Lecture 2 Cell Biology

Eukaryotic cell structure:Structure and function of cell organelles

Prepared by Mayssa Ghannoum

Structure and function of cell organelles All living organisms are made up of cells, and all living cells

come from preexisting cells. The cell: is the basic structural and functional unit of every

living organism. An organelle: is a specialized subunit within a cell that has a

specific function, and is usually separately enclosed within its own lipid bilayer (membrane).It is called “ little organ”, very small in sizeCan only be observed under electron microscope.Found only in eukaryotic cells.

Protoplasm

• The protoplasm of a cell consists of nucleus,  cell membrane and cytoplasm.

• Cytoplasm is a part of the protoplasm of a cell.• The protoplasm is bound by a plasma membrane or cell

membrane on all sides whereascytoplasm is the substance which is present surrounding the nucleus within a cell.

• Protoplasm is composed of a mixture of small molecules such as ions, amino acids, monosaccharides, water and macromolecules such as nucleic acids, proteins, lipids and polysaccharides

Protoplasm

• Inside the cell, the space between the organelles is a semifluid, jellylike substance called cytosol.

The Nucleus: Information central• Contains most of the genes in the eukaryotic cell.

• Usually one per cell.

• Control the normal activities of the cell.

• The most conspicuous organelle, about 5 µm in diameter.

• Bounded by a nuclear envelope, separating its contents from the cytoplasm.

• The nuclear envelope is a double membrane.The two membranes, each a lipid bilayer with associated proteins.The envelope is perforated by pore structures.

• Within the nucleus, the DNA is organized into units called chromosomes, structures that carry the genetic information.

• Each chromosome is made up of a material called chromatin, a complex of proteins and DNA.

• The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA) according to instructions provided by the DNA.

• The mRNA is then transported to the cytoplasm via nuclear pores. Once it’s in the cytoplasm, ribosomes translate it into a specific polypeptide.DNA RNA protein.

Ribosomes: Protein Factories

• Are complexes made of ribosomal RNA and protein.

• They Carry out protein synthesis.

• They have two cytoplasmic locales:1. free ribosomes are suspended in the cytosol2. bound ribosomes are attached to the rough endoplasmic reticulum.

The Endoplasmic Reticulum• The Endoplasmic reticulum membrane separates the

internal compartment of the ER, called ER lumen (cavity) from the cytosol.

• The ER membrane is continuous with the nuclear envelope.

• There are two distinct, connected regions of the ER that differ in structure and function: Smooth ER and Rough ER.

• Smooth ER: Is so named because its outer surface lacks ribosomes. Processing and transport of lipids.

• Rough ER: has ribosomes on the outer surface of the membrane.Transport of protein.

The Golgi apparatus: Shipping and Receiving Center

• Center of manufacturing, warehousing, sorting and shipping.

• Proteins are modified and stored and then sent to other destinations.

• Looks like a stack of plates.

• Molecules are transported to and from the Golgi by means of vesicles.

Lysosomes: Digestive compartments

• Is a membranous sac of hydrolytic enzymes that an animal cell uses to digest macromolecules, break wastes down and help recycle large proteins.

• Lysosomal enzymes work best in the acidic environment found in lysosomes.

• Hydrolytic enzymes and lysosomal membrane are made by rough ER and then transferred to Golgi apparatus for further processing.

• Garbage disposal of the cell.

Vacuoles• Storage container for water, food, enzymes, wastes,

pigments, etc.

• Plant cells contain large central vacuole which can play many roles:- Can hold reserves of important organic compounds- Disposal site for metabolic by-products that would endanger the cell if they accumulate.- Some vacuoles contain pigments that color the cell.- May help protect the plant against predators.- Has a major role in the growth of plant cells.

Mitochondria: Chemical energy conversion

• Sites of cellular respiration, the metabolic process that generates ATP by extracting energy from sugars, fats, and other fuels with the help of oxygen.

• Enclosed by two membranes, each a phospholipid bilayer with a unique collection of embedded proteins.

• The outer membrane is smooththe inner membrane is convoluted, with infoldings called cristae.

• Power house of the cell.

• Has its own strand of DNA

Chloroplasts: Capture of light energy• Found only in plant cells and algae.

• Sites of photosynthesis.

• They convert solar energy to chemical energy by absorbing sunlight and using it to drive the synthesis of organic compounds such as sugars from CO2 and water.

• Contain the green pigment:Chlorophyll.

• Bound by double membrane.

Peroxisomes: Oxidation

• Specialized metabolic compartments bounded by a single membrane.

• Roughly spherical and have a granularcore that is a dense collection of enzyme molecules.

• Contain enzymes that transfer hydrogen from various substrates to oxygen (O2).

• Produce hydrogen peroxide (H2O2) as a by-product.

• Peroxisomes have many functions: Some peroxisomes use oxygen to break down fatty acids

down into smaller molecules that can be then transported to mitochondria as fuel for cellular respiration.

Peroxisomes in the liver detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to oxygen.

H2O2 formed by peroxisomes is itself toxic, but the organelle also contains an enzyme that converts H2O2 to water.

• Peroxisomes may increase in number by splitting in two when they reach a certain size.

The cytoskeleton• Network of fibers extending throughout the cytoplasm.

• Plays a major role in organizing the structures and activities of the cell.

• It gives mechanical support to the cell and maintain its shape.

• It’s especially important for animal cells, which lack walls.

• The strength and resilience of the cytoskeleton is based on its architecture.

• It provides anchorage for many organelles and cytosolic enzymes.

• The cytoskeleton is more dynamic than an animal skeleton.

• Functions in cell motility (changes in cell location).

• Involved in the regulation of biochemical activities in the cell in response to mechanical stimulation.

• Composed of three types of molecular structures:microtubules, microfilaments, and intermediate filaments.

Cytoskeleton

Responsible for cell motility, and separation of chromosome during

cell division.

(Tubulin protein)

Support cell motility and transport

materials within the cell.

(Actin protein)

Reinforcing the cell shape and fixing

position of organelles.

(Fibrous protein)

MicrofilamentsIntermediate

filamentsMicrotubules

Thick Thin Middle

Centrosomes and Centrioles

• Found only in animal cells.

• Centrosome is a region located near the nucleus and considered “microtubule-organizing center”.

• Within the centrosome are a pair of centrioles, each composed of 9 sets of triplet microtubules arranged in a ring.

• Before an animal cell divides, the centrioles replicate.

• Aids in cell division.

Cilia and flagella• They both act as locomotors appendages.

• When cilia or flagella extend from cells that are held in place as part of a tissue layer, they can move fluid over the surface of the tissue.

• Motile cilia usually occur in large number on the cell surface.

• Flagella are usually limited to just one or few per cell.

• Cilia are about 0.25 µm in diameter and 2-20 µm long.

• Flagella are the same diameter but longer, 10-200 µm.

A comparison of the beating of flagella and cilia

Motion of flagella:a flagellum usually undulates, it’s snakelike motion driving a cell in the same direction as the axis of the flagellum.E.g.: propulsion of a human sperm cell.

Motion of cilia:cilia have a back-and-forth motion. The rapid power stroke moves the cell in a direction perpendicular to the axis of the cilium. Then, during slower recovery stroke, the cilium bends and sweeps sideways, closer to the surface.