Transfer Across Cell Membranes - Chapter 4 Flashcards
(88 cards)
1
Q
STRUCTURE OF CELL SURFACE MEMBRANES
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2
Q
What is a plasma membrane ?
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All membranes within (organelles) and around the cell
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Q
What is a cell-surface membrane ?
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The plasma membrane that surrounds the cell. It forms the boundary between the cell cytoplasm and the environment of the cell
4
Q
What are the components of the phospholipid bilayer ?
A
-hydrophilic heads
-hydrophobic tails
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Q
Info about hydrophilic heads?
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They point to the outside of the cell of the cell- surface membrane attracted by water on both sides
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Q
Info about hydrophobic tails?
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Point into the centre of the cell membrane, repelled by the water on both sides
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Q
What is the function of phospholipids ?
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- allow lipid -soluble substances to enter and leave the cell
-prevent water -soluble substances entering and leaving the cell
-make the membrane flexible and self-sealing
8
Q
What are the peripheral proteins ?
A
-occur in the surface of the bilayer and never extend completely across it
~~> provide mechanical support, or they are connected to proteins or lipids to make glycoproteins + glycolipids
9
Q
What are integral proteins ?
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-span the phospholipid bilayer from one side to the other
~~> Are protein channels or carrier proteins involved in the transport of molecule across the membrane
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Q
What are integral proteins (short answer) ?
A
Protein channels and carrier proteins
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Q
What are protein channels ?
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Form water-filled tubes to allow water-soluble ions to diffuse across the membrane
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Q
What are carrier proteins ?
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Bind to ions or molecules (like glucose and amino acids), then change shape in order to move these molecules across the membrane
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Q
What are the functions of the proteins in the membrane ?
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- provide structural support
- act as channels transporting
water-soluble substances across the membrane - allow active transport across the membrane through carrier proteins
- act as receptors
14
Q
What are the functions of proteins in plasma membranes ?
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- act as receptors
- enzymes
- carriers
-channels
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Q
What are glycoproteins?
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-protein with carbohydrate attached
-act as cell-surface receptors (more for hormones)
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Q
What are the function of glycoproteins?
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-act as recognition sites
-help cells to attach to one another and so form tissues
- allow cells to recognise each other
17
Q
What are glycolipids?
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-lipid with carbohydrate attached
- act as cell-surface receptors for specific chemicals
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Q
What are the function of glycolipids?
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-act as recognition sites
-helps cells to attach to one another and so form tissues
- helps maintain the stability of the membrane
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Q
What is cholesterol ?
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Cholesterol is an important component of the cell membrane and helps to regulate its fluidity
20
Q
What is the function of cholesterol?
A
-reduce the lateral movement of other molecules in the membrane
~~> useful as it makes the membrane less fluid at high temperatures
- prevents water and dissolved ions leaking out of the cell
21
Q
The cell-surface membrane is partially permeable, why do most molecules not freely diffuse across it ?
A
- not soluble in lipids, so cannot pass through the lipid bilayer
-too large to pass through the channels in the membrane
-electrically charged/ polar, difficulty passing through non-polar hydrophobic tails in phospholipid bilayer
-same charge as protein channels, repelled
22
Q
Why is the phospholipid called the fluid -mosaic model ?
A
Fluid: individual phospholipids + proteins can move around freely within the bilayer
Gives the membrane a flexible structure that is constantly changing shape
Mosaic: The pattern produced by the scattered protein molecules when the membrane is viewed from above - looks mosaic
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Q
DIFFUSION
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24
Q
What is diffusion ?
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The net movement of ions or molecules from a region of higher concentration to a region of lower concentration until evenly distributed
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Where does diffusion go with its concentration gradient
Diffusion occurs down/across its concentration gradient
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When does diffusion stop?
[There is a high concentration of CO2 inside the cell, and a low concentration of CO2 outside the cell. CO2 molecules diffuse from a region with a higher concentration to a region of lower concentration]
Eventually the concentration of molecules will be the same both inside and outside
~~> equilibrium has been reached, diffusion has stopped
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What is metabolic energy ?
Energy released by respiration
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Key points about diffusion ? [2 points]
1) Diffusion is a passive process, it does not require metabolic energy to take place
2) Some chemicals can easily diffuse through the membrane, but some cannot ; cell membrane is described as partially permeable
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What are the factors affecting the rate of diffusion ?
1)Temperature
2)Surface area
3)Difference in concentration
4)Thin exchange surface / diffusion distance
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How does temperature affect the rate of diffusion ?
- The higher the temperature, the more kinetic energy the molecules possess.
- Molecules move faster at higher temperature.
- Higher the temperature, the faster the rate of diffusion
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How does surface area affect the rate of diffusion ?
The greater the surface area of the exchange surface, the faster the rate of diffusion
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How does difference in concentration affect the rate of diffusion ?
- The greater the difference in concentration (on either side of the exchange surface ), the faster the rate of diffusion
( E.g in the intestine, blood continually transport the product of digestion from the intestine wall
A large difference in concentration is set up and the rate of diffusion is faster)
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How does thin exchange surfaces / diffusion distance affect the rate of diffusion ?
The thinner the exchange surface, the faster the rate of diffusion.
Exchange surfaces such as the epithelium (small intestine) or the alveoli (lungs) have very thin exchange surfaces
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What is the equation for Frick’s law ?
Rate of diffusion is proportional to = surface area X difference in conc. / length of diffusion path (membrane thickness)
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Why do not all substances diffuse across the cell membrane at the same rate?
Due to the centre of the cell membrane being hydrophobic (fatty acid tails)
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What adaptation is best for a molecule to have for a molecule to diffuse across the membrane faster ?
The smaller + less polar a molecule, the easier + faster it will diffuse across the cell membrane
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What is the difference between hydrophobic and hydrophilic substances diffusing across the cell membrane
- hydrophobic substances can diffuse rapidly across the cell membrane (non-polar)
- hydrophilic substances cannot diffuse across the cell membrane (polar )
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What molecules is an exception for diffusing across the membrane even though it is polar?
Water - they can diffuse across the membrane even though water molecules are polar , due to water molecules being very small
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How do you solve the problem with hydrophilic substances not being able to diffuse across the cell membrane?
By using facilitated diffusion
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Key points about facilitated diffusion :
- it is a passive process
- relies on kinetic energy of molecules that are diffusing
- occurs down/ across a concentration gradient
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In facilitated diffusion, hydrophilic substances diffuse across the cell membrane via what ?
Protein molecules (carrier proteins + channel proteins )
- these protein molecules allow the hydrophilic substances to cross the membrane without interacting with the hydrophobic centre of the phospholipid bilayer
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What does it mean by carrier proteins being selective ?
- They only open in the presence of a specific ion
- If the ion is not there, the channel remains closed
- The ions bind with the protein and it changes shape so that it closes one side and opens the other
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What do carrier proteins do (in detail ) ?
- carrier proteins span the plasma membrane
- When a molecule (e.g glucose) is present, the correct carrier protein will bind to it
- This changes it’s shape and releases the molecule into the inside of the membrane
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What happens in facilitated diffusion involving carrier proteins ?
- Molecule that is specific to carrier protein binds with it
- The carrier proteins then change shape
- The molecule is then released to the inside of the membrane
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OSMOSIS
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What is osmosis ?
Is the passage of water from a region where it has a high water potential to a region where it has a lower water potential through a partially permeable membrane
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What type of process is osmosis?
A passive process
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What is water potential + what is it measured in ?
The tendency of water molecules in a system to move
Measured in kilopascals
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What happens when water molecules are colliding with the membrane in osmosis?
In osmosis, there are water molecules on both sides of the membrane. These water molecules are colliding with the membrane.
- These collisions mean that the water molecules apply a pressure to the membrane,
The pressure = water potential
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How does osmosis stop / reach equilibrium ?
- Due to a high water potential on the left than the right (on the diagram), water molecules will move by osmosis from the left hand side to the right hand side
- At some point, both sides of the membrane will have the same water potential , reached equilibrium, osmosis stops
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What has the highest water potential and why ?
Pure water, because pure water has the highest possible concentration of water molecules
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Due to pure water being the highest water potential, what does this mean for a water potential of any solution ?
Water potential of any solution must be less than 0kPa ( must be a negative number )
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True or false : A more concentrated solution has a lower water potential than a less concentrated solution
True
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Example:
P= -250 kPa Q= -50 kPa
Where does water move ?
[ P has a lower water potential than Q)
Water molecules will move from Q to P by osmosis from a region of high water potential to a region of low water potential across a partially permeable membrane
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Osmosis in plant cells
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What happens if plant cells are placed in pure water?
Water will enter by osmosis,
The cell becomes turgid and this pressure resists the entry of more water
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What happens when there is equal water potential inside and outside the cell (equilibrium) ?
The protoplast is normal and the cell is in a stare of incipient plasmolysis
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What happens if plant cells are placed in a concentrated solution ?
Water leaves the cell by osmosis, and the pressure decreases (cell is flaccid)
Eventually the cell would become plasmolysed as the protoplast pulled away from the cell wall
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When is a solution hypotonic ?
When it has a lower concentration than cell cytoplasm
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When is a solution isotonic?
When it has the same concentration as cell cytoplasm
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When is a solution hypertonic?
When it has a higher concentration than cell cytoplasm
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Question :
A scientist placed some red blood cells in a solution and viewed the cells with a light microscope. After a few minutes, she noticed that many of the cells burst.
Explain why
-solution has higher water potential then the red blood cell cytoplasm (hypertonic)
-water entered the cells by osmosis, down the water potential gradient
-the cell membrane cannot withstand the pressure and bursts
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What is pressure potential?
Affected by the pressure applied to it,
The greater the pressure, the higher the water potential.
This is called the pressure potential and is always a positive value.
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In plant cells, what is the pressure potential a result of ?
It is the result of the cell wall exerting pressure on the cytoplasm
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How do you calculate water potential ?
Water potential = solute potential + pressure potential
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What is solute potential ?
Affected by the amount of solute it contains,
The greater the amount of solute, the lower the water potential.
It is always a negative value
This is because water molecules bind to the solute molecules, reducing the number of water molecule that are free to diffuse
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ACTIVE TRANSPORT
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What do calcium ions have to do with active transport ?
Cells are constantly transferring calcium ions from the cytoplasm to outside the cell (moving from a region of low concentration to a region of high concentration) for the role of cell signalling.
Cells use a process called active transport for this to take place
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What is cell signalling ?
In order for organisms to survive, cells must communicate with each other.
Cells can signal to each other by direct contact or via messengers
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What is active transport ?
The movement of molecules or ions inside the cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
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Active transport key points:
- It is an active process, it requires metabolic energy, which is provided by the molecule ATP
- molecules go against its concentration gradient
- In active transport, carrier proteins in the membrane transport a chemical from a region of lower concentration to a region of higher concentration against the concentration gradient
- It is a selective process, with specific substances being transported
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Contrast the processes of facilitated diffusion and active transport
- FD is a passive process, AT is an active process
- FD movement of molecules is from a high to low concentration, AT movement of molecules is from a low to high concentration
- FD involves channel and carrier proteins, AT only uses carrier proteins
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How does active transport work?
1) Molecule or ion binds to carrier protein in the plasma membrane
2) ATP binds to the protein on the other side which causes it to split by hydrolysis, into ADP and phosphate, releasing energy
3) The protein changes shape and opens on the opposite side of the membrane
4) The molecule or ion is released on the other side of the membrane
5) ATP is reformed and the protein returns to its original shape
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What are some examples of active transport?
- Absorption of amino acids from the gut into the blood
- Absorption of mineral ions by the roots of the plant
- Endocytosis + Exocytosis
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What are some factors that affect the rate of active transport ?
- number of carrier proteins
- speed of individual carrier proteins
- rate of respiration (factors that affect the rate of respiration : temperature, surface area )
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CO-TRANSPORT
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What is villi ?
The epithelial cells that line the villi possess microvilli
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How are microvilli/villi adapted?
-increase SA (greater diffusion)
-Thin walled (quicker diffusion)
-Good blood supply (maintains the concentration gradient)
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Adaptations of the small intestine for efficient absorption:
- long ~> Large SA for efficient absorption
- Folded ~> further increases SA
- Villi ~> finger-like projections which increases SA
- Villi has a good blood supply ~> maintains concentration gradient ( glucose is removed from the SI by blood flow)
- Thin epithelium, one cell thick ~> creates a short diffusion pathway
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Adaptations for transport :
-cells have microvilli which increases SA
- cells have lots of mitochondria, site of respiration, releasing energy (ATP)
- number of carrier proteins
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Co -transport key points:
- Active process
- requires ATP, but uses it indirectly as it is the movement of sodium ions which actually powers the movement of glucose into the cell
- two molecules are involved : glucose + sodium
- Na+ ions move across their concentration gradient
- glucose molecules move against their concentration gradient
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Why is the rest of glucose absorbed by co- transport and not facilitated diffusion when moved into the small intestine ?
-lots of glucose is moved into the small intestine by facilitated diffusion
- This does not absorb all available glucose because diffusion would stop at equilibrium
- The rest is absorbed by co -transport
Facilitated diffusion is passive process, so reaches equilibrium. Active /co transport requires additional energy (ATP), so is an active process, which results in non-equilibrium.
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Steps in co-transport (look at diagram in biology book):
1) Na + ions pumped out of epithelial cells into the blood
This requires ATP and the enzyme ATPase
This results in low concentration of Na + ions in epithelial cells
2) Na + ions diffuse down concentration gradient from lumen of small intestine into epithelial cells, bringing glucose with the,
3) Glucose diffuse into blood from epithelial cells by facilitated diffusion
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What is the Na+ glucose symporter?
2 glucose molecules and 2 Na+ ions must bind to the protein before they can be transported across the membrane
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What is the Na+ – k pump ?
-pumps out 3Na+ ions in exchange for 2k+ ions
-creates the low cone of Na+ in epithelial cells
- requires ATP (active transport)
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What is the glucose uniporter?
-passive process down concentration gradient from epithelial cells to blood
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OVERALL QUESTION
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Name and describe 5 ways substances can move across the cell-surface membrane into a cell.
1. (Simple) diffusion of small/non-polar molecules down a concentration gradient
2. Facilitated diffusion down a concentration gradient via protein carriers/channels
3. Osmosis of water down a water potential gradient
4. Active transport against a concentration gradient via protein carrier using ATP
5. Co-transport of 2 different substances using a carrier protein
