Chapter 3: Cells – Structure, Function, and Physiology

Cells: Structure, Function, and Physiology

Definition and Overview of Cells

The cell is the fundamental structural and functional unit of all living organisms. Cells carry out essential life processes and vary in size, shape, and function depending on their role in the body.

• Cell: The smallest unit of life, capable of independent existence and performing vital functions.

• Example: Human cells include muscle cells, nerve cells, and epithelial cells.

Plasma Membrane: Structure and Function

The plasma membrane is a selectively permeable barrier that surrounds the cell, maintaining homeostasis and mediating communication with the environment.

• Structure: Composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Function: Regulates entry and exit of substances, facilitates cell signaling, and provides structural support.

• Example: Transport proteins allow glucose to enter the cell.

Cell Junctions: Tight Junctions, Desmosomes, and Gap Junctions

Cell junctions connect adjacent cells and facilitate communication and structural integrity.

• Tight Junctions: Seal adjacent cells to prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow direct passage of ions and small molecules between cells for communication.

• Comparison Table:

\]Junction Type	Main Function	Location
Tight Junction	Barrier to fluid	Intestinal lining
Desmosome	Mechanical strength	Skin, heart muscle
Gap Junction	Cell communication	Cardiac muscle

Cell Transport Mechanisms

Cells use various mechanisms to move substances across the plasma membrane, classified as passive or active transport.

• Passive Transport: DUnderstanding the movement of water and solutes is essential for cell physiology.

• Osmolarity: The concentration of solute particles in a solution.

• Hydrostatic Pressure: The pressure exerted by a fluid due to gravity.oes not require energy; includes diffusion, osmosis, and facilitated diffusion.

• Active Transport: Requires energy (ATP); includes primary and secondary active transport, endocytosis, and exocytosis.

• Comparison: Passive transport moves substances down their concentration gradient, while active transport moves substances against their gradient.

Key Terms in Cell Transport

• Osmotic Pressure: The pressure required to prevent water movement across a semipermeable membrane.

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

• Hypertonic Solution: Higher solute concentration than the cell; water moves out, cell shrinks.

• Hypotonic Solution: Lower solute concentration than the cell; water moves in, cell swells.

Membrane Potential

The membrane potential is the electrical potential difference across the plasma membrane, crucial for nerve and muscle function.

• Resting Membrane Potential: Established by the unequal distribution of ions, mainly sodium (Na+) and potassium (K+).

• Maintenance: The sodium-potassium pump ($Na^+/K^+$ ATPase) actively transports ions to maintain the potential.

• Equation: $V_m = E_K$ (Nernst equation for potassium)

Glycocalyx

The glycocalyx is a carbohydrate-rich layer on the cell surface, important for cell recognition and protection.

• Role: Facilitates cell-cell recognition, protects the cell, and aids in immune response.

Cytosol and Inclusions

Cytosol is the fluid component of the cytoplasm, containing dissolved substances and inclusions.

• Composition: Water, ions, proteins, carbohydrates, and lipids.

• Inclusions: Non-living substances such as glycogen granules, lipid droplets, and pigments.

Cell Organelles: Structure and Function

Organelles are specialized structures within cells that perform distinct functions.

• Ribosomes: Sites of protein synthesis; composed of rRNA and proteins.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Mitochondria: Powerhouse of the cell; site of ATP production.

• Golgi Body: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes for breaking down waste.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Cytoskeleton Elements: Microfilaments, intermediate filaments, and microtubules provide structural support and facilitate movement.

• Centrioles: Organize microtubules during cell division.

• Cilia: Short, hair-like structures for movement of substances across cell surfaces.

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

• Microvilli: Increase surface area for absorption.

Endomembrane System

The endomembrane system is a network of membranes involved in synthesis, transport, and modification of cellular products.

• Components: Nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Function: Coordinates production and transport of proteins and lipids.

Nucleus and Associated Structures

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

• Nucleus: Contains DNA and directs cell activities.

• Nuclear Envelope: Double membrane with pores for transport.

• Nucleolus: Site of ribosome synthesis.

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

Cell Life Cycle and Mitosis

The cell life cycle includes phases of growth, DNA replication, and division.

• Phases:

  • Interphase: Cell growth and DNA replication.

  • Mitosis: Division of the nucleus.

  • Cytokinesis: Division of the cytoplasm.

• Mitosis Phases:

  • Prophase: Chromatin condenses, spindle forms.

  • Metaphase: Chromosomes align at the center.

  • Anaphase: Sister chromatids separate.

  • Telophase: Nuclear envelopes reform.

DNA Replication

DNA replication is the process by which a cell duplicates its DNA before division.

• Process: DNA unwinds, each strand serves as a template for a new complementary strand.

• Enzyme: DNA polymerase synthesizes new DNA.

• Equation: $DNA_{parent} ightarrow DNA_{daughter}$

Genes and Genetic Code

Genes are segments of DNA that code for proteins, and the genetic code is the set of rules by which DNA sequences are translated into proteins.

• Gene: A sequence of DNA that encodes a functional product.

• Genetic Code: The correspondence between nucleotide triplets and amino acids.

Protein Synthesis

Protein synthesis occurs in two phases: transcription and translation.

• Transcription: DNA is copied into messenger RNA (mRNA).

• Translation: mRNA is decoded to build a protein.

• Process:

  1. DNA unzips and mRNA is synthesized (transcription).

  2. mRNA travels to ribosome, where tRNA brings amino acids (translation).

• Triplets, Codons, Anticodons:

  • Triplet: Three DNA nucleotides coding for one amino acid.

  • Codon: Three mRNA nucleotides corresponding to an amino acid.

  • Anticodon: Three tRNA nucleotides complementary to mRNA codon.

Extracellular Material

Extracellular material surrounds cells and provides structural and biochemical support.

• Composition: Includes extracellular fluid, matrix proteins (collagen, elastin), and ground substance.

• Function: Supports cells, facilitates communication, and regulates cell behavior.

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for college-level Anatomy & Physiology students.