Cell Biology - The Plasma Membrane (1988)

[Music] cells whether in an animal's body or living independently are surrounded by a watery environment what keeps the cell's contents which is mostly water from escaping into its surroundings is a clear envelope called the plasma membrane the membrane itself is too thin to be seen with a light microscope what we are actually seeing is the boundary between the cell cytoplasm and the surrounding fluid that a membrane is present becomes evident as this amoeba is squeezed under a cover glass rupturing its plasma membrane and spilling its contents into the surrounding water the membrane not only contains and protects it's also a Gateway for molecular traffic one kind of molecular traffic that has a very immediate effect is the movement of water molecules through the membrane water molecules are in constant motion causing particles suspended in water to dance the particles are responding to the bombardment of speeding water molecules [Music] the effect of this molecular movement is also apparent if we drop some clear water into a dye solution because of their motions the molecules tend to spread out evenly this is called diffusion the dye particles slow down the water molecules they're dissolved in reducing their motion so the higher energy molecules in the clear water tend to move toward the molecules with lower energy in the dye solution but let's separate these two fluids with a membrane that lets water molecules pass through but restricts the larger dye molecules this is called a selectively permeable membrane the membrane is filled with dissolution plus some dissolved sugar which will slow down the water molecules even further the rising fluid shows that there is a net flow of higher energy water molecules in the beaker into the sugar dye solution with its lower energy water molecules [Music] this diffusion of water across a selectively permeable membrane is called osmosis [Music] osmosis allows the movement of soil water into the root cells of plants it's also the process that allows the water you drink to pass into your blood and then into your cells osmosis is basic to life processes but it can cause problems in organisms that live where the concentration of dissolved substances varies when the concentration of salt molecules is greater outside the cell than inside water molecules will diffuse out through the membrane under these conditions the membrane of a plant cell will pull away from the cell wall as water leaves the cell eventually the cytoplasm may be compressed Beyond recovery when the concentration of salt molecules is lower outside the cell the net movement of water is into the cell in this situation the plant cell has the advantage of a rigid cell wall outside of the plasma membrane which can withstand the increased pressure unlike plant cells animal cells have no cell wall and respond dramatically to concentrated Salt Solutions normally the concentration of salts inside blood cells is about the same as the concentration in the surrounding plasma under these ballast conditions water enters and leaves the cells at the same rate but add distilled water to the cell's environment and the concentration of salt becomes lower outside the cell in response water diffuses into the cell at a greater rate than it diffuses out causing the blood cells to balloon into well-stretched spheres they swell until they rupture leaving clear empty sacks of membrane called membrane ghosts if this can happen to blood cells how can single cell organisms like protozoans exist in fresh water the fact is they only do so because of special pumps called contractile vacuoles that rid the cell of excess water entering by osmosis foreign [Music] animals the blood carries excess water to specialized cells where it is collected and discharged through tubes back into the environment in mammals kidneys perform this function keeping the water content of their body fluids in balance with their tissue cells but suppose the concentration of salt becomes higher outside of an animal or protozoan cell this is exactly what happens when a paramecium is carried Downstream to the Sea an event we can simulate by adding a drop of salt water as the paramecia encounter the higher concentration of salt in the sea water they lose water through osmosis shrivel and die drink enough seawater and the same will happen to your cells [Music] so osmosis can cause a decrease or an increase in a cell's water content Red Cell ghosts produced by osmotic rupture have provided biologists with pure samples of plasma membrane for use in studying its structure and chemistry an electron micrograph of a section through the plasma membrane shows that it is composed of a double layer of molecules biochemists picture them like this a double layer of fat molecules the question is how do water molecules get through this fatty membrane the accepted theory is that water molecules are able to shoot through gaps or pores irregularly distributed in the membrane but this does not Explain how other molecules cross the membrane barrier into a cell the mechanism puzzled cell biologists until the discovery of proteins embedded in the membrane the protein acts as a lock when a passing molecule fits it like a key the lock opens sending the molecule through into the cytoplasm this is called facilitated diffusion in facilitated diffusion when molecules move from greater concentration outside the cell to lower concentration inside no consumption of energy is required however in some environments substances to be imported may exist in lower concentrations outside the cell than inside it to move such molecules in requires a cell to expend energy this requires a special pump and an input of energy from ATP the cell's energy carrier molecules energy assisted gating is called active transport [Music] a special kind of active transport found in all cells involves ion pumps these are membrane proteins that move charged atoms or ions through the membrane this model of a two-way ion pump shows how ions of sodium and potassium are thought to move through the membrane keeping the cell's proper sodium balance [Music] these forms of active transport account for the way cells are able to maintain an internal environment that is quite different from their surroundings active transport is also important for bringing building block molecules to cells sugar molecules synthesized in the green bodies called chloroplasts are moved out of the food making cell by active transport the sugar fuel is distributed to other cells which take it in again by active transport in protozoans and in all animals active transport plays a direct role in acquiring needed molecules in many kinds of animals including humans and worms nutrients broken down by digestion diffuse through the plasma membranes of intestinal cells into the blood [Music] as blood flows through capillaries these molecules are taken in through the plasma membranes of adjacent cells however certain types of white blood cells cells lining the gut of some simple animals and protozoans take their food in larger bites this kind of intake in which whole organisms are often engulfed is called phagocytosis gobbling up other organisms is a way of life for amoeba this one is hunting for its next meal one or more of the small green euglenoids here one has been surrounded and the plasma membrane has pinched off trapping it in a little Sac called a food vacuole the vacuole membrane still contains its protein pumps and Gates so that once the euglena has been adjusted its molecular building blocks will be absorbed into the cytoplasm [Music] phagocytic white cells use the same process to engulf invading bacteria during phagocytosis the plasma membrane exhibits its great flexing and self-sealing abilities [Music] [Music] you can visualize these special properties by looking at a more familiar kind of fatty membrane soap bubbles for example bubbles have extraordinary flexibility they easily fuse together and they are self-sealing just how self-repairing the plasma membrane is can be seen in this large cell which is being gradually squeezed forcing its membrane to bubble out and break open but amazingly the membrane does not totally disintegrate even when violently shoved around by an expanding air bubble [Music] thank you [Music] phagocytosis provides solid food for cells another method of nutrient intake is by pinocytosis the engulfment of droplets of fluid that's what these cells are doing they live in one of the most nutrient-rich habitats on earth a sewage treatment plant where they help process human wastes an electron micrograph shows how pinocytic vacuoles form by in-pocketing of the plasma membrane these tiny fluid-filled bubbles pinch off carrying dissolved nutrients into the cytoplasm pinocytosis also occurs in cells lining in animal's intestine another environment rich in nutrient fluids a third form of engulfment involves receptors on the membrane's outer surface the receptor proteins fish for specific kinds of molecules such as hormones using a lock and key type of recognition system as its receptors become loaded the membrane In Pockets carrying a highly specific and carefully selected cargo of molecules into the cell the plasma membrane is a deceptively simple structure just two layers of fat molecules in which are embedded proteins that act as Gates pumps and receptors controlling the flow of molecules entering and leaving the cell yet this amazing organelle allows a cell to maintain an internal environment vastly different from its surroundings an environment where the chemical reactions of Life can take place foreign [Music]