Chapter 3: Cell Structure, Function, and Energy

3.1 The Unit of Life (Cells)

• Cells 

  • A cell is the smallest unit of life that can function independently.

  • Every living thing is made up of one or multiple cells.

  • Every cell can do basic processes such as metabolism, growth, and reproduction.

• Types of Cells

  • Two major types of cells in nature are: Prokaryotic cells and Eukaryotic cells

    • Prokaryotes are the most ancient forms of life.

      • All prokaryotes are unicellular.

      • They lack a nucleus and many cell organelles.

      • Their cells contain a circular ring of DNA free in the cytoplasm, and the cell wall is normally present.

      • Example: Bacteria and Archaea groups

    • Eukaryotes are more complex cells.

      • These organisms can be unicellular and also multicellular.

      • They have cells with a nucleus and other membranous organelles.

      • DNA is located in their nucleus, bounded by a nuclear envelope.

      • All organelles are bounded in membranes, known as compartmentalization. 

        • Every organelle is in charge of a specific role such as making a product, cleaning up, watching for antigens, passing products from one place to another or another cell, etc.

      • Example: Eukaryotic cells are human, animal, and plant cells.

3.2 Cell Organelles and Function

3.3 Solute, Solvent, and Solution

• In biology, all chemical reactions occur in solutions.

  • Solute - a substance dissolved in a liquid

  • Solvent - the liquid portion of the solution (usually water)

  • Concentration - the measure of how much solute is present per volume of solvent

• Types of Solutions

  • Hypertonic - the solution with a higher concentration of solute.

  • Hypotonic - the solution with a lower concentration of solute.

  • Isotonic - when both solutions have the same concentration of solute. 

3.4 Types of Transport: Passive vs. Active

• Passive Transport

  • No energy required.

  • Movement is due to a gradient.

    • Differences in concentration, pressure, or change.

  • Movement occurs to equalize the gradient.

    • High concentration moves toward low concentration.

• Diffusion

  • Movement of solute from higher concentration to lower concentration.

• Osmosis

  • Movement of water (solvent) across a semipermeable membrane.

• Facilitated Diffusion

  • Where the solute particles are moved with the help of transport proteins.

• Active Transport

  • Energy in the form of ATP is required for this kind of transport.

  • Movement is against the concentration gradient (low to high)

    • Example: Sodium potassium pumps in our body are a way of active transport.

• Endocytosis

  • Endo stands for movement into a cell.

  • Three types of endocytosis:

    • Phagocytosis (cell eating) - when cells engulf particles

      • The cell membrane surrounds the particle and pinches off to form an intracellular vacuole.

    • Pinocytosis (cell drinking) - movement of liquids into a cell

      • The cell membrane surrounds a small volume of fluid and pinches off, forming a vesicle. 

    • Receptor-mediated endocytosis - uptake of substances by the cell is targeted to a single type of substance that binds at the receptor on the external cell membrane

• Exocytosis

  • Movement out of the cell

  • A vesicle transports a substance and fuses with the cell wall, releasing the substance into the extracellular fluid outside of the cell.

3.5 Different types of Metabolism

• Metabolism

  • All the biochemical reactions taking place in an organism.

  • There are two metabolic pathways:

    • Anabolic - small molecules are built into large ones.

      • Energy is required.

    • Catabolic - Large molecules are broken down into small ones.

      • Energy is released. 

3.6 Enzymes

• Enzymes

  • Biological catalysts.

  • Their job is to lower the activation energy and therefore favor the formation of products.

  • They catalyze all chemical reactions in cells.

• Properties

  • Enzymes are made of proteins.

  • Contain a metal ion (cofactor).

    • Or an organic molecule (coenzyme).

  • Enzymes are reusable.

  • Enzyme activity is highly specific.

  • Enzymes have an active site.

  • Enzymes are used in very small accounts.

• Substrate Complexes

  • Enzymes increase the probability of a chemical reaction.

  • Enzymes bind to the substrate forming enzyme substrate complex.

  • The complex lowers activation energy.

  • Enzymes are substrate specific.

• Enzyme Activity Can Be Regulated

  • Environmental factors like pH, temperature, salt concentration, and, in some cases, cofactors or coenzymes.

  • Competitive inhibition - an inhibitor binds at an active site and competes with substrate.

  • Non-competitive inhibition - allosteric inhibition or allosteric activation 

• Enzymes often team up in metabolic pathways

  • Metabolic pathway - a sequence of chemical reactions.

3.7 Aerobic Cellular Respiration

• Energy is required for all living organisms to power all life’s activities

  • All of this energy is used in the cell.

• It happens through two big processes: Cellular respiration and photosynthesis

• Cellular Respiration

  • Whether an organism makes its own carbohydrates (autotrophs) or gets carbohydrates by eating other organisms (heterotrophs), they must still break down the carbohydrates within their cells for energy.

  • Molecules in food (such as glucose) are used to produce ATP, the energy carrier molecule used to power cell activities. 

  • This process is known as cellular respiration, and the oxygen we breathe is vital to complete it.

    • The carbon dioxide we exhale every breath is the by-product or residual from this process, and the water we release in urination is another product as well. 

  • Chemical reaction: C₆H₁₂O₆ + O₂ = CO₂ + H₂O + ATP

    • Reactants are glucose + oxygen and the main product is ATP.

    • Water and carbon dioxide are by-products released to the environment.

• Steps of Cellular Respiration

  • Glycolysis: Glucose (a six-carbon sugar) undergoes a series of chemical transformations and is converted into two molecules of pyruvate ( a three-carbon sugar).

    • Whether oxygen is present or not, cells can always do glycolysis which occurs in the fluid cytosol of the cell.

    • In this process, the six-carbon glucose molecule is broken in half and rearranged to make two pyruvic acid molecules.

    • This process requires 2 ATP to get started but ultimately it produces 4 ATP from ADP and phosphate, and it produces 2 NADH from NAD+ 

  • Pyruvate Oxidation: Each pyruvate from glycolysis goes into the mitochondrial matrix (the innermost compartment of mitochondria). This is where it’s converted into a two-carbon molecule known as acetyl CoA. 

    • Carbon dioxide is released and NADH is generated.

  • Krebs Cycle (Citric Acid Cycle): The acetyl CoA combines with a four-carbon molecule and goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule.

    • ATP, NADH, and FADH₂ are produced, while carbon dioxide is released.

  • Oxidative Phosphorylation: The NADH and FADH₂ deposit their electrons into the electron transport chain, turning back into their “empty” forms (NAD⁺ and FAD).

    • As electrons move down the chain, energy is released and used to pump protons out of the matrix, forming a gradient

    • Protons flow back into the matrix through an enzyme called ATP synthase, making ATP.

    • At the end of the electron transport chain, oxygen accepts electrons and takes up protons to form water. 

• Aerobic Respiration

  • This pathway of cellular respiration happens in three stages: glycolysis, Krebs cycle (Citric acid cycle), and electron transport chain also known as oxidative phosphorylation or chemiosmosis.

    • Important: Although ATP is made at each stage, most of the ATP is made during the last stage which is oxidative phosphorylation.

• What Happens with Excess Energy?

  • Excess energy is either accumulated or excreted

    • It’s accumulated as fat and many migratory animals use it during their long travels, such as penguins, whales, salmon, etc. Others use it for hibernation, such as bears, badgers, raccoons, squirrels, beavers, etc.

• Anaerobic Respiration

  • This chemical reaction of respiration occurs without oxygen to release energy, creating ATP.

  • Fermentation is the pathway for anaerobic respiration where only the glycolysis is present (which is why it’s called universal).

    • It simply allows glycolysis to continue, producing small amounts of ATP.

    • There is no Krebs cycle or ETC present.

  • There are two types of anaerobic respiration:

    • Alcoholic fermentation - converts pyruvate to CO₂ and ethanol.

    • Lactic acid fermentation - converts pyruvate to lactic acid.

3.8 ATP is Cellular Energy

• Energy in the form of ATP is required for:

  • Movement

  • Cell division

  • Protein synthesis, etc. 

• ATP is made up of three high energy phosphate groups, adenine as a nitrogen base, and ribose as a sugar.

• The cleavage of each high energy phosphate releases energy when ATP is hydrolyzed to ADP.