7 3 cell boundaries

Editor's Notes

• #4: While cell membranes might be around every cell, cell walls made of cellulose are only found around plant cells. Cell walls are made of specialized sugars called cellulose. Cellulose provides a protected framework for a plant cell to survive. It's like taking a water balloon and putting it in a cardboard box. The balloon is protected from the outside world. Cellulose is called a structural carbohydrate (complex sugar) because it is used in protection and support. Cell walls also help a plant keep its shape. While they do protect the cells, cell walls and cellulose also allow plants to grow to great heights. While you have a skeleton to hold you up, a 100-foot tall redwood tree does not. It uses the strong cell walls to maintain its shape. For smaller plants, cell walls are slightly elastic. Wind can push them over and then they bounce back. Big redwoods need strength in high winds and sway very little (except at the top). You may hear about cell walls in other areas of biology. Bacteria also have a structure called a cell wall. Fungi and some ptotozoa also have cell walls. They are not the same. Only plant cell walls are made out of cellulose. The other walls might be made from proteins or a substance called chitin. They all serve the same purpose of protecting and maintaining structure, but they are very different molecules.
• #5: A cell wall is not a fortress around the delicate plant cell. There are small holes in the wall that let nutrients, waste, and ions pass through. Those holes are called plasmodesmata. These holes have a problem: water can also be lost. But even when the plant cell loses water, the basic shape is maintained by the cell walls. So if a plant is drooping because it needs water, it can recover when water is added. It will look just the same as when it started.
• #6: The Plasma Membrane (Fluid Mosaic Model) The plasma membrane forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It protects and supports the cell and also controls everything that enters and leaves the cell. It allows only certain substances to pass through, while keeping others in or out. The ability to allow only certain molecules in or out of the cell is referred to as selective permeability or semipermeability. To understand how the plasma membrane controls what crosses into or out of the cell, you need to know its composition. The Phospholipid Bilayer The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. As shown in Figure below, each phospholipid molecule has a head and two tails. The head “loves” water (hydrophilic) and the tails “hate” water (hydrophobic). The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell. Molecules that are hydrophobic can easily pass through the plasma membrane, if they are small enough, because they are water-hating like the interior of the membrane. Molecules that are hydrophilic, on the other hand, cannot pass through the plasma membrane—at least not without help—because they are water-loving like the exterior of the membrane. Photo: Phospholipid Bilayer. The phospholipid bilayer consists of two layers of phospholipids, with a hydrophobic, or water-hating, interior and a hydrophilic, or water-loving, exterior. The hydrophilic (polar) head group and hydrophobic tails (fatty acid chains) are depicted in the single phospholipid molecule. The polar head group and fatty acid chains are attached by a 3-carbon glycerol unit.
• #8: Each phospholipid has a head and two tails. Head: Hydrophilic Tails: Hydrophobic Two layers of phospholipids in the cell membrane The hydrophobic tails point inward and the hydrophilic heads point outward. Molecules that are hydrophobic can easily pass through the membrane if they are small enough. This is because they are water hating like the interior of the bilayer Molecules that are hydrophilic do not pass through the membrane without help This is because they are water loving like the exterior of the membrane These molecules are mostly lipids and proteins ex: The lipid cholesterol that help the plasma membrane keep its shape. Various proteins help other molecules pass through the plasma membrane.
• #9: Other Molecules in the Plasma Membrane The plasma membrane also contains other molecules, primarily other lipids and proteins. The light blue chain-shaped molecules in Figure here,for example, are the lipid cholesterol. Molecules of cholesterol help the plasma membrane keep its shape. Many of the proteins in the plasma membrane assist other substances in crossing the membrane. Some plasma membrane proteins are located in the lipid bilayer and are called integral proteins. Other proteins, called peripheral proteins, are outside of the lipid bilayer. Peripheral proteins can be found on either side of the lipid bilayer: inside the cell or outside the cell. Membrane proteins can function as enzymes to speed up chemical reactions, act as receptors for specific molecules, or transport materials across the cell membrane. Cholesterol molecules are important for maintaining the consistency of the cell membrane. They strengthen the membrane by preventing some small molecules from crossing it. Cholesterol molecules also keep the phospholipid tails from coming into contact and solidifying. This ensures that the cell membrane stays fluid and flexible. Carbohydrates, or sugars, are sometimes found attached to proteins or lipids on the outside of a cell membrane. That is, they are only found on the extracellular side of a cell membrane. Together these carbohydrates form the glycocalyx. The glycocalyx of a cell has many functions. It can provide cushioning and protection for the plasma membrane, and it is also important in cell recognition. Based on the structure and types of carbohydrates in the glycocalyx, your body can recognize cells and determine if they should be there or not. They glycocalyx can also act as a glue to attach cells together. Cholesterol - stiffens the membrane by connecting phospholipidsGlycolipids - signal molecules ----- Meeting Notes (12/11/14 11:42) ----- i love you bae deformed elephant
• #10: Glycoproteins - have an attached chain of sugar (antibodies) Proteins embedded in membrane serve different functions 1. Channel Proteins - form small openings for molecules to difuse through2. Carrier Proteins- binding site on protein surface "grabs" certain molecules and pulls them into the cell, (gated channels)3. Receptor Proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel proteins)4. Cell Recognition Proteins - ID tags, to idenitfy cells to the body's immune system5. Enzymatic Proteins - carry out metabolic reactions
• #11: Solution: mixture of two or more substances in which the molecules of the substances are equally distributed. Solute: substance that is dissolved in a solvent to make a solution. Solvent: substance in which a solute is dissolved to form a solution. Transport Across Membranes If a cell were a house, the plasma membrane would be walls with windows and doors. Moving things in and out of the cell is an important role of the plasma membrane. It controls everything that enters and leaves the cell. There are two basic ways that substances can cross the plasma membrane: passive transport and active transport.
• #12: If you dissolve 12g of salt in 3 liters of water, the concentration of the resulting solution is 12g/3L or 4g/L. If you dissolve 12g of salt in 6 L of water, the concentration of the resulting solution is 12g/6L or 2g/L. Which has a higher concentration?
• #13: Passive Transport Passive transport occurs when substances cross the plasma membrane without any input of energy from the cell. No energy is needed because the substances are moving from an area where they have a higher concentration to an area where they have a lower concentration. Concentration refers to the number of particles of a substance per unit of volume. The more particles of a substance in a given volume, the higher the concentration. A substance always moves from an area where it is more concentrated to an area where it is less concentrated. It’s a little like a ball rolling down a hill. It goes by itself without any input of extra energy. There are several different types of passive transport, including simple diffusion, osmosis, and facilitated diffusion. Each type is described below. You can also watch an animation of each type at this link: http://www.northland.cc.mn.us/biology/BIOLOGY1111/animations/passive1.swf.
• #14: Diffusion is the net movement of particles from an area of high concentration to an area of low concentration due to the random movement of particles. It is a passive process which means that no energy is required. When the concentration of the solute is the same throughout the system (in this case, on both sides of the cell membrane) the system is said to have reached equilibrium. Simple Diffusion - water, oxygen and other molecules move from areas of high concentration to areas of low concentration, down a concentration gradientFacilitation Diffusion - diffusion that is assisted by proteins (channel or carrier proteins)
• #15: Simple Diffusion Diffusion is the movement of a substance across a membrane, due to a difference in concentration, without any help from other molecules. The substance simply moves from the side of the membrane where it is more concentrated to the side where it is less concentrated. Figure below shows how diffusion works. Substances that can squeeze between the lipid molecules in the plasma membrane by simple diffusion are generally very small, hydrophobic molecules, such as molecules of oxygen and carbon dioxide. Diffusion Across a Cell Membrane. Molecules diffuse across a membrane from an area of higher concentration to an area of lower concentration until the concentration is the same on both sides of the membrane.
• #16: Facilitated Diffusion Water and many other substances cannot simply diffuse across a membrane. Hydrophilic molecules, charged ions, and relatively large molecules such as glucose all need help with diffusion. The help comes from special proteins in the membrane known as transport proteins. Diffusion with the help of transport proteins is called facilitated diffusion. There are several types of transport proteins, including channel proteins and carrier proteins. Both are shown in Figure below. Channel proteins form pores, or tiny holes, in the membrane. This allows water molecules and small ions to pass through the membrane without coming into contact with the hydrophobic tails of the lipid molecules in the interior of the membrane. Carrier proteins bind with specific ions or molecules, and in doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane. Facilitated Diffusion Across a Cell Membrane. Channel proteins and carrier proteins help substances diffuse across a cell membrane. In this diagram, the channel and carrier proteins are helping substances move into the cell (from the extracellular space to the intracellular space).
• #17: Usually, cells are in an environment where there is one concentration of ions outside and one inside. Because concentrations like to be the same, the cell can pump ions in an out to stay alive. Osmosis is the movement of water across the membrane. For a cell to survive, ion concentrations need to be the same on both sides of the cell membrane. If the cell does not pump out all of its extra ions to even out the concentrations, the water is going to move in. This can be very bad. The cell can swell up and explode. The classic example of this type of swelling happens when red blood cells are placed in water. The water rushes in to the cells, they expand and eventually rupture (POP!).
• #18: Osmosis Osmosis is a special type of diffusion — the diffusion of water molecules across a membrane. Like other molecules, water moves from an area of higher concentration to an area of lower concentration. Water moves in or out of a cell until its concentration is the same on both sides of the plasma membrane. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
• #21: Active Transport Active transport occurs when energy is needed for a substance to move across a plasma membrane. Energy is needed because the substance is moving from an area of lower concentration to an area of higher concentration. This is a little like moving a ball uphill; it can’t be done without adding energy. The energy for active transport comes from the energy-carrying molecule called ATP. Like passive transport, active transport may also involve transport proteins. You can watch an animation of active transport at the link below. http://www.northland.cc.mn.us/biology/BIOLOGY1111/animations/active1.swf Protein Pumps Endocytosis Exocytosis
• #22: Adenosine triphosphate - provides energy for cellular reactions and processes. ATP releases energy when it’s high energy bond is broken to release a phosphate group- Leaving behind ADP (we will talk about this is later units)
• #23: Sodium-Potassium Pump An example of active transport is the sodium-potassium pump. When this pump is in operation, sodium ions are pumped out of the cell, and potassium ions are pumped into the cell. Both ions move from areas of lower to higher concentration, so ATP is needed to provide energy for this “uphill” process. Figure below explains in more detail how this type of active transport occurs. The sodium-potassium pump. The sodium-potassium pump moves sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. First, three sodium ions bind with a carrier protein in the cell membrane. Then, the carrier protein receives a phosphate group from ATP. When ATP loses a phosphate group, energy is released. The carrier protein changes shape, and as it does, it pumps the three sodium ions out of the cell. At that point, two potassium ions bind to the carrier protein. The process is reversed, and the potassium ions are pumped into the cell. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
• #24: Vesicle Transport Some molecules, such as proteins, are too large to pass through the plasma membrane, regardless of their concentration inside and outside the cell. Very large molecules cross the plasma membrane with a different sort of help, called vesicle transport. Vesicle transport requires energy, so it is also a form of active transport. There are two types of vesicle transport: endocytosis and exocytosis. Both types are shown in Figure below and described below. Endocytosis is the type of vesicle transport that moves a substance into the cell. The plasma membrane completely engulfs the substance, a vesicle pinches off from the membrane, and the vesicle carries the substance into the cell. When an entire cell is engulfed, the process is called phagocytosis. When fluid is engulfed, the process is called pinocytosis. Exocytosis is the type of vesicle transport that moves a substance out of the cell. A vesicle containing the substance moves through the cytoplasm to the cell membrane. Then, the vesicle membrane fuses with the cell membrane, and the substance is released outside the cell. You can watch an animation of exocytosis at the link below.
• #25: Endocytosis Material moved into cell Forms vesicle Pinocytosis: Liquids Phagocytosis: Solids
• #26: Exocytosis Material released to outside of cell
• #27: Homeostasis and Cell Function For a cell to function normally, a stable state must be maintained inside the cell. For example, the concentration of salts, nutrients, and other substances must be kept within a certain range. The process of maintaining stable conditions inside a cell (or an entire organism) is homeostasis. Homeostasis requires constant adjustments, because conditions are always changing both inside and outside the cell. The processes described in this lesson play important roles in homeostasis. By moving substances into and out of cells, they keep conditions within normal ranges inside the cells and the organism as a whole.