Cell membranes1

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   --- Structure & Function

                              Dr J Raju

Cell theory is based upon the work of Matthias
    Schleiden, Theodore Schwann, and Rudolph

Cell theory states that:

      All organisms are composed of cells
      Cells are the basic unit of structure and
      function in organisms
      All cells come from preexisting cells through
      cell division
Three Major Parts of Cells

1.   Cell (plasma) membrane - barrier between inside and
     outside (skin)

2.   Cytoplasm - organelles, free proteins, ions (guts)

3.   Nucleus - Control center for decision-making,
     responding to environment and replicating genetic
     material (nervous system)
Basic unit of structure & function in body
Highly organized molecular factory
Cell is smallest unit having properties of life
  Why are cells so
• 10 – 100m
• Must be small to minimize
  energy consumption…
• But large enough to
  minimize heat loss
      Size determines rate of life
• Large enough to house
  organelles needed to
  eat, grow, reproduce
• Small enough that little
  energy & time is
  needed for transport of
  gases, food & waste
• Maximize volume
  Observing Cells
• Light microscope
   – Can observe living cells in true color
   – Magnification of up to ~1000x
   – Resolution ~ 0.2 µm – 0.5 µm

• Electron Microscopes
   – Preparation needed kills the cells
   – Images are black and white – may be colorized
   – Magnification up to ~100,000
      • Transmission electron microscope (TEM) 2D image
      • Scanning electron microscope (SEM) 3D image

Types of Eukaryotic cells

              Most only have plasma membranes
  Nucleus: Control center
• Holds genetic code and
  “machinery” for
  replication and
• Bounded by nuclear
  envelope (inner and
  outer membrane)
                             • Nuclear pores
• Present in every cell at
                             • Chromatin
  some life-stage
                             • Chromosomes
• Chromosomes?
  – Hypercoiled DNA
• Chromatin?
  – Histone coiled DNA
• 1 - 4 per nucleus

• Produce ribosomal
  RNA (rRNA)

• Site of large and
  small ribosomal
  subunit formation
  via attachment of
  rRNA and
  ribosomal protein.
     Where is the cytoplasm?
• Located between cell membrane & nuclear
• Consists of:
  – Cytosol: intracellular fluid (mostly H20, ions &
    buffering proteins)
  – Organelles: structures with specific functions;
    suspended in cytosol
  – Inclusions: Insoluble materials (lipids)
• Site of protein synthesis
• Consist of 2 subunits, each
  made of rRNA + protein

• Two varieties
  – Free ribosomes: produce
    proteins that travel to nucleus

  – Fixed ribosomes: produce
    proteins for export to
    Endoplasmic Reticulum (ER)
     Endoplasmic Reticulum (ER)
•   Network of membranes
    connected to nuclear
•   4 major functions
    1. Synthesis (proteins,
       carbohydrates, lipids)
    2. Storage
    3. Transport
    4. Detoxification
•   Two types
        - Smooth
        - Rough
           Smooth ER (SER)
• Why is it called smooth?
• Responsible for the synthesis and storage of:
  – Phospholipids and cholesterol for maintenance
    and growth of membranes of cell, ER, nucleus,
    Golgi apparatus (GA)
  – Steroid hormones: estrogens and androgens
  – Glycerides in liver and fat cells
  – Glycogen in skeletal muscle and liver cells
              Rough ER (RER)
• Workshop
• Synthesizes proteins (it
  has fixed ribosomes!!)
  and may chemically
  modify them.
  – Polypeptide chains
    migrate into cisternae,
    assume tertiary
    structure + additional
• Ships proteins to GA
  via transport vesicles
             Golgi Apparatus

• Packing & shipping depot
• Consists of 5-6 flattened
  membranous disks
     Functions of GA
•   Produces 3 export vesicles:
    1. Secretory - exocytosis
    2. Membrane renewal – replacement & remodeling
    3. Lysosomes - “Primary” contain inactive digestive
Lysosome Functions
 Abnormal lysosomes
• Lack, or have malfunctioning enzymes
   – Normal cell products accumulate & stifle
     (suffocate) cells
• Tay-Sachs disease
   – Lysosomes lack enzymes that break down lipids
     in nerve cells
• Pompe’s disease
   – Lysosomes lack hydrolytic enzyme that splits
• Contain digestive enzymes

• Functions:
  – Absorb and breakdown fatty acids and nucleic
    acids - produces H2O2 (danger!)
  – Convert free radicals to H2O2
  – Coverts H2O2 to harmless H2O and O2, using
Mitochondria = Powerhouse
    Mitochondria harness energy!
• “powerhouse” of the
  cell…makes ATP
• Double membrane
• Number per cell
  varies with metabolic
  activity (0% volume of RBC,
  20% volume of liver cell)
        Chloroplasts make food!
• Photosynthetic
• Converts light energy &
  CO2 to sugars
• Stroma: tubules &
  membranous disks
• Grana: stacks of disks;
  membranes chock full
  of chlorophyll, which
  traps solar energy

• Thinnest cytoskeletal elements (rodlike)
• Composed of the globular protein actin
• Enable cells to change shape and move
• Intermediate filaments
  – Present only in animal cells of
    certain tissues

  – Fibrous proteins join to form a
    rope-like structure
     • Provide internal structure
     • Anchor organelles in place.
• Microtubules – long hollow
  tubes made of tubulin proteins
  – Anchor organelles and act as
    tracks for organelle movement
  – Move chromosomes around
    during cell division
     • Used to make cilia and flagella
Plant cells:
Cell wall: maintains structure;
protection from environment;
limits water absorption
Central vacuole: storage of
nutrients; gets rid of waste
products; maintains pH;
enzymes for digestion;
contains pigments
Plastids: storages nutrients;
contains pigments; needed for
photosynthesis (chloroplast)
Amyloplast: makes starch
Do not contain lysosomes…in
animals only!!!
Occurring in sheets of tightly packed cells, epithelial
   tissue covers the outside of the body and lines
         organs and cavities within the body.

 – The cells of a epithelium are closely joined and in
   many epithelia, the cells are riveted together by tight

 – The epithelium functions as a barrier protecting against
   mechanical injury, invasive microorganisms, and fluid

 – The free surface of the epithelium is exposed to air or
   fluid, and the cells at the base of the barrier are
   attached to a basement membrane, a dense mat of
   extracellular matrix.
Epithelia are
classified by the
number of cell
layers and the
shape of the cells
on the free
• Some epithelia, called glandular epithelia, absorb or
  secrete chemical solutions.
   – For example, glandular epithelia lining tubules in the
     thyroid gland secrete a hormone that regulates fuel
   – The glandular epithelia that line the lumen of the
     digestive and respiratory tracts form a mucous
     membrane that secretes a slimy solution called mucus
     that lubricates the surface and keeps it moist.
      • The free epithelial surfaces of some mucous
        membranes have beating cilia that move the film of
        mucus along the surface.
      • In the respiratory tubes, this traps dust and particles.
Why cells must control materials?

  The plasma
  membrane is the
  boundary between
  the cell and its
It is the plasma membrane’s job to:
• allow a steady supply of glucose, amino acids,
  and lipids to come into the cell no matter what
  the external conditions are.

• remove excess amounts of these nutrients when
  levels get so high that they are harmful.

• allow waste and other products to leave the
This process of maintaining the cell’s
environment is called homeostasis.

Selective permeability is a process used to
maintain homeostasis in which the plasma
membrane allows some molecules into the
cell while keeping others out.

      Cell Membrane Functions
• Physical isolation - separates inner and outer
• Sensory receptor - membrane receptor
  proteins sense changes in external environment
  (encrusted with peripheral nerves)

• Regulates exchange with the environment
  - membrane channel proteins + carrier proteins
• Structural support - intercellular protein
    Structure of cell membrane
• 6-10 nm thick
• Contains lipids, proteins and carbohydrates
  – Lipids
     • Phospholipids; Cholesterol; Glycolipids
  – Proteins
     • Integral; Peripheral
  – Carbohydrates
     • Form glycocalyx (identity)
                                                                       oligosaccharide        cholesterol

                                    EXTRACELLULAR ENVIRONMENT

   (cytoskeletal pro-
   teins beneatch                 open    gated     gated      active      RECEPTOR             LIPID BILAYER
   the plasma         ADHESION
                                  channel channel   channel    transpor     PROTEIN
   membrane)           PROTEIN
                                  protein proten    proten     t protein
                                          (open)    (closed)                          RECOGNITION
    (area of                                                                            PROTEIN
                                     TRANSPORT PROTEINS                    CYTOPLASM

                                 PLASMA MEMBRANE
  Membrane is a collage of proteins & other molecules
    embedded in the fluid matrix of the lipid bilayer

 Glycoprotein        Extracellular fluid


                                             Filaments of
                         Cytoplasm           cytoskeleton
          Plasma Membrane
• Surrounds & gives cell form; selectively
• Formed by a double layer of phospholipids
  – restricts passage of polar compounds
    Plasma Membrane continued
• Proteins customize membranes
  – Provide structural support
  – Serve as transporters, enzymes, receptors &
    identity markers
     Plasma Membrane continued
• Carbohydrates in form of glycoproteins &
  glycolipids are part of outer surface
  – Impart negative charge to surface
 Structure of the Plasma Membrane
      Asymmetrical; the two halves are not identical

The plasma
membrane is
composed of two
layers of

 Phospholipids are lipids with a phosphate
            attached to them.
The lipids in a      Phosphate Group
plasma membrane
have a glycerol      Glycerol
backbone, two        Backbone
fatty acid chains,
and a phosphate          Two Fatty
group.                   Acid
Makeup of the phospholipid bilayer

The phosphate           Phosphate
group is critical for   Group
the formation and
function of the
Membrane Models
• Robertson- Unit membrane
• Singer and Nicolson - Fluid-Mosaic Model
    - Membrane structure is not rigid (fluid)

    - Membrane comprised of diff. molecules (mosaic)

    - Proteins float around the surface of the cells

    - Proteins, Carbohydrates, Phospholipids can be
      added/removed from the surfaces of cells
Membrane Models
   Makeup of the phospholipid bilayer
The fluid mosaic model describes the plasma
membrane as a flexible boundary of a cell. The
phospholipids move within the membrane.

FLUID- because individual phospholipids and proteins can
  move around freely within the layer, like it’s a liquid.

MOSAIC- because of the pattern produced by the
 scattered protein molecules when the membrane is
 viewed from above.
Other components of the plasma membrane

 Cholesterol plays the important role of
 preventing the fatty acid chains of the
 phospholipids from sticking together.

  Membrane Movement and Cholesterol
• Most of the lipids and some
  proteins can drift laterally in
  the plane of the membrane,
  but rarely flip-flop from one
  layer to the other.

• Cholesterol is wedged
  between phospholipids
  molecules in the plasma
  membrane of animals cells. It
  restrains the movement of
  the phospholipids in warm
  temps. and maintains fluidity
  by preventing tight packing at
  cold temps.
          Membranes are fluid
• The higher the concentration of unsaturated fats
  the more fluid the membrane.

• Fluidity of membrane structure helps maintain a
  pliable (flexible) membrane very important for
  example red blood cells.

       Glycolipids have a structure similar to
  phospholipids except that the hydrophilic head is
  a variety of sugars joined to form a straight or
           branching carbohydrate chain.
    Experiment to demonstrate lateral
         movement of proteins
• Tagged membrane receptors
  move in the membrane at
  about 2m per second

• When two cells (which have
  different receptor proteins)
  are fused.

• The receptors move and
  become evenly dispersed.
                 MEMBRANE PROTEINS
• Each cell: 10-50 different types of membrane proteins.
• Proteins determine membrane’s specific functions.
• Cell membrane & organelle membranes each have unique collections
  of proteins

Peripheral proteins (Cell surface identity marker (antigens))

- Attached loosely to membrane spanning proteins or polar
  regions of phospholipids
- Can be removed w/o destroying membrane
- Include enzymes and binding proteins
  that anchor cell to membrane.

Integral proteins (Transmembrane proteins)
- Tightly bound to phospholipid bilayer
- Can’t be removed w/o destroying membrane
- Most span the entire membrane
               Protein Functions
• Channel Proteins - Involved in passage of molecules
  through membrane.
• Carrier Proteins - Combine with substance to aid in
  passage through membrane.
• Cell Recognition Proteins - Help body recognize foreign
• Receptor Proteins - Allow molecule binding, causing
  protein to change shape and bring about cellular change.
• Enzymatic Proteins - Carry out metabolic reactions
   Many integral proteins are glycoproteins, which have
  an attached carbohydrate chain, similar to glycolipids.
  Therefore it can be said that the plasma membrane is
                      ‘Sugar coated’
Protein Functions
       Classes of Amino acids
What do these amino acids have in common?

          Nonpolar & hydrophobic
        Classes of amino acids
What do these amino acids have in common?

           Polar & hydrophilic
     Proteins domains anchor molecule
• Within membrane                                   Polar areas
                                                     of protein
  – nonpolar amino acids
      • hydrophobic
      • anchors protein
        into membrane

• On outer surfaces of membrane
  – polar amino acids
     • hydrophilic
     • extend into extracellular
       fluid & into cytosol

                                   Nonpolar areas of protein
 Membrane carbohydrates
Play a key role in cell-cell recognition
(The carbohydrate chains of glycolipids and glycoproteins
  serve as the “fingerprints” of the cell)

Glycolipids and glycoproteins vary from species to species
  and even from cell to cell in the same individual.

   – ability of a cell to distinguish one cell from another.
   – important in organ & tissue development.
   – basis for rejection of foreign cells by immune system.
   – Person’ particular blood group is due to the presence
     of particular glyoproteins in the membrane of RBC.
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