Cell Physiology

Chapter 2: Cell Physiology

Overview of Cells

  • Cells are the fundamental units of life and are composed of three key components:

    1. Plasma Membrane: The outer boundary of the cell.

    2. Nucleus: Acts as the control center for cellular activities.

    3. Cytoplasm: The intracellular component that surrounds the nucleus and constitutes the bulk of the cell.

  • The human body is made up of approximately 10^{14} (100 trillion) cells.

  • There are around 200 different types of cells in the human body, classified based on their unique structure and function.

Plasma Membrane

  • The plasma membrane defines the border of every human cell and separates intracellular fluid from extracellular fluid.

  • Structure:

    • Composed of a bilayer of lipid molecules:

    • Polar head region: Charged and hydrophilic; faces the cell surface and interior.

    • Non-polar tail region: Not charged and hydrophobic; consists of two chains of fatty acids that line up at the center of the membrane.

Nucleus

  • The nucleus is the largest organized component of the cell, typically containing a single nucleus, with exceptions including skeletal muscle cells and red blood cells.

  • Enclosed by a double-layered nuclear envelope with nuclear pores that control the passage of materials.

  • Functions:

    • Acts as a control center using genetic material.

    • DNA directs protein synthesis and serves as the genetic blueprint during cell division.

Cytoplasm

  • The cytoplasm is the portion of the cell's interior that is not occupied by the nucleus, consisting of two components:

    1. Organelles:

    • Highly organized and may be membrane-enclosed (membranous) or not (non-membranous).

    1. Cytosol:

    • A semi-liquid, gel-like substance where many chemical reactions occur.

  • The intracellular fluid is comprised of cytosol plus fluid within organelles and the nucleus.

Organelles

  • Membranous Organelles:

    • Endoplasmic Reticulum (ER):

    • Smooth ER (sER): Lacks ribosomes, involved in the synthesis of lipids and detoxification.

    • Rough ER (rER): Contains ribosomes, responsible for protein synthesis; abundant in cells specializing in protein secretion (e.g., pancreas).

    • Golgi Complex: Processes, modifies, sorts, and packages products from the ER.

    • Lysosomes: Membrane-bound organelles containing hydrolytic enzymes responsible for digesting cellular waste and pathogens.

    • Peroxisomes: Contain enzymes that detoxify byproducts of metabolism, like free radicals.

    • Mitochondria: The powerhouse of the cell that generates approximately 90% of ATP energy stores through cellular respiration.

  • Non-Membranous Organelles:

    • Ribosomes: The site of protein synthesis, existing freely in the cytosol or attached to the rER.

    • Cytoskeleton: A network of rods that provides structure, shape, and transport within the cell.

The Central Dogma of Molecular Biology

  • Describes the flow of genetic information from DNA to RNA to Protein.

Ribosomes

  • Serve as the site of protein synthesis where instructions are read from RNA.

  • Ribosomes can either be:

    • Free-floating: Synthesized proteins remain in the cytosol.

    • Attached to rER: Synthesized proteins are secreted from the cell.

Glycolysis and Cellular Respiration

  • Glycolysis:

    • Occurs in the cytosol and involves breaking down glucose into two pyruvate molecules through a series of 10 reactions.

    • Anaerobic Process that generates two molecules of NADH along with two ATPs from one molecule of glucose.

  • Citric Acid Cycle (Krebs Cycle):

    • Involves the conversion of pyruvate to Acetyl CoA, resulting in the release of carbon dioxide and generation of NADH and FADH2.

  • Oxidative Phosphorylation:

    • Takes place in the mitochondria, involving the electron transport chain and chemiosmosis to generate ATP.

Energy Production

  • ATP produced through controlled oxidation of food which releases about 40% of the energy, while 60% is released as heat.

  • Total ATP in the body is enough to maintain basic functions for approximately 90 seconds.

Aerobic vs. Anaerobic ATP Production

  • Anaerobic: When oxygen is not available, ATP production occurs mainly through glycolysis yielding 2 ATP per glucose.

  • Aerobic: In the presence of oxygen, ATP production can yield up to 38 ATP per glucose molecule.

Apoptosis and Necrosis

  • Apoptosis: Programmed cell death activated by an internal biochemical pathway, characterized by the release of cytochrome C from mitochondria, leading to cellular breakdown and phagocytosis by other cells.

  • Necrosis: Uncontrolled cell death resulting in inflammation and damage to surrounding cells.

Cytoskeleton

  • Provides structural support and organizes cell components, consisting of three elements:

    1. Microtubules: Support, shape, and facilitate vesicle transport within the cell.

    2. Microfilaments: Made of actin proteins, important for cell shape and movement.

    3. Intermediate Filaments: Provide mechanical support and maintain cell integrity.

Summary Data

  • Key metabolic processes generate ATP in varying amounts:

    • 1 glucose can yield 36-38 ATP under aerobic conditions.

    • Other substrates (fats, proteins, carbohydrates) contribute differently to energy production.

Additional Resources

  • A variety of additional multimedia resources are available for further understanding of cellular functions including the dynamics of the cell, respiration processes, and more. Links provided within classroom materials.