Cell Biology: Membrane and Cytoplasm Overview

Cell Membrane and Transport

• Phospholipid bilayer allows selective entry and exit of molecules that cannot easily penetrate.
• Cholesterol maintains fluidity of the bilayer, preventing stiffness.
• Transport proteins serve as carriers or channels:
  • Carrier proteins change shape to transport molecules.
  • Channel proteins provide passageways for specific molecules.
• Membrane proteins include:
  • Cell recognition proteins to detect pathogens.
  • Receptor proteins that trigger responses upon molecule binding.

Cytoplasm

• Gel-like substance consists of water, salts, proteins, and molecules.
• Occupies space between the cell membrane and nucleus, essential for biochemical reactions and energy production.
• Cytosol is the semi-fluid component where various organelles are suspended.
• Contains electrolytes, metabolites, RNA, and synthesized proteins.

Cytoskeleton

• Provides structure and shape to the cell with three major filaments:
  • Microfilament (7 nm) composed of actin, aids in contraction and movement.
  • Microtubule (25 nm) composed of tubulin, involved in cell division and transport.
  • Intermediate filament (10 nm) provides tensile strength and supports the nucleus.

Nucleus

• Control center of the cell, containing genetic information.
• Nuclear envelope consists of an outer and inner membrane.
• Nucleoplasm houses chromatin and nucleolus, essential for DNA replication.
• Chromatin condenses to form chromosomes during cell division, ensuring genetic continuity.

Nucleolus

• Membrane-free organelle in the nucleus, responsible for ribosome production.
• Contains fibrils (DNA for rRNA) and granules (rRNA and proteins).
• Absence of nucleolus hampers protein production due to lack of ribosomes.

Protein Synthesis

• Essential for repairing cellular structures and regulating functions.
• Ribosomes synthesize proteins based on mRNA instructions, critical for cellular processes.
• Two types of Endoplasmic Reticulum (ER):
  • Rough ER: Studded with ribosomes, involved in protein synthesis and modification.
  • Smooth ER: Specializes in lipid synthesis, carbohydrate metabolism, and detoxification.

Golgi Apparatus

• Processes and packages proteins and lipids for export from the cell.
• Works closely with ER for efficient processing and transportation.

Lysosomes

• Known as the “suicide bag” of the cell, originating from the ER.
• Functions include digesting food, recycling cellular components, and killing pathogens.

Cell Wall

• Provides additional support and protection for plant cells, located outside the membrane.
• Composed of cellulose, maintaining shape and regulating water uptake.
• Primary cell walls are flexible in young plants, while secondary walls are thicker in older plants.

Mitochondria

• Double-membrane organelles responsible for ATP production, essential for cellular energy.
• Numerous in active cells, particularly in the liver.

Flagella and Cilia

• Organelles for locomotion, composed of specialized arrangements of microtubules.
• Play a role in the movement of certain cells (e.g., sperm, some protists).

Peptidoglycan in Bacterial Cell Walls

• Composed of amino acids and sugars, providing structural integrity and protection to bacterial cells.