Chapter 3
Chapter Objectives
After studying this chapter, you will be able to:
Describe the structure and function of the cell membrane, including its regulation of materials into and out of the cell.
Describe the functions of the various cytoplasmic organelles.
Explain the structure and contents of the nucleus, as well as the process of DNA replication.
Explain the process by which a cell builds proteins using the DNA code.
List the stages of the cell cycle in order, including the steps of cell division in somatic cells.
Discuss how a cell differentiates and becomes more specialized.
List the morphological and physiological characteristics of some representative cell types in the human body.
Cellular Development and Differentiation
Human body development starts from a single fertilized egg cell, evolving into a complex organism composed of trillions of cells.
Early, undifferentiated cells undergo differentiation to become specialized in structure and function, contributing to various tissues.
Examples of specialized cells:
Squamous skin cells: Flat plate-shaped, live short lives, tightly packed, providing a protective barrier.
Nerve cells: Star-shaped, possess long processes for communication, can live for an organism's lifetime.
Theme: Form and Function - the shape of a structure is optimally suited to perform its assigned functions.
Each cell's primary responsibility is to contribute to homeostasis, a dynamic state of balance compatible with life.
Example: Cells require a water-based environment to survive, maintained by physiological mechanisms.
Deviations from homeostasis (e.g., blood pressure) can lead to illness or death.
Structure and Function of the Cell Membrane
The cell membrane, or plasma membrane, is a pliable structure comprised of a phospholipid bilayer.
Phospholipids: Comprised of a hydrophilic phosphate head and two hydrophobic fatty acid tails, arranged tail-to-tail.
Cholesterol: Found within the membrane, contributes to fluidity.
Proteins: Various functions embedded within the membrane.
Molecular Composition
Phospholipid: Features a phosphate group (polar and hydrophilic) and two nonpolar lipid tails (hydrophobic).
Amphipathic nature: Has both hydrophilic and hydrophobic regions.
Example of amphipathic behavior: Soap cleans grease by having a hydrophilic part that interacts with water and a hydrophobic part that interacts with grease.
Cell Membrane Structure
Composed of two layers of phospholipids; tails face inward, heads face outward toward intracellular and extracellular fluids.
Intracellular Fluid (ICF): The fluid within the cell.
Extracellular Fluid (ECF): The fluid surrounding the cell; Interstitial Fluid (IF) specifically refers to ECF not contained within blood vessels.
Cell membrane has a fluid mosaic model: lipids and proteins are not rigidly fixed.
Membrane Proteins
Integral Proteins: Embedded in the membrane; include channel proteins (for ion passage) and cell recognition proteins (mark cell identity).
Receptors: A type of integral protein that binds specific molecules, inducing chemical reactions within the cell.
Example: Dopamine receptor binding to dopamine opens ion channels.
Glycoproteins: Integral proteins with attached carbohydrates that function in cell recognition.
Glycocalyx: The fuzzy coat formed by glycoproteins and carbohydrates that aids bacteria binding, contains hormones, and has an identity function for immune recognition.
Peripheral Proteins: Found on the membrane's inner or outer surface, often linked to integral proteins, usually serving specific functions.
Transport Across the Cell Membrane
The cell membrane regulates concentration of various substances (ions, nutrients, waste).
Selectively Permeable: Allows certain substances to pass based on size, charge, and polarity.
Passive transport: Movement without energy expenditure; includes simple diffusion, facilitated diffusion, and osmosis.
Active transport: Movement requiring energy, usually against concentration gradients.
Passive Transport
Concentration Gradient: The difference in concentration across a space, driving diffusion, which is the movement from higher to lower concentration.
Examples: Perfume in a closed bathroom, sugar dissolving in tea.
Temperature affects diffusion rates; higher temperatures increase molecular motion.
Simple Diffusion: Movement of nonpolar substances through the lipid bilayer without energy.
Gases like O2 and CO2 easily diffuse due to their small size and nonpolarity.
Facilitated Diffusion: For larger or polar substances needing assistance from transport proteins (e.g., glucose, ions).
Example: Sodium ions use sodium channels to enter cells via facilitated diffusion due to charge.
Osmosis
Osmosis: The diffusion of water through a semipermeable membrane.
Hypertonic Solution: Higher solute concentration outside the cell, causing water to leave and cells to shrivel.
Hypotonic Solution: Lower solute concentration outside, allowing water into the cell, causing it to swell (risking bursting).
Isotonic Solution: Equal solute concentration, maintaining normal shape and function.
Filtration
Active transport differs from diffusion in that it uses hydrostatic pressure to push fluid and solutes from high to low pressure areas, crucial in circulatory and renal systems.
Active Transport
Requires ATP expenditure to move substances across membranes, often against their concentration gradient (e.g., sodium-potassium pump - Na+/K+ ATPase).
Transports 3 Na+ ions out and 2 K+ ions into the cell per ATP molecule utilized, essential for maintaining cell electrical gradients.
Important for nerve cells where maintaining ion gradients is critical for function.
Secondary Active Transport: Uses primary active transport to create gradients allowing passive transport of other substances (e.g., sodium-glucose symporter).
Organelles of the Endomembrane System
Cytoplasm: The internal environment of the cell composed of cytosol and organelles.
Endoplasmic Reticulum (ER): Continuous with the nuclear membrane; functions in transporting, synthesizing, and storing materials. Includes:
Rough ER: Studded with ribosomes, primarily synthesizes and modifies proteins.
Smooth ER: Lacks ribosomes, involved in lipid synthesis and detoxification of substances.
Golgi Apparatus
The Golgi apparatus modifies, sorts, and packages proteins from the rough ER; structurally resembles stacked discs.
New vesicles are formed at its opposite side to deliver proteins to destinations.
Lysosomes
Membrane-bound vesicles containing digestive enzymes; responsible for breaking down waste and cellular components through processes like autophagy and phagocytosis.
Mitochondria
Mitochondria are the cell's energy converters, executing cellular respiration converting nutrients to ATP, demand varies with cell type; more ATP is needed where energy use is higher.
Peroxisomes
Organelles containing enzymes that detoxify harmful substances and lipid metabolism; neutralize free radicals, with essential roles in metabolic processes.
Cytoskeleton
A network of fibrous proteins (microfilaments, intermediate filaments, microtubules) providing structural support, enabling motility, intracellular transport, and cell division.
Microtubules: Thickest, support cell shape, assist in moving organelles, contribute to cilia and flagella structures.
Microfilaments: Facilitate muscle movement and cell division; composed mainly of actin.
Intermediate Filaments: Provide tensile strength and anchor organelles; composed mainly of keratin.
Cell Membrane Review
Functions of the Cell Membrane: Provides a barrier, regulates substance movement, and enables communication.
Transport Mechanisms: Passive (no energy) methods include simple and facilitated diffusion and osmosis; active (energy-requiring) processes include pumps and vesicles.
Cytoplasm and Organelles Summary
Consists of cytosol and organelles, including the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and peroxisomes, working in tandem for cell function and homeostasis.