The Cellular Basis of Life

Definition and Major Regions of a Generalized Cell

Cells are the fundamental structural and functional units of all living organisms. Each cell consists of three major regions, each with distinct roles:

• Plasma Membrane: The outer boundary that regulates entry and exit of substances.

• Cytoplasm: The internal fluid containing organelles and cytosol.

• Nucleus: The control center containing genetic material (DNA).

Example: Human epithelial cells have all three regions, allowing them to form protective barriers and regulate material exchange.

The Plasma Membrane

Structure

The plasma membrane is described by the fluid mosaic model, which depicts it as a double layer of phospholipids with embedded proteins. This structure allows flexibility and selective permeability.

• Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.

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

• Carbohydrates: Attached to proteins and lipids, forming glycoproteins and glycolipids for cell recognition.

Functions: The plasma membrane controls the movement of substances, communication, and cell recognition.

Membrane Junctions

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

• Desmosomes: Anchor cells together, providing mechanical stability.

• Gap Junctions: Allow direct communication between cells via connexons.

Membrane Transport

Transport across the plasma membrane can be passive or active:

• Passive Transport: Movement down a concentration gradient without energy input (e.g., diffusion, facilitated diffusion, osmosis).

• Active Transport: Movement against a concentration gradient, requiring ATP (e.g., sodium-potassium pump).

Comparison Table:

Osmosis and Tonicity

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

• Tonicity: The ability of a solution to change the shape of cells by altering their internal water volume.

• Types:

  • Isotonic: No net water movement; cell shape unchanged.

  • Hypertonic: Water leaves cell; cell shrinks (crenation).

  • Hypotonic: Water enters cell; cell swells and may burst (lysis).

Vesicular Transport

• Endocytosis: Uptake of materials via vesicles (e.g., phagocytosis, pinocytosis, receptor-mediated endocytosis).

• Exocytosis: Release of substances from the cell via vesicles.

Membrane Potential

The membrane potential is the voltage difference across the plasma membrane, established by selective diffusion of ions (mainly Na+ and K+).

• Resting membrane potential is typically around -70 mV in neurons.

• Maintained by the sodium-potassium pump and selective permeability.

Cell-Environment Interactions

• Cell Adhesion Molecules (CAMs): Mediate cell-to-cell and cell-to-extracellular matrix interactions.

• Membrane Receptors: Bind signaling molecules (ligands) to initiate cellular responses.

The Cytoplasm

Composition and Organelles

The cytoplasm consists of cytosol (fluid), organelles, and inclusions. Each organelle performs specialized functions:

• Mitochondria: Site of ATP production via cellular respiration.

• Ribosomes: Synthesize proteins; can be free or attached to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Protein synthesis and modification.

  • Smooth ER: Lipid synthesis and detoxification.

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

• Lysosomes: Contain digestive enzymes for breakdown of waste.

• Peroxisomes: Detoxify harmful substances.

• Cytoskeleton: Provides structural support and facilitates movement (microtubules, microfilaments, intermediate filaments).

• Centrosome and Centrioles: Organize microtubules during cell division.

• Cellular Extensions:

  • Cilia: Move substances across cell surfaces.

  • Microvilli: Increase surface area for absorption.

The Nucleus

Structure and Function

• Nuclear Envelope: Double membrane with nuclear pores for material exchange.

• Nucleolus: Site of ribosomal RNA synthesis and ribosome assembly.

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

Cell Growth and Reproduction

The Cell Cycle

The cell cycle consists of interphase (growth and DNA replication) and the mitotic phase (cell division).

• Interphase: G1 (growth), S (DNA synthesis), G2 (preparation for division).

• Mitotic Phase: Mitosis (division of nucleus) and cytokinesis (division of cytoplasm).

Control of Cell Division

• Regulated by checkpoints, cyclins, and cyclin-dependent kinases (CDKs).

• Ensures accurate DNA replication and division.

Protein Synthesis

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

• Translation: mRNA is translated into protein at ribosomes in the cytoplasm.

• Key Molecules: DNA, mRNA, ribosomal RNA (rRNA), transfer RNA (tRNA).

Example: Hemoglobin synthesis in red blood cells involves transcription of the hemoglobin gene and translation of mRNA into the hemoglobin protein.

Apoptosis, Autophagy, and Proteasomes

• Apoptosis: Programmed cell death that removes unneeded or damaged cells.

• Autophagy: Degradation and recycling of cellular components.

• Proteasomes: Protein complexes that degrade unneeded or damaged proteins.

Additional info: These processes are essential for tissue homeostasis and prevention of disease.

Summary

-Cells are the smallest units of life.

-The fluid mosaic model depicts the plasma

membrane as a double layer of phospholipids with

embedded protein.

- Passive membrane transport is diffusion of

molecules down their concentration gradient.

-Active membrane transport directly or indirectly

uses ATP.

-Selective diffusion establishes the membrane

potential.

-Cell adhesion molecules and membrane receptors

allow the cell to interact with its environment.

-Cytoplasmic organelles each perform a specialized

task.

-Cilia and microvilli are two main types of cellular

extensions.

The nucleus includes the nuclear envelope, the

nucleolus, and chromatin.

-The cell cycle consists of interphase and a mitotic

phase•

-The cell cycle consists of interphase and a mitotic

phase.

-Messenger RNA carries instructions from DNA for

building proteins.

-Apoptosis disposes of unneeded cells; autophagy

and proteasomes dispose of unneeded organ