Cells: The Living Units – Structure, Function, and Processes

Ch. 3: Cells: The Living Units

Introduction to Cell Theory

Cells are the fundamental units of life, forming the basis of all living organisms. The cell theory provides a framework for understanding biological structure and function.

• Cell Theory: All living things are composed of cells; the cell is the smallest unit of life; cells arise only from pre-existing cells.

• Cell Diversity: The human body contains trillions of cells, classified into over 250 types, ranging from microscopic to meter-long nerve cells.

• Universal Features: All cells possess a plasma membrane, cytoplasm, and nucleus.

Cell Structure Overview

Cells contain specialized structures called organelles, each performing distinct functions necessary for cellular life.

• Plasma Membrane: Boundary separating the cell from its environment.

• Cytoplasm: Gel-like substance containing organelles.

• Nucleus: Control center housing genetic material.

Plasma Membrane

Structure and Components

The plasma membrane is a dynamic, selectively permeable barrier composed of lipids, proteins, and carbohydrates.

• Phospholipid Bilayer: Forms the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

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

• Carbohydrates: Glycoproteins and glycolipids contribute to cell recognition and signaling.

• Cholesterol: Stabilizes membrane fluidity.

Cell Junctions

Cells are joined by specialized junctions that facilitate communication and structural integrity.

• Tight Junctions: Prevent leakage between cells.

• Desmosomes: Provide mechanical strength by anchoring cells together.

• Gap Junctions: Allow direct communication via channels.

Membrane Transport

Selective Permeability

The plasma membrane regulates the movement of substances, allowing only specific molecules to pass.

• Passive Transport: Movement without energy input.

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

Passive Transport Mechanisms

Passive transport includes diffusion, facilitated diffusion, and osmosis.

• Simple Diffusion: Movement of small, nonpolar molecules across the membrane.

• Facilitated Diffusion: Uses channel or carrier proteins for larger or polar molecules.

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

Tonicity

Tonicity describes the effect of solutions on cell volume.

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

• Hypertonic: Water leaves cell; cell shrinks.

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

Active Transport Mechanisms

Active transport moves substances against their concentration gradients using energy from ATP.

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

• Secondary Active Transport: Uses energy from gradients established by primary transport.

Vesicular Transport

Vesicular transport involves the movement of large particles or volumes via membrane-bound vesicles.

• Endocytosis: Uptake of substances into the cell.

• Exocytosis: Release of substances from the cell.

Summary Table: Active and Vesicular Transport

The following table summarizes the main types of active and vesicular transport, their energy sources, descriptions, and examples.

Process	Energy Source	Description	Examples
Primary Active Transport	ATP	Directly uses ATP to move ions against gradient	Na+/K+ pump
Secondary Active Transport	Gradient (indirect ATP)	Uses gradient from primary transport to move other substances	Na+-glucose cotransport
Phagocytosis	ATP	Cell engulfs large particles	White blood cell engulfing bacteria
Pinocytosis	ATP	Cell takes in fluid and dissolved substances	Absorption in intestines
Receptor-mediated Endocytosis	ATP	Specific uptake via receptors	Cholesterol uptake
Exocytosis	ATP	Release of substances from cell	Neurotransmitter release

Membrane Potential

Establishment and Maintenance

Membrane potential is the voltage across the cell membrane, essential for nerve and muscle function.

• Voltage: Electrical potential energy from separation of charged particles.

• Leakage Channels: Allow K+ to leave cell, creating negative charge inside.

• Active Transport: Maintains gradient, typically at -90 mV for K+.

Nucleus and Genetic Material

Nucleus Structure and Function

The nucleus is the control center of the cell, storing genetic information and directing cellular activities.

• Nuclear Envelope: Double membrane with nuclear pores for transport.

• Nucleolus: Site of ribosome synthesis.

• Multinucleated: Cells with multiple nuclei.

• Anucleate: Cells without a nucleus.

Cell Cycle and Division

Phases of the Cell Cycle

The cell cycle consists of interphase and mitosis, governing cell growth and division.

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

• Mitosis: Division of nucleus and cytoplasm.

• G0: Resting phase for non-dividing cells.

DNA Replication

DNA replication is a semiconservative process ensuring genetic continuity.

• Initiation: Helicase unwinds DNA.

• Elongation: DNA polymerase synthesizes new strands.

• Termination: Two new DNA molecules are formed.

Mitosis

Mitosis is the process by which cells divide, producing two identical daughter cells.

• Phases: Prophase, Metaphase, Anaphase, Telophase.

• Purpose: Growth, repair, and maintenance.

Protein Synthesis

Transcription: DNA to RNA

Transcription is the process of copying genetic information from DNA to messenger RNA (mRNA).

• Initiation: Proteins bind to gene promoter.

• Elongation: RNA polymerase synthesizes mRNA.

• Termination: RNA polymerase releases mRNA.

RNA Processing

RNA transcripts undergo splicing to remove non-coding regions (introns).

• Splicing: Removal of introns, joining exons.

Translation: RNA to Protein

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• Initiation: Ribosome binds to mRNA.

• Elongation: tRNA brings amino acids, matching codons.

• Termination: Stop codon signals release of peptide.

Genetic Code

The genetic code matches nucleotide codons to their corresponding amino acids.

• Codon: Three-nucleotide sequence on mRNA.

• Anticodon: Complementary sequence on tRNA.

Cell Death

Mechanisms of Cell Death

Cells undergo programmed death to maintain tissue health and function.

• Autophagy: Self-digestion of cellular components.

• Ubiquitin-Proteasome Pathway: Protein degradation.

• Apoptosis: Programmed cell death.

Recap

• Cells are the basic units of life.

• Plasma membranes regulate transport and maintain potential.

• Cytoplasm contains organelles for specialized functions.

• Nucleus stores genetic material and directs cell activities.

• Cell cycle, DNA replication, and protein synthesis are essential for growth and function.

• Cell death mechanisms ensure tissue health.