Understanding Cells

Cells

•Living organisms are made up of one or more cells.•Cells are the most basic unit of organisms which can function on their own. Cells carry out life processes such as respiration, division, excretion and growth.•Cells are the building blocks of an organism.•Cells work together to keep an organism alive.•Cells of living things exist in many sizes and shapes. They can be round, oval, long, short, with tail and so on.

•Robert Hooke, a British Scientist, was the first person to study cells. The figure below shows some cork cells observed through a microscope by him.

Cork cells as seen by Robert Hooke

Using a microscope

•Cells are very tiny and cannot be seen with the naked eye.

•Cells of living organisms can be observed using a microscope.

A Microscope

Functions of the different parts of microscope

Parts of a microscope

Functions

Eye piece •To magnify the image of the specimen

Objective lens •To magnify the specimen

Body tube •To hold the eye piece at a fixed distance away from the objective lens

Coarse focus knob •To raise or lower the body tube gently for sharp focusing.

Arm and base •For holding and supporting the microscope

Clip To hold the slide on the stage in position

Stage To put the slide on.

Diaphragm To control the amount of light entering the lens.

Mirror To direct reflected light towards the diaphragm.

Condenser To concentrate light from the mirror onto the specimen

The correct way to use a compound light microscope

a. Place the microscope on a flat surface in a well- lighted area.

b. Position the low power objective lens above the hole of the stage.

c. Raise the condenser as high as it can go and open the iris of the diaphragm fully.

d. Look through the eye piece and adjust the mirror to obtain a bright field of view.

e. Place a slide with the specimen on the stage and hold it in place with the clip.

f. Lower the objective lens using the coarse focus knob until it is adjust above the slide.

g. Look through the eye piece. Turn the coarse focus knob counter – clockwise until the object is in focus.

h. If necessary, change to a high power objective lens.

i. Adjust the fine focus knob until a sharp image is obtained.

General structure of a cell•A typical cell is made up of cell membrane

which contains a living substance called protoplasm.a.Protoplasm consists of a nucleus and cytoplasm.

b.The nucleus is a spherical in shape and surrounded by a nucleus membrane.

c.Cytoplasm is a colourless jelly – like material surrounded by cell membrane.

d.The cell membrane is a thin film which is partially permeable to the surrounding substances.

Structure of animal cells•Generally, each animal cell is made up of

cell membrane and protoplasm (which consists of cytoplasm and nucleus.)

An animal cell

•Animal cells do not have cell walls or chloroplasts.•There is great variety of forms and functions among animal cells. The figure below shows some examples of animal cells

Paramecium

Red blood cell

Nerve cell

Structure of plant cells

•The figure below shows the general structure of a plant cell.

A plant cell

•The basic structure of plant cells is similar to that of animal cells in having a cell membrane, cytoplasm and a nucleus.

•All plant cells have a cell wall which gives them an almost fixed shape.

•Plant cells often have one large vacuole, whereas animal cells usually have many small ones.

•Most plant cells that are under the light contain chloroplasts which are green in colour.

•Plant cells vary in their shapes, sizes, structures and functions.

Epidermal cell in the root

Palisade cells

Guard cells

Examples of plant cells

The function of cell structures

Structure Present in Function Cell membrane All cells •Enclose the cytoplasm.

•Controls the movement of materials in and out of cells.

Cell wall ( a non – living layer of cellulose)

Plant cell only •Supports and give shape to the cells

Cytoplasm ( jelly – like substance)

All cells •This is where chemical reactions take place inside the cell.•Stores dissolved material

Cell membrane Cell wall

Cytoplasm

Structure Present in Function Nucleus Almost all cells

(mature red blood cells do not have nuclei)

•Controls all cellular activities

Chloroplast (contain green pigments called chlorophyll)

Most plant cells that are under light

•Chlorophyll absorbs light for photosynthesis to produce food and oxygen.

Vacuole (cell sap) Most plant cells and some animal cells

•Holds useful substances and wastes.•Supports the plant when it is full of water.

Nucleus Chloroplast

Vacuole

Comparison between animal cell and plant cells

Similarities Similarities

Animal cells Plant cells

•Have cell membrane

•Able to carry out activities to life

•Have protoplasm

Differences Differences

Animal cells Plant cells

•Generally smaller

•Usually irregular and may vary during life

•No cell wall•No chloroplast•With a small or no vacuoles

•Generally bigger

•Fixed by cell wall

•Have a cell wall•Many plant cell under light have chloroplasts•Often have a large vacuole

Size

Shape

Contents

•Usually at the centre of the cells

•Glycogen granules

•Maybe to one side of the cells

•Starch granules

Position of nucleus

Food storage