Anatomy and Physiology Ch 3

The Cellular Level of Organization

Introduction

• The study of cells is fundamental to understanding biology and encompasses their structure, function, and processes.

• Cells are the basic units of life, with each organism being composed of numerous cells.

Chapter Objectives

After studying this chapter, you should be able to:

• Describe the structure and function of the cell membrane, including material regulation.

• Detail the various functions of cytoplasmic organelles.

• Explain DNA structure, replication processes, and protein building from DNA codes.

• Outline the stages of the cell cycle and the specifics of cell division.

• Discuss cell differentiation and specialization in tissues.

• List representative cell types with notable characteristics in the human body.

Cell Development and Differentiation

• Development begins with a single fertilized egg cell, which differentiates into various specialized cells which form tissues and organs. This differentiation is crucial for maintaining the complexity of multicellular organisms.

• The interplay between structure and function in various cells exemplifies the theme of biology at all levels of organization. For example, skin cells (squamous) differ significantly in structure and function from nerve cells.

• Homeostasis, the maintenance of stable internal conditions, is vital for cell function.

History of the Cell Concept

• The term "cell" was first introduced by Robert Hooke in 1665 when observing cork tissue.

• Antonie van Leeuwenhoek later observed living cells, further laying the foundation for cell theory which posits that cells are the basic unit of life.

The Cell Membrane

Structure and Function

• The cell membrane (plasma membrane) acts as a protective barrier and regulates material movement in and out of cells.

• Composed of a phospholipid bilayer, it features cholesterol (which maintains fluidity) and proteins (which facilitate various functions).

Phospholipid Structure

• Each phospholipid has:

  • A hydrophilic (water-attracting) "head" made of a phosphate group.

  • Two hydrophobic (water-repellent) fatty acid "tails" that face inward in the bilayer.

• This amphipathic nature is crucial for membrane functionality.

Membrane Proteins

• Integral proteins penetrate the membrane; they include:

  • Channel proteins (allow ions to move in/out).

  • Cell recognition proteins (mark cells' identities).

  • Receptors that bind specific molecules, leading to cellular responses.

• Peripheral proteins are found on the membrane's surface and often function as enzymes or structural proteins.

Transport Across the Cell Membrane

Passive Transport

• Does not require energy; substances move along concentration gradients.

• Diffusion is the movement of particles from high to low concentration, while facilitated diffusion involves carrier proteins for larger or polar molecules.

• Osmosis is the diffusion of water across a semipermeable membrane.

Active Transport

• Requires energy (ATP) to move substances against their concentration gradients.

• Example: Sodium-potassium pump transports sodium out and potassium into the cell.

Endocytosis and Exocytosis

• Endocytosis: Cells assimilate material by engulfing it through their membrane, forming vesicles. This includes processes like phagocytosis and pinocytosis.

• Exocytosis: The expulsion of materials from the cell through vesicles.

Cystic Fibrosis

• Cystic fibrosis is caused by a mutation in the CFTR gene which affects ion channel function, leading to thick mucus buildup and complications in organs like lungs and pancreas.

• Treatments focus on managing symptoms and improving quality of life.

The Cytoplasm and Organelles

Overview of Organelles

• The cytoplasm consists of cytosol and organelles, each performing crucial functions for cell operation.

• Organelles include the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and peroxisomes, contributing to processes like protein synthesis, energy production, and detoxification.

Endoplasmic Reticulum (ER)

• Comprises Rough ER (with ribosomes, synthesizes proteins) and Smooth ER (synthesizes lipids, detoxifies).

Golgi Apparatus

• Functions like a post-office, modifying, sorting, and shipping proteins received from the ER.

Mitochondria

• Known as the powerhouse of the cell, involved in ATP production through cellular respiration, crucial for energy needs in active cells like muscles.

Peroxisomes

• Contain enzymes for lipid metabolism and detoxifying harmful substances, such as processing hydrogen peroxide.

The Cytoskeleton

• Composed of microtubules, microfilaments, and intermediate filaments, it provides structural support, aids in cell motility, and plays a role during cell division.

The Nucleus and DNA Replication

Nucleus Function

• The nucleus houses genetic material, regulating cellular functions by controlling the gene expression that translates to RNA and proteins.

DNA Structure

• Composed of double helical strands with complementary base pairs (A-T and C-G).

DNA Replication Process

1. Initiation: Helices unwinds the double helix.

2. Elongation: DNA polymerase synthesizes new strands using the original strands as templates.

3. Termination: Finishes replication producing two identical DNA molecules.

Protein Synthesis

Transcription

• mRNA production from a DNA template.

• Involves three stages: initiation, elongation, and termination.

Translation

• Synthesis of proteins by ribosomes using mRNA and tRNA.

• Steps include initiation, elongation, and termination.

Cell Growth and Division

The Cell Cycle

• Interphase (G1, S, G2) and the division phase (M), which includes mitosis and cytokinesis.

Mitosis Stages

1. Prophase: Chromosomes condense.

2. Metaphase: Chromosomes align at the equatorial plate.

3. Anaphase: Sister chromatids are pulled apart.

4. Telophase: Nuclei form in new cells.

Cell Cycle Regulation

• Controlled through checkpoints regulated by cyclins and cyclin-dependent kinases.

• Loss of control can lead to cancer, characterized by uncontrolled cell growth and division.

Cellular Differentiation

• Differentiation processes transform stem cells into specialized cells.

• Stem cells can be totipotent, pluripotent, multipotent, or unipotent depending on their differentiation potential.

• Significance in regenerative medicine and treatment development for various diseases like diabetes and cardiovascular conditions.