Chapter 8.1 Notes

Chapter 8.1 Notes

• The plasma membrane does not limit the diffusion of water.

• Water tries to reach equal concentration on both sides of the cell.

• The diffusion of water across a selectively permeable membrane is osmosis.

• Osmosis helps the cell to maintain homeostasis

• Molecules in liquids will always try to distribute themselves evenly through all the liquid.

• If a liquid such as water is separated by a selectively permeable barrier that only lets water though and one side of the container has a higher concentration of a substance than the other, then the water molecules will even out until there is a proportional amount of water molecules and the substance for each side. This uneven distribution of particles is called the concentration gradient.

• Isotonic solution – the concentration of a substance in a cell is the same as the concentration of the of a substance outside the cell. Isotonic solutions will not damage cells.

• Hypotonic solution – the concentration of a substance in in a cell is lower than the concentration of the substance outside the cell, causing the cell to pull in more of the substance. Cells will pull in water as well if they are in a hypotonic environment. If the concentration of the substance is high enough (or if it is pure water), the cell may go through cytolysis (bursting from high internal pressure) because it is pulling in too much water with the substance. This is unique to animal cells, as the cell wall in plant cells prevents the cell from bursting and will actually make the plant more firm.

• Hypertonic solution – the concentration of a substance in a cell is higher than the concentration of the substance outside the cell, causing the cell to put the substance into the environment outside the cell. Cells will expel water in a hypertonic environment, as well as the substance. If the concentration outside the cell is low enough, the cell will begin to shrivel and rupture from water loss. Plant cells will lose water mainly from the central vacuole and the plasma membrane will shrink away from the cell wall, which causes the plant to wilt.

• The movement of substances by diffusion is called passive transportation. The passive transport of substances that are not attracted to the phopholipid bilayer or are too large can still occur via the concentration gradient.
• The aid of transport proteins in order to diffuse substances is called facilitated diffusion. This is driven by the concentration gradient and is a common method of moving sugars and amino acids across membranes. The transport proteins simply provide a passage for the substances.
• Active transport is the movement of materials against the concentration gradient. It requires energy from the cell. Specialized transport proteins in the cell known as carrier proteins have specialized shapes that fit a certain molecule or ion. Chemical energy allows the carrier protein to change shape so that once the ion or molecule is in the "grasp" of the protein, it can morph so that an opening allows the substance into the cell. After the particle flows into the cell, the protein morphs back, ready to get more of the substance from the outside.
• Endocytosis is the process by which a cell surrounds and takes in materials from its environment, done by engulfing the substance with part of the plasma membrane, which breaks away. The substance is then enclosed in a vacuole and taken to an appropriate place for processing.
• Exocytosis is the expulsion of materials from a cell.
• These two forms of material transportation are both active.