Cell Structure and Genetic Control

Human physiology

Cell•Basic unit of structure and function of the

body.▫Highly organized molecular factory.

•Great diversity of function.▫Organ physiology derived from complex

functions of the cell.•3 principal parts:

▫Plasma membrane.▫Cytoplasm and organelles.▫Nucleus.

Plasma Membrane

• Is selectively permeable.• Composition:

▫ Double layer of phospholipids due to hydrophobic/hydrophilic parts. Restrict passage of H20 and H20 soluble ions.

▫ Proteins span or partially span the membrane. Provide structural support, transport molecules, serve as

receptors.▫Negatively charged carbohydrates attach to

the outer surface. Involved with regulatory molecules.

Plasma Membrane (continued)

Cytoplasm, Organelles, Nucleoli

• Cytoplasm:▫ Aqueous content of the cell.

• Organelles:▫ Sub-cellular structures within the cytoplasm.

• Nucleus:▫ Is a large spheroid body.▫ Largest of the organelles.▫ Contains the genetic material (DNA).▫Nucleoli:

Centers for production of ribosomes.

Cytoplasm, Organelles, Nucleoli (continued)

Bulk Transport

• Phagocytosis:▫ Phagocytic cells use pseudopods to surround and engulf

particles.▫ Pseudopods join, fuse, and surround ingested particle (food

vacuole). Lysosomes digest food vacuole.

▫ Protects from invading organisms.▫ Removes debris.

• Endocytosis:▫ Pinocytosis:

Nonspecific process. Plasma membrane invaginates, fuses, vesicle containing ECF

pinches off, and vesicle enters cell.

Bulk Transport (continued)

• Receptor-mediated endocytosis:▫ Interaction of molecules in ECF with specific membrane

receptor proteins.▫ Membrane invaginates, fuses, pinches off and forms

vesicle.▫ Vesicle enters cell.

• Exocytosis:▫ Process by which cellular products are secreted into

extracellular environment.▫ Proteins and other molecules to be secreted are packaged

in vesicles by Golgi complex.▫ Vesicles fuse with plasma membrane and release contents

into extracellular environment.

Cilia, Flagella, Microvilli

•Cilia:▫Tiny hair-like structures that project from

the surface of the cell. Stroke in unison.

Respiratory tract, uterine tube.

•Flagella:▫Simple whip-like structure that propels

sperm through its environment.•Microvilli:

▫Numerous folds (finger-like projections) increase surface area. Aid absorption.

Cytoplasm and Cytoskeleton

• Cytoplasm:▫ Jelly-like matrix within

the cell.▫ Includes organelles

and cytosol.▫ Highly organized

structure with microtubules and microfilaments that function as cytoskeleton.

• Cytoskeleton:▫ Actin and myosin

(microfilaments).▫ Spindle apparatus

(microtubules).

Lysosomes

▫Primary: Contain only digestive enzymes.

▫Secondary: Primary lysosome fuses with food vacuole or organelle.

Contain partially digested remnants of other organelles and organic material.

▫Residual body: Contain undigested wastes.

▫Autophagy: Process that destroys worn-out organelles, so that they

can be continuously replaced.▫Apoptosis (programmed cell death):

Lysosomes release digestive enzymes into the cell.

Peroxisomes

•Membrane-enclosed organelles.▫Contain specific enzymes that promote

oxidative reactions.▫Oxidize molecules and form H202.

•Catalase: converts H202 H20 + 02.•Oxidation of toxic molecules by

peroxisomes is an important function of liver and kidney cells.

Mitochondria

• Sites for energy production of all cells; but mature RBCs.

• Contain own DNA, can reproduce themselves.

• Structure:▫ Outer membrane:

smooth.▫ Inner membrane: cristae.▫ Cristae and matrix

compartmentalize mitochondrion space. Have different roles in

energy generation.

Ribosomes

•Protein factories:▫Proteins produced according to genetic

information contained in mRNA.▫Located in cytoplasm and on the surface of

endoplasmic reticulum.•rRNA molecules serve as enzymes

(ribozymes) required for protein synthesis.▫Contains 2 subunits composed of rRNA and

proteins.

Endoplasmic Reticulum (ER)

• Granular (rough) ER:▫ Bears ribosomes on

surface, in cells active in protein synthesis. Proteins enter cisternae

are modified for secretion.

• Agranular (smooth) ER:▫ Provides site for enzyme

reactions in steroid hormone production and inactivation.

▫ Storage of Ca2+ in striated muscle cells.

Golgi Complex

• Stacks of hollow, flattened sacks with cisternae.▫ One side of sack faces site

for entry of vesicles from ER that contain cellular products.

▫ Other site faces towards plasma membrane and releases vesicles of chemically modified products.

• Modifies proteins, separates according to destination, and packages into vesicles.

Cell Nucleus

Most cells have single nucleus.Enclosed by inner and outer membrane

(nuclear envelope).◦ Outer membrane is continuous with ER.

Nuclear pore complexes fuse inner and outer membranes together.◦ Selective active transport of proteins and RNA.

Regulation of gene expression. Transport of mRNA out of nucleus to ribosomes.

Nucleoli:◦ DNA contains the genes that code for the

production of mRNA.

Cell Death• Pathologically:

▫ Cells deprived of blood supply swell, the membrane ruptures, and the cell bursts (necrosis).

• Apoptosis:▫ Cells shrink, membranes become bubbled, nuclei

condense.• Capsases (“executioner enzymes”):

▫ Mitochondria membranes become permeable to proteins and other products.

• Programmed cell death:▫ Physiological process responsible for remodeling of

tissues during embryonic development and tissue turnover in the adult.