Cell Structure, Function, and Energy: Study Guide for Chapters 4–7

Cell Structure and Function

Why Cells Matter

Cells are the fundamental units of life, exhibiting all characteristics of living organisms. Understanding cell structure and function is essential for comprehending biological processes and human disease.

• Cells are to biology what atoms are to chemistry.

• Cellular structure and function underpin most aspects of biology and medicine.

Prokaryotic vs. Eukaryotic Cells

Cells are classified based on the presence or absence of a nucleus and membrane-bound organelles.

• Prokaryotic Cells: Found in Bacteria and Archaea. Lack a nucleus and membrane-bound organelles.

• Eukaryotic Cells: Found in Plants, Animals, Fungi, and Protists. Contain a nucleus and membrane-bound organelles.

Plant vs. Animal Cells

• Plant Cells: Have cell walls, chloroplasts, and large central vacuoles.

• Animal Cells: Have centrioles and lysosomes (rare in plants).

Microscopy: Light vs. Electron Micrographs

• Light Microscopy: Can view living or dead specimens; lower magnification.

• Electron Microscopy: Uses electron beams for higher resolution; only dead specimens.

Structure of Cell Membranes: Fluid Mosaic Model

The plasma membrane is a dynamic structure composed of a phospholipid bilayer with embedded proteins.

• Phospholipid Bilayer: Provides a semi-permeable barrier.

• Proteins: Embedded or attached, serve diverse functions (transport, signaling).

• Fluid Mosaic: Molecules move within the membrane; diversity of proteins creates a mosaic pattern.

Plant Cell Wall vs. Animal Extracellular Matrix (ECM)

• Plant Cell Wall: Made of cellulose; provides structural support and protection.

• Animal ECM: Meshwork of glycoproteins (e.g., collagen); supports and protects cells, holds tissues together.

Genetic Information Flow: Nucleus to Cytoplasm

• DNA in the nucleus is transcribed into mRNA.

• mRNA exits via nuclear pores, travels to ribosomes (cytoplasm or rough ER), where it is translated into protein.

Endomembrane System Components

The endomembrane system synthesizes, stores, and exports molecules.

• Nuclear Envelope: Encloses nucleus; continuous with ER.

• Rough ER: Studded with ribosomes; synthesizes membrane and secretory proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies toxins.

• Golgi Apparatus: Modifies, sorts, and packages proteins.

• Lysosomes: Contain digestive enzymes; break down food and damaged organelles.

• Vacuoles: Storage of water, nutrients, or waste.

Chloroplasts vs. Mitochondria

Both organelles are involved in energy conversion and have specialized structures for their functions.

• Chloroplasts: Site of photosynthesis; have outer and inner membranes, thylakoids (increase surface area).

• Mitochondria: Site of cellular respiration; have outer membrane with porins, inner membrane with cristae (increase surface area).

• Adaptive Advantage: Folded membranes (cristae/thylakoids) maximize energy production.

Endosymbiosis Theory

• Both mitochondria and chloroplasts have their own DNA, ribosomes, and double membranes, suggesting they evolved from free-living bacteria engulfed by ancestral eukaryotic cells.

Cytoskeleton, Cilia, and Flagella

• Cytoskeleton: Network of protein fibers (microtubules, microfilaments, intermediate filaments); provides structural support and mobility.

• Cilia: Short, numerous, "oar-like"; made of microtubules in a 9+2 arrangement.

• Flagella: Long, few, "whip-like"; also 9+2 microtubule arrangement.

Cellular Energy and Metabolism

Why Cellular Functions Matter

• Biological systems rely on various forms of energy.

• Energy coupling allows transfer from energy-yielding to energy-consuming processes.

• Enzymes catalyze reactions essential for life.

• Membranes regulate material passage and environmental response.

Energy Concepts

• Energy: Capacity to cause change or perform work.

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy based on position or structure.

• Conservation of Energy: Energy cannot be created or destroyed, only converted.

Equation:

$\text{Energy}_{\text{initial}} = \text{Energy}_{\text{final}}$

Entropy and Heat

• Entropy: Measure of disorder/randomness.

• Every energy transfer increases entropy; some energy is lost as heat (most disordered form).

Energy Use in Organisms vs. Machines

• Organisms convert external energy (sunlight/chemical bonds) into ATP, then into mechanical, electrical, and thermal energy.

• Humans: Cellular respiration breaks down food into ATP.

• Cars: Combustion breaks down gasoline for energy.

• Both use fuel, oxygen, and release CO2, H2O, and heat.

Energy Units: Calorie, Kilocalorie, Calorie

Unit	Definition
calorie (cal)	Energy to raise 1g water by 1°C
Kilocalorie (kcal)	1,000 calories
Calorie (C)	Food calorie; 1 Calorie = 1 kcal = 1,000 calories

ATP: Cellular Energy Currency

• ATP releases energy when its third phosphate is removed (becomes ADP).

• Used for mechanical, transport, and chemical work.

• ATP is recycled: Energy from food reattaches phosphate to ADP.

ATP Cycle Equation:

$\text{ATP} \xrightarrow{\text{hydrolysis}} \text{ADP} + \text{P}_i + \text{energy}$

$\text{ADP} + \text{P}_i + \text{energy} \xrightarrow{\text{cellular respiration}} \text{ATP}$

Enzymes: Biological Catalysts

• Enzymes speed up reactions by lowering activation energy.

• Essential for sustaining life; most reactions would be too slow otherwise.

Activation Energy Diagram: (Additional info: Activation energy is the energy barrier that must be overcome for a reaction to proceed.)

Enzyme Inhibitors and Poisons

• Competitive Inhibitors: Block active site.

• Non-competitive Inhibitors: Bind elsewhere, change enzyme shape.

• Poisons: Permanently inhibit essential enzymes (e.g., cyanide).

Transport Across Membranes

• Diffusion: Movement from high to low concentration; passive.

• Osmosis: Movement of water across a semi-permeable membrane.

• Passive Transport: No energy required; includes diffusion, osmosis, facilitated diffusion.

• Active Transport: Requires energy (ATP); moves substances against gradient.

• Hypertonic Solution: Higher solute concentration; cell loses water.

• Hypotonic Solution: Lower solute concentration; cell gains water.

• Endocytosis: Cell engulfs substances into vesicles.

• Exocytosis: Cell expels substances via vesicle fusion.

Cellular Respiration: Obtaining Energy from Food

Aerobic Capacity and Oxygen Limitation

• Aerobic Capacity: Maximum rate of oxygen intake and use during exercise.

• When oxygen is insufficient, cells switch to fermentation, producing lactate and causing muscle fatigue.

Photosynthesis and Ecosystems

• Photosynthesis converts solar energy to chemical energy, forming the basis of most food webs.

• Releases oxygen, essential for most life.

Autotrophs vs. Heterotrophs

• Autotrophs (Producers): Make organic molecules from inorganic sources (e.g., plants, algae).

• Heterotrophs (Consumers): Obtain energy by consuming other organisms (e.g., animals, fungi).

Photosynthesis and Cellular Respiration: Complementary Processes

• Products of photosynthesis (glucose, O2) are reactants for cellular respiration.

• Products of respiration (CO2, H2O) are reactants for photosynthesis.

Cellular Respiration Equation

Overall Equation:

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \rightarrow 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \text{ATP}$

• Reactants: Glucose (C6H12O6), Oxygen (O2)

• Products: Carbon dioxide (CO2), Water (H2O), ATP (energy)

Electron Transfer in Cellular Respiration

• Electrons from glucose are transferred to NAD+ (forming NADH).

• NADH carries electrons to the electron transport chain.

• Electrons ultimately reduce oxygen to form water.

Stages of Cellular Respiration

Stage	Location	Reactants	Products	ATP Yield
Glycolysis	Cytoplasm	Glucose	2 Pyruvate, 2 NADH, 2 ATP (net)	2
Citric Acid Cycle	Mitochondrial Matrix	Acetyl-CoA	CO2, NADH, FADH2, ATP/GTP	2
Electron Transport Chain	Inner Mitochondrial Membrane	NADH, FADH2, O2	ATP, H2O	~28

Additional info: In prokaryotes, citric acid cycle and ETC occur in the cytosol and plasma membrane, respectively.

Photosynthesis: Using Light to Make Food

Importance of Photosynthesis

• Produces ~160 billion metric tons of organic material annually.

• All human food traces back to photosynthetic plants.

• Provides materials for clothing, housing, pharmaceuticals, and more.

• Affects global environment and atmospheric composition.

Leaf and Chloroplast Structure

• Leaves are adapted to capture light, exchange gases, and transport water.

• Chloroplasts contain thylakoids (site of light reactions) and stroma (site of Calvin cycle).

Photosynthesis vs. Cellular Respiration

• Photosynthesis stores energy in glucose; respiration releases energy from glucose.

• Both processes are interconnected in the cycling of matter and energy.

Light Reactions vs. Calvin Cycle

Process	Reactants	Products
Light Reactions	Light, H2O, NADP+, ADP	O2, ATP, NADPH
Calvin Cycle	CO2, ATP, NADPH	Glucose (C6H12O6), ADP, NADP+

Wavelengths and Color

• Objects appear green because they reflect green wavelengths (~495–570 nm) and absorb others.

• Green is in the middle of the visible spectrum.

Photosynthetic Pigments

• Chlorophyll a: Main pigment; absorbs blue-violet and red light.

• Chlorophyll b: Accessory pigment; absorbs blue and orange light.

• Carotenoids: Accessory pigments; absorb blue-green light, protect against photo-damage.

• Pigments transfer photon energy within photosystems, driving ATP and NADPH production.

ATP Production: Thylakoid vs. Mitochondrial Membrane

• Both use chemiosmosis: proton gradient drives ATP synthase.

• Thylakoid membrane (chloroplasts): Light energy powers electron transport.

• Inner mitochondrial membrane: Chemical energy from NADH/FADH2 powers electron transport.

Calvin Cycle: Reactants and Products

• Reactants: CO2 (from atmosphere), ATP and NADPH (from light reactions).

• Products: Glucose (used for energy/storage), ADP and NADP+ (recycled to light reactions).