Question

Q 1 - What are the functions of different organelles in a cell ?

Q 2 - Described stepwise how you would make a slide of an onion peel and observe it under a microscope ?

Answer 1

1)



THE THREE MAIN COMPONENTS OF ANY PLANT OR ANIMAL CELL ARE:

1. PLASMA MEMBRANE/ CELL MEMBRANE

Structure- a bilipid membraneous layer composed of proteins and carbohydrates. It is fluid like.

Function - the cell membrane separates the cell from its external environment, and is selectively permeable (controls what gets in and out). It protects the cell and provides stability.

Proteins are found embedded within the plasma membrane, with some extending all the way through in order to transport materials.

Carbohydrates are attached to proteins and lipids on the outer lipid layer.



2. CYTOPLASM

Structure - The jelly-like substance composed of mainly water and found between the cell membrane and nucleus. The cytoplasm makes up most of the "body" of a cell and is constantly streaming.

Function - Organelles are found here and substances like salts may be dissolved in the cytoplasm.



3. NUCLEUS

Structure - The largest organelle in the cell. It is dark and round, and is surrounded by a double membrane called the nuclear envelope/membrane. In spots the nuclear envelope fuses to form pores which are selectively permeable. The nucleus contains genetic information (DNA) on special strands called chromosomes.

Function - The nucleus is the "control center" of the cell, for cell metabolism and reproduction.



THE FOLLOWING ORGANELLES ARE FOUND IN BOTH PLANT AND ANIMAL CELLS.

1. "ER" OR ENDOPLASMIC RETICULUM

The Endoplasmic Reticulum is a network of membranous canals filled with fluid. They carry materials throughout the cell. The ER is the "transport system" of the cell.

There are two types of ER: rough ER and smooth ER.

Rough Endoplasmic Reticulum is lined with ribosomes and is rough in appearance and smooth endoplasmic reticulum contains no ribosomes and is smooth in appearance.



2. RIBOSOMES

Ribosomes are small particles which are found individually in the cytoplasm and also line the membranes of the rough endoplasmic reticulum. Ribosomes produce protein. They could be thought of as "factories" in the cell.



3. GOLGI BODY / APPARATUS

Golgi bodies are stacks of flattened membranous stacks (they look like pancakes!). The Golgi Body temporarily stores protein which can then leave the cell via vesiciles pinching off from the Golgi.



4. LYSOSOMES

Lysosomes are small sac-like structures surrounded by a single membrane and containing strong digestive enzymes which when released can break down worn out organelles or food. The lysosome is also known as a suicide sac.



5. MITOCHONDRIA

The mitochondria are round "tube-like" organelles that are surrounded by a double membrane, with the inner membrane being highly folded. the mitochondria are often referred to as the "powerhouse" of the cell. the mitochondria releases food energy from food molecules to be used by the cell. This process is called respiration. Some cells( muscle cells) require more energy than other cells and so would have many more mitochondria.



6. VACUOLES

Vacuoles are fluid filled organelles enclosed by a membrane. They can store materials such as food, water, sugar, minerals and waste products.



ANIMAL CELLS ORGANELLES NOT FOUND IN PLANT CELLS:

CILIA AND FLAGELLA

Both cilia and flagella are hair-like organelles which extend from the surface of many animal cells. the structure is identical in both, except that flagella are longer and whiplike and cilia are shorter. There are usually only a few flagella on a cell, while cilia may cover the entire surface of a cell. The function of cilia and flagella ionclude locomotion for one-celled organisms and to move substances over cell surfaces in multi-celled organisms.



ORGANELLES AND OTHER FEATURES FOUND ONLY IN PLANT CELLS:

1. CELL WALL

The cell wall is a rigid organelle composed of cellulose and lying just outside the cell membrane. The cell wall gives the plant cell it's box-like shape. it also protects the cell. The cell wall contains pores which allow materials to pass to and from the cell membrane.



2. PLASTIDS

Plastids are double membrane bound organelles. It is in plastids that plants make and store food. Plastids are found in the cytoplasm and there are two main types:

Leucoplasts - colorless organelles which store starch or other plant nutrients. ( example - starch stored in a potato)

Chromoplasts - contain different colored pigments. The most important type of chromoplast is the chloroplast, which contains the green pigment chlorophyll. This is important in the process of photosynthesis.



3. CENTRAL VACUOLE

The central vacuole is a large fluid-filled vacuole found in plants.



2)

This activity is one of the first uses of an optical microscope that most school children encounter in a school biology lab. Onions are used because they have large cells that are easily visible under a student microscope.

An onion is made of many concentric layers. Each layer is separated by a thin skin or membrane. In this experiment you will make a slide and look at the cells of the membrane under a microscope.

Method
Take a small piece of onion and using forceps and peel off the membrane from the underside (the rough side).
Lay the membrane flat on the surface of a clean glass slide, and then add one drop of dye (iodine or methylene blue)be very careful; these dyes will stain your skin and clothes.
Using a pin, lower a thin glass cover slip onto the slide. Make sure there are no air bubbles.
Put the slide onto the stage of the microscope. Make sure the objective lense et on low power, and the microscope light is turned on.
Look through the eyepiece lens and turn the focusing wheel until you can see the cells. They should look like lizard skin.
Swap the objective lens for a high powered one so that you can see the cells at a greater magnification. You should be able to make out the nucleus.