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The Nature of a Cell

Intracellular Aqueous Environment

Extracellular Aqueous Environment

Cell BoundaryInsoluble and Semi-permeable

A cell is a compartment containing a variety of controlled chemical reactions.

All organisms are made of cells.

The Tasks of a Cells

Make specific biomacromolecules

Control and regulate chemical reactions

Produce energy to drive chemical reactions

Take in small molecules

Receive and respond to chemical signals

Produce useful products for export

Remove waste products

Grow, reproduce and pass on genetic information to the next generation of cells.


The Needs of a Cell

Chemical processes involve the interaction of atoms and molecules

Atoms and molecules need to move into, and around, the cell in order to reach their specific site of activity.

Atoms and molecules must be present in adequate concentrations if chemical reactions are going to occur at the right rate.

Cell structure must

* facilitate these movements * and maintain adequate concentrations

so that reactions can occur and the cell can function.


Size and Shape Matters

The surface area of a cell must provide enough exposure to the extracellular environment to allow sufficient movement of the molecules and atoms

necessary for its function.

l = 10, w = 10, h = 10

Surface Area = 600

Volume = 1000

SA:V = 0.6

l = 100, w = 10, h = 1

Surface Area = 2200

Volume = 1000

SA:V = 2.2

l = 1 w = 1, h = 1

Surface Area = 6

Volume = 1

SA:V = 6

Think about:

the size of eukaryotic cells

Think about:

the shape of ER

Think about:

the size of a prokaryotic cell

Size matters

Size and shape will have an effect on function as the cell strives to control and regulate complex chemical reactions.


Prokaryotic vs Eukaryotic Cells

My volume is small enough to meet the needs

of my molecules

I need lots of internal membranes and

compartments to meet the needs of molecules

Eubacteria and Archaebacteria lacks internal compartments. These cells are

less than 2μm in diameter

Animal, plants and fungi have cells from 10μm+ in diameter. They meet their needs by being full of compartment (organelles) that do specific tasks.


Prokaryotic Cells


Eukaryotic Cells (Plant)


Eukaryotic Cells (Animal)


Molecular Biology

Molecular Biology: Bonding Polarity

Molecules are groups of non-metal atoms covalently bonded together.

Molecules have a polarity that is determined by their overall charge.

What is important about these relationships is that

Nonpolar molecules* have no overall charge* are hydrophobic

Polar Molecules* have regions of positive and negative charge* are hydrophilic

Non-Polar Molecules: Hydrogen Gas


Nonmetal atoms form covalent bonds by sharing electrons.

The electrostatic forces of attraction between the positive nucleus and the negative electrons hold this arrangement together.

The electron attracting power of each atom (electronegativity) determines how equally the electrons are shared.

These hydrogen atoms have the same electronegativity- the electron pair will be shared equally, leaving the molecule nonpolar and hydrophobic.

Polar Molecules: Water


As an oxygen atom I have very high electronegativity

so I can hold the electrons closer and become slightly negative!

We hydrogen atoms cannot compete with

oxygen's pulling power for electrons. As a result we are

always slightly postive.

Because of the unequal sharing of electrons, water is a polar molecule.

Intermolecular Attraction

Molecular Biology: Intermolecular Attraction

The slight opposite charges on different regions of a water molecule lead to a weak bonding between the molecules.

This is called dipole-dipole bonding; or more specifically in this case hydrogen bonding.

Thus, there is a weak attraction between polar molecules.

Biomolecules: The Main Elements

Molecular Biology: Basics of Biomolecules

C

O

N

H

P

S

The main elements that make up biomolecules.

All are nonmetals and all are able to bond together by sharing outer shell electrons.

Sometimes the sharing is uneven.

Biomolecules: Why Carbon?


Forms strong, stable covalent bonds with other carbon atoms and nonmetal atoms.

Each carbon atom can form up to 4 single covalent bonds. They can also form double and triple bonds.

Can create chains, and other structures, by bonding to other carbon atoms.

Other groups of atoms can be attached to these structres to create different compounds.

C

C

C

Biomolecules: Hydrocarbons


Hydrocarbons form the backbone of many organic molecules.

Formed by bonding between hydrogen and carbon atoms these molecules can exist as branched or unbranched chains, or as ring (cyclic) structures.

In this form hydrocarbons are nonpolar and hydrophobic.

Hydrophilic & Hydrophobic Don't Mix!


Oil and Water do not mix.

Like oil, ethane (shown) is hydrophobic because it is made up of non-polar hydrocarbon molecules.

Water is polar. Water molecules will mix with other polar molecules that have a charge (because they will be hydrophilic)

Stay away from us!

We won't mix with you!

I'll mix with you! We can make hydrogen

bonds together!

Reactive Groups


Sometimes a hydrogen is substituted for a different group of atoms and the molecule acquires a new chemical character.

These groups are called “functional groups” and confer polarity on the molecule.

Main functional groups in Biology

OHCOOH

NH2

HS

C

H

H

H

H

Methane MethanolWith an OH group substituted in the molecule is now functions as

an alcohol.

Biomacromolecules

Molecular Biology: Biomacromolecules

Cells import water, mineral ions, and a host of small organic molecules like simple sugars, fatty acids and amino acids. Many other small organic molecules are made and altered in different chemical reactions in the cell.

Small molecules also store and distribute energy for cellular processes and other, such as hormones, act as signals in directing the activities of the cell.

Cells can only acquire biomacromolecules by making them. They are made in a condensation reaction.

Breaking down these biomacromolecules occurs in a reverse process called hydrolysis.

Condensation Reactions

Biomacromolecules: Synthesis

monomer O-HOH monomer O-HOH

H20

monomer monomer monomer

I donate a hydrogen from my OH groupI donate my OH group

We are removed from the reactants as a

water molecule

When functional groups react together a new chemical bond

is formed that links the subunits.

If a water molecule is lost, a condensation reaction is sometimes called a dehydration reaction.

Polymer (has repeating subunits)

Condensation & Hydrolysis Reactions

Biomacromolecules: Synthesis and Breakdown

Just as condensation reactions allow cells to build biomacromolecules by

removing water.....

.....by adding water, cells are able to break down biomolecules in a process

called hydrolysis.

Biomacromolecules are very large molecules that play an essential role in the structure and function of cells.

Biomacromolecules

Biomacromolecules: Features

Biomacromolecule Sub-units Bond Formed by Condensation

Cellular Functions

LipidsLipids are not polymers

Fatty Acids & Glycerol Ester linkage Energy storage, phospholipids, signalling molecules

Complex carbohydratesPolysaccharides

Simple sugar monomersMonosaccharides

Glycosidic linkage Energy storage, structral components (eg. cellulose)

Nucleic AcidsPolynucleotides

Nucleotide monomers Phosphodiester linkage

Genetic material

ProteinsPolypeptides

Amino Acid monomers Peptide linkage Diverse roles; control and regulation, transport, receptors, structural components

Lipids

Biomacromolecules: Lipids

• Lipids do not form polymers

• Lipids are usually hydrophobic because they consist mostly of hydrocarbons, which form nonpolar covalent bonds

• The most biologically important lipids are fats, phospholipids, and steroids

Lipids are constructed from two types of smaller molecules: glycerol and fatty acids

Triglycerides

Lipids: Glycerol & Fatty Acids

Ester linkage

A triglyceride is formed with the bonding of 3 fatty acids to a glycerol molecule.

• Fats separate from water because water molecules form hydrogen bonds with each other and exclude the fats

• Fatty acids vary in length (no. of carbons) and in the number and locations of double bonds

• Saturated fatty acids have no double bonds- the maximum number of hydrogen atoms possible is attached

• Unsaturated fatty acids have one or more double bonds

• The major function of fats is energy storage

Saturated and Unsaturated Fats

Lipids: Saturated and Unsaturated Fats

Saturated Fats

Lipids: Saturated Fats

• Fats made from saturated fatty acids are called saturated fats

• Most animal fats are saturated

• Saturated fats are solid at room temperature

• A diet rich in saturated fats may contribute to cardiovascular disease through plaque deposits

Unsaturated Fats

Lipids: Unsaturated Fats

• Fats made from unsaturated fatty acids are called unsaturated fats

• Plant fats and fish fats are usually unsaturated

• Plant fats and fish fats are liquid at room temperature and are called oils

Phospholipids

Lipids: Phospholipids

• In a phospholipid, two fatty acids and a phosphate group are attached to glycerol

• The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head

Phospholipid Bilayer

Lipids: Phospholipids

• When phospholipids are added to water, they self-assemble into a bilayer, with the hydrophobic tails pointing toward the interior

• The structure of phospholipids results in a bilayer arrangement found in cell membranes

• Phospholipids are the major component of all cell membranes

Steroids

Lipids: Steroids

• Cholesterol is a component in animal cell membranes

• Steroids are lipids with a carbon skeleton of four fused rings. They include cholesterol, estrogen and testosterone.

• Because they are non-polar and hydrophobic, steroids are soluble in fats and pass easily across the phospholipid bilayer.

The Plasma Membrane

The Plasma Membrane: Fluid Mosaic Model

Why are they important?

• Membranes define the cell boundary and provide a permeability barrier.

• Membranes have sites for specific functions

• Membranes regulate the transport of solutes

• Membranes detect electrical and chemical signals

• Membranes assist in cell to cell communication

The Bilayer Boundary

The Plasma Membrane

(3) Charged Atoms

Not permeable to large hydrophilic polar molecules (such as glucose) or charged groups of atoms and metallic ions.

(1) Size

Permeable to small molecules such as oxygen, carbon dioxide, ethanol, water. Rate of exchange may be decreased depending on polarity.

(2) Degree of Polarity

Permeable to hydrophobic molecules such as steroids.

Simple Diffusion

The Plasma Membrane: Transmembrane Migration

Animation: Simple Diffusion

• Simple diffusion is a passive process in which solutes move down a concentration gradient.

Here the ink is spreading by moving to areas of

lower ink solute concentration. Eventually

all the water will be evenly coloured.

http://www.indiana.edu/~phys215/lecture/lecnotes/lecgraphics/diffusion2.gif

Passive Facilitated Diffusion

Transmembrane Migration: Passive Facilitated Diffusion

Membrane proteins can assist the diffusion process

Polar, charged ions may cross the membrane through channel proteins.

Larger polar molecules may cross via carrier proteins.

Active Transport

Transmembrane Migration: Active Transport

Some membrane proteins can help a cell to push solutes against the concentration gradient.

These are often called “pumps” and require energy from ATP. A sodium-potassium pump is shown here.

Active Transport

Transmembrane Migration: Active Transport

Why should active transport be necessary?

• Uptake of essential nutrients against the concentration gradient

• Removal of secretory and waste products against the concentration gradient

• Maintain ion concentrations in a steady, non-equilbrium state.

Osmosis

Transmembrane Migration: Osmosis

• Is a special case of passive diffusion involving the migration of water molecules down a concentration gradient.

Hypertonic and Hypotonic Solutions


Hypertonic and Hypotonic Solutions


Identify• the cell types shown• the process occurring• the solution type in which the

cells have been placed.

Exocytosis & The Secretory Pathway

Transmembrane Migration: Bulk Transport

• During exocytosis a small membrane bound vesicle will fuse with the cell membrane and expel it contents into the extracellular space.

• This is a form of bulk transport and is especially important for the secretion of biomacromolecules such as hormones, antibody, neurotransmitters, mucous, growth regulators and even toxins.

Endoplasmic Reticulm

Exocytosis & The Secretory Pathway: Endoplasmic Reticulum

• ER is a series of folded membranes and tubules that extend out continuously from the nucleus

• Rough ER is studded with ribosomes.

• Cytosolic ribosomes will bind to the ER once it begins to synthesize a protein destined for the secretory pathway

• The rough ER will then send these products the Golgi body in small vesicles.

Endoplasmic Reticulm

Exocytosis & The Secretory Pathway: Endoplasmic Reticulum

• Smooth ER is free of ribosomes

• Smooth ER is the site of• lipid and steroid synthesis (inc. phospholipids and cholesterol)• synthesis and metabolism of some carbohydrates• drug and poison detoxification• regulation of calcium concentration

Golgi Body

Exocytosis & The Secretory Pathway: Golgi Body

• The Golgi body is a series of flat membrane compartments resembling tubes

• Proteins and lipids manufactured on the ER may be chemically modified (often by adding carbohydrate groups to form glycoproteins and glycolipids)

• These biomacromolecules are packaged into transport vesicles bound for the surface.

Secretory Pathway: Overview

Exocytosis & The Secretory Pathway

From p61 Nelson Biology

Endocytosis

Endocytosis: Bulk Transport

• During endocytosis, the plasma membrane sinks inward and engulfs the extracellular space. It encloses the material within to form an endocytic vesicles which transport the contents within the cytoplasm.

• If solids are engulfed it is phagocytosis

• If liquid is engulfed it is pinocytosis

Phagocytosis Pinocytosis

Lysosomes

Endocytosis: Lysosomes

• Solids that are engulfed are digested by in vesicular structures called lysosomes.

• Lysosomes fuse with the membranes of old organelles or the membranes created by phagocytosis and endocytosis.

• They empty their enzymes into the space and breakdown the molecules.

• Lysosomes recycle cellular materials and offer protection.

Additional Membrane Proteins

The Plasma Membrane: Membrane Proteins

Receptor proteins are important for cell signalling.

Recognition proteins allow cells to identify each other and interact- identifying the cell as self.

Adhesion proteins provide binding potential between the cells of multicellular organisms

Additional Membrane Proteins

The Plasma Membrane: Membrane Proteins

The Plasma Membrane

The Plasma Membrane: Fluid Mosaic Model

Lipids provide fluidity to the cell membrane. Proteins and cholesterol molecules are embedded in the membrane creating a mosaic. This is the

fluid mosaic model of the plasma membrane.

What were those funny worm things?!Eukaryotes have a network of protein fibres in the cytosol.

Microtubules: Movement of chromosomes, organelles, cilia, flagella

Intermediate Filaments: Adds tensile strength to maintain tissue shape and attachment of cells to each other.

Microfilaments: Muscle contraction, maintenance of cell shape, cellular movements.

The Cell Cytoskeleton

The Cell Cytoskeleton

When a white blood cell squeezes between the cell junction of a blood vessel it is the cytoskeleton that provides flexibility and returns the cell to its original shape.

The extracellular matrix (ECM) determines the shape and mechanical properties of tissues and organs.

Bone and cartilage are largely composed of ECM material.

The Extracellular Matrix

The Extracellular Matrix

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