More than just boundaries…. Biological Membranes More than just boundaries….

Organelles with Membranes Eukaryotes have their cells divided into organelles with membranes, including: The external cell membrane The nuclear membrane The golgi complex, vacuoles, lysosomes, the e.r., the mitochondrion, the chloroplast, and more! Prokaryotes (bacteria) lack these internal membranes, but they still have an external cell membrane

The Cell Membrane Fluid Mosaic Model (many separate pieces that can move) Phospholipid Bilayer with proteins embedded in it

Phospholipid Molecules Phosphate “Head” (Phosphorus + Oxygen) Hydrophilic (Loves Water) Lipid “Tails” (Fatty Acid Chains) Hydrophobic (Fears Water) Simplified Phospholipid Detailed Phospholipid

Phospholipid Bilayer Phosphate heads are attracted to water (hydrophilic) so they form the inner and outer layers of the membrane in contact with the intracellular and extracellular fluids Extracellular Fluid Lipid tails avoid water (hydrophobic) so they form the interior of the membrane where they are protected from water Intracellular Fluid (Cytoplasm)

Selective Permeability Permeable = Molecules can cross through it (a t-shirt is permeable to water) Impermeable = Molecules can not cross through it (plastic is impermeable to water) Semi-permeable = Some molecules can cross through it, others can’t Selectively Permeable = The membrane CHOOSES which molecules can cross through it – and it chooses which direction they can cross!

Membrane Proteins The phospholipid bilayer has many different proteins embedded in it. Each type of protein has its own function, such as choosing which molecules are allowed to enter or exit the cell.

Channel Proteins I Low Concentration High Concentration Channel Proteins allow molecules to cross the membrane by making a tunnel for them.

Channel Proteins II Molecules are moving from high to low concentration Energy is NOT needed Passive transport Facilitated diffusion

Molecules inside are exchanged for other molecules outside the cell Membrane Pump Molecules inside are exchanged for other molecules outside the cell Energy is required Active transport

Carrier Proteins Specific molecules are grabbed and carried across the membrane by this protein Energy is required Active transport

This changes the activities of the cell that receives the message Receptor Proteins Hormones and neurotransmitters send messages from one cell to another by binding with this protein This changes the activities of the cell that receives the message

Marker Proteins Cells can recognize each other with these proteins Different kinds of cells have different markers Very important to have matching marker proteins for organ transplants and blood transfusions

This steroid strengthens animal cell membranes Cholesterol This steroid strengthens animal cell membranes Diets high in cholesterol are associated with high blood pressure and heart disease

The entire membrane is involved with this process Endocytosis If the cell “swallows” small particles (like water) it is called “pinocytosis!” If the cell “swallows” large particles (or other cells) it is called “phagocytosis!” The entire membrane is involved with this process Large molecules are surrounded and engulfed when the cell forms a vacuole around them Amoebas get food this way, and your white blood cells attack bacteria by the same process

Exocytosis The golgi complex packages hormones, enzymes, and neurotransmitters into vesicles The vesicles merge with the cell membrane, and release the molecules outside of the cell where the molecules can go to work This is known as secretion, since these molecules are not waste molecules (if they were wastes, it would be called excretion)

The Concentration Gradient 5 % Salt The concentration gradient is the difference in concentration of a molecule between the inside and outside of a cell. 1 % Salt Hi Concentration Gradient 1.5 % Salt 1 % Salt Lo Concentration Gradient

Passive Transport vs. Active Transport No Energy Needed Molecules cross membrane from high to low concentration (with the concentration gradient) Includes diffusion, osmosis, and facilitated diffusion Energy from ATP required Molecules cross membrane low to high concentration (against the concentration gradient) Includes carrier proteins, endocytosis, and exocytosis

“The Virtual Cell Website” References All images courtesy of “The Virtual Cell Website” www.ibiblio.org/virtualcell/index.htm