5.5 Osmosis

Question 1

Define osmosis

Answer 1

Osmosis is the net movement of water molecules from an area of high concentration to an are of low concentration through a selectively permeable membrane down a water potential gradient, a PASSIVE PROCESS

Question 2

What is water potential?

Answer 2

Water potential is the pressure exerted by free water molecules as they collide with a membrane or a container, measure in pascals

Question 3

What is the water potential for pure water and thus how do others relate ?

Answer 3

Water potential for pure water is 0 kpa, and anything more concentrated than water will have less , thus negative kpa

The more negative the water potential the more concentrated the solution

Question 4

Why does water potential decrease as solute is added?

Answer 4

• Water molecules are likely to aggregate around the solute
• meaning the kinetic energy of water is lowered and the amount of FREE WATER MOLECULES is now decreased
• meaning overall a less pressure is exterted than pure

Question 5

What is hydrostatic pressure?

Answer 5

When water moves into a CLOSED SYSTEM LIKE A CELL, this results in an increase of inside pressure, and this is known as hydrostatic pressure. Now if this gets too high it can cause damage

Question 6

What happens if an animal cell is out into a less negative steer potential solution, HYPOTONIC ?
What’s the problem with animal size of membranes

Answer 6

Water will move into the cell via osmosis, increasing the HYDROSTATIC PRESSURE
- as animal cells membranes are very thin, only 7nm, and have NO CELL WALLS, the cells will eventually burst= CYTOLYSIS, cannot return

Question 7

What happens if animal placed in solution more negative, hypertonic solution?

Answer 7

Here water will move out by osmosis from the cytoplasm, leading to a reduction in the volume of the cell, leading to shrivel and shrinking known as cremation

Question 8

Istonic solution

Answer 8

= the same

Question 9

What is different for plant cells vs animal cells when it comes to osmosis

Answer 9

Plant cells cannot control the gradients around them unlike animal cells can, for example root cells will ALWAYS HAVE WATER AROUND THEM
- also plant cells have cellulose walls around them

Question 10

What happens when plant cell placed in less negative, hypotonic solution ?
What is tugor pressure and WHAT IT DOES?

Answer 10

• water moves in by osmosis
• hydrostatic pressure increases, pushing cell membrane to cell walls
• the new pressure AGAINST THE CELL WALLS IS CALLED TUGOR PRESSURE
  
  • tugor pressure increasing will RESIST further movement of water from entering
• at this state you say the cell is now TURGID

Question 11

What happens when plant placed in solution more negative than it, hypertonic?

Answer 11

Water moves out of the cell into the solution via osmosis

• reduction of volume in cytoplasm
• eventually pulls the cell surface membrane away from the cell as the contents collapse
• this is known as the cell to be PLASMOLYSED

Question 12

What happens when you put plant cel, in isotonic solution

Answer 12

Water potential is same so no water moves in or out

- thus as it is not at MAXIMUM PRESSURE IT TECHNICALLY IS FLACCID