Cell Membranes Flashcards
Which model describes the structure of cell membrane?
Fluid mosaic model
Why are cell membranes structure described as fluid mosaic?
Fluid- phospholipids can move freely past past each other (within a particular layer8
Mosaic - proteins embedded in the membrane are various shapes, sizes and patterns
Name the molecules involved in cell membranes
Phospholipids
Channel proteins (intrinsic)
Carrier protein ( intrinsic)
Extrinsic proteins
Glycolipids
Glycoproteins
Cholesterol
What are the roles of cell membranes?
-boundary separating cell contents from cytoplasm/outside world
- cell recognition and signalling
- holding components for some metabolic pathways
- regulation of cells in and out
What can pass through the phospholipid belayer
Lipid solvable (non polar) molecules can diffuse through
What is the purpose of the phospholipids in the bilayer
They make the cell membrane flexible and self-sealin
What are extrinsic proteins?
Proteins found on the surface of either side of the membrane bilayer
What are intrinsic proteins
Extends across both layers of the membrane (bilayer)
What are two types of intrinsic proteins?
Carrier proteins
Channel proteins
What is the function of channel proteins?
Allow water soluble molecules (polar/ionic) to pass through
What is the purpose of carrier proteins?
They bind to solute/molecules which fit into the binding site this causes a conformational (shape) change transferring the molecule from one side of the membrane to the other
What is a glycoprotein?
Proteins with a carbohydrate group/chain attached to it
What is a glycolipid?
A lipid with a carbohydrate chain (made from monosaccharides) attached to it
What is the function of a glycolipid?
- act as cell surface receptors for specific chemicals (basis of ABO blood system)
- Helps maintain stability of membrane and attach cells to one another to form tissue
What is the function of a glycoprotein?
- acts as cell surface receptors (e,g for hormones)
- helps attach cells to one another to form tissues
- Allows cells to recognise each other (e.g lymphocytes recognise organisms body cells)
Where is cholesterol found.
Between phospholipids tails
What is cholesterol?
It is a molecule made from 4 hydrocarbon rings
Has a polar head (due to OH groups) so can associate with phospholipid heads
What type of molecule is cholesterol?
Steroid
What is the function of cholesterol?
Adds strength and rigidity to the membrane by pulling fatty acid tail closer together
Prevents loss of water and dissolved ions from the cell (stops leaking)
What are the two types of transport across membranes?
Passive
Active transport
What does passive transport across a membrane refer to?
Does not require any external energy (ATP)
What does active transport across a membrane refer to?
Requires energy in the form of ATP (to move molecules against the concentration gradient)
What are the two types of passive transport across membranes?
Simple diffusion
Facilitated diffusion
Define diffusion
The net movement of particles from a region of high concentration to a region of low concentration down the concentration gradient
What is simple diffusion
Diffusion directly across the phospholipid bilayer (small, lipid soluble, uncharged molecules like oxygen
What type of molecules move by simple diffusion
Small
Lipid soluble
Uncharged molecules
What is facilitated diffusion?
Diffusion using channel and carrier proteins (small, water soluble molecules - mainly ions/polar)
What type of molecules are transported across membranes using channel proteins (by facilitated diffusion)
Small, water soluble molecules (mainly ions/polar)
E.g. Na+, H+ions
What type of molecules are transported across membranes using carrier proteins (by facilitated diffusion?
Larger, water soluble molecules like glucose and amino acids
Why does the transport rate plateau when facilitated diffusion occurs?
There are only so many transport proteins that become saturated.
Define active transport
The movement of particles into or out of a cell from a region of lower concentration to a region of higher concentration (against the concentration gradient) using ATP and carrier proteins
Explain the process of active transport
- the molecule or ion to be transported across the membrane binds to the specific carrier protein on one side of the membrane at the binding site.
- on the inside of the membrane ATP binds to the carrier protein at the ATP binding site, causing it to hydrolyse into ADP and Pi (releasing the ADP and energy)
- this causes the carrier protein to change shape and open to the opposite side of the membrane (releasing the molecule or ion to the opposite side)
- the Pi is released from the carrier protein causing it to revert back to its original shape.
- the process can be repeated
Describe the process of the sodium potassium pump
- 3 sodium ions bind to the binding site of carrier protein
- ATP bonds to the ATP binding site and is hydrolysed real easing ADP and energy while Pi remains
- carrier protein changes shape due to energy and 3 sodium ions are released against the concentration gradient
-whilst carrier protein is open to opposite side, the 2 potassium ions bind to the specific binding site - inorganic phosphate is released from the binding site causing it to revert back to the original shape,
- 2 potassium ins are released (against the concentration gradient)
Describe the process of glucose and amino acid co-transport
The sodium potassium pump moves sodium out of the epithelium cells into the bloodstream by facilitated diffusion while potassium is moved into the cell.
The sodium concentration inside the epithelium cells decrease while the sodium in the ileum is high. This leads to passive diffusion of sodium ions into the epithelial cells (down the concentration gradient)
There is a low concentration of glucose/amino acids in the ileum and a high concentration in the epithelial cells
Co-transport proteins move sodium ions (passively diffusing down the concentration gradient) along side glucose/amino acids against their concentration gradients.
Now there is a high concentration of glucose/amino acids in the epithelial cells causing it to diffuse through the phospholipid bilayer by facilitated diffusion into the bloodstream.
Define osmosis
The passive movement of water from a region of high water potential to a region of low water potential through a selectively permeable membrane
Which direction does water always move?
Down the water potential gradient
What does the symbol mean 🔱 ?
Water potential
What does water potential mean?
It is a measure of how free water molecules are to move (potential to do osmosis)
What is the water potential of pure water ?
🔱 = 0
How does the addition of solute do to the water potential?
It reduces it to a negative value
Why does adding solvent reduce the water potential?
As water is dipolar it is attracted to the solvent charge so is less able to move
What does it mean when the solution is hypotonic
There is a higher water potential outside the cell due to a lower solute concentration (than on the inside) causing the net movement of water to move into the cell.
What does it mean when the solution is hypertonic
There is a lower water potential outside the cell due to a higher solute concentration (than on the inside) causing the net movement of water to move out of the cell.
What does it mean when the solution is isotonic
There is the same water potential in the cell and outside the cell so no movement of water.
What happens to animal cells in isotonic conditions?
There is no net movement of water so the cells remain the same
What happens to animal cells in hypotonic solutions
Water enters the cell which may cause it to burst (lyses)
What happens to animal cells in hypertonic solution?
Water leaves the cell causing it to shrink/shrivel ( crenation)
What happens to plant cells in isotonic conditions?
There is no net movement of water so incipient plasmolysis occurs (cells membrane starts to pull away from the cell wall
What happens to plant cells in hypotonic conditions?
The vacuole fills with water, turgid pressure develops and the chloroplasts are seen next to the cell wall
What happens to plant cells in hypertonic conditions?
Vacuoles lose water and the cytoplasm shrinks (plasmolysis) and chloroplasts appear at the center of the cell because the cell membrane has pulled away from the cell wall
