Chapter 6

A TOUR OF THE CELL

Robert Hooke, 1665

Coined the term “Cell” after viewing cork tissue under a simple microscope.

Cell Theory

Schleiden and Schwann , 1838

1. The cell is the basic unit of life 2. All living organisms are either cells or made up of cells 3. All cells come from pre-existing cells

How we study cells

• Light Microscopes - Brightfield (Used for stained or unstained specimen) - Fluorescence - Phase – Contrast - Differential-interference – contrast - Confocal • Electron Microscopes - Scanning Electron Microscopes - Transmission Electron Microscopes

Types of Electron Microscopy Scanning (SEM)

The beam of electrons scans the surface of the specimen

Used for detailed study of external surfaces of the specimen – which could be a cell, an organelle or a whole organism

Types of Electron Microscopy Transmission (TEM)

Beam of electrons passes through a thinly prepared specimen

Used to view internal structures of the specimen – a cell, an organelle or a structure like a flagellum

Size Range of Cells

An unfertilized ostrich egg is the largest single cell on the planet, weighing in at a whopping 3 lbs!

s any shape come in m Cells

Cell Fractionation

1.

2.

3.

Tissue such as skin cells is lysed, so there is a homogenous mixture This mixture is spun, to pellet-out the larger components – the smaller ones are found in the supernatant The “sup” is removed and spun at a higher rpm, for longer period of time to pellet-out smaller and smaller components components

Two Fundamental Cell Types

Prokaryotic and Eukaryotic

Greek translation: “Karyon” = nucleus “Pro” = before “Eu” = true

Prokaryotes

1.

2.

3.

4.

5.

6.

7.

Smaller than eukaryotic cells No nucleus or membrane bound organelles Single, circular DNA DNA confined to a “nucleoid” region and is attached to the plasma membrane Smaller ribosomes Plasma membrane often surrounded by cell wall and capsule Lack a cytoskeleton (no microtubules or other such filaments)

Eukaryotes – animal cells

Eukaryotes – Plant Cells

Plants contain an organelle group called plastids, to which amyloplasts and chloroplasts belong Plants contain centrosomes but not centrioles Plants contain a central vacuole Plants, like prokaryotes have cell walls

• • • • •

Why are most cells microscopic?

The cell’s plasma membrane or surface area is very critical to the proper functioning of the cell It allows nutrients to move in, wastes to move out, it communicates with other cells, etc.

As a cell increases in size, its volume increases far more than its surface area – this decreases the ratio of surface area to volume Larger cells are not nearly as efficient as smaller cells Evolution has therefore favored the small size

The Cell Boundary

The Fluid mosaic model Phospholipid bi-layer embedded with Cholesterol, amphipathic proteins and glycoproteins • • Keeps cell contents and precious nutrients inside Allows certain atoms and molecules in and out (selectively permeable)

Cytosol

• Semi-fluid medium • Contains all organelles • Contains water, nutrients and building blocks (carbohydrates, lipids, amino acids, nucleotides, ATP, enzymes, ions such as Ca ++ , Na + , Cl , H + , OH , K + and many others) Cytoplasm: The cytosol-containing area outside the nucleus Nucleoplasm: The cytosol-containing area inside the nucleus

• • • • •

The Nucleus

Contains most of the cell’s DNA (Chromatin) Nuclear envelope is made up of a double membrane – each a phospholipid bi-layer The inside of the inner layer has a net-like protein (keratin and laminin) lining called the

nuclear lamina

Numerous “Nuclear pores” are punched through the envelop, to allow various molecules in and out Contains the nucleolus, where ribosomal RNA (rRNA) is synthesized and sent into cytoplasm to be assembled into ribosomes

The Ribosomes

• Made up of 2 units of proteins • Contain Ribosomal RNA (rRNA) • Sites of Protein synthesis (Translation of mRNA) • Some are “Bound” to the rough endoplasmic reticulum, while others are “Free” in the cytosol • The protein units of ribosomes are made in the cytoplasm, the rRNA of ribosomes are made in the nucleolus . The ribosomal proteins are then imported into the nucleus – The assembly of the ribosomes and rRNA begins in the nucleus and is completed in the cytoplasm Free ribosomes synthesize proteins that are to remain and be used in the cytosol.

Bound ribosomes synthesize proteins: •For secretion from the cell •To be used by different organelles •Meant to be membrane proteins

The Endomembrane System

• A network of sac-like membranes that are spread through-out the cell cytoplasm • Consists of: 1. The Nuclear envelope 2. The Rough Endoplasmic Reticulum 3. The Smooth Endoplasmic Reticulum 4. The Golgi Apparatus or Body 5. Lysosomes, vacuoles and other Transport Vesicles

The Rough Endoplasmic Reticulum

• Connected to the nuclear envelope • Labyrinth of sac-like or tube like membranous structures • The sacs are called

cisternae

and the insides are called

cisternal space lumen

or • Is studded with ribosomes (these are therefore called bound ribosomes)

Functions:

1.

Protein folding, tagging and transport for secretion 2.

Membrane production and transport

The Smooth Endoplasmic Reticulum • Structure similar to rough ER but does not house ribosomes • Synthesizes lipids : phospholipids, steroids and steroid hormones like testosterone and estrogen -

Cells of the ovary and testes are rich in smooth endoplasmic reticulum – for estrogen and testosterone production

• Hydrolyzes stored carbohydrates (glycogen in the liver cells of animals) so that blood glucose levels remain constant • Enzymes in the smooth ER also help detoxify alcohol and drugs cells – especially the smooth ER in liver

Liver- The detox center

•Detoxification involves adding OH groups to the drug, so it becomes soluble and can be washed out of the system. Drugs like Phenobarbital (used for epilepsy) are removed from the liver in this manner

Alcoholics and drug abusers have highly proliferated smooth ERs

The Golgi Apparatus

Proteins that have sugar added to them are called

glycoproteins. Budding vesicle

•Flattened sac-like structures •Has a “cis-face” that faces the ER and is the “receiving” side •Has a “trans-face” that is the “shipping” side •Receives material from the ER and further processes it (like adding sugar molecules to proteins), stores it and ships it to its final destination in transport vesicles.

These transport vesicles have proteins tags that help them “dock” with specific organelles or plasma membrane

A glycoprotein

• A protein that has taken on its final conformation in the rough ER, is then tagged with a small carbohydrate (an oligosaccharide) • The Golgi body alters these generic oligosaccharides to more specific ones

Lysosomes

Membrane-bound sacs of various hydrolytic enzymes. They can digest fat, proteins, carbohydrates or nucleic acids.

Their primary role is digestion

Hydrolysis is the opposite of dehydration synthesis

1.

2.

They can digest: Contents of food vacuoles that are a result of phagocytosis and pinocytosis Old and dead organelles – this is called autophagy

Tay-Sachs Disease

• Lysosomes lack an important fat hydrolyzing enzyme • Lipids accumulate in neurons and other cells in and around the brain • Afflicted children become blind, deaf, and unable to swallow. Muscles begin to atrophy and paralysis sets in. Death usually occurs before the age of 4 or 5.

• Autosomal recessive disease, common in Ashkenazi Jews.

Red-spot on retina

• Lipids accumulate in neurons around the retina • Only one normal spot of the retina is visible – this is the red spot

Vacuoles

Membrane-bound sacs that contain various cellular materials including: • Solid nutrients • Liquid (water, dissolved nutrients) Food Vacuoles Contractile vacuoles These are found in freshwater protists. They use them to pump out excess water

Vacuoles – cont’d.

Plants have a “central vacuole” surrounded by a membrane called the tonoplast.

Tonoplast Central Vacuole The central vacuole stores water and nutrients for the plant cell, for release into the cytosol as and when needed. It helps the cell elongate by filling itself up and creating “turgor pressure”

The Mitochondrion

The “Powerhouse” of the cell • • • • •

Inner and outer membrane. Inner membrane has many folds called cristae The inner membrane surrounds nutrient-rich matrix Has its own DNA – which is circular and resides in the matrix. The DNA is bound to the inner membrane. Divides on its own Has it own ribosomes (small like prokaryotic ones) All our mitochondria came from our mothers Uses the energy (e-) stored in the chemical bonds of food molecules such as Glucose, to produce ATP, the energy currency of the cell

The Mitochondrion

• • • • •

Inner and outer membrane. Inner membrane has many folds called cristae The inner membrane surrounds nutrient-rich matrix Has its own DNA – which is circular and resides in the matrix. Divides on its own. The mDNA usually exists in multiple copies Site of cellular respiration which produces ATP* – the universal cellular unit of energy *Adenosine triphosphate

Chloroplasts

Belong to a family of organelles called Plastids. Amyloplasts are colorless and store starch (amylose). Chromoplasts such as chloroplasts store various pigments such as chlorophyll Found in plant cell and cells of protists that photosynthesize

• • • • • •

Chloroplasts: Have a double membrane -the inner membrane the outer membrane They contain flat disc-shaped membrane sacs called thylakoids. A stack of thylakoids is called a granum The thylakoid membrane contains pigments that can trap light energy, such as chlorophyll. Chlorophyll is green and lends its color to the plant cell have their own DNA Have their own ribosomes (smaller - like the ribosomes of prokaryotes) they make their own enzymes required for photosynthesis Use carbon dioxide and water to produce glucose. Light energy drives the reaction

Chloroplast

• Belong to the Plastid family, which consists of

amyloplasts

and

chromoplasts

. Amyloplasts store starch, in roots and tubers, while chromoplasts contain pigments of various types. Chloroplasts are a type of chromoplast.

• Inner and Outer membrane enclose a jelly-like matrix called

Stroma

(Matrix in mitochondrion) • Stacks of disc-shaped

Thylakoids

actually contain the pigment chlorophyll, making the organelle look green • Chloroplasts also contain their own circular DNA, and they can replicate on their own • The thylakoid membrane and the stroma are the sites of

Photosynthesis

Endosymbiosis

The proof: Mitochondria and chloroplasts have their own DNA and ribosomes Their ribosomes are smaller than those in the cytosol (normal eukaryotic) They divide on their own – whether or not the cell is dividing They have double membranes – a clear indication of “phagocytosis”

1.

2.

Peroxisomes

Enzymatically transfer hydrogens from metabolic wastes and combine them with oxygen, to produce H 2 O 2 They then use enzymes to convert H 2 O 2 to water (since is H 2 O 2 toxic to cells) 3.

They use oxygen to break down fatty acids into smaller components that can be transported into mitochondria to be used as fuel for ATP generation (cellular respiration) 4.

Peroxisomes in the liver also detoxify alcohol and other toxins (like the smooth ER) 5.

Peroxisomes split in two, when they reach a certain size.

• Peroxisomes are shown in green

Glyoxysomes

• Plant seeds contain a relative of peroxisomes called glyoxysomes – which convert fatty acids to sugar for the growing seedling as a source of food, until the seedling can photosynthesize.

The Cytoskeleton

• • A scaffolding made of protein fibers and filaments that gives the

eukaryotic

cell its structural integrity, mobility and support for its organelles.

Three types of cytoskeletal fibers: 1. Microtubules 2. Microfilaments 3. Intermediate filaments

Microtubules

• Each microtubule is a straight hollow rod, made up of dimers of the protein “Tubulin” • Found in cilia, flagella, centrioles and spindle fibers

Centrioles are the animal cell’s microtubule organizing centers. They can elongate the microtubule by adding tubulin dimers or shorten it by removing them. Plant cells lack centrioles.

Cilia and Flagella

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They are made up of microtubules arranged in a 9+2 arrangement and are covered by an extension of the plasma membrane.

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They are motile and designed either to move the cell itself (as in flagella) or to move substances over or around the cell (as in cilia).

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The primary purpose of cilia in mammalian cells is to move fluid, mucous, or cells over their surface.

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The major difference between cilia and flagella : Cilia are much shorter and exist in large numbers on the surfaces of certain cells. Flagella are very long and motile cells have only one or two of them.

Centrioles

• They are made up of microtubules arranged in 9 triplets and are not covered by an extension of the plasma membrane. • Most plants lack centrioles • Usually exist in pairs • They are the microtubules organizing centers. • They organize the spindle fiber apparatus on which chromosomes move during mitosis and meiosis

Centrioles and Basal Bodies have the same structure (9 triplets)

Microfilaments

• • • •

Made of globular proteins called actin Long chains of the actin molecules are intertwined in a helix to form individual microfilaments. In association with another protein called myosin, microfilaments create cellular contractions and basic cell movements , such as the contractions of muscle cells.

They enable a dividing cell to pinch off into two cells and are involved in amoeboid movements of certain types of cells.

Amoeboid Movement and Cytoplasmic Streaming • • •

Locomotion in certain cells is brought about by the continuous assembly and disassembly of actin and myosin As a result of this activity, pseudopodia are formed at the front end of the moving cell As the pseudopodia forms, it fills up with the fluid region of the cytoplasm SOL – the SOL is then converted to a more viscous form called GEL. Locomotion is therefore possible because of the SOL-GEL transitions as well as the assembly and disassembly of actin microfilaments

Cytoplasmic streaming is promoted by actin filaments and allows plant cells to circulate the nutrients in their cytoplasm

Intermediate Filaments

• Made of 5 different types of protein, including keratin and laminin (Nuclear lamina made of this) • Permanent structures – cannot disassemble like others • Offer structure support to cell and organelles

The Cell Surface

Many cells have additional boundaries over their plasma membrane for extra protection, communication or interaction with neighboring cells or recognition.

•Cell walls (Bacteria, plants and fungi) •Glycocalyx (Bacteria, Animal cells) •Intercellular junctions (multicellular organisms)

Cell Walls

Plant cells: Cellulose fibers Bacteria: Peptidoglycan Fungal cells: Chitin

Glycocalyx

• Oligosaccharides that are attached to certain membrane proteins make a thick exterior coat for cells • This coat protects the cell, allows it to recognize other cells and sometimes helps it communicate with other cells

Intercellular junctions

Help integrate cells into higher levels of structure and function

Plasmodesmata in Plants

• Perforations within plant cell walls and membranes • These connect adjacent plant cells and allows them to exchange water, small solutes, RNA and proteins. • Similar function to gap junctions in animal cells

Extracellular Matrix (ECM)

• The extracellular matrix is the defining feature of connective tissue in animals. – provides structural support and anchorage to the cells – performs various other important functions. • The ECM is primarily made up of glycoproteins secreted by the cells, such as: – Collagen – Proteoglycans • Collagen fibers are embedded in a network of proteoglycans – Fibronectin • A very important family of proteins that is a part of the ECM is Integrin

Integrins

•

Integrins

are protein cell surface receptors that interact with the extracellular matrix (ECM) and the interior of the cell.

• These are integral membrane proteins (transmembrane). • Integrin plays a role in the attachment of cells to other cells • They also transmit signals between the ECM and the cytoskeleton • Besides the attachment role, integrin also plays a role in signal transduction, a process by which a cell transforms one kind of signal or stimulus into another.

Signal Transduction

The End