Introduction
- Some organisms use an energy-converting reaction 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
- Bioluminescence is an example of the multitude of energy conversions that a cell can perform
- Many of the cell’s reactions
- Take place in organelles
- Use enzymes embedded in the membranes of the these organelles
Membranes are fluid mosaics of lipids and proteins with many functions
- Membranes are composed of
- A bilayer of phospholipids with embedded and attached proteins in a structure biologists call a fluid mosaic
- Membrane proteins perform many functions
- Some proteins help maintain cell shape and coordinate changes inside and outside the cell through their attachment to the cytoskeleton and extracellular matrix
- Some proteins function as receptors for chemical messengers from other cells
- Some membrane proteins function as enzymes
- Some membrane glycoproteins are involved in cell-cell recognition
- Membrane proteins may participate in the intercellular junctions that attach adjacent cells to each other
- Membranes may exhibit selective permeability, allowing some substances to cross more easily than others
Membranes form spontaneously, a critical step in the origin of life
- Phospholipids 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
Passive transport is diffusion across a membrane with no energy investment
- Diffusion is the tendency of particles to spread out evenly in available spaces
- Particles move from an area of more concentrated particles to an area where they are less concentrated
- This means that particles diffuse down their concentration gradient
- Eventually, the particles reach equilibrium where the concentration of particles is the same throughout
- Diffusion, across a cell membrane does not require energy, so it is called passive transport
- The concentration gradient itself represents potential energy for diffusion
Osmosis is the diffusion of water across a membrane
- One of the most important substances that crosses membranes is water
- The diffusion of water across a selectively permeable membrane is called osmosis
- If a membrane is permeable to water, but not a solute, separates two solutions with different concentrations of solute
- Water will cross the membrane, moving down its own gradient until the solute concentration on both sides is equal
Water balance between cells and their surrounding is crucial to organisms
- Tonicity is a term that describes the ability of a solution to cause a cell to gain or lose water
- Tonicity mostly depends on the concentration of solute on both sides of the membrane
- How will animal cells be affected when placed into solutions of various tonicities?
- Isotonic solution
  - The concentration of solute is the same on both sides of the membrane
  - The cell volume will not change
- Hypotonic solution
  - The solute concentration is lower outside the cell
  - Water molecules move into the cell and the cell will expand
- Hypertonic Solution
  - The solute concentration is higher outside the cell
  - Water molecules move out of the cell and the cell will shrink
- For an animal cell to survive in a hypotonic or hypertonic environment, it must engage in osmoregulation
- The control of water balance
- 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
- In a hypertonic environment, plant and animal cells both shrivel
Transport proteins can facilitate diffusion across membranes
- Hydrophobic substances easily diffuse across a cell membrane
- Polar or charged substances do not easily cross cell membranes, and, instead, move across membranes with the help of specific proteins in a process called facilitated diffusions
- Does not require energy
- Relies on the concentration gradients
- Some proteins function by becoming a hydrophilic tunnel for the passage of ions or other molecules
- Other proteins bind their passenger, change shape, and release their passenger on the other side
- 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
Research on another membrane protein led to the discovery of aquaporins
- 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
Cells expend energy in the active transport of a solute
- In active transport, a cell must expend energy to move a solute against its concentration gradient
Exocytosis and endocytosis transport large molecules across membranes
- A cell uses two mechanisms to move large molecules across membranes
- Exocytosis
  - Used to export bulky molecules, such as proteins or polysaccharides
- Endocytosis
  - 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
Three kinds of endocytosis
- Phagocytosis
- Engulfment of a particle by wrapping cell membrane around it, forming a vacuole
- Pinocytosis
- Same thing as phagocytosis except fluids are taken into small vesicles
- Receptor-mediated endocytosis
- Uses receptors in a receptor-coated pit to interact with a specific protein, initiating the formation of a vesicle
Cells transform energy as they perform work
- Cells are small units, a chemical factory, housing thousands of chemical reactions
- Cells uses these reactions for cell maintenance, manufacturing of cellular parts, and cell replication
- Energy is the capacity to cause change or to perform work
- There are two kinds of energy
- Kinetic energy
  - Energy of motion
- Potential energy
  - Energy that matter possesses as a result of its location or structure
- Heat, or thermal energy, is a type of kinetic energy associated with the random movement of atoms or molecules
- Light is also a type of kinetic energy, and can be harnessed to power photosynthesis
- 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
- Thermodynamics is the study of energy transformations that occur in a collection of matter
- Scientists use the word
  - System for the matter under study
  - Surroundings for the rest of the universe
- Two laws govern energy transformations in organisms
- First law of thermodynamics
  - Energy in the universe is constant
- Second law of thermodynamics
  - Energy conversions increase the disorder of the universe
- Entropy
- The measure of disorder, or randomness
- Cells use oxygen in reactions that release energy from fuel molecules
- In cellular respiration, the chemical energy stored in organic molecules is converted to a form that the cell can use to perform work
Chemical reactions either release or store
- Chemical reactions either release energy (exergonic reactions) or require an input of energy and store energy (endergonic reactions)
- Exergonic reactions release energy
- These reactions release the energy in covalent bonds of the reactants
- Burning woods releases 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
- 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
- End with products that contain more chemical energy
- Photosynthesis is a type of endergonic process
- Energy-poor reactants, carbon dioxide, and water are used
- Energy is absorbed from sunlight
- Energy-rich sugar molecules are produced
- A living organism carries out thousands of endergonic and exergonic chemical reactions
- The total of an organism’s chemical reaction is called metabolism
- A metabolic pathway is a series of chemical reactions that either
- Builds a complex molecule
- Breaks down a complex molecule into simpler compounds
- Energy coupling uses the energy release from exergonic reactions to drive essential endergonic reactions, usually using the stored ATP molecules
ATP drives cellular work by coupling exergonic and endergonic reactions
- Adenosine triphosphate
- Powers nearly all forms of cellular work
- ATP consists of
- Nitrogenous bases adenine
- Five carbon sugar ribose
- Three phosphate groups
- Hydrolysis of ATP releases energy by transferring the third phosphate from ATP to some other molecule in a process called phosphorylation
- Most cellular work depends on ATP energizing molecules by phosphorylating them
- There are three main types of cellular work
- Chemical
- Mechanical
- Transport
- ATP drives all three of these types of work
- 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
Enzymes speed up the cell’s chemical reactions by lowering energy barriers
- Although biological molecules possess much potential energy, it is not released spontaneously
- An energy barrier must be overcome before a chemical reaction can begin
  - This energy is called the activation energy
- We can think of the activation energy 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
- Enzymes
- Function as a biological catalyst by lowering the activation energy needed for a reaction to begin
- Increase the rate of a reaction without being consumed by the reaction
- Are usually proteins
  - Although some RNA molecules can function as enzymes
A specific enzyme catalyzes each cellular reaction
- An enzyme 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 substrate molecules
- 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
- Most human enzymes work best at 35-40℃
- Many enzymes require nonprotein helpers called cofactors, which bind to the active site and function in catalysis
- Some cofactors are inorganic
- Ex: zinc, iron, copper
- If a cofactor is an organic molecule, such as most vitamins, it is called a coenzyme
Enzyme inhibitors can regulate enzyme activity in the cell
- A chemical that interferes with an enzyme’s activity is called an inhibitor
- Competitive inhibitors
- Block substrates from entering the active site
- Reduce the enzyme’s productivity
- Noncompetitive inhibitors
- Bind to the enzyme somewhere other than the active site
- Change the shape of the active site
- Prevent the substrate from binding
- 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
Many drugs, pesticides, and poisons are enzyme inhibitors
- Many beneficial drugs act as enzyme inhibitors
- Ibuprofen
  - Inhibiting the productions of prostaglandins
- Some blood pressure medications
- Some antidepressants
- Many antibiotics
- Protease inhibitors used to fight HIV
- Enzyme inhibitors have also been developed as pesticides and deadly poisons for chemical warfare