Cell Membrane: Structure and Function Chapters 1.14-1.16 Objectives

Cell Membrane Structure

A cell cannot survive if it is totally isolated from its environment. Maintaining a consistent interior environment is crucial to the survival of the cell. Why?

HOMEOSTASIS is the process by which a constant internal environment is maintained despite changes in the internal and external environment.

The cell membrane acts as a barrier separating the cell from its external environment. Border control for the cell!

The cell maintains a consistent internal environment by controlling the movement of substances in and out of the cell

It controls the ease with which substances pass into and out of the cell. Some substances easily cross the membrane, while others cannot cross at all.

Find a video Cell membranes are mostly composed of; LIPIDS, PROTEINS, and CARBOHYDRATES PHOSPHOLIPIDS are the most abundant lipids in most membranes.

What if the cell membrane was only phospholipids? hardly anything could move in or out of the cell proteins act as channels to let bigger substances move in and out

Passive Transport Diffusion and Osmosis

Simple Diffusion The cell membrane is said to be SELECTVELY PERMEABLE because SOME things can pass through while others cannot. Like Nylon or Gore-Tex

Which molecules can pass? Many small, uncharged molecules can pass through the membrane freely. They simply diffuse from one side of the membrane to another. See figure 3, page 59 Which molecules can pass? Water Oxygen Carbon dioxide Non-charged molecules Which cannot? Macromolecules (nucleic acids, lipids) Glucose Charged molecules and ions

Simple diffusion = the movement of particles from an area of high concentration to low concentration. (Tries to “equal out”.) https://www.youtube.com/watch?v=_v0MRZxU2bc A concentration gradient is the difference in concentration between two areas. Diffusion always occurs WITH the concentration gradient. Osmosis = the diffusion of WATER molecules from high to low concentration across a membrane https://www.youtube.com/watch?v=w3_8FSrqc-I

Three Conditions for Osmosis A few special terms are commonly used to describe differences in solute concentration. Isotonic = solutions have equal solute concentrations. Movement of water (osmosis) occurs equally in BOTH directions across a membrane. Hypertonic = solutions have higher solute concentrations Hypotonic = solutions have lower solute concentrations *See handout questions.

What is hemolysis? What happens to red blood cells in each of the three conditions above? *Pg. 63

Facilitated Diffusion Some large polar substances like glucose or large ions (charged) such as sodium and chloride cannot go through the membrane by simple diffusion because they cannot pass easily through the hydrophobic middle region of the membrane. To compensate, some proteins span the lipid bilayer and act as a pathway for large or charged materials to pass. These such proteins are called transmembrane proteins (carrier proteins). The process by which large molecules pass through a transmembrane protein to gain entry to the cell is called facilitated diffusion. No energy is used, but the natural diffusion (with the concentration gradient) is facilitated by the protein.

Facilitated Diffusion!

Your Task Let’s Summarize the findings of our demo. Read pages 59-64 Answer questions 1, 3, 5-8, 10