Notes on Cellular Processes and Organelles

Active vs Passive Processes

• Passive Processes:

  • Do not require cellular energy ($ATP$).

  • Substances move down their concentration gradient (from high to low concentration).

  • Simple Diffusion: Movement of small or lipophilic molecules through the lipid bilayer.

  • Facilitated Diffusion: Movement of larger or charged molecules via membrane proteins (channels or carriers).

  • Osmosis: Specifically the diffusion of water across a semi-permeable membrane.

• Active Processes:

  • Require energy expenditure in the form of $ATP$.

  • Substances move against their concentration gradient (from low to high concentration).

  • Primary Active Transport: Direct use of $ATP$, such as the $Na^+/K^+$ pump.

  • Vesicular Transport: Includes endocytosis (bringing substances into the cell) and exocytosis (releasing substances out of the cell).

Skin Reaction to Water Exposure

• Skin Swelling and Sagging in Tub:

  • Mechanism: When soaked in hypotonic fresh water, osmosis occurs. Water moves into the keratinized cells of the stratum corneum (the outermost layer of the skin), causing them to expand and the skin to appear wrinkled or swollen.

• Dryness and Tightness after Ocean Swimming:

  • Mechanism: Seawater is hypertonic relative to the fluid inside skin cells. Through osmosis, water is drawn out of the skin and into the salty environment, leading to dehydration of the tissue and a characteristic tight sensation.

Cell Identity Marker

• Function: Often comprised of glycoproteins (proteins with carbohydrate chains attached) located on the outer surface of the plasma membrane.

• Major Histocompatibility Complex (MHC): These specific markers allow the immune system to recognize "self" vs. "non-self" cells, preventing the body from attacking its own tissues.

Organelles in Cells

• Overview: Metabolically active internal structures that perform specific functions necessary for cellular homeostasis.

• Detailed List of Organelles:

  1. Nucleus:

  • The "control center" containing the cell's genetic material ($DNA$).

  • Surrounded by a nuclear envelope with pores for regulated transport.

  1. Ribosomes:

  • Sites of protein synthesis where $RNA$ is translated into amino acid chains.

  • Found free in the cytoplasm or attached to the Rough Endoplasmic Reticulum.

  1. Endoplasmic Reticulum (ER):

  • Smooth ER: Lacks ribosomes; specializes in lipid metabolism, steroid hormone synthesis, and calcium storage.

  • Rough ER: Studded with ribosomes; modifies and folds proteins destined for secretion or membrane insertion.

  1. Golgi Apparatus:

  • Composed of flattened sacs (cisternae).

  • Acts as a shipping and receiving center, modifying products from the ER and sorting them into vesicles.

  1. Mitochondria:

  • Double-membrane organelle that generates most of the cell's supply of adenosine triphosphate ($ATP$).

  • Contains its own mitochondrial $DNA$ ($mtDNA$).

  1. Lysosomes:

  • Spherical bodies containing digestive enzymes (acid hydrolases).

  • Functions in autophagy (digesting old organelles) and breaking down phagocytized material.

  1. Peroxisomes:

  • Smaller than lysosomes; contain enzymes like catalase to neutralize toxins and break down hydrogen peroxide ($H<em>2O</em>2$).