Cell Membrane Transport and Organelles Detailed Study Notes

Molecular Movement Through Membranes

Molecules and gasses, such as oxygen (O₂) and carbon dioxide (CO₂), move across membranes based on:
- Concentration Gradient: Movement occurs from an area of high concentration to an area of low concentration.
- Random Movement: The random kinetic motion of molecules facilitates diffusion.

Infusion is the driving factor for the exchange of gases in living organisms:
- Oxygen moves from the lungs into the blood.
- Carbon dioxide moves from the blood into the lungs to be exhaled.
- In tissues, oxygen diffuses from blood into tissue fluid, while carbon dioxide diffuses from tissue fluid into the blood.
Mechanism: No external force is required for this diffusion; it occurs naturally due to concentration differences and random molecular movement.

While molecular diffusion is common, ions also diffuse across membranes:
- Cells have specialized pumps, which are proteins that use ATP (adenosine triphosphate) to facilitate this movement.

Lipid-Soluble Molecules: Can pass directly through the plasma membrane without aid.
Polar Molecules: May diffuse if they can enter through a specific channel or protein pathway.
- Water Movement: Water is a polar molecule that diffuses through specialized proteins called aquaporins, allowing controlled water movement into and out of cells.

The movement of water across a semipermeable membrane is termed osmosis.
In biological systems, water is usually the solvent, with solutes being the substances dissolved in it.
Since water is the solvent, it follows specialized rules in movement, which are not covered in this class.

A type of passive diffusion that still follows the principle of moving from higher concentration to lower concentration:
- Does not require ATP.
- Involves carrier proteins that change shape to transport polar molecules across the membrane.

The energy for shape changes in carrier proteins comes from the kinetic energy of the particles being transported, not ATP.
This phenomenon can be confusing, as it seems counterintuitive that proteins can change shape without ATP.

Movement from low to high concentration requires energy input (ATP), known as active transport.
- Example: Moving water out of a flooded cellar requires energy, whether by using pumps or manually removing water.
- Active transport systems are vital for maintaining ionic balance within cells, affecting their functionality.
- Essential ions include sodium (Na⁺), potassium (K⁺), calcium (Ca²+), magnesium (Mg²+), and chloride (Cl⁻).

Proteins called pumps actively transport ions against their concentration gradients, employing ATP as the energy source.
- Without these pumps, biological functions such as muscle contraction and neuronal signaling would fail.

Involves ATP and enables substances to be moved in bulk within vesicles:
- Endocytosis: Bringing substances into the cell.
- Types of Endocytosis:
  - Pinocytosis (Cell Drinking): Engulfs liquid and closes off to form vesicles.
  - Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor binding, where the membrane caves in once receptors are occupied.
  - Example: HIV uses receptor-mediated endocytosis for cellular entry.
Exocytosis: The reverse process, pushing materials out of cells by vesicle fusion with the plasma membrane. Often used to export proteins like insulin.

Differentiates from pinocytosis as it involves the intake of solid particles, often performed by white blood cells to eliminate pathogens.

Provides structural support and intracellular transport.
- Microfilaments: Smallest filaments that help maintain cell shape.
- Intermediate Filaments: Intermediate size, providing mechanical support.
- Microtubules: Largest filaments, serving as tracks for vesicle transport and maintaining cell shape.
- Motor Proteins: Travel along cytoskeletal elements, facilitating efficient cellular transport.

Sites of protein synthesis, consisting of rRNA and proteins:
- Free Ribosomes: Produce proteins for use within the cytosol.
- Fixed Ribosomes: Attached to the ER, synthesize proteins destined for secretion or for use in the membrane.

Known as the powerhouse of the cell; key points include:
- Generate ATP through aerobic respiration, using oxygen.
- Have double membranes: an outer capsule-like membrane and a folded inner membrane (cristae) that increases surface area for energy production.
- Contains its own DNA (mtDNA) and ribosomes for internal protein synthesis.

Functions mainly to store DNA and oversee ribosome production in the nucleolus:
- Surrounded by a double phospholipid bilayer, with pores allowing for transport of materials between the nucleus and cytoplasm.
- Nucleolus is the site of ribosome assembly.

Membrane transport can be classified into passive (e.g., diffusion, facilitated diffusion) and active (e.g., active transport, vesicular transport) categories, connected by their usage of energy (ATP) and directionality relative to concentration gradients.
Understanding these transport mechanisms and organelle functions is essential for grasping cellular activity and biological processes.