Transport Across Cell Membranes Flashcards
Can you explain what’s in the fluid mosaic model and their function?
Glycoproteins- branching carbohydrate portion of a protein which acts as a recognition site for chemicals e.g. hormones.
Glycolipid- acts as a recognition site e.g. for cholera toxins.
Cholesterol- adds strength
A protein molecule was partly embedded.
Hydrophobic tails of phospholipid molecules- point inwards.
Channel protein molecules spanning the phospholipid layer.
Protein molecule lying on the surface.
Hydrophilic heads of phospholipid molecules- point outwards.
The role of cholesterol in the phospholipid bilayer.
Very hydrophobic so play a role in preventing the loss of water and dissolved ions from cell.
Pull together fatty acid tails of phospholipid molecules, limiting their movement and movement of other molecules without making the membrane too rigid.
Make membrane less fluid at high temperatures.
Simple diffusion
For substances that are small and non-polar.
Only lipid-soluble molecules can pass through the phospholipid bilayer.
Net movement of molecules or ions from an area of high to low concentration.
E.g. Oxgen, carbon dioxide, urea.
Water (small but polar)
Fatty acids (large but non-polar).
Facilitated diffusion
Plasma membranes aren’t permeable to large, polar molecules because the phospholipid tails in the membrane are hydrophobic, only small non-polar molecules that are lipid soluble.
The movement of molecules that can’t enter by simple diffusion is made easier by protein channels and carrier proteins.
Facilitated diffusion is a passive process and relies on kinetic energy of the diffusing molecules (no input of ATP).
Occurs down a concentration gradient through protein channels and carrier proteins.
Protein channels
Form hydrophilic channels across the membrane that allow specific water soluble ions to pass through.
The channels only open in the presence of a specific ion and remains closed if not present: the ions bind with the protein and the protein changes shape in a way that closes it to one side and opens it to the other side.
Carrier proteins
When a molecule that is specific to the protein (e.g. glucose) is present, it binds with the protein.
This causes it to change shape in a way that the molecule is released to the inside of the membrane.
No external energy needed (ATP)
From a region of high to low concentration using kinetic energy from diffusing molecules.
Osmosis
Higher water potential to lower water potential through selectively permeable membrane. (Down a water potential gradient).
Permeable to water molecules and small molecules but not to larger molecules.
Addition of solute to pure water lowers the water potential because the solution is more concentrated.
More negative the value, the lower the water potential.
When water potentials on either side are equal, dynamic equilibrium is established and there’s no net movement of water.
Active transport
Movement of molecules into/ out of a cell from low to high concentration (against concentration gradient) using energy from ATP and carrier proteins.
- Molecule/ion binds to receptor site on carrier protein.
- On inside of cell, ATP binds to protein causing it to split to ADP and inorganic phosphate so the protein molecule changes shape and opens to the opposite side of the membrane.
- Molecule/ ion released to other side of membrane.
- The phosphate is released from the carrier protein, reverting the protein to the original shape ready to repeat the process.
- Inorganic phosphate recombines with ADP to form ATP during respiration.
Cotransport
- Sodium ions transported out of epithelial cells by the sodium potassium pump into the blood. This takes place in a protein carrier molecule in the cell surface membrane.
- This maintains a higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cell.
- Sodium ions diffuse into epithelial cells down concentration gradient through cotransport protein in the cell surface membrane. As sodium ions diffuse in through the 2nd carrier protein, they carry amino acid molecules or glucose molecules into the cell with them. Glucose/ amino acids move against their concentration gradient. The sodium ion concentration gradient powers the movement of glucose or amino acids into the cell (indirect active transport).
- The glucose/ amino acids pass into the blood plasma by facilitated diffusion using another type of carrier.
How are cells adapted for rapid transport across their internal or external membranes?
The epithelial cells lining the ileum posses microvilli: finger-like projections of the cell surface membrane around 0.6 micrometres long. Microvilli provide more surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can take place.
Increasing the number of protein channels and carrier proteins in any given area of membrane also increases the transport across the membrane.
How does differences in concentration gradient increase the overall movement across plasma membranes?
As carbs and proteins are digested continuously, there’s a larger concentration of glucose/ amino acids in the ileum than in the blood. There’s therefore a concentration gradient down which glucose moves by facilitated diffusion from the ileum to the blood. This glucose absorbed is continuously removed as blood is circulated by the heart- it’s removed by cells as they use it during respiration. This helps maintain the concentration gradient between the ileum and the blood and means the movement by facilitated diffusion across the cell surface membrane increases.
How does water potential affect the rate of movement across cell membranes.
In osmosis, water molecules diffuse from an area of high water potential to an area of lower water potential across a semi-permeable membrane until the water potentials on either side of the membrane are equal. A dynamic equilibrium is reached and there’s no net movement of water. Animal cells are living in liquid with the same water potential outside the cell as inside the cell so it doesn’t shrivel or burst.
Osmosis and plant cells
Water entering by osmosis causes the protoplast to swell and press on the cell wall. The cell wall is only capable of limited expansion so a pressure builds up on it that resists the further entry of water. The protoplast is then kept pushed against the cell wall keeping the cell turgid.
- If the plant cell is placed in a solution with lower water potential, water leaves by osmosis and the volume of the cell decreases.
- When the protoplast is no longer pressed on the cellulose cell wall, the cell is at incipient plasmolysis.
- Further loss of water will cause the cell contents to shrink further and the protoplast pulls away from the cell wall. The cell is plasmolysed.
What happens to plant cells when placed in pure water?
Water enters by osmosis across semipermiable membrane because of the lower (more negative) water potential inside the cell.
External solution water potential is equal to cell.
Water neither enters or leaves.
Protoplast beginning to pull away from the cell.
Cell is in incipient plasmolysis.
