THE CELL

THE CELL

Introduction

  • The cell is the basic unit of an organism
  • All living organisms are made up of cells
  • Some organisms are made up of one cell and others are said to be multicellular
  • Other organisms are made of many cells and are said to be multicellular
  • Cells are too little to see with the naked eye
  • They can only be seen with the aid of a microscope

The microscope

The microscope is used to magnify objects

Magnification

  • The magnifying power is usually inscribed on the lens
  • To find out how many times a specimen is magnified, the magnifying power of the objective lens is multiplied by that of the eye piece lens
  • If the eye piece magnification lens is x10 and the objective lens is x4, the total magnification is x40
  • Magnification has no units
  • It should always have the multiplication sign

e.g.x40

Microscope parts and their functions

To View the Object

  • Turn the low power objective lens until it clicks into position
  • Looking through the eye piece, ensure that enough light is passing through by adjusting the mirror
  • This is indicated by a bright circular area known as the field of view
  • Place the slide containing the specimen on stage and clip it into position
  • Make sure that the specimen is in the centre of the field of view
  • Using the coarse adjustment knob, bring the low power objective lens to the lowest point
  • Turn the knob gently until the specimen comes into focus
  • If finer details are required, use the fine adjustment knob
  • When using high power objective always move the fine adjustment knob upwards

Care of a Microscope

  • Great care should be taken when handling it
  • Keep it away from the edge of the bench when using it
  • Always hold it with both hands when moving it in the laboratory
  • Clean the lenses with special lens cleaning paper
  • Make sure that the low power objective clicks in position in line with eye piece lens before and after use
  • Store the microscope in a dust-proof place free of moisture

Cell membrane (Plasma membrane):

  • This is a thin membrane enclosing cell contents
  • It controls the movement of substances into and out of the cell

Cytoplasm:

  • This is a jelly-like substance in which chemical processes are carried out
  • Scattered all over the cytoplasm are small structures called organelles
  • Like an animal cell, the plant cell has a cell membrane, cytoplasm and a nucleus

vacuole

  • Plant cells have permanent, central vacuole

It contains cell sap where sugars and salts are stored

Cell wall:

  • This is the outermost boundary of a plant cell
  • It is made of cellulose
  • Between the cells is a middle lamella made of calcium pectate

Chloroplasts;

  • With special staining techniques it is possible to observe chloroplasts
  • These are structures which contain chlorophyll, the green pigment responsible for trapping light for photosynthesis

The Electron Microscope (EM)

  • Capable of magnifying up to 500,000 times
  • The specimen is mounted in vacuum chamber through which an electron beam is directed
  • The image is projected on to a photographic plate
  • The major disadvantage of the electron microscope is that it cannot be used to observe living objects
  • However, it provides a higher magnification and resolution (ability to see close points as separate) than the light microscope so that specimen can be observed in more detail
See also  MODE OF NUTRITION AND PLANT NUTRITION

The Plasma Membrane

  • Under the electron microscope, the plasma membrane is seen as a double layer
  • This consists of a lipid layer sandwiched between two protein layers
  • This arrangement is known as the unit membrane and the shows two lipid layers with proteins within
  • Substances are transported across the membrane by active transport and diffusion

The Endoplasmic Reticulum (ER)

  • This is a network of tubular structures extending throughout the cytoplasm of the cell
  • It serves as a network of pathways through which materials are transported from one part of the cell to the other
  • An ER encrusted with ribosomes it is referred to as rough endoplasmic reticulum
  • An ER that lacks ribosomes is referred to as smooth endoplasmic reticulum
  • The rough endoplasmic reticulum transports proteins while the smooth endoplasmic reticulum transports lipids

The Ribosomes

  • These are small spherical structures attached to the ER
  • They consist of protein and ribonucleic acid (RNA)
  • They act as sites for the synthesis of proteins

Goigi Bodies

  • Golgi bodies are thin, plate-like sacs arranged in stacks and distributed randomly in the cytoplasm
  • Their function is packaging and transportation of glycol-proteins
  • They also produce lysosomes

Mitochondria

  • Each mitochondrion is a rod-shaped organelle
  • Made up of a smooth outer membrane and a folded inner membrane
  • The foldings of the inner membrane are called cristae
  • They increase the surface area for respiration
  • The inner compartments called the matrix
  • Mitochondria are the sites of cellular respiration, where energy is produced

Lysosomes

  • These are vesicles containing hydrolytic enzymes
  • They are involved in the breakdown of micro-organisms, foreign macromolecules and damaged or worn-out cells and organelles

The Nucleus

  • The nucleus is surrounded by a nuclear membrane which is a unit membrane
  • The nuclear membrane has pores through which materials can move to the surrounding cytoplasm
  • The nucleus contains proteins and nucleic acid deoxyribonucleic acid (DNA) and RNA
  • The chromosomes are found in the nucleus
  • They are the carriers of the genetic information of the cell
  • The nucleolus is also located in the nucleus but it is only visible during the non-dividing phase of the cell

The Chloroplasts

  • These are found only in photosynthetic cells
  • Each chloroplast consists of an outer unit membrane enclosing a series of interconnected membranes called lamellae
  • At various points along their length the lamellae form stacks of disc like structures called grana
  • The lamellae are embedded in a granular material called the stromal
  • The chloroplasts are sites of photosynthesis
  • The light reaction takes place in the lamellae while the dark reactions take place in the stroma

Also See:

THE DIFFERENCES BETWEEN ANIMALS AND PLANTS

DETERMINATION OF THE GENOTYPE OF A DOMINANT PHENOTYPE

VARIATION IN POPULATION

CLASSIFICATION OF ANIMALS AND ORGANIZATION OF LIFE

CLASSIFICATION OF LIVING ORGANISMS

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