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Chapter 3

Cell Structure and Function

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Outline• Cellular Organization

– Plasma MembraneFunctions

– Nucleus– Endomembrane System– Cytoskeleton– Cilia and Flagella

• Cellular Respiration• Fermentation

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Cell Size

• Relatively small cell size is explained by surface-to-volume-ratio.– As cells get larger in volume,

proportionate amount of surface area decreases.

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Microscopy and Cell Structure

• Three common types of microscopes:– Compound light microscope.– Transmission electron microscope.– Scanning electron microscope.

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Cellular Organization

• Plasma membrane surrounds the cell and regulates entrance and exit of substances.– Nucleus is the centrally located structure

containing chromosomes and is the control center of the cell.

– Cytoplasm is the portion of the cell between the nucleus and plasma membrane. Consistency: semifluid gel, like wet Jello.

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Cellular Organization

– Organelles are small membranous structures, each with a specific function.

– Cytoskeleton is a network of interconnected filaments and microtubules in the cytoplasm that maintain cell shape.

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Plasma Membrane

• Plasma membrane is a phospholipid bilayer with attached or embedded proteins.– Polar head and non-polar tails.

Form spherical bilayer when placed in water.

– Plasma membrane proteins form receptors, conductors, or enzymes in metabolic reactions.

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Plasma Membrane Model

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Plasma Membrane Functions

• Plasma membrane is selectively permeable, and regulates movement of molecules and ions across the cell membrane.– Diffusion is the random movement of

molecules from an area of higher concentration to an area of lower concentration until they are equally distributed.

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Plasma Membrane Functions

– Osmosis is the diffusion of water across a plasma membrane.

Tonicity is the degree to which a solution’s solute concentration causes water to move into or out of cells.

Cells gain water in hypotonic solutions, and lose water in hypertonic solutions.

Osmotic pressure is the force exerted on a selectively permeable membrane due to the osmotic movement of water.

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Tonicity

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Plasma Membrane Functions

– Transport by Carriers.Facilitated transport occurs when a

molecule is transported across the plasma membrane down a concentration gradient.

Active transport occurs when a molecule is moving against a concentration gradient with the input of energy.

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Active Transport

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Plasma Membrane Functions

– Transport by Carriers.Endocytosis occurs when a portion of the

plasma membrane invaginates to envelop a substance and then pinches off to form an intracellular vesicle.

Exocytosis occurs when a vesicle fuses with the plasma membrane as secretion occurs.

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The Nucleus

• The nucleus stores genetic information that determines body cell characteristics and metabolic functioning.– Contains nucleolus: site of ribosome

formation.– Contains chromatin: uncoiled DNA– Nucleus is separated from the cytoplasm by

a nuclear envelope.Contains nuclear pores to permit passage

of proteins and ribosomal subunits.

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Nucleus and Nuclear Membrane

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Ribosomes

• Ribosomes are found singly or in groups (polyribosomes) in the cytoplasm.– Function in protein synthesis.

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The Endomembrane System

• Endoplasmic Reticulum.– System of membranous channels and

saccules.Rough ER is studded with ribosomes

and synthesizes proteins.Smooth ER synthesizes phospholipids,

as well as various other functions.

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The Endomembrane System

• The Golgi Apparatus.– Consists of a stack of three to twenty

curved saccules, along with vesicles.Receives protein and or lipid vesicles

that bud from the ER.• Lysosomes.

– Membranous sacs produced by the Golgi apparatus that contain hydrolytic digestive enzymes.

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Endomembrane System

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Mitochondria

• Mitochondria are double-membrane organelles involved in cellular respiration.– Site of ATP production.

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The Cytoskeleton

• The cytoskeleton is formed of several types of filamentous structures that give the cell its shape and organelles the ability to move about the cell.– Microtubules.– Actin Filaments.

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Centrioles

• Centrioles are short cylinders composed of microtubules that are involved in spindle formation during cell division and give rise to basal bodies.

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Cilia and Flagella

• Cilia and flagella (cilium, flagellum) are projections that move in an undulating or whipping fashion.– Cilia tend to be shorter while flagella tend

to be longer.

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Cellular Metabolism

• Cellular metabolism includes all the chemical reactions that occur in a cell.– Often organized into metabolic pathways.

Most regulated by feedback inhibition.

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Enzymes and Coenzymes

• When an enzyme speeds up a reaction, the participating reactants are called the enzyme’s substrates.– Many enzymes require cofactors for

assistance.Some cofactors may accept or donate

atoms to the reaction.Many vitamins are cofactors

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Enzymatic Action

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Cellular Respiration

• Glucose breakdown requires three sub-pathways.– Glycolysis.– Citric Acid Cycle (Krebs cycle).– Electron Transport System.

• Altogether, the breakdown of one glucose molecule results in 36 ATP molecules.

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Cellular Respiration

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Fermentation

• Fermentation is an anaerobic process that results in the buildup of lactate.– Lactate is toxic to cells and causes

muscle cramps and fatigue.– Only produces two ATP per glucose

molecule.

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Review

• Cellular Organization– Plasma Membrane

Functions– Nucleus– Endomembrane System– Cytoskeleton– Cilia and Flagella

• Cellular Respiration• Fermentation

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