Cells: The Living Units

Generalized Cell Structure

All human cells share certain structural features, which can be divided into three major regions. Each region has specialized functions essential for cell survival and activity.

• Plasma Membrane: The outer boundary of the cell, responsible for regulating the movement of substances into and out of the cell.

• Cytoplasm: The cellular material between the plasma membrane and the nucleus, containing cytosol, organelles, and inclusions.

• Nucleus: The control center of the cell, containing genetic material (DNA) and directing cellular activities.

Plasma Membrane: Structure and Function

The plasma membrane is a selectively permeable barrier composed primarily of lipids and proteins. Its structure is crucial for its diverse functions.

• Chemical Composition: The membrane consists of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipids: Form the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

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

• Cholesterol: Stabilizes membrane fluidity.

• Carbohydrates: Attached to proteins and lipids, forming the glycocalyx for cell recognition.

• Functions: Physical barrier, selective transport, cell communication, and structural support.

Membrane Transport Mechanisms

Substances move across the plasma membrane by passive or active processes, depending on energy requirements and direction of movement.

• Passive Transport: Does not require cellular energy (ATP). Includes simple diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires ATP to move substances against their concentration gradient.

Passive Transport Mechanisms

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer from high to low concentration.

• Facilitated Diffusion: Movement of larger or polar molecules (e.g., glucose, ions) via protein channels or carriers.

• Osmosis: Diffusion of water through a selectively permeable membrane, often via aquaporins.

Example: Oxygen entering cells by simple diffusion; glucose entering cells by facilitated diffusion.

Active Transport Mechanisms

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

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

Vesicular Transport

• Endocytosis: Process by which cells take in substances by vesicle formation.

• Exocytosis: Process by which cells expel substances via vesicles fusing with the plasma membrane.

• Types of Endocytosis:

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

  • Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

  • Receptor-Mediated Endocytosis: Specific uptake of substances via receptor binding.

Cytoplasm: Composition and Organelles

The cytoplasm is the site of most cellular activities and contains various components with specialized functions.

• Cytosol: The viscous, semitransparent fluid in which organelles are suspended; composed mainly of water, proteins, salts, and sugars.

• Inclusions: Non-living chemical substances (e.g., glycogen granules, lipid droplets, pigment) stored in the cytoplasm.

• Organelles: Specialized structures performing specific functions.

Cytoskeletal Elements

• Microfilaments: Thin filaments of actin; involved in cell movement and shape.

• Intermediate Filaments: Rope-like fibers; provide mechanical support.

• Microtubules: Hollow tubes of tubulin; maintain cell shape, form spindle fibers, and serve as tracks for organelle movement.

Cell Extensions: Cilia and Microvilli

Cells may possess surface extensions that increase surface area or aid in movement.

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

• Microvilli: Finger-like extensions of the plasma membrane that increase surface area for absorption (e.g., intestinal lining).

Comparison: Cilia are motile and move fluid; microvilli are non-motile and increase absorptive capacity.

Cell Life Cycle

The cell life cycle consists of a series of events that lead to cell growth and division.

• Interphase: Period of cell growth and DNA replication; includes G1, S, and G2 phases.

• Mitotic Phase (M phase): Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Phases of Mitosis: Prophase, Metaphase, Anaphase, Telophase.

DNA Replication

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

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

• Enzymes involved: Helicase (unwinds DNA), DNA polymerase (adds nucleotides), ligase (joins fragments).

Equation:

Genes and the Genetic Code

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

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

• Function: Genes direct the synthesis of proteins, which determine cell structure and function.

Protein Synthesis

Protein synthesis occurs in two main phases: transcription and translation, involving DNA, mRNA, tRNA, and rRNA.

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

• Translation: mRNA is decoded by ribosomes in the cytoplasm to assemble amino acids into a polypeptide chain.

• Roles:

  • DNA: Template for mRNA synthesis.

  • mRNA: Carries genetic information from DNA to ribosomes.

  • tRNA: Brings amino acids to the ribosome during translation.

  • rRNA: Structural and enzymatic component of ribosomes.

Extracellular Materials

Cells are surrounded by various substances that support and influence their function.

• Extracellular Fluid (ECF): Includes interstitial fluid, blood plasma, and cerebrospinal fluid; provides nutrients and removes waste.

• Cellular Secretions: Substances such as saliva, mucus, and enzymes.

• Extracellular Matrix (ECM): A network of proteins and polysaccharides that provide structural support to tissues.