Chapter 5
Introduction The Working Cell Some organisms use energy-converting reactions to produce light in a process called bioluminescence. – Many marine invertebrates and fishes use bioluminescence to hide themselves from predators. – Scientists estimate that 90% of deep-sea marine life produces bioluminescence.
The light is produced from chemical reactions that convert chemical energy into visible light.
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko © 2012 Pearson Education, Inc.
Figure 5.0_1
Figure 5.0_2
Chapter 5: Big Ideas
Cellular respiration
Membrane Structure and Function
Energy and the Cell
How Enzymes Function
Introduction Bioluminescence is an example of the multitude of energy conversions that a cell can perform. Many of a cell’s reactions
MEMBRANE STRUCTURE AND FUNCTION
– take place in organelles and – use enzymes embedded in the membranes of these organelles.
This chapter addresses how working cells use membranes, energy, and enzymes.
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5.1 Membranes are fluid mosaics of lipids and proteins with many functions
5.1 Membranes are fluid mosaics of lipids and proteins with many functions
Membranes are composed of
Many phospholipids are made from unsaturated fatty acids that have kinks in their tails.
– a bilayer of phospholipids with
These kinks prevent phospholipids from packing tightly together, keeping them in liquid form.
– embedded and attached proteins, – in a structure biologists call a fluid mosaic.
In animal cell membranes, cholesterol helps – stabilize membranes at warmer temperatures and – keep the membrane fluid at lower temperatures.
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Figure 5.1
CYTOPLASM Enzymatic activity Fibers of extracellular matrix (ECM)
5.1 Membranes are fluid mosaics of lipids and proteins with many functions Membrane proteins perform many functions.
Phospholipid Cholesterol
Cell-cell recognition
Receptor
2. Some proteins function as receptors for chemical messengers from other cells.
Signaling molecule
Transport
Attachment to the cytoskeleton and extracellular matrix (ECM)
Signal transduction
1. Some proteins help maintain cell shape and coordinate changes inside and outside the cell through their attachment to the cytoskeleton and extracellular matrix.
ATP
3. Some membrane proteins function as enzymes.
Intercellular junctions Glycoprotein
Microfilaments of cytoskeleton
CYTOPLASM
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5.1 Membranes are fluid mosaics of lipids and proteins with many functions 4. Some membrane glycoproteins are involved in cell-cell recognition. 5. Membrane proteins may participate in the intercellular junctions that attach adjacent cells to each other. 6. Membranes may exhibit selective permeability, allowing some substances to cross more easily than others.
5.2 EVOLUTION CONNECTION: Membranes form spontaneously, a critical step in the origin of life Phospholipids, the key ingredient of biological membranes, spontaneously self-assemble into simple membranes. The formation of membrane-enclosed collections of molecules was a critical step in the evolution of the first cells.
Animation: Signal Transduction Pathways
Animation: Overview of Cell Signaling © 2012 Pearson Education, Inc.
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Figure 5.2
Figure 5.2Q
Water
Water
Water-filled bubble made of phospholipids
5.3 Passive transport is diffusion across a membrane with no energy investment
5.3 Passive transport is diffusion across a membrane with no energy investment
Diffusion is the tendency of particles to spread out evenly in an available space.
Diffusion across a cell membrane does not require energy, so it is called passive transport.
– Particles move from an area of more concentrated particles to an area where they are less concentrated.
The concentration gradient itself represents potential energy for diffusion.
– This means that particles diffuse down their concentration gradient. – Eventually, the particles reach equilibrium where the concentration of particles is the same throughout. Animation: Diffusion
Animation: Membrane Selectivity © 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
Figure 5.3A
Figure 5.3B
Molecules of dye
Membrane
Pores
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
3
5.4 Osmosis is the diffusion of water across a membrane
5.4 Osmosis is the diffusion of water across a membrane
One of the most important substances that crosses membranes is water.
If a membrane permeable to water but not a solute separates two solutions with different concentrations of solute,
The diffusion of water across a selectively permeable membrane is called osmosis.
– water will cross the membrane, – moving down its own concentration gradient, – until the solute concentration on both sides is equal.
Animation: Osmosis © 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
Figure 5.4
Lower Higher concentration concentration of solute of solute
Equal concentrations of solute
5.5 Water balance between cells and their surroundings is crucial to organisms Tonicity is a term that describes the ability of a solution to cause a cell to gain or lose water.
H2O Solute molecule
Tonicity mostly depends on the concentration of a solute on both sides of the membrane.
Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules
Osmosis © 2012 Pearson Education, Inc.
5.5 Water balance between cells and their surroundings is crucial to organisms
5.5 Water balance between cells and their surroundings is crucial to organisms
How will animal cells be affected when placed into solutions of various tonicities? When an animal cell is placed into
For an animal cell to survive in a hypotonic or hypertonic environment, it must engage in osmoregulation, the control of water balance.
– an isotonic solution, the concentration of solute is the same on both sides of a membrane, and the cell volume will not change, – a hypotonic solution, the solute concentration is lower outside the cell, water molecules move into the cell, and the cell will expand and may burst, or – a hypertonic solution, the solute concentration is higher outside the cell, water molecules move out of the cell, and the cell will shrink. © 2012 Pearson Education, Inc.
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5.5 Water balance between cells and their surroundings is crucial to organisms
Figure 5.5
Hypotonic solution
The cell walls of plant cells, prokaryotes, and fungi make water balance issues somewhat different. – The cell wall of a plant cell exerts pressure that prevents the cell from taking in too much water and bursting when placed in a hypotonic environment.
H2O
Isotonic solution
Hypertonic solution
H2O
H2O
H2O
Animal cell Normal
Lysed
– But in a hypertonic environment, plant and animal cells both shrivel.
Shriveled Plasma membrane
H2O
H2O
H2O
Plant cell
Video: Chlamydomonas
Video: Plasmolysis
Video: Paramecium Vacuole
Video: Turgid Elodea
Turgid (normal)
Flaccid
Shriveled (plasmolyzed)
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5.6 Transport proteins can facilitate diffusion across membranes
5.6 Transport proteins can facilitate diffusion across membranes
Hydrophobic substances easily diffuse across a cell membrane.
Some proteins function by becoming a hydrophilic tunnel for passage of ions or other molecules.
However, polar or charged substances do not easily cross cell membranes and, instead, move across membranes with the help of specific transport proteins in a process called facilitated diffusion, which
Other proteins bind their passenger, change shape, and release their passenger on the other side.
– does not require energy and
In both of these situations, the protein is specific for the substrate, which can be sugars, amino acids, ions, and even water.
– relies on the concentration gradient.
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5.6 Transport proteins can facilitate diffusion across membranes
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Figure 5.6
Solute molecule
Because water is polar, its diffusion through a membrane’s hydrophobic interior is relatively slow. The very rapid diffusion of water into and out of certain cells is made possible by a protein channel called an aquaporin.
Transport protein
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5.7 SCIENTIFIC DISCOVERY: Research on another membrane protein led to the discovery of aquaporins
Figure 5.7
Dr. Peter Agre received the 2003 Nobel Prize in chemistry for his discovery of aquaporins. His research on the Rh protein used in blood typing led to this discovery.
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5.8 Cells expend energy in the active transport of a solute
Figure 5.8_s1
In active transport, a cell Transport protein
– must expend energy to – move a solute against its concentration gradient.
The following figures show the four main stages of active transport. Solute 1
Solute binding
Animation: Active Transport © 2012 Pearson Education, Inc.
Figure 5.8_s2
Figure 5.8_s3
Transport protein
Transport protein
P ADP Phosphate attaching
ATP Solute 1
Solute binding
2
P ADP Phosphate attaching
ATP Solute 1
Solute binding
2
P Protein changes shape. 3
Transport
6
Figure 5.8_s4
5.9 Exocytosis and endocytosis transport large molecules across membranes A cell uses two mechanisms to move large molecules across membranes.
Transport protein
– Exocytosis is used to export bulky molecules, such as proteins or polysaccharides. P ADP Phosphate attaching
ATP Solute 1
Solute binding
2
P Protein changes shape. 3
Transport
Phosphate P detaches. 4
Protein reversion
– Endocytosis is used to import substances useful to the livelihood of the cell.
In both cases, material to be transported is packaged within a vesicle that fuses with the membrane.
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5.9 Exocytosis and endocytosis transport large molecules across membranes
Figure 5.9
Phagocytosis EXTRACELLULAR FLUID Pseudopodium
There are three kinds of endocytosis.
Food being ingested
CYTOPLASM
“Food” or other particle Food vacuole
1. Phagocytosis is the engulfment of a particle by wrapping cell membrane around it, forming a vacuole.
Pinocytosis Plasma membrane
2. Pinocytosis is the same thing except that fluids are taken into small vesicles. 3. Receptor-mediated endocytosis uses receptors in a receptor-coated pit to interact with a specific protein, initiating the formation of a vesicle.
Vesicle
Plasma membrane
Receptor-mediated endocytosis Coat protein Receptor
Coated vesicle
Animation: Exocytosis and Endocytosis Introduction
Coated pit
Animation: Exocytosis
Animation: Pinocytosis
Animation: Phagocytosis
Animation: Receptor-Mediated Endocytosis
Specific molecule
Coated pit Material bound to receptor proteins
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Figure 5.9_1
Figure 5.9_2
Pinocytosis
Phagocytosis EXTRACELLULAR FLUID Pseudopodium
CYTOPLASM
“Food” or other particle
Food being ingested
Plasma membrane
Vesicle Food vacuole Plasma membrane
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Figure 5.9_3
Figure 5.9_4
Receptor-mediated endocytosis Coat protein
Plasma membrane Food being ingested
Coated vesicle
Receptor
Coated pit
Specific molecule
Coated pit Material bound to receptor proteins
Figure 5.9_5
Figure 5.9_6
Plasma membrane
Coated pit
Material bound to receptor proteins
Plasma membrane
5.10 Cells transform energy as they perform work
ENERGY AND THE CELL
Cells are small units, a chemical factory, housing thousands of chemical reactions. Cells use these chemical reactions for – cell maintenance, – manufacture of cellular parts, and – cell replication.
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5.10 Cells transform energy as they perform work
Figure 5.10
Energy conversion
Fuel
Waste products
Heat energy
Energy is the capacity to cause change or to perform work.
Carbon dioxide
Gasoline
There are two kinds of energy.
Combustion Kinetic energy of movement
Water
Oxygen Energy conversion in a car
1. Kinetic energy is the energy of motion. Heat energy
2. Potential energy is energy that matter possesses as a result of its location or structure.
Cellular respiration Glucose
Oxygen
Carbon dioxide ATP
ATP
Energy for cellular work
Water
Energy conversion in a cell © 2012 Pearson Education, Inc.
Figure 5.10_1
Figure 5.10_2
Fuel Energy conversion
Fuel
Waste products
Heat energy
Heat energy Carbon dioxide
Gasoline
Energy conversion
Waste products
Combustion Kinetic energy of movement
Water
Oxygen
Glucose
Oxygen
Cellular respiration Carbon dioxide ATP
ATP
Energy for cellular work
Water
Energy conversion in a car Energy conversion in a cell
5.10 Cells transform energy as they perform work
5.10 Cells transform energy as they perform work
Heat, or thermal energy, is a type of kinetic energy associated with the random movement of atoms or molecules.
Chemical energy is the potential energy available for release in a chemical reaction. It is the most important type of energy for living organisms to power the work of the cell.
Light is also a type of kinetic energy, and can be harnessed to power photosynthesis.
Animation: Energy Concepts © 2012 Pearson Education, Inc.
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5.10 Cells transform energy as they perform work
5.10 Cells transform energy as they perform work
Thermodynamics is the study of energy transformations that occur in a collection of matter.
Two laws govern energy transformations in organisms. According to the
Scientists use the word – system for the matter under study and – surroundings for the rest of the universe.
– first law of thermodynamics, energy in the universe is constant, and – second law of thermodynamics, energy conversions increase the disorder of the universe.
Entropy is the measure of disorder, or randomness.
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5.10 Cells transform energy as they perform work
5.11 Chemical reactions either release or store energy
Cells use oxygen in reactions that release energy from fuel molecules.
Chemical reactions either
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5.11 Chemical reactions either release or store energy Exergonic reactions release energy. – These reactions release the energy in covalent bonds of the reactants. – Burning wood releases the energy in glucose as heat and light. – Cellular respiration – involves many steps, – releases energy slowly, and – uses some of the released energy to produce ATP.
– release energy (exergonic reactions) or – require an input of energy and store energy (endergonic reactions).
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Figure 5.11A
Potential energy of molecules
In cellular respiration, the chemical energy stored in organic molecules is converted to a form that the cell can use to perform work.
Reactants Amount of energy released Energy Products
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An endergonic reaction – requires an input of energy and – yields products rich in potential energy.
Endergonic reactions – begin with reactant molecules that contain relatively little potential energy but – end with products that contain more chemical energy.
Figure 5.11B
Potential energy of molecules
5.11 Chemical reactions either release or store energy
Products
Energy
Amount of energy required
Reactants
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5.11 Chemical reactions either release or store energy
5.11 Chemical reactions either release or store energy
Photosynthesis is a type of endergonic process.
A living organism carries out thousands of endergonic and exergonic chemical reactions.
– Energy-poor reactants, carbon dioxide, and water are used. – Energy is absorbed from sunlight. – Energy-rich sugar molecules are produced.
The total of an organism’s chemical reactions is called metabolism. A metabolic pathway is a series of chemical reactions that either – builds a complex molecule or – breaks down a complex molecule into simpler compounds.
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5.11 Chemical reactions either release or store energy
5.12 ATP drives cellular work by coupling exergonic and endergonic reactions
Energy coupling uses the
ATP, adenosine triphosphate, powers nearly all forms of cellular work.
– energy released from exergonic reactions to drive – essential endergonic reactions, – usually using the energy stored in ATP molecules.
ATP consists of – the nitrogenous base adenine, – the five-carbon sugar ribose, and – three phosphate groups.
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Figure 5.12A_s1
5.12 ATP drives cellular work by coupling exergonic and endergonic reactions
Adenosine Triphosphate
ATP:
Phosphate group
Hydrolysis of ATP releases energy by transferring its third phosphate from ATP to some other molecule in a process called phosphorylation.
P
P
P
Adenine Ribose
Most cellular work depends on ATP energizing molecules by phosphorylating them.
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Figure 5.12A_s2
5.12 ATP drives cellular work by coupling exergonic and endergonic reactions
Adenosine Triphosphate
ATP:
Phosphate group P
P
There are three main types of cellular work:
P
Adenine
1. chemical,
Ribose
2. mechanical, and
P
3. transport.
H2 O
Hydrolysis
ATP drives all three of these types of work.
P
P
Energy
Adenosine Diphosphate
ADP:
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Figure 5.12B
Chemical work
Mechanical work
Transport work
ATP
ATP
ATP
Solute P
Motor protein
P P
Reactants
Membrane protein
5.12 ATP drives cellular work by coupling exergonic and endergonic reactions ATP is a renewable source of energy for the cell. In the ATP cycle, energy released in an exergonic reaction, such as the breakdown of glucose,is used in an endergonic reaction to generate ATP.
P P P
Product Molecule formed
ADP
P
Protein filament moved
ADP
P
Solute transported
ADP
P © 2012 Pearson Education, Inc.
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Figure 5.12C
HOW ENZYMES FUNCTION ATP
Energy from exergonic reactions
ADP
Energy for endergonic reactions
P
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5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers
5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers
Although biological molecules possess much potential energy, it is not released spontaneously.
We can think of EA
– An energy barrier must be overcome before a chemical reaction can begin. – This energy is called the activation energy (EA).
– as the amount of energy needed for a reactant molecule to move “uphill” to a higher energy but an unstable state – so that the “downhill” part of the reaction can begin.
One way to speed up a reaction is to add heat, – which agitates atoms so that bonds break more easily and reactions can proceed but – could kill a cell.
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Figure 5.13A
Figure 5.13A_1
Activation energy barrier Activation energy barrier
Enzyme
Energy
Energy
Products Without enzyme
Reactant Energy
Activation energy barrier reduced by enzyme
Reactant
Reactant
Products With enzyme
Products Without enzyme
13
Figure 5.13A_2
Figure 5.13Q
Enzyme
Energy
Reactant
a Energy
Activation energy barrier reduced by enzyme
b Reactants c
Products Products Progress of the reaction
With enzyme
5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers
5.14 A specific enzyme catalyzes each cellular reaction
Enzymes
An enzyme
– function as biological catalysts by lowering the EA needed for a reaction to begin, – increase the rate of a reaction without being consumed by the reaction, and – are usually proteins, although some RNA molecules can function as enzymes.
Animation: How Enzymes Work © 2012 Pearson Education, Inc.
5.14 A specific enzyme catalyzes each cellular reaction
– is very selective in the reaction it catalyzes and – has a shape that determines the enzyme’s specificity.
The specific reactant that an enzyme acts on is called the enzyme’s substrate. A substrate fits into a region of the enzyme called the active site. Enzymes are specific because their active site fits only specific substrate molecules. © 2012 Pearson Education, Inc.
Figure 5.14_s1
1
Enzyme available with empty active site Active site
The following figure illustrates the catalytic cycle of an enzyme. Enzyme (sucrase)
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Figure 5.14_s2
Figure 5.14_s3
1
Enzyme available with empty active site Active site
1
Substrate (sucrose) 2
Enzyme available with empty active site Active site
Substrate (sucrose)
Substrate binds to enzyme with induced fit
2
Enzyme (sucrase)
Substrate binds to enzyme with induced fit
Enzyme (sucrase)
H2O
3
Figure 5.14_s4
1
Enzyme available with empty active site Active site
5.14 A specific enzyme catalyzes each cellular reaction Substrate (sucrose) 2
Glucose
Substrate is converted to products
Substrate binds to enzyme with induced fit
Enzyme (sucrase)
For every enzyme, there are optimal conditions under which it is most effective. Temperature affects molecular motion. – An enzyme’s optimal temperature produces the highest rate of contact between the reactants and the enzyme’s active site.
Fructose H2O 4
– Most human enzymes work best at 35–40ºC.
Products are released 3
Substrate is converted to products
The optimal pH for most enzymes is near neutrality. © 2012 Pearson Education, Inc.
5.14 A specific enzyme catalyzes each cellular reaction
5.15 Enzyme inhibitors can regulate enzyme activity in a cell
Many enzymes require nonprotein helpers called cofactors, which
A chemical that interferes with an enzyme’s activity is called an inhibitor.
– bind to the active site and – function in catalysis.
Some cofactors are inorganic, such as zinc, iron, or copper.
Competitive inhibitors – block substrates from entering the active site and – reduce an enzyme’s productivity.
If a cofactor is an organic molecule, such as most vitamins, it is called a coenzyme.
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5.15 Enzyme inhibitors can regulate enzyme activity in a cell
Figure 5.15A
Substrate Active site
Noncompetitive inhibitors Enzyme
– bind to the enzyme somewhere other than the active site,
Allosteric site Normal binding of substrate
– change the shape of the active site, and Competitive inhibitor
– prevent the substrate from binding.
Noncompetitive inhibitor
Enzyme inhibition
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5.15 Enzyme inhibitors can regulate enzyme activity in a cell
Figure 5.15B
Enzyme inhibitors are important in regulating cell metabolism. In some reactions, the product may act as an inhibitor of one of the enzymes in the pathway that produced it. This is called feedback inhibition.
Feedback inhibition
Enzyme 1 Reaction 1 Starting molecule
Enzyme 2 B
A
Enzyme 3 C
Reaction 2
D Reaction 3 Product
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5.16 CONNECTION: Many drugs, pesticides, and poisons are enzyme inhibitors
Figure 5.16
Many beneficial drugs act as enzyme inhibitors, including – Ibuprofen, inhibiting the production of prostaglandins, – some blood pressure medicines, – some antidepressants, – many antibiotics, and – protease inhibitors used to fight HIV.
Enzyme inhibitors have also been developed as pesticides and deadly poisons for chemical warfare. © 2012 Pearson Education, Inc.
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You should now be able to
You should now be able to
1. Describe the fluid mosaic structure of cell membranes.
5. Explain how osmosis can be defined as the diffusion of water across a membrane.
2. Describe the diverse functions of membrane proteins.
6. Distinguish between hypertonic, hypotonic, and isotonic solutions.
3. Relate the structure of phospholipid molecules to the structure and properties of cell membranes.
7. Explain how transport proteins facilitate diffusion.
4. Define diffusion and describe the process of passive transport.
8. Distinguish between exocytosis, endocytosis, phagocytosis, pinocytosis, and receptor-mediated endocytosis.
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© 2012 Pearson Education, Inc.
You should now be able to
You should now be able to
9. Define and compare kinetic energy, potential energy, chemical energy, and heat.
13. Explain how ATP functions as an energy shuttle.
10. Define the two laws of thermodynamics and explain how they relate to biological systems. 11. Define and compare endergonic and exergonic reactions. 12. Explain how cells use cellular respiration and energy coupling to survive.
14. Explain how enzymes speed up chemical reactions. 15. Explain how competitive and noncompetitive inhibitors alter an enzyme’s activity. 16. Explain how certain drugs, pesticides, and poisons can affect enzymes.
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Figure 5.UN01
Figure 5.UN02
Passive transport (requires no energy) Diffusion
Facilitated diffusion
HIgher solute concentration
Active transport (requires energy)
Osmosis HIgher free water concentration
HIgher solute concentration
Solute
Water Lower solute concentration
Lower free water concentration
ATP Lower solute concentration
ATP cycle
Energy from exergonic reactions
ATP
ADP
P
Energy for endergonic reactions
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Figure 5.UN03
Figure 5.UN04
Molecules cross cell membranes by
may be
c.
by
b.
passive transport
(a) moving down
moving against
a.
requires
d.
ATP
(b) uses
of
(c)
Table 5.UN05
(d)
uses
f.
(e) e.
of
polar molecules and ions
Figure 5.UN06
Rate of reaction
diffusion
0
1
2
3
4 pH
5
6
7
8
9
10
18
