Chapter 3: The Cell – Structure, Function, and Processes

Chapter 3: The Cell

Module 3.1 Introduction to Cells

The cell is the basic structural and functional unit of all living organisms. Understanding its components and their functions is fundamental to anatomy and physiology.

• Main Parts of a Cell:

  • Plasma Membrane: Serves as the outer boundary, regulating entry and exit of substances.

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

  • Nucleus: The control center, housing genetic material (DNA) and directing cellular activities.

• Intracellular vs. Extracellular Fluid:

  • Intracellular Fluid (ICF): Fluid within cells, mainly cytosol.

  • Extracellular Fluid (ECF): Fluid outside cells, including interstitial fluid and plasma.

• Cytoplasm vs. Cytosol:

  • Cytoplasm: Includes cytosol, organelles, and inclusions.

  • Cytosol: The fluid portion of cytoplasm, where many metabolic reactions occur.

Module 3.2 Structure of the Plasma Membrane

The plasma membrane is a dynamic structure that separates the cell from its environment and controls molecular traffic.

• Lipid Distribution and Function:

  • Phospholipids: Form a bilayer, providing a semi-permeable barrier.

  • Cholesterol: Stabilizes membrane fluidity and integrity.

• Carbohydrates and Proteins:

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

  • Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), involved in cell recognition and signaling.

• Fluid Mosaic Model: Describes the membrane as a mosaic of lipids, proteins, and carbohydrates that can move laterally within the layer, allowing flexibility and dynamic function.

Module 3.3 Transport Across the Plasma Membrane

Cells regulate the movement of substances across the plasma membrane through various transport mechanisms.

• Passive Transport: No energy required.

  • Simple Diffusion: Movement of small, nonpolar molecules down their concentration gradient.

  • Facilitated Diffusion: Movement of larger or polar molecules via membrane proteins.

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

• Tonicity Effects:

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

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

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

• Active Transport: Requires energy (usually ATP).

  • Primary Active Transport: Direct use of ATP to move substances against their gradient (e.g., sodium-potassium pump).

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

• Vesicular Transport: Movement of large particles via vesicles.

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

  • Exocytosis: Release of substances from the cell.

Module 3.4 Cytoplasmic Organelles

Organelles are specialized structures within the cytoplasm that perform distinct cellular functions.

• Definition: Organelles are membrane-bound or non-membrane-bound structures with specific roles.

• Major Organelles and Functions:

  • Mitochondria: Site of ATP production via cellular respiration.

  • Ribosomes: Protein synthesis; 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: Break down fatty acids and detoxify harmful substances.

• Endomembrane System: Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles; these organelles work together to modify, package, and transport lipids and proteins.

Module 3.5 The Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, facilitates movement, and organizes cellular components.

• Components:

  • Microfilaments (actin filaments): Support cell shape and enable movement.

  • Intermediate Filaments: Provide mechanical strength.

  • Microtubules: Maintain cell shape, form spindle fibers, and serve as tracks for organelle movement.

• Centrioles, Cilia, and Flagella:

  • Centrioles: Organize microtubules during cell division.

  • Cilia: Short, hair-like structures for movement of substances across cell surfaces.

  • Flagella: Longer structures for cell movement (e.g., sperm cell).

• Role in Cellular Motion: The cytoskeleton enables cell movement, intracellular transport, and changes in cell shape.

Module 3.6 The Nucleus

The nucleus is the largest organelle, containing the cell's genetic material and coordinating activities such as growth and reproduction.

• Structure and Function: Surrounded by a double membrane (nuclear envelope) with nuclear pores for material exchange.

• Chromatin and Chromosomes:

  • Chromatin: DNA and associated proteins in a loose, threadlike form during interphase.

  • Chromosomes: Condensed chromatin visible during cell division.

  • Sister Chromatids: Identical copies of a chromosome, joined at the centromere during mitosis.

• Nucleolus: Dense region within the nucleus; site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Module 3.8 The Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide, ensuring genetic continuity.

• Interphase: Period of cell growth and DNA replication; consists of G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• Stages of the Cell Cycle:

  • Mitosis: Division of the nucleus into two genetically identical daughter nuclei.

  • Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

• DNA Replication: The process by which DNA makes an exact copy of itself during the S phase of interphase.

  • Key Steps: Unwinding of DNA, complementary base pairing, and formation of two identical DNA molecules.

Example: During mitosis, a human cell with 46 chromosomes duplicates its DNA and divides to produce two daughter cells, each with 46 chromosomes.