Explain the main functions of the cell membrane and the cell wall. Describe what happens during diffusion and osmosis. Describe what happens during active and passive transport. All cells are surrounded by a thin, flexible barrier known as the cell membrane. Many cells also produce a strong supporting layer around the membrane known as the cell wall. The cell membrane regulates what enters and leaves the cell and also provides protection and support (maintains homeostasis). Nearly all cell membranes are made of a double- layered sheet called a phospolipid bilayer. A phospholipid is made of a polar head which is hydrophilic or “water loving” and two nonpolar fatty acid chains or tails that are hydrophobic or “water fearing”. The phospholipids organize themselves in a bilayer to hide their hydrophobic tails from and expose their hydrophilic heads to the water found on both sides of the cell membrane.
In addition to the lipids, most cell membranes contain protein molecules that are embedded in the lipid bilayer. Some of these proteins form channels and pumps that help move material across the cell membrane.
Cell walls are present in plants, algae, fungi, and many prokaryotes (but not animals). Cell walls lie outside the cell membrane. Most cell walls are porous enough to allow water, oxygen, carbon dioxide, and certain other substances through easily. The main function of the cell wall is to provide support and protection for the cell. Most cell walls are made from fibers of carbohydrates and protein. These substances are produced inside the cell and released at the surface of the cell membrane where they are assembled to form the wall. Plant cell walls are composed mostly of cellulose, a tough carbohydrate fiber. Cellulose is the main component of wood and paper. Plant Cell Prokaryote Every living cell exists in a liquid environment that it needs to survive. Even in deserts, the cells of cactus plants, scorpions and vultures are bathed in liquid. One of the most important functions of the cell membrane is to regulate the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side. The cytoplasm of a cell contains a solution of many different substances in water. A solution is a mixture of two or more substances. The substances dissolved in the solution are called solutes. The concentration of a solution is the mass of the solute in a given volume of solution, or mass/volume. Diffusion is the movement of the molecules of a substance from areas of higher concentration to areas of lower concentration. When the concentration of the solute is the same throughout a system, the system has reached equilibrium.
If a substance is present in unequal concentrations on either side of a cell membrane it will diffuse from areas of higher concentration to areas of lower concentration, assuming the substance can cross the cell membrane.
Lower Concentration
Higher Concentration Because diffusion depends on random particle movements, substances diffuse across membranes without requiring the cell to use any energy (passive transport). Once equilibrium is reached, particles continue to move across the membrane in both directions with equal numbers moving in each direction, resulting in no further concentration changes. Some substances can diffuse across biological membranes while others are too large or too strongly charged to cross the lipid bilayer. If a substance is able to diffuse across a membrane, the membrane is said to be permeable to it. A membrane is impermeable to substances that cannot pass across it. Most biological membranes are selectively permeable, meaning some substances can pass through them while others cannot. Small polar molecules such as water and
small, uncharged molecules such as 02 and CO2 pass easily across most membranes, but charged ions (Na+, K+, Ca2+) and many large molecules such as proteins and sugars cannot.
Osmosis is the diffusion of water through a selectively permeable membrane. There are more sugar molecules on the left side of the membrane. That means the concentration of water is lower on the left/higher on the right. The membrane is permeable to water, but not to sugar. As a result, water will diffuse across the cell membrane until equilibrium is reached (because the sugar can’t). When equilibrium is reached the two solutions are isotonic (same strength). Before osmosis occurred, the more concentrated sugar solution on the left was hypertonic (above strength) and the more dilute sugar solution on the right was hypotonic (below strength). 10% NaCL ENVIRONMENT 90% H2O
CELL NO NET 10% NaCL MOVEMENT
90% H2O
What is the direction of water movement?
The cell is at ______.equilibrium 2 9 10% NaCL
90% H2O
CELL
20% NaCL
80% H2O
What is the direction of water movement? 3 0 15% NaCL ENVIRONMENT
85% H2O
CELL
5% NaCL
95% H2O
What is the direction of water movement?
3 1 Isotonic Solution Hypotonic Hypertonic Solution Solution NO NET MOVEMENT OF
H2O (equal amounts The cell The cell entering & leaving) swells (gains shrinks- The cell remains mass) the dehydrates and the same size cell could loses mass burst 3 2
For organisms to survive, they must have a way to balance the intake and loss of water. Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of a selectively permeable membrane. Osmotic pressure can cause serious problems for a cell. Because the cell is filled with salts, sugars, proteins, and other molecules, it will almost always be hypertonic to fresh water. This means that the solution (fresh water) surrounding the cell has a lower solute concentration (and a higher water concentration) than the solution inside the cell which will result in osmotic pressure producing a net movement of water into the cell. This could cause the cell to swell and burst. Hypotonic Hypertonic
Outside cell Inside cell
Solution is hypotonic, cell interior is hypertonic Fortunately, cells in large organisms do not come into contact with freshwater and are not in danger of bursting. Most such cells are bathed in fluids, such as blood, that are isotonic. Water in = water out Paramecium do come into contact with freshwater. They eliminate excess water through contractile vacuoles. Other cells, such as plant cells and bacteria, which do come into contact with fresh water, are surrounded by a tough cell wall which prevents the cell from expanding and bursting. This osmotic pressure in plants supports leaves and flowers. Increased osmotic pressure also makes these cells vulnerable to cell wall injuries. The process by which protein channels in the cell membrane allow certain, specific molecules to cross the cell membrane that ordinarily would be too large or too strongly charged. This is still diffusion and requires no energy to be used by the cell. For example, red blood cells have a cell membrane protein with an internal channel that allows glucose to pass through it. Only glucose can pass through this channel. Hundreds of different protein channels have been found that allow particular substances to cross different membranes.
The process by which cells move small molecules and ions (charged atoms) in the opposite direction – against a concentration difference. Active transport requires ATP or the cell to use energy. Carried out by transport proteins or “pumps” that are found in the cell membrane. Many cells use protein pumps to move calcium, potassium and sodium ions against a concentration gradient. Much of the energy used by cells in their daily activities is devoted to active transport.
A form of active transport where larger molecules and solid clumps of material are taken into the cell by means of infoldings, or pockets, of the cell membrane.
Extensions of cytoplasm surround a particle, package it within a food vacuole and engulf it.
Process by which cells take in liquid from the surrounding environment when tiny pockets form along the cell membrane, fill with liquid, and pinch off to form vacuoles within the cell. A form of active transport in which cells release large amounts of material when vacuoles/vesicles surrounding the material fuse with the cell membrane, forcing the contents out of the cell.
Shooting a BB through a chain link fence. Morbidly obese woman with a pork chop bone tumor in her hip. Drug tunnel under a border wall. Pushing a boulder up a hill. A waterfall tumbling over a cliff. Popping a zit. Water flowing through a fence. Getting water from a well or sucking water through a straw. Active transport, diffusion, phagocytosis, osmosis, facilitated diffusion, exocytosis, pinocytosis,