2.04 Membrane Transport

hypertonic solution

A solution in which the concentration of solutes is greater than that of the cell that resides in the solution aka nonpenetrating solutes

types of transport

Although the two main types of transport are active and passive, there are a few more that fall into these two categories. Throughout the lesson, you will review the types of transport across the cell membrane. You will also see how it impacts daily life.: The two types of transport across a membrane are passive transport and active transport. Passive transport is the movement of a substance across a membrane with the concentration gradient and does not require energy expenditure. Active transport is the movement of a substance across a cell membrane against its concentration gradient. This type of transport requires transport proteins and an expenditure of energy.

Introduction

Cell membranes are made up of carbohydrates, phospholipids, and proteins. The membrane is selectively permeable; that is, it allows some substances to pass through but not others. A cell's ability to allow some substances in and keep others out is essential to life. One of the most important functions of the cell membrane is to regulate the conditions inside the cell by controlling the movement of molecules in and out. There are two ways a substance can move through a cell's membrane: by passive transport or active transport.

Enduring Understanding

Cells have membranes that allow them to establish and maintain internal environments that are different from their external environments.

Learning Objectives

Describe the mechanisms that organisms use to maintain solute and water balance. Describe the mechanisms that organisms use to transport large molecules across the plasma membrane.

diffusion

Diffusion is the movement of particles spreading out to all available space. This is an example of passive transport.

endoycytosis

Endocytosis is the cellular uptake of biological molecules in the form of vesicles from the plasma membrane.

exoycytosis

Exocytosis is the cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane.

facilitated diffusion

Facilitated diffusion is the passage of molecules across a membrane with help of transport proteins. This is a form of passive transport.

osmosis

Osmosis is the diffusion of water across a selectively-permeable membrane. This is a form of passive transport.

Essential Knowledge

Passive transport is the net movement of molecules from high concentration to low concentration without the direct input of metabolic energy. Passive transport plays a primary role in the import of materials and the export of wastes. Active transport requires the direct input of energy to move molecules from regions of low concentration to regions of high concentration. The selective permeability of membranes allows for the formation of concentration gradients of solutes across the membrane. The processes of endocytosis and exocytosis require energy to move large molecules into and out of cells.

the membrane potential acts like

a battery an energy source that affects the raffic of all charged substances across the membrane. cuz the inside of the cell is negative compared w the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell

two forces that drive the diffusion of ions across a membrane

a chemical force (the ions concentration gradient which has been our sole consideration) and an electrical force (the effect of the membrane potential on the ions movement)

carrier proteins like glucose transporter seem to undergo

a subtle change in shape that somehow translocates the solute binding site across the membrane. a change in shape may be triggered by the binding and release of the transported molecule. like ion channels, carrier proteins involved in facilitated diffusion resulted din the net movement of a substance down its concentration gradient. where no energy input is required aka passive transport

cotransport

a transport protein can couple the downhill diffusion of the solute to the uphill transport of a second substance against its own concentration gradien. for instance, a plant cell can use gradient of h+ to the transport of sucrose into the cell. this protein can translocate sucrose into the cell against its concentration gradient but only if the sucrose molecule travels in the company of h+. he H+ uses the transport protein as an avenue to diffuse down its own electrochemical gradient which is maintained by the proton pump. plants use h+ to load sucrose produced by photosynthesis into cells in the veins of leaves. the vascular tissue of the plant can then distribute the sugar to roots and other non-photosynthetic organs that do not make their own food

electrogenic pump

a transport protein that generates voltage across a membrane. the sodium potassium pump appears to be the major electrogenic pump of animal cells

in the case of ions, we must refine our concept of passive transport

an ion diffuses not simply down its concentration gradient but more exactly down its electrochemical ingredient

a similar contransporter in aniimals transport glucose into intestinal cells together w sodium

an ion that is moving down its concentration gradient into the cell. the glucose and sodium then exit the cell into the blood on the opp side, the glucose facilitated diffusion and the na by active transport. water follows these solutes into the cell and then into the blood by osmosis. our understanding of glucose cotransporters hels us find effective reatements for serious life threatening problems that can cause dehydration if patients drink lots of water

an important use of proton gradients in the cell for ATP synthesis during cellular respiration

another type of membrane traffic is called cotransport

a solute that exists in different concentrations across a membrane can do work

as it moves across the membrane by diffusion down its concentration gradient

cholesterol accumulates in the blood contributing to early

atherosclerosis which is the build up of lipids within blood vessel walls which narrows the space in the vessels and impedes blood flow resulting in heart damage or stroke

a cell without rigid cell walls

can tolerate neither excessive uptake nor excessive loss of water

plasmolysis

causes the plant to wilt and can lead to plant death. the walled cells of bacteria and fungi also plasmolyse in hypertonic environments

the cells of plants prokaryotes fungi and some protists surrounded by

cell walls

two types of transport proteins

channel proteins and carrier proteins

ion channels

channel proteins that transport ions

human cells use receptor mediated endocytosis to take in cholesterol for membrane synthesis and the synthesis of oher steroids

cholesterol travels in the blood in particles called low density lipoproteins each a complex of lipids and a protein. LDL bind to the LDL receptors on plasma membranes and then enter the cells by endocytosis. in the inherited disease familial hypercholesteromia characterised a high level of cholesterol in the blood, LDL cannot enter cells cuz the LDL receptor proteins r defective or missing

explaining the behavior of a cell in a solution

consider solute concentration and membrane permeability. both factors r taken into account in the concept of tonicity. if there is a higher concentration of nonpenetrating solutes in the surrounding solution, water will tend to leave the cell and vice versa

voltage

electrical potential energy - a separation of opposite charges. the cytoplasmic side of the membrane is negative in charge relative to the extracellular side cuz of an unequal distribution of anions and cations on the two sides

many secretory cells use exocytosis to export products

ex. the cells in the pancreas that make insulin secrete it into the extracellular fluid by exocytosis ex. nerve cells use exocytosis to release neurotransmitters that signal other neurons or or muscle cells, when plant cells r making cell walls, exocytosis delivers some of the necessary proteins and carbs from golgi vesicles to the outside of the cell

sodium potassium pump

exchanges sodium and potassium across the plasma membrane of animal cells.

despite the help of transport proteins

facilitated diffusion is considered passive transport cuz the solute is moving down its concentration gradient, a process that requires no energy

in hypertonic or hypotonic environments, organisms that lack rigid cell walls must

have other adaptions like osmoregulation. for example, unicellular protist lives in pond water which is hypotonic to the cell. water continually enters the cell. the protist cell doesnt burst cuz it is equipped w a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it enters by osmosis

when such a cell is immersed in a hypotonic solution like bathed in rain water the cell wall

helps maintain the cells water balance - a plant cell like an animal cell swells as water enters by osmosis, however the relatively inelastic cell wall will expand only so much before it exerts a back pressure on the cell called turgor pressure that opposes further water uptake. at this point the cell is turgid aka very firm which is the healthy state for most plant cells. plants that r not woody depend for mechanical support on cells kept turgid by a surrounding hypotonic solution. if a plants cells and their surroundings r isotonic, there is no net tendency for water to enter and the cells become flaccid ; the plant wilts

example

imagine a synthetic membrane separating pure water from a solution of a dye in water. to see how diffusion would result in equal concentrations of dye molecules in both solutions. at that point a dynamic equilibrium will exist with as many dye molecules crossing per second in one direction as in the other

rule of diffusion

in the absence of other forces a substance will diffuse from where it is more concentrated to where it is less concentrated. a substance diffuses down its concentration gradient it is a spontaneous process needing no input of energy unaffected by concentration gradients of other substances

a cell wall is no advantage if the cell is immersed in a hypertonic environment

in this case a plant cell like an animal cell will lose water to its surroundings and shrink. as the plant cell shrivels, its plasma membrane pulls away from the cell wall at multiple places called plasmolysis

water and small solutes enter and leave the cell by diffusing through the plasma membrane or by being moved across iy by transport proteins

large molecules like proteins and polysaccharides as well as larger particles, cross the membrane in bulk packaged in vesicles like active transport these processes need energy

active transport enables a cell to

maintain internal concentrations of small solutes that differ from concentrations in its environment - an animal cell has a much higher concentration of potassium ions and lower sodium ions than its surroundings. the plasma.membrane helps maintain these steep gradients by pumping sodium out of the cell and potassium into the cell

gated channels

many ion channels function as gated channels which open or close in response to a stimulus. for some gated channels the stimulus is electrical. - in a nerve cell an ion channel opens in response to an electrical stimulus allowing a stream of potassium ions to leave the cell restoring the cells ability to fire again other gated channels have a chemical stimulus which open or close when a specific substance not the one transported binding to the channel. gated channels r also important to functioning of the nervous system

facilitated diffusion; passive transport aided by proteins

many polar molecules and ions impeded by lipid bilayer of the membrane diffuse passively w the help of transport proteins that span the membrane by facilitated diffusion

endocytosis and exocystosis also provide

mechanisms for rejuvenating or remodeling the plasma membrane which processes occur continually in most eukaryotic cells yet the amount of plasma membrane in a nongrowing cell remains fairly constant. the addition of membrane by one process appears to the offset the loss of membrane by the other

some membrane proteins that actively transport ions contribute to the

membrane potential = an example is the sodium potassium pump, the pump does not translocate sodium and potassium but pumps three sodium ions out of the cell for every two potassium ions i pumps into the cell. w each crank of the pump there is a net transfer of one positive charge from the cytoplasm to the extracellular fluid a process that stores energy as voltage

molecules have thermal energy to their constant motion

one result of this motion is diffusion. each molecule moves randomly yet diffusion of a population of molecules may be directional

the ATP hydrolysis supplies the energy for most active transport

one way ATP can power active transport is when its terminal phosphate group is transferred directly to the transport protein. this can induce the protein to change its shape in a manner that translocates a solute bound to the protein across the membrane. one transport system that works this way is the sodium potassium pump

movement of water across cell membranes and balance of water between the cell and its environment r crucial to

organisms

to treat diarrhea

patients r given a solution to drink containing therapeuutic conecntrations of salt and clucose

all cells have voltages across their

plasma membranes

to see how two solutions w different solute concentrations interact picture u shaped glass tube w a selectively permeable artifical membrane seperating two sugar solutions

pores in the synthetic membrane r too small for sugar molecules to pass through but large enough for water molecules. tight clustering of water molecules around the hydrophilic solute molecules make some of the water unavailable to cross the membrane. thus the solution w a higher solute concentration has a lower free water concentration. water diffuses across the membrane from the region of higher free water concentration aka lower solute concentration to that of lower free water concentration aka higher solute concentration till the solute concentrations on both sides of the membrane r more nearly equal.

channel proteins

provide corridors that allow specific molecules or ions to cross the membrane. the hydrophilic passageways provided by these proteins can allow water molecules or small ions to diffuse very quickly from one side of the membrane to the other.

the problem of water balance is solved if such a cell lives in isotonic surroundings

sea water is isotonic to many marine invertebrates. the cells of most terrestrial animals r bathed in an extracellular fluid that is isotonic to cells.

facilitated diffusion speeds transport of a solute by provdiding efficient passage through the membrane, but it doesnt alter the direction of transport

some other transport proteins can move solutes against their concentration gradients across the plasma membrane from the side where they r less concentrated to the side where they r more concentrated whether inside or out

if a cell without a cell wall like an animal cell is immersed in an environment

that is an isotonic environment

tonacity

the ability of a surrounding solution to cause a cell to gain or lose water which depends in part on its concentration of solutes that cannot cross the membrane aka nonpenetrating solutes relative to that inside the cell

exocytosis

the cell secretes certain biological molecules by the fusion of vesicles w the plasma membrane through this process

endocytosis

the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane. although the proteins involved in the two processes r different, the events of endocytosis look like the reverse of exocytosis. a small ares of the plasma membrane sinks inwards to form a pocket, the pocket deepens pinching in forming a vesicle contaning material that had been outside the cell

putting a cel in a solution that is hypertonic to it means

the cell will lose water, shrivel and probs die which is why an increase in saltiness of a lake can kill animals there; if lake water becomes hypertonic to animals cells they could shrivel and die. however taking up too much water can be just as hazordous

electrochemical gradient

the combination of forces acting on an ion

osmoregulation

the control of solute concentrations and water balance

passive transport

the diffusion of a substance across a biological membrane cuz it requires no energy. the concentration gradient itself reps potential energy

osmosis

the diffusion of free water across a selectively permeable membrane, whether artificial or cellular

proton pump

the main electrogenic pump of plants, fungi, and bacteria which actively transports protons out of the cell. the pumping of h+ transfers positive charge from the cytoplasm to the extracellular solution. by generating voltage across membranes, electrogenic pumps help store energy that can be tapped for cellular work

diffusion

the movement of particles of any substance so that they spread out into the available space

concentration gradient

the region along which the density of a substance increases or decreases

membrane potential

the voltage across a membrane which ranges from abt -50 to -200. millivolts

aquaporins

the water channel proteins facilitate the massive levels of diffusion of water osmosis that occur in plant cells and in animal cells like red blood cells. certain kidney cells also have many aquaporin molecules allowing them to reclaim water from urine before it is excreted. if the kidenys didnt perform this function u would secret too much pee

if we place the cell in a hypotonic solution then

the water will enter the cell faster than it leaves and the cell will swell and burst

isotonic

there will be no net movement of water across the plasma membrane and in an isotonic environment, the volume of an animal cell is stable

most transport proteins r very specific

they transport some substances but not others

active transport

this type of membrane traffic where transport proteins move solutes against their concentration gradients r all carrier proteins rather than channel proteins. this makes sense cuz when channel proteins r open they merely allow solutes to diffuse down their concentration gradients rather than picking them up and transporting them against their gradients

the need for energy in active transport

to pump a solute across a membrane against its gradient requires work; the cell must expand energy

a transport vesicle that budded from the golgi apparatus moves along microtubules of the cytoskeleton

to the plasma membrane; when the vesicle membrane and the plasma membrane come into contact specific proteins rearrange the lipid molecules of the two bilayers so that the two membranes fuse. the contents of the vesicle spill to the outside of the cell, and the vesicle membrane becomes part of the plasma membrane

membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules

water can diffuse very rapidly across the membranes of some cells w aquaporins compared w diffusion in the absence of aquaporins. the movement of water across the plasma membrane has important consequences

much of the traffic across cell membranes occurs by diffusion

when a substance is more concentrated on one side of a membrane there is a tendency for it to diffuse across, down its concentration gradient assuming the membrane is permeable to that substance. - one important example is the uptake of oxygen by a cell performing cellular respiration. dissolved oxygen diffuses into the cell accross the plasma membrane. as long as cellular respiration consumes the o2 as it enters, diffusion into the cell will continue because the concentration gradient favors movement in that direction

for example, the concentration of na inside a resting nerve cell is much lower than outside it

when the cell is stimulated, gated channels open hat faciliated sodium diffusion. sodium ions then fall down their elecrochemical gradient, driven by the concentration gradient of na and attraction of these cations to the negative side of the membrane. in both electrical and chemical contributions to the electrochemical gradient act in the same direction across the membrane but that is not always so,. in cases where electrical forces due to the membrane potential oppose the simple diffusion of an ion down its concentration gradient, active transport by be necessary