biology 120 - lesson 4

A term that describes a situation in which the concentration of a solute is higher on one side of a membrane than the other is --

transmembrane gradient.

true statements comparing simple and facilitated diffusion

- Both simple and facilitated diffusion require concentration gradients. - Facilitated diffusion, but not simple diffusion, requires a transport protein.

categories of proteins that assist in the movement of molecules and ions across membranes.

- channels - transporters

To move a substance through a membrane against a concentration gradient, which of the following are necessary?

- energy - a transport protein

tonicity (in reference to the solution)

- isotonic: equal water and solute concentrations on either side of the membrane - hypertonic: solute concentration is higher (and water concentration lower) on one side of the membrane - hypotonic: solute concentration is lower (and water concentration higher) on one side of the membrane - In the explanation of osmosis, the area of high water concentration is referred to as hypotonic, while the area of low water concentration is referred to as hypertonic. Once the net movement of water across the membrane stops and all the water concentrations equilibrate, the solution is said to be isotonic. When a cell is placed in a hypotonic solution (the concentration of water is greater in the solution than it is inside the cell), water will flood into the cell causing it to swell, and the cell may burst. When a cell is placed in a hypertonic solution (the concentration of water is greater inside the cell than in the solution), water will leave the cell, causing it to shrink. When a cell is placed in an isotonic solution, however, nothing will happen (see figures 5.14, "Relative solute concentrations outside and inside cells," and 5.15, "The phenomena of osmosis," in the textbook). The concentration of water, therefore, is used to regulate cell size and shape. If a cell actively pumps solute into the cytoplasm (the inside), it lowers the water concentration in the cell. Osmosis will then cause the cell to take in water, making the cell larger. If osmosis is regulated (which it is in healthy cells), the cell will not end up bursting, but will stay at a relatively constant water concentration regardless of the solute molecules coming in and out of the cell.

functions of the selectively permeable plasma membrane:

- maintain a proper balance of ions within the cell - maintain a proper supply of essential molecules such as glucose and amino acids - remove waste products from within the cell

Many Factors Affect Membrane Permeability

- number of double bonds between the carbons in the phospholipid's hydrophobic tail-kinks tail so more permeable - length of the tail-short more permeable - number of cholesterol molecules in the membrane-decreases permeability - temperature-increase permeability - double bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid - shorter acyl tails are less likely to interact, which makes the membrane more fluid - in studies with liposomes they discovered that adding colesterol in the polar head decreases permeability while increasing the temperature will increase permeability. + cholesterol tends to stabilize membranes + effects vary depending on temperature

Selective Permeability of Lipid Bilayers

- permeability: ability of a substance to pass through Phospholipid bilayers have selective permeability. - small and non-polar molecules move across phospholipid bilayers quickly. - charged and large polar substances cross slowly, if at all. - Activity-act out membrane Line up in row: legs/arms are tails When kinked, farther apart, easier to cross Large, "popular" hard to get through Cholesterol, large presence hard to pass through Length makes harder to get through

reasons why it is more appropriate to describe membranes as semifluid:

- phospholipids usually move in two dimensions (the movement in fluids move in three directions) - molecules move laterally within the membrane leaflet

membrane structure

- the framework of the membrane is the phospholipid bilayer - phospholipids are amphipathic molecules - hydrophobic (water-fearing) region faces in - hydrophilic (water-loving) region faces out - membranes also contain proteins and carbohydrates

plasma membrane

- the plasma membrane, or cell membrane, separates life from nonlife - it basically separates interior of the cell to the exterior of the cells. It serves as a selective barrier. - membrane is considered a mosaic of lipid, protein, and carbohydrate molecules - membrane resembles a fluid because lipids and proteins can move relative to each other within the membrane

About how fast does water cross the lipid bilayer?

10^-2 cm/sec

Osmosis describes the movement of water from areas where the solute concentration is -- to areas in which the solute concentration is ---.

1: lower 2: higher

What could provide energy to fuel active transport?

ATP

cell membrane

Cell membranes literally make cells—they're the "skin" that keeps the inside in and the outside out. They are made up of phospholipids, which are made up of hydrocarbons.

exocytosis

Cells also release proteins in a process called exocytosis. When a cell "drinks," or engulfs water, in a phagocytic way, it is referred to as pinocytosis. In some cases, this is facilitated by protein channels called aquaporins in the cell membrane since it is relatively impermeable to water otherwise. One such aquaporin is called CHIP28. The discovery and characterization of CHIP28 as an aquaporin is described in your text. Note how the researchers started with a basic question resulting in a hypothesis (a testable claim). The hypothesis led to the design of an elegant experiment from which data was generated. The thoughtful interpretation of the data resulted in a conclusion that was published as a new discovery.

What property of biological membranes describes the ability of individual molecules to move within the membrane?

Fluidity

What is the significance of being able to gate a channel protein?

Gating a channel protein allows the cell to regulate the movement of solutes.

Which of the following molecules are most likely to pass through the membrane by facilitated diffusion?

H2O and sucrose

hydrocarbons

Hydrocarbons are molecules that are exclusively composed of hydrogen and carbon. The electrons that are in bonds between hydrogen and carbon atoms are shared equally making hydrocarbons nonpolar and resistant to the solvent properties of water. Because of this, we refer to hydrocarbons as hydrophobic (water-fearing). An important example of a hydrocarbon is fatty acids. Fatty acids have a carboxyl group on the end of the molecule. This is what makes fatty acids acidic.

phospholipids

In cells, hydrocarbons are used to make phospholipids, the molecular building blocks of cell membranes. Phospholipids possess unique characteristics that make them particularly useful to cells. Collectively, phospholipids form bilayers that regulate what materials go into and out of cells (permeability). Some of the factors that regulate permeability include temperature, concentration gradients, the size and charge of particles, the concentration of unsaturated fatty acids, and the presence of cholesterol. Phospholipids are composed of a glycerol molecule, two fatty acids, a phosphate group, and the amino acid serine. Although fatty acids are hydrophobic, the glycerol, phosphate group, and serine are hydrophilic (water-loving). The quality of having both hydrophobic and hydrophilic components is referred to as amphipathic (dual-symmetry).

Diffusion

Movement of a substance from a region where its concentration is high to a region where its concentration is low Chemical solutions are made of solvents (like water) and solutes (the substances dissolved in water). When membranes are in the way of solutes dispersing evenly throughout the solution, concentration gradients on either side of the membrane result. Random motion of solute molecules in solution causes a net movement from the area of high solute concentration on one side of the lipid bilayer to the area of low solute concentration on the other side.

Facilitated diffusion

Movement of a substance from a region where its concentration is high to a region where its concentration is low through a passageway provided by a transport protein

Active transport

Movement of a substance from a region where its concentration is low to a region where its concentration is high with the aid of a transport protein and a source of energy

diffusion

Movement of molecules from an area of higher concentration to an area of lower concentration. a spontaneous movement of solutes (molecules and ions) from high to low concentration - a difference in solute concentrations create concentration gradient - diffusion is a form of passive transport. - Diffusion Solutes move from high- concentration regions to low-conc. Regions along their own concentration gradient until they reach equilibrium. Diffusion along a concentration gradient increases entropy and is thus spontaneous.

Facilitated Diffusion

Movement of specific molecules across cell membranes through protein channels - transport proteins - transmembrane proteins that provide a passageway for the movement of ions and hydrophilic molecules across membranes - two classes based on type of movement + channels + Transporters - sugars, aa, nucleotides

Which of the following is true of the fluid mosaic model of membrane structure?

Proteins can easily move laterally through membranes.

passive transport

Requires NO energy, Movement of molecules from high to low concentration, Moves with the concentration gradient class: - requires no input of cellular energy - down or with gradient + passive diffusion - Diffusion without transport protein + facilitated diffusion - Diffusion with the aid of a transport protein

Saturated vs. Unsaturated Hydrocarbons

Saturated hydrocarbons are those where every carbon is covalently bonded to at least two hydrogen atoms. When a hydrogen atom is stripped off, a double bond forms between the carbon and one of its neighboring carbon atoms, resulting in a kink in the hydrocarbon. We would then say that the hydrocarbon is unsaturated. If many of the hydrocarbons are unsaturated, the lipids cannot orient themselves as close to one another. This predictably increases permeability.

Permeability

Some molecules can pass through membranes while others cannot. This characteristic makes lipid bilayers selectively permeable. Generally, lipid bilayers are more permeable to hydrophobic and small molecules and less permeable to ionic and large molecules. Permeability, however, is regulated by a number of different factors.

Isotonic solution

The solute concentration outside a cell is equal to the solute concentration inside the cell.

Hypertonic solution

The solute concentration outside a cell is higher than the solute concentration inside the cell.

Hypotonic solution

The solute concentration outside the cell is lower than the solute concentration inside the cell.

endocytosis or phagocytosis

There are also types of transport for even larger molecules or groups of molecules. For example, some white blood cells actively "eat" viruses by encompassing each particle with a section of membrane, creating a small vesicle with the virion (a single virus particle) inside. This type of transport, when a cell appears to "eat," is referred to as endocytosis or phagocytosis. In some cases, the particle/protein/etc. are phagocytized after binding to specific receptors on the cell surface in a process called receptor-mediated endocytosis

An animal cell is placed in a hypertonic medium. Predict which way water will move.

Water will move from the inside of the cell to the outside.

Electrochemical Gradients

When solutes are ionic (charged), a gradient also has electrochemical properties. When ions move across a membrane through ion-channel proteins, they can create an action potential that is similar to electrons flowing down a wire to turn on a light bulb. Such gradients are responsible for many biological processes, including the making of ATP.

Transporter proteins transport solutes across the membrane via:

a conformational change triggered by solute binding

What is the basic structure of the plasma membrane?

a phospholipid bilayer

The transport of a substance that requires an input of energy is called --- transport.

active

types of transport that require a transport protein.

active and facilitated

Moving a solute from areas of low concentration to high concentration requires:

an input of energy

The difference between passive transport and active transport is that active transport requires:

an input of energy

When the concentration of a solute is higher on one side of membrane than the other, a(n) ______ gradient is established.

concentration

Transporter proteins bind their solutes in a hydrophilic pocket and then undergo a(n) --- change that switches the exposure of the pocket from one side of the membrane to the other.

conformational

osmosis

diffusion of water through a selectively permeable membrane - specialized form of diffusion - water diffuses from an area with more water to less water i.e. assume that the membrane is impermeable to the solutes, but imbalanced ●Water moves from regions of low solute/high solvent (water) concentration to regions of high solute/low solvent (water) concentration. ●This movement dilutes the higher concentration, thus equalizing the concentration on both sides of the bilayer ●Osmosis only occurs across a selectively permeable membrane (Solutes move first but if other things can't move, water will!) - If solutes are prevented from moving across the membrane, water will move across until solvent concentrations are the same on either side of the membrane

Lipid Bilayers

double layer of lipids arranged tail-to-tail; structural foundation of all cell membranes Because of the amphipathic nature of phospholipids, they can form bilayers spontaneously when they interact with water. (They can also form micelles, which are single-layered spheres, but we won't worry about that.) The important part about lipid bilayers is that all of the hydrocarbon chains orient themselves (as a result of their common fear of water) toward one another, creating a hydrophobic zone within which water cannot reside (see figure 5.1, "Fluid-mosaic model of membrane structure," in the text). It also allows water to interact with the hydrophilic heads of the phospholipids on either side of the bilayer. This mutual orientation makes membranes composed of phospholipids stable yet fluid; the hydrophobic interactions keep the phospholipids in the proper orientation, but there are no bonds between building blocks so their necessary movement is possible. In fact, when lipid bilayers are agitated by shaking, bilayers reform spontaneously.

Moving a solute against its chemical or electrochemical gradient requires a source of --

energy

equilibrium

equilibrium is established once the molecules or ions are randomly distributed throughout a solution. Molecules are still moving randomly but there is no more net movement.

Aquaporin allows water molecules to move very rapidly across a plasma membrane. What would be the best definition of this process?

facilitated diffusion using a channel

When channel proteins are ---, it means they can open and close to regulate the movement of ions and molecules across the cell membrane.

gated

Phospholipid bilayers are an effective barrier to many charged or polar solutes because of their --- interior region.

hydrophobic, non-polar

lipid bilayer

in a lipid bilayer, the hydrophilic heads of phospholipids face out, toward water, while the hydrophobic tails face in, away from water. Cell membranes consist in part of phospholipid bilayers.

property of a biological membrane that is fluid

individual molecules can move within the membrane.

Most phospholipids move freely within a semifluid membrane..

laterally along the plane of the membrane in two dimensions

micelles

micelles are tiny, globule-like structures. They form if amphipathic compounds have extremely short hydrophobic tails, because the hydrophilic heads face toward water while the hydrophobic tails face in.

Ways to move across membranes:

passive transport and active transport

The selective --- of the plasma membrane allows the cell to maintain a favorable internal environment.

permeability

phospholipids

phospholipids have a: - phosphate head that is o hydrophilic o "water loving" polar head - two fatty acid tails o that are hydrophobic and nonpolar o "water fearingAmphipathic Lipids Contain Hydrophilic and Hydrophobic Elements.

The three main types of macromolecules that comprise the plasma membrane are:

proteins, phospholipids, and carbohydrates

The plasma membrane of a cell is considered to be:

selectively permeable

All cells are surrounded by a ----- permeable plasma membrane, meaning that the membrane allows the passage of some molecules and ions but not others.

semi

active transport

the movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy. class: - requires cellular energy - up or against gradient + primary uses ATP to transport against the gradient + secondary Uses a pre-existing gradient to drive the active transport of another solute - movement of a solute across a membrane against its gradient from a region of low concentration to higher concentration - energetically unfavorable and requires the input of energy - primary active transport uses a pump + directly uses energy to transport solute - secondary active transport uses a different gradient + uses a pre-existing gradient to drive transport

Osmosis is the flow of --- across a membrane.

water, H2O, water molecules, or H20