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Part 1:
Part 1:
Cell Membrane Structures
Cell Membrane Structures
Among the most important parts of a cell are its borders, which separate the cell from its surroundings.
Among the most important parts of a cell are its borders, which separate the cell from its surroundings.
CELL (PLASMA) MEMBRANE: a thin, flexible barrier that surrounds ALL cells.
CELL (PLASMA) MEMBRANE: a thin, flexible barrier that surrounds ALL cells.
Cell Membrane
Cell Membrane
The cell membrane regulates what enters and leaves the cell and also provides protection and support.
The cell membrane regulates what enters and leaves the cell and also provides protection and support.
The plasma membrane separates the internal environment of the cell from the external environment. In doing so, it regulates the entrance and exit of molecules from the cell. It helps the cell maintain homeostasis.
The composition of nearly all cell membranes is a double-layered sheet called the PHOSPHOLIPID BILAYER. The lipid bilayer gives cell membranes a flexible structure that forms a strong barrier between the cell and its surroundings.
The Phospholipid Bilayer is made up of two layers of PHOSOPLIPIDS a type of lipid with both hydrophobic (water fearing) and hydrophilic (water loving) properties.
The Phospholipid Bilayer is made up of two layers of PHOSOPLIPIDS a type of lipid with both hydrophobic (water fearing) and hydrophilic (water loving) properties.
The phospholipids of the membrane forms a bilayer, with the HYDROPHILIC (water loving) POLAR HEADS of the phospholipid molecules facing the outside and inside of the cell (where water is found), and the HYDROPHOBIC (water fearing) NONPOLAR TAILS in between/in the middle.
Phospholipids are Amphiphilic /Amphipathic because they are molecules that have both hydrophobic (nonpolar) and hydrophilic (polar) regions.
Heads of Phospholipids…
Heads of Phospholipids…
Hydrophilic (Polar) = Water Loving
Tails of Phospholipids...
Tails of Phospholipids...
Hydrophobic (Nonpolar) = Water Fearing
DINB Help! - Phospholipid Bilayer
DINB Help! - Phospholipid Bilayer
Name: Phospholipid Bilayer
Structure: Two layers made up of phospolipids. The HYDROPHOBIC tails face inwards towards each other and the HYDROPHILIC heads face outwards.
Function: The bilayer seperates the inside of the cell from the EXTERNAL environment. This allows the cell to have a carefully CONTROLLED internal environment. It also holds other MOLECULES, e.g. proteins.
CARBOHYDRATES
CARBOHYDRATES
CARBOHYDRATES (sugar) molecules can be attached to one of these proteins or to lipids in the cell membrane. Typically these carbohydrates act like chemical identification cards, allowing individual cells to identify one another. Both phospholipids and proteins can have attached carbohydrate (sugar) chains.
If the carbohydrate is attached to a lipid together they form a GLYCOLIPIDS.
If the carbohydrate is attached to a protein together they form a GLYCOPROTEINS. Both glycolipids and glycoproteins play key roles in cell signaling and cell recognition.
DINB Help! - Glycoprotein
DINB Help! - Glycoprotein
Name: Glycoprotein
Structure: It is a PROTEIN that has a CARBOHYDRATE chain attached to its surface.
Function: The carbohydrate chain extends into the external environment and allows the cell to COMMUNICATE with its environment and other cells. It is important for cell ADHESION.
DINB Help! - Glycolipid
DINB Help! - Glycolipid
Name: Glycolipid
Structure: It is a LIPID that has a carbohydrate CHAIN attached to its surface.
Function: The carbohydrate chain EXTENDS into the external environment and allows the cell to communicate with other CELLS. It also helps STABILIZE the cell membrane.
CHOLESTEROL
CHOLESTEROL
The phospholipid bilayer has a fluid consistency. The fluidity of the membrane is regulated by steroids such as CHOLESTEROL, which serve to stiffen and strengthen the membrane while also ensuring the membrane maintains it flexibility and fluidity.
DINB Help! - Cholesterol
DINB Help! - Cholesterol
Name: Cholesterol
Structure: It is a type of steroid and has a basic structure of FOUR rings of CARBON atoms. It is a small molecule and is found mixed in with the PHOSPHOLIPIDS.
Function: Cholesterol helps to regulate the FLUIDITY of the cell membrane. At LOW temperatures it stops the phospholipids from packing together too TIGHTLY. At high temperatures it facilitates this, keeping the membrane together.
PROTEINS
PROTEINS
In addition to lipids, most cell membranes contain protein molecules that are embedded or partially embedded in the lipid bilayer. Some of the proteins form channels and pumps that help to move material across the cell membrane or play a role in cell structure.
PERIPHERAL PROTEINS are associated with only one side of the plasma membrane. Tend to play a role in cell structure (when connected to cytoskeleton filaments) or can be used in cell recognition.
INTEGRAL PROTEINS are proteins that are embedded in the cell membrane and can stick out from one or both sides.
TRANSMEMBRANE PROTEINS are integral proteins that span the entire membrane. They play an important role in transporting materials from one side of the cell to the other.
Examples of transmembrane/integral proteins include:
Channel Proteins
Carrier Proteins
Receptor Proteins
DINB Help! - Channel Protein
DINB Help! - Channel Protein
Name: (Integral Protein) Channel Protein
Structure: It is a transmembrane protein (it travels COMPLETELY through the cell membrane) and has a "CHANNEL" through the middle of it.
Function: Channel proteins TRANSPORT molecules from one side of the cell membrane to the other. They move large or POLAR molecules and ions that CANNOT pass through the cell membrane on their own.
DINB Help! - Carrier Protein
DINB Help! - Carrier Protein
Name: (Integral Protein) Carrier Protein
Structure: It is a TRANSMEMBRANE protein (it travels completely through the cell membrane) and has BINDING site for specific molecules.
Function: Like a channel protein, carrier proteins also transport LARGE polar molecules and IONS across the cell membrane. However, it is more SPECIFIC in the molecules it can transport.
DINB Help! - Receptor Protein
DINB Help! - Receptor Protein
Name: (Integral Protein) Receptor Protein
Structure: This is TRANSMEMBRANE protein - some span the membrane numerous times or just once. They have a RECEPTOR that extends into the external envrionment.
Function: Receptor proteins are for cell SIGNALING - when a signaling molecule, e.g. a hormone or nutrient, ATTACHES to the receptor, the protein changes shape and causes a CHANGE inside the cell.
There are so many kinds of molecules in the cell membrane that scientists describe their understanding of the membrane as the "FLUID MOSAIC MODEL." This is because the combination of proteins, steroids, and phospholipids makes up a mosaic pattern.
There are so many kinds of molecules in the cell membrane that scientists describe their understanding of the membrane as the "FLUID MOSAIC MODEL." This is because the combination of proteins, steroids, and phospholipids makes up a mosaic pattern.
Part 2:
Part 2:
Cell Transport
Cell Transport
Every living cell exists in a liquid environment that it needs to survive. 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. This is important because a cell is only able to survive if it can maintain its normal composition.
The plasma membrane is able to carry out this very important function because it is SELECTIVELY PERMEABLE/SEMIPERMEABLE.
The plasma membrane is able to carry out this very important function because it is SELECTIVELY PERMEABLE/SEMIPERMEABLE.
SELECTIVELY PERMEABLE/SEMIPERMEABLE: some substances can pass through while others cannot.
If a substance is able to cross the cell membrane then the membrane is PERMEABLE to it.
- In general small, noncharged molecules (ex: carbon dioxide, oxygen, alcohol) can freely cross the cell membrane. AKA the cell membrane is PERMEABLE to such substances.
A membrane is IMPERMEABLE to substances that cannot pass across it.
Large molecules and some ions and charged molecules are unable to freely cross the membrane. AKA the membrane is IMPERMEABLE to such substances. They can only cross the membrane through channel proteins, or in vesicles.
DIFFUSION
DIFFUSION
The cytoplasm of a cell contains a solution of many different substances in water. Remember a solution is a mixture of two or more substances. The substances that are dissolved in the solution are the SOLUTES. Also remember that the CONCENTRATION of a solution is the mass (amount) of solute in a given volume of solution, or mass/volume.
In a solution, particles move constantly. They collide with one another and spread out. As they move, the particles tend to go from an area where they are more concentrated to an area where they are less concentrated. This is known as DIFFUSION.
DIFFUSION: the movement of molecules from a higher concentration to a lower concentration. AKA the movement of solute down its concentration gradient.
DIFFUSION: the movement of molecules from a higher concentration to a lower concentration. AKA the movement of solute down its concentration gradient.
These molecules are said to go "DOWN" their concentration gradient as they move from an area where their concentration is high to an area where their concentration is low.
Some molecules are able to go "UP" their concentration gradient or move from an area where their concentration is low to an area where their concentration is high, but this requires energy.
When the concentration of the solute is the same throughout a system or mixture, the mixture has reached EQUILIBRIUM.
When the concentration of the solute is the same throughout a system or mixture, the mixture has reached EQUILIBRIUM.
Because diffusion depends upon random particle movements, substances diffuse across membranes WITHOUT requiring the cell to use energy.
Even when equilibrium is reached, particles of a solution will continue to move across the membrane in both directions. However, because an almost equal number of particles move in each direction, there is no further change in concentration.
Several factors influence the rate of diffusion:
temperature, pressure
electrical currents
and molecular size
(Ex: As temperature increases the movement of molecules increase which also increases the rate of diffusion.)
Facilitated Diffusion:
Facilitated Diffusion:
Diffusion with Protein Channel Helpers!
Diffusion with Protein Channel Helpers!
A few molecules, like glucose or amino acids, seem to pass through the cell membrane much more quickly than they should. Given their composition you'd think they wouldn't be able to pass through because they're too large and/or strongly charged to cross the membrane. But somehow they are able to cross very easily. How does this happen?!
Study Tip:
Study Tip:
Remember during facilitated diffusion molecules such as glucose that cannot diffuse across the cell membrane's lipid bilayer on their own move through protein channels instead.
Facilitated diffusion explains how molecules like glucose and amino acids are able to cross the membrane.
FACILITATED DIFFUSION is the process by which molecules that are too big or too polar (like glucose) can pass through the membrane through transmembrane proteins that allow them to cross the membrane without "upsetting" the selective phospholipid bilayer.
FACILITATED DIFFUSION is the process by which molecules that are too big or too polar (like glucose) can pass through the membrane through transmembrane proteins that allow them to cross the membrane without "upsetting" the selective phospholipid bilayer.
Although facilitated diffusion is fast and specific, it is still diffusion! Therefore, a net movement of molecules across cell membrane will occur only if there is a higher concentration of the particular molecules on one side than on the other. In other words it only works when molecules are moving down their concentration gradient. Thus this movement does not require energy.
Both traditional diffusion and facilitated diffusion are examples of PASSIVE TRANSPORT.
Both traditional diffusion and facilitated diffusion are examples of PASSIVE TRANSPORT.
They are passive because they DO NOT REQUIRE ENERGY.
They are passive because they DO NOT REQUIRE ENERGY.
(Osmosis is another form of passive transport.)
OSMOSIS
OSMOSIS
Water, a polar molecule, would not be expected to readily cross the mostly nonpolar cell membrane. While the small size of water may allow some molecules to diffuse across, the majority of cells have special channel proteins called AQUAPORINS that allow water to quickly cross the membrane.
Look at the picture to the left. In the beaker on the left there are more sugar molecules on the left side of the membrane than on the right side. That means that the concentration of water is lower on the left side than it is on the right. The membrane is permeable to water but not sugar.. This means that water can cross the membrane in both directions but sugar cannot. As a result, there is a net movement of water from the area of high concentration to the area of lower concentration.
OSMOSIS: the diffusion of water through a selectively permeable membrane (Cell/Plasm Membrane).
OSMOSIS: the diffusion of water through a selectively permeable membrane (Cell/Plasm Membrane).
Water will tend to move across the membrane until equilibrium is reached. At that point, the concentrations of water and sugar will be the same on both sides of the membrane.
For organisms to survive, they must have a way to balance the intake and loss of water. Osmosis exerts a pressure known osmotic pressure on the more concentrated side of a selectively permeable membrane.
OSMOTIC PRESSURE: the pressure that develops in a system due to osmosis.
OSMOTIC PRESSURE: the pressure that develops in a system due to osmosis.
The greater the osmotic pressure, the more likely it is that water will diffuse in that direction.
Hypotonic Solutions
Hypotonic Solutions
"Below Strength"
Refers to a solution with a LOWER concentration of solute (higher concentration of water) than inside the cell.
If a cell is place in a HYPOTONIC SOLUTION, water enters the cell.
The net movement of water is from the outside to the inside of the cell. These solutions can cause cell to swell, or even burst due to an intake of water.
Isotonic Solutions
Isotonic Solutions
"Same Strength"
In an ISOTONIC SOLUTION, the solute concentration and the water concentration both inside and outside the cell are equal.
If a cell is placed in an ISOTONIC SOLUTION there is not net gain or loss of water.
Hypertonic Solutions
Hypertonic Solutions
"Above Strength"
Refers to a solution with a HIGHER percentage of solute (lower concentration of water) than the cell.
If a cell is placed in a HYPERTONIC SOLUTION, water leaves the cell.
The net movement of water is from inside to the outside of the cell. These solutions cause cells to shrink or shrivel due to loss of water.
How do these solutions affect cells?
How do these solutions affect cells?
HYPOTONIC SOLUTIONS...
HYPOTONIC SOLUTIONS...
Animal cells placed in such solution expand and sometimes burst or lyse due to the buildup of pressure. The term CYTOLYSIS is used to refer to such cells.
The swelling of a plant cell in a hypotonic solution creates TURGOR PRESSURE. When in this type of solution the plants cytoplasm expands because the large central vacuole gains water and the plasma membrane pushes against the rigid cell wall. It is this cell wall that prevents plants, bacteria, and other organisms with cell walls from bursting due to the increase of osmotic pressure.
Turgor pressure is extremely important in plants because it allows them to maintain their erect position (helps them "stand" up straight).
HYPERTONIC SOLUTIONS...
HYPERTONIC SOLUTIONS...
Animal cells placed in such a solution shrink. The term CRENATION refers to the shriveling of a cell in a hypertonic solution.
When a plant cell is placed in a hypertonic solution, the plasma membrane pulls away from the cell wall as the large central vacuole loses water. This is an example of PLASMOLYSIS, which is the shrinking of the cytoplasm due to osmosis.
Study Tip:
Study Tip:
The term "tonicity" refers to the osmotic pressure or tension of the solution. iso- means "the same as". hypo- means "less than" and hyper- means "more than".
The term "tonicity" refers to the osmotic pressure or tension of the solution. iso- means "the same as". hypo- means "less than" and hyper- means "more than".
Remembering these prefixes and what they mean will better help you remember how these solutions can affect the cell.
ACTIVE TRANSPORT
ACTIVE TRANSPORT
Sometimes diffusion isn't enough for the cell. Cells sometimes must move materials in the opposite direction; against their concentration gradient AKA up their concentration gradient. This is accomplished by a process known as ACTIVE TRANSPORT.
Sometimes diffusion isn't enough for the cell. Cells sometimes must move materials in the opposite direction; against their concentration gradient AKA up their concentration gradient. This is accomplished by a process known as ACTIVE TRANSPORT.
Active Transport requires ENERGY. This is why it's ACTIVE!
Active Transport requires ENERGY. This is why it's ACTIVE!
The active transport of small molecules or ions across a cell membrane is generally carried out by integral transmembrane proteins or carrier proteins that are found embedded in the cell membrane. Larger molecules and clumps of material can also be actively transported across the cell membrane by processes known as endocytosis and exocytosis. The transport in these larger molecules sometimes involves changes in the shape of the cell membrane.
Remember Active Transport moves in the opposite direction of diffusion. This is because molecules are being moved UP their concentration gradient!
Remember Active Transport moves in the opposite direction of diffusion. This is because molecules are being moved UP their concentration gradient!
ENERGY = ATP
ENERGY = ATP
Both carrier proteins and energy are needed to transport molecules against their concentration gradient. Usually, this chemical energy is in the form of ATP.
Both carrier proteins and energy are needed to transport molecules against their concentration gradient. Usually, this chemical energy is in the form of ATP.
In these cases, ATP is required for the carrier protein to combine with the substance to be transported.
Active Transport: Pumping Materials UP (against) their Concentration Gradient!
Active Transport: Pumping Materials UP (against) their Concentration Gradient!
This process requires ATP Energy!
This process requires ATP Energy!
Proteins involved in active transport are often called pumps because, just as a water pump uses energy to move water against the force of gravity, proteins use energy to to move substances against their concentration gradient.
Sodium-Potassium Pump
Sodium-Potassium Pump
One type of pump that is active in all animal cells, but is especially associated with nerve and muscle cells, moves sodium ions (Na+) to the outside of the cell and potassium ions (K+) to the inside of the cell. These two events are linked and the carrier protein is called a SODIUM-POTASSIUM PUMP.
How do large macromolecules like polypeptides and polysaccharides enter or exit a cell?
How do large macromolecules like polypeptides and polysaccharides enter or exit a cell?
VESICLE FORMATION! A form of Active Transport that uses Vesicles!
VESICLE FORMATION! A form of Active Transport that uses Vesicles!
A VESICLE is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicle formation occurs because some materials are too big to be transported by carrier proteins and are instead transported into and out of the cell by vesicles.
A VESICLE is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicle formation occurs because some materials are too big to be transported by carrier proteins and are instead transported into and out of the cell by vesicles.
Vesicle formation does require the cell to use energy.
Vesicle formation does require the cell to use energy.
ENDOCYTOSIS
ENDOCYTOSIS
Large molecules, clumps of food, and even whole cells can be taken up and INTO the cell via endocytosis. Two examples of endocytosis are PHAGOCYTOSIS and PINOCYTOSIS.
PHAGOCYTOSIS: "cell eating"; Occurs when the material taken in by endocytosis is large, such as food particles or another cell.
PINOCYTOSIS: Occurs when vesicles form around liquid or other very small particles. In this form of endocytosis tiny pockets form along the cell membrane, fill with liquid or small particles and pinch off from the membrane and enter the cytoplasm.
Many cells also release large amounts of materials from the cell. This process is known as EXOCYTOSIS. During exocytosis the membrane of the vesicle surrounding the material fuses with the cell membrane, forcing the material out of the cell.
Many cells also release large amounts of materials from the cell. This process is known as EXOCYTOSIS. During exocytosis the membrane of the vesicle surrounding the material fuses with the cell membrane, forcing the material out of the cell.
Hormones, neurotransmitters, and digestive enzymes are secreted from cells using this process. The golgi apparatus often produces the vesicles that carry these cell products to the membrane.
The Different Types of Cell Transport:
The Different Types of Cell Transport:
Passive (no energy) vs. Active (needs energy)
Passive (no energy) vs. Active (needs energy)
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