Unit 2: Plasma Membranes Wkst - AP Bio

Signal transduction

A membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein.

How fluid is the membrane? How do saturated and unsaturated phospholipids, along with cholesterol, maintain this fluidity?

A membrane remains fluid as temperature decreases until the phospholipids settle into a closely packed arrangement and the membrane solidifies. Factors that affect membrane fluidity include unsaturated versus saturated hydrocarbon tails and cholesterol within the animal cell membrane. Unsaturated hydrocarbon tails (kinked) prevent packing, enhancing the membrane fluidity. Saturated hydrocarbon tails pack together, increasing membrane viscosity. Because of kinks in the tails where double hydrocarbons are located, unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, making the membrane more fluid. Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, but at low temperatures it hinders solidification by disrupting the regular packing of phospholipids.

Define "selective permeability."

A property of biological membranes that allows them to regulate the passage of substances across them.

Enzymatic activity

A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. Sometimes, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.

Transport of a protein

A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Other transport proteins shuttle a substance from one side to another by changing shape.

Define active transport. How is it similar to and different from facilitated diffusion?

Active transport is the movement of a substance across a cell membrane against its concentration or electrochemical gradient, mediated by specific transport proteins and requiring an expenditure of energy. Active transport differs from facilitated diffusion in the way that active transport requires an expenditure of energy and facilitated diffusion does not. However, they both are the transport of a substance across a biological membrane.

What is "amphipathic?"

Amphipathic is defined as having both a hydrophilic region and a hydrophobic region.This term is a good description for a phospholipid molecule because the structure of a phospholipid consists of two hydrophobic tails and a hydrophilic head.

How do cells use electrochemical gradients?

An ion diffuses down its electrochemical gradient.

How does co-transport work?

Cotransport is when a transport protein can couple the "downhill" diffusion of the solute to the "uphill" transport for a second substance against its own concentration gradient. There is a resulting H+ gradient representing potential energy that can be used for active transport- in this case, of sucrose. Thus, ATP hydrolysis indirectly provides the energy necessary for cotransport.

Define diffusion, concentration gradient, and passive transport.

Diffusion is the movement of particles of any substance so that they spread out into the available space. Concentration gradient is the region along which the density of a chemical substance increases or decreases. Passive transport is the diffusion of a substance across a biological membrane.

Describe endocytosis, exocytosis, phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Endocytosis- the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane. Exocytosis- the cell secretes certain molecules by the fusion of vesicles with the plasma membrane Phagocytosis - A cell engulfs a particle by extending pseudopodia around it and packaging it within a membranous sac called a food vacuole. The particle will be digested after the food vacuole fuses with a lysosome containing hydrolytic enzymes . Pinocytosis - A cell continually "gulps" droplets of extracellular fluid into tiny vesicles, formed by infoldings of the plasma membrane. Receptor-mediated endocytosis- This is a specialized type of pinocytosis that enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid.

What is endosymbiosis? Describe how endosymbiosis would have given rise to mitochondria and plastids.

Endosymbiosis is when one organism actually lives inside the other. Endosymbiosis explains how mitochondria and plastids developed from bacterial cells.

On a cellular level, how are eukaryotes distinct from prokaryotes.

Eukaryotes contain membrane-bound organelles. Eukaryotes do not have nucleoids. Eukaryotic cells also have a cytoskeleton and an ER.

Describe facilitated diffusion.

Facilitated diffusion is defined as the passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins, requiring no energy expenditure.

Try to visualize how a glycoprotein or glycolipid would be positioned on the inside or outside of the membrane.

Glycolipids are more towards the border of the membrane.

What's the fluid mosaic model of membrane structure? How do proteins fit into the phospholipid portion of the membrane?

In a fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.The fluid mosaic model is constantly being refined as new research reveals more about membrane structure. The proteins reside in the phospholipid bilayer with their hydrophilic regions protruding. Groups of proteins are associated with long-lasting, specialized patches, where they carry out common functions.

Distinguish between integral and peripheral proteins. What are transmembrane proteins?

Integral proteins penetrate the hydrophobic interior of the lipid bilayer. Other integral proteins extend only part way into the hydrophobic interior. The hydrophobic regions of an integral protein consist of one or more stretches of non-polar amino acid usually coiled into alpha helices. Peripheral proteins are not embedded in the lipid bilayer at all. They are loosely bound to the surface of the membrane, often to exposed parts of integral proteins. Transmembrane proteins span the membrane

What are ion channels? Gated channels?

Ion channels are channel proteins that transport ions. Gated channels open or close in response to a stimulus and many ion channels function as this.

Describe the functions of membrane carbohydrates.

Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units. Some are covalently bonded to lipids, forming molecules called glycolipids. Most are covalently bonded to proteins, which are glycoproteins. The carbohydrates on the extracellular side of the membrane vary from species to species. The diversity of the molecules and their locations on the cell's surface allows membrane carbohydrates to function as markers that distinguish one cell from another.

Intercellular joining

Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions. This type of binding is more long-lasting.

Attachment to the cytoskeleton and extracellular matrix (ECM)

Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes.

Describe the types of molecular movements that happen within membranes.

Most of the lipids and some proteins can shift about sideways, like partygoers elbowing their way through a crowded room. Rarely, a lipid may flip-flop across the membrane, switching from one phospholipid layer to another. Phospholipids can move sideways at a rapid pace. Adjacent phospholipids switch positions about 107 times per second, which indicates that a phospholipid can travel about 2 µm in one second. Proteins are larger than lipids and move more slowly, but some membrane proteins do drift. Some proteins move in a highly directed matter, possibly driven along cytoskeletal fibers in the cell by motor proteins. Although, many proteins in the membrane seem to be held immobile by their attachment to the cytoskeleton or to the extracellular matrix.

List the substances that can and can't pass through the lipid bilayer.

Nonpolar molecules, like hydrocarbons, CO2, and O2, are hydrophobic, so they can all dissolve in the lipid bilayer of the membrane and cross it easily. However, the hydrophobic interior of the membrane impedes direct passage through the membrane of ions and polar molecules, which are hydrophilic. Polar molecules, like glucose and other sugars pass slowly through a lipid bilayer, and even water does not cross rapidly. A charged atom and its surroundings shell of water are even less likely to penetrate the hydrophobic interior of the membrane.

Define/describe osmoregulation: why is it so important, and how is it accomplished in cells with and without cell walls?

Osmoregulation is the control of solute concentrations and water balance. This maintains the homeostasis of an organism's water content. Cells in organisms like bacteria have cellular mechanisms that balance the internal and external solute concentrations to ensure that water does not move out of the cell. The cell wall in cells expands only so much before it exerts a back pressure on the cell that opposes further water uptake.

Define osmosis, hypertonic, hypotonic, isotonic.

Osmosis is the diffusion of free water across a selectively permeable membrane, whether it is artificial or cellular. Hypertonic is when the cell is placed in a solution when it will lose water, shrivel, and probably die. Hypotonic is when the cell is placed in a solution where water will enter the cell faster than it leaves, and the cell will swell and burst like an overfilled water balloon. An isotonic solution refers to two solutions having the same osmotic pressure across a semipermeable membrane.

How thick is the plasma membrane? How does that compare to the size of a typical prokaryotic cell? Eukaryotic cell?

Plasma membranes are usually 7.5-10 nm thick with two layers of lipid molecules containing different types of protein molecules. In diameter, prokaryotic cells are 0.1-5.0 µm and eukaryotic cells are 10-100 µm.

Cell-cell recognition

Some glyco-proteins serve as identification tags that are specifically recognized by membrane proteins of other cells. This type of cell-cell binding is usually short-lived.

Explain how the sodium-potassium pump works.

Step 1) Cytoplasmic Na+ binds to the sodium-potassium pump. The affinity of Na+ is high when the protein has this shape. Step 2) Na+ binding stimulate phosphorylation by ATP. Step 3) Phosphorylation leads to a change in protein shape, reducing its affinity for Na+, which is released outside. Step 4) The new shape has a high affinity for K+, which binds on the extracellular side and triggers the release of the phosphate group. Step 5) Loss of the phosphate group restores the protein's original shape, which has a lower affinity for K+. Step 6) K+ is released; affinity for Na+ is high again, and the cycle repeats.

Electrochemical gradients

The diffusion gradient of an ion, which is affected by both the concentration difference of an ion across a membrane and the ion's tendency to move relative to the membrane potential.

Describe the steps involved in the origin of multicellular animals.

The first multicellular organisms were small red algae and then fossils of larger and more diverse multicellular animals appeared in the fossil record about 600 million years ago. The rise of large eukaryotes in the Ediacaran period represents an enormous change in the history of life.

Explain the formation of lipid bilayers from phospholipid molecules.

The lipid bilayer can be a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water.Amphipathic proteins can reside in the phospholipid bilayer with their hydrophilic regions protruding. In a fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

What holds the membrane together?

The membrane is held together mainly by hydrophobic interactions, which are weaker than covalent bonds.

Describe two characteristics shared by chloroplasts and mitochondria. Consider both function and membrane structure.

They both have their own DNA and both chloroplasts and mitochondria use their DNA to produce proteins and enzymes that each organelle requires.

Summarize the passage explaining familial hypercholesterolemia. If anyone in your family takes a statin like lipitor or simvastatin they may have this condition.

This is a genetic disorder caused by a defect on chromosome 19. This disorder makes the body unable to remove low density lipoprotein cholesterol from the blood.

Membrane potential

This is the voltage across a membrane and ranges from about -5o to -200 millivolts.

Describe the functions of various transport proteins, including aquaporins and carrier proteins.

Transport proteins function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane. The passage of water molecules through the membrane in certain cells is facilitated by channel proteins known as aquaporins. Without aquaporins, only a tiny fraction of water molecules would pass through the same area of the cell membrane in a second, so the channel protein brings about a tremendous increase in rate. Carrier proteins hold onto their passengers and change shape in a way that shuttles them across the membrane.

Describe conditions under which plant cells will become turgid or flaccid. What's plasmolysis?

Turgid is when the cell wall expands only so much before it exerts a back pressure on the cell and opposes further water uptake. This is the healthy state for most plant cells. If a plant cell's and surroundings are isotonic, there is no net tendency for water to enter and the cells become flaccid (limp) and the plant wilts. Plasmolysis causes the plant to wilt and can lead to plant death.

Do plant cells have mitochondria? Explain.

Yes, they do have mitochondria. Mitochondria break down the sugar that is made to make energy for the cell.