3. Cellular level of organization 2025

Chapter 3: The Cellular Level of Organisation

Learning Objectives

• List the functions of the cell membrane

• Describe the structure of the cell membrane (components and functions)

• Describe the permeability of the cell membrane

• Explain the origin and significance of the transmembrane potential

Functions of the Cell Membrane

• Physical Isolation: Acts as a barrier separating the inside of the cell from the extracellular fluid (ECF).

• Regulation of Exchange: Controls the entry of ions and nutrients, elimination of wastes and release of secretions.

• Sensitivity to Environment: The first part of the cell affected by changes in ECF composition.

• Structural Support: Provides stability to tissues.

Structure of the Cell Membrane

• Phospholipid Bilayer: Composed of hydrophobic tails and hydrophilic heads; forms the basic structure of the membrane.

• Integral Proteins: Embedded in the membrane; involved in transport and communication.

• Peripheral Proteins: Attached to the membrane’s surface; help in cellular signaling.

• Membrane Carbohydrates: Form glycolipids and glycoproteins; involved in cell recognition and signaling.

Membrane Permeability

• Selectively Permeable: Allows certain molecules to pass while restricting others.

• Water-soluble molecules (e.g., amino acids) cannot cross the membrane, while lipid-soluble molecules (e.g., O2, CO2) can.

Transmembrane Potential

• Definition: Potential difference across the plasma membrane; created by ion concentration differences.

• Resting Potential: Typically -70mV in neurons, -90mV in cardiac muscle cells.

• Significance: Essential for transmitting information in the nervous system and muscle contraction.

Cytosol and Extracellular Fluid Comparisons

• Distinct Differences:

  1. Higher K+ concentration in intracellular fluid (ICF)

  2. Lower Na+ concentration in ICF

  3. Higher concentration of proteins in ICF

  4. Small amounts of carbohydrates and reserves in ICF, none in ECF.

Components of the Cytoplasm

• Cytoplasm: Material between the plasma membrane and nucleus, containing cytosol and organelles.

• Organelles: Carry out specific functions:

  • Non-Membranous Organelles: Directly contact cytosol (e.g., cytoskeleton, ribosomes).

  • Membranous Organelles: Isolated from cytosol by membranes (e.g., endoplasmic reticulum, mitochondria).

The Cytoskeleton

• Function: Provides internal framework; assists with cell movement and mitosis.

• Components: Includes microfilaments, intermediate filaments, and microtubules.

Microvilli and Cilia

• Microvilli: Finger-shaped projections that increase surface area for absorption.

• Cilia: Move fluids across cell surfaces, important in respiratory and reproductive tracts.

Ribosomes

• Types: Free ribosomes (in cytoplasm) and Fixed ribosomes (on rough ER).

• Function: Protein synthesis.

Endoplasmic Reticulum (ER)

• Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

• Rough ER: Has ribosomes; synthesizes and modifies proteins.

Golgi Apparatus

• Functions:

  1. Modifies and packages proteins and enzymes.

  2. Renews or modifies the plasma membrane.

  3. Packages enzymes into vesicles for cytoplasmic use.

Lysosomes and Peroxisomes

• Lysosomes: Contain digestive enzymes; clean up and recycle cellular debris.

• Peroxisomes: Break down fatty acids and other compounds; smaller than lysosomes.

Mitochondria

• Function: Powerhouses of the cell; responsible for ATP production through aerobic metabolism.

The Nucleus

• Function: Control center of the cell; stores genetic material necessary for protein synthesis.

• Contents: DNA, nucleoplasm, nucleoli (produce rRNA and ribosomal subunits).

The Genetic Code

• DNA Base Sequence: Determines amino acid structure; triplet code is pivotal.

• Gene Activation: Uncoiling of DNA to access genes for protein synthesis.

Protein Synthesis: Transcription and Translation

• Transcription: Copied mRNA from DNA; occurs in the nucleus.

• Translation: mRNA is translated to proteins at ribosomes in the cytoplasm.

The Cell Life Cycle

• Phases: G1, S (DNA replication), G2, and Mitosis.

• Differentiation: Process through which cells develop specific structures for functions; essential for tissue formation.

Cell Transport Mechanisms

• Passive Transport: Movement without energy required (e.g., diffusion, osmosis).

• Active Transport: Movement requiring energy (electron flow against gradient).

• Types of Transport:

  • Diffusion: Molecules move from high to low concentration.

  • Filtration: Fluid movement based on pressure.

  • Carrier-mediated Transport: Involves integrative proteins; facilitated diffusion and active transport.

  • Vesicular Transport: Movement into/out of the cell via vesicles (endocytosis and exocytosis).