1. Membrane Transport and Function
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2. Cell Wall
The cell wall is a secondary boundary surrounding certain cells.
It lies outside the cell membrane and is porous enough to allow water, oxygen, carbon dioxide, and certain other substances to pass through.
The cell wall is made of cellulose in plants, chitin in fungi, and peptidoglycan in bacteria.
The purpose of the cell wall is to provide protection and support.

3. The Cell Membrane
The cell membrane (or plasma membrane) is the major barrier for all cells.
It is composed of a phospholipid bilayer.
A phospholipid is a lipid (fat) that has replaced one fatty acid chain with a phosphate group.
The phosphate “head” is hydrophilic and attracted to water. The fatty acid “tails” are hydrophobic and are repelled by water.
The cell membrane is made of two sheets of phospholipids that have formed a bilayer with the fatty acids towards the middle of the bilayer and the phosphates towards the exterior and interior of the cell.

5. Further Membrane Structure
The plasma membrane has proteins embedded throughout it as well.
These proteins aid the membrane in transporting substances in and out of the cell.
The membrane is also supported by cholesterol molecules.

7. Properties of the Plasma Membrane
Because the plasma membrane is able to control what passes through and lipids are hydrophobic, the cell membrane is called semi-permeable (or selectively permeable).
Only certain substances are able to pass directly through the plasma membrane. These are usually small nonpolar molecules such as oxygen and carbon dioxide.
Charged particles, polar molecules and large molecules are unable to pass directly through the membrane and require special transport proteins.
Because the proteins are not bound to a specific location in the membrane and are free to float around, the membrane is described as a fluid-mosaic.

8. Movement of Substances
Substances are always moving by diffusion.
Diffusion is the random movement of molecules from an area of high concentration to an area of low concentration.
Concentration is usually expressed as a molar concentration (symbolized by a capital M) or as a percentage.

9. Diffusion Across the Membrane
Many substances enter cells by diffusion.
When the concentration of substances is the same on both sides of the membrane, equilibrium is reached.
However, substances still pass back and forth across the membrane after equilibrium has been reached. Because the concentration of substances on either side of the membrane doesn’t change this is called dynamic equilibrium.
Osmosis is a special case of diffusion. It occurs when water diffuses across a semi-permeable membrane.

10. Cell Behavior
Cells will behave differently based on the concentration of the solution they are placed in.
An isotonic solution is one where the concentration of substances outside the cell is the same as the concentration of substances inside the cell.
A hypertonic solution is one where the concentration of substances outside the cell is higher than the concentration of substances inside the cell.
A hypotonic solution is one where the concentration of substances outside the cell is lower than the concentration of substances inside the cell.
Water will diffuse across the cell membrane until dynamic equilibrium is reached and the exterior solution is isotonic.

11. If cells are placed in isotonic solutions, water is moving into the cell at the same rate water is leaving the cell. The cell remains unchanged.
If cells are placed in hypotonic solutions, water is moving into the cells at a higher rate than water is leaving the cell. The cell swells. Animal cells can burst. Cell walls prevent plant cells from bursting. The cells become turgid, and the plant as a whole stands upright.
If cells are placed in hypertonic solutions, water is moving out of the cells at a higher rate than water is entering the cells. The cells shrivel and can die. In plant cells, the membrane pulls away from the cell wall, and the plant as a whole wilts.

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13. Material Acquisition
Cells have two methods to obtain materials from their environment.
Passive transport is a method that does not require energy. Substances move by diffusion with the concentration gradient.
Active transport is a method that does require energy. Substances move against the concentration gradient (low concentration to high concentration).

14. Passive Transport
Passive transport occurs anytime substances pass through the membrane with the concentration gradient.
Some substances cannot pass directly through the membrane. They travel through special transport proteins. Because this occurs with the concentration gradient, it is called facilitated diffusion.

16. Active Transport
Active transport requires energy to move substances into a cell against the concentration gradient.
Active transport will always use some form of transport protein.