Cell Structure, Membrane Transport, and Genetic Processes: Study Notes for Anatomy & Physiology

Cell Structure and Function

Main Parts of a Cell

The cell is the basic structural and functional unit of living organisms. It consists of three main parts: the plasma (cell) membrane, cytoplasm, and nucleus.

• Plasma (Cell) Membrane: Serves as a selective barrier, regulating the movement of substances in and out of the cell.

• Cytoplasm: Contains cytosol (fluid portion) and organelles; site of most cellular activities.

• Nucleus: Contains genetic material (DNA) and controls cellular activities such as growth, metabolism, and reproduction.

Chemical Composition and Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which depicts the membrane as a dynamic structure composed of lipids, proteins, and carbohydrates.

• Lipids: Primarily phospholipids, forming a bilayer that provides fluidity and barrier properties.

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

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

Plasma Membrane Proteins

Membrane proteins have various functions:

• Structural Proteins: Provide support and maintain cell shape.

• Receptor Proteins: Bind signaling molecules and initiate cellular responses.

• Channel Proteins: Facilitate the movement of ions and molecules across the membrane.

Cellular Organelles

Organelles are specialized structures within the cytoplasm that perform distinct functions necessary for cell survival.

• Mitochondria: Site of ATP production (cellular respiration).

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

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

• Lysosomes: Contain digestive enzymes for breaking down waste.

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

Membrane Transport Mechanisms

Diffusion and Facilitated Diffusion

Cells exchange materials with their environment through various transport mechanisms.

• Simple Diffusion: Movement of molecules from high to low concentration without energy input or assistance.

• Facilitated Diffusion: Movement of molecules via membrane proteins (channels or carriers); still passive, but allows transport of larger or polar molecules.

Comparison Table:

Mechanism	Material Moved	Energy Source
Simple Diffusion	Small, nonpolar molecules (O2, CO2)	Concentration gradient
Facilitated Diffusion	Larger or polar molecules (glucose, ions)	Concentration gradient

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane. It differs from simple diffusion in that it specifically involves water molecules moving from an area of lower solute concentration to higher solute concentration.

Primary, Secondary Active Transport

Active transport requires energy to move substances against their concentration gradients.

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses the energy from the movement of one substance down its gradient to drive another substance against its gradient.

Comparison Table:

Type	Material Moved	Energy Source
Primary Active Transport	Ions (Na+, K+)	ATP
Secondary Active Transport	Glucose, amino acids	Ion gradient (created by primary transport)

Osmolarity and Tonicity

Osmolarity refers to the concentration of solute particles in a solution. Tonicity describes the effect of a solution on cell volume:

• Hypertonic: Higher solute concentration outside the cell; cell shrinks.

• Isotonic: Equal solute concentration; cell volume remains unchanged.

• Hypotonic: Lower solute concentration outside; cell swells.

Vesicular Transport: Exocytosis, Endocytosis, Phagocytosis, Pinocytosis

Cells use vesicles to transport large molecules or particles.

• Exocytosis: Movement of substances out of the cell via vesicles.

• Endocytosis: Uptake of substances into the cell by engulfing them in vesicles.

• Phagocytosis: "Cell eating"; engulfment of large particles or cells.

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

Comparison Table:

Process	Material Moved	Direction	Energy Source
Exocytosis	Proteins, waste	Out of cell	ATP
Endocytosis	Macromolecules	Into cell	ATP
Phagocytosis	Large particles	Into cell	ATP
Pinocytosis	Fluid	Into cell	ATP

Genetic Processes

Gene Code, Transcription, and Translation

The genetic code is the set of rules by which information encoded in DNA is translated into proteins. The processes include:

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

• Translation: mRNA is decoded by ribosomes to assemble amino acids into proteins.

Equation:

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

RNA Synthesis and Types of RNA

RNA synthesis (transcription) is the process of creating RNA from a DNA template. Types of RNA include:

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

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

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

Cell Cycle and DNA Replication

The cell cycle consists of phases that prepare a cell for division:

• G1 phase: Cell growth

• S phase: DNA replication

• G2 phase: Preparation for mitosis

• M phase: Mitosis (cell division)

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

Equation:

$\text{DNA} \xrightarrow{\text{replication}} \text{2 DNA molecules}$

Chromatin, Chromosomes, and Chromatids

Genetic material exists in different forms:

• Chromatin: Loosely packed DNA and proteins; present during interphase.

• Chromosomes: Condensed chromatin; visible during cell division.

• Chromatids: Each chromosome consists of two sister chromatids after DNA replication.

Mitosis and Cytokinesis

Mitosis is the process of nuclear division resulting in two genetically identical daughter cells. Cytokinesis is the division of the cytoplasm, completing cell division.

• Prophase: Chromatin condenses into chromosomes; spindle forms.

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

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

• Telophase: Nuclear envelopes reform; chromosomes decondense.

• Cytokinesis: Cytoplasm divides, forming two cells.

Equation:

$\text{Parent cell} \xrightarrow{\text{mitosis}} 2 \text{daughter cells}$

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