Exchange Across Cell Membrane - Diffusion Flashcards
What is an example of passive transport?
- Facilitated diffusion
- Simple diffusion
What does ‘passive’ mean?
Energy comes from the natural, inbuilt motion of particles (kinetic energy)
rather than the external sources such as ATP
net movement of particles from high to low concentration
Properties of diffusion particles
- All particles are constantly in motion due to the kinetic energy they possess
- Motion is random - no set pattern in which the particles move around
- Particles are constantly bouncing off one another as well as objects e.g side of vessel they are contained
What is the definition of diffusion?
Net movement of molecules/ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed
What molecules can diffuse straight through the cell membrane?
(simple diffusion)
Small and non-polar molecules (makes them soluble in lipids - dissolve in hydrophobic bilayer)
e.g oxygen
Why do charged ions and polar molecules do not diffuse easily?
Hydrophobic nature of the fatty acid tails of the phospholipids in the membrane
What factors affect facilitated diffusion?
- Concentration gradient
- Number of channel or carrier proteins
The concentration gradient is one of these factors – how does the concentration gradient affect the rate of diffusion?
(e.g Ventilation of air in lungs)
The steeper the concentration gradient the higher the rate of diffusion.
How does the concentration gradient affect diffusion?
- The higher it is the faster the rate of diffusion
- As diffusion takes place the difference between two sides decreases until it reaches an equilibrium (both sides are equal)
- Diffusion slows down over time
How does surface area affect the rate of diffusion?
(Microvilli , Alveoli , cristae , thylakoids,
Larger surface area the faster rate of diffusion
How does concentration gradient affect facilitated diffusion?
Higher concentration gradient , faster rate
As equilibrium reached , rate of diffusion will level off
How does thickness of the exchange surface affect the rate of diffusion?
The thinner the exchange surface (i.e shorter distance the particles have to travel) the faster the rate of diffusion
How does microvilli increase the rate of diffusion?
Some cells e.g epithelial cells have microvilli (projections in cell surface membrane folding up on itself)
It gives cell a large surface area - increase rate of diffusion
What is facilitated diffusion?
The passive movement of molecules crosses the cell membrane in a specific transmembrane protein
What molecules are used for facilitated diffusion?
- Charged ions
- Polar molecules
Where does diffusion occur with concentration gradient?
Down a concentration gradient
What two proteins are involved in facilitated diffusion?
- Carrier proteins
- Channel proteins
What do channel proteins form?
Water-filled hydrophobic channels across the membrane
What type of substances do channel proteins pass through?
Water-soluble susbtances
small charged or polar molecules e.g ions Na + , K + , Ca2+
How are channel proteins selective in the opening of the presence of a specific ion?
If the particular ion is not present , channel remains closed - control entry and exit of ions
How do carrier proteins transport large molecules across cell membrane?
Large molecule bind with the protein causing it to change shape
Releases molecule on the opposite side of the membrane
What type of molecules are used with carrier proteins?
e.g glucose specific to carrier protein
Causes to change shape and molecule released to the inside of the membrane
What factors does simple diffusion depend on?
- Concentration gradient
- Thickness of the exchange surface
- Surface area
- Microvilli - increase diffusion
With reference to the key factors that affect the rate of diffusion discussed in question one, discuss how the alveoli is adapted for efficient gas exchange.
– Short diffusion pathway
– each alveolus is surrounded by a dense capillary network.
-Steep concentration gradient
– dense capillary network means that blood is rapidly transported away, maintaining the concentration gradient.
- Large surface area
– alveolar walls provide a large SA/ large SA: Volume ratio
People who suffer from emphysema, a disease usually caused by smoking or exposure to air pollution, have a greatly reduced amount of elastin in their alveolar walls.
Why do people who suffer from emphysema often suffer from an increased rate of breathing?
– Destroyed elastin reduces the surface area of the alveoli
- Destroyed elastin reduces efficiency of ventilation (higher residual volume in lungs)
- gas exchange is less efficient - Less oxygen is transported into the blood and to the respiring tissues.
- Increased breathing rate is required to provide sufficient oxygen
Why would ions diffuse slowly in the cell membrane?
Ions and polar molecules are water soluble
centre of bilayer is hydrophobic
What is carrier proteins used?
Move large molecules down their concentration gradient
different carrier proteins facilitate the diffusion of different molecules
Why would channel proteins be moved?
Facilitate the diffusion of charged particles
Why increasing channel proteins help to increase the rate of facilitated diffusion in kidney cells?
- Aquaporins in a special type of channel protein allow facilitated diffusion of water through the cell membrane
- Some kidney cells are adapted to have lots of aquaporins - allow to reabsorb a lot of water that would otherwise excreted by body
How does a number of channel or carrier proteins affect facilitated diffusion?
Once all of proteins in use , faciliated diffusion can’t happen faster even if you increase concentration gradient
Increase: faster rate
Diffusion of oxygen:
Simple diffusion through the cell membrane
Diffuse straight through
Random movement of oxygen molecules
down the concentration gradient
Facilitated diffusion of glucose
Using carrier proteins
Attach to protein in membrane causing it change shape
Release towards opposite side of membrane
Describe the difference between facilitated diffusion and active transport
- Facilitated - use channel and carrier
- Active - only use carrier
- Active transport - Active process - requires ATP to move substances against the concentration gradient
- Facilitated diffusion requires no ATP - ‘passive’ requires own kinetic energy for it
Give two differences between facilitated diffusion and osmosis
- Facilitated diffusion requires the use of channel or carrier proteins
- Facilitated diffusion moves larger molecules compared with osmosis with molecules diffusing through the partially permeable membrane
Explain why molecules of oxygen and carbon dioxide is able to diffuse through the membranes?
Small and non-polar
Use simple diffusion to diffuse through phospholipid bilayer
Explain why ventilation of the lungs increase the efficiency of gas exchange
Alveoli causing the greater concentration of diffusion between alveoli and blood capillaries
One similarity between active transport and facilitated diffusion
Involve transport proteins
Give one evidence why it is entering the cell by facilitated diffusion and not diffusion
Concentration levels off above external concentration
Give two functions of proteins in plasma membrane
- Channel protein : Is able to transport ion/water by facilitated diffusion
- Carrier protein helps to transport two molecules
Exam Questions
Ventilation and Circulation of air and blood
- Maintain a high concentration gradient
- Alvelo and capillary have thin walls (‘one cell thick)
- Reduces diffusion distance
- Alveoli - (folded) Large Surface Area
What do both protein channel and carrier proteins have in common?
- High to low concentration
- Specific - each molecule has its own transport protein
- Same protein can be used for active transport (against the concentration gradient using ATP)
