Cell Structure and Components

Cell Theory

  • 1. All living organisms have one or more cells
  • 2. Cells are the basic unit of life
  • 3. Cells come from pre-existing cells

There are a few exceptions but nonetheless, cell theory is still useful and has not been rejected.

Common Cell Structures

    1. Plasma Membrane

  • Outer boundary of the cell and all of its contents
  • Controls the entry and exit of substances
  • Maintains the concentration gradient in the cell
  • Made of a phospholipid bilayer

    2. Cytoplasm

  • Composed mainly of water
  • Many substances are dissolved/suspended in cytoplasm (e.g. enzymes)

    3. DNA (Nucleic Acids)

  • Genes contain the information needed for a cell to carry out all of its functions
  • Many genes hold instructions for making proteins
  • The usage of DNA as genetic material is universal and common to all cells
    • However the location of DNA may vary between cell types

Prokaryotic vs Eukaryotic Cell Structure

    Prokaryotes

  • DNA Location: In the nucleoid
  • Ribosomes: 70S
  • Has Membrane-Bound Organelles: No
  • Internal Structure: Simple, organelles are exposed and suspended in cytoplasm

    Eukaryotes

  • DNA Location: In the nucleus
  • Ribosomes: 80S
  • Has Membrane-Bound Organelles: Yes
  • Internal Structure: Complex, eukaryotes have a compartmentalized internal structure

Processes of Life in Organisms

  • Homeostasis - maintaining a constant internal environment in an organism
  • Metabolism - the sum of all biochemical reactions occurring in an organism
  • Nutrition - supplying nutrients needed for growth, energy, and repair in an organism
  • Excretion - the removal of waste products of the metabolism from an organism
  • Growth - the increase in the size or number of cells
  • Response to Stimuli - responding to stimuli in the environment
  • Reproduction - the production of offspring, either sexually or asexually

Organelles in Different Types of Cells

    Plastids

  • A family of organelles with two outer membranes and internal membrane sacs

  • Plants: Contain plastids of varied types
  • Fungi: None
  • Animals: None

    Cell Wall

  • A rigid layer outside the plasma membrane to strengthen and protect the cell

  • Plants: Composed of cellulose
  • Fungi: Composed of chitin
  • Animals: None

    Vacuoles

  • Flexible fluid-filled compartments surrounded by a single membrane

  • Plants: There is often a large permanent vacuole. Vacuoles here are used for the storage of substances and pressurizing the cell.
  • Fungi: There is often a large permanent vacuole. Vacuoles here are used for the storage of substances and pressurizing the cell.
  • Animals: Small temporary vacuoles expel excess water or digest food

    Centrioles

  • Cylindrical organelles that organize an assembly of structures composed of microtubules

  • Plants: Only present in male gametes, which have centrioles at the base of the flagellum
  • Fungi: Only present in male gametes, which have centrioles at the base of the flagellum
  • Animals: Centrioles are used to construct spindles that move chromosomes during mitosis

Organelles

    Nucleus

  • Contains chromosomes with DNA and histone proteins
  • Uncoiled chromosomes are spread widely throughout the nucleus
  • Denser areas contain coiled chromosomes

    Rough Endoplasmic Reticulum

  • Consists of flattened membrane sacs (cisterna) with ribosomes attached
  • Main function is to synthesize proteins
  • Proteins are passed through cisternae and carried by vesicles to the golgi apparatus

    Smooth Endoplasmic Reticulum

  • Consists of a branched network of tubular membranes
  • No ribosomes attached
  • Used to synthesize lipids, phospholipids, and steroids

    Lysosome

  • Spherical shape, formed from golgi vesicles
  • Contains a high concentration of digestive enzymes that are used to break down digested food, organelles, or sometimes whole cells

    Mitochondrion

  • Double membrane
  • Fluid inside is called the matrix
  • Spherical/ovoid shape
  • Produces ATP for the cell via aerobic cellular respiration
  • Fat is also digested here if it is being used as an energy source

    Ribosome

  • Constructed in the nucleolus
  • Free ribosomes synthesize proteins and release them into the cytoplasm

    Chloroplast

  • Double-membraned plastid
  • Spherical/ovoid shape
  • Contains flattened sacs of membrane called thylakoids
  • Produces glucose and other organic compounds via photosynthesis
  • May contain starch grains if found photosynthesizing rapidly

    Vacuole/Vesicles

  • Single membrane, contains fluid
  • Used for various reasons such as food digestion/storage or excretion, depending on the cell type
  • Vesicles are very small vacuoles used to transport items within the cell

    Microtubules/Centrioles

  • Cytoplasm contains small cylindrical fibers called microtubules
  • Have various roles, typically in mitosis or as part of cilia/flagella

    Cytoskeleton

  • Constructed from several types of protein fibre
    • Tubulin for microtubules and actin for microfilaments
  • Can be easily constructed or deconstructed
  • Guides the movement of components within the cell
  • A layer of microfilaments inside the plasma membrane helps animal cells maintain their structure

    Cilia and Flagella

  • Whip-like structures projecting from a cell's surface
  • Contain a ring of 9 double microtubules in addition to 2 central ones
  • Flagella are larger and usually only one is present
  • Cilia are smaller and many are present
  • Used for movement

Atypical Cell Structures in Eukaryotes

    Red Blood Cells

  • Nucleus is removed in the late stage of development
  • Allows for increased flexibility
  • Due to the lack of a nucleus, these cells cannot repair themselves when damaged
    • Results in a life span of 100 - 120 days

    Phloem Sieve Tubes

  • Conduct sap through tubular vessels
  • Made from columns of cylindrical cells
  • Dividing walls between cells are removed
  • The plasma membrane and cell contents break down, making the tube hollow
  • Sieve tubes are connected with pores to adjacent cells that contain a nucleus and mitochondria
    • This helps with the survival of sieve tube cells

    Skeletal Muscle

  • Multinucleate cells that are formed when cells fuse together
    • These structures are called syncytium
    • Muscle fibres develop in this way
  • Columns of cells, each with a nucleus, are formed by cell division
  • Cells then fuse together to form muscle fibres

    Aseptate Fungal Hyphae

  • Cells grow and divide their nucleus without cell division
  • Thread-like hyphae of some fungi develop like this
  • Walls that divide hyphae into uninucleate cells are called septa
  • Hyphae without these walls are multinucleate and aseptate