Bio Unit 1.3 - Cell Membranes and Transport Flashcards
Why are the hydrophilic heads pointing in important in the phospholipid bilayer?
Interacts with water in the cytoplasm
Why are the hydrophilic heads pointing out important in the phospholipid bilayer?
Interacts with water surrounding the cell
Which way do hydrophobic tails point?
Toward each other, to centre of membrane
What does the phospholipid component of the bilayer allow to cross?
Allows lipid soluble molecules across but not water soluble molecules
Extrinsic Proteins (on surface of bilayer)
Provide structural support and form recognition sites (by identifying cells) and receptor sites hormone attachment
Intrinsic Protein (extends across both layers)
Include transport proteins which use active/passive transport to move molecules/ions across cell membrane
2 reasons why phospholipid bilayer is known as ‘fluid-mosaic’ model
- Fluid - individual phospholipid molecules can move within layer relative to one another
- Mosaic - proteins embedded in bilayer vary in shape and size and in their distribution
What do both plant and animal cells contain in their cell membrane?
Sterols, glycoproteins, glycolipids
What is the sterol in animal membrane?
Cholesterol
What does cholesterol do?
Occurs between phospholipid molecules, making membrane more stable to high temperatures and more fluid at low temperatures
What is the carbohydrate layer around animal cells and what does it do?
Glycocalyx - works as hormone receptors
How can small molecules travel across the membrane? (O2 and Co2)
Move between phospholipid molecules and diffuse across membrane
How do lipid soluble molecules (vitamin A) move across?
Dissolve in phospholipid and diffuses across membrane. The phospholipid layer is hydrophobic so lipid soluble molecules move through easier.
How do water soluble molecules pass through the membrane?
Must pass through intrinsic proteins forming water filled channels across the membrane (why cell surface is selectively permeable)
Where do glycoproteins connect to?
Extrinsic Proteins
Diffusion
The passive movement of a molecule/ion down a concentration gradient from a region of high concentration to a region of low concentration until they are equally distributed
How does concentration gradient affect rate of diffusion?
Greater difference in concentration of molecules in two areas - more molecules diffuse in given time
What affects rate of diffusion?
Concentration gradient, thickness of exchange surface, surface area of membrane
Other irregular factors that might affect rate of diffusion?
Size of molecule, nature of diffusing molecules, temperature
Facilitated Diffusion
Passive transfer of molecules down a concentration gradient across a membrane by channel or carrier protein molecules in the membrane
Where does facilitated diffusion occur?
Sites on membrane where transport proteins are present (numbers limit rate of facilitated diffusion.)
2 types of facilitated diffusion?
Channels proteins and Carrier Proteins
Channel Protein facilitated diffusion
Molecules with pores lined with polar groups, channels are hydrophilic so water soluble ions can pass through
Carrier Protein facilitated diffusion
Allows diffusion of larger polar molecules (sugars and amino acids)
How does carrier protein f.d work?
Molecules attaches to its binding site, protein changes shape and releases molecule on the other side of membrane before resuming original shape
Do carrier proteins allow active transport?
Yes
What different molecules do channel and carrier proteins transport?
Channel-water soluble
Carrier - soluble and insoluble
Active Transport
Movement of molecules against a concentration gradient using energy from hydolysis of ATP made by cell respiration
Features of active transport
- Low to high concentration
- Requires energy from ATP
- Occurs through intrinsic carrier proteins
- Rate is limited by availability of carrier proteins
Process of Active Transport through Carrier Protein
- molecule combines with specific carrier protein outside the membrane
- ATP transfers phosphate group to the carrier protein on inside of membrane
- Carrier protein changes shape and takes molecule across membrane
- Molecule released into cytoplasm
- Phosphate ion is released from carrier molecule back to cytoplasm and forms ATP
- Carrier protein returns to original shape
Effect of cyanide?
Respiratory inhibitor, will prevent aerobic respiration and without production of ATP active transport cannot take place
Co transport
A transport mechanism in which facilitated diffusion brings molecules and ions (glucose and Na+ ions) across cell membrane together (on the same transport protein molecule) into cell
Why is glucose and sodium co transport important?
Significant for absorbing these substances across cell membranes and into the blood in the ileum and kidney nephron
Process of co-transport
- Glucose/Sodium ions bind to carrier protein in cell membrane
- Carrier changes shape and deposits both inside the cell (facilitated diffusion)
- Both substances separately diffuse through cytoplasm to the opposite membrane
- Glucose passes into blood by facilitated diffusion
- Sodium ions carried out of epithelial cell via active transport by same carrier that moves potassium in
Why is sodium level low in epithelial cell?
S0 more sodium ions can move in and bring in more glucose (so facilitated diffusion can continue)
Water Potential
measure of free energy of water molecules and the tendency of water to move (kPa)
Osmosis
The net passive diffusion of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential
What is water potential of pure water?
Zero
What does addition of solute to pure water do to water potential?
Lowers it into a negative value
Higher concentration impact on water potential
More concentrated, fewer water molecules free to move, pull on water molecules greater, water potential more negative
Solute Potential
Measure of osmotic strength of a solution. The reduction of water potential due to the presence of solute molecules
More solute impact of solute potential?
more solute, more tightly water molecules are held, lower tendency of water to move out therefore higher concentration has more negative solute potential
Pressure potential
Pressure of when water enters cell by osmosis and expands the vacuole, pushing against the cell wall. Cell wall can only expand a little so pressure outwards builds up, resisting entry of more water (cell becomes turgid)
What does pressure potential increase?
Increases tendency of water to move out
Potential Equation
Solute potential + Pressure potential = Water potential
If water potential of external solution is less negative than inside solution?
External solution is hypotonic to cell and water flows in.
If water potential externally is more negative than inside then?
External solution is hypertonic to cell and water flows out
Same water potentials externally and internally?
External solution and cell are isotonic
Plasmolysis
Retraction of cytoplasm and cell membrane from cell wall as cell loses water by osmosis
Incipient Plasmolysis
Cell membrane and cytoplasm are partially detached from cell wall due to insufficient water to make cell turgid
After playmolysis…
Flaccid
Turgid
Cytoplasm pushes against cell wall
Why is pressure potential not considered in animal cells?
No cell wall to be pushed against
Hameolysis
Bursting of (eg. red blood cells) without a cell wall after water enters through osmosis
When will red blood cell burst?
In pure water (too much water moves in)
When will cell shrink?
In concentrated solution (too much water moves out)
Endocytosis
active process of cell membrane engulfing material, bringing it into cell in a vesicle
Exocytosis
active process of vesicle fusing with cell membrane , releasing the molecule it contains
2 types of endocytosis
Phagocytosis and Pinocytosis
Phagocytosis
Uptake of solid material too large to be taken in by diffusion
Granulocytes engulf bacteria, a lysosome fuses with vesicle formed and enzymes digest cells, products absorbed into cytoplasm
What does the cell membrane have to do when exocytosis and endocotosis take place?
Change shape and requires energy (uses ATP produced by respiration)
Pinocytosis
Uptake of liquid by the same mechanism (vesicles are smaller though)
