Osmosis: The Cell's Water Pump43009RK

Osmosis: The Cell's Water Pump


- self-sufficent lifestyles are called "autotrophic"

- "different nourishment", or "heterotrophic"  lifestyles describe animals, fungi, and microorganisms that depend on products made by photosynthesizing plants

Requirements of autotrophs:

1. water
2. minerals
3. carbon dioxide

Osmosis, (from the Greek word "to push") allows water molecules to cross into the root's epidermal cells.

From your handout, trace or sketch 'water uptake by roots on page 153.


Water entering a cell is stored in the large, central vacuole which expands and presses the cytoplasm against the cell wall. When a cell becomes turgid, the cell is fully inflated with water, and water continues to diffuse into the cell.

Why don't plant cells burst from too much water? 

Turgor pressure, a counteracting internal force, will squeeze water out, acting like a safety valve to prevent bursting.

What examples of turgid and nonturgid cells can you describe?

Turgid: crisp lettuce or celery

Nonturgid: wilted lettuce

Why do plants wilt?

When too much salt accumulates in soils, roots lose water. Just like what happened in your osmosis lab, salt is able to suck the water molecules (reverse osmotic flow).

Root Pressure

- root hairs draw water from the soil by osmosis

- when the epidermal cells are turgid, they discharge water into spaces between cortex cells

Draw the path of water across a root in your science notebook. See page 155 in the 'botany for gardeners' book

- endodermis is the 2nd osmotic pump that directs water into the hollow, tubular cells of the xylem

- root pressure: the effect of root pressure is seen when you cut the stem of a rose and water oozes out from the slice

- transpiration: when water is discharged from the turgid mesophyll, the sun's heat turns it to vapor which saturates the internal spaces in the leaf and ultimately is released through the stomata

Mesophyll contains the palisade cells

Palisade cells are spongy
cells that are packed with carbon dioxide, oxygen, and water vapor.
Waxy cuticle layer on epidermis prevents evaporation. Pores of the leaf
surface are called the stomata.

How many stomata are involved?

-39,000 stomata per sq cm of lower epidermis - apple

- 25,000 stomata per sq in of bean

- 45,000 orange and 27,000 on a pumpkin leaf

Stomata openings permit gases to enter the leaf. Stomata routinely close at night.