Activator: Word splash LEQ: How does the cell membrane function as a selectively-permeable barrier? Reading: 3.4-3.5 Activator: Word splash Write three sentences about cells & biomolecules using the following terms: Hydrophilic, hydrophobic, phospholipid, fatty acid, water, polar, nonpolar, Key terms – fluid mosaic model, selective permeability,

Cells evolved when a membrane could enclose a unique chemical system: The lipid membrane hypothesis describes how cells became unique, sustained chemical systems Cells evolved when a membrane could enclose a unique chemical system: Phospholipids If a cell is going to evolve, it must have acquired a membrane at some point. The lipid membrane hypothesis accounts for some basic properties that are essential to the membranes of cells today: phospholipids. Whether you are a metabolism-first or RNA-world proponent, membrane chemistry is also very important to consider. The phospholipid is what makes a membrane possible as a structure.

phospholipid bilayer: Cell membranes are barriers that separate the inside of a cell from the external environment. phospholipid bilayer: Two sided w/ heads facing toward water  hydrophilic Fatty acid tails in middle  hydrophobic! cell membrane protein cholesterol carbohydrate chain protein channel Cell membranes consist of a phospholipid bilayer: two layers of phospholipids arranged much like an oreo cookie with the fatty acid tails directed into the center of the bilayer (above, “cream filling”). Note that the hydrophilic heads are arranged so that they form a uniform layer on either side of the membrane. Their chemistry cause the heads to naturally turn toward water to interact with it, turning the tails away from water into the middle of the bilayer. Tails must be sufficiently packed in tight to create stable conditions allowing the lipids to settle into this shape.

Barrier enclosing unique chemistry regulates passage of chemicals Two major functions: Barrier enclosing unique chemistry regulates passage of chemicals cell membrane outside cell Any cell membrane serves two basic functions: A physical barrier that encloses the chemistry of a cell (the cytoplasm, any organelles, and all associated reactions with the cell’s metabolism). This permits a cell to retain distinct chemical properties and function as a unique chemical system separate, but connected to the outside world. A regulator of chemicals that must pass through the membrane for a cell to continue to supply reactants for chemical reactions, substrates for enzymes, export waste chemicals, etc. It is the emergent properties of membrane chemistry that make this possible. inside cell

The fluid mosaic model describes the membrane’s chemical behavior. “assemblage of small pieces” Membranes behave like oil at room temp (fluid). Biomolecules float in the membrane cell membrane protein cholesterol carbohydrate chain protein channel Membranes are currently understood as behaving in a way best explained by two principles: fluids – which flow as liquids, but stick together and can form surfaces, sheets, in much the same way water does. Mosaics – a term often associated with ancient art constructed from pieces of small glass, shells, tile, etc. Mosaics are a great example of emergence. Each individual piece is remarkable only in its shape, color, and texture, but they can be assembled to form a higher order structure with complex meaning and emotion. See the following slide for examples of mosaics. The model is called the fluid mosaic model: the membrane behaves like an oily fluid at room temperature, but consists of small parts with distinct shapes and properties such as proteins, carbohydrates, and various types of lipids.

The cell membrane is selectively permeable. More on how the membrane controls the passage of chemicals across the membrane: Membranes are selectively permeable to certain chemicals because of their chemistry. Since the fatty acid tails for a hydrophobic barrier, v ery few chemicals can pass through the membrane without aid. For example, small nonpolar molecules such as molecular oxygen (O2) can pass through the membrane unaided, but a polar molecule like glucose (C6H12O6) will require some means of cross the hydrophobic area of the membrane. Small nonpolar molecules can cross the membrane while hydrophilic molecules cannot.

An example of a mosaic made on a computer

Summary: Why do phospholipids form a bilayer when they are dissolved in water? If membranes solidified why would homeostasis be disrupted in a cell? How does this relate to the characteristics of organisms?

Phospholipids form a double layer in response to the presence of polar water molecules surrounding them. The polar heads of phospholipids interact with the water inside and outside the cell, forming transient hydrogen bonds. The nonpolar tails are repelled by water and interact with each other inside the membrane Membrane receptors bind to a signal molecule on the outside of a cell. Upon binding, the membrane receptor changes shape, which sends a message inside the cell. Both enzymes and receptors are proteins that bind to a specific ligand or substrate, change shape in response to binding, and cause some sort or action or response. If proteins were rigid, they would be unable to change shape. Therefore, they could not effectively transmit a message to the cell’s interior.

The body would no longer receive the message from insulin The body would no longer receive the message from insulin. Cells would not take up sugar, blood sugar levels would rise, and death would result.

Chemical signals are transmitted across the cell membrane. Signal (ligand) binds to a protein receptor: Membrane receptors – hydrophilic signals (left) Intracellular receptors – hydrophobic signals (right) Molecules that act as chemical signals to change cellular behavior must also pass their message across the membrane. The general mechanism involves a molecule bonding to a protein receptor on the membrane surface or inside the cell. This association is reversible. The lock-and-key analogy is useful in remembering how the signal associates with the receptor. In this relationship, the signal can be referred to as a ligand since it has a complementary fit to the receptor and they combine together to form a larger signaling complex. Membrane receptors positioned on the cell surface bind with specific signals of a hydrophilic nature. This is typical of some signals that promote the growth and division of a cell. Intracellular receptors found within the cell can freely associate with signal molecules that can pass across the cell membrane without aid. This is typical of steroid hormones, which are made of lipids and can pass through the hydrophobic part of a membrane.