Cells Membrane: its Structure & Function

Anatomy of the Cell

•Cells are not all the same

•All cells share general structures

•Cells are organized into three main regions

–Nucleus

–Cytoplasm

–Plasma membrane

Plasma Membrane

•Barrier for the cell contents

•Consists of a double phospholipid layer

–Hydrophilic heads

–Hydrophobic tails

•Also contains protein, cholesterol, and glycoproteins

Selective Permeability

•Acts as a gatekeeper

•The plasma membrane allows some materials to pass while excluding others

•This permeability includes movement into and out of the cell

Plasma Membrane

•Phospolipid Bilayers

–Block the passage of most molecules

–Can isolate cell contents from the external environment

–Very small molecules may pass through freely:

•water and uncharged lipid-soluble molecules

Cell Membrane

The mosaic membrane is embedded with protein molecules that:

1. Aid in transport of molecules

2. Play a role in cell’s responses to

substances in its environment

•Transport proteins

•Receptor proteins

•Recognition Proteins

Plasma Membrane Transport

•Transport proteins – allows water-soluable molecules to cross the plasma membrane by carrying them across

•Receptor proteins – specific to chemical messages (hormones).

•Recognition Proteins – act as identification tags. Recognize your own cells from invading disease causing organisms

Solutions and Transport

•Solution – homogeneous mixture of two or more components

–Solvent – dissolving medium

–Solutes – components in smaller quantities within a solution

•Intracellular fluid – nucleoplasm and cytosol

•Interstitial fluid – fluid on the exterior of the cell

Cell Membrane Transport

•Membrane Transport – movement of substance into and out of the cell

•Transport is by two basic methods

–Passive transport

•No energy is required

–Active transport

•The cell must provide metabolic energy

Passive Transport:

Diffusion

Simple diffusion – nonpolar and lipid-soluble substances

•Unassisted process

•Diffuse directly through the lipid bilayer

•Diffuse through channel proteins

•Molecules disperse evenly

Passive Transport Processes

•Diffusion

–Particles tend to distribute themselves evenly within a solution

–Movement is from high concentration to low concentration, or down a concentration gradient

Passive Transport Processes

•Types of diffusion

–Facilitated diffusion

•Substances require a protein carrier for passive transport

Passive Transport

Diffusion through Osmosis

•Occurs when concentration of a solvent is different on opposite sides of a membrane

•Diffusion of water across a selective membrane down reduce the concentration of water, from a higher concentration of water to a lower concentration of water.

•Allows the passage of some molecules but prevents the passage of other molecules

•Osmolarity – Tonicity

Passive Transport Processes

•Filtration

–Water and solutes are forced through a membrane by fluid, or hydrostatic pressure

–A pressure gradient must exist

•Solute-containing fluid is pushed from a high pressure area to a lower pressure area

Active Transport Processes

•Transport substances that are unable to pass by diffusion

–They may be too large

–They may not be able to dissolve in the fat core of the membrane

•Two common forms of active transport

–Solute pumping – chemical exchanges

–Bulk transport - exocytosis

Active Transport Processes-Solute pumping-

Exocytosis

Endocytosis

THE LIVING HUMAN (ANIMAL) CELL

Cytoplasm

•Cytoplasm – material between plasma membrane and the nucleus

•Cytosol – viscous semi-fluid, largely water with dissolved protein, salts, sugars, and other solutes

•Cytoplasmic organelles – metabolic machinery of the cell

•Inclusions – chemical substances such as glycosomes, glycogen granules, and pigment

Cytoplasmic Organelles

•Membranous - mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi apparatus

•Nonmembranous - cytoskeleton, centrioles, and ribosomes

Mitochondrion

•Double membrane structure with shelf-like folds - cristae

•Provide most of the cell’s ATP via aerobic cellular respiration

•Contain their own DNA and RNA

RibosomesGranules containing protein and rRNA

•Site of protein synthesis

•Free ribosomes synthesize soluble proteins

•Membrane-bound ribosomes synthesize proteins to be incorporated into membranes

Endoplasmic Reticulum (ER)

•Interconnected tubes and parallel membranes enclosing cristernae (cristae)

•Continuous with the nuclear membrane

•Two varieties –

Rough (ER)

•External surface studded with ribosomes

•Manufactures all secreted proteins

•Responsible for the synthesis of integral membrane proteins and phospholipids for cell membranes

Smooth (ER)

Catalyzes the following reactions in various organs of the body:

•Liver – lipid & cholesterol metabolism, breakdown of glycogen, detoxification of drugs

•Testes – synthesis steroid-based hormones

•Intestinal cells – absorption, synthesis, and transport of fats

•Skeletal and Cardiac muscle – storage and release of calcium

Golgi Apparatus

•Stacked, flattened membranous sacs

•Modifies concentration of proteins and packages them

•Transport vesicles from the ER and are received by Golgi apparatus

Lysosomes

•Spherical membranous bags containing digestive enzymes

•Digest ingested bacteria, viruses, and toxins

•Degrade nonfunctional organelles

•Breakdown glycogen and release thyroid hormone

•Autolysis – self-digestion of the cell

•Breakdown nonuseful tissue

•Breakdown bone to release Ca2+

•Secretory lysosomes are found in white blood cells, immune cells, andmelanocytes

Peroxisomes

“Peroxide bodies”

•Membranous sacs containing oxidases and catalases

•Detoxify harmful or toxic substances

•Neutralize dangerous free radicals

•Free radicals – highly reactive chemicals with unpaired electrons

Cytoskeleton

•The “skeleton” of the cell

•Dynamic, elaborate series of rods running through the cytosol

•Consists of microtubules, microfilaments, and intermediate filaments

1. Microtubules

•Dynamic, hollow tubes made of the spherical protein tubulin

•Determine the overall shape of the cell and distribution of organelles

2. Microfilaments

Dynamic strands of protein Actin

•Attached to the cytoplasmic side of the plasma membrane

•Braces and strengthens the cell surface

Cytoskeleton (cont.’)

Intermediate Filaments

•Tough, insoluble protein fibers with high tensile strength

•Resist pulling forces on the cell and help form desmosomes

3. Centrioles

•Small barrel-shaped organelles located in the centrosome near the nucleus

•Pinwheel array of nine triplets of microtubules

•Organize mitotic spindle during mitosis

Cellular MotionCELIA

•Cellular extensions that provide motility in a whiplike motion.

•Typically found in large numbers

•Located in the exposed surface of the cell

•Move substances in one direction across cell surface

Example: paramecium organism, ciliated epithelial cells found in the trachea.

Flagella

•Projections are longer

• A single propulsive flagellum

• Movement is achieved by propelling itself across the surface or environment

• Basal bodies in the centrioles form the bases for ceia and flagella

Example: the sperm

Nucleus

•The control center containing genetic

•Largest cytoplasmic organelle - 5µm

•Nuclear envelop –dbl membrane barrier

•Nucleoli – DNA & RNA for genetic synthesis

•Chromatin – threadlike coils that form chromosomes in cell division. Genes

DNA