04. The plasma membrane & material transport.Source: Campbell et al. (2011) Ian Anderson Saint Ignatius College Geelong
Knowledge and skills. Describe the molecular structure of plasma membranes. Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of plasma membranes. List the functions of membrane proteins. Describe the different ways that molecules or substances cross membranes, including • Diffusion • Active transport • Osmosis • Endocytosis • Facilitated diffusion • Exocytosis
Plasma membrane. The boundary that separates a cell from its surroundings. Maintains the internal environment of a cell by controlling the movement of substances into and out of cell (i.e. partially permeable). Enabling the cytosol to have a different composition from the surrounding environment. ~7-9 nm thick.
Fluid-mosaic model. Composed of a double layer (bilayer) of lipids (phospholipids), called a phospholipid bilayer. The phospholipid molecules are in constant motion, sliding past one another and the other molecules embedded in the plasma membrane. Proteins can Traverse the plasma membrane and stick out from both surfaces; be partially submerged in the inner or outer surface; or be found on the surface. Main roles are in helping transport molecules across the plasma membrane; acting as attachment points for other cells; and functioning as identity tags for cells.
Fluid-mosaic model. Carbohydrates are attached to some proteins that protrude to the outside of the cell (together called glycoproteins). Play an important role in cell-to-cell interactions Cholesterol molecules can be found in the middle between the two phospholipid bilayers. Helps to stabilize the plasma membrane and keep it flexible.
Fluid-mosaic model. Source: Enger et al. (2011)
Phospholipids. Lipids that contain a phospate group at one end of each molecule. Hydrophilic (‘water loving’) at their phosphate end. Hydrophobic (‘water fearing’) along their fatty acid tail region. When in an aqueous solution, the phospholipid molecules line up with their hydrophobic tails pointing away from the solution phospholipid bilayer.
Movement in and out of cells. All cells must be able to exchange (take in and expel) substances with their environment in order to survive, grow and reproduce. Movement of substances across the plasma membrane depends on Surface area available for exchange Chemical properties of the substance being exchanges, plus Concentration gradients, temperature, etc.
Surface area-volume ratio. Surface area-volume ratio of a cell influences the rate of entry and exit of substances into and out of it. As a structure increases in size, its SA:V decreases. The SA:V ratio differs according to the shape of the structure. Ratio is highest in flattened shapes and lowest in spheres.
Movement across the plasma membrane. Plasma membrane is a partially permeable boundary. i.e. only some dissolved materials are able to cross it. Various processes used to allow materials to cross plasma membrane Diffusion Facilitated diffusion Osmosis Active transport Endocytosis and exocytosis
Diffusion.The net movement of molecules along a concentration gradient, from a region of high concentration to a region of low concentration. Does not require energy (i.e. passive). Occurs due to the random movement of molecules. Always occurs down a concentration gradient. The larger the concentration gradient, the more rapid the rate of diffusion.
Diffusion. Diffusion across the plasma membrane will occur as long as the molecule can pass through. Lipid-soluble substances (such as vitamins A & D and alcohol) diffuse through the lipid bilayer. Small molecules such as water, O2 and CO2 can diffuse between the phospholipid molecules. Large molecules, polar molecules and small ions generally are unable to diffuse across the plasma membrane without assistance.
Facilitated diffusion. The movements of a substance across the plasma membrane from a region of higher concentration of a substance to a region of lower concentration, assisted (or facilitated) by specific membrane proteins. Membrane proteins are either carrier proteins or ion channels. Carrier proteins attaches to the molecule, with the resultant change in shape allowing the molecule to be shifted across the plasma membrane and then released. Ion channels act like gates by opening and closing the channel. Does not require energy.
Facilitated diffusion. Facilitated diffusion is more rapid than simple diffusion. The protein channels through the plasma membrane are specific for particular molecules. Mainly involves substances that cannot diffuse across the plasma membrane by dissolving in the lipid bilayer of the membrane. Polar molecules will diffuse through via carrier proteins and small ions via the ion channels. e.g. the movement of glucose across the membrane of red blood cells.
Facilitated diffusion. Source: Campbell et al. (2011)
Osmosis. The net movement of free water molecules through a partially permeable membrane, from a dilute to a more concentrated solution. A special case of diffusion (i.e. movement of water molecules not the movement of solute molecules). Osmotic gradient; osmotic pressure. e.g. Absorption of water from food in the gut; reabsorption of water in kidneys.
Osmosis. Osmotic influence on cells. Cells are affected by the amount of dissolved materials in the water that surrounds them. Hypertonic v hypotonic v isotonic solutions. Hypertonic soln = more dissolved material & less water. Hypotonic soln = less dissolved material and more water. Isotonic soln = both solutions the Source: Enger et al. (2011) same concentration of water.
Active transport. The net movement of dissolved substances into or out of a cell against the concentration gradient. Requires energy. Enables cells to maintain stable internal conditions in spite of extreme variation in the external surroundings. Occurs through protein channels which are very selective for specific ions and molecules. e.g. Sodium ions are pumped out of cells and potassium ions are pumped in, both against the concentration gradient. Plant root cells take up nitrate ions from very dilute solutions in the soil.
Active transport. e.g. active transport of salt occurs in many freshwater organisms. Freshwater fish lose salt by diffusion across their skin- cell plasma membranes into their surrounding freshwater environment. Energy (in the form of ATP) is used to actively transport salt molecules against the concentration gradient. Some molecules can be transported across plasma membranes either by facilitated diffusion or active transport e.g. glucose (most cells v liver cells).
Active transport. Source: Walpole et al. (2011)
Endocytosis & exocytosis. Movement of large molecules and particles in bulk across the plasma membrane. Requires energy. Exocytosis = movement of a substance out of a cell. Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents. e.g. Digestive enzymes in a transport vesicle. Endocytosis = movement of a substance into a cell. Molecules enter cells within vesicles that pinch inward from the plasma membrane. Phagocytosis v pinocytosis e.g. white blood cells engulf microorganisms by phagocytosis.
Endocytosis & exocytosis. Endocytosis and exocytosis in a white blood cell. Source: Enger et al. (2011)