Download plant cells.

Cell Structure
The Cell




ESSENTIAL to the
study of biology
Simplest form of life
Every organism’s basic
unit of structure and
function
Named by Robert
Hooke in 1665 after
observing cork cells
(cell walls) under
microscope.
Prokaryotic vs. Eukaryotic
Cells


Prokaryote
 “before” “nucleus”/ NO
NUCLEUS/few organelles
 Bacteria
 DNA is concentrated in
nucleoid (non membranebound)
Eukaryote
 “true” “nucleus” / many
membranous organelles
 Protists, plants, fungi,
animals
 Nucleus with nuclear
membrane holds DNA
Why so small?

Metabolism requires
that cells stay small


As a cell grows, its
volume grows
proportionately more
than its surface area
Cells need a high
surface area to
volume ratio to
exchange materials
with their
environment through
plasma membrane.
Compartmental Organization of
Cells

Compartments (ORGANELLES)
provide different local environments
(pH, etc.)

Incompatible but equally important
processes can occur next to each
other in different “rooms”
Cytoskeleton & Related
Organelles

Cytoskeleton




Maintains shape of cell
Responsible for
movement of cell and
movement of organelles
within cell
Made of three types of
protein fibers:
Microtubules,
microfilaments, &
intermediate filaments
Cytoskeleton & Related
Organelles

Microtubules



Hollow tubes
Made up of A- and Btubulin
Responsible for:
• Cell motility
• cilia/flagella
• Chromosome
movements (mitosis)
• centrioles
• Movement of
organelles
• Maintenance of cell
shape
Cytoskeleton & Related
Organelles

Components of Cytoskeleton:
Microtubules – 25 nm diameter
 Intermediate Filaments – 8 – 12 nm
diameter
 Microfilaments – 7 nm diameter

Cytoskeleton & Related
Organelles

Intermediate
Filaments



Made up of fibrous
proteins
Made up of keratin
Responsible for:
• Structural support
• Maintenance of cell
shape
• Anchors nucleus &
certain organelles
Cytoskeleton & Related
Organelles

Microfilaments
Made up of 2 intertwined strands of
actin
 Responsible for:

•
•
•
•
•
Muscle contraction
Cytoplasmic streaming
Cell motility (pseudopodia)
Cell division (cleavage furrow)
Maintenance of/changes in cell shape
Centrioles





Only found in animal
cells
Visible only during cell
division
9+0 arrangement of
microtubules
May give rise to cilia &
flagella
May be involved in
formation of spindle
fibers in animal cells, but
not plants!
Flagella and Cilia
microtubule

Structures for cell
motility

Flagella (long & few in
#)

Cilia (short & numerous)

9 + 2 internal structure

Basal body has 9+0
structure like centrioles
Figure 4.25
Page 73
dynein
Cellular
Organelles

Nucleus:




“control center” of the cell
Surrounded by a nuclear
envelope
Contains DNA
Nucleolus: site of ribosome
synthesis
Cellular Organelles

Ribosomes


Site of protein assembly
Free and bound ribosomes
• Free: float through cytoplasm (make proteins for use inside
that cell)
• Bound: attached to Rough ER (make proteins to be
transported out of the cell)
Cellular Organelles

Endoplasmic Reticulum:
Made up of membranous tubules and
cisternae (sacs)
 Smooth ER: NO ribosomes attached

• Synthesis and transport of lipids
• Controls glucose glycogen conversion in
liver & muscles
• Detoxification of drugs and other poisons
• Sarcoplasmic reticulum (muscle ER) stores
calcium needed in muscle contraction.

Rough ER: ribosomes attached
• Synthesis & transport of proteins
Endomembrane System

Smooth and
Rough ER
Endomembrane System

Golgi
Apparatus:


Products of the
Endoplasmic
Reticulum are
modified and
stored here
Modifies &
packages
proteins
Endomembrane System

Lysosomes:



Used by cells to
digest
macromolecules
Sac of hydrolytic
enzymes
Apoptosis:
• Programmed cell
death

Usually found only
in animal cells
Endomembrane System

Vacuoles:



Food vacuoles
(storage)
Contractile vacuoles
(pump extra water out
of cells in freshwater
protists)
Central vacuole
(plant cells)
• Stores organic
compounds, inorganic
ions (K+, Cl-), and
water
• Surrounded by
tonoplast
Endomembrane System

Peroxisomes:
Contain enzymes that transfer
hydrogen from various substances to
oxygen, producing H2O2 as a
byproduct
 Various functions:

• Break fatty acids down into smaller
molecules for cellular respiration
• Detoxify alcohol in liver
Energy-related organelles

Mitochondria



Site of cellular
respiration (Energy
from the breakdown of
organic molecules is
used to phosphorylate
ADP to produce ATP)
“powerhouse of the
cell”
More metabolic activity
= more mitochondria
Energy-related organelles

Mitochondrial
Structure:
 Outer membrane
 Inner membrane:
• Cristae = large
surface area
makes more
efficient at
producing energy
 Intermembrane
space
 Mitochondrial
matrix
Energy-related organelles

Chloroplasts:



Found in plants and
eukaryotic algae
Site of photosynthesis
Contain the green
pigment chlorophyll
Energy-related organelles

Chloroplast
Structure

Thylakoids
• Grana = stacks of
thylakoids
• (Light Dependent
Phase)

Stroma
• Fluid outside the
thylakoids
• (Calvin Cycle)
Cellular
Organelles

Cell Wall






Found only in plant cells
Protects the cell Maintains
cell shape
Prevents excessive uptake
of water
Holds plant up against
gravity
Primary Cell Wall-thin;
cellulose
Secondary Cell Wallthicker; found in woody
plants
Cellular Organelles

Extracellular Matrix:


Found in animal cells
Made up of
glycoproteins (collagen)
& proteoglycans
• Proteins + carbohydrates


Provides support and
anchorage for cells
Differs from one cell
type to another
Intercellular Junctions


Neighboring cells are
connected to one
another
Plant cells:

Plasmodesmata:
• Channels in the cell
wall through which
strands of cytoplasm
pass through and
connect the living
contents of adjacent
cells
Intercellular Junctions (Animal Cells)



Tight junctionsmembrane proteins
interlock
Desmosomes,
(anchoring
junction)intermediate filaments
“sew” membranes
together
Gap junctionschannels align
allowing materials to
flow between cells
Intercellular Junctions

Tight junctions:



Membranes of
neighboring cells are
fused
Form a continuous
“belt” around a cell
Example: junction
between epidermis of
the skin
Intercellular Junctions

Desmosomes




Anchoring
junctions
Act as rivets
Muscle cells
are held
together by
desmosomes.
What happens
when a
muscle is
torn?
Intercellular Junctions

Gap junctions



Communicating
junctions
Cytoplasmic
channels
between adjacent
cells
Salts, sugars,
AAs, etc. can
pass through
Membrane Structure
& Function
Functions of the Cell
Membrane
Isolates the cytoplasm from the
external environment
 Regulates the flow of materials into
and out of the cell
 Communicates with other cells

The Fluid Mosaic Model




Currently accepted model of
the cell membrane
Proposed by Singer and
Nicolson in 1972
Phospholipid bilayer
 Hydrophilic “head” –
exposed to the outside
 Hydrophobic “tail” –
hides inside
Membrane proteins are
randomly dispersed in
phospholipid bilayer
Fluid Mosaic Model
Fluidity of the Membrane



The lipids and
proteins can drift
throughout the
membrane
Membrane is NOT
stiff/rigid
Cholesterol makes
the membrane
stronger by limiting
the movement of
phospholipids
Membrane as a Mosaic


Lipid bilayer has
membrane
proteins
embedded in it
Integral
proteins


Go through the
membrane
(both sides)
Peripheral
proteins

attached to the
surface of the
membrane
Selective Permeability of the
Cell Membrane
The cell membrane can “choose” what
enters and exits a cell
 “Gatekeeper of the Cell”

Passive Transport

Def’n:


Diffusion of a substance that does
NOT require the input of energy by the
cell
3 types of passive transport:
Diffusion
 Osmosis
 Facilitated diffusion

Diffusion
Movement of molecules from high to
low concentration until equilibrium is
reached.
Passive Transport= no energy required
What substances may diffuse across
membrane? Nonpolar (non-charged)
molecules; small polar molecules
Diffusion

Each substance
diffuses down its
OWN concentration
gradient and is
unaffected by
concentration
gradients of other
substances
Diffusion

Does all movement
stop once
equilibrium is
reached??


NO!!
Equal rates in all
directions
Osmosis

Def’n:


The passive transport of
water across a
selectively permeable
membrane
Hyper-, hypo-, iso- tonic
• RELATIVE TERMS!!
• Always referring to solute
concentration

Water moves from areas of
lower concentration of solutes
(hypotonic) to areas of higher
solute concentration
(hypertonic)
Osmosis in Plant and Animal
Cells
 Animal Cells:


Cell crenate/shrinks
Cytolysis
• Occurs when a cell is in
a hypotonic solution
• Water goes from
solution into cell

Plant Cells:
 Turgid- vacuole
swells
 Flaccid-vacuole
shrinks (plasmolysis)
Facilitated Diffusion

Def’n:
The diffusion of large molecules
across the cell membrane using
transport proteins
 Glucose; ions


Does NOT require an input of energy

Solute is still moving down its
concentration gradient
Facilitated Diffusion



Transport proteins are
specific for their solutes
Transport proteins can
become saturated
Some are gated
channels:

Chemical or electrical
stimulus causes them
to open
Example



Which direction
will sucrose
move?
Which direction
will glucose
move?
Which direction
will fructose
move?
Pop Quiz – Passive
Transport
1.
2.
3.
4.
What is passive transport across
the cell membrane?
Describe 3 types of passive
transport discussed yesterday.
If a blood cell is placed in a
hypotonic solution what would be
the result?
What type of solution is preferred
in plant cells.
Active Transport

Def’n:
The pumping of solutes against their
gradients
 Requires an input of energy by the cell
 Used so cells can “stockpile” extra
supplies (storage of any substance in
a very high concentration; i.e. Iodine
stored in thyroid; glycogen stored in
liver
 Sodium/Potassium Pump

Electrogenic Pumps
Voltage across membranes = stored
energy that can be used for cellular
work
 Sodium-Potassium Pump:

3 Na+ OUT of the cell for every 2 K+
pumped in
 Net transfer of one positive charge
from cytoplasm to extracellular fluid


Very important for transferring signals
between nerve cells
Sodium-Potassium Pump
Cotransport


Substance that has
been pumped
across a
membrane can do
work as it “leaks”
back by diffusion
Another substance
“hitches a ride”
Endocytosis & Exocytosis

Def’n:
The movement of large molecules
(polysaccharides, proteins, etc.)
across the membrane using vesicles
 Endocytosis = cell takes in
macromolecules
 Exocytosis = cell secretes
macromolecules

Endocytosis




Cell takes in macromolecules
by forming vesicles made
from the plasma membrane
Phagocytosis = “cell eating”
 Large molecules
Pinocytosis = “cell drinking”
 Small molecules & liquids
Receptor-mediated
endocytosis = seeks out
specific molecules
Exocytosis


The cell secretes
macromolecules by
the fusion of
vesicles with the
plasma membrane
Used to release
hormones,
chemical signals,
etc.
Pop Quiz 2
1.
2.
3.
4.
What is Active Transport across the cell
membrane?
What does it mean when molecules are
moving up the concentration gradient?
Describe (in detail) how the sodium
potassium pump works in nerve cells.
What is the difference between
phagocytosis and pinocytosis?