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Biology Set 8
Biology Set 8
Terms in this set (51)
Why is the cell membrane important?
It separates the cell from its surroundings
It controls the flow of substances into and out of the cell
Cell Membrane Structure
Composed of lipids, proteins, and carbohydrates
The most abundant lipids are phospholipids
A molecule that is a constituent of the inner bilayer of biological membranes, having a polar, hydrophilic head and a nonpolar, hydrophobic tail.
It is amphipathic.
having both hydrophilic and hydrophobic regions
Fluid Mosaic Model of Membrane Structure
Model postulated by Singer and Nicolson (1972), who proposed that amphipathic proteins lie inside the phospholipid bilayer with their hydrophilic region protruding into the aqueous surrounding. fluid refers to the fluidity of the bilayer. The clustering or diffusing of proteins and lipids give a mosaic pattern to the membrane proteins. The fluidity allows the membrane to adjust to alter the number and pattern of receptors.
Freeze-fractured cell membranes show a cobblestone appearance under electron microscopy. The "cobblestones" are the hydrophobic parts of the transmembrane proteins protruding into the interior of the bilayer.
History of the Fluid Mosaic Model of Membrane Structure
In 1915, chemical analysis of membranes isolated from RBC found them to be composed of lipids and proteins
In 1925, researchers reasoned that a cell membrane must be a phospholipid bilayer (hydrophobic shielded from water, phosphate heads exposed to water)
Real cell membranes are more strongly attracted to water than simple phospholipid bilayers. Why?
Proteins are in cell membranes. Perhaps
proteins cause the greater attraction to water.
How Fast Do Lipids and Proteins Move Around the Membrane
Phospholipids move quickly around the membrane. They can move the length of a bacterial cell in 1 second. Proteins are much larger and move more slowly, but most do drift around the cell membrane.
Why Do Some Membrane Proteins Have
Directed Movement, While Others Do Not Seem to Move at All?
Perhaps they are pushed along cytoskeletal fibers by motor proteins connected to the cytoplasmic regions of the membrane protein.
Perhaps they are held in place by an attachment to the cytoskeleton or extracellular matrix.
What factors influence the temperature at which solidification occurs?
The temperature at which solidification occurs depends on the specific type of phospholipids in the membrane. Phospholipids with unsaturated, and thus kinky, hydrocarbon tails cannot pack together as closely at phospholipids with straight tails. This makes the membrane more fluid.
What role does cholesterol play in determining membrane fluidity?
The steroid cholesterol is wedged between phospholipid molecules in animal cell membanes. At high temperature, such as 37° C, it blocks phospholipid movement, but at colder temperatures it hinders close packing of phospholipids.
Why does membrane fluidity matter?
To work properly, a cell membrane has to be fluid. A solidified membrane is not as permeable and enzymatic proteins in the membrane may stop functioning if they need to move within the membrane.
A solidified membrane blocks movement into and out of the cell.
Too much fluidity is also not a good thing.
A cell membrane that is too fluid cannot support proper protein function.
Whitefish live in deep cold water in the North American Great Lakes. Their cell membranes have a large proportion of lipids with unsaturated hydrocarbon tails. This example indicates that...
..the type of cell membrane lipids varies from species to species and is influenced by natural selection.
Membrane Proteins: What are they made of and how are they arranged?
There are many different proteins embedded in the cell membrane, often clustered together in groups.
Phospholipids form the fabric of the membrane. Proteins determine the membrane's function.
Different types of cells have
different sets of membrane proteins.
What are the two major types of membrane proteins?
Integral membrane proteins and peripheral membrane proteins
What are integral membrane proteins?
Integral proteins penetrate the hydrophobic interior of the membrane. Most integral proteins are transmembrane proteins that span the entire membrane.
What are peripheral membrane proteins?
Peripheral proteins are not embedded in the membrane. Instead, they are loosely bound to the surface of the membrane, often to the exposed part of integral proteins.
Why are some membrane proteins held in place, and how is this done?
Some membrane proteins are held in place - by attachment to the cytoskeleton inside the cell and to fibers of the extracellular matrix outside the cell. These give cells a strong framework.
How many functions do membrane proteins perform?
Membrane Proteins perform six major functions.
A single protein can perform several functions, and a single cell can have membrane proteins carrying out several different functions.
Define the "transport" function of a membrane protein.
Some transport proteins provide a hydrophilic channel across the membrane that is selective for certain solutes. Other transport proteins move substances by changing shape. Some of these use ATP as an energy source to actively pump substances across the membrane.
Define the "enzymatic activity" function of a membrane protein.
Some membrane proteins act as enzymes. Sometimes, several enzymes in the membrane are organized as a team that carries out sequential steps in a metabolic pathway.
Define the "signal transduction" function of a membrane protein.
Membrane protein receptors bind a specific chemical messenger, such as a hormone. Once the signal molecule is bound, the receptor changes shape and typically binds a cytoplasmic protein. This relays the signal to the interior of the cell.
Define the "intercellular joining" function of a membrane protein.
Membrane proteins of adjacent cells may hook together in gap or tight junctions to create long-lasting binding.
Define the "attachment of the cytoskeleton and extracellular matrix" function of a membrane protein.
Membrane proteins can bond to the cytoskeleton or to molecules of the extracellular matrix. This helps maintain the shape of the cell and stabilizes the membrane proteins.
Define the "cell-cell recognition" function of a membrane protein.
A glycoprotein is a membrane protein with an attached short branched carbohydrate chain. Some glycoproteins are identification tags that are specifically recognized and bound temporarily by surface proteins of other cells. The carbohydrate portion of glycoproteins varies from species to species, among individuals of the same species, and even among different cell types within a single individual. In this way, glycoproteins allow cells to recognize each other and foreign cells, important in the immune response.
How do the four human blood types relate to cell-cell recognition?
The four human blood types: A, B, AB, and O arise from variation in the carbohydrate part of glycoproteins on the surface of red blood cells.
The plasma membrane lets some materials flow through freely, but restricts passage to others;
restrictions based on: size, electrical charge, molecular shape, and lipid solubility;
very low permeability for ions/most polar molecules;
nonpolar substances diffuse down their concentration gradient.
Small molecules and ions move continually across a plasma membrane into and out of the cell.
Sugars, amino acids, and other nutrients
enter the cell.
Waste products leave the cell.
What influence does membrane structure have on selective permeability?
The structure of a cell membrane allows it to have selective permeability.
Selective permeability is an emergent property that arises when molecules arrange into a phospholipid bilayer studded with proteins.
What is the relationship between the lipid bilayer and selective permeability?
The lipid bilayer itself is selectively permeable.
Nonpolar molecules, including carbon dioxide and oxygen, dissolve in the lipid bilayer and cross easily into and out of the cell.
Polar molecules, including glucose and other sugars, and charged molecules are not able to penetrate the hydrophobic interior of the membrane.
What is the relationship between transport proteins and selective permeability?
Transport proteins allow selective permeability.
Transport proteins span the cell membrane and help certain ions and polar molecules to enter and exit the cell.
A type of transport protein called channel proteins serve as a hydrophilic channel that polar molecules can use like a tunnel through the membrane.
What are aquaporins?
Channel proteins called aquaporins facilitate the movement of water molecules into and out of cells - up to 3 billion water molecules per second!
What are carrier proteins?
Carrier proteins, another type of transport proteins, grab their special molecule and hold onto it while they change shape in a way that moves the molecule across the membrane. Each carrier protein is specific for a certain substance.
What is the relationship between the plasma membrane and selective permeability?
A type of carrier protein in the plasma membrane of red blood cells transports glucose, and only glucose, across the membrane 50,000 times faster than the molecule can move on its own.
The spontaneous tendency of a substance to move down its concentration gradient from where it is more concentrated to where it is less concentrated.
Many solutes cross the cell membrane by simple diffusion. This is called passive transport because the cell does not have to expend energy to make it happen.
How are the human lungs an example of diffusion?
When cells perform respiration, oxygen is consumed, lowering O2 concentration. Oxygen moves into the cell by diffusion.
Polar molecules and ions cannot pass through the lipid bilayer of the plasma membrane. However, they can diffuse passively into the cell with the help of transport proteins in a process called facilitated diffusion. Transport proteins involved in facilitated diffusion are specific. They carry only certain substances.
Channel proteins provide a hydrophilic corridor through which water molecules and small ions can pass.
A part of facilitated diffusion.
Channel proteins that transport ions are called ion channels. These proteins are often gated channels that open and close in response to a signal, either electrical or a specific substance. A part of facilitated diffusion.
Channel proteins that transport ions are called ion channels. These proteins are often gated channels that open and close in response to a signal, either electrical or a specific substance.
A part of facilitated diffusion.
Cystinuria and Carrier Proteins
Individuals with the inherited disorder cystinuria lack the carrier protein that transports cysteine and some other amino acids across the membranes of kidney cells. The amino acids accumulate and crystalize in the kidneys, forming painful stones.
Active transport moves substances across the plasma membrane from an external solution where they are at low concentration into the cell where they are at high concentration. This requires energy. Active transport allows cells to maintain internal concentrations of substances that are different than external concentrations.
The Sodium-Potassium Pump
The sodium-potassium pump moves Na+ and K+ against their concentration gradients across animal cell membranes. Sodium ions (Na+) are pumped out of the cell and potassium ions (K+) are pumped into the cell. The energy to drive the pump is released by hydrolysis of ATP.
Intracellular fluid: Low concentration Na+, high concentration K+
Extracellular fluid: High concentration Na+
An example of carrier proteins performing active transport.
Large molecules, such as proteins and polysaccharides, and large particles move across the cell membrane in vesicles.
This requires energy.
A vesicle containing molecules to be excreted buds from the Golgi apparatus, moves along microtubules of the cytoskeleton, to the plasma membrane. The lipid bilayer of the vesicle fuses with the lipid bilayer of the plasma membrane, and the contents of the vesicle spill outside the cell.
Name an example of secretory cells using exocytosis to export products.
Example 1: The cells in the pancreas that produce insulin secrete it through exocytosis.
Example 2: Neurons secrete neurotransmitters through exocytosis.
Cells take in large molecules and particles by forming new vesicles from the plasma membrane.
A small area of the plasma membrane sinks, forming a pocket. The pocket pinches away from the plasma membrane, forming a vesicle containing material from outside the cell.
Three Types of Endocytosis
3. receptor-mediated endocytosis
The cell engulfs a particle by wrapping pseudopodia around it and packaging it in a membrane-surrounded sac called a food vacuole. The food vacuole fuses with a lysosome containing enzymes that digest the particles.
The cell "gulps" droplets of extracellular fluid into small vesicles. In this way, the cell takes in molecules dissolved in the droplets. This process is non-specific. The cell takes in all solutes that are dissolved in the extracellular fluid.
Membrane proteins with receptor sites specific to the target substance cluster in pits in the membrane that are coated on the cytoplasmic side with proteins. The pit forms a vesicle containing the bound substance. After the substance is digested, the receptors are recycled.
Receptor-mediated Endocytosis allows cells to accumulate large quantities of substances that are at low concentration in the environment.
Cholesterol and Endocytosis
Human cells use receptor-mediated endocytosis to take in cholesterol from the blood. Special receptors bind with low-density lipoproteins which are taken into the cell by endocytosis. Humans with the inherited disorder familial hypercholesterolemia lack functional LDL receptors. Since their cells cannot take in LDL, cholesterol accumulates in the blood and builds up in deposits along the blood vessels, which impedes blood flow.
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