topic 4 Flashcards
what do organisms need to exchange in their envoirnments?
- Gases (oxygen and carbon dioxide)
- Nutrients (glucose, amino acids, fatty acids, vitamins, minerals)
- Waste products (urea and carbon dioxide)
- Heat
- All of these must be exchanged through the body surface in some way, either passively (diffusion or osmosis) or actively (active transport
how does sa:volume ratio change as organisms get larger
- As organisms get larger their volume increases by the third power of the increase in size. Their surface area on the other hand only increases by the second power.
- This means that larger organisms ‘do not have as much’ surface area for each unit of volume and so cannot rely upon simple exchange through their surface.
name solutions to SA:V issues
- There are two possible solutions to the SA:V problem:
- You can be very small i.e. single-celled organisms
- You can have a low metabolic rate
- Additionally, there is the problem that as organisms get larger, the cells inside are too far away from the surface for diffusion to reach easily. Adaptations are needed:
- You can have a flattened body i.e. the tapeworm
- You can evolve specialised exchange surfaces and transport systems to go with them i.e. lungs/gills and the circulatory system,
describe how a surface can be effective at exchanging substances
- For a surface to be effective at the exchange of substances it ought to have the following characteristics:
- Large surface area
- Thin surface
- Permeable to the substance
- Maintenance of the concentration gradient through the movement of either/both the internal and external media.
- Exchange surfaces in larger organisms are often found protected inside the organism.
describe the difference between active and passive transport
- Diffusion and Osmosis are forms of Passive Transport.
- This means that the energy originates from the inherent kinetic energy of the particles involved.
- Active Transport is an Active process.
- This means that the energy must come from some external source (respiration in the cell).
define diffusion
Diffusion is the net movement of molecules or ions from a region of high concentration to a region of lower concentration.
•This occurs due to the natural motion of particles in solution. They move randomly and bounce off the walls and other particles.
what is net movement
- When there is a difference in concentration the probability of a particle moving from high to low is greater than in the opposite direction. We say there is net movement in one direction.
- When the concentrations are equal there is an equal probability of movement in each direction. This is a dynamic equilibrium.
explain the factors that affect the rate of diffusion
- The rate at which diffusion occurs depends upon a number of factors:
- Concentration Gradient – the larger the difference in concentration the quicker it will occur.
- Surface Area – the greater the surface area, the faster the rate.
- Thickness of Exchange Surface – the thinner the surface, the less distance the substance must travel, and the faster diffusion occurs.
- You must remember are be able to use Fick’s Law:
Rate of Diffusion ∝ (surface area ×conc. difference)/(diffusion distance)
what is facilitated diffusion
- This is diffusion which occurs through specific channel proteins in the cell membrane.
- These provide water-filled channels through which water-soluble substances (glucose, proteins etc.) can pass.
- This speeds up the rate of diffusion of these molecules across the membrane but also allows for control of their passage as the channels can be closed.
- Some channel proteins change shape as the molecule passes through (similar to carrier proteins) but it is still a passive process.
define osmosis
The passage of water from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
- Osmosis is sometimes referred to as a special case of diffusion involving water molecules.
- Plasma membranes are partially permeable, they allow the movement of water and other small molecules only.
- Cell walls are fully permeable, they allow the passage of all substances.
define and explain water potential
- Water potential (Ψ) is a measure of the pressure created by the water molecules in the solution.
- It is therefore expressed in units of pressure, kiloPascals (kPa).
- Pure water at standard temperature and pressure (25°C and 100kPa) has a water potential of 0kPa, the highest possible. All values for water potential must be lower than this.
- As you add more solute the water potential decreases.
- Water will always move from higher to lower water potentials.
explain how osmosis works in animal cells
- As the plasma membrane of a cell is partially permeable and water can move across freely, osmosis is a very important process for cells.
- If an animal cell loses too much water it shrinks and takes on a shrivelled appearance.
- Conversely, if too much water moves into the cell then the cell membrane will simply burst, emptying the contents of the cell.
- For this reason cells are normally found bathed in a solution of approximately equal water potential.
explain how osmosis works in plant cells
- Plant cells have a rigid cell wall, changing the way that osmosis affects them.
- The strength of plant cells comes from the fact that they are often turgid. This means that water has entered the cell by osmosis causing it to push outwards against its own cell wall giving it strength.
- If the extracellular solution is isotonic then the cell enters a state of incipient plasmolysis. It may become flaccid and is not as rigid.
- In extreme cases where water leaves the cell the cell membrane may shrink away from the cell wall. It is said to be plasmolysed.
define and explain active transport
The movement of molecules or ions from a region of lower to a region of higher concentration, across a membrane, using energy (in the form of ATP) and carrier proteins.
- Active transport is a fundamentally different process to passive transport:
- It requires energy in the form of ATP (adenosine triphosphate)
- It goes against the concentration gradient
- Intrinsic proteins known as carrier proteins move specific substances across the membrane
Active transport can be direct or co-transport
describe and explain direct active transport
- A trans-membrane protein accepts the molecule/ion to be transported.
- ATP binds to the inner portion of the molecule and splits into ADP (adenosine diphosphate) and an inorganic phosphate.
- This causes the molecule to change shape and release the molecule/ion across the membrane.
- The phosphate is released from the protein causing the protein to revert back to its original shape.
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•This can also occur with more than one molecule/ion in the same direction or in opposite direction (sodium-potassium pump)
explain how co-transport works in the small intestine
- Towards the end of the small intestine the concentration of glucose in the lumen become too low for diffusion to occur.
- As glucose is an important molecule the body ensures that as much glucose as possible is absorbed by the body using active transport.
- There is no ‘directly ATP-powered’ pump to move glucose into the cell so co-transport is utilised.
- Sodium ions are actively transported out of the epithelial cells into the blood by sodium-potassium pumps.
- This lowers the sodium concentration of the epithelial cells below that of the lumen.
- Sodium ions diffuse from the lumen into epithelial cells through carrier proteins (co-transport proteins) bringing glucose molecules with them.
- These glucose molecules then diffuse into the blood via facilitated diffusion.
•As ATP is used to set up the sodium ion gradient as opposed to moving the glucose this is an example of indirect active transport.
