Onion Cell Plasmolysis Lab

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Name: ____________________________ Period: ______ Date: ________________

Onion Cell Plasmolysis

Objectives

In this laboratory experiment, students will be able to

observe

the process of osmosis and

distinguish

between hypertonic, hypotonic, and isotonic solutions.

Purpose

You are a laboratory assistant who has been selected to be part of newly formed research team. You have been asked to give a lecture on the process of osmosis. To show that water molecules move in and out of a cell by osmosis, you will set up the following demonstrations.

Background

Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cells. Some substances can cross the cell membrane without any input of energy by the cell. The movement of such substances across the membrane is known as

passive transport

. The simplest type of passive transport is diffusion.

Diffusion

is the movement of molecules from an area of higher concentration to an area of lower concentration. A second type of passive transport is osmosis.

Osmosis

is the process by which water molecules diffuse across a cell membrane from an area of higher concentration to an area of lower concentration. Molecules must move about in a cell in order for a cell to survive. ______________________________________________________________________________

Plasmolysis

is the shrinking of the cytoplasm of a plant cell in response to diffusion of water out of the cell and into a hypertonic solution (high solute concentration) surrounding the cell as shown in the figure below. During plasmolysis the cellular membrane pulls away from the cell wall. In the next lab exercise you will examine the details of the effects of highly concentrated solutions on diffusion and cellular contents.

Red Onion Membrane in Distilled Water and Salt Water Solutions

1.

Prepare a slide of red onion membrane. Add 2 drops of

distilled water

, cover with a coverslip and observe under the microscope (

high power

). Draw a picture of your observations below (

use colored pencils

) and

label

the

cell wall, cell membrane,

and

cytoplasm

. 2.

Using the prepared slide from above and a dropper, place a drop of

salt water

solution at one edge of the coverslip. Place a small piece of paper towel on the opposite edge to draw the salt water solution under the coverslip. Repeat with a second drop of salt water solution. Observe the onion cells under the microscope (

high power

) for several minutes. Draw a picture of your observations below (

use colored pencils

) and

label

the

cell wall, cell membrane,

and

cytoplasm

. 3.

Explain (

in detail

) the changes you observed in the

distilled water

solution. Please be sure to discuss the positioning of the

cell wall, cell membrane,

and

cytoplasm

. Refer to your handout on hypotonic, hypertonic, and isotonic solutions for additional help. 4.

Explain (

in detail

) the changes you observed in the

salt water

solution. Please be sure to discuss the positioning of the

cell wall, cell membrane,

and

cytoplasm

. Refer to your handout on hypotonic, hypertonic, and isotonic solutions for additional help. 5.

In the winter, grass often dies near roads that have been salted to remove ice. What causes this to happen?

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