Cell Biology: Structure and Function of Cell Membrane and Transport

Posted on Aug 31, 2024 in Biology

Functions of Nucleotides

Nucleotides play crucial roles in various cellular processes:

  • Form the building blocks of nucleic acids (DNA and RNA).
  • Provide energy (ATP is the primary energy currency for animals and plants).
  • Act as coenzymes in metabolic reactions.
  • Serve as cellular messengers (e.g., cyclic AMP in hormone signaling).

Composition and Molecular Features of Nucleotides

Nucleotides consist of three main components:

  • A nitrogenous base (purines or pyrimidines)
  • A pentose sugar (ribose or deoxyribose)
  • A phosphate functional group

A nucleoside is similar to a nucleotide but lacks the phosphate group.

Cell Membrane

Features

  • Surrounds the cell, separating the intracellular and extracellular fluids.
  • Composed mainly of cholesterol and phospholipids.
  • Contains proteins with amphipathic character, contributing to its dynamic and fluid nature (liquid-like gel state).

Types of Membrane Proteins

  • Integral or transmembrane proteins
  • Peripheral globular proteins

Carbohydrates in the Cell Membrane

  • Oriented towards the extracellular environment.
  • Contribute to membrane asymmetry.
  • Involved in cell signaling.

Molecular Composition

  • Protein: 55%
  • Fat: 42%
  • Carbohydrates: 3%

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is an extension of the cell membrane into the cell’s interior. It is involved in protein and lipid synthesis and has two main types:

Types of ER

  • Smooth Endoplasmic Reticulum (SER): Forms flattened cisternae and is involved in lipid synthesis.
  • Rough Endoplasmic Reticulum (RER): Has ribosomes attached to its cisternae and is involved in protein synthesis.

Features of ER

  • Protein synthesis and modification (including glycosylation and compaction).
  • Lipid synthesis (cholesterol and phospholipids).
  • Enzyme activity due to its membrane-like structure.

Golgi Apparatus

The Golgi apparatus is composed of stacked, flattened membrane sacs (cisternae) and works in conjunction with the ER. It has two main functions:

Functions of the Golgi Apparatus

  • Synthesis of large polysaccharides (proteoglycans).
  • Processing of secretions from the ER, including the formation of secretory vesicles, lysosomes, and peroxisomes.

Transport Across the Membrane

The cell membrane is selectively permeable, allowing only small, nonpolar molecules (like O2 and CO2) to pass freely. Other molecules require specific transport mechanisms.

Types of Diffusion

  • Passive Diffusion (no energy required):
    • Simple diffusion through the membrane (O2, CO2)
    • Simple diffusion through channel proteins
    • Facilitated diffusion (requires membrane proteins but no energy)
  • Active Transport (requires energy):
    • Movement of molecules against their concentration gradient.
    • Example: Na+/K+ pump

Factors Affecting Passive Diffusion

  • Permeability of the membrane
  • Membrane surface area
  • Concentration gradient (chemical gradient)
  • Electrical potential difference (electrical gradient)
  • Pressure difference

Osmosis is a crucial passive diffusion mechanism involving the movement of water across a semipermeable membrane.

Na+/K+ Pump (Active Transport)

The Na+/K+ pump is an essential active transport mechanism with two main functions:

  • Controls cell volume by regulating intracellular sodium levels.
  • Creates and maintains the membrane potential (electrical potential difference) crucial for nerve and muscle cell function.

Membrane Potential

Neurons and muscle cells can transmit information by altering their membrane potential. This ability to change the electrical potential is known as the action potential.

Factors Influencing Membrane Potential

  • Permeability to K+: The chemical and electrical gradients influence the movement of potassium ions.
  • Na+/K+ Pumps: Contribute to the electronegativity inside the cell by pumping out three positive sodium ions for every two positive potassium ions pumped in.
  • Permeability to Na+: Although the membrane is less permeable to sodium, the chemical and electrical gradients favor its entry into the cell, slightly reducing the internal electronegativity.