Science , Grade 8 Chapter No 5 STRUCTURE OF CELLS Science , Grade 8 Chapter No 5

INTRODUCTION What are cells? All organisms are made of small units called cells. Just like bricks build up a building, cells build up our body. Cells are basic units of an organisms Unicellular organisms are made of one cell whereas multicellular organisms are made of many cells. Most of the cells are very small and they are not visible to the naked eye .They can only be observed under a microscope. Micron is the unit to measure the size of a cell.

Activity! Classify the following into unicellular and multicellular organisms: .

Answer: Unicellular: Amoeba and diatom Multicellular: hydra and ant

What do cells do ? Cells perform activities like nutrition respiration and cell division, which contribute to growth of an organism Hence cells are called structural and functional units of organism.

Cell Theory Schleiden and Schwann put forth cell theory in 1839 Cell theory states that Body of living organisms is formed from one or more cells. All the cells arise only from pre existing cells through cell division.

VARIOUS SHAPES OF CELLS Different cells have different shapes to carry out different functions For example: Red blood cell is round in shape and its function is to carry blood to different parts of the body whereas nerve cell is long and its function is to carry information from brain to different parts of the body.

ELECTRONIC MICROPSCOPE Cell and its parts can be seen clearly with the help of an electronic microscope. It uses beam of electrons to get the image of an object. It magnifies objects upto 2000 times.

PARTS OF A CELL 1)There are three prominent parts of a cell Cell membrane Cytoplasm Nucleus 2)Cell is surrounded by a cell membrane(also called plasma membrane ) 3) It helps maintain the shape of a cell and controls the movement of substances in and out of it. 4) Hence,Cell membrane is described as a semi permeable membrane.

ANIMAL CELL

PLANT CELL

Movement of molecules across the cell membrane occurs by a process called diffusion. It is the process in which molecules move from region of higher concentration to region of lower concentration This process requires energy and is called ACTIVE TRANSPORT. Osmosis is a special type of diffusion in which molecules move from lower concentration region to higher concentration region.

Difference between plant and animal cell In Plant cell, in addition to cell membrane , there is cell wall. Cell wall is thick,rigid and surrounds the cell membrane and mainly composed of cellulose. It provides shape,strength ,protection and rigidity to the cell.

Cytoplasm It is jelly like substance found within cell membrane and it is the seat for most of the cellular activities. It consists of 80% water and a number of smaller living and nonliving components . Non living substances include reserve food, secretions and several chemicals such as resin,gum,starch grains,oils droplets,calcium carbonate

ORGANELLS Living components of cytoplasm are called organells. They carry out specific functions which are grouped under 2 categories 1)Membranous Organells 2)Non membranous Organells(not covered by membrane)

Membranous Organells Organells covered by membrane are called membranous organells. Eg: endoplasmic reticulum,mitochondria,lysosomes,plastids

1)Endoplasmic reticulum: It is a network of membranes and it extends from the cell membrane to the nuclear membrane. It forms the supporting framework of the cells and transports substances from one part of the cell to the other and from one cell to another.

2)Mitochondrion: occurs in various shapes. It is the site of respiration which uses oxygen to break down glucose into carbon dioxide and water. This process releases energy which is stored in a molecule called adenosine triphosphate(ATE). It is bounded by double membrane, outer is smooth but inner is folded. Folds are called cristae and inner membrane encloses a mass known as matrix.

Mitochondrion is also called power house of a cell.

3)Golgi complex Consists of different types of structure Some are like tubes and some are like granules Main function is to secrete chemicals required for cellular activities

4)Lysosome: Found in animal cells . Spherical sac like organelles. Contain a variety of hydrolytic enzymes which help in the digestion of organic substances present in the cell. They destroy themselves when they become weak, damaged or diseased. Hence they are called suicide bags of the cell.

5)Plastids: Found only in plant cells. Some plastids without coloring element are called leucoplasts and occur in regions which are not exposed to sunlight and their main function is to store food. Some plastids with green color pigment are called chloroplasts and the color is due to a green colored pigment called chlorophyll and they are found mostly on the leaves of the plant.

Chloroplast is covered by a double membrane. Inner membrane encloses matrix which consists of stroma and grana. Grana has chlorophyll which plays an important role in preparing food.

Non membranous cell organelles 1)Ribosomes: Grain like structure attached to endoplasmic reticulum. They play an important role in the synthesis of protein hence called “protein factories of cell”. 2)Centriole: 2 centrioles located near the nucleus which help during cell division.

Nucleus

It is the largest organelle in a cell. It is covered by a double membrane, nuclear membrane which encloses a fluid called nucleoplasm. Nucleoplasm has thread like structures called chromatin During cell division chromatin undergoes coiling and super coiling and becomes short and thick to form chromosomes. Chromosomes contain genetic material called DNA which has genes.

Genes determine particular characteristic of an organism. Nucleus coordinates the activities of a cell. Nucleus has a dense spherical object called nucleolus which isn't covered by a membrane. Nucleolus encloses ribosomes and thus involves in synthesis of protein.

CONCLUSION IMPORTANT THINGS TO REMEMBER Parts of cell role of each organell for overall activity of the cell Structure and functions of mitochondra and chloroplasts 4.difference between plant and animal cell 5. role diffusion and osmosis process 6. Drawing of plant and animal cell