Chapter 3

CELLS

How we study cells

• Light Microscopes • Electron Microscopes

Size Range of Cells

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

Eukaryotes – animal cells

TEM of Phospholipid Bilayer

The fluid mosaic model

Membranes have the consistency of cooking oil!

(Protein + Oligosaccharide = Glycoprotein)

Cholesterol

(Oligosaccharide added in the Golgi body) (Part of cytoskeleton)

Plasma Membrane - The Cell Boundary

• • Phospholipid bi-layer embedded with Cholesterol, proteins and some other important molecules 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 Extracellular fluid (Interstitial fluid): The watery liquid that surrounds cells

• •

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.

2.

3.

Microtubules (Made of the protein Tubulin) Microfilaments (Made of the protein Actin) Intermediate filaments (Made of many different proteins like laminin)

Cytoskeleton

Types of Membrane Proteins

• Receptor Proteins – Receive and transmit signals • Integral Proteins – Form pores, channels, and carriers in cell membranes • Cellular Adhesion Molecules (CAMS) – Enable cells to stick together • Cell Surface Proteins – Establish “SELF”

Intercellular junctions

Help integrate cells into higher levels of structure and function (Extracellular fluid)

Intercellular Junctions, cont’d.

• Tight Junctions – Extremely tight, belt-like areas of fusion – Cells of the digestive tract have this junction – Cells of capillaries of the brain have this – preventing passage of many chemicals – “Blood Brain Barrier” • Desmosomes – “riveted” spots between membranes of adjacent cells, connected to intermediate filaments inside cytoplasm • Gap Junctions – tubular channels found between membranes of adjacent cells which allow the passage of nutrients and other small molecules between cells

Tight junctions are found in all tissues, but those of particular relevance to drug delivery include: •Nasal tissue •Gastrointestinal tissue - where oral drugs are absorbed •Blood vessels •Blood-brain barrier

Desmosomes

Desmosomes – “riveted” spots between membranes of adjacent cells, connected to intermediate filaments inside cytoplasm

Gap Junctions

Gap junctions Proteinaceous tubes that connect adjacent cells. These tubes allow material to pass from one cell to the next without having to pass through the plasma membranes of the cells. Dissolved substances such as ions or glucose can pass through the gap junctions. Large organelles such as mitochondria cannot pass.

The Nucleus

• Contains most of the cell’s DNA (Chromatin) • Nuclear envelope is made up of a double membrane – each a phospholipid bi-layer • Numerous “Nuclear pores” are punched through the envelop, to allow various molecules in and out • Contains the nucleolus, where ribosomal RNA (rRNA) is synthesized to make ribosomes

The Ribosomes

• Made up of 2 units of proteins • Contain Ribosomal RNA (rRNA) • Sites of Protein synthesis • Some are “Bound” others are “free” in the cytosol

The Endomembrane System

• A network of sac-like membranes that are spread through-out the cell cytoplasm • Consists of: 1. The Nuclear envelop 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 • Is studded with ribosomes (these are therefore called bound ribosomes)

Functions:

1. Protein folding, and transport 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.

• The smooth ER is also responsible for detoxification of drugs and alcohol Cells of the ovary and testes are Rich in smooth endoplasmic reticulum – for estrogen and testosterone production

The Golgi Apparatus – UPS

•Flattened sac-like structures •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.

Milk Secretion – how organelles work together

Lysosomes – The clean-up crew

Membrane-bound sacs of various hydrolytic (digestive) 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

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

Peroxisomes

Small membrane-bound organelle filled with enzymes and performs some critical functions: 1. Gets rid of excess Hydrogen in the cell by binding it to oxygen to make H 2 O 2 2. H 2 O 2 (Hydrogen peroxide) is toxic to the cell, so it uses an enzyme called catalaze to neutralize it into water and oxygen 3. Like the smooth ER, it also participates in detoxifying alcohol, drugs and other toxins 4. It breaks down long fatty acids 5. Makes bile acids in liver cells

The Mitochondrion – The Power House

• Site for cellular respiration, a process through which the mitochondrion uses the energy in food to convert ADP into ATP – the universal molecule of energy. The mitochondrion needs O 2 for cellular respiration and produces CO 2 as wastes and H 2 O • Contains many copies of its own DNA (circular) C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + ATP (energy)

Centrioles

• • • • •

Form in a region called a centrosome Usually exist in pairs They organize the spindle apparatus on which chromosomes move during cell division (Mitosis) Made up of microtubules Not membrane-bound

Cilia and Flagella



They are made up of microtubules and are covered by an extension of the plasma membrane.



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).

Cilia and Flagella

The major differences between cilia and flagella :

•

The primary purpose of cilia in mammalian cells is to move fluid, mucous, or cells over their surface (Resp.tract, fallopian tubes, etc). Flagella move the entire cell (sperm)

•

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.

Movement Into and Out of the Cell

PASSIVE TRANSPORT

• • •

Passive transport is the movement of molecules down their electrochemical gradient Passive transport requires no energy expenditure on the part of the cell. “Free” energy is used – the energy of the system Examples of passive transport:

–

Diffusion

–

Osmosis

–

Facilitated diffusion (Protein channels involved)

Diffusion

• Molecules have the natural tendency (due to random molecular motion) of moving from an area where they are highly concentrated, to an area where their concentration is low – they move down their concentration gradient+ •Once the molecules are evenly dispersed in the environment, they reach a state of equilibrium – they continue to move, but it is equal in every direction – so no net change

High free energy Low free energy – stable system

Osmosis

• The diffusion of water molecules • The tendency of water molecules (due to random molecular motion) to move from an area where their concentration is high (higher free energy) , to an area where their concentration is lower (lower free energy) – until equilibrium is reached (no net movement of water) • Movement of water molecules is down their concentration gradient

Low solute High solute Isotonic Solution solute and solvent balanced (Also a form of Passive Transport) (of water molecules) As solute concentration increases, “free” water concentration decreases – so water potential decreases Water then moves from an area of high water potential to an area of low water potential



Inside the cell is lower, because of solutes in the cytosol Water molecules always move from an area of higher water potential to an area of lower water potential, so water rushes into the “cell” from the outside (Net movement is inwards)



Inside the cell is higher than the outside, because the outside has more solute particles Water will therefore move out of the cell to an area of lower



(Net movement is outwards)



Is equal on both sides, so no net movement

Is the “cell” hypertonic, hypotonic or isotonic with respect to its environment?

Plasmolysis

When a cell is placed in a hypertonic environment – more solute outside than inside: - Water potential is greater inside - Water will move from where water potential is greater, to where it is lower - Water will move out of the cell, causing plasma membrane to collapse (low pressure potential) - Cell wall will keep cell from losing its shape – animal cell loses shape

Facilitated Diffusion

• Ions and small polar (Hydrophilic) molecules use facilitated diffusion • Membrane channel proteins are used • Requires no cellular energy (ATP, GTP, etc.) • Diffusion is down concentration gradient

Facilitated Diffusion, Cont’d.

ACTIVE TRANSPORT

• Uses cellular energy (ATP, GTP, etc.) • Uses integral membrane proteins • Specific proteins for specific molecules • Molecules can be moved against electrochemical gradient their • Ion pumps – like the Na+ / K+ pump and the Proton pump (H+) an example of active transport are • Concentration of

Na+ has to be higher outside

gradient) the cell whereas that of

K+ has to be higher inside

the cell – so active transport is used to maintain these concentrations (pumping against electrochemical

1.

Na+ binds to the transport protein at specific binding sites

2.

Na+ binding causes ATP to phosphorylate protein

6.

When K+ exits its binding site, it causes the release of the inorganic phosphate group

Active Transport Cont’d.

3.

Phosphorylation causes conformational change in protein, which moves the Na+ out of the cell

4.

When Na+ exits the binding site, the binding site for K+ is made accessible and K+ binds to sites

5.

When K+ binds, it causes another conformational change, which moves K+ into cell

Endocytosis = Phagocytosis + Pinocytosis

Pinocytic vesicle forming Endocytosis is active transport – needs energy expenditure

A lymphocyte attacking E.coli

SEM of stained prep.

TEM of lymphocyte – E.coli being ingested

All somatic cells reproduce mitotically

• Somatic cells are

all

the cells of the body,

except

the gametes (egg and sperm) • Skin cells, liver cells, cells that line the G.I. tract, etc. are constantly dividing, to replace dead cells • Other cells such as neurons, adipose cells, muscle cells, etc. never or rarely divide

The Cell Cycle

• A

typical

human cell undergoes a division about every 24 hours (there are many exceptions!) • The cell cycle is basically an alternation of 2 major phases – Mitosis and Interphase • Interphase is the phase in which the cell spends 23 of the 24 hours – the cell grows, carries out its “housekeeping duties” and its specialized activities • Mitosis takes about 1 hour

The Cell Cycle, cont’d.

Interphase can be broken down into 3 distinct sub-phases: – G1 (Known as gap 1) – S (for

synthesis

– G2 (gap 2) of DNA)  Cells that do not divide are considered to be in a phase called G0 – where they carry on normal housekeeping and do not prepare to divide

The Cell Cycle

Phases of Interphase

•

G1 phase:

The period prior to the synthesis of DNA. In this phase, the cell prepares for cell division - proteins are synthesized - the cell increases in mass •

S phase:

The period after G1, where all genetic material (DNA) is synthesized •

G2 phase:

The period after DNA synthesis has occurred but prior to the start of mitosis. - cell continues to increase in size - centrosome divides into 2 - In animal cells, each centrosome has 2 centrioles

Phases of Mitosis

• Prophase • Prometaphase • Metaphase • Anaphase • Telophase (followed immediately by cytokinesis)

G2 of Interphase

Phases of Mitosis

Prophase Prometaphase Metaphase Anaphase Telophase & beginning of cytokinesis Completion of cytokinesis

Prophase

1.

2.

3.

4.

Nuclear chromatin starts to become organized and condenses into thick strands that eventually become chromosomes observable in the optical microscope. The nucleoli, primarily responsible for the production of ribosomal RNA, begin to disappear as the chromosomes condense. The

mitotic spindle

, which is assembled by the centrosomes begins to appear along the periphery of the nuclear membrane. These are called asters or stars Centrosomes begin to move apart

Prometaphase

• Nuclear membrane begins to fragment • This allows spindle fibers to invade the nuclear space and interact with chromosomes • Chromosomes are extremely dense and each sister chromatid has a protein complex at the centromere called a kinetochore • Some microtubules (spindle fibers) attach to chromosome kinetochores • Other microtubules (spindle fibers) interact with those from the opposite pole of the mitotic spindle

Metaphase

• Centrosomes are at opposite poles • The chromosomes, attached to the kinetochore microtubules, begin to align in a single plane (known as the

metaphase plate

) midway between the spindle poles • Each sister chromatid’s kinetochore is attached to a spindle fiber coming from opposite poles

Anaphase

• • Sister chromatids pull apart and are now considered daughter chromosomes • *

Hypothesis proteins

- the

motor

in the kinetochore move the chromosome along the microtubule toward the poles.

Nonkinetochore microtubules

lengthen, pushing the centrosomes further apart. • At the end of anaphase, each group of chromosomes is clustered at opposite poles.

Telophase

• In animal cells, the cleavage furrow begins to form due to an actin ring (microfilaments) • In plant cells there is no cleavage furrow – a cell plate forms (discussed later) • Nuclear membrane begins to re form • The mitotic spindle begins to disassemble • Chromosomes begin to return to chromatin state • Nucleolus begins to reappear

Interphase

Centrioles will replicate once the cell is ready to divide again

• Nucleus contains chromatin • Only one set of centrioles (one centrosome) • Fully formed nuclear membrane • Fully formed nucleolus

Stem & Progenitor Cells

• Stem cells are totipotent • Stem cells can divide and give rise to more stem cells or one daughter stem cell and a progenitor cell (partially specialized) • A Progenitor cell is pluripotent a.k.a. committed. It can only become a

set

number of cells (restricted to a number of cell types)

THE END