Chapter 4 Transport Across Cell Membrane Flashcards
Why are membranes described as a fluid mosaic model
Fluid: as the individual phospholipid molecules can move relative to one another giving the membrane a flexible structure that is constantly changing shape
Mosaic: the proteins that are embedded in the phospholipid bilayer vary in shape, size, pattern
How is the cell membrane created
All molecules arranged within the phospholipid bilayer create the partially permeable membrane that is the cell surface membrane
Components of membrane / components in phospholipid bilayer
Components of the membrane
- phospholipid bilayer
- cholesterol
- peripheral proteins
- integral proteins (protein channels and protein carriers)
Found in phospholipid bilayer:
- cholesterol
- peripheral proteins
- integral proteins (protein channels and protein carriers)
Function of phopholipid bilayer
- It aligns as a bilayer (2 layers) due to the hydrophilic heads being attracted to water and the hydrophobic tails being repelled by water , so hydrophilic head will face outside of the cell and the tail will face inside of the cell
- allows lipid soluble substances to enter and leave the cell
- prevents water soluble substances entering and leaving the cell
- makes the membrane flexible and self sealing
Why are the phospholipid heads hydrophilic and the tails hydrophobic
- hydrophilic head - the head contains a phosphate group which has a negative charge and a glycerol molecule causing the head to have an overall negative charge and so is attracted to water which makes it hydrophilic
- hydrophobic- as the tails contain fatty acid chains so do not have a charge so will repel water
How does cholesterol contribute to the cell membrane
- it restricts the lateral movement of other molecules in the membrane
- this makes it useful as it makes the membrane less fluid at high temperatures which is important as if membrane was too fluid then the gaps between phospholipids will become too large and water and dissolved ions will leak out of the cell, causing the cell to shrivel or burst
How does peripheral proteins contribute to the cell membrane and where are they found
- they are found in the surface of the bilayer and never extend across it
- By either providing mechanical support or are connected to proteins or lipids to make glycoproteins and glycolipids which provides the function of cell recognition
How does integral proteins contribute to the cell membrane and where are they found
- they completely spam the bilayer from one side to the other
- They are protein carriers or channel proteins which is involved in the transport of molecule across the membrane
What do protein channels do
They form tubes that fill with water to enable polar molecules to diffuse into the cell
What do proteins carriers do
- they bind to ions causing the carrier protein to change shape and transport the molecule to the other side
Why is the cell membrane partially permeable
Due to the components in the phospholipid bilayer only lipid soluble substances and very small molecules can diffuse into the cell
What molecules cannot pass through the membrane through diffusion
- water soluble (polar) substances
- large molecules
- ions
Function of glycoproteins
- act as recognition sites
- help maintain the stability of the membrane
- help cells to attach to one another and form tissue
Structure of glycolipids
They are made up of a carbohydrate covalently bonded with a lipid, the carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as the cell surface receptor for specific chemicals
Function of glycolipids
- act as cell recognition sites
- helps cells attach to one another to form tissues
- allows cells to recognise one another
Key types of transport
- simple diffusion
- facilitated diffusion
- active transport
- osmosis
What is simple diffusion
The net movement of molecules or ions from a region of higher concentration to a region of lower concentration until equilibrium is reached
Key points of simple diffusion
- for the molecules to move they have kinetic energy which enables them to constantly be in motion
- ## the molecules are lipid soluble and small which enables them to diffuse across the membrane
How does facilitated diffusion differ from simple diffusion
- as proteins are used to transport molecules
- this means that it can transport ions and polar molecules which cannot simply diffuse across the membrane using protein channels and carrier proteins
How are protein channels involved in facilitated diffusion
- they are filled with water it enables polar molecules to pass through the membrane which cannot pass through the phospholipid bilayer
- gated protein channels are only open in the presence of certain ions when they bind to a certain protein
- continuous protein channels allow ions to transport continuously into the cell through facilitated diffusion as they are always open
What is osmosis
The movement of water from an area of higher water potential to an area of a lower water potential across a partially permeable membrane
What is water potential
The pressure created by water molecules and is measured in kPa
Hypertonic hypotonic and isotonic
Hypotonic - when the solute concentration outside the cell is lower than the solute concentration inside the cell
Hypertonic- when the solute concentration outside the cell is higher than the solute concentration inside the cell
Isotonic- when there is no net movement of water molecules across the membrane.
How to tell the concentration of solutes using water potential
The more solutes dissolved in water the more negative the water potential
Pure water has the water potential of 0 kPa
What would happen if an animal cell was placed in a hypotonic, and , hypertonic solution
Hypotonic- water will move in the cell by osmosis and as animal don’t have a cell wall the pressure will cause the cell to burst
Hypertonic - animal cells will shrivel due to large volumes of water leaving the cell by osmosis
What is active transport
The movement of molecules and ions from an area of lower concentration to an area of higher concentration, against the concentration gradient using ATP and carrier proteins
Role of carrier proteins in active transport
They act as a pump to move substances across the membrane which is very selective as only certain molecules can bind to carrier proteins to be pumped
How is ATP and carrier proteins used in active transport (process of active transport)
1) certain molecules that are complementary to the receptor site can bind to the receptor site on carrier proteins
2) ATP will bind to the carrier protein on the inside of the membrane and is hydrolysed into ADP and Pi
3) This causes the protein to change shape and open towards the inside of the membrane
4) this causes the molecule to be released to the other side of the membrane
5) The Pi molecule (inorganic phosphate) is then released from the protein and results the protein reverting to its original shape
Type of active transport
- co transport
What happens in the co transport of glucose and sodium ions in the ileum
1) the sodium ions are actively transported out of the epithelial cell (layer of cells that cover the wall of the ileum) against their conc gradient by the sodium potassium pump into the blood, causing the potassium ions to move into the epithelial cell, maintaining the electrochemical gradient, through active transport
2) this reduces the sodium ion concentration in the epithelial cell
3) the sodium ions from the lumen diffusion down their concentration gradient into the epithelial cell by a co transporter protein through facilliated diffusion
4)the co transporter protein also enables the glucose or amino acids to attach to their complementary receptor site on the protein at the same time and be transported into the epithelial cell against their conc gradient
5) the glucose/amino acids pass into the blood plasma by facilliated diffusion
Why is the conc of glucose in the blood lower than in epithelial cells
As the blood is continuously flowing so carries away the absorbed glucose to cells that need it for respiration which maintains a conc gradient between ileum and blood
how is the rate of movement increased across membranes in the co transport of glucose and sodium ions
- the epithelial cells on the ileum contain microvilli which increase the surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport take place, so there will be an increase in carrier proteins increases the rate of movement
- As the blood is continuously flowing so carries away the absorbed glucose to cells that need it for respiration which maintains a conc gradient between ileum and blood so the rate of movement by facilitated diffusion is increased
what is co transport
- the movement of molecules using a conc gradient that has been set up by active transport using a protein
how does active transport differ from passive transport
- ATP is required
- substances are moved against the conc gradient
- carrier proteins are involved
- the process is very selective
what is passive transport and an example
- the movement of molecules where the energy comes from the natural kinetic energy of the particles rather from an external source
examples: diffusion (S or F) and osmosis
what happens when plant cells are placed in an
hypertonic solution
cell contents will shrink and the protoplast (cell membrane, nucleus and cytoplasm) will pull away from the cell causing the cell to be plasmolysed
what happens when plant cells are placed in an
hypotonic solution
water moving in the plant causes the protoplast (cell membrane, nucleus and cytoplasm) to swell and press on the cell wall and as the cell wall is only capable of limited expansion a pressure builds up that prevents the entry of more water causing the cell to be turgid
what happens when plant cells are placed in an
isotonic solution
the volume of the cell will decrease and the protoplast (cell membrane, nucleus and cytoplasm) will no longer touch the cell wall causing the cell to reach incident plasmolysis
How are carrier proteins used in facilitated diffusion
Carrier proteins allow small polar molecules to be transported across the membrane by facilitated diffusion.
Once the polar substance is attached to the carrier protein it causes the protein to change shape and transport the molecule across the membrane
