Membrane Structure and Function Unit D Chapter 4

Main structure of the cell membrane The cell membrane is made up mostly of fats and proteins The arrangement of these materials is referred to as a phospholipid bilayer

Phospholipid bilayer:

Phospholipids Phospholipids are molecules that consist of a polar (hydrophilic) phosphate head and two non-polar (hydrophobic) tails.

Phospholipids Hydrophilic: likes water, polar Hydrophobic: likes oil (doesn’t like water), non polar

Transmembrane Proteins: All the proteins embedded in the lipid bilayer. Examples: Glycoproteins Channel proteins Carrier proteins Receptor proteins Enzymatic proteins

Glycoproteins:

Glycoproteins: AKA: recognition proteins Proteins with carbohydrate (oligosaccharide) molecules attached to the outer surface of animal cell membranes. Function: cell recognition (like the “finger print” of the cell), they serve as markers on the cell membrane for identifications So cells can recognize one another Example: antigens on red blood cells

Channel Proteins: Allows a particular molecule or ion to freely cross the cell membrane through a pore as it enters or exits the cell Transport of lipid insoluble materials across the membrane ie: water, ions, amino acids Always open Don’t require energy (passive transport)

Carrier Protein: A protein that selectively interacts with a specific molecule or ion so that is can cross the cell membrane to enter or exit the cell. Glucose enters the cell using carrier proteins Not always open Sometimes require energy

Carrier Protein: Certain molecules bind to a proteins triggering a change in shape that transports the molecule across the membrane Outside cell Inside cell

Receptor Proteins: A protein that is shaped in such a way that a specific molecule can bind to it. The binding of the molecule, such as a hormone, that can influence the metabolism of the cell. “little people” were once believed to be short because they did not produce enough growth hormone “Little people” are now believed to be short because their cell membrane growth hormone receptors are faulty and cannot interact with the hormone.

Receptor Proteins They bind with external molecules and then activate cellular activity. Viruses must often attach to receptors before they enter a cell.

Enzymatic Proteins: A protein that catalyzes a specific reaction within the cell. The active site is usually inside the cell they allow specific reactions to happen Figure 4.2 page 62 in text.

Glycolipids:

Glycolipids: Carbohydrate (oligosacharide) molecules that are attached to phospholipids on the outer surface of the cell membrane Functions: they are markers for cell to cell communication, cell to pathogen communication and provide energy One of the glycolipids found in human red blood cells is involved in the antigens of the ABO blood type. Some glycoproteins are also involved with the antigens as well.

Cholesterol: lipid (steriod) molecules found in animal cell membranes , mostly in the hydrophilic region. Function: they lend stability to the lipid bilayer and prevent a drastic decrease in fluidity at low temperatures Can comprise up to 50% of animal plasma membrane Hydrophilic OH groups toward surface Smaller than a phospholipid and less amphipathic (having both polar and non-polar regions of the molecule)

Methods of moving molecules across the cell membrane Molecule movement across a membrane can be separated into two categories: Passive transport: does not need energy Active transport: requires energy

Comparsion Passive Transport includes: Simple diffusion (including osmosis) Facilitated Diffusion Active Transport includes: Active Transport Endocytosis Phagocytosis Pinocytosis Exocytosis

Passive Transport Across a Membrane: Diffusion: the net movement of a substance from a region of higher concentration to a region of lower concentration. The molecules move down a concentration gradient until they are equally distributed.

Diffusion across a membrane:

Net flow across a membrane:

Diffusion across a membrane:

Diffusion: Only certain molecules can pass through the cell membrane by simple diffusion: Lipid-soluble molecules (ie. Alcohols) Oxygen O2 Carbon Dioxide CO2 You need to know the 8 factor that affect cell diffusion (in workbook)

Osmosis: The movement of water across a differentially permeable membrane Still moves from high concentration to lower concentration Requires the concentration gradient. Example: movement of water

Important terms Solvent: a fluid, such as water, that dissolves other molecules (solutes) Solute: A substance dissolved in a solvent to form a solution.

Osmotic pressure: The measure of the number of collisions water molecules make against the membrane surface. The greater the difference in concentration of the solute, the greater the number of collisions, the higher the osmotic pressure.

Turgor Pressure: AKA Turgidity The force the cytoplams exerts against the inside of the cell membrane Large sustained changes in turgidity can affect cell function.

Tonicity: Refers to the strength of a solution in relationship to osmosis There are three options Isotonic solutions: cell is stable (no change to cell) Hypotonic solutions: cells swell Hypertonic solutions: cells shrivel

Hypotonic Solutions A blood cell (1% salt) is placed in pure water, what will happen to the cell? Hypo means “less than” AFTER BEFORE

Hypertonic Solutions A blood cell (1% salt) is placed in a beaker with a 10% salt solution, what will happen to the cell? AFTER BEFORE

Comparison of tonicity in animal and plant cells (figure 4.6 in text) (crenation)

Before After

Facilitated Transport: Direction: molecules move from higher concentration to lower concentration Requires: carrier protein and a concentration gradient Examples: sugar (glucose) and amino acids NO ENERGY REQUIRED

Active Transport: Direction: from low concentration to higher concentration Requirements: carrier protein and energy Examples: sugars, amino acids, and ions

Endocytosis Substances too large to pass through the membrane are taken into the cell forming vesicles (ie. food vacuoles) Direction: towards inside of cell Requirements: energy and vesicle formation Phagocytosis: AKA “cell eating” Ingestion of larger food vacuole (large vesicle) The food vacuole fuses with a lysosome and digests the contents Pinocytosis: AKA “cell drinking” Ingestion of smaller objects or fluids A smaller vesicle is formed

Endocytosis

Pinocytosis:

Exocytosis: When vesicles containing wastes and products for secretion fuse with the cell membrane, open its surface, and spill their contents to the other side. The opposite process of endocytosis Direction: towards outside of cell Requirements: Energy and vesicle fusing with cell membrane Examples: macromolecules, hormones

Exocytosis

Exocytosis of tear fluid: