Cellular Organization

Organisms Are Made of Cells Some organisms are single-celled (unicellular) Some are made of many cells (multicellular) Everything that an organism does is ultimately happening at the cellular level.

Some Types of Cells

A. Cells – the smallest functional unit of life 1. Eukaryote – organism with a nucleus, membrane-bound organelles, and more than one chromosome. Usually either a plant or an animal. Ex. – anything BUT bacteria 2. Prokaryote – unicellular organism with NO nucleus, NO membrane-bound organelles, and circular-stranded DNA. Ex. – bacteria

Differences Between a Plant Cell & Animal Cell Has both a cell membrane AND a cell well Has a giant vacuole for water storage and structure Usually a rectangular shape Has only a cell membrane Has several smaller vacuoles for water, food, or waste storage Can come in any shape

Why are most cells so small? As the radius of a cell increases, the volume of the cell increases faster than the surface area. If the volume becomes too large, the intake of nutrients and output of wastes can’t keep up. Video The yolk of an ostrich egg is the largest cell in existence today!

II. Cell Membranes A. Structure: Fluid-Mosaic Model – portrays the cell membrane as a thick liquid or gel in which specific proteins float like icebergs. 1. Phospholipid Bilayer – has one polar end and one nonpolar end. The nonpolar, hydrophobic, tails of each molecule face each other to form the “filling of the sandwich”. The polar, hydrophilic, heads form the “bread”. The heads are phosphate, the tails are lipid. Hence, phospholipid bilayer.

Structure Continued 2. Cholesterol molecules embedded in the bilayer give it more of a rigid structure, so that it doesn’t just collapse like mush. 3. Carbohydrates attach to the bilayer itself, forming glycolipids, or to the floating proteins, forming glycoproteins. 4. The combination of glycolipids and glycoproteins is called the glycocalyx. Every cell has its own unique glycocalyx, thus it is termed the cell’s “fingerprint”. This fingerprint allows for coordinating cell behavior and communication between cells.

B. Functions 1. Regulates material moving in or out of the cell. i. selective permeability – lets some things in but keeps others out; essential for maintaining homeostasis.

Functions Continued 2. Provides surface area for chemical reactions to occur. 3. Receives signals from other cells (glycocalyx). 4. How one cell recognizes another cell.

Diffusion The movement of substances from a high concentration to a low concentration. A solution reaches equilibrium when the solute moves back and forth at a constant rate.

III. Movement Across Membranes – Passive Transport A. Simple Diffusion – molecules that are SMALL ENOUGH to fit through the phospholipid bilayer move from areas of high concentration to low concentration. Requires no energy input. B. Facilitated Diffusion – a molecule diffuses through a protein channel. Still moving from a high to low concentration gradient, requiring no energy input.

Movement Across Membranes – Active Transport C. Active Transport – uses a membrane protein to act as a pump to move molecules AGAINST the concentration gradient from LOW to HIGH; requires ATP (energy).

Movement of H2O Across the Cell Membrane - Osmosis Osmosis – movement of water, from a high concentration to a low concentration; requires no energy input; this is how a cell maintains homeostasis! 1. isotonic – the solute (salt, sugar, etc) concentration is the same both inside and outside the cell; a state of equilibrium. Water moves in and out of the cell at a constant rate. 2. hypertonic (more H2O) – higher concentration of water inside the cell, so it moves to a lower concentration OUT of the cell; causes the cell to shrink 3. hypotonic (less H2O) – low concentration of water inside the cell, so water moves INTO the cell to even it out; causes the cell to swell up, if it’s an animal cell, maybe even burst! If it’s a plant cell, the cell wall is strong enough to keep it from bursting.

Movement Across Membranes Cont’d F. Endocytosis – involves the movement of large or multiple substances into the cell; often used by white blood cells to engulf bacteria and viruses. 1. phagocytosis – the intake of solids, “cell eating”; arms of the cell engulf the material and bring it in. 2. pinocytosis – the intake of fluid, “cell drinking”; arms of the cell engulf the liquid and pull it in. 3. receptor-mediated endocytosis – involves a specific receptor protein on the cell membrane (glycocalyx) that recognizes the molecule and binds with it, allowing it access into the cell. Then, the arms of the cell surround the material and bring it in.

Movement Across Membranes Cont’d Exocytosis – the movement of materials out of the cell