Cell Structure and Function

Nucleus and Nuclear Structures

  • Nucleus Overview

    • Contains nuclear envelope (also called nuclear membrane).

    • Comprised of a double membrane with nuclear pores allowing for molecular transport.

    • Functions:

    • A: Allowing substances in.

    • B: Allowing substances out.

    • RNA can pass through nuclear pores to ribosomes to synthesize proteins.

  • Nuclear Lamina

    • A network of protein filaments within the nucleus.

    • Structure resembles a fishnet, providing shape stability to the nucleus.

  • Nucleoplasm

    • The substance within the nucleus containing chromatin and other materials.

Chromatin and DNA

  • Chromatin

    • DNA combined with proteins (histones).

    • Appears uncoiled for most of the cell cycle, often described as a "ball of yarn" morphology.

    • When cells divide, chromatin condenses into visible chromosomes.

    • Example of DNA wrapping: Analogous to thread wrapping on a fishing reel, preventing entanglement.

Ribosomes and Protein Synthesis

  • Ribosomes

    • Composed of RNA and proteins.

    • Function: Synthesize proteins from mRNA.

    • The nucleus produces heavily for ribosomal RNA (rRNA) which has high DNA content dedicated to making ribosomes due to high protein synthesis demand in cells.

Endoplasmic Reticulum (ER)

  • Endoplasmic Reticulum (ER)

    • Includes Rough ER and Smooth ER.

  • Rough ER

    • Studded with ribosomes.

    • Main function: Synthesis of proteins destined for secretion or for use in lysosomes.

  • Smooth ER

    • Lacks ribosomes.

    • Functions:

    • Synthesis of lipids and steroids.

    • Detoxification of drugs and alcohol.

    • In muscle cells, regulates calcium storage.

Golgi Apparatus

  • Golgi Complex

    • Composed of stacks of membrane-bound vesicles.

    • Function: Modifies, sorts, and ships proteins synthesized in the ER.

    • Operates via vesicular transport:

    • Vesicles form when parts of the membrane bulge and pinch off.

    • Vesicles can transport substances between the ER and Golgi, and then onward to their destinations.

    • Role: Chemical modification of proteins and addition of address labels for destination specification.

    • Components:

    • Cis side: Entry side of the Golgi.

    • Trans side: Exit side of the Golgi.

Lysosomes and Cellular Breakdown

  • Lysosomes

    • Contain hydrolytic enzymes for digestion of cellular waste and foreign materials.

    • Function: Breakdown of macromolecules and recycling of cell components through autophagy.

  • Hydrolysis vs. Dehydration Synthesis:

    • Hydrolysis introduces water to break down molecules.

    • Dehydration synthesis removes water to form molecules.

Mitochondria

  • Mitochondria

    • Double membrane structure responsible for ATP production (energy currency of the cell).

    • Considered the powerhouses of the cell.

    • Process:

    • Sugars are broken down in the cytosol and enter mitochondria for further breakdown in the presence of oxygen (aerobic respiration).

    • Energy yield: Can produce approximately 34 ATP molecules per glucose molecule when oxygen is present.

Centrioles and Cell Division

  • Centrioles

    • Structures involved in cell division.

    • Help organize the mitotic spindle and pull chromosomes apart during cell division.

Plasma Membrane

  • Plasma Membrane

    • Vital for maintaining homeostasis within the cell.

    • Composed primarily of phospholipids, proteins, and cholesterol.

    • Functions as a selective barrier regulating substance entry and exit.

  • Phospholipids

    • Have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

    • Arranged in a bilayer formation.

  • Fluidity of the Membrane

    • Influenced by saturated and unsaturated fatty acids.

    • Cholesterol maintains and regulates fluidity at varying temperatures.

  • Membrane Proteins

    • Peripheral: Attached on the surface.

    • Integral: Span across the membrane; serve as receptors, enzymes, or channels.

    • Glycoproteins and glycolipids: Contribute to cell recognition and signaling.

Summary of Key Concepts

  • Cells require specific organizational structures (like the nucleus and organelles) for efficient function.

  • Various organelles cooperate in processes like protein synthesis, modification, and transport.

  • Understanding the structure and function of these components is foundational for studying cellular biology and physiology.