Cell Structure, Function, and Processes: Study Notes for Anatomy & Physiology

Cell Structure and Composition

Definition of a Cell

The cell is the basic structural, functional, and biological unit of all living organisms. Cells can exist as independent units of life (as in unicellular organisms) or as part of multicellular organisms, where they specialize and cooperate.

Extracellular Materials

Extracellular materials refer to substances found outside the cell, including extracellular fluid (ECF), interstitial fluid, plasma, and extracellular matrix. These materials provide structural support, mediate cell signaling, and facilitate nutrient and waste exchange.

• Extracellular matrix (ECM): A network of proteins (e.g., collagen, elastin) and polysaccharides that provide structural and biochemical support to surrounding cells.

• Extracellular fluid: The fluid environment outside cells, including plasma and interstitial fluid.

Major Regions of a Generalized Cell

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

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

• Plasma membrane: The outer boundary of the cell, regulating entry and exit of substances.

Plasma Membrane Structure and Function

Chemical Composition of the Plasma Membrane

The plasma membrane is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipids: Form the basic structure, providing fluidity and selective permeability.

• Proteins: Serve as channels, carriers, receptors, and enzymes.

• Cholesterol: Stabilizes membrane fluidity.

• Carbohydrates: Attached to proteins and lipids, involved in cell recognition.

Membrane functions include protection, communication, transport, and cell recognition.

Cell Junctions

Cell junctions are specialized structures that connect adjacent cells.

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

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap junctions: Allow direct communication between cells via channels.

Membrane Transport Processes

Passive Transport

Passive transport involves movement of substances across the membrane without energy input.

• Simple diffusion: Movement of molecules from high to low concentration.

• Facilitated diffusion: Movement via membrane proteins (channels/carriers).

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

Direction: Always down the concentration gradient.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Primary active transport: Direct use of ATP (e.g., sodium-potassium pump).

• Secondary active transport: Uses energy from the movement of another substance.

Tonicity

Tonicity describes the effect of a solution on cell volume.

• Isotonic: No net water movement; cell volume remains unchanged.

• Hypotonic: Water enters the cell; cell swells and may burst.

• Hypertonic: Water leaves the cell; cell shrinks.

Endocytosis and Exocytosis

These are forms of bulk transport across the plasma membrane.

• Endocytosis: Uptake of materials into the cell (e.g., phagocytosis, pinocytosis).

• Exocytosis: Release of materials from the cell.

Direction: Endocytosis brings substances in; exocytosis expels substances out.

Membrane Receptors

Membrane receptors are proteins that bind signaling molecules and initiate cellular responses. G protein-coupled receptors (GPCRs) are a major class involved in signal transduction.

Cytosol and Organelles

Cytosol Composition

Cytosol is the fluid portion of the cytoplasm, containing water, ions, proteins, and nutrients.

Mitochondria

Mitochondria are double-membraned organelles responsible for ATP production via cellular respiration.

Ribosomes, Endoplasmic Reticulum, and Golgi Apparatus

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

• Endoplasmic reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.

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

These organelles are functionally interconnected in the synthesis, modification, and transport of biomolecules.

Lysosomes and Peroxisomes

• Lysosomes: Contain digestive enzymes for breaking down waste and cellular debris.

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

Cell Extensions

Cilia and Microvilli

• Cilia: Hair-like structures that move substances across the cell surface.

• Microvilli: Finger-like projections that increase surface area for absorption.

Structure: Cilia contain microtubules; microvilli contain actin filaments.

Nucleus and Genetic Material

Nuclear Envelope, Nucleolus, and Chromatin

• Nuclear envelope: Double membrane surrounding the nucleus.

• Nucleolus: Site of ribosomal RNA synthesis.

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

DNA Replication

DNA replication is the process of copying DNA prior to cell division, occurring during the S phase of the cell cycle.

Genes and Genetic Code

• Gene: A segment of DNA that codes for a specific protein.

• Genetic code: The sequence of nucleotides in DNA and RNA that determines the amino acid sequence of proteins.

• Karyotype: The number and appearance of chromosomes in the nucleus; used to detect chromosomal abnormalities.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information:

• DNA → RNA → Protein

Processes involved:

• Transcription: DNA is transcribed to messenger RNA (mRNA).

• Translation: mRNA is translated into a protein.

Cell Division: Mitosis

Stages of Mitosis

Mitosis is the process of nuclear division resulting in two genetically identical daughter cells. The stages are:

1. Prophase: Chromatin condenses into chromosomes; nuclear envelope breaks down.

2. Metaphase: Chromosomes align at the cell's equator.

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

4. Telophase: Nuclear envelopes reform; chromosomes decondense.

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

Summary Table: Cell Structures and Functions

Structure	Main Function	Key Features
Nucleus	Genetic control center	Contains DNA, nucleolus
Plasma Membrane	Regulates entry/exit	Phospholipid bilayer, proteins
Mitochondria	ATP production	Double membrane, cristae
Ribosomes	Protein synthesis	Free or bound to ER
Golgi Apparatus	Protein modification & sorting	Stacked cisternae
Lysosomes	Digestion	Enzymes for breakdown
Peroxisomes	Detoxification	Oxidative enzymes
Cilia	Movement	Microtubules
Microvilli	Absorption	Actin filaments

Key Equations and Concepts

• Diffusion rate: $J = -D \frac{dC}{dx}$

• Osmosis: $\pi = iMRT$ (osmotic pressure)

• Sodium-potassium pump: $3 \text{Na}^+ \text{out} / 2 \text{K}^+ \text{in}$ per ATP hydrolyzed

• Central dogma: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Example: In a hypotonic solution, a red blood cell will swell due to water influx and may undergo lysis.

Additional info: Academic context and definitions have been expanded for clarity and completeness.