chapter three - cells : the living units

Cell Theory and Generalized Cell Structure

Cell Theory

The cell theory is foundational to biology, stating that the cell is the basic structural and functional unit of life. Organismal activity depends on the individual and collective activity of cells, and the biochemical activities of cells are dictated by their subcellular structures. Continuity of life is maintained through cellular reproduction.

• Basic Unit: All living organisms are composed of cells.

• Function: Cells carry out all vital functions.

• Structure: Subcellular structures determine cell function.

• Reproduction: Cells arise from pre-existing cells.

Plasma Membrane and Membrane Proteins

Plasma Membrane Structure

The plasma membrane separates intracellular from extracellular fluids and plays a dynamic role in cellular activity. The fluid mosaic model describes the membrane as a double bilayer of lipids with embedded proteins. Key components include phospholipids, cholesterol, and glycolipids.

• Phospholipids: Have hydrophobic tails and hydrophilic heads.

• Cholesterol: Provides membrane stability.

• Glycolipids: Lipids with bound carbohydrates, found only on the outer membrane surface.

• Glycocalyx: Glycoprotein area for cell recognition.

Membrane Proteins and Junctions

Membrane proteins perform many tasks, including transport, communication, and structural support. Membrane junctions include:

• Tight Junctions: Impermeable junctions encircling cells.

• Desmosomes: Anchoring junctions scattered along cell sides.

• Gap Junctions: Allow chemical substances to pass between cells.

Membrane Transport

Passive Transport

Plasma membranes are selectively permeable. Passive transport does not require cellular energy (ATP) and substances move down their concentration gradient. Transports molecules from high to low concentration. Types include:

• Simple Diffusion: Movement from higher to lower concentration.

• Facilitated Diffusion: Carrier-mediated or channel-mediated.

• Osmosis: Movement of water based on solute concentration.

Osmosis and Tonicity

Osmosis is the movement of water across a membrane. Tonicity describes the effect of a solution on cell volume:

• Isotonic: Same solute concentration as cytosol; cells retain normal size.

• Hypertonic: Higher solute concentration; cells shrink.

• Hypotonic: Lower solute concentration; cells swell and may burst.

Active Transport and Vesicular Transport

Active Transport

Active transport requires ATP and moves solutes against their concentration gradient. Transports molecules from low to high concentration. Types include:

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

• Secondary Active Transport: Indirect use of ATP via ion gradients.

The sodium-potassium pump maintains electrochemical gradients essential for muscle and nerve function.

Vesicular Transport

Vesicular transport moves large particles, macromolecules, and fluids across membranes. Types include:

• Exocytosis: Transport out of cell.

• Endocytosis: Transport into cell.

• Transcytosis: Transport into, across, and out of cell.

• Vesicular Trafficking: Transport within cell.

Membrane Potential

Resting Membrane Potential

Membrane potential is the separation of oppositely charged ions across a membrane, creating potential energy measured as voltage. The resting membrane potential (RMP) ranges from –50 to –100 mV and is established mainly by potassium ion diffusion and active transport.

Cytoplasm and Organelles

Cytoplasm

The cytoplasm is the material between the plasma membrane and nucleus, containing cytosol, organelles, and inclusions.

• Cytosol: Water with dissolved proteins, salts, sugars, and other solutes.

• Organelles: Specialized cellular compartments.

• Inclusions: Chemical substances such as glycogen granules and pigments.

Major Organelles

• Mitochondria: Double membrane, site of ATP production via aerobic respiration, contains DNA and RNA.

• Ribosomes: Granules of protein and rRNA, site of protein synthesis.

• Endoplasmic Reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid metabolism, detoxification).

• Golgi Apparatus: Modifies, concentrates, and packages proteins.

• Lysosomes: Digestive enzymes, breakdown of waste and cellular debris.

• Peroxisomes: Detoxify harmful substances, neutralize free radicals.

Cytoskeleton and Cellular Extensions

Cytoskeleton

The cytoskeleton is an elaborate network of rods throughout the cytosol, providing structural support and facilitating movement.

• Microfilaments: Actin strands, cell motility, shape changes.

• Intermediate Filaments: Tough, ropelike fibers, resist pulling forces.

• Microtubules: Hollow tubes, determine cell shape and organelle distribution.

Cellular Extensions

• Cilia: Move substances across cell surfaces.

• Flagella: Propel whole cells (e.g., sperm tail).

• Microvilli: Increase surface area for absorption.

Nucleus and Genetic Material

Nucleus Structure

The nucleus is the gene-containing control center of the cell, dictating protein synthesis. It contains the nuclear envelope, nucleoli, and chromatin.

• Nuclear Envelope: Double membrane with pores, encloses nucleoplasm.

• Nucleoli: Site of ribosome production.

• Chromatin: DNA and histones, forms chromosomes during cell division.

Cell Cycle and Division

Cell Cycle

The cell cycle consists of interphase (growth, DNA synthesis, preparation for division) and the mitotic phase (mitosis and cytokinesis).

• Interphase: G1 (growth), S (DNA synthesis), G2 (preparation), G0 (cease dividing).

• Mitotic Phase: Mitosis (nuclear division), Cytokinesis (cytoplasmic division).

DNA Replication

DNA replication is semiconservative, producing two identical DNA molecules. Key enzymes include helicase (unwinds DNA), DNA polymerase (synthesizes new strand), and ligase (joins fragments).

• Leading Strand: Synthesized continuously.

• Lagging Strand: Synthesized in segments.

Mitosis

Mitosis is essential for growth and repair. The phases are Prophase, Metaphase, Anaphase, and Telophase (PMAT). Cytokinesis divides the cytoplasm.

Protein Synthesis

From DNA to Protein

Protein synthesis involves transcription (DNA to RNA) and translation (RNA to protein). Messenger RNA (mRNA) carries genetic information, transfer RNA (tRNA) brings amino acids, and ribosomal RNA (rRNA) forms ribosomes.

• Transcription: RNA polymerase synthesizes RNA from DNA template.

• Translation: Ribosomes assemble polypeptides from mRNA codons and tRNA anticodons.

Cell Diversity

Cell Diversity

Cells vary in size, shape, and function, reflecting their specialized roles in the body.

Summary Table: Membrane Transport Mechanisms

The following table summarizes active and vesicular transport mechanisms:

Process	Energy Source	Description	Example
Primary Active Transport	ATP	Direct use of ATP to move ions against gradient	Na+-K+ pump
Secondary Active Transport	Ion gradient	Uses energy from ion gradient to transport other substances	Na+-glucose symport
Phagocytosis	ATP	Cell engulfs large particles	White blood cell engulfing bacteria
Pinocytosis	ATP	Cell "drinks" drops of fluid	Absorption in intestines
Receptor-mediated Endocytosis	ATP	Selective uptake of specific molecules	Cholesterol uptake
Exocytosis	ATP	Release of substances from cell	Neurotransmitter release