Transport across membranes Flashcards
What kind of transport is simple diffusion known as?
Simple
Why is simple diffusion known as passive transport?
The energy for the movement comes from the natural, inbuilt motion of particles, rather than other sources such as ATP
Define some necessary details of particles
- All particles are constantly in motion due to the kinetic energy they posess
- This motion is random
- Particles are constantly bouncing off one another as well as other objects
What will particles that are concentrated together in a closed vessel do?
Distribute themselves evenly throughout the vessel as a result of diffusion
What is 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 evenly distributed
Which molecules can easily pass through the cell-surface membrane?
Small, non-polar molecules e.g. water and carbon dioxide
Why do charged ions and polar molecules not diffuse easily across a cell-surface-membrane?
Due to the hydrophobic nature of the fatty acid tails
How is the movement of charged ions and polar molecules made easier?
Through facilitated diffusion. The movement is made easier by transmembrane channels and carriers that span the membrane
What kind of process is facilitated diffusion?
Passive
It relies only on the inbuilt motion of the diffusing molecules. No external input of ATP from respiration
Where does facilitated diffusion occur?
It occurs down a concentration gradient but it occurs at specific points on the plasma membrane where there are special protein molecules
Which types of proteins are involved in facilitated diffusion?
Protein channels and carrier proteins
How do protein channels help in facilitated diffusion?
These proteins form water-filled hydrophilic channels across the membrane
What do protein channels allow to diffuse across?
Specific water-soluble ions
The channels are selective, each opening in the presence of a specific ion. If the particular ion is not present, the channel remains closed
What do protein channels allow for in cell-surface membranes?
They allow for control over the entry and exit of ions. The ions bind with the proteins causing it to change shape in a way that closes it to one side of the membrane and opens it to the other
What is an alternative way, aside from protein channels, for facilitated diffusion?
Another way involves carrier proteins that span the plasma membrane
How do carrier proteins carry out facilitated diffusion?
- When a molecule such as glucose that is specific to the protein is present, it binds with the protein
- This causes it to change shape so that the molecule is released to the inside of the membrane
- No external energy is needed
- The molecules move from a region where they are highly concentrated to one of lower concentration, using only the kinetic energy of the molecules themselves
How can cells be adapted for rapid transport across their internal or external membranes?
By an increase in surface area, or by an increase in the number of protein channels and carrier molecules in their membranes
What is 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 does selectively permeable mean?
Permeable to water molecules and a few other small molecules, but not larger molecules (cell - surface membranes and other plasma membranes are selectively permeable)
What is a solute?
Any substance dissolved in a solvent
What do the solute and the solvent form together?
A solution
What is water potential and what is it measured in?
It is the pressure created by water molecules and it is measured in units of pressure (usually kPa)
It is represented by the greek letter psi
When does pure water have a potential of zero?
Under standard conditions of temperature and pressure (25 degrees celsius and 100 kPa)
What will lower the water potential of pure water?
The addition of solute
What value must the water potential of a solution always be?
A negative value - less than zero
If you add more solute to a solution, what will happen?
Its water potential will get lower
How do you find the water potential of cells or tissues?
- Place them in a series of solutions of different water potentials
- When there is no net gain or loss of water from the cells or tissues, the water potential inside the cells/tissues is the same externally
What do animal cells contain in their watery cytoplasm?
A variety of dissolved solutes
What will happen if a RBC is placed in pure water?
It will absorb water by osmosis as it has a lower water potential
Why will the cell surface membrane break is a RBC is placed in pure water?
- They are very thin and although they are flexible, they cannot stretch to any great extent
- It will therefore break, bursting the cell and releasing its contents
- In RBC’s, this is called haemolysis
How is the bursting of animal cells prevented?
They normally live in a liquid which has the same water potential as the cells
What will happen if a RBC is placed in a solution with a water potential lower than its own?
Water leaves by osmosis and the cell shrinks and becomes shrivelled
What is the protoplast?
It consists of the outer cell-surface membrane, nucleus, cytoplasm and the inner vacuole membrane
What happens when a plant cell is placed in pure water?
- Water is absorbed by osmosis
- So the protoplast swells and presses on the cell wall
- The cell wall is only capable of very limited expansion and so a pressure builds up on it that resists the entry of further water
- So, the protoplast is kept pushed against the cell wall
- And the cell is said to be turgid
What happens when a plant cell is placed in a solution with the same water potential as itself?
No water leaves or enters by osmosis
The cell is said to be at incident plasmolysis
What happens when a plant cells is placed in a solution with a lower water potential than itself?
- Loss of water by osmosis will cause the cell contents to shrink and the protoplast will pull away from the cell wall
- The cell is said to be plasmolysed
What is 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 is ATP used for in active transport?
- To directly move molecules
- Individually move molecules using a concentration gradient which has already been set up by (direct) active transport (known as co-transport)
How does active transport differ from passive transport?
- Metabolic energy in form of ATP is needed
- It’s against a concentration gradient
- Carrier protein molecules which act as ‘pumps’ are involved
- The process is very selective, with specific substances being transported
What do carrier proteins do in terms of active transport?
They span the plasma membrane and bind to the molecule or ion to be transported on one side of it
What does the molecule/ion do at the beginning of active transport?
It binds to receptor sites on the carrier protein
What does ATP cause to happen on the inside of the cell at the beginning of active transport?
- ATP binds to the protein
- It splits into ADP and a phosphate molecule
- As a result, the tertiary structure of the protein molecule changes shape and opens to the opposite side of the membrane
What happens as a result of the protein changing shape in active transport?
The molecule/ion is then released to the other side of the membrane
What happens to the phosphate molecule bound to the carrier protein after active transport?
- It is released from the protein which causes the protein to revert to its original shape, ready for the process to be repeated
- The phosphate molecule then recombines with the ADP to form ATP during respiration
Can more then one molecule/ion be moved in the same direction at the same time in active transport?
Yes
Can a molecule/ion move into a cell at the same time as a different one is moving out of the cell in active transport?
Yes
What is the sodium-potassium pump?
Sodium ions are actively removed from the cell/organelle while potassium ions are actively taken in from the surroundings
Why is the sodium-potassium pump an important process?
It is essential to a number of important processes in the organism, including the creation of a nerve impulse
What do the epithelial cells lining the ileum possess that increases the rate of transport?
Microvilli
What are microvilli?
Finger like projections of the cell-surface membrane about 0.6 micrometers in length
What are microvilli collectively termed as?
A ‘brush border’
(when viewed under a microscope, they look like the bristles on a brush)
How do microvilli increase transport across membranes?
They provide more surface area for the insertion of carrier proteins
Apart from increased surface area, what mechanism can also be used to increase transport across membranes?
Increase the number of protein channels and carrier proteins in any given area of membrane
Why is there normally a greater concentration of glucose and amino acids in the ileum than in the blood?
Carbohydrates and proteins are being digested continuously
How does glucose move into the blood from inside the ileum?
There is a concentration gradient so it moves down by facilitated diffusion
What is the effect of constant circulation of blood on glucose?
- As it is constantly circulated, the glucose moved into it is continuously being removed by the cells as they use it up during respiration
- This helps to maintain the concentration gradient between the inside of the ileum and the blood
- So the rate of movement by facilitated diffusion is increased
Why can’t all the available glucose and amino acids be absorbed into the blood by diffusion?
- Diffusion only results in the concentrations either side of the intestinal epithelium cell becoming equal
- Not all is absorbed into the bloodstream and some may pass out of the body
How else are amino acids and glucose absorbed into the blood aside from diffusion?
By active transport
What is the actual mechanism for the absorption of glucose and amino acids into the blood from the small intestine?
It is an example of co-transport
Why is the term co-transport used for the absorption of glucose and amino acids into the blood from the small intestine?
As either glucose or amino acids are drawn into the cells along with sodium ions that have been actively transported out by the sodium-potassium pump
How is a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells maintained?
- Sodium ions are actively transported out of epithelial cells, by the sodium potassium pump, into the blood
- This takes place in one type of protein carrier molecule found in the cell surface membrane of epithelial cells
How do the sodium ions help glucose molecules to move into the epithelial cells?
- Sodium ions diffuse into the cells down this concentration gradient through a different type of protein carrier (co-transport protein) in the cell-surface membrane
- As the sodium ions diffuse in through this second carrier protein, they carry either amino acids or glucose into the cell with them
- They carry amino acids/glucose/galactose/fructose with them as otherwise, they can’t diffuse in as the channel proteins will not permit it
How do the glucose/amino acid molecules travel into the blood after being taken into the epithelial cells by the sodium ions?
Through facilitated diffusion using another type of carrier
What kind of active transport is the absorption of glucose molecules into the blood from the intestinal lumen known as?
Indirect active transport
Why is this known as indirect active transport?
(What kind of active transport is the absorption of glucose molecules into the blood from the intestinal lumen known as?)
- Both sodium ions and glucose/amino acids move into the cell, but sodium ions move down a concentration gradient whilst glucose/amino acids move against a concentration gradient
- It is the sodium ion concentration gradient, rather than ATP directly, that powers the movement of glucose/amino acids into the cells
