Topic 4: Transport across membranes Flashcards
What is the cell-surface membrane?
- separates cytoplasm on the inside of cell and the environment.
- Controls movement of substances in and out of cell
What is the cell surface memrane made out of?
- Phospholipids
- Proteins
- Cholesterol
- Glycolipids
- Glycoproteins
Describe and explain the structure of the phospholipid bilayer
- Hydrophillic heads pointing to the outside of the membrane, because they are attracted to water
- Hydrophobic tails pointing inside, because they are repelled by the water.
Functions of phospholipids
- To allow lipid-soluble substances through (small, non-polar)
- Make the membrane flexible and self-healing
- Prevent water soluble substances from entering and leaving
What are the main two ways that proteins are integrated into the membrane
- Only through one layer of the membrane, giving mechanical support or acting as receptor molecules for homones
- Stretch through the whole phospholipid bilayer (carrier / channel proteins)
Functions of proteins in the membrane
- Provide structural support
- Act as channels for transport
- Allow active transport
- Form cell-surface receptors for identification
- Attatch cells to each other
- Acts as receptors
What is cholesterol
Steroid molecule which is within the phospholipid bilayer, adding strength to the membrane. cholestrol is hydrophobic, so it stops water soluble substances moving across, and also adds anther hydrophobic interaction holding the tails together.
Functions of cholesterol
- Reduce movement across the membrane of other molecules (water soluble)
- Make the membrane more fluid at higher temperatures.
- Prevent leakage of water and dissolved ions from the cell.
What is a glycolipid?
what its made from, where its found, what it does
Carbohydrate covalently bonded with a lipid, with the carbohydrate extending from the phospholipid bilayer into the watery environment. Acts as a receptor for certain chemicals
Functions of glycolipids
- Act as identification / recognition sites
- Help maintain stability of the membrane
- Help cells attatch to one another, and so form tissues
What is a glycoprotein?
- Carohydrate chains are attatched to many proteins on the outer surface of the membrane
- Glycoproteins acts as cell-surface receptors for hormones and neurotransmitters
Functions of glycoproteins
- Act as recognition sites
- Help cells stick together
- Allow cell-recognition
What things affect the rate of diffusion
- Temperature
- SA
- Szie of the particle
- Nature of particle (polar non polar)
Define facillitated diffusion
The transport of substances across a membrane by a specifiic trans-membrane protien molecule.
Facillitated diffusion is still passive
Used when molecules too big
What are Channel proteins
+ What do they do
Form a water-filled pore or channel in the membrane. This allows charged substances (usually ions) to diffuse across membranes. Most channels can be gated (opened or closed), allowing the cell to control the entry and exit of ions.
What are Carrier proteins
+ what do they do
-have a binding site for a specific solute and constantly flip between two states so that the site is alternately open to opposite sides of the membrane. The substance will bind on the side where it at a high concentration and be released where it is at a low concentration.
Define Osmosis
Osmosis is the net movement of water from a high water potential (ψ) to a low water potential through a semi permeable membrane.
How does adding a solute to a solution affect the water potential
Water can move through the membrane freely as it is a small molecule.
However, when solutes are added to a solution, water surrounds them, interacting with the charged regions, forming **hydration shells. **
Water is then unable to move freely. The potential for water to move from that region has been reduced thus.
Define a concentrated solution
A solution rich with solute and a low water potential
Define a dilute solution
A solution with a low solute concentration, and with a high water potential
What is the water potential of pure water
0
In what direction does water move in reference to the water potential
Water will move from a high to low concentration, aka from a less negative to a more negative water potential
What are the three possible concentration (states) of a solution
- Isotonic
- Hypertonic
- Hypotonic
Define an isotonic solution
Solution of equal concentration to the cell
Define a Hypertonic solution
Solution of higher concentration than inside the cell, (water moves out)
Define a Hypotonoc solution
Lower concentration of solution than inside the cell, so the cell gains water
Define active transport
Movement of molecules from an area of low concentration to high concentration, using ATP and carrier proteins
How does the protein pump briefly work
- The protein binds a molecule on one side of the membrane
- changes shape
- releases it on the other side.
How many protein pumps are there for each molecule and why
These proteins are highly specific, so there is a different protein pump for each molecule to be transported. There are as many as possible to allow for maximum efficiency and maximum transport
How does the carrier protein change shape
The carrier protein is an ATPase enzyme, which breakd ATP into ADP + Phosphate, which releases energy. This release of energy will change the shape of the carrier protien, and pump the molecule.
In what ways is active transport different to passive transport (+ facillitated diffusion)
- Moves molecules against the concentration gradient (low to high)
- Using a specific carrier protein
- The molecule binds to the protein
- ATP is used to cause a conformational change in the shape of the carrier to move the molecule
Where is the sodium potassium pump found?
Every cell of the body
What and how does the sodium potassium pump transport
- Actively transports three Na+ ions out of the cell
- Passively transports two K+ ions into the cell
What can you also call co-transport
It is a specififc case of (secondary) active transport
How does co-transport allow for the movement of molecules such as glucose
By maintaining a concentration gradient using sodium cations, and then this gradient drives the transport of other molecules
What does the NA-K-pump actively transport out of the cell?
Three Na+ Ions
What does NA-K-pump passively transport into the cell?
Two K+ ions
Explain co-transport
(in detail)
- The co-transport antiport protein (Na/K Pump) pumps Na+ out of the epithelial cells, K+ into the cell using energy released by hydrolysis of ATP. Large build-up of Na+ in the lumen of the gut.
- The facillitated diffusion symport protein, (Na Glucose transporter) only found in the membrane of the epithelial cells of the ileum. Protein has two binding sites for glucose and Na, diffuse down conc gradient
- One direction due to the conc gradient, drives the sodium-glucose cotransporter.
- Sodium ions diffuse down conc gradient, glucose molecules carried up conc gradient
- Glucose diffuses through the epithelial cells, into tissue fluid of villus by facillitated diffusion through a carrier protein that in only found on the inner surface of the epithelial cells.
- Glucose enters blood capillary by diffusing through gaps between capillary endothelial cells. Glucose carried in the blood to every cell, needed for respiration
