Topic 5

Topic 5: The Working Cell

Textbook Reference: Chapter 5 (p127- p150), Chapter 6 (p159-p173)
License: This work is licensed under a Creative Commons Attribution-NonCommercial- ShareAlike 4.0 International License. Slides are adapted from…

Lecture Objectives:

By the end of this lecture students will be able to:

  • Discuss in detail the components of the plasma membrane, their function and arrangement.
  • Explain why the plasma membrane is considered a semi-permeable membrane and discuss which types of molecules can cross it without assistance.
  • Explain in detail the different types of passive and active membrane transport.
  • Define osmosis and discuss in detail its role in cell homeostasis.
  • Compare and contrast the different types of energy.
  • Explain what is chemical energy and how does an animal cell harvest this type of energy.
  • Describe the structure and function of ATP.
  • Compare and contrast endergonic and exergonic reactions.
  • Discuss the mechanism by which enzymes facilitate chemical reactions.
  • Discuss how the activity of an enzyme is regulated.
  • Compare and contrast the two types of receptors and their signal transduction pathways.
  • Give an example of an intracellular receptor and explain its role in development.

Introduction to Cell Function

The Cell is more than puzzle pieces assembled. There are several processes and chemical reactions that are critical for the cell to survive.

  1. How do Molecules Cross the Plasma Membrane?
  2. What is Energy and Why does the Cell need Energy?
  3. How do Cells Control Chemical Reactions?
  4. How do Cells Communicate with each other?

1. How do Molecules Cross the Plasma Membrane?

A. Plasma Membrane Structure and Function

  • Plasma Membrane Composition:
    • Phospholipid Bilayer: Forms the basic structural integrity of the plasma membrane.
    • Embedded Components:
    • Cholesterol: Impacts fluidity and stability of the membrane.
    • Proteins: Including peripheral and integral proteins.
    • Glycoproteins and Glycolipids: Involved in communication and recognition.
  • Plasma Membrane is semi-permeable: Allows some substances to pass while blocking others.

B. The Fluid Mosaic Model

  • Membrane Fluidity Factors:
    • Phospholipid Type: Saturated vs. Unsaturated Fatty Acids affect the fluidity.
    • Cholesterol Content: Helps to maintain membrane fluidity across temperature changes.
    • Temperature: Higher temperatures increase fluidity while lower temperatures decrease it.

C. Types of Transport Across Membranes

  1. Passive Transport:
    • Definition: Movement of molecules without needing cellular energy.
  2. Active Transport:
    • Definition: Movement of molecules requiring cellular energy.
  3. Bulk Transport:
    • Mechanism: Utilizes vesicles to transport larger substances.
    • Types:
      • Endocytosis: Movement into the cell.
      • Phagocytosis: Type of endocytosis for large particles.
      • Exocytosis: Movement out of the cell.

D. Passive Transport

  • Mechanism: Molecules move down their concentration gradient.
  • Diffusion: Movement from high to low concentration until equilibrium is reached.
    • Equilibrium: Achieved when concentration is equal throughout a solution.

E. Types of Molecules in Passive Transport

  • Only small nonpolar molecules (e.g. O2, CO2) and lipid hormones can diffuse through the membrane.
  • Polar and Charged Molecules: Cannot readily pass through the hydrophobic core of the membrane.

F. Facilitated Passive Transport

  • Transport Proteins: Integral proteins aid in transporting specific substances.
  • Channel Proteins:
    • Characteristics: Specific to the substance transported, may be open all the time or gated, e.g., Aquaporin.
  • Carrier Proteins:
    • Function: Transport specific molecules; e.g., Glucose Transporter (GLUT).
    • Some carrier proteins are also involved in active transport.

1. How do Molecules Cross the Plasma Membrane? (Continued)

G. Osmosis

  • Definition: Diffusion of water across a semi-permeable membrane.
  • Mechanism: Water moves from an area of high solvent concentration to low solvent concentration; also described as moving from low solute concentration to high solute concentration.

H. Osmolarity

  • Definition: Refers to the total solute concentration in a solution.
  • Types of Solutions:
    • Isotonic: Equal osmolarity inside and outside the cell.
    • Hypertonic: Higher osmolarity outside the cell; leads to cell shrinkage.
    • Hypotonic: Lower osmolarity outside the cell; can cause cell swelling.

2. Energy Conversion and Storage

A. Concepts of Energy

  • Energy: Capacity to do work.
  • Kinetic Energy: Energy of motion.
  • Potential Energy: Stored energy that can be converted to kinetic energy.
  • All energy conversions generate heat as a byproduct.

B. Energy Types and Sources

  • Chemical Energy: Specifically derived from arrangements of atoms, utilized by cells for metabolic processes.

2. ATP and Cellular Energy

A. ATP (Adenosine Triphosphate)

  • Function: Acts like an energy shuttle, storing energy obtained from food and releasing it as needed.
  • Process: Hydrolysis of ATP converts it to ADP + P + energy (exothermic reaction).
  • Structure:
    • Composed of ribose sugar, three phosphate groups connected by high-energy bonds.

B. Why Does the Cell Require Energy?

  • Metabolism: Sum of all chemical reactions within an organism.
    • Anabolism: Building larger molecules from smaller ones (energy requiring).
    • Catabolism: Breakdown of larger molecules to smaller ones (energy releasing).

3. Enzymes Facilitate Biochemical Reactions

A. Definition and Function of Enzymes

  • Enzymes: Proteins that act as biological catalysts, speeding up chemical reactions without being consumed in the process.
  • Catalysis: Enzymes reduce the activation energy needed for reactions.

B. Enzyme Structure

  • Active Site: Specific shape complementary to the substrate; upon binding, the enzyme may change conformation to enhance fit.
  • Binding and Catalytic Sites: Positions to orient substrates and reduce activation energy.

C. Enzyme Regulation

  • Enzyme Inhibitors: Molecules that decrease enzyme activity.
    • Noncompetitive Inhibitors: Bind at a site other than the active site.
    • Competitive Inhibitors: Compete with substrate for active site.
  • Allosteric Regulators: Molecules that induce a change in the active site by binding at alternative sites.

4. Cell Communication

A. Mechanisms of Cell Communication

  • Receptor Proteins: Specialized proteins that bind specific ligands, initiating a signaling cascade.
    • Signaling Molecules: Can be hydrophobic (e.g., hormones) or hydrophilic (e.g., neurotransmitters).
    • Signal Transduction Pathways: Chemical reactions leading to the cellular response.

B. Types of Membrane Receptors

  1. Ion Channel Gated Receptors
  2. Enzyme-Linked Receptors
  3. G-Protein Coupled Receptors

C. Example of an Intracellular Receptor

  • Androgen Receptor:
    • Function in Muscle: Increases muscle mass and strength, decreases fatty acid catabolism.
    • Function in Fat Tissue: Reduces triglyceride synthesis while increasing triglyceride hydrolysis.