Bio10Lecture Notes 4: Cells and EnergySRJC

4.) Cell Transport

Concentration Gradient

Different numbers of molecules or ions in different regions

Substances tend to move down gradient - from higher to lower concentration

Diffusion

Net movement of molecules or ions down a concentration gradient

Diffusion RateFactors

Steepness of concentration gradient

  • Steeper gradient, faster diffusion

Molecular size

  • Smaller molecules, faster diffusion

Temperature

  • Higher temperature, faster diffusion

Electrical or pressure gradients

Transport Proteins

Span the lipid bilayer

Interior can open to either side

Change shape when they interact with solute

Move water-soluble substances across a membrane

Passive and Active Transport

Passive Transport

  • Doesn’t require energy inputs
  • Solutes diffuse through a channel inside the protein’s interior
  • Net movement is down concentration gradient

Active Transport

  • Requires ATP
  • Protein is an ATPase pump
  • Pumps solute against its concentration gradient

Membrane Traffic

Exocytosis

  • Vesicle fuses with membrane, releasing substance into intracellular fluid

Endocytosis

  • Membrane forms vesicle, bringing substance into cell

Types of Endocytosis

Bulk-phase endocytosis

Receptor-mediated endocytosis

Phagocytosis

5.) Enzymes

Energy Laws

Energy: the capacity to do work

Total amount of energy in the universe is constant

Energy flows from higher to lower energy forms

ATP

Main energy carrier in cells

Can give up phosphate group to another molecule

Phosphorylation energizes molecules to react

The Cell’s Energy Currency

ATP couples energy inputs and outputs

ATP/ADP cycle regenerates ATP

Energy Changes

Endergonic reactions require energy

  • Synthesis of glucose from carbon dioxide and water during photosynthesis

Exergonic reactions release energy

  • Breakdown of glucose to carbon dioxide and water by aerobic respiration

Electron Transfers

Oxidation: loss of an electron

Reduction: gain of an electron

Electron transfer chains are vital to the formation of ATP during photosynthesis and aerobic respiration

Participants in Metabolic Pathways

Reactants

Intermediates

Products

Energy carriers

Enzymes

Cofactors

Transport proteins

Reactions: Forward and Reverse

Most chemical reactionsare reversible

Direction of reaction depends upon

  • Energy content of participants
  • Reactant-to-product ratio

Chemical Equilibrium

Reaction rate is the same in both directions

Conversions continue, but proportions of reactant and product do not change

Usually amounts of reactant and product are not equal

Metabolic Pathways

Biosynthetic (anabolic) pathways

  • Require energy inputs
  • Assemble large molecules from subunits
  • Photosynthesis

Degradative (catabolic) pathways

  • Release energy
  • Breakdown large molecules to subunits
  • Aerobic respiration

Enzymes

Catalyze (speed up) reactions

Recognize and bind specific substrates

Act repeatedly

Most are proteins

Activation Energy

For a reaction to occur, an energy barrier must be surmounted

Enzymes make the energy barrier smaller

Factors Influencing Enzyme Activity

Coenzymes and cofactors

Allosteric regulators

Temperature

pH

Salt concentration

Allosteric Control

Activator or inhibitor binds to an enzyme

Binding changes enzyme shape

Change hides or exposes active site

Feedback inhibition

Product shuts off enzyme by binding to activation site

Effect of Temperature

Small increase in temperature increases molecular collisions, reaction rates

High temperatures disrupt bonds ad destroy the shape of active site

Enzymes and the Environment

Most enzymes require specific activation conditions

  • Certain temperature or pH extremes can shut down activity

6.) Photosynthesis

Sunlight and Survival

Plants are photoautotrophs; they use sunlight and CO2 to produce sugar in the process of photosynthesis

Many kinds of energy

Wavelengths of visible light

Visible Light

Wavelengths humans perceive as different colors

  • Violet (380 nm) to red (750 nm)
  • Longer wavelengths, lower energy

Pigments

Visible color is from wavelengths not absorbed (they reflect the color we see)

Pigments capture light energy from absorbed wavelengths

Light energy destabilizes bonds and boosts electrons to higher energy levels

Variety of Pigments

Chlorophylls ; green, yellow

Carotenoids; red, orange, yellow

Xanthophylls; yellow, brown, purple, blue

Anthocyanins, red, purple, blue

Phycobilins; red or blue-green

Light Receptors

Pigments capture light energy

Photosynthesis Equation

6 CO2 + 12 H2O + light energy  C6H12O6 + 6 O2 + 6 H2O

Two Steps in Photosynthesis

Light-dependent reactions

Light-independent reactions

Light-Dependent Reactions

Cyclic pathway

  • ATP forms
  • Requires one type of photosystem

Noncyclic pathway

  • ATP and NADPH form
  • Water is split and oxygen released
  • Requires two types of photosystems

Chloropasts

Organelle of photosynthesis in plants and algae

Light-dependent reactions take place in thylakoids

Light independent reactions take place in stroma

Photosystems

Thylakoid Membrane Section

Role of Electron Transfer Chains

Adjacent to photosystems

Acceptor molecule accepts electrons from reaction center

As electrons pass along chain, energy released drives synthesis of ATP

Cyclic Electron Flow

Electrons are donated by chlorophyll a in photosystem I to an acceptor moleculeflow through electron transfer chain and back to photosystem

Electron flow drives ATP formation

No NADPH is formed

Noncyclic Electron Flow

Two-step pathway for light absorption and electron excitation

Uses type I and type II photosystems

Produces ATP and NADPH

Involves photolysis (splitting of water) and releases oxygen as a byproduct

ATP Formation in the Noncyclic Pathway

Photolysis and electron transfer chains create electrical and H+ concentration gradients across thylakoid membrane

H+ flows down gradients into stroma through ATP synthases

Flow of ions drives formation of ATP from ADP and phosphate

Light-Independent Reactions

Synthesis part of photosynthesis

Can proceed in the dark using energy stored in light reactions

Take place in stroma

Calvin-Benson cycle

Calvin-Benson Cycle

Reactants

Carbon dioxide

ATP

NADPH

Products

  • Glucose
  • ADP
  • NADP+

The C3 Pathway

In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA

Because the first intermediate has three carbons, the pathway is called the C3 pathway

Photorespiration in C3 Plants

On hot, dry days stomata close

Inside leaf

  • Oxygen levels rise
  • Carbon dioxide levels drop

Rubisco attaches RuBP to oxygen instead of carbon dioxide

Only one PGAL forms instead of two

C4 Plants

Carbon dioxide is fixed twice

  • In mesophyll cells, carbon dioxide is fixed to form 4-carbon oxaloacetate
  • Oxaloacetate is transferred to bundle-sheath cells

Carbon dioxide is released and fixed again in Calvin-Benson cycle

CAM(Crassulacean Acid Metabolism) Plants

Desert plants like cacti keep stomata closed during the day

Carbon is fixed twice (in same cells)

Night

  • Carbon dioxide is fixed by repeated turns of a type of C4 cycle

Day

  • Carbon dioxide is released and fixed in Calvin-Benson cycle

Summary of Photosynthesis

Linked Processes

Photosynthesis

  • Energy-storing pathway
  • Releases oxygen
  • Requires carbon dioxide

Aerobic Respiration

  • Energy-releasing pathway
  • Requires oxygen
  • Releases carbon dioxide

7.) Cellular Respiration: How Cells Release Chemical Energy

Main Types of Energy-Releasing Pathways

Anaerobic pathways

  • Evolved first
  • Don’t require oxygen
  • Start with glycolysis in cytoplasm
  • Completed in cytoplasm

Aerobic pathways

  • Evolved later
  • Require oxygen
  • Start with glycolysis in cytoplasm
  • Completed in mitochondria

ATP:Universal Energy Source

Photosynthesizers get light energy from the sun, store it as chemical energy, and produce ATP

Animals eat plants or other animals and transform chemical energy to ATP

Making ATP

Plants make ATP during photosynthesis

Anaerobes make ATP by fermentation

Cells of most organisms make ATP by aerobic respiration of carbohydrates, fats, and protein

Aerobic Respiration

Glycolysis; partial breakdown of glucose

  • Occurs in the cytoplasm
  • Produces 2 ATP

Krebs Cycle (citric acid cycle)

  • Break down of glycolysis byproducts to CO2produces NADH and FADH

Electron Transport Chain

  • uses NADH and FADH from Krebs cycleto produce ATP

Summary Equation for Aerobic Respiration

C6H12O6 + 6O2  6CO2 + 6H2O + energy (ATP)

The Role of Coenzymes

NAD+ and FAD accept electrons and hydrogen

Become NADH and FADH2

Deliver electrons and hydrogen to the electron transfer chains

Glycolysis Occurs in Two Stages

Energy-requiring steps

  • ATP energy activates glucose and its 6-carbon derivatives

Energy-releasing steps

  • The products of the first part are split into 3-carbon pyruvate molecules
  • ATP and NADH form

Glucose

A simple sugar

  • (C6H12O6)

Atoms held together by covalent bonds

Energy-Requiring Steps

Energy-Releasing Steps

Glycolysis: Net Energy Yield

Energy requiring steps:

  • 2 ATP used

Energy releasing steps:

  • 2 NADH formed
  • 4 ATP formed

Net yield: 2 ATP + 2 NADH

Mitochondria

Organelles where the next two phases of aerobic respiration proceed (Krebs cycle and electron transport chain)

Produces 34 more energy molecules ATP

Second Stage Reactions

Preparatory reactions

  • Pyruvate is oxidized into 2-carbon acetyl-CoA + CO2
  • NAD+ is reduced

Krebs cycle

  • Acetyl-CoA is oxidized to two CO2
  • NAD+ and FAD are reduced

The Krebs Cycle

Overall Reactants

  • Acetyl-CoA
  • 3 NAD+
  • FAD
  • ADP and Pi

Overall Products

  • Coenzyme A
  • 2 CO2
  • 3 NADH
  • FADH2
  • ATP

Results of the Second Stage

All of the carbon molecules in pyruvate end up in CO2

Coenzymes are reduced (they pick up electrons and hydrogen)

One molecule of ATP is formed

4-carbon oxaloacetate is regenerated

Coenzyme Reductions During First Two Stages

Glycolysis / 2 NADH
Preparatory reactions / 2 NADH
Krebs cycle / 2 FADH2 + 6 NADH
Total / 2 FADH2 + 10 NADH

Third Stage

Electron Transfer Phosphorylation

Occurs in mitochondria

Coenzymes deliver electrons to electron transfer systems

Electron transfer sets up H+ ion gradients

Flow of H+ down gradients powers ATP formation

Creating an H+ Gradient

Making ATP

Importance of Oxygen

Electron transport phosphorylation requires oxygen

Oxygen withdraws spent electrons from the electron transport system, then combines with H+ to form water

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

Anaerobic Pathways

Alcoholic Fermentation

Fermentation Pathways

  • Begin with glycolysis
  • Are anaerobic: don’t require oxygen
  • Yield only 2 ATP from glycolysis
  • Steps after glycolysis only regenerate NAD+

Alcoholic Fermentation

Lactate Fermentation

Alternative Energy Sources

Carbohydrates, fats, and proteinsare digested and enter aerobic respiration

Evolution of Metabolic Pathways

Earliest organisms used anaerobic pathways

Later, noncyclic pathway of photosynthesis increased atmospheric oxygen

Aerobic respiration evolved due to selective pressure by oxygen

Anaerobic Archaeans

Use hydrogen sulfide as energy source

Aerobic Respiration

Uses products of photosynthesis

A. CarranzaPage 111/4/2018