Biology AS Chapter 4 - Transport Across Cell Membranes Flashcards
What are the 3 functions of phospholipids in the cell surface membrane?
- Allow lipid soluble substances to enter and leave the cell
- Prevent water soluble substances entering and leaving the cell
- Make the membrane flexible and self sealing
What is the different between protein channels and carrier proteins?
Protein channels form water filled tubes to allow water soluble ions to diffuse across the membrane. Carrier proteins bind to ions and then change shape to move these molecules across the membrane
What are the 6 functions of proteins in the membrane?
- Provide structural support
- Act as channels to transport water soluble substances across the membrane
- Allow active transport through carrier proteins
- Form cell surface receptors for identifying cells
- Help cells adhere together
- Act as receptors
What are the 3 functions of cholesterol in the membrane?
- Reduce lateral movement of other molecules, including phospholipids
- Make the membrane less fluid at high temperatures
- Prevent leakage of water and dissolved ions from the cell as they are very hydrophobic
What are the 3 functions of glycolipids in the membrane?
- Act as recognition sites
- Help maintain the stability of the membrane
- Help cells attach to one another to form tissues
What are the 3 functions of glycoproteins in the membrane?
- Act as recognition sites
- Help cells attach to one another and so form tissues
- Allows cells to recognise one another, for example, lymphocytes can recognise an organism own cells
What are the 4 reasons why most molecules cannot freely diffuse across the membrane?
- Not soluble in lipids so cannot pass through the phospholipid bilayer
- Too large to pass through the channels in the membrane
- Of the same charge as the charge on the protein channels and so are repelled
- Electrically charged (polar) so cannot pass through non-polar hydrophobic tails in the phospholipid bilayer
Why is the fluid mosaic model named this way?
Fluid: individual phospholipid molecules can move relative to one another, meaning the membrane is flexible
Mosaic: the proteins embedded vary in shape, size, and pattern
Define diffusion
The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until they are evenly distributed
Define facilitated diffusion
The movement of molecules from an area of high concentration to low concentration, facilitated by transmembrane channels and carriers in the membrane. No ATP is required
Define osmosis
The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane
What is a solute?
Any substance dissolved in a solvent
What is water potential measured in?
kPa
Define active transport
The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
What are the 4 ways active transport differs from passive forms of transport?
- Metabolic energy in the form of ATP is required
- Substances are moved against a concentration gradient
- Carrier proteins are involved
- The process is very selective
Describe the process of the direct active transport of a single molecule/ion
- The carrier proteins span the plasma membrane and the molecule/ion binds to the receptor sites on the carrier protein
- On the inside of the cell/organelle, ATP bind to the protein, causing it to be hydrolysed into ADP and a phosphate molecule. As a result, the protein molecule changes shape and opens to the opposite side of the membrane
- The molecule/ion is then released to the other side of the membrane
- The phosphate molecule is released from the protein which causes it to revert back to its original shape, ready for the process to be repeated. ATP is then re-synthesised
How do microvilli increase the rate of transport across membranes?
The provide a larger surface area for the insertion of carrier proteins
Give a detailed explanation using an example of the role of diffusion in absorption in the ileum
As carbohydrates and proteins are being digested continuously, there is usually a greater concentration of glucose and amino acids in the ileum than the blood. Glucose therefore moves down the concentration gradient by facilitated diffusion. Given that the blood is constantly being circulated, the glucose absorbed is continuously being removed by cells as they use it up during respiration. This helps to maintain the concentration gradient between the inside of the ileum and the blood and so the rate of movement by facilitated diffusion across epithelial cell surface membranes is increased
How are glucose and amino acids absorbed from the small intestine when there is not a diffusion gradient?
Co-transport
Describe the process by which the sodium-potassium pump allows glucose and amino acids to pass into the blood (co-transport)
- Sodium ions are actively transported out of the epithelial cells, by the sodium-potassium pump, into the blood. This takes place in one type of protein carrier molecule found in the membrane of epithelial cells
- This maintains a much higher concentration of sodium ions in the lumen of the intestine than in the epithelial cells
- Sodium ions diffuse into the epithelial cells down this concentration gradient through a co-transport protein. As they diffuse through this and into the cell, they carry with them either amino acid or glucose molecules
- The glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier
What are 3 causes of diarrhoea?
- damage to the epithelial cells lining the intestine
- loss of microvilli due to toxins
- excessive secretion of water due to toxins
Why is drinking water to treat diarrhoeal diseases ineffective?
- water is not being absorbed from the intestine
- drinking water does not replace the electrolytes (ions) which are being lost from the intestinal cells
What does a rehydration solution need to contain and why?
- Water - to rehydrate the tissues
- Sodium ions - to replace those lost from the epithelium of the intestine and optimise the use of sodium-glucose carrier proteins
- Glucose - to stimulate the uptake os sodium ions from the intestine and provide energy
- Potassium ions - to replace those lost and stimulate appetite
- Other electrolytes - such as chloride ions and citrate ions to help prevent an imbalance
