Transcript Cells: `Structure & Function`

Membranes and Cell Organelles
Chapter 2: Pages 32 - 59
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Chapter 2 - Membranes and cell
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APOPTOSIS: LIFE OR DEATH OF A CELL
Apoptosis: the natural death of cells, also known as ‘programmed cell death’
Details:
 Natural feature of cells

Death and cell reproduction equal in mature organsisms

Cancer occurs when ‘apoptosis’ is prevented

Auto Immune Disease occurs when too much apoptosis occurs (i.e
Alzheimers’ disease)

Signals initiating apoptosis may come from either inside or outside a cell.

Proteins are suspected of controlling Apoptosis
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LOOKING AT CELLS
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(Definition)
 Cells: basic structural and functional units of all living
organisms
(Definition)
 Organelles: structural unit in cells that performs a key
function in cellular metabolism
organelle (ôr g -n l )
A structure or part that is enclosed within its own membrane inside a cell and has a
particular function. Organelles are found only in eukaryotic cells and are absent
from the cells of prokaryotes such as bacteria. The nucleus, the mitochondrion, the
chloroplast, the Golgi apparatus, the lysosome, and the endoplasmic reticulum are
all examples of organelles. Some organelles, such as mitochondria and
chloroplasts, have their own genome (genetic material) separate from that found in
the nucleus of the cell. Such organelles are thought to have their evolutionary origin
in symbiotic bacteria or other organisms that have become a permanent part of the
cell.
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
Living cells can be divided into 2 distinct groups
according to the organisation of their internal
structures.

Prokaryotic Cells – Lack of a nucleus or clear
structure to house their DNA. (ie bacteria)

Eukaryotic Cells - Contain structures called
organelles and the DNA is enclosed inside a membrane
bound nucleus. (ie plants, animals, fungi & protists)
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Comparing Prokaryotic and Eukaryotic Cells
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This particular eukaryotic cell happens to be an animal cell, but the
cells
of plants,
fungiStructure
and protists
are also eukaryotic.
Chapter
2 - Cells:
& Function
Prokaryotic Cells

Are smaller than eukaryotic cells

Have no nucleus to house their DNA

Often have a protective cell wall on the outside of their
plasma membrane

Include all bacteria and Archae
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Typical Sizes of Cells
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Image courtesy of WebPath
The light microscope
The light microscope has
a limit of resolution of
about 200 nm (0.2 µm).
This limit is due to the
wavelength of light (0.40.7 µm). Cells observed
under a light microscope
can be alive, or fixed and
stained
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Image courtesy of WebPath
The Transmission Electron Microscope (TEM)

The Transmission Electron
Microscope (TEM) has a limit of
resolution of about 2nm. This is
due to limitations of the lens
used to focus electrons onto the
sample. A TEM looks at replicas
of dead cells, after fixation and
heavy metal ion staining.
Electrons are scattered as they
pass through a thin section of
the specimen, and then detected
and projected onto an image on
a13fluorescent screen. Chapter 2 - Membranes and cell
organelles
Image courtesy of CIPE
The Scanning Electron Microscope (SEM)

The Scanning Electron
Microscope (SEM) also has
a limit of 2nm. Like the
TEM, the SEM allows you
to look at replicas of dead
cells, after fixation and
heavy metal ion staining.
With this technique,
electrons are reflected off
the surface of the
specimen.
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CELL MEMBRANES
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The Cell Membrane


The boundary of all living cells is the cell membrane.
It controls entry of dissolved substances into and out of the
cell.



also known as the Plasma Membrane OR Liquid Lipid Bilayer
Surrounds the cytoplasm of the cell
Is partially (or semi) permeable

The cell membrane is ultra thin (thickness <0.01
micrometers).

A cell membrane contains both lipids and protein and
carbohydrates .
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The Cell Membrane
Fig 2.4 – Fluid
Mosaic Model
p25
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A phospholipid molecule
This is a simple representation of a phospholipid. the yellow
structure represents the hydrophillic or water loving section of the
phospholipid.
The blue tails that come off of the sphere represent the
hydrophobic or water fearing end of the Phospholipid. Below
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Chapter 2 - Cells: Structure & Function
If you mix phospholipids in water they will form these double layered structures.
The hydrophillic ends will be in contact with water.
The hydrophibic ends will face inwards touching each other
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Chapter 2 - Cells: Structure & Function
Membrane Proteins
Floating around in the cell membrane are different kinds of proteins.
These are generally globular proteins.
They are not held in any fixed pattern but instead float around in the phospholipid layer.
Generally these proteins structurally fall into three catagories...
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Chapter 2 - Cells: Structure & Function
There are carrier proteins that regulate transport and diffusion
Marker proteins that identify the cell to other cells
And receptor proteins that allow the cell to receive instructions
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Chapter 2 - Cells: Structure & Function
& Embedded Cholesterol
Steroids are sometimes a component of cell membranes in the form of cholesterol.
When it is present it increases the fluidity of the membrane.
Not all membranes contain cholesterol.
Q. In what kinds of environment would cholesterol be a vital membrane
commponent?
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Chapter 2 - Cells: Structure & Function
Role of Cell Membranes
1.
Identification
2.
Communication
3.
Regulation of movement of substances into/ out of
cell
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1. Recognising Self or
Non-self
(also known as protein markers)
On its outer surface, a plasma
membrane has substances,
often called Antigens (or
MHC Markers) that identify
the cell as belonging to one
particular organism.
Usually consist of proteins
mixed with carbohydrates
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2. Communication Between Cells:
Receptor Proteins
These proteins are used in intercellular communication. In this
animation you can see the a hormone binding to the receptor.
This causes the receptor protein release a signal to perform
some action.
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Chapter 2 - Cells: Structure & Function
3. Control Movement In and Out of Cells

In order to survive, cells need to
take in and expel substances.

Cells generally let in particular dissolved substance. It is
therefore said to be partially permeable.

Dissolved substances that pass across the cell membrane,
does so by two processes – Diffusion & Active
Transport.
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Chapter 2 - Cells: Structure & Function
TYPES OF MOVEMENT INTO & OUT OF CELLS
PASSIVE
(Without energy)
a.
b.
c.
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Diffusion
Osmosis
Facilitated diffusion
1. Channel mediated
2. Carrier Mediated
ACTIVE
(With energy – ATP)
D.
E.
Active Transport with–
1. Carrier proteins
2. Ion pumps
Active Transport with
1. Endocytosis
2. Exocytosis:
a.Phagocytosis/
b.Pinocytosis
Chapter 2 - Cells: Structure & Function
Passive Transport
a. – Diffusion
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
Diffusion is the net movement of a substance (solution),
from a region of high concentration to a region of low
concentration.

Diffusion does not require energy.

Net: means overall movement (because movement does continue both ways!)
Chapter 2 - Cells: Structure & Function
b. Passive Transport - Osmosis

One special case of diffusion is known as osmosis.

Osmosis is the movement of water molecules from an area of low solute
(high water) concentration to an area of high solute (low water)
concentration.
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Chapter 2 - Cells: Structure & Function
c. Facilitated Diffusion

Solid particles (ie molecules) and those that cannot enter via
the lipid layer, enter the cell through a specific carrier protein in
the membrane. This is known as facilitated diffusion.

Fig 2.9a – Facilitated diffusion

Facilitated diffusion travels from an area of high concentration
to an area of low concentration.

Two forms of passive facilitated diffusion: Channel Mediated
& Carrier Mediated (see page 41)
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Chapter 2 - Cells: Structure & Function
Examples of Facilitated Diffusion cont…
3a. Channel Mediated Passive Transport
Channel proteins extend through the bilipid layer. They form a
pore through the membrane that can move molecules in several
ways.
In some cases the channel proteins simply act as a passive pore.
Molecules will randomly move through the opening in a process
called diffusion. This requires no energy, molecules move from an
area of high concentration to an area of low concentration nd is
known as passive transport
Direction: with
the concentration
gradient
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Chapter 2 - Cells: Structure & Function
Examples of Facilitated Diffusion cont..
3b. Carrier Mediated Passive Transport
out
In this case a molecule that is moving
naturally into the cell through diffusion is used
to drag another molecule into the cell. This
hitching of a ride doesn’t use energy. The
molecule is facilitated by one that is moving
from a high concentration to a low
concentration.
in
Protein channel
Phospholipid
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Chapter 2 - Cells: Structure & Function
ACTIVE TRANSPORT

Active transport is the net
movement of dissolved
substances into or out of the
cell against a concentration
gradient (opposite to facilitated
diffusion).

Active transport requires
energy (ATP) to perform.

Active transport enables cells
to maintain stable internal
conditions in spite of extreme
external surroundings.
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Chapter 2 - Cells: Structure & Function
EXAMPLES OF ACTIVE TRANSPORT Cont…
Bulk Transport – Cytosis into & out of cells
MOVEMENT INTO CELL
MOVEMENT OUT OF CELL
Endocytosis
Exocytosis
i. Phagocytosis “cell eating”
ii. Pinocytosis “cell drinking”
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Chapter 2 - Cells: Structure & Function
Bulk Transport – into cells

When bulk material is
taken into a cell as a solid,
the process is termed
Phagocytosis’.

When bulk fluid is taken
into a cell as fluid, the
process is termed
‘Pinocytosis’.
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Chapter 2 - Cells: Structure & Function
Bulk Transport - Endocytosis

Unicellular protists (Amoeba) obtain their energy in the
form of relatively large food particles that they engulf.

The food gathers at the lipid layer of the membrane and
is drawn in and enclosed within a sac where the food is
digested.

This process of bulk material into a cell is known as
endocytosis.
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Chapter 2 - Cells: Structure & Function
Bulk Transport - Endocytosis

Fig 2.11a – Example of Endocytosis
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Chapter 2 - Cells: Structure & Function
Bulk Transport - Endocytosis

Fig 2.11b – Example of Endocytosis
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Chapter 2 - Cells: Structure & Function
Bulk Transport - Exocytosis

Bulk transport out of cells (export or waste material) is
called exocytosis.

In exocytosis, vesicles formed within a cell fuse with the
plasma membrane before the contents of the vesicles are
released from the cell.
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Chapter 2 - Cells: Structure & Function
Bulk Transport - Exocytosis

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Fig 2.13 – Exocytosis
Chapter 2 - Cells: Structure & Function
Cell Organelles
Cell Walls

The cell membrane forms the exterior of animal cells.

However, in plants, fungi and bacteria, a rigid cell wall
lies outside the cell membrane.

The cell wall varies in composition between plants, fungi
and bacteria.
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Cell Walls
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Chapter 2 - Cells: Structure & Function
Structure of Cells Wall

Plants have rigid cell walls.

the primary cell wall of adjacent cells are held together
tightly by a layer of pectin, a sticky carbohydrate
(polysaccharide).

Secondary cell walls are layed down (inside) on the
cytosol side of the primary wall, causing quite a gap
between two sides.
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STRUCTURE OF PLANT CELL WALL
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Chapter 2 - Cells: Structure & Function
Nucleus

The control centre of the cells of animals, plants, algae
and fungi is the nucleus.

forms a distinct spherical structure that is enclosed
within a double membrane known as the nuclear
envelope.

Cells that have a membrane bound nucleus are called
eukaryote cells.

Some cells have more than one nucleus (WBC!); whilst
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2 - Cells:
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(mature
RBC)
Nucleus - function

This organelle has two major functions:
1. it stores the cell's hereditary material, or DNA, and
2. it coordinates the cell's activities, which include growth,
intermediary metabolism, protein synthesis, and
reproduction (cell division).
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Chapter 2 - Cells: Structure & Function
The nucleus is of primary
importance in the cell because it is
the control center that oversees
the metabolic functioning of the
cell and ultimately determines the
cell's characteristics. Within the
nucleus, there are masses of
threads called chromatin, which is
indistinct in the nondividing cell,
but it condenses to chromosomes
at the time of cell division. This is
where the DNA resides. The
nucleolus is the specialized part of
chromatin in which the ribosomal
RNA (rRNA), is produced.
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Nucleolus

The nucleus also contains one or more large inclusions
known an nucleoli which are composed of ribonucleic
acid (RNA).
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Mitochondrion
Note: Sketch diagrams

rod-shaped organelles that can be considered the power
generators of the cell, converting oxygen and nutrients into
adenosine triphosphate (ATP).

Contains DNA
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Ribosomes




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All living cells contain ribosomes,
They are tiny organelles composed of approximately 60 percent
RNA and 40 percent protein.
They are not membrane bound
In eukaryotes, ribosomes are made of four strands of RNA. In
prokaryotes, they consist of three strands of RNA
Chapter 2 - Cells: Structure & Function
Ribosomes
(don’t copy)

Ribosomes are mainly found bound to the endoplasmic reticulum
and the nuclear envelope, as well as freely scattered throughout the
cytoplasm, depending upon whether the cell is plant, animal, or
bacteria.

The organelles serve as the protein production machinery for the
cell and are consequently most abundant in cells that are active in
protein synthesis, such as pancreas and brain cells.

Some of the proteins synthesized by ribosomes are for the cell's
own internal use, especially those that are produced by free
ribosomes.

Many of the proteins produced by bound ribosomes, however, are
transported outside of the cell.
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Chapter 2 - Cells: Structure & Function
Endoplasmic Reticulum

The endoplasmic reticulum is a network of sacs that manufactures,
processes, and transports chemical compounds for use inside and
outside of the cell.

It is connected to the double-layered nuclear envelope, providing a
pipeline between the nucleus and the cytoplasm.
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Smooth ER

A network of interconnected membranes forming
channels within the cell.

A site for synthesis and metabolism of lipids.

Also contains enzymes for detoxifying chemicals including
drugs and pesticides
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Chapter 2 - Cells: Structure & Function
Golgi Complex

A series of stacked membranes.

Vesicles (small membrane surrounded bags) carry
materials from the RER to the Golgi apparatus.

Vesicles move between the stacks while the proteins
are "processed" to a mature form.

Vesicles then carry newly formed membrane and
secreted proteins to their final destinations including
secretion (pinocytosis) or membrane localization.
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Chapter 2 - Cells: Structure & Function
Golgi Complex
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Chapter 2 - Cells: Structure & Function
Lysosomes

A membrane bound organelle that is responsible for
degrading (break down) proteins and membranes in the
cell, and also helps degrade materials ingested by the cell.
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Chapter 2 - Cells: Structure & Function
Chloroplasts

Surrounded by a double membrane, containing stacked
thylakoid membranes.

Responsible for photosynthesis, the trapping of light
energy for the synthesis of sugars.

Contains DNA, and like mitochondria is believed to
have originated as a captured bacterium.
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VACUOLES
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Chapter 2 - Cells: Structure & Function
Functions of vacuoles

Removing unwanted structural debris

Isolating materials that might be harmful to the cell

Containing waste products


Maintaining internal hydrostatic pressure (turgor
pressure) within the cell

Maintaining an acidic internal pH

Containing small molecules

Exporting unwanted substances from the cell.

Enabling the cell to change shape.

Movement in some cells
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CILIA & FLAGELLUM
Cilia and flagellum are structures that aide cell movement
Scanning electron micrograph of ciliate
A sperms flagellum
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Cytoskeleton
•
•
•
•
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Each cell has an internal framework of protein microtubules & microfilaments.
These supply the inner strength and support for the cell.
They are also important in cell division.
This supporting structure is called the Cytoskeleton.
Chapter 2 - Cells: Structure & Function
Connections between animal cells
Although some cells like blood cells, are free to move as individuals
around the body, most cells remain as a group. To assist this ‘
community’ of cells are three types of junctions (joins) including:
Occluding junctions: simply where one cell comes into contact with
another (no movement of material between them)
Communicating junctions (gap junctuions): also known as gap
junctions. Consist of protein lined pores between the membrane of
adjacent cells. The proteins are aligned like a series of rods with a
gap (pore) in the middle of them. This bridge between cells enables
the passage of ions, sugars, amino acids, other small particles and
even electrical signals between the two cells. (example: electrical
impulse in the heart)
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Anchoring Junctions (desmosomes): most common form of junction
in epithelial cells (skin, uterus, lining cells!). Dense proteins
protrude from the cytosol of one cell into the other to ‘anchor’ the
cells together.
Chapter 2 - Membranes and cell
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
Communicating
Junctions right ..

Anchoring
junctions below
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Connections between plant cells

Plasmodesmata (singular: plasmodium) is the name given
for the structure that joins plant cells. Because of the way
in which cell walls are built, the gap or pore between two
cells is continuos and lined with cell membrane. The
structure that bridges the gap is continuos with smooth
endoplamsic reticulum.
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Cells in Multicellular organisms

In a unicellular organism, one cell carries out all of the
functions of life.

In contrast, most cells in a multicellular organism are
specialized to perform one or a few functions – more
efficiently.

Because of cell specialization, the cells of multicellular
organisms depend on other cells in the organism for their
survival.
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Chapter 2 - Cells: Structure & Function
Surface Area to Volume

A few types of cells are large enough to be seen by the unaided eye.

The human egg (ovum) is the largest cell in the body, and can (just) be seen
without the aid of a microscope.

Most cells are small for two main reasons:
a). The cell’s nucleus can only control a certain volume of active cytoplasm.
b). Cells are limited in size by their surface area to volume ratio. A group of small cells has
a relatively larger surface area than a single large cell of the same volume. This is
important because the nutrients, oxygen, and other materials a cell requires must enter
through it surface.
As a cell grows larger at some point its surface area becomes too small to allow these
materials to enter the cell quickly enough to meet the cell's need. (= Fick’s Law –
something you need to learn well).

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Rate of diffusion α
Surface Area x Concentration Difference
Distance
Chapter 2 - Cells: Structure & Function
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Chapter 2 - Cells: Structure & Function
Tissue, Organs and Organ Systems

In most Multicellular Organisms, we find the following organization:

Cellular Level: The smallest unit of life capable of carrying out all
the functions of living things.

Tissue Level: A group of cells that performs a specific function in
an organism.

Organ Level: Several different types of tissue that function
together for a specific purpose.

Organ System Level: Several organs working together to
perform a function. The different organ systems in a multicellular
organism interact to carry out the processes of life
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Chapter Revision
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BioChallenge & Chapter Review
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