2.1 - Cell Structure and Function

Cells are defined as the basic units of life. Imagine cells as all the tiles needed to build a house. Similarly, millions of cells are needed to make up an organism. They can be mainly divided into animal cells and plant cells.

Animal Cells

Figure 1 - Animal cell and its basic cell components.

Cell Components/Organelles

• Plasma membrane

  • Outer membrane that separates the inside and outside of the cell
  • Made up of proteins and phospholipid bilayer
  • Semi-permeable
  • Functions:
    • Enclose all cell components within the cytoplasm.
    • Regulate exchange of ions, nutrients, respiratory gases and waste materials between the cell and external environment.

• Nucleus (Plural: nuclei)

  • Largest component in the cell
  • Contains nucleolus, chromosomes (DNA) and nucleoplasm
  • Surrounded by nuclear membrane
  • Functions:
    • Control all cell activities, which include the characteristics of the cell and metabolic processes.

• Cytoplasm

  • Gel-like fluid that fills the cells
  • Contains proteins, ions, sugars, amino acids, nucleotides and mostly water
  • Functions:
    • Support the internal structure of the cell.
    • Maintain shape and consistency of the cell.
    • Stores various chemical substances essential for life.
    • Medium for metabolic activities.

• Rough endoplasmic reticulum (RER)

  • Its membrane is continuous with the nuclear membrane
  • Contains ribosomes on the surface and hence the rough surface observed
  • Functions:
    • Presence of ribosomes help to synthesize proteins.
    • Transport proteins to Golgi apparatus for modification.
    • Provide surface for enzyme attachment and biochemical processes.

• Smooth endoplasmic reticulum

  • Membrane-bound network of tubules
  • Absence of ribosomes on its surface
  • Functions:
    • Synthesize lipids, steroids, carbohydrates.
    • Involved in the metabolism of drugs or toxins.

• Mitochondrion(Plural: mitochondria)

  • Membrane-bound organelle
  • Contains two layers of membrane – outer and inner membranes
  • Functions:
    • Generate energy in the form of adenosine triphosphate (ATP) during cellular respiration.

• Golgi apparatus

  • Membrane-bound organelle
  • Single layer membrane
  • Functions:
    • Modify, package and transport carbohydrates, proteins and lipids into vesicles to be transported out of the cell.

• Centrioles

  • Tiny paired cylindrical organelles located near nuclear membrane
  • Made up of microtubules
  • Absent in plant cells
  • Functions:
    • Form spindle fibres to separate chromosomes during cell division.

• Lysosome

  • Small circular membrane-bound organelle
  • Contains digestive enzymes called lysozymes
  • Functions:
    • Destroy bacteria and viruses.
    • Break down damaged or dead cells in the body.
    • Digest macromolecules such as proteins, lipids and nucleic acids.

• Ribosome

  • Can be found in the cytoplasm or on the surface of RER
  • Made up of two subunits – small and large ribosomal subunits
  • Functions:
    • Involved in protein synthesis.

*** The above listed organelles can also be found in plant cells, except centrioles

Plant Cells

Figure 2 - General structure of a plant cell.

Cell Components/Organelles

• Cell wall

  • Only exist in plant cells
  • Outer membrane that surrounds the plasma membrane
  • Made up of cellulose
  • Fully permeability
  • Functions:
    • Protect the cell from osmotic and mechanical stress.
    • Help to maintain the shape of the cell.

• Chloroplast

  • Can be found only in plant cells
  • Double layer membrane
  • Contains chlorophyll which gives the plant its green color
  • Functions:
    • Convert light energy to chemical energy during photosynthesis.

• Vacuole

  • A space within cell that does not contain cytoplasm
  • Surrounded by tonoplast membrane
  • Larger vacuole in plant cells; smaller in animal cells
  • Contains water, enzymes, ions, respiratory gases and waste products
  • Functions:
    • Store water to maintain turgidity of the cell.
    • Excrete unwanted substances out of the cell.

*** The above listed organelles can only be found in plant cells, except vacuole

Similarities and differences between animal and plant cells

Similarities

Both have plasma membrane, nucleus, cytoplasm, Golgi apparatus, mitochondria, endoplasmic reticulum and ribosomes

Differences

• Animal cell

  • Irregular shape
  • Absent cell wall
  • Absent chloroplast
  • Smaller or absent vacuoles
  • Carbohydrate storage form: Glycogen
  • Present centrioles

• Plant cell

  • Fixed shape (due to presence of cell wall)
  • Present cell wall
  • Present chloroplast
  • Larger vacuoles
  • Carbohydrate storage form: Starch
  • Absent centrioles