The Cellular Level of Organization

Introduction

The cell is the fundamental unit of life in all living organisms. Understanding the structure and function of cells is essential for studying anatomy and physiology. This section covers the main components of cells, the plasma membrane, cellular transport mechanisms, the cytoskeleton, gene expression, and the cell cycle.

Comparing Cell Types

Animal Cells, Plant Cells, and Prokaryotic Cells

• Similarities: All have a plasma membrane, cytoplasm, genetic material (DNA), and ribosomes.

• Differences:

  • Animal Cells: Eukaryotic, lack cell walls, contain centrioles, have small vacuoles.

  • Plant Cells: Eukaryotic, have cell walls (cellulose), chloroplasts, large central vacuole.

  • Prokaryotic Cells: Lack a nucleus and membrane-bound organelles, usually smaller, DNA is circular and found in the nucleoid region.

Basic Structure of an Animal Cell

Three Main Parts

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

• Cytoplasm: Jelly-like fluid containing organelles and cytosol.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

Cell Organelles and Their Functions

Major Organelles

• Nucleus: Stores DNA, site of transcription.

• Mitochondria: Site of ATP (energy) production.

• Endoplasmic Reticulum (Rough and Smooth): Protein and lipid synthesis.

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

• Lysosomes: Digestion of macromolecules.

• Peroxisomes: Breakdown of fatty acids and detoxification.

• Ribosomes: Protein synthesis.

• Cytoskeleton: Structural support, cell movement.

Additional info: Plant cells also contain chloroplasts and a large central vacuole; prokaryotes lack membrane-bound organelles.

The Plasma Membrane

Structure and Properties

• Fluid Mosaic Model: Describes the membrane as a flexible, dynamic structure with proteins embedded in or attached to a bilayer of phospholipids.

• Lipid Bilayer: Composed of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward.

• Amphipathic: Phospholipids have both hydrophilic and hydrophobic regions.

• Selective Permeability: Allows some substances to pass while restricting others.

Membrane Transport Mechanisms

Passive vs. Active Transport

• Passive Transport: Movement of substances down their concentration gradient without energy input.

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2).

  • Facilitated Diffusion: Movement via membrane proteins (channels or carriers); e.g., glucose transport.

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

• Active Transport: Movement against the concentration gradient, requiring energy (ATP).

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

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

Factors Influencing Diffusion: Concentration gradient, temperature, molecular size, membrane permeability.

Equilibrium: Diffusion stops when concentrations are equal, but molecules continue to move randomly (dynamic equilibrium).

Membrane Transport Proteins

• Channel Proteins: Form pores for ions or water to pass through.

• Carrier Proteins: Bind and transport specific molecules across the membrane.

Bulk Transport Mechanisms

Endocytosis and Exocytosis

• Receptor-Mediated Endocytosis: Specific molecules are ingested into the cell after binding to receptors.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

Additional info: Pinocytosis (cell drinking) and phagocytosis (cell eating) are other forms of endocytosis.

The Cytoskeleton

Components

• Microfilaments: Actin filaments; support cell shape and movement.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Tubulin polymers; maintain cell shape, form cilia/flagella, and guide chromosome movement during cell division.

Genetic Material: Chromatin vs. Chromosomes

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

• Chromosomes: Condensed, visible structures during cell division.

The Endomembrane System

Definition and Pathway

• Definition: A network of membranes inside the cell that work together to modify, package, and transport lipids and proteins.

• Components: Nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, plasma membrane.

• Secretory Protein Pathway: Synthesized in rough ER → transported to Golgi apparatus → packaged into vesicles → exocytosed at plasma membrane.

Protein Synthesis (Gene Expression)

Major Steps

• Transcription: DNA is copied into mRNA in the nucleus.

• RNA Processing: mRNA is modified (splicing, capping, polyadenylation).

• Translation: mRNA is decoded by ribosomes to synthesize proteins in the cytoplasm.

Sub-Steps in Transcription

• Initiation: RNA polymerase binds to promoter region.

• Elongation: RNA strand is synthesized.

• Termination: RNA polymerase releases the completed mRNA.

Sub-Steps in Translation

• Initiation: Ribosome assembles around mRNA and first tRNA.

• Elongation: Amino acids are added one by one to the growing polypeptide chain.

• Termination: Ribosome reaches a stop codon and releases the polypeptide.

Roles of RNA Types

• mRNA (messenger RNA): Carries genetic code from DNA to ribosome.

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

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

Gene Expression Steps and Major Events

The Cell Cycle

Overview

• Definition: The sequence of events in the life of a cell from one division to the next.

• Main Stages: Interphase (G1, S, G2), Mitosis (M phase), Cytokinesis.

Pie Chart Representation: Interphase occupies the majority of the cell cycle; mitosis and cytokinesis are shorter phases.

Stages of Interphase and Mitosis

Stages of Meiosis

Comparing Mitosis and Meiosis

• Mitosis: Produces two genetically identical diploid cells; for growth and repair.

• Meiosis: Produces four genetically unique haploid cells; for sexual reproduction.

• Key Differences: Meiosis includes two divisions and crossing over; mitosis has one division and no crossing over.

Aging and the Cell

• Effects of Aging: Accumulation of DNA damage, decreased cellular repair, reduced cell division, and increased apoptosis.