Chapter 3: The Cell

Module 3.1 Introduction to Cells

The cell is the basic structural and functional unit of all living organisms. Understanding the components and organization of cells is fundamental to the study of anatomy and physiology.

• Plasma Membrane: The outer boundary of the cell, separating the internal environment from the external environment.

• Cytoplasm: The region between the plasma membrane and the nucleus, containing cytosol, organelles, and inclusions.

• Cytosol: The fluid portion of the cytoplasm, where many metabolic reactions occur.

• Organelle: Specialized structures within the cell that perform specific functions (e.g., mitochondria, endoplasmic reticulum).

• Cytoskeleton: A network of protein filaments providing structural support, shape, and movement for the cell.

• Nucleus: The control center of the cell, containing genetic material (DNA).

Intracellular fluid (ICF) is the fluid within cells, while extracellular fluid (ECF) is found outside cells (e.g., plasma, interstitial fluid).

Module 3.2 Structure of the Plasma Membrane

The plasma membrane is a dynamic structure that controls the movement of substances into and out of the cell.

• Lipid Distribution: The membrane is primarily composed of a phospholipid bilayer, with hydrophilic heads facing outward and hydrophobic tails inward.

• Cholesterol: A lipid that stabilizes membrane fluidity and integrity.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, forming the glycocalyx for cell recognition.

• Proteins:

  • Integral proteins: Span the membrane and function as channels, carriers, or receptors.

  • Peripheral proteins: Loosely attached to the membrane surface, involved in signaling or structural support.

• Fluid Mosaic Model: Describes the membrane as a flexible, dynamic structure with proteins and lipids moving laterally within the bilayer.

Module 3.3 Transport across the Plasma Membrane

Cells regulate the movement of substances across the plasma membrane through various transport mechanisms.

• Selectively Permeable Membrane: Allows some substances to pass while restricting others.

• Passive Transport: Movement of substances down their concentration gradient without energy input (e.g., diffusion, osmosis).

• Active Transport: Movement of substances against their concentration gradient, requiring ATP.

• Diffusion: The net movement of molecules from an area of higher concentration to lower concentration.

• Concentration Gradient: The difference in concentration of a substance across a space.

• Simple Diffusion: Direct movement of small, nonpolar molecules through the membrane.

• Facilitated Diffusion: Movement of larger or polar molecules via membrane proteins.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Osmotic Pressure: The force exerted by solutes drawing water across the membrane.

• Tonicity: The ability of a solution to change the shape of cells by altering their water content.

• Hypertonic Solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.

• Isotonic Solution: Equal solute concentration; no net water movement.

• Hypotonic Solution: Lower solute concentration outside; water enters the cell, causing it to swell.

• Primary Active Transport: Direct use of ATP to move substances (e.g., sodium-potassium pump).

• Vesicular Transport: Movement of large particles via vesicles.

  • Endocytosis: Uptake of materials into the cell.

  • Phagocytosis: "Cell eating"; engulfing large particles.

  • Pinocytosis: "Cell drinking"; uptake of fluid and dissolved substances.

  • Receptor-mediated Endocytosis: Specific uptake via receptor binding.

  • Exocytosis: Release of substances from the cell.

  • Transcytosis: Transport of substances across the cell via vesicles.

Module 3.4 Cytoplasmic Organelles

Organelles are specialized structures within the cytoplasm that perform distinct cellular functions.

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles. These organelles work together to modify, package, and transport lipids and proteins.

• Packaging enzymes and lipids in vesicles prevents cellular damage and allows targeted delivery.

Module 3.5 The Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support, shape, and movement for the cell.

• Microfilaments: Thin filaments (actin) involved in cell movement and shape.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Hollow tubes that maintain cell shape and serve as tracks for organelle movement.

• Centrioles: Organize microtubules during cell division.

• Microvilli: Fingerlike extensions that increase surface area for absorption.

• Cilia: Short, hairlike structures that move substances across the cell surface.

• Flagella: Long, whip-like structures for cell movement (e.g., sperm).

• Movement Mechanisms:

  1. Actin-myosin interactions (muscle contraction, cell crawling)

  2. Microtubule-based transport (vesicle movement)

  3. Ciliary and flagellar beating

Module 3.6 The Nucleus

The nucleus is the largest organelle and serves as the control center of the cell.

• Structure: Surrounded by a double membrane (nuclear envelope) with nuclear pores for material exchange.

• Function: Stores genetic information and coordinates cell activities.

• Chromatin: Loosely coiled DNA and proteins present during interphase.

• Chromosomes: Condensed chromatin visible during cell division.

• Nucleolus: Dense region within the nucleus; site of ribosome synthesis.

Module 3.7 Protein Synthesis

Protein synthesis is the process by which cells build proteins based on genetic instructions.

• Genetic Code: The sequence of nucleotides in DNA that specifies the amino acid sequence of proteins.

• Transcription: The process of copying DNA into messenger RNA (mRNA); occurs in the nucleus.

• Translation: The process by which ribosomes use mRNA to assemble amino acids into proteins; occurs in the cytoplasm.

• rRNA (ribosomal RNA): Forms the core of ribosomes and catalyzes protein synthesis.

• mRNA (messenger RNA): Carries genetic information from DNA to ribosomes.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

Example: The codon AUG on mRNA codes for the amino acid methionine, which is the start signal for protein synthesis.

Module 3.8 The Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide.

• Interphase: The period of cell growth and DNA replication; consists of G1, S, and G2 phases.

• G0 Phase: A resting state where cells exit the cycle and do not divide.

• G1 Phase: Cell grows and carries out normal functions.

• S Phase: DNA is replicated.

• G2 Phase: Final preparations for cell division.

• M Phase (Mitosis): Division of the nucleus and its contents.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

• Relationship to Cancer: Uncontrolled cell division due to loss of cell cycle regulation can lead to cancer.

Additional info: The cell cycle is tightly regulated by checkpoints and proteins such as cyclins and cyclin-dependent kinases (CDKs).