Transport across Cell Membranes Flashcards
What is the fluid-mosaic model?
This is a model to describe the structure of cell membranes. The fluid is the phospholipids that are constantly moving via diffusion which form a bilayer. The mosaic is the proteins of different sizes and shapes embedded into the phospholipid bilayer.
What are the key components of cell membranes?
- Phospholipid bilayer - made up of phospholipids with hydrophilic phosphate heads and hydrophobic fatty acid tails so water-soluble substances can’t pass through
- Cholesterol - between phospholipid molecules to add stability by reducing fluidity of membrane
- Integral proteins - embedded into the membrane e.g. carrier and channel proteins
- Peripheral proteins - found on the outer surface of the membrane which can act as receptors and detect chemicals released from other cells
- Glycolipids - lipids with carbohydrate chains, which project out of the membrane for cell adhesion (attachment of two cells), recognition and signalling as receptor molecules
- Glycoproteins - proteins with carbohydrate chains, which project out of the membrane for cell adhesion (attachment of two cells), recognition and signalling as receptor molecules
What is a phospholipid bilayer?
- they are composed of two layers of hydrophobic fatty acid tails facing inwards and hydrophilic phosphate heads facing outwards, meaning water-soluble substances cannot pass through but lipid-soluble can
- they form compartments to establish the boundary of each cell and organelle
What are the roles of cell membranes?
- cell surface membranes - surround cells to act as a barrier between the cell and its environment, and controls what substances enter and leave
- membranes around organelles - surround organelles to act as a barrier between it and the cytoplasm and forms different compartments
- they are both partially permeable so some molecules can pass through but others can’t
What does cholesterol do?
- regulates the fluidity of the membrane
- prevents phospholipids from packing too closely together at low temperatures so they don’t freeze and fracture
- stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid as they bind to the hydrophobic tails so they pack more closely together
What is diffusion?
The net movement of a substance from a region of higher concentration to a region of lower concentration. This means particles move down a concentration gradient and the process is passive (no energy needed)
What are the two types of diffusion?
- simple diffusion - molecules can diffuse directly across cell membranes as they are small (so pass through spaces between phospholipids) or non-polar (as they are soluble in the phospholipid bilayer)
- facilitated diffusion - large (like glucose and amino acids) and polar (ions) molecules cross cell membranes with the help of carrier proteins and channel proteins
What factors affect the rate of diffusion and how?
- concentration gradient - the steeper the concentration gradient, the faster the rate of diffusion
- temperature - at higher temperatures, molecules and ions have more kinetic energy so they move faster, increasing the rate of diffusion
- surface area - the larger the surface area, the more particles can cross the membrane at once, increasing the rate of diffusion
- thickness of membrane - the thicker the membrane, the longer the particles have to travel, which takes longer so this decreases the rate of diffusion
- number of carrier and channel proteins - more proteins will increase the rate of facilitated diffusion
How does facilitated diffusion take place?
- Large molecules cross the cell membrane with the help of carrier proteins. The large molecule will attach to it, which causes the carrier protein to change shape and then will release the molecule on the opposite side of the membrane. This process can only take place down a concentration gradient.
- Ions cross the cell membrane with the help of channel proteins, which form pores in the membrane which ions can travel through. They can open and close.
What is osmosis in terms of concentration?
The diffusion of water molecules from a high concentration to lower concentration across a partially permeable membrane, meaning only small molecules, like water can cross it. Water moves down a concentration gradient, which makes it a passive process.
What is osmosis in terms of water potential?
It is the net movement of water molecules from a region of lower water potential (concentrated) to higher water potential (dilute) through a partially permeable membrane.
What are some factors that affect membranes?
- temperature - as the temperature increases, phospholipids have more kinetic energy so move faster and become more fluid. Below zero degrees, and above 45 degrees, phospholipids are packed closely together and the proteins can denature, which increases the permeability of the membrane
- solvents - non-polar molecules can insert themselves into the phospholipid bilayer, pushing the phospholipids out of their orderly placement, which increases its permeability. Solvents can also disrupt bonds in the proteins, which can denature them.
What is water potential?
It is the potential or likelihood of water molecules diffusing in or out of a solution. It is measured in kilopascals.
- high water potential is a low concentration of solute
- low water potential is a high concentration of solute
- pure water has a water potential of 0kPa and as more solute is added, this value becomes more negative
What factors affect the rate of osmosis?
- water potential gradient - the higher the water potential gradient, the faster the rate of osmosis
- membrane thickness - the thicker the membrane, the water molecules have to travel further so diffuse slower and the rate is decreased
- surface area - the larger the surface area, more molecules can cross at once and the faster the rate of osmosis
What types of solutions are there?
- hypotonic - solution has a higher water potential (more dilute) than the inside of the cell
- isotonic - solution has the same water potential than the inside of the cell
- hypertonic - solution has a lower water potential (more concentrated) than the inside of the cell
What happens during the osmosis of animal cells in different solutions?
- hypotonic - water will move into the cell by osmosis and the cell will swell up and eventually be lysed (burst) as the membrane cannot withstand the water pressure
- isotonic - there will be no net movement of water
- hypertonic - water will move out of the cell by osmosis and the cell will shrivel as they lose water from the cytoplasm
What happens during the osmosis of plant cells in different solutions?
- hypotonic - water will move into the cell by osmosis and the cell will swell up but doesn’t burst as the cell wall is strong, so it becomes turgid
- isotonic - there will be no net movement of water
- hypertonic - water will move out of the cell by osmosis and the cell is protected from shrinking although the vacuole will shrink and the cell membrane will pull away from the cell wall, and the cell becomes plasmolyzed
What are serial dilutions?
This is any dilution in which the concentration decreases by the same factor in each successive step.
Serial dilution = initial volume of the stock solution / final solution volume
What is active transport?
The movement of particles from an area of lower concentration to higher concentration, which is an active process that requires energy from respiration in the form of ATP. Active transport needs carrier proteins to change shape to move a molecule across membranes, which needs energy.
How do carrier proteins help active transport?
- the molecule or ion binds to the carrier protein
- ATP binds to the carrier protein
- ATP is hydrolysed to ADP and phosphate, which causes the carrier protein to change shape and release the molecule or ion on the opposite side of the membrane
- The phosphate is released from the carrier protein, which causes it to return to its original shape to be used again
What is co-transport?
Active transport uses special carrier proteins called co-transporters which can bind two molecules at the same time. An antiporter is a co-transporter that moves the two molecules in opposite directions and a symporter moves two molecules in the same direction.
What is an example of co-transportation?
Co-transportation of glucose in the small intestine:
1. sodium ions move out of the epithelial cells in the ileum to the blood by active transport by the sodium-potassium pump
2. a concentration gradient is created as there are more sodium ions in the ileum than blood
3. therefore, the sodium ions diffuse from the lumen in the ileum into the epithelial cell down a concentration gradient via sodium-glucose co-transporter and glucose enters the cell with the sodium
4. there is a higher concentration gradient of glucose inside the epithelial cell than blood so glucose diffuses into the blood down a concentration gradient via a protein channel by facilitated diffusion
What factors affect the rate of active transport?
- speed of carrier proteins - the faster they work, the faster the rate of active transport
- number carrier proteins - the more proteins, the faster the rate of active transport
- rate of respiration - the more respiration, the more ATP available so the faster the rate of active transport
What is exocytosis and endocytosis?
Exocytosis - how cells transport signalling chemicals or waste products from inside the cell to outside in vesicles, which fuse with the plasma membrane
Endocytosis - various types of active transport that move particles into a cell by enclosing them in a vesicle made out of plasma membrane. This forms a pocket around the target particles, which is pinched off by proteins, leaving it enclosed.
