Chapter 1: Cell Structure

All living organisms are made of cells, there are several different types of cells, some of them share some common features. Humans are made up of eukaryotic cells. All eukaryotic cells contain a nucleus and membrane-bound organelles. A more detailed structure of cells called the ultrastructure can be obtained by using a microscope.

Light microscopes

These are good for observing samples in a lab as they are cheap and portable. They have lower magnification and resolution than electron

microscopes, however.

 Electron microscopes

These are good for examining organelles in high detail. They have a high magnification and resolution, but samples must be placed in a vacuum

and prepared first. This technique can be very expensive.

The Ultrastructure of Plant cell and Animal cell

The figure represents the ultrastructure of the plant cell and animal cell.

 

Animal cells and plant cells have the following common features:

  • The cell surface membrane surrounds the cell and controls what enters and exits.
  • The nucleus is surrounded by a double membrane called the envelope containing pores which enable molecules to enter and leave the nucleus, the nucleus also contains chromatin and a nucleolus which is the site of ribosome production. The darker part of the nucleus is called the nucleolus.

  • Rough endoplasmic reticulum which is a series of flattened sacs enclosed by a membrane with ribosomes on the surface. RER folds and processes proteins made on the ribosomes. The ribosomes on RER produce protein by linking amino acids.

  • Smooth endoplasmic reticulum is a system of membrane bound sacs. SER produces and processes lipids. They are less extensive than RER.
  • Golgi apparatus is a series of fluid filled, flattened & curved sacs with vesicles surrounding the edges. The Golgi apparatus modifies the protein by adding carbohydrate groups.
  • Mitochondria are usually oval shaped, bound by a double membrane called the envelope.  They have an envelop of an outer membrane and inner membrane. The inner membrane is folded to form projections called cristae with matrix on the inside containing all the enzymes needed for respiration.
  • Ribosomes are composed of two subunits and are the site of protein production.
  • Lysosome is a vesicle containing digestive enzymes bound by a single membrane. It helps to digest the food taken into cell. It also helps to break down unwanted organelles within the cell.
  • Some animal cells may contain cilia on their surface membrane. These are small hair-like structures composed of microtubules in a ‘9+2’ formation. This allows movement of cilia therefore allowing movement of substances along the surface of the cell.

Difference between plant cell and animal cell

  • The plant cell contains cell wall which is made of cellulose micro fibrils. Its role is to strengthen the cell and prevent bursting due to osmosis.
  • The vacuole is a fluid-filled sac present in plant cells, surrounded by a membrane called the tonoplast. It contains mineral salts, sugars, amino acids, waste substances and pigments. Its role is to colour the cell to attract pollinating insects, act as a temporary food store and provide support through turgidity.
  • Centrioles are only found in animal cell. They are made of microtubules arranged in a circular pattern.
  • Chloroplast are found in some plant cell. They may contain starch grain which helps during photosynthesis.

Fundamentally different type of cells

Prokaryotic cell

Bacteria

  • It contains cell wall which is rigid outer covering made out of peptidoglycan.
  • It contains capsule which is a protective layer.
  • It has ribosomes which produces protein.
  • It has a plasmid which is the circular piece of DNA.
  • Flagella is a tail like structure which helps in the movement of the cell.

Viruses

 Viruses are non-living structures which consist of nucleic acid (either DNA or RNA) enclosed in a protective protein coat called the capsid, sometimes covered with a phospholipid layer called the envelope.

 

Difference between:

Prokaryotes

Eukaryotes

Polysaccharide cell wall.

No cell wall in animal cell, cellulose cell wall in plant cell.

DNA is circular and lies free in cytoplasm.

DNA is not circular and is contained in a nucleus.

DNA is naked

DNA is associated with protein, forming structure called chromosome.

There is no ER.

ER is present.