Lecture 4- Movement Across Membranes Flashcards Preview

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Flashcards in Lecture 4- Movement Across Membranes Deck (24):

What are the different ways substances are transported across a membrane?

1. Simple diffusion
2. Mediated transport
-facilitated diffusion (no energy required) through ion channels, porins, permeases
-active transport (energy required)- against the concentration gradient
3.water transport- osmosis
-transporting epithelia- glucose transport


List and explain the functions of membrane proteins

1. Structure- link membrane to cytoskeleton
2. Enzymes- membrane associated enzymes catalyst reactions on cells external surface or inside cytoplasm
3. Receptors- specific for certain molecule or family of molecules (eg hormones) called ligands
4. Transporters- molecules must enter/ leave cells via


Give definition of diffusion

Movement of molecules from an area of higher concentration to one of lower concentration of molecules.
-the molecules are in a constant state of motion
-the driving force for diffusion is the molecules kinetic energy- the force is an electrochemical gradient
-it's a passive process


What are 4 properties of diffusion? Ie concentration gradient, rate of diffusion, passive process and speed of diffusion

1. Molecules move from a higher concentration to a lower concentration; the difference in concentration = concentration gradient. The rate of diffusion depends on magnitude of concentration difference
2. Diffusion is a passive process ie doesn't require energy. It happens using only the kinetic energy that the molecules possess. Ie bouncing of each other
3.there will be a net movement of molecules untill the concentration is equal everywhere (ie system reaches equilibrium)
Molecules still move at this state, however for each molecule that enters, another leaves
4. Diffusion is rapid over short distances but much slower over long distances


What are the 5 major functions of epithelia

1. Protective- outer surface of body and opening cavities eg mouth
2. Exchange- rapid exchange of gases eg lungs, blood vessels
3. Transporting- exchange of no gaseous materials; selective e.g gut and kidney
4. Ciliated- move fluid across the surface
5. Secretory- exocrine= releases secretory products to external environment: serous or mucous
Endocrine= internal environment (extra cellular space > blood)


Properties of diffusion. How temp, size of molecules, and kind of system affects rate

1. Temperature: higher temp means molecules move faster, increasing rate of diffusion
2. Diffusion is inversely related to molecules size: large molecules move slower therefore slower rate of diffusion
3. Diffusion can occur in open system. Ie movement of substance throughout a room, or across a partition which separates two systems


Simple diffusion and its properties. Ie . What affects rate of diffusion
1. Ability of a molecule to dissolve in membrane lipid layer
2. Surface area of membrane
3. Thickness of membrane

1. Rate depends on ability of molecule to dissolve in membrane lipid layer. Ie substances that are hydrophilic (dissolve in h20) tend to be lipophobic ie don't dissolve in lipids, meaning that hydrophilic substances can't enter the cytoplasm.
Two properties of molecule influence it's movement across a membrane
A) Size: v. Small lipid soluble molecules cross directly through belayer of membrane
Larger less lipid soluble molecules excluded from transfer, unless cell has a specific process e.g protein transport, for moving them across
B) lipid solubility or polarity: only non polar lipid soluble molecules can cross membrane
Membranes with high cholesterol content though impermeable to water eg kidney
2. Rate of diffusion is proportional to surface area of membrane. Ie larger sa = greater diffusion
3. Rate of D is inversely proportional to thickness of membrane. Ie thicker = slower diffusion


Mediated transport (facilitated diffusion). List the three main proteins that assist these molecules and how each functions
PIP pip ions her permed pores?

1. Ion channels: amino acids make up protein 'cylinders' that surround a water filled channel
-allows movement of water and some specific ions -via selective electrical charge. Small molecules generally
-membrane channels have regions on cytoplasmic side that act as gates.
-these channels can be opened or closed. 3 kinds of channels
A) voltage- gated channels: open/close in response to membrane potential
B) ligand-gated channels: open/close in response to specific molecules
C) mechano-gated channels- open/close via interactions with subcellular proteins in cytoskeleton eg change in shape due to muscular activity e.g hearing n touch receptors
2. Porins: larger channels that move larger molecules. Eg aqua porins move water
3. Permeases (carrier proteins) bind a substrate, causes a quick change, and this then releases the substrate into the other side. Eg glucose, amino acids


Mediated transport: facilitated diffusion
List and Explain the 3 factors which determine the rate of facilitated diffusion

1. The rate at which individual carriers can transport the molecule. Eg glucose transporters 10 000 a second
2. The number of carriers or channels in the membrane
3. Magnitude of the concentration gradient of the transported substance
Rate, number, and concentration gradient


Mediated transport (facilitated diffusion) Properties.
These 3 properties are common to both facilitated diffusion and active transport.
Specificity, competition and saturation
Also what is competitive inhibition

1. Specificity: ie one carrier can only move one kind of molecule or family of molecules
2. Competition: eg glucose and galactose are transported on the same membrane protein, but the carrier rathers (has a high affinity) for glucose. So when glucose is added, the rate of galactose transport decreases.
Competitive inhibition: the competing molecule is not transported but BLOCKS the molecule being transported
3. Saturation: were concentration of substrate exceeds transporters capacity. Ie when all carriers are full of substrate


Mediated transport: active transport
How is active transport different to passive

-carrier proteins move molecules against concentration gradient
-requires energy input in form of adenosine triphosphate (ATP)


What are the two factors that affect rate of transport?

1. The rate at which the individual pump proteins transport the substance
2. The number of pumps in the membrane


Mediated transport PRIMARY active transport
What is primary active transport?

In primary active transport, hydrolysis of ATP results in the changing of shape of the transport protein, so that it pumps the bound solute across the membrane. Eg the Na+ and K+ pump is most important transport protein in the body. Nerve cells use Na+ gradient for generation of action potentials.
-uses ATP to run this pump
-causes not only a chemical gradient, but an electrical gradient also.


Mediated transport SECONDARY active transport

-they couple the kinetic energy of one molecule moving down its concentration gradient, to the movement of another molecule moving against the concentration gradient.
-most common are driven by Na+
-as Na eters a cell it brings one or molecules with it or exchanges with molecules exiting the cell
-ie when sodium bonds to the carrier protein it makes a space for the glucose to bond also. Refer to pictures for diagram


What are the 5 major functional types of epithelia?
Pip exchanges T coz she
What is transepithelial transport?

1. Protective: outer surface of body and opening of cavities
2. Exchange: rapid exchanges of gases
3. Transporting: exchange of non gaseous material; selective eg gut and kidney.
4. Ciliated- moves fld across a surface eg respiratory tract, reproductive tract
5. Secretory- exocrine: releases products to externals environment And endocrine: internal environment ie extra cellular space to blood
Transepithelial transport is when a molecule has to cross more than one membrane ie kidney and gut


What is the surface and base of the epithelial layer called?

Surface is apical (mucosal) surface
Bottom is called basolateral (serosal surface)
This is an example of a double membrane layer using exchange epithelia


Transporting epithelia cells (eg kidney and intestine)

-transporting epithelial cell are said to be polarized
-this is because the apical and basolateral surfaces have different properties, this is due to the uneven distribution of membrane proteins on the surfaces.


Why is polarization needed to transport materials from lumen to the extracellular fluid or the other way.



If the molecule can be transported through a protein channel or on a carrier protein, then the 2 step process usually has:

1. An uphill step requiring energy
2. A downhill step in which molecules move passively down concentration gradient
Eg transport of glucose across epithelium


Transepithelial transport of glucose
What are the 3 different transport systems involved in the transportation of glucose across a membrane.

1. Active transport: of Na+ across basolateral surface by Na+ and K+ ATPase pump.
2. Secondary active transport of glucose with Na+ from lumen into cell at apical membrane
3. Facilitated diffusion of glucose across basolateral surface and into the ECF (and eventually blood supply)
From diagram:
1. Glucose enters the cell on the Na+ glucose symporter
2. The symporter uses the energy stored in the Na+ gradient to push glucose against its concentration gradient as Na+ moves down its gradient
3. Na+ is pumped out of the cell by the Na+ K+ ATPase while glucose diffuses out using a facilitated diffusion carrier.


What is osmosis, when does it occur? What are the factors which affect this movement of water

Osmosis is diffusion of water through a selectively permeable membrane
Osmosis occurs when the water concentration differs on both sides of a membrane; bulk flow then occurs
Factors affecting:
1. Amount of solutes dissolved in the water
2. Physical pressure/ tension exerted on the water
-water concentration and solute concentration are inversely related
-in osmosis water moves across a membrane to dilute the area I'd more concentrated solute


What is the difference between osmolarity and osmolality

Osmolarity: osmoles per litter of solution
= molarity (M) x no. Of particles/ molecules
Eg 1mol/L solution of glucose:
1 M glucose x 1 particle in solution = 1 Osm/L
Eg a 1M NaCl x 2 ions in solution = 2 Osm/L
Osmolality: osmoles per kg of solution


3 rules of osmotic pressure:

1. If solute concentration increases OP ->
2. OP depends on total solute concentration rather than molecular identities in solution
3. OP greater in solutions with ionized molecules


Osmolarity vs. tonicity. What is the difference

Osmolarity takes into account the total concentration of penetrating solutes AND non penetrating solutes

Tonicity takes into account the total concentration of ONLY non-penetrating solutes