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The Cell Membrane
Cells require protection, nutrients, water, elimination of waste, and response to stimuli. The cell membrane performs all these functions for the cell. The cell membrane acts as a barrier between the outside environment and the cell.
Cell Membrane Components:
Cell membrane consists of a double layer of phospholipids. Remember that a phospholipid consists of a phosphate group, glycerol, and two fatty acid chains.
The head of the phosphate is glycerol and the phosphate group. The head is hydrophilic (water loving). The tail of the phosphate is the two fatty acid chains. The tail is hydrophobic (water hating).
The Cell membrane is oriented in such a way that the two layers of phospholipids sandwich themselves. The heads will touch the outside environment as well as the cytoplasm inside the cell. The fatty acid chains are attracted to each other. The heads of the phospholipids are like the bread of the sandwich and the chains are the fillings in the sandwich.

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The cell membrane is responsible for allowing substances in and out of the cell. This is important since the cell needs to maintain equilibrium.
The cell will need to take in nutrients and expel waste or excess amount of substances.
There are molecules located inside the cell membrane that serve various functions: o Cholesterol: help to strengthen the membrane o Proteins: these proteins are embedded in the membrane to help transport molecules across. Other proteins are on the surface of the cell membrane which act as receptors or are a part of the cytoskeleton. o Carbohydrates: are attached on the surface of the membrane, they act as identification tags to distinguish one cell type from the next.

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Fluid Mosaic Model
Scientists have termed the cell membrane characteristics as the fluid
mosaic model for two reasons: o The cell membrane is flexible not rigid. Phospholipids can move from side to side and pass each other. The membrane acts as a fluid. o The numerous proteins and molecules that line the membrane and are embedded make the membrane colourful and display a mosaic of shapes and textures.
The cell membrane is selectively permeable, meaning that it allows some substances to cross but not others. The membrane is also called semi-permeable. Selectively permeable allows the cell to maintain
homeostasis, which is the ability to maintain equilibrium. Cells need to have specific conditions to carry out certain functions therefore it must control the input and output of certain ions.
The movement of a molecule across the membrane will depend on its size, polarity, and concentration of the molecule inside compared to outside the cell. Small non-polar molecules can pass thru on their own, while small polar molecules will need to be transported across the membrane via proteins. Large molecules will be transported via vesicles.
Chemical Signals Across the Membrane

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The cell membrane is also the site of chemical signals. The carbohydrates on the outside of the membrane act as identifiers. A receptor will recognize the carbohydrates on the surface and send its own signal to perform a response. A receptor will only bind to certain molecules. The molecule that the receptor binds to is known as a ligand. When the receptor binds to the ligand they change shape. The change in shape is important because it determines how the receptor interacts with other molecules.
There are two types of receptors:
1) Intracellular receptor
An intracellular receptor is located inside the cell; intra meaning ‘within’.
Many hormones are intracellular receptors.
2) Membrane receptor
Molecules that cannot cross the membrane will bind to a receptor (protein) embedded in the membrane. When the ligand binds, the receptor will send a signal to inside the cell. The receptor will also change shape causing molecules inside to respond.
Transportation of Molecules Across the Cell Membrane:
Passive transport: Does not require energy. It is the movement of molecules across the membrane without the input of energy.

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Types of Passive Transport:
1.
Diffusion
Is the movement of molecules from a region of high concentration to a region of low concentration.
Concentration: is the number of molecules in a given volume.
Concentration gradient: is the difference in concentration of a substance in one area to another. Molecules will naturally diffuse down their concentration gradient; high concentration to low concentration.
Most small non polar molecules move across the membrane by diffusion. i.e oxygen.
2.
Osmosis
Is the diffusion of water molecules from an area of high concentration to an area of low concentration. the
Note: the higher the concentration of particles, the LOWER water concentration and vice versa.

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3.
Facilitated Diffusion
Is the diffusion of molecules across the cell membrane through transport proteins. Molecules are still moving DOWN a concentration gradient.
Some molecules cannot pass through the membrane on their own, they need to use the channels in membrane proteins to enter or leave.
Descriptions of Solutions:
Isotonic Solution: The solution has the same number of solutes as the cell. Water will diffuse in and out of the cell at equal rates.
Hypertonic Solution: The solution has a higher concentration of solutes than the cell. Water will diffuse out at a higher rate than into the cell.
Hypotonic Solution: The solution has a lower concentration of solutes than the cell. Water will diffuse into the cell at a higher rate than out of the cell.

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Active Transport
Some molecules need to move against their concentration gradient.
These molecules need to use transport proteins, known as pumps.
Active transport moves molecules across the cell membrane from an area of low concentration to an area of high concentration of solutes.
The transport proteins are powered by chemical energy. Most use energy from the molecule, ATP.

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Uptake or Removal of Large Molecules:
Endocytosis:
Exocytosis:
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