• Chapter 3, Part A
• Structure and Function
of Cells
• Cell Doctrine
• All living things are composed of cells
• A single cell is the smallest unit that exhibits all of the characteristics of life
• All cells come only from preexisting cells
• Two Basic Cell Types Classified by Internal Organization
Prokaryotic Cells
• Plasma membrane
• No nucleus
• Cytoplasm: fluid within membrane
• No true organelles
Eukaryotic Cells
• Plasma membrane
• Nucleus: information center
• Cytoplasm: fluid within membrane
• Organelles: structures with specialized functions
• All human cells are eukaryotic
• Cell Structure Reflects Cell Function
• Though eukaryotic cells are remarkably similar, there are structural differences
– Examples:
• Muscle cells
– Contain numerous organelles providing energy needed for muscle contraction
• Nerve cells
– Long and thin to carry impulses over distance
• Small size is efficient
• Cells Remain Small to Stay Efficient
• Cells cannot be seen without magnification
• Microscopes enable visualization and study of cells
– Light microscope
• Magnifies up to 1000X
– Transmission electron microscope
• Magnifies up to 100,000X
– Scanning electron microscope
• Magnifies up to 100,000X
– Provides 3-D view of cell surface
• Cells Remain Small to Stay Efficient
• Small cells have a higher surface:volume ratio
• High surface:volume ratio promotes efficiency in
– Acquisition of nutrients
– Disposal of wastes
• Plasma Membrane Surrounds the Cell
• Separates a cell from its environment
• Selectively permeable
– Permits movement of some substances into and out of the cell, but blocks others
• Enables transfer of information between environment and cell
• A Plasma Membrane Surrounds the Cell
• Plasma membrane is a lipid bilayer
– Phospholipids: polar head and nonpolar tail
– Cholesterol: makes membrane a bit more rigid
– Proteins: provide means of transport through membrane
– Carbohydrates: recognition patterns for cells and organisms
• Nonrigid
• Fluid mosaic
• Molecules Cross the Plasma Membrane in Several Ways
• Passive transport
– Cell does not need to expend energy for this
• Diffusion
• Osmosis
• Active transport – cell must expend energy
• Bulk transport
– Involves membranous vesicles to move larger substances
• Endocytosis
• Exocytosis
• Passive Transport Moves with the Concentration Gradient
• Passive transport is powered by the concentration gradient. In the cell it occurs as
– Diffusion through lipid layer
– Diffusion through protein channels
– Facilitated transport
• Transport or carrier proteins in the membrane assist in moving molecules across the membrane, down the concentration gradient, without expending energy
• Active Transport
• Active transport moves substances from an area of lower concentration to an area of higher concentration
– Requires a membrane protein (transporter)
– Requires ATP or other energy source
• Endocytosis and Exocytosis Move Materials in Bulk
• Used to move larger molecules
– Endocytosis: brings substances into the cell
– Exocytosis: expels substances from the cell
• Information Transfer Across the Plasma Membrane
• Receptor proteins span membrane – required for transmission of information to and from cell
• Receptor sites (on receptor proteins) – interact specifically with signal molecules
• A change is triggered within the cell as a result of binding of signal molecule to receptor site
• Different cell types have different receptor proteins
• The Sodium–Potassium Pump:
Helps Maintains Cell Volume
• Sodium–potassium pump expels unwanted ions, keeps needed ones, and maintains cell volume
• ATP is used to expel 3 sodium ions for every 2 potassium ions brought into the cell
• Increase in cell volume = increase in water in cytoplasm by decreasing pumping and allowing more sodium inside cell
• Decrease in cell volume = less water in cytoplasm by increasing pumping and expelling more sodium ions
• Isotonic Extracellular Fluid Maintains Cell Volume
• Tonicity: relative concentration of solutes in two fluids
• Isotonic
– Extracellular and intracellular ionic concentrations are equal
– Cells maintain a normal volume in isotonic extracellular fluids
– Regulatory mechanisms maintain extracellular fluid that is isotonic with intracellular fluid
• Isotonic Extracellular Fluid Maintains Cell Volume
• Variations in tonicity
– Hypertonic
• Extracellular ionic concentration higher than intracellular
• Water will diffuse out of cell
• Cell will shrink and die
– Hypotonic
• Extracellular ionic concentration lower than intracellular
• Water will diffuse into cell
• Cell may swell and burst
• Nucleus
• Functions
– Contains the genetic information of the cell
– Controls the cell
• Structural features
– Double-layered nuclear membrane
– Nuclear pores
– Chromosomes/chromatin
– Nucleolus
• Ribosomes
• Site of protein synthesis
• Location
– Free: floating I cytoplasm
– Bound: attached to outer surface of endoplasmic reticulum
• Endoplasmic Reticulum (ER)
• Two types of endoplasmic reticulum (ER)
– Rough ER
• Has ribosomes on surface
• Protein manufacturing, modifications
– Smooth ER
• No ribosomes on surface
• Lipid synthesis
• Packages the proteins
• Golgi Apparatus
• Refines synthesized products
• Packaging and shipping center
• Products are packaged into vesicles and shipped to other locations within the cell or to the cell membrane for export
• Vesicles
• Storage and shipping vesicles
• Secretory vesicles
• Endocytic vesicles
• Peroxisomes
– Contain enzymes that detoxify
• Lysosomes
– Contain digestive enzymes
• Mitochondria
• “Power plant” of the cell
• Surrounded by a double membrane
• Utilizes O2 and produces CO2
• Generates ATP
• Fat and Glycogen: Sources of Energy
• Fat
– Triglycerides
– Long-term energy storage in animals
• Glycogen
– Carbohydrate storage
– Short-term energy storage in animals
• Cell Structures for Support and Movement
• Cytoskeleton
– Microtubules
– Microfilaments
• Cilia
– Short, many
– Found on cells lining airways
• Flagella
– Long, single
– Enable spermatozoa to swim
• Centrioles
• Cells Use and Transform Matter
and Energy
• Anabolism
– Requires enzymes
– Used in making/assembling large molecules
– May require energy (ATP)
– Used in building and assembling cell components
– Used in storing energy
• Cells Use and Transform Matter
and Energy
• Catabolism
– Requires enzymes
– Breakdown of molecules
– May release energy
– Used in breaking down nutrients and recycling cell components
– Used to access energy storage
• Glucose Provides the Cell with Energy
• Glucose provides energy for the cell
• Energy in glucose is used to generate ATP
• In absence of glucose, other carbohydrates, fats, and protein can be catabolized to generate ATP
• Glucose Provides the Cell with Energy
• Four stages of cellular respiration
– Glycolysis
– Preparatory step
– Citric acid cycle
– Electron transport system
• Glycolysis
• Occurs in the cytoplasm
• Series of 10 reactions that split glucose into two molecules of pyruvic acid
• 2 ATP are produced (net)
• High-energy electrons and hydrogen ions are removed and picked up by a coenzyme NAD+, forming NADH
• Preparatory Step for Citric Acid Cycle
• Pyruvate enters mitochondria
• Pyruvate converted to acetyl group and CO2
• High-energy electrons and hydrogen ions are removed and picked up by a coenzyme NAD+, forming NADH
• Acetyl group joined to coenzyme A to form acetyl CoA
• Acetyl CoA will enter citric acid cycle
• Citric Acid Cycle
• Occurs in mitochondria
• Also known as Krebs cycle
• Extracts high-energy electrons to form NADH and FADH2
• Produces two ATP and carbon dioxide
• Electron Transport System
• Located in inner mitochondrial membrane
• Takes electrons from NADH and FADH2
• Movement of electrons from one electron carrier to the next releases energy that is harvested to generate ATP
• Final electron acceptor is O2, which forms water upon receiving electrons and hydrogen ions
• ATP generated by ATP synthase enzyme
• Process also known as oxidative phosphorylation
• Summary of Energy Production
from Glucose
• Over 20 enzyme-catalyzed reactions
• Approximately 36 ATP (net) produced from each molecule of glucose
• Oxygen consumed, carbon dioxide produced
• Cellular respiration: cellular process that uses oxygen and produces carbon dioxide in the process of making ATP
• Additional Energy Sources
• Glycogen
• Fats
– Triglycerides have twice the energy of carbohydrates
• Proteins
– Have the same energy as carbohydrates
• Anaerobic Pathways Make Energy Available without Oxygen
• Cellular respiration cannot continue in the absence of O2
• Glycolysis will continue, pyruvate will build up
• Pyruvate will be converted to lactic acid
• Lactic acid buildup in muscles will cause a burning sensation
• 2 ATP produced per molecule of glucose
• When O2 is available, lactic acid will be metabolized aerobically