LECTURE Chapter 3 - Cell Structure and Function - PART 1

Cells: The Living Unit

Introduction to Cells

Cells are the fundamental structural and functional units of all living organisms. In human anatomy and physiology, understanding the cell is essential for grasping how the body operates at its most basic level.

• Definition: A cell is the smallest unit of life capable of carrying out all vital functions.

• Cell Theory:

  • All living things are composed of one or more cells.

  • Cells arise only from pre-existing cells.

  • The cell is the basic unit of structure and function in organisms.

• Diversity: The human body contains 50 to 100 trillion cells, with over 250 different types, each specialized in size, shape, and function.

Generalized Cell Structure

Common Features of Human Cells

Despite their diversity, all human cells share certain structural components that enable them to function effectively.

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

• Cytoplasm: The intracellular fluid containing organelles, which are specialized structures that perform distinct cellular functions.

• Nucleus: The control center of the cell, containing DNA and regulating cellular activities.

Structure of the Generalized Cell

The generalized cell includes various organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes, each contributing to the cell's survival and function.

• Mitochondria: Powerhouse of the cell, responsible for ATP production.

• Endoplasmic Reticulum (ER): Involved in protein and lipid synthesis.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Lysosomes: Contain digestive enzymes to break down waste materials.

• Peroxisomes: Detoxify harmful substances.

Extracellular Materials

Substances Found Outside Cells

Extracellular materials are crucial for cell support, communication, and transport.

• Extracellular Fluids (ECFs):

  • Interstitial Fluid: Bathes and surrounds cells.

  • Blood Plasma: Fluid component of blood.

  • Cerebrospinal Fluid: Surrounds nervous system organs.

• Cellular Secretions: Includes substances like saliva, mucus, and gastric fluids.

• Extracellular Matrix: A network of proteins and polysaccharides that acts as a glue to hold cells together and provides structural support.

Plasma Membrane

Structure and Function

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell.

• Separates: Intracellular fluid (ICF) from extracellular fluid (ECF).

• Dynamic Role: Controls what enters and leaves the cell, maintaining homeostasis.

• Also Known As: The cell membrane.

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which highlights its dynamic and complex structure.

• Phospholipid Bilayer: Composed mainly of phospholipids with hydrophilic (water-loving) heads and hydrophobic (water-hating) tails.

• Cholesterol: Interspersed within the bilayer, providing membrane stability and fluidity.

• Proteins: Embedded within the membrane, serving various functions such as transport, communication, and structural support.

• Glycocalyx: Surface sugars attached to lipids (glycolipids) and proteins (glycoproteins), functioning as biological markers for cell recognition.

Membrane Proteins

Membrane proteins are essential for cell communication, transport, and structural integrity.

• Integral Proteins: Span the membrane and are involved in transport, acting as channels or carriers, and as receptors for signal transduction.

• Peripheral Proteins: Loosely attached to the membrane, functioning as enzymes, motor proteins, and in cell-to-cell connections.

Functions of Membrane Proteins

• Transport: Move substances across the membrane via channels and carriers.

• Receptors for Signal Transduction: Bind chemical messengers and initiate cellular responses.

• Enzymatic Activity: Catalyze metabolic reactions.

• Cell-Cell Recognition: Glycoproteins serve as identification tags.

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

• Attachment to Cytoskeleton and ECM: Maintain cell shape and stabilize membrane proteins.

Glycocalyx

The glycocalyx is a carbohydrate-rich area on the cell surface, crucial for cell recognition and immune response.

• Biological Markers: Allow cells to identify each other and distinguish "self" from "nonself".

Intercellular Junctions

Types of Cell Junctions

Cell junctions are specialized structures that connect adjacent cells, providing communication and structural support.

• Tight Junctions: Integral proteins fuse adjacent cells, forming an impermeable barrier that prevents the passage of substances between cells.

• Desmosomes (Anchoring Junctions): Linker proteins interlock like a zipper, anchoring cells together and preventing tearing.

• Gap Junctions: Connexons form channels allowing ions and small molecules to pass directly between cells, facilitating rapid communication (especially in cardiac and smooth muscle).

Membrane Transport

Overview

Cells must exchange materials with their environment to survive. Membrane transport occurs via passive or active mechanisms.

• Passive Transport: Does not require energy (ATP).

• Active Transport: Requires energy (ATP).

Passive Membrane Transport

Passive transport relies on the natural movement of molecules down their concentration gradients.

• Simple Diffusion: Nonpolar, lipid-soluble substances (e.g., oxygen, carbon dioxide) diffuse directly through the lipid bilayer.

• Facilitated Diffusion: Polar or charged molecules (e.g., glucose, ions) move via carrier or channel proteins.

• Osmosis: Movement of water across the membrane, either through the lipid bilayer or via aquaporins.

Factors Affecting Diffusion Rate:

• Concentration gradient

• Molecular size

• Temperature

Equilibrium: Achieved when there is no net movement of molecules.

Selective Permeability

The plasma membrane allows selective passage of substances, maintaining internal conditions and preventing entry of harmful materials.

• Lipid-soluble and small molecules: Cross unassisted (simple diffusion).

• Water-soluble and large molecules: Require assistance (facilitated diffusion).

Osmosis and Tonicity

Osmosis is vital for maintaining fluid balance in cells. Tonicity describes how a solution affects cell volume.

• Isotonic Solution: Same osmolarity as the cell; no net water movement.

• Hypertonic Solution: Higher osmolarity than the cell; water leaves the cell, causing shrinkage (crenation).

• Hypotonic Solution: Lower osmolarity than the cell; water enters the cell, causing swelling and possible lysis (bursting).

Active Membrane Transport

Overview

Active transport moves substances against their concentration gradients, requiring energy (ATP).

• Primary Active Transport: Direct use of ATP to move ions via pumps (e.g., Na+-K+ pump).

• Secondary Active Transport: Indirect use of ATP; uses ion gradients created by primary transport to move other substances (cotransport).

Primary Active Transport: Sodium-Potassium Pump

The Na+-K+ ATPase pump is essential for maintaining cellular electrochemical gradients.

• Pumps 3 Na+ out and 2 K+ into the cell per ATP molecule hydrolyzed.

• Maintains resting membrane potential, crucial for nerve and muscle function.

Equation:

$3\text{Na}^+_{\text{in}} + 2\text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Secondary Active Transport

Uses the energy stored in ion gradients to transport other molecules.

• Symporters: Move two substances in the same direction.

• Antiporters: Move substances in opposite directions.

• Commonly transports glucose, amino acids, and ions.

Vesicular Transport

Vesicular transport moves large particles or volumes across membranes using vesicles and requires ATP.

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

• Exocytosis: Transport out of the cell.

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

• Vesicular Trafficking: Movement within the cell between organelles.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Examples
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions
Osmosis	No	Down gradient	Water
Primary Active Transport	Yes (ATP)	Against gradient	Na+-K+ pump
Secondary Active Transport	Indirect (ATP)	Against gradient	Glucose/Na+ symport
Vesicular Transport	Yes (ATP)	Bulk movement	Endocytosis, exocytosis

Additional info: These notes expand on the provided slides with definitions, examples, and a summary table for clarity and exam preparation.