B4 Transport Across Cell Membranes Flashcards
(42 cards)
What are membranes described as
The membranes are described as a fluid-mosaic model due to the mixture and movement of the phospholipids, proteins, glycoproteins and glycolipids it is made of.
All of the these molecules arranged within the phospholipid bilayer create the partially permeable membrane, that is the cell-surface membrane.
Components of membrane
The phospholipids align as a bilayer due to the hydrophilic heads being attracted to water and the hydrophobic tails being repelled by water.
Cholesterol in membrane
Cholesterol is present in some membranes too and this will restrict the lateral movement of other molecules in the membrane. This is useful as it makes the membrane less fluid at high temperatures and prevents water and dissolved ions leaking out of the cell.
Proteins in membrane
Proteins are embedded across the cell surface membrane either peripheral (do not extend completely across the membrane) and integral (span across from one side of the bilayer to the other).
The peripheral proteins provide mechanical support, or they are connected to proteins or lipids to make glycoproteins and glycolipids. The function of these is cell recognition, as receptors.
The integral proteins are protein carriers or channel proteins involved in the transport of molecule across the membrane.
Protein channels form tubes that fill with water to enable water-soluble ions to diffuse, whereas the carrier proteins will bind with other ones and larger molecules, such as glucose and amino acids, and change shape to transport them to the other side of the membrane.
Partially permeable
Molecules that pass through the plasma membrane:
Lipid soluble substances (e.g. some hormones) and very small molecules (e.g. CO2, 02, H20)
Molecules that cannot pass through the membrane:
Water soluble (polar) substances (Sodium ions) and large molecules (glucose)
What are the 4 key types of transport across membranes
Simple diffusion
Facilitated diffusion
Active transport
Osmosis
Simple diffusion
This is the net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached. This process does not require ATP.
For the molecules to move they do have energy, but this is due to the kinetic energy that they possess to enable them to constantly move in fluids.
For molecules to diffuse across the membrane they must be lipid soluble and small.
Facilitated diffusion
This is a passive process (it does not require ATP) but it differs from simple diffusion as proteins are used to transport molecules. The movement of ions and polar molecules, which cannot simply diffuse, can be transported across membranes by facilitated diffusion using protein channels and carrier proteins.
Protein channels form tubes filled with water and this enables water-soluble ions to pass through the membrane. This is still selective, as the channel proteins only open in the presences of certain ions when they bind to the protein.
Carrier proteins will bind with a molecule, such as glucose, which causes a change in the shape of the protein. This shape change enables the molecule to be released to the other side of the membrane.
What’s osmosis
Osmosis is the movement of water from an area of higher water potentials to an area of lower water potential across a partially permeable membrane.
Water potential is the pressure created by water molecules and is measured in kPa and represented with the symbol . Pure water has a water potential of zero, so when solutes are dissolved in water the water potential will become negative. The more negative the water potential, the more solute must be dissolved in it.
What is an isotonic solution
An isotonic solution is when the water potential is the same in the solution and the cell within the solution.
What is a hypotonic solution
Hypotonic is when the water potential of a solution is more positive (closer to zero) than the cell.
What is a hypertonic solution
Hypertonic is when the water potential of a solution is more negative than the cell.
What happens when animal cells are placed in hypertonic/hypotonic solutions
In animal cells, if they are placed in a hypotonic solution such as pure water, a lot of water will move into the cell by osmosis.
As animal cells do not have a cell wall the pressure will cause the cell to burst, plants cells do not because of the strengthened cell wall and instead become turgid.
Both animal and plant cells will shrink and become shrivelled if they are placed in hypertonic solutions, due to large volumes of water leaving the cell by osmosis.
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.
The carrier proteins act as pumps to move substances across the membrane. This is very selective, as only certain molecules can bind to the carrier proteins to be pumped.
Certain molecules can bind to the receptor site on carrier proteins. ATP will bind to the protein on the inside of the membrane and is hydrolysed into ADP and Pi.
This causes the protein to change shape and open towards the inside of the membrane. This causes the molecule to be released on the other side of the membrane.
The Pi molecule is then released from the protein, and this results in the protein reverting to its original shape. This is how ATP and carrier proteins are used in active transport.
What’s active transport
The movement of a substance from a low concentration to a high concentration using metabolic energy and a carrier protein.
Process of active transport
I. Transport is through carrier proteins spanning the cell membrane
- Molecule binds to a receptor complementary in shape on the protein
- ATP binds to the carrier protein from the inside of the cell and it is hydrolysed into ADP +Pi
- This causes the carrier protein to change shape and release the molecule to the other side
- The phosphate ion is then released and the protein returns to its original shape
Co-transport of glucose and Na+ in ileum - process
I. Sodium ions are actively transported out of the epithelial cell into the blood
2. This reduces the sodium ion concentration in the epithelial. cell.
3. Sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell
4. The protein the sodium ions diffuse through is a co-
transported protein, so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient
5. Glucose then moves by facilitated diffusion from the epithelial cell to the blood.
CO-TRANSPORT OF GLUCOSE AND SODIUM IONS IN THE ILEUM
To absorb glucose from the lumen to the gut there must be a higher concentration of glucose in the lumen compared to the epithelial cell (for facilitated diffusion).
BUT
There is usually more glucose in the epithelial cells.
That is why active transport and co-transport are required!
Extra points about CO-TRANSPORT OF GLUCOSE AND SODIUM IONS IN THE ILEUM
The concentration of glucose in the blood is lower than in the epithelial cells because the blood flows and carries away absorbed glucose.
Microvilli on the epithelial cell increase the surface area for co-transporter proteins.
Why is active transport across a carrier protein possible
Active transport across a carrier protein is possible because when ATP is hydrolysed to ADP Pi it causes the carrier protein to change shape and release molecules to the other side of the cell membrane.
Describe fluid mosaic model of membranes
Fluid: phospholipid bilayer in which individual phospholipids can move = membrane has flexible shape
Mosaic: extrinsic & intrinsic proteins of different sizes and shapes are embedded
Explain role of cholesterol & glycolipids in membranes
Cholesterol : steroid molecule is some plasma membranes; connects phospholipids and reduces fluidity to make bilayer more stable
Glycolipids = cell signalling and cell recognition
Explain role of extrinsic and transmembrane proteins in membranes
Extrinsic:
- binding sites/receptors e.g. for hormones
- antigens (glycoproteins)
- bind cells together
- involved in cell signalling
Intrinsic:
- electron carriers (respiration/photosynthesis)
- channel proteins (facilitated diffusion)
- carrier proteins (facilitated diffusion/active transport)
Explain functions of membranes within cells
- provide internal transport system
- selectively permeable to regulate passage of molecules into/out of organelles
- provide reaction surface
- isolate organelles from cytoplasm for specific metabolic reactions
