 1972 Singer and Nicolson developed the fluid mosaic model Main Function: Isolate the cell’s contents from the external environment and regulate what moves in and out of the cell.  “Fluid portion”- phospholipid bilayer  “Mosaic portion”- membrane proteins

Fluid Mosaic Model

A. Phosopholipid bilayer- isolates the cell interior from the external environment. Structure of the Plasma membrane  Structure: 2 layers of phospholipids  hydrophilic heads form the outer borders of the bilayer and the  hydrophobic “tails” form the inner portion of the bilayer.

Phospholipids make up Cell Membranes

B. Membrane Proteins - responsible for moving substances across the membrane and communicating with other cells Transport Proteins –Form Channels or act as Carriers Ex. Glucose enters our RBCs through a transport protein that acts as a carrier. Inside Cell Outside Cell Glucose

B. Membrane Proteins Receptor Proteins when a specific molecule binds to them it triggers a specific cellular response Ex. secretion of hormones and initiation of cell division

Pancreas Pancreas cells secreting hormones

Recognition Proteins (many are glycoproteins) serve as cell ID tags or cell surface attachment sites Ex. our WBCs recognize certain bacteria as invaders from the other cells of our body that belong there

Recognition Proteins Ex. WBCs recognize invaders

Fluid Mosaic Model

C. Cholesterol makes the bilayer stonger, more flexible, and less permeable to water-soluble substances such as ions and simple sugars.

Fluid Mosaic Model

Does the Plasma Membrane allow most dissolved substances to pass through? No! The plasma membrane is selectively permeable. Only certain substances are allowed to pass through the membrane freely Ex. Water, gases (CO 2, and O 2 ), and some small dissolved substances

Glucose Water Plasma Membrane

How are substances brought across the membrane? A. Passive Transport - does not require ATP Ex. Simple Diffusion Osmosis Facilitated Diffusion 1. Simple diffusion is the movement of molecules from an area of high concentration to an area of low concentration

Requirements for simple diffusion  Concentration gradient - difference in concentration of a substance b/w 2 areas  Brownian motion - random motion of molecules

What factors might affect the rate of diffusion? Temperature Concentration gradient Size of molecules

2. Osmosis is the movement of water molecules from an area of high concentration to an area of low concentration through a selectively permeable membrane What is Osmosis and how is it different than simple diffusion? Osmosis is affected by temperature and a change in the concentration gradient also.

Parts of a solution Solute- dissolved substances Solvent- a liquid in which another substance may be dissolved Ex. Sugar water (Hummingbird food) * Solute = sugar * Solvent = water

Cellular Environment Isotonic –equal concentrations of solute outside and inside the cell (Result: Dynamic equilibrium - permeable molecules move in and out of the cell but result in no net change) Plasma membrane 50% Sugar

Cellular Environment Plasma membrane Hypertonic –a higher concentration of solute outside the cell than inside the cell 100% water 50% Sugar 50% Water Result: water moves out of the cell; cell shrinks or shrivels

Cellular Environment Marine fish must deal with saltwater environment Cartilaginous fish reabsorb urea Bony fish excrete salt

Cellular Environment Plasma membrane 50% sugar 100% Water Hypotonic –a lower concentration of solute outside the cell than inside the cell Result: water moves into the cell; cell expands Some cases cell may burst (cell lysis)

Cellular Environment

Response of plant cell Hypotonic EnvironmentHypertonic Environment

Plants vs. Animal Cells Animal cells are better in an isotonic environment. Plant cells in fleshy parts of plants are better in a hypotonic environment. These cells need the influx of water to remain turgid to support non- woody parts of the plant.

How are substances brought across the membrane? A. Passive Transport - does not require ATP Simple Diffusion Osmosis Facilitated Diffusion B. Active Transport- requires ATP Ion Pumps C. Endocytosis - requires a vesicle Phagocytosis Pinocytosis Receptor-mediated Endocytosis D. Exocytosis - requires a vesicle

A. Passive Transport - does not require ATP Ex. Simple Diffusion Osmosis Facilitated Diffusion- molecules move through the membrane with the aid of a transport protein from high to low concentration. Ex. Glucose enters our RBCs through a transport protein that acts as a carrier.

Facilitated Diffusion –Carrier Protein

B. Active Transport: Ion Pumps Specialized carrier proteins require energy (ATP) to move specific particles and molecules against their concentration gradient.

Na+ Active transport sets up a concentration/electrical gradient

- Na+ - - Na+ rushes back into the cell causing an electrical impulse.

C. Endocytosis- requires a vesicle (3types) Pinocytosis- “cell drinking” the cell engulfs liquid by formation of a vesicle.

Phagocytosis- “cell eating” the cell engulfs large particles (including whole microorganisms) by surrounding the particle with extensions of the cytoplasm (pseudopodia). Ex. Amoeba and WBCs engulf large particles. Video

Receptor-mediated endocytosis- the cell takes up specific substances that bind to a receptor protein in the membrane. Ex. Cells take up LDL complexes to obtain cholesterol for the membrane Video

D. Exocytosis Involved in the elimination of waste or the secretion of a substance.

So, how does a cell take in nutrients and eliminates wastes? Through its _________________________ plasma membrane or surface Why are cells so small? Maybe we could answer this question better with an analogy.

Which canned drink would get colder faster? 40 0z. 10 0z. A cell takes in nutrients and eliminates wastes through its surface like a canned drink exchanges heat with the environment.

A cell needs to have more surface area than volume (high SA/V ratio) in order to obtain nutrients and eliminate waste efficiently with the environment.

Specialized cell surfaces How are our cells held together to form tissues? - Desmosomes

Specialized cell surfaces How is it that our intestines or bladder doesn’t leak? - Tight junctions

Specialized cell surfaces How do adjacent cells communicate with one another? - Gap junctions and Plasmodesmata (plants)