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Cells
Topic 2
Cell Theory
Schleiden, Schawnn, and Virchow
Three components:
(1) All living things are made of cells.
(2) Cells are the basic unit of structure and
function in living organisms.
(3) Cells come from other cells.
Cells are microscopic
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Most cells can’t be seen without a microscope
Smallest cells are bacteria with diameters as
small as .2 um.
The bulkiest cells are bird eggs
Longest human cells are certain muscle and
nerve cells.
Most plant and animal cells (10 to 100 um)are
ten times larger than most bacteria
Refer to figure 4.2A on p. 54
Surface area to volume ratio of cells
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The max. size of a cell is influenced by its
requirement for enough surface area to obtain
adequate nutrients and oxygen from the
environment and dispose of wastes.
Cells must be very tiny to ensure the surface area
is much larger than the volume for efficiency in
a cell.
Prokaryotic Cells
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Bacteria and Archaebacteria
Very small cells, can only be seen with electron
microscope
Range from 1 to 10 um in length, about 1/10 the
size of a eukaryotic cell
Lack a nucleus and membrane bound organelles.
DNA is coiled in a nucleoid
Have ribosomes, cell wall, capsule, pili, and
flagella
Reproduce asexually via binary fission
Prokaryotic cells
Cell Wall
Protects the cell and gives it shape
Capsule
Surrounds the cell wall and further protects the cell surface
Pili
Short projections used to attach bacteria to each other
Flagella
Longer projections used for movement
Ribosomes
Synthesize proteins
Eukaryotic Cells
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Protists, Fungi, Plants, and Animals
Have nucleus and membrane-bound organelles
Much larger and more complex than prokaryotic
cells.
Reproduce sexually and asexually
Eukaryotic Organelles
General Function: Manufacturing
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Nucleus
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Nucleolus
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Control center of cell; contains most of the cell’s
DNA
Location where ribosomes are synthesized
Nuclear pore
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Allows RNA to move in and out of nucleus
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Ribosomes
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Protein synthesis
Rough ER
Comprised of a network of tubes and flattened sacs.
 Continuous with plasma membrane and nuclear
membrane
 Site of protein synthesis (consists of ribosomes)
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Eukaryotic Organelles
General Function: Manufacturing
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Smooth ER
Site of lipid and carbohydrate metabolism
 No ribosomes
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Golgi Apparatus
Connected with ER; flattened disc-shaped sacs,
stacked one on top of the other
 Modification, storage, and packaging of proteins.
 “tags” proteins so they go to the correct destination.
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Eukaryotic Organelles
General
Function:
Breakdown
 Lysosomes (in animal cells and some
protists)
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Peroxisomes
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Digestion of nutrients, bacteria, and damaged
organelles; destruction of certain cells during
embryonic development
Diverse metabolic processes with breakdown of H2O2
by-product
Vacuoles
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Digestion (like lysosomes); storage of chemicals, cell
enlargement; water balance
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Eukaryotic Organelles
General Function: Energy
Processing
Chloroplasts
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Conversion of light energy to chemical energy of
sugars (site of photosynthesis)
Mitochondria
Conversion of chemical energy of food to chemical
energy of ATP
 “Power House” of cell
 Bound by double membrane
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Eukaryotic Organelles
General Function: Support,
Movement, and Communication
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Cytoskeleton (including cilia, flagella, and
centrioles in animal cells)
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Maintenance of cell shape; anchorage for organelles;
movement of organelles within cells; cell movement;
mechanical transmission of signals from exterior of
cell to interior.
Cell walls (in plants, fungi, and protists)
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Maintenance of cell shape and skeletal support;
surface protection; binding of cells in tissues
Eukaryotic Organelles
General Function: Support,
Movement, and Communication
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Extracellular matrix
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Binding of cells in tissues; surface protections;
regulation of cellular activities
Cell junctions
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Communication between cells; binding of cells in
tissues.
Cell Membranes
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Membranes provide the structural basis for
metabolic order.
In eukaryotes, most organelle’s are made from
membranes
Many enzymes are built right into the
membranes of these organelles
Selective permeability
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Cell membranes control what goes in and out of
the cell
It allows some substances to cross more easily
than others
Cell membrane is amazingly thin
Membrane phospholipids form a
bilayer
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Lipids, mainly phopholipids, are the main
structural components of membranes
Phospolipid has a phosphate group and only
two fatty acids
Head, with a charged phosphate group, is
hydrophillic
 Fatty acid tails are nonpolar and hydrophobic
 Thus, the tail end is pushed away by water, while the
head is attracted to water
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Phospholipid Bilayer
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Phospholipids form a two-layer sheet called a
phospholipid bilayer.
Hydrophillic heads face outward, exposed to the
water on both sides of a membrane
 Hydrophobic tails point inward, mingling together
and shielded from water.
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Phospholipid Bilayer
Phospholipid bilayer
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Hydrophobic interior of the bilayer is one
reason membranes are selectively permeable.
Nonpolar, hydrophobic molecules are lipidsoluble can easily pass through membranes
Polar molecules and ions are not lipid-soluble
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Ability to pass through membrane depends on
protein molecules in the phospholipid bilayer.
Fluid Mosaic Model
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Plasma membrane is described as a “Fluid
Mosaic”
Mosaic denotes a surface made of small
fragments, like pieces of colored tile
A membrane is considered “mosaic” because it
has diverse protein molecules embedded in a
farmework of phospholipids.
A membrane mosaic is “fluid” in that most of the
individual proteins and phospholipids can can
drift literally in the membrane
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Tails of phospholipids are kinked.
Kinks make the membrane more fluid by
keeping adjacent phospholipids from packing
tightly together.
In animal cells, the steroid cholesterol stabilize
the phospholipids at body temperature and also
keep the membrane fluid at lower temperatures.
In a cell, phospholipid bilayer remains about as
fluid as salad oil at room temperature.
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Outside surface of plasma membrane has
carbohydrates bonded to proteins and lipids in
the membrane.
A protein with attached sugars is called a
glycoprotein, whereas a lipid with sugars is
called a glycolipid.
Function as cell identification tags that are
recognized by other cells.
Significant for cells in an embryo to sort themselves
to tissues and organs.
 Also functions in the immune system to recognize
and reject foreign cells.
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Proteins make the membrane a
mosaic of function
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Mosaic refers both to positioning of proteins in
membrane but also the activities of these
proteins.
Proteins perform most of the functions of a
membrane.
Different cells contain different sets of
membrane proteins, and membranes within cells
have unique proteins
Functions of Membrane Proteins
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Attaching the membrane to cytoskeleton and
external fibers
Providing identification tags
Forming junctions between adjacent cells
Enzymes, functioning in catalytic teams for
molecular assembly lines
Receptors for chemical messengers from other
cells
Help move substances across the membrane, like
water and glucose
Receptor proteins
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Has a shape that fits the shape of a specific
messenger, such as a hormone, just as an
enzyme fits its substrate
Binding of messenger to the receptor triggers a
chain reaction involving other proteins, which
relay the message to a molecule that performs a
specific activity inside the cell (signal
transduction)
Passive Transport
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Diffusion of a substance across a biological
membrane
Diffusion is the movement of particles from
high concentration to low concentration.
Moves with a concentration gradient
No energy input required
Eventually reaches equilibrium
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Molecules continue to move back and forth, but no
net change in concentration will occur
Passive Transport
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Necessary in our cells; lungs transport oxygen in
red blood cells and carbon dioxide out via
diffusion.
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Small, nonpolar molecules that easily diffuse across
plasma membranes
Passive Transport: Facilitated
Diffusion
 Facilitated diffusion
Many substances can’t diffuse freely across membrane
because of their size, polarity, or charge
 Need the help of specific transport proteins in the
membrane to move across the membrane
 Does not require energy
 Goes with the concentration gradient (high to low)
 Some sugars, amino acids, ions, and even water use
facilitated diffusion
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Since water is polar, movement through hydrophobic
interior is slow. Aquaporins allow for rapid transport into
and out of cell.
Passive Transport: Facilitated
Diffusion
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Transport proteins (figure 5.15)
Have to be specific to molecule moving across the
membrane
 One example: provides a pore, or tunnel, for the
passage of a solute
 Another example: protein binds molecule, changes
shape, and releases molecule on the other side
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Passive Transport: Osmosis
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Osmosis: Diffusion of water molecules across a
selectively permeable membrane
With concentration gradient
Requires no energy
Tonicity
Describes the tendency of a cell in a given solution
to lose or gain water.
 Isotonic, hypertonic, and hypotonic
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Passive Transport: Osmosis
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Isotonic solution
Equal concentration of solvent inside and outside of
cell; water goes in and out
 Cell’s volume remains the same; equilibrium
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Hypertonic solution
Solute concentration is lower inside cell (solvent
concentration is higher inside cell) ;Water goes out
 Cell shrivels
 Causes plasmolysis in plant cells
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Passive Transport: Osmosis
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Hypotonic solution
Solute concentration is greater inside the cell
(solvent concentration is lower inside the cell); water
goes in
 Cell swells and may lyse
 Causes cytolysis in animal cells
 Refer to figure 5.17
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Passive Transport: Osmosis
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Osmoregulation
Control of water balance
 Method by which animals survive in hypertonic and
hypotonic environments
 Example; freshwater fish have kidneys and gills that
work constantly to prevent excessive buildup of
water in the body
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Active Transport
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Requires that a cell expend energy to move
molecules across a membrane
Moves against the concentration gradient.
ATP supplies the energy
Moving molecules from low to high
concentration
Active transport
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Simple model, figure 5.18
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2.
3.
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Process begins when solute on cytoplasmic side of
the plasma membrane attaches to a specific
binding site on the transport protein.
ATP phoshporylates the transport protein
Causing it to change shape in such a way that the
solute is released on the other side of the
membrane
Phosphate group detaches and the transport
protein returns to its original shape
Active Transport
Active Transport: Exocytosis
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Exocytosis
Used to export bulky materials
 Examples: insulin secretion into blood from
pancreas; tear glands secrete salty sol’n containing
proteins
1. A membrane-enclosed vesicle filled with
macromolecules moves to the plasma membrane
2. Vesicle fuses with plasma membrane
3. Vesicles contents spill out of the cell
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Endocytosis
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Endocytosis
A cell takes in macromolecules or other particles by
forming vesicles or vacuoles from its plasma
membrane
 Three kinds: phagocytosis, pinocytosis, receptormediated endocytosis
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Phagocytosis
Endocytosis
“cellular eating”
 Molecules of food move into the cell
 Example: amoeba
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Pinocytosis
“cellular drinking”
 Molecules of fluid move into the cell; not specific
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Receptor-mediated endocytosis
Highly specific; materials come in through indented
pit in plasma membrane. Pit is lined with receptor
proteins, pinch close to form vesicle containing
molecules to be brought into the cell
 Example: cholesterol
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Summary of Cell Transport
Cell Division
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Cell Division
Virchow: Cells can only come from preexisting cells
 In unicellular organisms, can reproduce an entire
organism
 Allows multicellular organisms to reproduce
asexually
 Basis of sexual reproduction sperm and egg
 Allows fertilized egg, or zygote, to develop into an
adult organism
 Replaces worn-out or damaged cells
 Enables multicellular organism to grow to adult size
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Prokaryotes reproduce by binary
fission
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Binary fission
Process by prokaryotes reproduce by cell division.
 Steps:
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Duplication of chromosomes and separation of copies.
 Cell elongates
 Divides into two daughter cells
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Chromosomes
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Human cells carry about 20,000 genes to make
100,000 proteins.
Almost all genes are located in the nucleus
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Very small amount found in mitochondria
Genes are found on DNA
Chromatin
Diffuse mass of long, thin fibers, not seen under the
microscope, less tightly coiled
 Combination of DNA and protein
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Chromosomes
Chromosomes
Rod-shaped structure
 Coiled up, compact forms of chromatin
 Contains one long DNA molecule bearing hundreds
or thousands of genes.
 DNA is attached to protein molecules (histones)
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Sister chromatids
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Each duplicated chromosome contains two identical
copies.
Centromere
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The point by which two chromatids are joined.
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What is the difference between diploid and
haploid and what are those numbers in
humans?
Diploid 2 sets of chromosomes (2n); 46 in humans
 Haploid 1 set of chromosomes (n); 23 in humans
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What is the process called in Eukaryotes, when
cells divide to make cells exactly alike (diploid)?
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Mitosis!
What is the process called in Eukaryotes, when
cells divide to make haploid cells?
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Meiosis!
Cell Cycle
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In your own body, millions of cells must divide
every second to maintain the total number of
about 100 trillion cells.
Some cells divide once a day, and some do not
at all (mature muscle cells, liver cells, brain cells)
Interphase
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Occurs when the cell is between cell division
Interphase stages:
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G1: Cells grow to mature size
S: DNA is copied
G2: Cell prepares for division
Cells exit the cell cycle via…
G0: Cells do not copy DNA or prepare for mitosis, but are still alive (e.g.
nervous system)
Interphase
Prophase
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What does the cell look like?
Centrioles and spindle fibers appear
 Nuclear envelope disappears, and chromosomes are
visible
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Prophase
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What happens to the DNA and nucleus?
Chromosomes form when chromatin tightens and
coils
 Nuclear membrane breaks down and disappears
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What two things appear near where the nucleus
was?
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Centrioles and spindle fibers
Prophase
Metaphase
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What does the cell look like?
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Chromosomes move to the middle
Where are the chromosomes during metaphase?
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Middle of the cell
Metaphase
Anaphase
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What does the cell look like?
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Chromosomes move to the end of cell
What happens to the chromosomes?
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Chromosome splits at centromere into 2 chromatids
and moves to end of cell
Anaphase
Telophase
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What does the cell look like?
Cell starts to pinch in
 Nucleus starts to reform
 Chromosomes are at opposite ends
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What happens to the chromosomes and
nucleus?
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Nucleus forms back around single chromatids
Telophase
After Telophase
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What is cytokinesis?
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Cytoplasm and contents (other organelles) divide
What’s special about cytokinesis in plants?
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Cell wall also divides with new cell plate in middle
Growth Factors signal the cell
cycle
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Growth factors are the main signals that
stimulate cell division
Cells have a series of checkpoints that are
regulated by proteins.
These checkpoints determine whether or not
cell division will occur.
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Cancer
Mutations in the genes of these checkpoint
proteins may lead to cancer:
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The uncontrolled growth of cells.
Tumor: an abnormally growing mass of body
cells
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Benign tumor
If abnormal cells remain at original site
 Can be problematic if disrupt certain organs, but usually
easily removed by surgery
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Malignant tumor
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If abnormal cells spread into other tissues and body parts,
interrupting organ function
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Metastasis
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Cancer found in the external or internal coverings of
the body; skin, lining of intestine
Sarcomas
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Cancer that spreads via the circulatory system
Carcinomas
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Cancer
Cancer found in tissues that support the body; bone
and muscle
Leukemias and lymphomas
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Cancer of blood forming tissues, bone marrow,
spleen, and lymph nodes
Cancer
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Usually caused by a loss of inhibitory factors
(tumor suppressor genes), or an increase in
growth factors (oncogenes).
Surgery, radiation, and chemotherapy are typical
treatments
Cell Differentiation
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During repeated cell division that lead from a
zygote to a multicellular adult, individual cells
must undergo differentiation
Become specialized in structure and function
 Results from selective gene expression, the
turning on and off of genes
 Each differentiated cell expresses genes that
allow the cell to perform certain functions:
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Beta cells expressed genes that allow it to produce the
hormone insulin
Cell Differentiation
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About 50 cell division produce approximately
100 billion cells in an adult.
During the development of an embryo, the cells
specialize to take on the roles of 220 different
cell types.
Some cells stop dividing at a certain point in the
adult life, some continue to divide throughout
lifespan
Genes, either turned on or off, control the
destined role of the cell.
Stem Cells
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Adult Stem Cells (ACS)
Cells present in adult tissues that generate
replacemetns for nondividing diffrentiated cells.
 multipotent
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Embryonic Stem Cells (ES cells)
Harvested from the blastocyst
 Give rise to all the different kinds of specialized
cells of the body
 pluripotent
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Stem Cells
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Induced Pluripotent Stems Cells (iPS)
Stem cells created via de-differentiation of an adult
cell (eg. Skin cell).
 Pluripotent
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Cloning
Nuclear transplantation is used:
Replacing nucleus of an egg cell or zygote with the
nucleus of an adult somatic cell.
 After sever days, a blastocyst forms which may be used
for many purposes
 If stem cells in blastocyst are used for the birth of a new
individual: reproductive cloning
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Can be used to create genetically identical individuals for
experimentation.
If stem cells in blastcyst are induced to form specialized
cells: therapeutic cloning:
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Used to make essential cells: beta cells to make insulin,
dopamine neurons for Parkinson’s patients.