers), enzymes, or receptors.

• Peripheral proteins: Loosely attached to integral proteins. Function as enzymes, motor proteins for cell shape change, cell division, and muscle contraction; also involved in cell-to-cell connections.

The Glycocalyx

• Glycocalyx: "Sugar covering" at the cell surface, composed of lipids and proteins with attached carbohydrates (sugar groups).

• Each cell type has a different pattern of sugars, serving as specific biological markers for cell-to-cell recognition.

• Allows the immune system to recognize "self" and "non-self"; cancerous cells may change it continuously.

Membrane Transport

Overview

• Plasma membranes are selectively permeable.

• Some molecules pass through easily; others do not.

• Two major types of transport:

  • Passive processes

  • Active processes

Passive Processes

• Simple diffusion: Movement of molecules from high to low concentration directly through the lipid bilayer.

• Facilitated diffusion: Movement of molecules via protein channels or carriers.

• Osmosis: Movement of solvent (e.g., water) across a selectively permeable membrane.

Osmosis Details:

• Water diffuses through plasma membranes via the lipid bilayer and through specific water channels called aquaporins (AQPs).

• Occurs when water concentration differs on the two sides of a membrane.

Active Processes

• Require energy (usually ATP).

• Move substances against their concentration gradient.

• Require carrier proteins (solute pumps) that bind specifically and reversibly with substances.

Vesicular Transport

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

• Exocytosis: Transport out of the cell.

Functions:

• Hormone secretion, neurotransmitter release, mucus secretion, and excretion of wastes.

Types of Endocytosis

• Phagocytosis: Cell engulfs a large particle by forming pseudopods and enclosing it in a phagosome.

• Pinocytosis: Cell "drinks" a droplet of extracellular fluid; no receptors involved.

• Receptor-mediated endocytosis: Substance binds to specific receptors, triggering vesicle formation.

Cytoplasm

Components

• Located between the plasma membrane and nucleus.

• Composed of:

  • Cytosol: Gel-like solution of water, salts, and sugars.

  • Organelles: Specialized structures performing specific cellular functions.

  • Inclusions: Temporary structures (e.g., glycogen granules, pigments, lipid droplets).

Cytoplasmic Organelles

Classification

• Membranous organelles: Mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, Golgi apparatus.

• Nonmembranous organelles: Cytoskeleton, centrioles, ribosomes.

Membranes allow crucial compartmentalization within cells.

Mitochondria

• Provide most of the cell's ATP via aerobic cellular respiration.

• Contain their own DNA, RNA, and ribosomes.

• Capable of cell division called binary fission.

Ribosomes

• Site of protein synthesis.

• Free ribosomes: Synthesize soluble proteins for use in cytosol or other organelles.

• Membrane-bound ribosomes: Form rough ER; synthesize proteins for membranes, lysosomes, or export.

Endoplasmic Reticulum (ER)

• Interconnected tubes and parallel membranes; continuous with nuclear membrane.

• Two varieties:

  • Rough ER: Studded with ribosomes; manufactures all secreted proteins and membrane integral proteins and phospholipids. Assembled proteins move to ER interior, enclosed in vesicle, and go to Golgi apparatus.

  • Smooth ER: Network of tubules; its enzymes (integral proteins) function in lipid metabolism, cholesterol and steroid-based hormone synthesis, making lipids of lipoproteins, absorption/synthesis/transport of fats, detoxification, and storage/release of calcium.

Golgi Apparatus

• Modifies, concentrates, and packages proteins and lipids from rough ER.

Summary Table: Membrane Lipids Composition

Key Equations

• Diffusion Rate (Fick's Law):

• Osmosis (Osmotic Pressure):

Where is osmotic pressure, is the van 't Hoff factor, is molarity, is the gas constant, and is temperature.

Additional info:

• Binary fission in mitochondria is similar to bacterial cell division, reflecting their evolutionary origin.

• Compartmentalization by organelle membranes allows specialized functions to occur efficiently within the cell.