4.3/3/5 osmosis, active transport and co-transport

What is osmosis?

the passage of water from a region of high water potential to lower water potential through a selectively permeable membrane

what is water potential?

The pressure created by water molecules, represented by psi and measured in kPa usually

- under standard conditions of 25c temperature and 100kPa pressure water has a water potential of 0
- adding solute makes this value negative, so water moves from the less negative area to more negative area

What is solute potential?

A solutes contribution to the water potential (Ψs)

It always reduces the water potential so will always be negative

What is a hypertonic solution?

Hypertonic solutions are where there is a higher concentration of solutes or the water potential is lower (more negative)

What are isotonic solutions?

Isotonic solutions are where there is the same concentration so no net movement of water

What are hypotonic solutions?

Hypotonic solutions are where there is a lower concentration of solutes or a higher water potential (less negative)

What happens to an animal cell in a hypotonic solution to its cytosol?

Water will move in and cause the cell to burst

What happens to an animal cell in an isotonic solution to its cytosol?

Nothing, water going in and out is the same so there is no net change

What happens to an animal cell in an hypertonic solution to its cytosol?

Water will move out of the cell, causing it to shrink

What happens to a plant cell in a hypotonic solution to its cytosol?

Water will enter the cell and fill the vacuole, causing the cell membrane to push against the cell wall making the plant cell turgid

What happens to a plant cell in an isotonic solution to its cytosol?

Nothing will change, this state is called incipient plasmolysis and the cell membrane (protoplast) is starting to pull away from the cell wall

What happens to a plant cell in a hypertonic solution to its cytosol?

Water moves out and the vacuole becomes flaccid, stopping it from pushing against the cell wall (this state is called plasmolysis)

What is a cells cytosol?

the aqueous component of the cytoplasm of a cell, within which various organelles and particles are suspended.

What is active transport?

the movement of ions or molecules across a cell membrane into a region of higher concentration, using a carrier protein and ATP

How does active transport differ from osmosis and diffusion?

- Osmosis and diffusion are passive, active transport requires energy
- Substances are moved against a concentration gradient, passive transport goes with it
- carrier proteins are involved
- selective process

What is the process of active transport?

- The carrier proteins span the membrane and bind to the molecule or ion to be transported

- Molecule or ion binds to the receptor sites on the carrier protein

- On the inside of the cell, ATP binds to the protein causing it to split into ADP and a phosphate molecule, this hydrolysis provides enough energy to change the shape of the protein and opening it to the opposite side of the membrane
- Molecule is released to other side of membrane
- Phosphate molecule is released from the protein causing it to revert back to its original shape, the phosphate then recombines with the ADP to make ATP again

What factors affect active transport?

- Concentration gradient
- Electrical charge
- Energy availability (affected by temperature, oxygen availability and cellular glucose concentrations)

What is co-transport?

When two molecules are transported at once, either in the same or opposite directions

Example of where co-transport happens

sodium potassium pump, sodium ions are moved out of the cell or organelle and potassium is moved in from surroundings

Adaptations that increase the rate of diffusion

- increased concentration gradient
- shorter distances for diffusion
- increased surface area, providing more space for insertion of carrier and channel proteins
- increased density of channel and carrier proteins

Structure of the small intestine and how is it specialised for absorption

- walls are folded into very thin finger-like projections called villi (1mm long)
- very thin walls made of epithelial cells, decreasing distance of diffusion
- they are able to move to maintain a concentration gradient
- on the villi are smaller projections called microvilli which are about 0.6 micrometres long, increasing the surface area further

What is the role of active transport in the absorption of glucose?

1. sodium ions are actively transported out of epithelial cells into the bloodstream by a sodium potassium pump which uses energy
2. Due to this there is a lower concentration of sodium ions in the epithelial cell, compared to lumen of ileum
3. The sodium ions move down their concentration gradient from the lumen into the epithelial cell by a carrier protein, bringing a glucose molecule with it
4. The glucose then passes into the blood via facilitated diffusion as it is more concentrated in the epithelial cell than the bloodstream


in this case, the sodium gradient powers the movement of the glucose, not ATP directly