Core Concepts: Cell Membranes and Transport Flashcards
What do phospholipids consist of
- A glycerol bonded to two fatty acids and a phosphate group.
- The fatty acids are hydrophobic - repel water and polar molecules.
The phosphate group is hydrophilic - attract water and polar molecules Phospholipids have a hydrophilic ‘head’ and hydrophobic ‘tails
What happens if 1 layer of isolated phospholipids is placed on water
They form a monolayer with the hydrophilic heads in the water and the tails which repel water sticking out into the air
How are Phospholipids arranged in a cell membrane
In cells, as cytoplasm is watery and cells are bathed in watery solutions, phospholipids arrange themselves in a bilayer – with the hydrophilic heads on the outsides and the hydrophilic tails in the centre
Where are extrinsic proteins found in the phospholipid bilayer
Extrinsic proteins are found in one side of the bilayer or on the surface of the bilayer
Where are intrinsic proteins found in the phospholipid bilayer, and what can some form
Intrinsic proteins span the membrane from one side to the other. Some intrinsic proteins form channels through the membrane as shown in the diagram.
Where are hydrophylic parts of proteins with polar/charged R groups located in the Phospholipid Bilayer
Around the polar heads
Where are hydrophobic parts of proteins with non-polar R groups located in the Phospholipid Bilayer
Near/between the hydrophobic tails
Why is the model of membrane structure called the ‘fluid-mosaic’ model
- The individual phospholipid molecules can move within a layer relative to one another (fluid)
- The proteins embedded in the bi-layer vary in shape and size in their distributionamong the phospholipids (mosaic)
Why are cell membranes described as selectively permeable
- The hydrophobic layer in the middle of the bilayer is impermeable to polar and charged particles.
- Non-polar molecules like oxygen, carbon dioxide and lipid-soluble vitamins (A, D, E and K) dissolve in the hydrophobic layer and can cross the membrane by simple diffusion
- Water-soluble/charged/polar substances e.g glucose can’t diffuse through the phospholipids and must pass through the water-filled chanels across the membrane
How do channel proteins transport molecules across the membrane
Channel proteins have hydrophilic R groups lining the space down the middle of the protein. These channels are specific to particular charged or polar particles and allow them to diffuse across the membrane in facilitated diffusion.
Some proteins are involved in active transport and facilitated diffusion across the membrane; these are specific to the polar or charged molecules carried and are termed protein carriers
What is a Glycoprotein
Carbohydrate attached to a protein
What is a Glycolipid
Phospholipids attached to carbohydrates
What is the function of glycoprotein
Cell Signalling. This is why it is only found on the side of the bilayer that faces out of the cell. The carbohydrates act as specific antigens
Where is Cholesterol found in the phospholipid bilayer
Between fatty acid tails
What is the function of cholesterol in the phospholipid bilayer
Molecules of cholesterol regulate the fluidity of the membrane.
What is the width of the phospholipid bilayer
7 nm
Describe an investigation into the permeability of cell membranes using beetroot
- Cylinders of beetroot are cut to 1 cm length. This controls the surface area of the tissue.
- The cylinders are rinsed to wash away pigment from damaged cells.
- A thermostatically controlled water bath should be used to maintain the temperature of the water.
- After 30 minutes, the tubes of water are agitated and the beetroot removed
- Use a colorimiter to measure absorbtion - as temperature increases, so does absorbance
Describe the results obtained during an investigation into the permeability of cell membranes using beetroot
At temperatures below 40°C
* The increased kinetic energy of the phospholipids and proteins causes them to move more
* This creates gaps between the phospholipids
* The pigment will also have more kinetic energy so will move more and diffuse out of the cell into the water through these gaps
At temperatures above 40°C
* Protein components of the membrane begin to denature
* Forming pores through which the pigment can diffuse out more easily
* The graph will level off as all proteins denature and the cell membrane becomes fully permeable to the pigment
What is Diffusion
Diffusion is the random movement of particles from an area of high concentration to an area of low concentration down a concentration gradient
What part of the membrane is used for diffusion
Through the Phospholipid bilayer only
What tye of molecules are transported by diffusion
- Small molecules
- Hydrophobic molecules
- Non-polar molecules
- Lipid soluble molecules
E.g Oxygen, CO2
Is diffusion an active or passive process
Passive
What factors affect diffusion
- Temperature
- Surface Area
- Concentration Gradient
- Size of molecule
- Lipid solubility
- Diffusion distance
Explain how Surface Area affects diffusion
A higher surface area increases the number of phospholipids through which diffusion can happen and increases the rate of diffusion
Explain how Temperature affects diffusion
Increase in temperature increases the kinetic energy of the molecule and increases the rate of diffusion
Explain how Concentration Gradient affects diffusion
Larger concentration gradient increases diffusion
Explain how Size of molecule affects diffusion
Smaller molecules diffuse more quickly than larger ones since it’s it’s easier for them to fit through the gaps between the phospholipids
Explain how Lipid Solubility affects diffusion
An increase in lipid solubility will increase the rate of diffusion becuase the molecules will find it easier to dissolve and diffuse through the bilayer
Explain how Diffusion Distance affects diffusion
The shorter the diffusion distance the greater the diffusion rate
What is facilitated diffusion
The movement of molecules or ions down a concentration gradient by channel or carrier proteins in the membranes
What area of the membrane is used for facilitated diffusion
Channel proteins or Carrier Proteins
Is Facilitated diffusion Active or Passive
Passive
What type of molecule is transported by Facilitated Diffusion
- Polar Molecules
- Charged Molecules
E.G Ions, Glucose, Amino Acids
What factors affect Facilitated Diffusion
- Temperature
- Number of specific proteins in the membrane
Explain how temperature affects Facilitated Diffusion
Increase in temperature increases the kinetic energy of the molecules and increases the rate of diffusion
Explain how the number of specific Proteins in the membrane affects facilitated diffusion
If you increase the number of specific proteins, the rate of diffusion increases
However if this number remains constant but the concentration gradient of the molecules increases, there will be a point where the max rate is achieved
How do channel proteins work
Molecules with mores lined with polar groups. Channels are hydrophyllic so ions can pass through
How do carrier proteins work
Allow diffusion of larger polar molecules e.g amino acids or sugars across a membrane. Molecules attatch to binding site. Carrier protein changes shape and releases molecule on other side. Then changes back to original shape
What is co-transport
A transport mechanism in which facilitated diffusion brings molecules and ionsacross a cell membrane together
What is active transport
The movement of molecules or ions across a membrane against a concentratuion gradient using energy from the hydrolysis of ATP made by the cell in respiration
What part of the membrane is involved in active transport
Carrier Proteins - Coded & Specific
Is Active Transport active or passive
Active - requires ATP
What types of molecule are transported by Active Transport
- Polar Molecules
- Charged Molecules
- Ions
- NOT Water
What factors affect Active Transport
- Temperature
- Number of Specific Proteins in the membrane
- Availability of ATP
How does Temperature affect Active Transport
Increasing temperature increases the kinetic energy of the molecules and increases the rate of diffusion
How does the number of Specific Proteins in the membrane affect Active Transport
By increasing the number of specific channel proteins you will increase the rate of diffusion.
However if the number remains constant, but the cct. gradient increases, at some point all the channel proteins will be saturated at one time
How does the availability of ATP affect Active Transport
If a respiratory inhibitor is added e.g cyanide, this will stop aerobic respiration, and stop ATP synthesis and therefore stop active transport of molecules
What is Osmosis
Osmosis is the net movement of water from an area of high water potential to an area of low water potential across a selectively permeable membrane down a water potential gradient
Is Osmosis active or passive
Passive
Why is the overall movement of water in osmosis described as net modement
The water molecules move in both directions across the membrane
Through what part of the membrane does osmosis happen
Channel Proteins - aquaporins
Some can move through the bilayer
What factors affect osmosis
- Temperature
- Solute concentration (water potential gradient)
- Number os specific aquaporins present
- High Solute Concentration = Low water potential in the cell
What is Water Potential
The tendency for water to move into a system. Water moves from a solution with higher water potential (less negative value) to one with a lower water potential (more negative value). Water Potential is decreased by the addition of solute.
The higher the concentration of solute in a solution or cell, the ……………. the water potential
The higher the concentration of solute in a solution or cell, the lower the water potential
If glucose or ions are pumped or diffuse into a cell, what happens to the water potential of that cell
The water potential of that cell gets lower (more negative)
Where does the water move by osmosis
Ψ A = -400kPa
Ψ B = -380kPa
ΨC = -360kPa
C → B → A
What are the units for Water Potential (w) Solute Potential (s) and Pressure Potential (p)
kPa
What does Water Potential depend on
- Solute Potential
- Pressure Potential
What is the Pressure Potential
The force of the cell wall on the cell
What is the equation for the water potential of a plant cell
ψcell = ψs + ψp
What does ψcell represent
Water Potential of a plant cell
What does ψs represent
Solute potential of the cytoplasm
What does ψp represent
Pressure potential
What is the Water Potential for pure water
0kPa
What molecules diffuse by osmosis
ONLY Water
What is a Hypotonic Solution and what is the movement of water
- Lower solute concentration (dilute)
- Higher water potential than the cytoplasm
- Movement of water Into the cell by osmosis down a water potential gradient
What is an Isotonic Solution and what is the movement of water
- A solution with the same water potential as the cytoplasm
- Movement of water in and out of the cell by osmosis down a water potential gradient but with no net gain
What is a Hypertonic Solution
- Higher solute concentration (concentrated)
- Lower water potential than the cytoplasm
- Movement of water out of the cell by osmosis down a water potential gradient
What happens if blood cells are placed in a hypotonic solution
- Net movement of water is from the higher water potential of the solution to the lower water potential in the cells, by osmosis.
- The cells will expand
- As they lack a cell wall, they will burst because of the increased pressure in the cells.
- This is called haemolysis
What happens if blood cells are laced in a hypertonic solution
- Net movement of water is from the higher water potential in the cells to the lower water potential in the solution by osmosis.
- The cells shrivel
- This is called crenating
What happens if blood cells are placed in an isotonic solution
No net movement of water
In which solution would a blood cell burst?
A - Isotonic
B - Hypertonic
C - Hypotonic
C - Hypotonic
Why do red blood cells burst when they are placed in a solution with a higher water potential
They have no cell wall
Which of there water potentials is the highest?
- -750 kPa
- -650 kPa
- -800 kPa
- -700 kPa
-650 kPa
A red blood cell with a water potential of -400 kPa is placed in a solution with a water potential of -450 kPa. What will happen to the cell
Water will leave the cell by omosis and it will shrivel
What does turgid mean
A plant cell that holds as much water as possible. The cell wall can’t expand any further.
What does turgid mean
A plant cell that holds as much water as possible. The cell wall can’t expand any further.
What happens if a plant cell is placed in a Hypotonic Solution
- Plant cells become turgid
- Water enters by osmosis and the cytoplasm and vacuole expand.
- When it is not physically possible for any more water to enter the cell, the water potential of the cell is 0
What happens if a plant cell is placed in a Hypertonic Solution
- Water leaves the cells by osmosis, from a higher water potential inside the cell to a lower water potential in the solution
- The vacuole and cytoplasm shrink because of the loss of water and the cytoplasm pulls away from the cell wall - Plasmolysis
- Down a microscope there is a visible gap between the cell membrane and cell wall
What happens when a plant cell is placed in an Isotonic Solution
- Incipient Plasmolysis - when ψ just reaches 0 kPa
- 50% of the cells are plasmolysed
What is Bulk Transport
the movement of solids or liquids
What is Endocytosis
The active process when the cell membrane engulfs material, bringing it into the cell in a vescicle
What are the 2 types of Endocytosis
- Phagocytosis
- Pinocytosis
What is Phagocytosis
The active process of the cell membrane engulfing large particles, bringing them into the cell in a vescicle
What is Pinocytosis
The active process of the cell membrane engulfing droplets of fluid, bringing them into the cell in a vescicle
How does Endocytosis work
- The cell membrane extends around the particles being taken into the cell
- Eventually the particles are surrounded
- The membrane of the cell fuses around the particle, forming a vesicle in the cytoplasm containing the particle
What is Exocytosis
The active process of a vescicle fusing with the cell membrane, releasing the molecules it contains. Large particles such as enzymes and hormones are secreted from cells
EHow does exocytosis work
The vesicle containing the particles migrates to the cell membrane and fuses with it, releasing the particles to the outside of the cell
Does Exocytosis, or Endocytosis require ATP and why
Yes because the cell membrane has to change shape requiring energy.
