Unit II: The Cell

All organisms are made of cells, the organism’s basic

unit of structure and function.

Size range of cells

How We Study CellsMicroscopes - light microscope - electron microscope (TEM/SEM)

A Panoramic View of the Cell

Prokaryotic Eukaryotic+only in bacteria/archaebacteria Protists, Fungi, Plantae, Animalia

+no true nucleus/nuclear envelope true membrane-bound nucleus

+genetic material in nucleoid region genetic material in nucleus

+no organelles many organelles

Animal vs. Plant CellUnique to Plant Cells:- cell wall- large central vacuole-Chloroplasts-Plasmodesmata

Unique to Animal Cells:-Centrioles-Lysosomes- flagella and/or cilia

How organelles are

fractionated (isolated)

The Nucleus and RibosomesNucleus- enclosed by nuclear envelope- contains most of the genes that control the entire cell

+ DNA organized with proteins into chromatin

- nucleolus

Nuclear lamina – protein filamentsthat give structure to the inner nuclear membrane

The Nucleus and Ribosomes (con’t)

Ribosomes- build proteins- RNA/protein complexes- free/bound

The Endomembrane SystemIncludes:•nuclear envelope

•Endoplasmic reticulum

•Golgi apparatus

•Lysosomes

•Vacuoles

•Plasma membrane*

Endomembrane System (con’t)Endoplasmic reticulum• manufactures membranes • two distinct regions + smooth ER

- synthesis of lipids- carbohydrate metabolism- detoxify drugs/poisons- stores calcium ions

+ rough ER- manufacture proteins for secretion - membrane production

Endomembrane System (con’t)Golgi apparatus• finishes, sorts, and ships cell products• two poles + cis face + trans face

Enzymes in the Golgi modify products of the ER in stagesas they move through the Golgi stack from cis to trans face.

Endomembrane System (con’t)Lysosomes – contain hydrolytic enzymes• digestive compartments + intracellular digestion + recycle cell material + program cell destruction (apoptosis)

Apoptosis – programmed cell death

Endomembrane System (con’t)Vacuoles• function in cell maintenance + food vacuole + contractile vacuole + central vacuole (pictured)

- tonoplast

Endomembrane System Summary

Other Membranous OrganellesPeroxisomes•consume oxygen for metabolism + contain specialized teams of enzymes - peroxide-producing oxidases and catalase

RH2 + O2 -oxidase-> R + H2O2

2H2O2 -catalase-> 2H2O + O2

Mitochondria and Chloroplasts•energy transformers of cells + double membranes + contain ribosomes/DNA

Mitochondrial DNA is only passed on by mom

The CytoskeletonCytoskeleton•provides structural support for motility and regulation + network of fibers

- microtubules - microfilaments

- intermediate filaments

Cytoskeleton (con’t)

Centrosomes

• Found in animal & plant cells

• Produce microtubules during cell reproduction

• Animal cells contain centrioles whichcontain 9 sets of 3microtubules

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Flagellum Structure

Cell Surfaces and

Junctions

Cell Walls (1° & 2°)•cellulose fibers•Plasmodesmata

•Middle lamella Made of pectin (sticky polysaccharide)

Pectin holds cell wallstogether like concrete

Cell Surfaces and JunctionsECM (Extra Cellular Matrix)•meshwork of macromolecules outside plasma membrane

+ mostly glycoproteins (Collagen & proteoglycan)

+ support/anchorage (Fibronectin & integrin)

Cell Surfaces and Junctions

Tight junctions – prevent fluid from moving between cell layers in a tissue

Desmosomes – anchor adjacent cells

Gap junctions – allows the movement of cytoplasm, ions, sugars, amino acids from one cell to the next

Tight Junctions

Desmosomes

Gap junctions

Membrane Structure and Function Collage

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proteoglycan

Fibronectin

Membrane Structure and Function (con’t)

Plasma Membrane•boundary that separates living cell fromits non-living surroundings + 8 nm thick + selectively permeable + unique structure relates to function

Phospholipid

Glycerol

Fatty Acid Chains

Amphipathic molecule – has hydrophilic & hydrophobic regions

Hydrophilic vs Hydrophobic

Membrane Structure and Function (con’t)

Fluid Mosaic Model•The Fluid Quality of Membranes + held together by hydrophobic interactions - lipids/proteins drift about laterally

+ unsaturated hydrocarbon tails - maintain fluidity at low temperatures

+ cholesterol- stabilizes the membrane

• restrains movement at high temp.• hinders close packing at low temp.

Membrane Structure and Function (con’t)

Fluid Mosaic Model• Membranes as Mosaics + membrane is collage of proteins

- integral proteins•transmembrane

- peripheral proteins• appendages

Plasma membrane synthesis

Membrane Structure and Function (con’t)

Functions of Membrane Proteins• Transport

• Enzymatic Activity

• Signal Transduction

• Intercellular joining

• Cell-cell recognition

• Attachment to the cytoskeleton and ECM

Signal Transduction with G proteins

Types of Membrane Proteins

Voltage Gated Ion Channels

Glycoprotein

Glycolipid

Cholesterol

•Reduces membrane fluidity by reducing phospholipid movement

•Hinders solidification at low temperatures

Cholesterolis a steroid

Traffic Across MembranesSelective Permeability

• hydrophobic, small, lipid molecules pass easily across the membrane

• hydrophilic, large, charged (polar) molecules cannot pass easily

How do cells get the materials they need inside?

Traffic Across Membranes (con’t)Passive TransportDiffusion + the tendency for molecules of any substance to spread out into the available space

- concentration gradient

Passive Transport

• Requires no energy• Occurs due to natural concentration gradient• Molecules move from high concentration to low

concentration (DOWN the gradient)

3 Types Diffusion Osmosis Facilitated Diffusion

Diffusion

Diffusion

• A.K.A. simple diffusion

• Movement of small molecules across a selectively permeable membrane from an area of HIGH concentration to an area of LOW concentration w/o the use of energy (DOWN the concentration gradient)

e.g. O2, CO2, urea, & alcohol

Traffic Across Membranes (con’t)

Passive Transport• Osmosis + the diffusion of water

- hypotonic,hypertonic, isotonic

Tonicity – the ability of a solution to cause a cell to gain or lose water

Osmoregulation – the control of water balance e.g. contractile vacuole in paramecium

Osmosis The diffusion of WATER across a selectively

permeable membrane

OSMOTIC PRESSURE

The pressure exerted on plasma membranes in solution• Isotonic solution• Hypertonic solution• Hypotonic solution

Water Potential

• The physical property predicting the direction in which water will flow, controlled by the solute concentration

Water potential (ψ) = pressure potential (ψp ) + solute potential (ψs )

Water potentials (ψ) are a way of measuring the free-energy of water. Water will flow spontaneously from a high potential to a low potential, like a ball rolling down a hill.

(ψ) = Greek letter psi

Physical pressure due to air & the container

“Water potential" (Ψ) is a measure of the free energy of water; pure water  (which has a high amount of free energy) is arbitrarily assigned a water potential of zero; units pressure: MPa; can be positive or negative

Factors that determine plant water potential:  1) amount of solutes- increasing concentrations will lower  the free energy (water potential); termed osmotic potential (ψs)

2)  turgor pressure (ψp) in plant cell- positive pressure inside plant cells; increases free energy; loss of turgor = wilting

Water Potential

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Solute Concentr ation

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Cells in Isotonic Solution

•If the concentration of solute (salt) is = on both sides, there will be no net movement of water

•"ISO" means the same

Cells in Hypertonic Solution

Hypertonic Solutions• More solute (salt) molecules outside the cell, which causes

water to be sucked out of the cell.

• In plant cells, the central vacuole loses water and the cells shrink, causing plasmolysis resulting in the plant wilting.

• In animal cells, the cells also shrink.

• In both cases, the cell may die.

• This is why it is dangerous to drink sea water

Plasmolysis

A phenomenon in plant cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall when the cell loses water to a hypertonic environment.

Cells in Hypotonic Solution

Hypotonic Solutions• There are less solute (salt) molecules outside the cell,

since salt sucks, water will move into the cell.

• The cell will gain water and grow larger. In plant cells, the central vacuoles will fill and the plant becomes stiff and rigid, the cell wall keeps the plant from bursting

• In animal cells, the cell may be in danger of bursting, organelles called CONTRACTILE VACUOLES will pump water out of the cell to prevent this.

Osmotic Potential

The tendency of water to move across a selectively permeable membrane into a solution

Determined by measuring the pressure required to stop the osmotic movement of water into the solution.

Osmosis Review

Traffic Across Membranes (con’t)

Passive Transport• Facilitated Diffusion + diffusion with the help of transport proteins

- gated channels

Facilitated DiffusionProtein Channel or Pore

Facilitated DiffusionProtein Carrier

Traffic Across Membranes (con’t)

Active Transport• energy-requiring process + ATP • pumps molecules against concentration gradient + Na+/K+ pump

Active Transport

• Requires cell energy (ATP) to move molecules AGAINST the concentration gradient; from an area of LOW concentration to an area of HIGH concentration

• Sodium–Potassium pump (Exchange 3 sodium ions for 2 potassium ions)

• Hydrogen ion, or proton pump (Pump hydrogen ion against the concentration gradient)

Phosphorylation

The addition of a phosphate (PO4) group

(From ATP) to a protein or a small molecule

This changes the protein shape

Active Transport (Uniport)

Active Transport Na-K Pump

Na-K Pump Antiport

Glucose-Sodium Symport

Traffic Across Membranes (con’t)

Membrane Potential• electrogenic pump + proton pumps (H+)• electrochemical gradient + cotransport

Traffic Across Membranes (con’t)

Bulk Media Transport

• Endocytosis – Vesicle is created from the invagination of the plasma membrane, which pinches off, bringing large molecules into the cell

• Pinocytosis – Cell drinking (endocytosis)

• Phagocytosis – Cell eating (endocytosis)

• Receptor Mediated Endocytosis – Substrate binds to receptor found on the plasma membrane to be brought into the cell

• Exocytosis – Vesicle binds to the plasma membrane releasing the contents outside of the cell

Endocytosis

Pinocytosis

Phagocytosis

Receptor Mediated Endocytosis

Ligand – molecule that binds specifically to a receptor site of another molecule

LDL uses receptors to enter cells (hypercholesterolemia is due to receptor defect)

Exocytosis

Traffic Across Membranes (con’t)

Transport of Large Molecules• Exocytosis + the cell exports macromolecules using vesicles from Golgi apparatus• Endocytosis + the cell takes in macromolecules by forming new vesicles from membrane

- phagocytosis (“cellular eating”)- pinocytosis (“cellular drinking”)- receptor-mediated endocytosis

+ ligands