Cells: The Living Units

Introduction to Cells

Cells are the basic structural and functional units of all living organisms. In human anatomy and physiology, understanding cell structure and function is essential for grasping how tissues, organs, and systems operate.

• Cell Theory: All living things are composed of cells; cells are the smallest units of life; all cells arise from pre-existing cells.

• Generalized Human Cell: Consists of three main parts: plasma membrane, cytoplasm (with organelles), and nucleus.

• Example: Muscle cells contain many mitochondria due to high energy demands.

Plasma Membrane Structure and Function

Plasma Membrane Composition

The plasma membrane is a fragile barrier that separates the cell's internal environment from the external environment. It is primarily composed of a phospholipid bilayer with embedded proteins.

• Phospholipid Bilayer: Provides fluidity and selective permeability.

• Membrane Proteins: Integral proteins span the membrane; peripheral proteins are loosely attached to the surface.

• Functions: Transport, cell recognition, receptor sites for hormones, cell adhesion.

Membrane Junctions

Cells are connected by specialized junctions that help maintain tissue integrity and facilitate communication.

• Tight Junctions: Prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow ions and small molecules to pass between cells for communication.

Plasma Membrane Transport Processes

Transport across the plasma membrane can be passive or active, depending on whether energy is required.

• Passive Transport: Does not require energy. Includes:

  • Simple diffusion

  • Facilitated diffusion

  • Osmosis

  • Filtration

• Active Transport: Requires energy (ATP). Includes:

  • Primary active transport (e.g., sodium-potassium pump)

  • Secondary active transport

  • Vesicular transport (endocytosis, exocytosis, phagocytosis, pinocytosis)

Osmosis and Red Blood Cells

Osmosis is the diffusion of water across a selectively permeable membrane. The effect of osmosis on red blood cells (RBCs) depends on the tonicity of the surrounding solution.

Membrane Potential

The plasma membrane helps generate and maintain the resting membrane potential, which is essential for nerve and muscle function.

• Resting Membrane Potential: Electrical potential difference across the membrane, typically −20 to −200 mV (inside negative).

• Key Ions: Sodium (Na+) higher outside; Potassium (K+) higher inside.

• Sodium-Potassium Pump: Maintains ion gradients by pumping Na+ out and K+ in.

$\text{Na}^+/\text{K}^+ \text{ pump:} \, 3\, \text{Na}^+ \text{ out}, \, 2\, \text{K}^+ \text{ in per ATP}$

Cytoplasm and Organelles

Cytoplasm

The cytoplasm is the cellular material between the plasma membrane and the nucleus. It contains cytosol (fluid), organelles, and inclusions.

• Cytosol: Semi-transparent fluid in which organelles are suspended.

• Inclusions: Chemical substances such as glycogen granules, lipid droplets.

Major Cytoplasmic Organelles

• Mitochondria: "Powerhouse" of the cell; site of ATP production via aerobic respiration.

• Endoplasmic Reticulum (ER):

  • Rough ER (RER): Studded with ribosomes; synthesizes proteins.

  • Smooth ER (SER): Lacks ribosomes; synthesizes lipids, detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes; perform intracellular digestion.

• Peroxisomes: Contain oxidase enzymes; detoxify harmful substances.

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

• Centrioles: Paired cylindrical bodies; organize microtubules during cell division and form cilia and flagella.

Nucleus and Genetic Material

Nucleus Structure and Function

The nucleus is the control center of the cell, containing genetic material (DNA) and directing cellular activities.

• Nuclear Envelope: Double membrane that regulates passage of materials.

• Nucleolus: Site of ribosome subunit assembly.

• Chromatin: DNA and associated proteins (histones).

Cell Life Cycle

Cells undergo a life cycle that includes growth, DNA replication, and division.

• Interphase: Period of cell growth and DNA replication.

• S Phase: DNA synthesis (replication).

• M Phase: Mitosis (cell division) to produce two identical daughter cells.

DNA Replication

DNA replication ensures that each daughter cell receives an identical set of genes.

• Process: DNA strands separate; each serves as a template for synthesis of a complementary strand.

• Enzymes: DNA polymerase builds new strands.

$\text{DNA} \rightarrow \text{DNA (replicated)}$

Genes and the Genetic Code

A gene is a segment of DNA that contains instructions for making a specific protein. The genetic code is the sequence of nucleotide bases that determines protein structure.

• Nucleotide Bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T).

• Codon: Sequence of three bases that specifies an amino acid.

• Genetic Code: The sequence of codons forms a "sentence" that tells the cell how to build a protein.

Protein Synthesis

Protein synthesis occurs in two main steps: transcription and translation.

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

• Translation: mRNA is read by ribosomes in the cytoplasm; transfer RNA (tRNA) brings amino acids to build the polypeptide chain.

$\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Protein Degradation

Cellular proteins are eventually degraded when they are damaged or no longer needed.

• Ubiquitin: Tags proteins for destruction.

• Proteasome: Degrades tagged proteins into peptides.

Extracellular Materials

Types of Extracellular Materials

Cells are surrounded by various extracellular materials that support and protect them.

• Interstitial Fluid: Bathes cells, providing nutrients and removing wastes.

• Blood Plasma: Fluid component of blood.

• Cell Secretions: Include digestive enzymes, mucus, and serous fluids.

• Extracellular Matrix: Jellylike substance composed of proteins and polysaccharides; provides structural support.

Cell Differentiation and Aging

Cell Differentiation

Cells develop specialized structures and functions through differentiation, guided by chemical signals and gene expression.

• Process: Cells in different regions are exposed to different signals, leading to specialization.

• Example: Nerve cells, muscle cells, and epithelial cells have distinct features and functions.

Cell Aging

Cell aging is influenced by genetic and environmental factors, including chemical insults and accumulation of free radicals.

• Effects: Damage to DNA and proteins, weakened immune response, and programmed cell death (apoptosis).

• Additional info: Aging is a complex process involving both intrinsic genetic programs and extrinsic factors such as oxidative stress.