Cell organelles

Nucleus

• Largest organelle in eukaryotic cells

• Control center of the cell

• Contains DNA scattered throughout as Chromatin

• Consists of:

• 1. 2 membranes surrounding the nucleus: each a phospholipid bilayer

– Outer bilayer continues into the endoplasmic reticulum

– Inner bilayer defines the nucleus

 • NUCLEAR PORES are found on the membrane

– regulates movements of material between nucleus and cytosol

• 2. Nucleolus

• Condensed area of chromatin in the nucleus• Synthesizes:

– Ribosomal RNA (rRNA)– Ribosomal Protein/subunits

• **These travel through the nuclear pores into the cytosol.

Chromosomes:

• Linear structures composed of DNA molecules

• Found in nuclei of eukaryotic cells

• Total DNA in the chromosomes of an organism is referred to as its genome

• DNA is wound around Proteins called histones. Histones help keep the DNA organized.

• A series of histones and DNA is called a nucleosomes.

• A Chromosome is composed of many nucleosomes.

• Each chromosome consists of 2 identical sister Chromatids (Each consisting of one double-stranded DNA molecule)

• Chromatids are attached at a specific region called a centromere.

• Note: When cell division is not happening, chromosomes are uncoiled and may be called chromatin.

Cytoplasm:

• Gel-like material• Found between the nucleus and cell membrane• Consists:

– Mostly of water– Many organelles– Protein-rich – Enzymes

• Creates the chemical environment in which the other cell structures function

Ribosomes:

• Location:

• Free ribosomes in the cytoplasm are attached to cytoskeletal filaments

• Ribosomes can also be attached to the endoplasmic reticulum making it ‘Rough’ Endoplasmic reticulum.

• Structure:• 2 Subunits

– 1 large subunit (50S)– 1 small subunit (30S)Total of 80S**In prokaryotes, ribosomes contain 2 subunits but only add up to 70S

• Both composed of – Ribosomal RNA and – Protein

  

• Function:

• Translate RNA (coming from the nucleus) into protein. (Protein synthesis)

• Ribosomes attached to the Endoplasmic Reticulum (ER) produce proteins that move into the ER.

Endoplasmic Reticulum (ER)• Membrane bound network of interconnected vesicles• Enzymes are found embedded on the surface of the ER.• Materials synthesized here include:

– Membrane phospholipids and cellular lipids- Sex hormones (testosterone and estrogen) (In Specialized cells

ex. testicular cells produce testosterone)- Production of Insulin (ex. liver)Other functions at this site include:- Storage of calcium ions until they are needed in muscle

contractions.- Transporation of lipids to different parts of the cell.

• The ER keeps these materials from the rest of the cell. If not, they may have harmful effects to the cell.

• There are 2 types:• 1. Rough Endoplasmic Reticulum (RER)

• Ribosomes are attached to the ER – ‘Rough’ These ribosomes synthesize:– Proteins – Digestive Enzymes

• As they are synthesized they move into the ER to become modified

• 2. Smooth ER

• Synthesis of : – Membrane lipids (fatty acids and

phospholilpids)– Membrane Proteins

• Once synthesized and modified in the ER, the materials are carried to the golgi complex through vesicles

• Vesicles are formed from portions of membrane from the ER budding off to form a small membrane bound transport ‘vehicle’.

Golgi Apparatus or Golgi Complex

• Location and structure:

• Series of flattened saucer shaped sacs

• Located near the nucleus

Function:• Vesicles transfers

macromolecules from the ER to one golgi complex sac. (The Cis side)

• Golgi complex completes the processing of these molecules making them ‘functional’.

• They also get sorted, packaged and transported out of the complex by a second set of transport vesicles. (On the opposite side called the Trans side)

• Vesicles then travel to the cell membrane and secrete their contents into the extracellular fluid

Lysosomes:

• Structure:– Special vesicle formed from the golgi complex

Function:

• Transports cellular digestive enzymes safely through the cytoplasm

• Degrades:– Worn out cellular components – Foreign molecules

• I.e.• Contains enzymes such as:• 1. Nuclease: Degrades RNA & DNA into

nucleotides• 2. Proteases: Degrades Proteins and

peptides• 3. Phosphatases: Remove phosphate

groups from nucleotides, phospholipids

Tay-Sachs Disease

• Commonly found in the Jewish Population• Non-Jewish people are 100 times less likely to

have the Tay-Sachs gene• Children Born with Tay-sachs:

– Appear normal at birth

– Central nervous system begins to deteriorate rapidly

– Motor development slows down

– Most children that have Tay Sachs die by the age of 2 or 3 years.

How does it occur?

• A recessive gene is inherited from both parents• Lacks the gene to produce a lysosomal enzyme

that breaks down a certain glycolipid called gangliosides found in nerve cells

• Nerve cells are therefore, greatly enlarged with swollen lipid-filled lysosomes

• Nerve cells are eventually destroyed

Cell Organelles Continued

Mitochondria

FYI:• Evidence shows, mitochondria evolved from

bacteria that were endocytosed into ancestral cells containing a eukaryotic nucleus.

• Over time,• Some Genetic Material transferred to the nucleus

and some stayed in the mitochondria

***Ribosomes found here are of the 70S

• Function:• Responsible for producing energy for the cell • Transforming energy from macromolecules into ATP

(Adenosine triphosphate) • ‘Power Plant’ of the cell • They replicate themselves by dividing down the middle to

create 2 daughter mitochondria 

• Structure: • Contains:• 1. Two membranes: 

– Outer Membrane

• Composed of lipid and protein• Permeable, allowing molecules to pass

through• Separates the chemical reactions occurring

here from the rest of the cytoplasm 

– Inner membrane

• Composed of lipid and protein• Less permeable• The inner membrane folds many times to

create CRISTAE– Cristae increases the surface area for enzymes to

produce ATP THEREFORE,

– Larger surface area = more chemical Reaction (ATP production)

• 2. Their own set of Ribosomes

• 3. Circular DNA

• DID YOU KNOW?

• White Fat Tissue

• Stores fat

• Few mitochondria

• More commonly found in adult humans

• Brown Fat Tissue

• Colour is due to presence of many mitochondria

• Specialized for the generation of heat (Thermogenesis)

• The inner-membrane protein thermogenin is responsible for converting energy into heat

• Seen commonly in :– Cold adapted animals (ie. Rats, snakes)– Hibernating mammals (i.e. Bear)– Human infants (Thermogenesis is vital for

newborns to survive)

Chloroplast:

• Has the same evolutionary line as mitochondria

• Location:

• Characteristic organelles in plants and green algae

 

• Structure and Function: • Contains its own ribosomes and DNA• Surrounded by 2 membranes: (outer and inner

membrane)• Internal system of membrane bound sacs called

Thylakoids– Thylakoids are flattened to form disks– Contains the pigment chlorophyll (gives green colour

and absorbs solar energy to carry out photosynthesis)

•  

– Thylakoids are:– Grouped in stacks called Grana

– These stacks (or Grana) are embedded in the matrix called the Stroma

Intermediate Filaments

Cytoskeleton

• Internal movements are essential to the growth and differentiation of cells

• All cell movements• 1. Require fuel – ATP and • 2. Proteins that convert the energy

stored in ATP in to motion  

• Structure and Function:

• System of fibers found in the cystoplasm• Extending from the nucleus to the cell membrane• Supports the cell membrane• Forms tracks on which organelles and other

elements move throughout the cytosol• Can Disassemble and Reassemble in seconds or

minutes

• Consists of 3 types of cytosolic fibers:• 1. Actin Filaments

– Supports the cell membrane– Determines shape of cell– Ability to contract (I.e. found in muscle cells)

 • 2. Intermediate filaments

– Anchor organelles in different areas of the cell– Supports nuclear envelope and cell membrane

• 3. Microtubules– Act like ‘tracks’ where organelles can move

along

Cilia and Flagella

• Structure:

 • Flexible membrane extensions of the cell• Both consist of a bundle of microtubules

 • Cells either possess • 1 or 2 single flagellum or• Many short cilia

• Function:1. Used to move certain cells by propelling them

with a beating action– (Speed of about 1mm/s)

2. Also used to bring in Food Particles

• I.e. Respiratory Passages are lined with many short cilia that move out particles that collect in the mucus secretions of the tissues.

 

Cell Wall

• Found in • Plants• Fungal Cells• Some single celled eukaryotes

• Structure:• Consists of Celulose fibres• Found outside the plasma membrane

• Function:

 

• Adds strength and rigidity to the cell

• Prevents cells from bursting in hypotonic environments

• Prevents cells from Shrinking in hypertonic solutions

• Other chemicals are found in cell walls…..

• Lignin – Strength and rigidity (I.e. Woody trees)

• Waxes – Prevents plant tissues and proteins from dying out (I.e. Pine needles)