Cells and how they work

Chapter 3

CELLS AND HOW THEY WORK

Chapter Outline

Cells: Organized for life

The cytoskeleton: support and movement

The plasma membrane: a lipid bilayer

The plasma membrane is a mix of lipids and proteins

Membrane proteins carry out most membrane functions

DNA is organized in chromosomes

Endoplasmic reticulum (ER) : A protein and lipid assembly line

Golgi Bodies: Packing and shipping Mitochondria: the cell’s energy factories

Mitochondria: the cell’s energy factories

ATP—The cell’s energy currency

How cells make ATP

Cells make ATP in three steps

Step 1: Glycolysis breaks glucose down to pyruvate

Step 2: The Krebs cycle produces energy-rich transport molecules

Step 3: Electron transport produces a large harvest of ATP

• Smallest unit of life

• Can survive on its own or has potential

to do so

• Is highly organized for metabolism

• Senses and responds to environment

• Has potential to reproduce

Cell

Structure of Cells

All start out life with:– Plasma membrane

– Region where DNA is stored

– Cytoplasm

Two types:– Prokaryotic

– Eukaryotic

PLASMA MEMBRANE GOLGI BODY LYSOSOME

ENDOPLASMIC RETICULUM (ER)

nuclear envelope

nucleolusNUCLEUS MITOCHONDRION

microfilaments

microtubules

components ofcytoskeleton

plasma membrane

mitochondrion

nuclear envelope

nucleolus

DNA + nucleoplasm

NUCLEUS

vesicle

lysosome

rough ER

ribosomes

smooth ER

vesicle

Golgi body

pair ofcentrioles

Table 3.2 Common Features of Eukaryotic Cells

ORGANELLES AND THEIR MAIN FUNCTIONS:

Nucleus

Endoplasmic reticulum (ER)

Golgi body

Various vesicles

Mitochondria

Contains the cell’s DNA

Routes and modifies newly formed polypeptide chains; also, where lipids are assembled

Modifies polypeptide chains into mature proteins; sorts and ships proteins and lipids for secretion or for use inside cell

Transport or store a variety of substances; break down substances and cell structures in the cell; other functions

Produce ATP

Table 3.2 Common Features of Eukaryotic Cells

OTHER STRUCTURES AND THEIR FUNCTIONS:

Ribosomes

Cytoskeleton

Assemble polypeptide chains

Gives overall shape and internal organization to cell; moves the cell and its internal parts

Electron MicroscopeElectron Microscope

Light MicroscopeLight Microscope

• Basis for cell shape

• Enables organelle movement within cells and, in some cases, cell motility

• Main elements are microtubules, microfilaments, and filaments

Cytoskeleton

microtubules

intermediate filaments

microfilaments

Figure 3.13Figure 3.13

Flagella and Cilia

• Structures for

cell motility

• 9+2 internal

structure

• Arise from

centriolesdynein

microtubule

one of the outer ring’s pairs of microtubules(doublets)

dynein arm

two centralmicrotubules

central sheath

base of flagellum or cilium

plasma membrane

basal body

plasma membrane

• Main component of cell membranes

• Gives membrane its fluid properties

• Two layers of phospholipids– Hydrophilic heads face outward

– Hydrophobic tails in center

Lipid Bilayer

lipid bilayer

water

water

one layer of lipids

one layer of lipids

Plasma Membrane

• Extremely thin

• Mosaic of proteins and lipids

• Lipids give membrane its fluid quality

• Proteins carry out most membrane

functions

Membrane Proteins

• Transport proteins

• Receptor proteins

• Recognition proteins

• Adhesion proteins

EXTRACELLULAR FLUID

cytoskeletal proteins just beneath the plasma membrane

adhesion protein

phospholipid cholesterol

LIPID BILAYER

recognition protein

receptor protein

CYTOPLASMtransport proteins

In-text figurePage 50oxygen, carbon dioxide,

and other small, nonpolar molecules; some water

molecules

glucose and other large, polar, water-soluable molecules; ions

(e.g., H+, Na+, K+, Ca++, CI–)

Membrane Proteins

• Transport proteins

• Receptor proteins

• Recognition proteins

• Adhesion proteins

glucose, more concentrated outside cell than inside

glucosetransporter

When the glucose bindingsite is again vacant, the protein resumes its original shape.

Glucose binds to a vacant site inside the channel through the transport protein.

Glucose becomes exposed to fluid on the other side of the membrane.

Bound glucose makes the protein change shape. Part of the channel closes behind the solute.

Passive Transport

• Flow of solutes through the interior of passive transport proteins down their concentration gradients

• Passive transport proteins enable solutes to move both ways

• Does not require any energy input

Active Transport

• Net diffusion of solute is against concentration gradient

• ATP gives up phosphate to activate protein

• Binding of ATP changes protein shape and affinity for solute

higher concentration of calcium outside cell

lower concentration of calcium inside cell

The pump returns toits resting shape.

ATP binds to a calcium pump.

Shape change permits calcium release at opposite side of membrane. Phosphate group and ADP are released.

Calcium enters tunnel through pump.

ATP transfers a phosphate group to pump. This energy input will cause pump’s shape to change.

Components of Nucleus

Nuclear envelope

Nucleoplasm

Nucleolus

nuclear pore (protein complex that spans both lipid bilayers)

one of two lipid bilayers(facing cytoplasm)

one of two lipid bilayers(facing nucleoplasm)

NUCLEAR ENVELOPE

• Keeps the DNA molecules of eukaryotic cells separated from metabolic machinery of cytoplasm

• Makes it easier to organize DNA and to copy it before parent cells divide into daughter cells

Functions of Nucleus

Endoplasmic Reticulum

• In animal cells, continuous with nuclear

membrane

• Extends throughout cytoplasm

• Two regions: rough and smooth

Rough ER

• Arranged into flattened sacs

• Ribosomes on surface give it a rough

appearance

• Some polypeptide chains enter and are

modified

cytoplasm ribosomevesicle

RNA messages from the nucleus

the cell nucleus rough ER

Smooth ER

• A series of interconnected tubules

• No ribosomes on surface

• Lipids assembled inside tubules

Golgi Bodies

• Put finishing touches on proteins and lipids that arrive from ER

• Package finished material for shipment to final destinations

internal space

budding vesicle

5 Vesicles from the Golgi body transport products to the plasmamembrane. Products arereleased by exocytosis.

4 Proteins and lipids take onfinal form inside Golgi body. Modifications enable them to be sorted out and shipped to proper destinations.

3 Vesicles bud from the ERmembrane and transportunfinished proteins and lipids to a Golgi body.

2 In the membrane of smoothER, lipids are assembled.

1 Some polypeptide chainsenter the rough ER. Modifications begin.

Endocytic vesicles format plasma membrane andmove into the cytoplasm. They might fuse withthe membrane of otherorganelles or remain intact, asstorage vesicles.

Exocytic vesicles budfrom ER and Golgimembranes, travel to and fuse with plasma membrane.Their contents are therebyreleased from the cell.

DNA instructions forbuilding polypeptidechains leave the nucleusand enter the cytoplasm.

assorted vesicles

Golgibody

smooth ER

rough ER

Chains are assembled on ribosomes in cytoplasm.

Vesicles

• Membranous sacs that move

through the cytoplasm

• Lysosomes

• Peroxisomes

©2005 Brooks/Cole - Thomson

plasma membrane

endocytic vesicle forming

exocytic vesicle leaving cytoplasm

Mitochondrial Structure

• Outer membrane faces cytoplasm

• Inner membrane folds back on itself

• Membranes form two distinct compartments

• ATP-making machinery is embedded in the inner mitochondrial membrane

• ATP-producing powerhouses

• Double-membrane system

• Carry out the most efficient

energy-releasing reactions

• Reactions require oxygen

Mitochondria

cristae

inner membraneouter mitochondrial membrane

outer compartment

inner compartment

inner compartment outer compartment cytoplasm

outer mitochondrial membrane

inner mitochondrial membrane

• Glucose is absorbed into blood

• Pancreas releases insulin

• Insulin stimulates glucose uptake by cells

• Cells convert glucose to glucose-6-phosphate

• This traps glucose in cytoplasm where it can

be used for glycolysis

Carbohydrate Breakdown and Storage

Glycolysis

• Occurs in cytoplasm

• Reactions are catalyzed by enzymes

Glucose 2 Pyruvate

(six carbons) (three carbons)

ATPUniversal Energy Currency

• ATP is earned in reactions that yield energy, and

spent in reactions that require it

P P P

ribose

adenine

Mitochondrial Reactions

• Reactions begin when pyruvate enters a mitochondrion

How Cells Make ATP

• Step 3: Electron transport produces many ATP molecules.

– The final stage of cellular

respiration occurs in the

electron transport

systems embedded in

the inner membranes

(cristae) of the mitochondrion.

Glucose

ATP

ATP

ATP

ATP

ADP

ADP

P

P P

P P

Energy in (2 ATP)

Net Energy Yield: 2

PGAL:

pyruvate

NAD+ NAD+

NADH NADH

Intermediates donate phosphate to ADP, making 4

To second set of

reactions

CYTOPLASM

MITOCHONDRION

GLYCOLYSIS

ELECTRON TRANSPORT

PHOSPHORYLATION

KREBS CYCLE ATP

ATP

energy input to start reactions

2 CO2

4 CO2

2

32

water

2 NADH

8 NADH

2 FADH2

2 NADH 2 pyruvate

e- + H+

e- + oxygen

(2 ATP net)

glucose

TYPICAL ENERGY YIELD: 36 ATP

e-

e- + H+

e- + H+

ATP

H+

e- + H+

ATP2 4

Overview of Aerobic Respiration

C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water

dioxide

Creating an H+ Gradient

NADH

OUTER COMPARTMENT

INNER COMPARTMENT

Making ATP

ATP

ADP+Pi

INNER COMPARTMENT

How Electron Transport Forms ATP

©2005 Brooks/Cole - Thomson

Krebs Cycle

NADH

NADH

NADH

ATP ATP

ATP

ATP

ADP + Pi

INNER COMPARTMENT

OUTER COMPARTMENT

acetyl-CoA

free oxygen

6 H+ flows back into inner compartment, through ATP synthases. Flow drives ATP formation.

1 Pyruvate from cytoplasm enters inner mitochondrial compartment.

3 NADH and FADH2 give up electrons and H+ to electron transport systems.

2 Krebs cycle and preparatory steps: NAD+ and FADH2

accept electrons and hydrogen. ATP forms. Carbon dioxide forms.

5 Oxygen accepts electrons, joins with H+ to form water.

4 As electrons move through the transport system, H+ is pumped to outer compartment.

Summary of Energy Harvest(per molecule of glucose)

• Glycolysis– 2 ATP formed by substrate-level phosphorylation

• Krebs cycle and preparatory reactions– 2 ATP formed by substrate-level phosphorylation

• Electron transport phosphorylation– 32 ATP formed

Diffusion

Click to view animation.

Animation

Factors Affecting Diffusion Rate

• Concentration gradient

• Molecular size

• Temperature

• Electrical or pressure gradients

selectively permeable membranebetween two compartments

proteinmolecule

watermolecule

Osmosis

98% water2% sucrose

100% water(distilled)

90% water10% sucrose

98% water2% sucrose

HYPOTONICCONDITIONS

HYPERTONICCONDITIONS

ISOTONICCONDITIONS

Enzyme Structure and Function

• Enzymes speed the rate at which certain reactions occur

• Nearly all are proteins

• An enzyme recognizes and binds to only certain substrates

• Reactions do not alter or use up enzyme molecules

Participants in Metabolic Pathways

• Enzymes

• Energy Carriers

• Cofactors

• Substrates

• Intermediates

• End Products

Factors Influencing Enzyme Activity

Temperature

pH

Salt concentration

Coenzymes and cofactors

productmolecule

©2005 Brooks/Cole - Thomson

two substrate molecules

active site

substratescontactingactive siteof enzyme

substrates briefly bindtightly to enzyme active site

enzyme unchanged by the reaction