Chapter 4 - The Energy of Life

All cells capture and use energy

$Energy - the ability to do work/to move matter$

Energy has different forms

$Kinetic energy -$ energy of motion/movement

$Potential energy$ - stored energy that is available to do work

Chemical bonds are potential energy

$Molecules like glucose and triglycerides store energy in their bonds.$ To release the energy, the cell breaks the bonds. If the cell cannot capture the energy being released, it will be lost as heat.

Energy is converted from one form to another

Energy changed form within its biological system.

$First law of thermodynamics$ - Energy is never created or destroyed

 Energy from the sun cannot be used directly by cells. It must be captured, stored, and converted before it is usable for cellular work.

$Notice that heat energy is lost at each step. Heat energy is disordered, which means it cannot be used or converted back to a useful form of energy.$

• It takes a lot of energy to stay “ordered”
• Cells constantly use energy to build their molecules perform chemical reactions, and carry out life’s processes

Metabolism includes all chemical reactions in cells

Chemical reactions rearrange atoms. Building complex molecules out of simple parts (like monomers) forms new chemical bonds. Breaking complex molecules into into parts (like monomers) breaks apart chemical bonds.

Chemical reactions can require or release energy

$Endergonic$ - reactions that form bonds to build molecules require energy input

$Exergonic$ - reactions that break bonds to release energy stored in the bond

Some chemical reactions transfer electrons

Most energy transformations in organisms occur in $oxidation - reduction$ reactions.

Oxidation reactions release energy

$Oxidation$ - is the loss of electrons from an atom or molecule, releasing energy.

The energy released by the oxidized molecule is stored in the $electrons$

Reduction reactions require energy

$Reduction$ - is the gain of electrons by an atom or molecule, which requires energy.

The reduced molecule gains the energy stored in the electrons.

Oxidations and reductions occur simultaneously (“redox”)

An $electron transport chain$ is an series of membrane proteins participating in sequential, linked oxidation - reduction reactions.

Photosynthesis and cellular respiration both use electron transport chains. As energy is released, cells store it and use it in other reactions.

ATP is the cellular energy currency

$Adenosine triphosphate (ATP)$ - is a nucleotide that temporarily stores energy. It is the form of energy that cells can use.

All cells rely on the $potential energy$ stored in ATP to power chemical reactions.

ATP releases stored energy

Removing the endmost phosphate group by hydrolysis releases the potential energy stored in ATP. The cell uses this energy to do work.

$ATP is formed during cellular respiration$

Cellular respiration is a series of chemical reactions that release energy from sugar, producing ATP from ADP

ATP is coupled with chemical reactions

ATP breakdown is coupled with endergonic reactions.

$Energy released from ATP breakdown is used to power - endergonic reactions.$

ATP hydrolysis is coupled with endergonic reactions

1. ATP energizes target molecule, making it more likely to bond with other molecules.
2. ATP donates a phosphate group that changes the shape of the target molecule

Enzymes speed biochemical reactions

Chemical reactions in cells must occur very quickly to sustain life.

Enzymes are reusable

An $enzyme$ - a protein that acts as a catalyst

$Catalyst$ - speeds up a chemical reaction without being consumed, such as a protein

Enzymes speed biochemical reactions

Each enzyme fits the shape of a substrate

$Substrate$ molecules bind to the enzyme’s $active site,$ where the chemical reaction occurs. The substrate is what the enzyme acts on.

Enzymes alter substrates to form products

Once the chemical reaction occurs, $product$ molecules are released. The enzyme retains its original form. Products are what the reaction produces.

Enzymes speed a variety of reactions

Note that not all enzymes break a single substrate into two products. Some enzymes combine two substrates into one product, for example.

Enzymes lower the activation energy

$Activation energy$ - is the energy required to start a reaction.

Without the enzyme, activation energy is high. When an enzyme binds to the substrate, activation energy is lowered.

Some enzymes require cofactors

$Cofactors$ - Partners of enzymes that help catalyze reactions to increase enzyme activity.

• Metal ions and vitamins are common cofactors

Cells control their biochemical reactions

Only some chemical reactions are occurring in a cell at a given time. $Enzyme inhibition is one way to prevent unneeded reactions from taking place.$

Many factors affect enzyme activity

Cells control the rate of biochemical reactions is by limiting the activity of enzymes.

Inhibitors lower enzyme activity

Substrate molecules typically bind to the $active site$ of enzymes… but what if another molecule prevents it?

Noncompetitive enzyme inhibitors change the shape of the active site

Cells can produce molecules that bind to an enzyme outside of its active site to alter enzyme shape and prevent the substrate and enzyme from binding together.

Competitive enzyme inhibitors block access to the active site =

Cells can produce molecules that bind to an enzyme right at the active site, keeping the substrate and enzyme from binding together.

Inhibitors shut down unneeded reactions

In many cases, the $inhibitor$ is the product of the reaction that the enzyme catalyzes. Once there is enough product in the cell, it would be a waste of energy to produce more of that product

Substances enter and exit cells by multiple methods

A cell’s interior is chemically different from its exterior. It maintains this difference by regulating transport of dissolved substances (solutes) across its membrane.

• Regulating what is inside of a cell is an example of homeostasis

The cell membrane forms a barrier

Only certain substances can pass through a cell membrane. Solutes enter and exit by different methods, depending on two factors:

• Concentration gradients
• The chemical nature of the substance (its polarity, charge, and size)

“Gradient” describes a difference between neighboring regions

 The tea is more highly concentrated near the teabag than in the rest of the cup, forming a concentration gradient.

• Gradients are directional: This arrow points $down the concentration gradient$ since it starts at high concentration and ends at low concentration of tea

Concentration gradients dissipate without energy input

 This is how it looks after time has passed. The concentration of tea is uniform throughout the cut (no gradient).

• The dissipation occurs by random motion
• The dissipated gradient is an example of: $entropy$

Maintaining a gradient requires energy

Maintaining gradients $requires energy$ since the concentration gradient has a tendency to dissipate

Crossing a cell membrane can occur in several ways

By passive transport, $DOWN$ a concentration gradient: Simple diffusion, Osmosis, Facilitated Diffusion

By active transport, $AGAINST$ a concentration gradient: In vesicles, by endocytosis or exocytosis

Simple diffusion does not require energy

Passive transport includes simple diffusion, which occurs when concentration gradients dissipate across a biological membrane.

 Molecules move down their concentration gradient

The size and chemical properties of small, nonpolar molecules allow them to pass through the membranes hydrophobic barrier. This is impossible for large polar molecules.

Osmosis does not require energy

Passive transport includes $osmosis$ which is a simple diffusion of water across a selectively permeable membrane (down its concentration gradient)

Osmosis is driven by the concentration gradient of water

In an $isotonic solution,$ water moves equally into and out of cells

 In a $hypotonic solution,$ water rushed into cells from outside

 In a $hypertonic solution$, water rushes out of cells

Osmosis determines water content in plant cells

Plants usually keep the solute concentration inside their cells lower than the outside, so that water enters the cells.

Hypertonic surroundings result in a loss of water, shrinking the large central vacuoles.

This causes cells to lose $turgor pressure$, causing the plant to wilt.

Facilitated diffusion does not require energy

Passive transport includes $facilitated diffusion$ which occurs when membrane proteins transport substances across a cell membrane.

Substances move down their concentration gradient

Facilitated diffusion requires membrane proteins

The hydrophobic tails of phospholipids in the cell membrane repel hydrophilic substances. Therefore, ions and polar molecules must pass through a protein channel in order to move across the membrane down their concentration gradient

Active transport requires energy

Active transport occurs when membrane proteins use cellular energy to transport substances across a cell membrane.

This type of membrane transport protein is often referred to as a “pump”

Substances are moved against their concentration gradient

The sodium - potassium pump is an example of active transport

Endocytosis requires energy

Cells form $vesicles$ as they engulf fluids and large molecules to bring them into the cell. Substances can be moved against of down their concentration gradients this way.

Exocytosis requires energy

Cell form $vesicles$ to secrete large polar molecules such as proteins out of cells. Substances can be moved against or down their concentration gradients this way.

Exocytosis brings sodium channels to the membrane

When there a lot of sodium channels in a neuron cell membrane, lots of Na+ ions enter the cell.

Neurons are activated by the Na+ influx

The electrical organ produces high electrical fields.

Endocytosis removes sodium channels from the membrane

When there a few sodium channels in a neuron cell membrane, less Na+ ions can enter the cell.

Neurons activated less.

The electrical organ produces low electrical fields.

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