Chapter 6 A Tour of the Cell

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Chapter 6 A Tour of the Cell
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Faith is a fine invention when gentlemen can
see, but microscopes are prudent in an
emergency.Emily Dickinson
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Observation

• Is the keystone of science.
• Need Techniques to observe
  cells.

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Question ?

• Can cells be seen with the naked eye?
• Yes, a few are large enough, but most require the
  use of a microscope.

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Microscope History

• 1590 - Janseen Brothers invent the compound
  microscope.
• 1665 - Robert Hooke discovers cells in cork.
• Early 1700s - von Leeuwenhoek makes many
  observations of cells including bacteria.

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Light Microscope - LM

• Uses visible light to illuminate the object.
• Relatively inexpensive type of microscope.
• Can examine live or dead objects.

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Light Microscope
Occular Lens
Objective Lens
Stage with specimen
Light Source
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Magnification

• Increase in diameter or size.

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Resolution

• Ability to detect two discrete points as separate
  from each other.
• As Magnification increases, resolution decreases.
  
• LM working limits are 100 - 1000X.

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Limitations - LM

• Miss many cell structures that are beyond the
  magnification of the light microscope.
• Need other ways to make the observations.

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Light Microscope Variations

• Fluorescence uses dyes to make parts of cells
  glow.
• Phase-contrast enhances contrasts in density.
• Confocal uses lasers and special optics to focus
  only narrow slides of cells.

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Electron Microscopes

• Use beams of electrons instead of light.
• Invented in 1939, but not used much until after
  WWII.

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TEM
SEM
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Advantages

• Much higher magnifications.
• Magnifications of 50,000X or higher are possible.
  
• Can get down to atomic level in some cases.

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Disadvantages

• Need a Vacuum.
• Specimen must stop the electrons.
• High cost of equipment.
• Specimen preparation.

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Transmission Electron Microscope - TEM

• Sends electrons through thinly sliced and stained
  specimens.
• Gives high magnification of interior views. Many
  cells structures are now visible.

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TEM Limitations

• Specimen dead.
• Specimen preparation uses extreme chemicals so
  artifacts are always a concern.

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Scanning Electron Microscope - SEM

• Excellent views of surfaces.
• Produces 3-D views.
• Live specimens possible.

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Limitations

• Lower magnifications than the TEM.

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EM Variations

• High Voltage TEM
• Tunnel SEM
• Elemental Composition SEM

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TEM - interior
SEM - surface
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Cell Biology or Cytology

• Cyto cell -
  ology study of
• Should use observations from several types of
  microscopes to make a total picture of how a cell
  is put together.

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Other Tools for Cytology

• Cell Fractionation
• Chromatography
• Electrophoresis

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Cell Fractionation

• Disrupt cells.
• Separate parts by centrifugation at different
  speeds.
• Result - pure samples of cell structures for
  study.

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Cell Fractionation
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Chromatography

• Technique for separating mixtures of chemicals.
• Separates chemicals by size or degree of
  attraction to the materials in the medium.
• Ex - paper, gas, column,
• thin-layer

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Electrophoresis

• Separates mixtures of chemicals by their movement
  in an electrical field.
• Used for proteins and DNA.

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History of Cells

• Robert Hooke - Observed cells in cork.
• Coined the term "cells in 1665.

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History of Cells

• 1833 - Robert Brown, discovered the nucleus.
• 1838 - M.J. Schleiden, all plants are made of
  cells.
• 1839 - T. Schwann, all animals are made of cells.
  
• 1840 - J.E. Purkinje, coined the term
  protoplasm.

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Cell Theory

• All living matter is composed of one or more
  cells.
• The cell is the structural and functional unit of
  life.
• All cells come from pre-existing cells

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Types of Cells

• Prokaryotic - lack a nucleus and other membrane
  bounded structures.
• Eukaryotic - have a nucleus and other membrane
  bounded structures.

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Prokaryotic
Eukaryotic
Nucleus
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Eukaryotic
Prokaryotic
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How small can a cell be?

• Mycoplasmas - bacteria that are .1 to 1.0 mm.
  (1/10 the size of regular bacteria).

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Why Are Cells So Small?

• Cell volume to surface area ratios favor small
  size.
• Nucleus to cytoplasm consideration (control).
• Metabolic requirements.

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Basic Cell Organization

• Membrane
• Nucleus
• Cytoplasm
• Organelles

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Animal Cell
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Plant Cell
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Membrane

• Separates the cell from the environment.
• Boundary layer for regulating the movement of
  materials in/out of a cell.

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Cytoplasm

• Cell substance between the cell membrane and the
  nucleus.
• The fluid part of a cell. Exists in two forms
  
• gel - thick
• sol - fluid

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Organelle

• Term means "small organ Formed body in a cell
  with a specialized function.
• Important in organizational structure of cells.

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Organelles - function

• Way to form compartments in cells to separate
  chemical reactions.
• Keeps various enzymes separated in space.

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Nucleus

• Most conspicuous organelle.
• usually spherical, but can be lobed or irregular
  in shape.

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Structure

• Nuclear membrane
• Nuclear pores
• Nucleolus
• Chromatin

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Nuclear Membrane

• Double membrane separated by a 20-40 nm space.
• Inner membrane supported by a protein matrix
  which gives the shape to the nucleus.

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Nuclear Pores

• Regular holes through both membranes.
• 100 nm in diameter.
• Protein complex gives shape.
• Allows materials in/out of nucleus.

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Nucleolus

• Dark staining area in the nucleus.
• 0 - 4 per nucleus.
• Storage area for ribosomes.

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Chromatin

• Chrom colored
• - tin threads
• DNA and Protein in a loose format. Will form
  the cells chromosomes.

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Nucleus - Function

• Control center for the cell.
• Contains the genetic instructions.

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Ribosomes

• Structure 2 subunits made of protein and rRNA.
  No membrane.
• Function protein synthesis.

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Subunits

• Large
• 45 proteins
• 3 rRNA molecules
• Small
• 23 proteins
• 1 rRNA molecule

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Locations

• Free in the cytoplasm - make proteins for use in
  cytosol.
• Membrane bound - make proteins that are exported
  from the cell.

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Endomembrane System

• Membranes that are related through direct
  physical continuity or by the transfer of
  membrane segments called vesicles.

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Endomembrane System
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Endoplasmic Reticulum

• Often referred to as ER.
• Makes up to 1/2 of the total membrane in cells.
• Often continuous with the nuclear membrane.

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Structure of ER

• Folded sheets or tubes of membranes.
• Very fluid in structure with the membranes
  constantly changing size and shape.

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Types of ER

• Smooth ER no ribosomes.
• Used for lipid synthesis, carbohydrate storage,
  detoxification of poisons.
• Rough ER with ribosomes.
• Makes secretory proteins.

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Golgi Apparatus or Dictyosomes

• Structure parallel array of flattened cisternae.
  (looks like a stack of Pita bread)
• 3 to 20 per cell.
• Likely an outgrowth of the ER system.

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Structure Has 2 Faces

• Cis face - side toward the nucleus. Receiving
  side.
• Trans face - side away from the nucleus. Shipping
  side.

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Function of Golgi Bodies

• Processing - modification of ER products.
• Distribution - packaging of ER products for
  transport.

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Golgi Vesicles

• Small sacs of membranes that bud off the Golgi
  Body.
• Transportation vehicle for the modified ER
  products.

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Lysosome

• Single membrane.
• Made from the Trans face of the Golgi apparatus.

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Function

• Breakdown and degradation of cellular materials.
• Contains enzymes for fats, proteins,
  polysaccharides, and nucleic acids.
• Over 40 types known.

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Lysosomes

• Important in cell death.
• Missing enzymes may cause various genetic enzyme
  diseases.
• Examples Tay-Sachs, Pompes Disease

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Vacuoles

• Structure - single membrane, usually larger than
  the Golgi vesicles.
• Function - depends on the organism.

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Protists

• Contractile vacuoles - pump out excess water.
• Food vacuoles - store newly ingested food until
  the lysosomes can digest it.

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Plants

• Large single vacuole when mature making up to 90
  of the cell's volume.
• Tonoplast - the name for the vacuole membrane.

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Function

• Water regulation.
• Storage of ions.
• Storage of hydrophilic pigments.
  (e.g. red and blues in flower petals).

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Function Plant vacuole

• Used to enlarge cells and create turgor pressure.
• Enzymes (various types).
• Store toxins.
• Coloration.

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Microbodies

• Structure single membrane.
• Often have a granular or crystalline core of
  enzymes.

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Function

• Specialized enzymes for specific reactions.
• Peroxisomes use up hydrogen peroxide.
• Glyoxysomes lipid digestion.

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Enzymes in a crystal
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Mitochondria

• Structure 2 membranes. The inner membrane has
  more surface area than the outer membrane.
• Matrix inner space.
• Intermembrane space area between the membranes.

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Inner Membrane

• Folded into cristae.
• Amount of folding depends on the level of cell
  activity.
• Contains many enzymes.
• ATP generated here.

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Function

• Cell Respiration - the release of energy from
  food.
• Major location of ATP generation.
• Powerhouse of the cell.

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Mitochondria

• Have ribosomes.
• Have their own DNA.
• Can reproduce themselves.
• May have been independent cells at one time.

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Chloroplasts

• Structure - two outer membranes.
• Complex internal membrane.
• Fluid-like stroma is around the internal
  membranes.

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Inner or Thylakoid Membranes

• Arranged into flattened sacs called thylakoids.
• Some regions stacked into layers called grana.
• Contain the green pigment chlorophyll.

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Function

• Photosynthesis - the use of light energy to make
  food.

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Chloroplasts

• Contain ribosomes.
• Contain DNA.
• Can reproduce themselves.
• Often contain starch.
• May have been independent cells at one time.

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Plastids

• Group of plant organelles.
• Structure - single membrane.
• Function - store various materials.

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Examples

• Amyloplasts/ Leucoplasts - store starch.
• Chromoplasts - store hydrophobic plant pigments
  such as carotene.

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Ergastic Materials

• General term for other substances produced or
  stored by plant cells.
• Examples
• Crystals
• Tannins
• Latex
• Resins

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Cytoskeleton

• Network of rods and filaments in the cytoplasm.

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Functions

• Cell structure and shape.
• Cell movement.
• Cell division - helps build cell walls and move
  the chromosomes apart.

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Components

• Microtubules
• Microfilaments
• Intermediate Filaments

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Microtubules

• Structure - small hollow tubes made of repeating
  units of a protein dimer.
• Size - 25 nm diameter with a 15 nm lumen. Can be
  200 nm to 25 mm in length.

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Tubulin

• Protein in microtubules.
• Dimer - a and b tubulin.

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Microtubules

• Regulate cell shape.
• Coordinate direction of cellulose fibers in cell
  wall formation.
• Tracks for motor molecules.

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Microtubules

• Form cilia and flagella.
• Internal cellular movement.
• Make up centrioles, basal bodies and spindle
  fibers.

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Cilia and Flagella

• Cilia - short, but numerous.
• Flagella - long, but few.
• Function - to move cells or to sweep materials
  past a cell.

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Cilia and Flagella

• Structure - 92 arrangement of microtubules,
  covered by the cell membrane.
• Dynein - motor protein that connects the tubules.

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Dynein Protein

• A contractile protein.
• Uses ATP.
• Creates a twisting motion between the
  microtubules causing the structure to bend or
  move.

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Centrioles

• Usually one pair per cell, located close to the
  nucleus.
• Found in animal cells.
• 9 sets of triplet microtubules.
• Help in cell division.

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Basal Bodies

• Same structure as a centriole.
• Anchor cilia and flagella.

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Basal Body
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MTOCs

• Microtubule Organizing Centers - sites that
  microtubules grow from.
• Assist in cell division by anchoring spindle
  fibers.
• May be anchored by centrioles.

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Microfilaments

• 5 to 7 nm in diameter.
• Structure - two intertwined strands of actin
  protein.

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Microfilaments are stained green.
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Functions

• Muscle contraction.
• Cytoplasmic streaming.
• Pseudopodia.
• Cleavage furrow formation.
• Maintenance and changes in cell shape.

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Intermediate Filaments

• Fibrous proteins that are super coiled into
  thicker cables and filaments 8 - 12
  nm in diameter.
• Made from several different types of protein.

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Functions

• Maintenance of cell shape.
• Hold organelles in place.

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Cytoskeleton

• Very dynamic changing in composition and shape
  frequently.
• Cell is not just a "bag" of cytoplasm within a
  cell membrane.

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Cell Wall

• Nonliving jacket that surrounds some cells.
• Found in
• Plants
• Prokaryotes
• Fungi
• Some Protists

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Plant Cell Walls

• All plant cells have a Primary Cell Wall.
• Some cells will develop a Secondary Cell Wall.

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Primary Wall

• Thin and flexible.
• Cellulose fibers placed at right angles to
  expansion.
• Placement of fibers guided by microtubules.

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Secondary Wall

• Thick and rigid.
• Added between the cell membrane and the primary
  cell wall in laminated layers.
• May cover only part of the cell giving spirals.
• Makes up "wood.

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Middle Lamella

• Thin layer rich in pectin found between adjacent
  plant cells.
• Glues cells together.

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Cell Walls

• May be made of other types of polysaccharides
  and/or silica.
• Function as the cell's exoskeleton for support
  and protection.

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Extracellular Matrix - ECM

• Fuzzy coat on animal cells.
• Helps glue cells together.
• Made of glycoproteins and collagen.
• Evidence suggests ECM is involved with cell
  behavior and cell communication.

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Intercellular Juctions

• Plants-Plasmodesmata

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Plasmodesmata

• Channels between cells through adjacent cell
  walls.
• Allows communication between cells.
• Also allows viruses to travel rapidly between
  cells.

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Intercellular Juctions

• Animals
• Tight junctions
• Desmosomes
• Gap junctions

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Tight Junctions

• Very tight fusion of the membranes of adjacent
  cells.
• Seals off areas between the cells.
• Prevents movement of materials around cells.

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Desmosomes

• Bundles of filaments which anchor junctions
  between cells.
• Does not close off the area between adjacent
  cells.
• Coordination of movement between groups of cells.

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Gap Junctions

• Open channels between cells, similar to
  plasmodesmata.
• Allows communication between cells.

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Summary

• Answer Why is Life cellular and what are the
  factors that affect cell size?
• Be able to identify cellular parts, their
  structure, and their functions.