Cells: The Living Units – Chapter 3 Study Notes

Cells: The Living Units

Introduction to Cells

Cells are the basic living, structural, and functional units of the body. All organisms are composed of cells, which are specially adapted to perform particular functions. In human anatomy and physiology, understanding the structure and function of cells is foundational.

• Eukaryotic cells: Cells with a membrane-bound nucleus and organelles.

• Over 250 different types of human cells, varying in size and shape according to their function.

• Cells are mostly comprised of water and are surrounded by a plasma membrane.

Levels of Organization in the Human Body

Hierarchy of Biological Organization

The human body is organized into a hierarchy of structural levels, each building upon the previous.

• Atom: The smallest unit of matter.

• Molecule: Two or more atoms bonded together.

• Macromolecule: Large, complex molecules (e.g., proteins, DNA).

• Organelle: Specialized structures within cells (e.g., mitochondria).

• Cell: Basic unit of life.

• Tissue: Groups of similar cells performing a common function.

• Organ: Composed of two or more tissue types.

• Organ system: Groups of organs working together.

• Organism: The complete living being.

Example: The digestive system includes organs such as the stomach and intestines, which are made up of tissues composed of cells.

Cell Structure and Function

Major Components of a Cell

Cells contain various structures that perform specific functions necessary for life.

• Plasma membrane: Selectively permeable barrier separating the cell from its environment.

• Cytoplasm: Watery fluid (cytosol) plus organelles inside the cell.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

Plasma Membrane Structure

The plasma membrane is a flexible but sturdy barrier composed of several components:

• Phospholipids: Make up about 75% of the membrane, forming a lipid bilayer with hydrophilic (polar) heads and hydrophobic (non-polar) tails.

• Glycolipids and cholesterol: Provide stability and fluidity.

• Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

Example: The lipid bilayer allows selective passage of substances, maintaining homeostasis.

Cell Junctions

Types of Cell Junctions

Cell junctions are specialized structures that connect cells together, allowing communication and maintaining tissue integrity.

• Gap Junctions: Communicating junctions that allow ions and small molecules to pass between cells for intercellular communication. Common in heart and nerve cells.

• Tight Junctions: Impermeable junctions that prevent molecules from passing through the intercellular space. Found in digestive tract lining.

• Desmosomes: Anchoring junctions that bind adjacent cells together and help form an internal tension-reducing network of fibers. Found in tissues subject to mechanical stress (e.g., skin, heart).

Transport Across the Cell Membrane

Types of Membrane Transport

Cells regulate the movement of substances across their membranes through various transport mechanisms.

• Simple Diffusion: Passive movement of particles from high to low concentration. No energy required.

• Facilitated Diffusion: Passive transport of substances via channels or carrier proteins.

• Osmosis: Passive movement of water across a selectively permeable membrane.

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

• Vesicular Transport: Movement of large particles via vesicles (endocytosis and exocytosis).

Equation (Diffusion Rate): $J = -D \frac{dC}{dx}$ Where $J$ is the rate of diffusion, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Tonicity and Its Effects on Cells

Tonicity describes the ability of a solution to change the shape or tone of cells by altering water volume.

Type of Solution	Description	Effect on Cell
Isotonic	Same concentration of non-penetrating solutes inside and outside the cell	Cell retains its shape
Hypotonic	Lower concentration of non-penetrating solutes outside the cell	Cell swells and may burst (lyse)
Hypertonic	Higher concentration of non-penetrating solutes outside the cell	Cell shrinks (crenates)

Cell Organelles and Their Functions

Major Organelles

Organelles are specialized structures within the cytoplasm that perform specific functions.

• Ribosomes: Sites of protein synthesis; can be free or attached to rough ER.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER (with ribosomes) synthesizes proteins, smooth ER synthesizes lipids.

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

• Lysosomes: Contain digestive enzymes to break down waste and cellular debris.

• Vacuoles: Store substances; more common in plant cells.

• Mitochondria: "Powerhouses" of the cell; produce ATP via cellular respiration.

• Cytoskeleton: Network of protein rods (microfilaments, intermediate filaments, microtubules) that provide structural support.

• Centrosome and Centrioles: Organize microtubules and are involved in cell division.

Example: Mitochondria generate ATP, the energy currency of the cell.

Nucleus and Genetic Control

Structure and Function of the Nucleus

The nucleus is the control center of the cell, containing genetic material and regulating cellular activities.

• Nuclear Envelope: Double-layered membrane with nuclear pores for selective transport.

• Nucleolus: Site of ribosome production.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

DNA Functions

DNA must accomplish three key functions:

• Replication: Copying itself for cell division.

• Mutation: Ability to change and adapt.

• Protein Synthesis: Directing the production of proteins responsible for cellular functions.

Cell Extensions

Types of Cell Extensions

Cells may have specialized extensions to aid in movement or absorption.

• Cilia: Short, hair-like projections that move substances across the cell surface (e.g., tracheal cells).

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

• Microvilli: Finger-like extensions that increase surface area for absorption (e.g., intestinal cells).

Cell Cycle and Division

Phases of the Cell Cycle

The cell cycle is the process by which cells grow, replicate, and divide.

• Interphase: Preparation phase, includes G1 (growth), S (DNA synthesis), and G2 (final preparations).

• M Phase (Mitosis): Division of the nucleus into two identical daughter cells.

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

Steps of Mitosis

• Prophase: Chromatin condenses into chromosomes; nuclear envelope dissolves; spindle forms.

• Metaphase: Chromosomes align at the cell's equator (metaphase plate).

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Chromosomes de-condense; nuclear envelope reforms; spindle disappears.

Equation (DNA Replication): $DNA_{parent} \xrightarrow{replication} DNA_{daughter}$

Protein Synthesis

Transcription and Translation

Protein synthesis is the process by which cells make proteins, occurring in two main steps:

• Transcription: DNA is coded into messenger RNA (mRNA) in the nucleus.

• Translation: mRNA is decoded by ribosomes in the cytoplasm to assemble proteins.

Equation (Central Dogma): $DNA \xrightarrow{transcription} mRNA \xrightarrow{translation} Protein$

Mutations

A mutation is a permanent alteration in the DNA sequence, which can affect protein function and lead to changes in cell behavior.

• Mutations may result in disease or adaptation.

Additional info: Some details, such as the specific roles of organelles and the molecular basis of transport, have been expanded for academic completeness.