Isabel Hoyt
Period 4
Membranes
1. What does selective permeability mean and why is that important to cells?
The plasma membrane exhibits selective permeability, it allows some substances to cross it more easily than others. This ability of the cell to discriminate in its chemical exchanges with its environment is fundamental to life, and it is the plasma membrane and its component molecules that make this selectivity possible.
2. What is an amphipathic molecule?
The most abundant lipids in most membranes are phospholipids. A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic region. Other types of membrane lipids are also amphipathic, Most of the proteins of membranes have both hydrophobic and hydrophilic regions.
3. How is the fluidity of cell’s membrane maintained?
The membrane is a fluid mosaic model, the membrane is a fluid structures with a “mosaic” of various proteins embedded in or attached to a double layer of phospholipids. A membrane is held together by hydrophobic interactions, which are much weaker than covalent bonds. Most of the lipids and proteins can drift laterally. The lateral movement of phospholipids within the membrane is rapid. Proteins move slower than lipids, but some do drift. A membrane remains fluid as temperature decreases, until the phospholipids settle into a closely packed arrangement and the membrane solidifies. Because of kinks in the tails where double bonds are located, unsaturated hydrocarbons cannot pack together as closely as saturated hydrocarbons, this makes the membrane more fluid. At warm temperatures cholesterol makes the membrane less fluid by restraining the movement of phospholipids. Cholesterol is a “temperature buffer” for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature.
4. Label the diagram below – for each structure – briefly list it’s function:
Integral Protein- penetrates the hydrophobic core of the lipid bilayer, completely span the membrane. Channels for transport of molecules.
Peripheral Proteins- are not embedded in the lipid bilayer; they are appendages loosely bound to the surface of the membrane. Cell recognition, enzymatic activity.
Extracellular matrix – Connects the cell, surrounds it.
Carbohydrate – Short, branched chains that are covalently bonded to lipids, forming molecules called glycolipids.
Glycoplipids- Cell-cell recognition
Glycoprotein – Glycolipids are covalently bonded to proteins, which are thereby glycoprotein. Cell-cell recognition
Cytoskeleton – intercellular structure for shape and support
Cholesterol – fluidity of the cell
5. List the six broad functions of membrane proteins.
Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to the cytoskeleton and extracellular matrix.
6. How do glycolipids and glycoproteins help in cell to cell recognition?
Cell-cell recognition, a cell’s ability to distinguish one type of neighboring cell from another, it’s crucial to the functioning of an organism. The way cells recognize other cells is by binding to surface molecules, often carbohydrates, on the plasma membrane. Carbohydrates are covalently bonded to lipids forming molecules called glycolipids. Most are covalently bonded to proteins which are glycoproteins. The diversity of the molecules and their location on the cell’s surface enable membrane carbohydrates to function as markers that distinguish one cell from another.
7. Why is membrane sidedness an important concept in cell biology?
When vesicle fuses with the plasma membrane, the outside layer of the vesicle becomes continuous with the cytoplasmic layer of the plasma membrane. Therefore, molecules that start out on the inside face of the ER end up on the outside face of the plasma membrane.
8. What is diffusion and how does a concentration gradient relate to passive transport?
Passive transport is diffusion of a substance across a membrane with no energy investment. Diffusion is the tendency for molecules of any substance to spread out through a concentration gradient into available space. The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy and drive diffusion.
9. Why is free water concentration the “driving” force in osmosis?
Free water concentration causes the water to move. Water diffuses across the membrane from the region of lower solute concentration to that of higher solute concentration.
10. Why is water balance different for cells that have walls as compared to cells without walls?
If a cell without a wall, such as an animal cell, is immersed in an environment that is isotonic to the cell, there will be no net movement across the membrane, but at the same rate. In hypertonic cell, the cell will lose water to its environment and probably die. If it is hypotonic to the cell, water will enter the cell faster than it leaves, and the cell will swell and burst. A cell without rigid walls can tolerate neither excessive uptake nor excessive loss of water. In an isotonic state, a cell wall helps maintain water balance. Plant cell swells as water enters by osmosis, but the elastic wall will expand only so much before it exerts a back pressure on the cell that opposes further water uptake- turgid. Will a wall (plant cell) it is healthies in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.
11. Label the diagram below:
12. What is the relationship between ion channels, gated channels and facilitated diffusion?
Facilitate diffusion is when the membrane diffuses passively with the help of transport proteins that span the membrane. Ion channels, many which function as gate channels stimulus causes them to open or close.
13. How is ATP specifically used in active transport?
Active transport uses energy to move solutes against their gradients. Some proteins can move solutes against their concentration gradients, across the plasma membrane from the side where they are less concentrated to the side where they are more concentrated. – Active transport, expend energy to pump up a concentration gradient.
14. Define and contrast the following terms: membrane potential, electrochemical gradient, electrogenic pump and proton pump
The voltage across a membrane, called a membrane potential, acts like a battery, any energy source the affects the traffic of all charged substances across the membrane. The membrane potential favors the passive transport of cations into the cell and anions out the cell- 2 forces drive the diffusion, chemical force and electrical force. The combination of forces acting on an ion is called the electrochemical gradient. An ion diffuses down its electrochemical gradient. Some membrane proteins that actively transport ions contribute to the membranes potential. A transport protein that generates voltage across a membrane is called an electrogenic pump. The main electrogenic pump of plants, fungi and bacteria is a proton pump, which actively transports hydrogen ions out of the cell.
15. What is cotransport and why is an advantage in living systems?
A single ATP- powered pump that transports a specific solute can indirectly drive the active transport of several other solutes in a mechanism called cotransport.
16. What is a ligand?
Any molecule that binds specifically to a receptor site of another molecule. Cholesterol travels in the blood in particles called low-density lipoproteins, complexes of lipids and proteins- act as ligands.
17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated endocytosis.
Phagocytosis is a type of endocytosis involving large, particulate substances, accomplished mainly by macrophages, neutrophils, and dendritic cells. Pinocytosis is also a part of endocytosis, in which the cell ingests extracellular fluid and its dissolved solutes. Receptor-mediated endocytosis is the movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taken in, enable a cell to acquire bulk quantities of specific substances.
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