Plasma Membrane Structure and Function Separates the internal environment of the cell from its surroundings. Regulates what materials enter and leave a cell; maintains homeostasis Made of 2 layers of phospholipids (phospholipid bilayer) with proteins embedded throughout. Fluid consistency (like a light oil) and a mosaic pattern of proteins.

Because of consistency and pattern of components, referred to as fluid-mosaic model of membrane structure fluidity.swf

The Fluid Mosaic Model A. Nonpolar (hydrophobic) tails B. Polar (hydrophilic) heads C. Protein D. Protein E.Carbohydrates F.Ion/small polar molecule G. Diffusion H. Glycoprotein (carbohydrate attached to a protein) I. Glycolipid (carbohydrate attached to a lipid) J. Phospholipid K. Phospholipid bilayer Note – this diagram does not show the following: Proteins on inner or outer surface of membrane Cholesterol (would be scattered amongst nonpolar tails of phospholipid)

Cells live in fluid environments, with water inside and outside the cell. Components of plasma membrane: –2 layers of phosphlipids Glycerol backbone, 2 fatty acid chains, and a phosphate group Polar head and nonpolar tail –Integral proteins –Peripheral proteins –Cholesterol –Carbohydrates

Phospholipid bilayer: Hydrophilic (water-loving) polar heads Face outside and inside of cell. Hydrophobic (water-fearing) nonpolar tails Extend to the interior of the plasma membrane.

Cholesterol – helps prevent fatty acid tails of phospholipid bilayer from sticking together; makes membrane fluid; reduces permeability of membrane to H + and Na + Carbohydrates: Define the cell’s characteristics and help cells identify chemical signals; might help disease- fighting cells recognize & attack a potentially harmful cell Glycoproteins – proteins with carbohydrates attached Glycolipids – phospholipids with carbohydrates attached

Proteins: Proteins on inside surface Anchor the plasma membrane to the cell’s internal support structure; give the cell its shape Proteins on outside surface Called receptors; transmit signals to the inside of the cell

Proteins that span the entire membrane: Called transport proteins; create tunnels through which certain substances can enter and leave the cell Remove needed substances or waste materials through the plasma membrane Contributes to the selective permeability of the membrane

Functions of membrane proteins Some help to transport materials across the membrane. Channel Protein – allows certain molecule or ion to cross membrane freely Carrier Protein – interacts with certain molecule or ion to help move it across membrane

Others receive specific molecules, such as hormones. Receptor Protein – has certain shape so that specific molecule, such as a hormone or some other signaling molecule, can attach Attachment can cause protein to change shape & cause cell to perform certain action

Still other membrane proteins function as enzymes. Enzymatic Protein - Can carry out metabolic reactions

The Permeability of the Plasma Membrane Selectively permeable – meaning only certain materials can cross the membrane. Two mechanisms of transport: –Active – requires ATP (chemical energy) –Passive – does not require chemical energy

Examples of passive transport: –Diffusion (no carrier protein needed) –Facilitated transport (which usually requires a carrier protein) Active transport: –Requires a carrier protein –May sometimes involve the formation of vesicles to take in or get rid of materials (for example endocytosis and exocytosis). **More on these types of transport later in chapter**

How do materials get across the plasma membrane? Small, uncharged molecules pass through the membrane, following their concentration gradient (gradual change in chemical concentration from one area to another – molecules tend to move from area of high to low concentration). Larger macromolecules or tiny charged molecules rely on proteins to help them to get across.

How molecules cross the plasma membrane

Diffusion and Osmosis Diffusion is the passive movement of molecules from a higher to a lower concentration until equilibrium is reached. –Equilibrium – state in which all materials are evenly concentrated (sometimes called dynamic equilibrium) Movement of molecules still occurs, but there is no NET movement of molecules Gases move through plasma membrane by diffusion.

Osmosis Defined as the diffusion of water across a differentially permeable membrane due to concentration differences. Molecules always move from higher to lower concentration. Water enters cells due to osmotic pressure (pressure that develops in a system due to osmosis) within cells.

Osmosis demonstration

Rate of diffusion affected by: Concentration – higher the concentration, more collisions between particles, higher the rate of diffusion Temperature – at higher temperatures, particles move faster, so more collisions occur between particles; rate of diffusion increases Pressure – higher the pressure, the closer the particles are to one another; they collide more often; rate of diffusion increases

Osmosis in cells A solution contains a solute (solid) and a solvent (liquid). Cells are normally isotonic to their surroundings, and the solute concentration is the same inside and out of the cell. –Cell is in equilibrium; there is no net movement of water across the cell membrane. Video clip

Isotonic = same X = Solute X = water No net movement of water

Cells in isotonic solutions

Elodea in fresh water

Hypotonic solutions cause cells to swell & possibly burst; “hypo” means less than. –Lower concentration of solute, higher concentration of water outside than inside cell. –Net movement of water from outside to inside the cell. –Animal cells - may undergo lysis (cell is disrupted – may even burst) –Plant cells - increased turgor pressure (makes plant cell rigid); plant cells do not burst because they have a cell wall.

Hypotonic = less than X = Solute X = water Water moves in (hypo/hippo – swells like a hippo)

Cells in a hypotonic solution

Elodea in distilled water

Hypertonic solutions cause cells to lose water; “hyper” means more than –Higher concentration of solute, lower concentration of water outside than inside cell. –Net movement of water from inside to outside of cell. –Animal cells - undergo crenation (shrivel) –Plant cells - undergo plasmolysis, the shrinking of the cytoplasm. –Turgor pressure is lost as plant cells shrink.

Hypertonic = more than X = Solute X = water Water moves out

Cells in a hypertonic solution

Elodea in salt water

Transport by Carrier Proteins Some materials cannot enter or leave cell due to their size and/or nature. Channel proteins – open and close to allow substances to diffuse across plasma membrane Carrier proteins are specific and combine with only a certain type of molecule; change shape as they function to move substances across membrane. Facilitated transport and active transport both require carrier proteins.

Facilitated transport Substances pass through a carrier protein following their concentration gradients (high  low concentration). Does not require energy. Brings in materials such as glucose & amino acids; proteins are specific to molecules taken in. Some molecules can be taken in faster than others – differential permeability. Animation

Active transport Ions or molecules are moved across the membrane against the concentration gradient – from an area of lower to higher concentration. Energy in the form of ATP is required for the carrier protein to combine with the transported molecule. Occurs in cells such as kidney cells (taking sodium from urine), thyroid gland cells (taking in iodine), and cells in digestive tract (absorbing nutrients).

Active transport

Carrier proteins involved in active transport are called pumps. The sodium-potassium pump is active in all animal cells (particularly nerve & muscle cells), and moves sodium ions to the outside of the cell and potassium ions to the inside. The sodium-potassium pump carrier protein exists in two conformations; one that moves sodium to the outside, and the other that moves potassium into the cell.

The sodium-potassium pump sources/animations/ion_pump/ionpump.html

Exocytosis and Endocytosis For molecules that are too large to be transported with carrier proteins – transported using vesicle formation – requires energy. Exocytosis - vesicles fuse with the plasma membrane for secretion. Causes cell membrane to enlarge – process occurs during growth. Molecules released become part of cell membrane, part of matrix surrounding cell, or nourish nearby cells.

Exocytosis

Some cells are specialized to produce and release specific molecules. In some cases, release of molecules may occur only in the presence of signals received by the plasma membrane. Examples include release of digestive enzymes from cells of the pancreas, or secretion of the hormone insulin in response to rising blood glucose levels.

Endocytosis - cells take in substances when a portion of the plasma membrane folds in, and forms a vesicle around the substance. When vesicle fuses with lysosome, digestion occurs. Endocytosis occurs as: Phagocytosis – large particles (food or other cells) Pinocytosis – small particles or liquids Receptor-mediated endocytosis – form of pinocytosis for specific particles such as vitamins, hormones, or lipoproteins hill.com/sites/ /student_view0/chapter6/animations.html

Phagocytosis Seen in unicellular organisms like amoebas. White blood cells can use this process to take in bacteria & worn out red blood cells

Pinocytosis Used by root cells of plants. Seen in blood cells, cells that line kidneys and intestines.

Receptor-mediated endocytosis Seen during exchange of maternal & fetal blood in placenta; & intake of cholesterol in body cells (failure to do so results in high blood pressure, blocked arteries, and heart attacks).