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Cells
Palestine Polytechnic University
Department of Environmental Engineering
Technology
Course: Environment & Biological
Diversity
Instructor
Dr. Ayman Salah
Chapter 4
Cells
The Working Units of Life
• The cell theory
 Cells are the
fundamental units of
life (the structural
unit).
 All organisms are
composed of cells.
 All cells come from
preexisting cells.
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Cells are the
fundamental units
of life
The surface area –
to- volume ratio
• Cells have
different sizes.
• Most cells are
very small and
therefore have a
high ratio of
surface to cell
volume.
Cells
Cells are the fundamental units of life
• The surface area –to- volume ratio
• What is the biological significance of this
phenomenon?
1. The volume of a cell determines the amount of
chemical activity it carries out per unit of time.
2. The surface area of a cell determines the amount
of substances the cell can take in from the outside
environment and the amount of waste products it
can release to the environment.
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Cells
Cells are the fundamental units of life
Basic features of all cells:
• – Plasma membrane
• – Semifluid substance called cytosol
• – Chromosomes (carry genes)
• – Ribosomes (make proteins)
Cells
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Cells
Cells are the fundamental units of life
Characteristics of Prokaryotic Cells
Cells
Cells are the fundamental units of life
Characteristics of Eukaryotic Cells
Cells are prokaryotic or eukaryotic
• Characteristics of Prokaryotic Cells
 The plasma membrane encloses the cell
 The nucleoid contains the hereditary material
(DNA) of the cell.
 Cytoplasm; contain the ribosomes, dissolved
ions, small molecules and soluble
macromolecules
 Cell wall Composed of peptidoglycan, a
polymer of amino sugars
Eukaryotic cells are characterized by:
• DNA in a nucleus that is bounded by a
• membranous nuclear envelope
• Cytoplasm in the region between the plasma
membrane and nucleus
• Compartmentalization is the key to eukaryotic cell
function
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Cells
Characteristics of Eukaryotic Cells
What is Compartmentalization ?
• eukaryotic cells have compartments within the
cytoplasm whose interiors are separated from the
cytosol by a membrane.
• These membranous compartments, as well as
other structures (such as ribosomes) that lack
membranes but possess distinctive shapes and
functions, are called organelles.
• Each organelle has specific roles in the cell
• Some organelles process information
Cells
Characteristics of Eukaryotic Cells
Nucleus
• the largest organelle.
• The roles of the Nucleus:
 the site of DNA replication.
 the site of genetic control
of the cell's activities.
 A region within the nucleus, the nucleolus,
begins the assembly of ribosomes from
RNA and specific proteins.
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Cells
Cells
Characteristics of Eukaryotic Cells
Characteristics of Eukaryotic Cells
The Endomembrane system
• Nuclear envelope
• Endoplasmic reticulum
• Golgi apparatus
• Lysosomes
• Plasma membrane
• Vacuoles
Endoplasmic Reticulum (ER)
• a network of membranes branching throughout
the cytoplasm forming tubes and flattened sacs.
• There are two distinct regions of ER; Rough ER
and Smooth ER
• Rough ER is important in protein synthesis
• These components are either continuous
or connected via transfer by vesicles
Cells
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Cells
Characteristics of Eukaryotic Cells
Characteristics of Eukaryotic Cells
Endoplasmic Reticulum (ER)
• a network of membranes
branching throughout the
cytoplasm forming tubes
and flattened sacs.
• There are two distinct
regions of ER; Rough ER
and Smooth ER
• Important in protein
synthesis
Golgi Apparatus
• consists of flattened membranous sacs
• The Golgi apparatus has several roles:
 receives proteins from the ER and may further
modify them.
 concentrates, packages, and sorts proteins before
they are sent to their cellular or extracellular
destinations.
 Synthesis of polysaccharides for the plant cell
wall.
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Cells
Characteristics of Eukaryotic Cells
Lysosomes = cellular digestive systems
• Originate from the Golgi apparatus.
• contain digestive enzymes, that hydrolyze
macromolecules -proteins, polysaccharides,
nucleic acids, and lipids- into their monomers.
• Lysosomes are sites for the breakdown of food
and foreign objects taken up by the cell
• Lysosomes are also where the cell digests its own
material.
Cells
Characteristics of Eukaryotic Cells
Some organelles
transform energy
• Mitochondria
• Plastids - Chloroplasts
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• Describe the structure and function
of the nucleus?
• Describe the structure and function
of the components of the
endomembrane system?
Chapter 5
The Dynamic Cell Membrane
The Structure of Biological Membranes
• The constituents of all biological membranes are
lipids, proteins, and carbohydrates.
• This general design of biological membranes is
known as the fluid mosaic model.
• The lipids in biological membranes are usually
phospholipids which form a bilayer
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The Dynamic Cell Membrane
The phospholipid bilayer
The Dynamic Cell Membrane
The Structure of Biological Membranes
• All biological membranes have a similar structure,
• Membranes from different cells or organelles may
differ in their lipid composition:
 Phospholipids can differ in terms of fatty acid
chain length, degree of unsaturation in the fatty
acids, and the polar groups present.
 The amount of cholesterol in the membrane.
The Structure of Biological Membranes
The Dynamic Cell Membrane
The Structure of Biological Membranes
• The phospholipid bilayer is flexible, not rigid.
 the fatty acids of the phospholipids make the
hydrophobic interior of the membrane somewhat
fluid.
 This fluidity permits some molecules to move
laterally (side to side) within the plane of the
membrane.
 The fluidity of a membrane is affected by its lipid
composition and by its temperature.
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The Dynamic Cell Membrane
The Structure of Biological Membranes
Membrane proteins
• Membrane proteins are asymmetrically
distributed.
• The amount of proteins in the membrane depends
on the membrane function.
• Many membrane proteins are embedded in, or
extend across, the phospholipid bilayer.
• These proteins have both hydrophilic and
hydrophobic regions
The Dynamic Cell Membrane
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The Dynamic Cell Membrane
The Structure of Biological Membranes
Membrane carbohydrates
• The carbohydrates are located on the outer
surface of the membrane
• covalently bonded to:
 lipids (Glycolipids) or
 proteins
(Glycoproteins).
• Serve as recognition
sites for other cells and
molecules.
The Dynamic Cell Membrane
The Structure of Biological Membranes
The Passive Processes of Membrane Transport
Membranes are dynamic
• Membranes are constantly
 forming,
 transforming from one
type to another,
 fusing with one
another, and
 breaking down
The roles of plasma membrane
• allows the cell to maintain constant internal
environment
• acts as a selectively permeable barrier, preventing
some substances from crossing it while permitting
other substances to enter and leave the cell.
• important in communicating with adjacent cells
and receiving signals from the environment.
• responsible for binding and adhering to adjacent
cells
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The Dynamic Cell Membrane
The Passive Processes of Membrane Transport
• There are two fundamentally different processes
by which substances cross biological membranes:
1- The passive transport processes; (not active) do
not require any input of outside energy to drive
them.
 simple diffusion
 facilitated diffusion
2- The active transport processes; require the input
of chemical energy from an outside source.
The Dynamic Cell Membrane
The Passive Processes of Membrane Transport
1- Diffusion
• Diffusion is defined as the net movement from
regions of greater concentration to regions of
lesser concentration
• It is a random movement toward a state of
equilibrium
• after equilibrium equal numbers of molecules
move in each direction, so there is no net change
in concentration
• For example, if a drop of ink.
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The Dynamic Cell Membrane
The Passive Processes of Membrane Transport
• The diffusion of
solutes across
membranes
• the motion of each
individual particle
is absolutely
random, but the
net movement of
particles is
directional until
equilibrium is
reached.
The Dynamic Cell Membrane
The Passive Processes of Membrane Transport
Diffusion across membranes
• How fast a substance diffuses depends on four
factors:
1. The diameter
2. The temperature
3. The electric charge
4. The concentration gradient in the system.
What molecules pass through the phospholipid
bilayer?
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The Dynamic Cell Membrane
The Passive Processes of Membrane Transport
Diffusion across membranes
• The molecules that pass by diffusion are:
 small molecules pass through the phospholipid
bilayer
 hydrophobic, molecules (soluble in lipids).
• The molecules that do not pass are:
 electrically charged
 polar molecules, such as amino acids, sugars,
and ions.
• What about water movement???
The Dynamic Cell Membrane
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The Dynamic Cell Membrane
The Passive Processes of Membrane Transport
Osmosis
• Water will diffuse from
a region of its higher
concentration (with a
lower concentration of
solutes) to a region of
its lower concentration
(with a higher
concentration of
solutes).
The Dynamic Cell Membrane
The Passive Processes of Membrane Transport
The Passive Processes of Membrane Transport
Osmosis
• Osmosis is the diffusion of water across
membranes
• Osmosis depends on the number of solute
particles present, not on the kinds of particles
• It occurs when a solute (example: salt, sugar,
protein, etc.) cannot pass through a membrane but
the solvent (water) can.
• Water will diffuse from a region of its higher
concentration (with a lower concentration of
solutes) to a region of its lower concentration
(with a higher concentration of solutes).
Osmosis
• Three terms are used to compare the solute
concentrations of two solutions separated by a
membrane:
• Isotonic solutions have equal solute
concentrations.
• A hypertonic solution has a higher solute
concentration than the other solution with which it
is being compared.
• A hypotonic solution has a lower solute
concentration than the other solution with which it
is being compared.
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What happen to the cells when they are in solutions?
The Dynamic Cell Membrane
The Passive Processes of Membrane Transport
Facilitated diffusion
Osmosis Can Modify the Shapes of Cells
The Dynamic Cell Membrane
The Passive Processes of Membrane Transport
What determine the speed of
diffusion?
1. The concentration gradient
of the ion.
2. The electrochemical
gradient; when there is an
overall imbalance in the
charged substances between
the outside and inside of the
cell.
Ex. Cl- inside is high, so there is
a tendency for K+ to stay inside
the cell to balance out the
The Dynamic Cell Membrane
The Passive Processes of Membrane Transport
Facilitated diffusion
Facilitated diffusion
• Diffusion may be aided by channel proteins.
• polar substances such as amino acids and sugars
and charged substances such as ions do not
readily diffuse across membranes.
• But they do cross by the help of some proteins
(Channels and carrier )
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• Facilitated diffusion has three essential
characteristics;
1. It is passive;
2. It is specific; each protein carrier is specific for a
particular ion.
3. It saturates;
 There are only a limited number of carrier protein
per unit of membrane area.
 The rate of diffusion reaches a maximum when all
the carrier molecules are fully loaded with solute
molecules
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The Dynamic Cell Membrane
How Do Substances Cross Membranes against a
Concentration Gradient?
Active transport
• Active transport is the movement of a substance
across a biological membrane against a
concentration gradient
• Requires:
 chemical energy (energy-rich molecule ATP)
 membrane proteins
The Dynamic Cell Membrane
Active transport: example, Na+ - K+ pump
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The Dynamic Cell Membrane
Active transport: example, Na+ - K+ pump
• once the Na+ - K+ pump establishes a concentration
gradient of Na+ ions, the passive diffusion of some
Na+ back into the cell can provide energy for the
transport of glucose into the cell.
This is the
way to uptake
amino acids
and sugars
The Dynamic Cell Membrane
What are the differences between diffusion and
active transport?
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The Dynamic Cell Membrane
How Do Large Molecules Enter and Leave a Cell?
• Too large and too
charged or polar to
pass through
biological
membranes are
transported by
means of vesicles
(Endocytosis and
Exocytosis)
• Why does putting salt on meat
preserve it from spoilage by
bacteria?
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