unit_1_cell

The Cell

Dr. S. Francis

Typical Cell

Cytoplasm

• Portion of cell interior not occupied by the nucleus

• Consists of– Organelles

• “little organs”• Distinct, highly organized, membrane-enclosed

structures– Cytosol

• Complex, gel-like mass in which the cytoskeleton is found

Cytosol

• Occupies about 55% of total cell volume• Semi-liquid portion of cytoplasm that surrounds

the organelles• Contains cytoskeleton and cell nutrients• Activities associated with gelatinous portion of

cytoplasm– Enzymatic regulation of intermediary metabolism– Ribosomal protein synthesis– Storage of fat, carbohydrate, and secretory vesicles

Cytoskeleton

• Complex protein network protein of cytosol that acts as “bone and muscle” of cell

• Three distinct elements– Microtubules– Microfilaments– Intermediate filaments

Element FunctionMicrotubules •Transport secretory vesicles

•Movement of specialized cell projections•Form mitotic spindle during cell division

Microfilaments •Contractile systems•Mechanical stiffeners

Intermediate filaments

•Help resist mechanical stress

Examples of Organelles• Endoplasmic reticulum• Golgi complex• Lysosome• Peroxisome• Ribosomes• Mitochondrion• Vault

Endoplasmic Reticulum (ER)• Elaborate fluid-filled membranous system

distributed throughout the cytosol• Primary function

– Protein and lipid manufacture• Two types

– Smooth ER (lipid synthesis)• Mesh of tiny interconnected tubules

– Rough ER (protein synthesis)• Projects outward from smooth ER as stacks of relatively

flattened sacs• Surface has attached ribosomes

Endoplasmic Reticulum (ER)

Ribosomes

• Consist of two subunits– Large and a small subunit

• Involved in Protein Synthesis

Golgi Complex

• Closely associated with ER• Consists of a stack of flattened, slightly curved,

membrane-enclosed sacs called cisternae• Number of Golgi complexes per cell varies

with the cell type• Functions

– Processes raw materials into finished products– Sorts and directs finished products to their final

destinations

Golgi Complex

Lysosomes

• Membranous sacs containing hydrolytic enzymes

• Serve as intracellular digestive system• Extracellular material attacked by lysosomes

enters cell by endocytosis– Pinocytosis– Receptor-mediated endocytosis– phagocytosis

Peroxisomes

• Membranous sacs that house oxidative enzymes that detoxify various waste products

Vaults• Shaped like octagonal barrels• May serve as extracellular transport vehicles• May contribute to multi-drug resistance

sometimes displayed in cancer cells• Exact function is not clear

Mitochondria

• Energy organelle– Major site of ATP

production– Contains enzymes for citric

acid cycle and electron transport chain

• Enclosed by a double membrane– Inner infolded membrane

is called the cristae

Nucleus

• Typically largest single organized cell component

• Enclosed by a double-layered nuclear envelope

• Contains cell’s genetic material, DNA– DNA functions

• Directs protein synthesis• Serves as genetic blueprint during cell replication

Plasma Membrane

• Also called the cell membrane• Surrounds every cell• Separates cell contents from its surroundings

– Separates ICF and ECF• Controls movement of molecules into and out

of cell

Plasma Membrane Structure• Most abundant are the lipids (phospholipids)

• Polar end of phospholipid is hydrophilic• Nonpolar end of phospholipid is hydrophobic .

• Fluid lipid bilayer embedded with proteins• Also has small amount of carbohydrates

– On outer surface only• Cholesterol

– Tucked between phospholipid molecules– Contributes to fluidity and stability of cell

membrane

Plasma Membrane• Functions of the Plasma Membrane

– Physical isolation• Barrier

– Regulates exchange with environment• Ions and nutrients enter• Wastes eliminated and cellular products released

– Monitors the environment• Extracellular fluid composition• Chemical signals

– Structural support • Anchors cells and tissues

Plasma Membrane

• Membrane Lipids

– Double layer of phospholipid molecules

• Hydrophilic heads—toward watery environment, both

• Hydrophobic fatty-acid tails—inside membrane

• Barrier to ions and water—soluble compounds

Plasma Membrane• Membrane Carbohydrates

– Proteoglycans, glycoproteins, and glycolipids• Extend outside cell membrane

• Form sticky “sugar coat” (glycocalyx)

– Functions of the glycocalyx• Lubrication and protection

• Anchoring and locomotion

• Specificity in binding (receptors)

• Recognition (immune response)

Plasma Membrane Structure

Plasma Membrane

• Membrane Proteins

– Integral proteins

• Within the membrane

– Peripheral proteins

• Bound to inner or outer surface of the membrane

Plasma Membrane• Membrane Proteins

– Anchoring proteins (stabilizers)• Attach to inside or outside structures

– Recognition proteins (identifiers) • Label cells as normal or abnormal

– Enzymes • Catalyze reactions

– Receptor proteins• Bind and respond to ligands (ions, hormones)

– Carrier proteins • Transport specific solutes through membrane

– Channels • Regulate water flow and solutes through membrane

Membrane Transport• The plasma (cell) membrane is a barrier, but

– Nutrients must get in

– Products and wastes must get out

• Permeability determines what moves in and out of a cell, and

a membrane that

– Lets nothing in or out is impermeable

– Lets anything pass is freely permeable

– Restricts movement is selectively permeable

Membrane Transport• Plasma membrane is selectively permeable

– Allows some materials to move freely– Restricts other materials

• Selective permeability restricts materials based on– Size– Electrical charge– Molecular shape– Lipid solubility

Membrane Transport: Fat-and Water-Soluble Molecules

Membrane Transport• Transport through a plasma membrane can

be– Active (requiring energy and ATP)– Passive (no energy required)

• Diffusion (passive)• Carrier-mediated transport (passive or active)• Vesicular transport (active)

Membrane Transport• All molecules are constantly in motion • Molecules in solution move randomly• Random motion causes mixing• Concentration is the amount of solute in a

solvent• Concentration gradient

– More solute in one part of a solvent than another

Diffusion• Diffusion is a Function of the Concentration

Gradient– Diffusion

• Molecules mix randomly

• Solute spreads through solvent

• Eliminates concentration gradient

• Solutes move down a concentration gradient

Membrane Transport: Diffusion

Diffusion

Figure 3–14 Diffusion.

Diffusion• Factors Affecting Diffusion

– Distance the particle has to move

– Molecule size • Smaller is faster

– Temperature • More heat, faster motion

– Gradient size • The difference between high and low concentrations

– Electrical forces • Opposites attract, like charges repel

Diffusion• Diffusion Across Plasma Membranes

– Can be simple or channel mediated• Materials that diffuse through plasma membrane by

simple diffusion:– lipid-soluble compounds (alcohols, fatty acids, and steroids)– dissolved gases (oxygen and carbon dioxide)

• Materials that pass through transmembrane proteins (channels):

– are water–soluble compounds– are ions

Diffusion

• Diffusion across Plasma Membranes

– Factors in channel-mediated diffusion

• Passage depends on:

– size

– charge

– interaction with the channel

Diffusion

Figure 3–15 Diffusion across the Plasma Membrane

Diffusion• Osmosis: A Special Case of Diffusion

– Osmosis is the diffusion of water across the cell membrane

• More solute molecules, lower concentration of water molecules

• Membrane must be freely permeable to water, selectively permeable to solutes

• Water molecules diffuse across membrane toward solution with more solutes

• Volume increases on the side with more solutes

Membrane Transport

• Osmosis – Net diffusion of water down its own concentration gradient

Diffusion

• Osmosis: A Special Case of Diffusion

– Osmotic Pressure

• Is the force of a concentration gradient of water

• Equals the force (hydrostatic pressure) needed to block

osmosis

Diffusion

FIGURE 3–16 Osmosis.

Diffusion• Tonicity

– The osmotic effect of a solute on a cell:

– Isotonic (iso- = same, tonos = tension)• A solution that does not cause osmotic flow of water in or out of a

– Hypotonic (hypo- = below)• Has less solutes and loses water through osmosis

– Hypertonic (hyper- = above) • Has more solutes and gains water by osmosis

Diffusion• Osmolarity and Tonicity

– A cell in a hypotonic solution:

• Gains water

• Ruptures (hemolysis of red blood cells)

– A cell in a hypertonic solution:

• Loses water

• Shrinks (crenation of red blood cells)

Carriers and Vesicles• Carrier-Mediated Transport

– Carrier-mediated transport of ions and organic substrates• Facilitated diffusion • Active transport

– Characteristics• Specificity:

– one transport protein, one set of substrates

• Saturation limits: – rate depends on transport proteins, not substrate

• Regulation: – cofactors such as hormones

Carriers and Vesicles

• Carrier-Mediated Transport

– Cotransport• Two substances move in the same direction at the

same time

– Countertransport• One substance moves in while another moves out

– Facilitated diffusion• Passive

• Carrier proteins transport molecules too large to fit through channel proteins (glucose, amino acids):

– molecule binds to receptor site on carrier protein

– protein changes shape, molecules pass through

– receptor site is specific to certain molecules

Membrane Transport: Facilitated Diffusion

FIGURE 3–18 Facilitated Diffusion.

– Active transport• Active transport proteins:

– move substrates against concentration gradient

– require energy, such as ATP

– ion pumps move ions (Na+, K+, Ca2+, Mg2+)

– exchange pump countertransports two ions at the same time

Membrane Transport: Active Transport

– Active transport

• Sodium-potassium exchange pump

– active transport, carrier mediated:

» sodium ions (Na+) out, potassium ions (K+) in

» 1 ATP moves 3 Na+ and 2 K+

Figure 3–19 The Sodium–Potassium Exchange Pump

– Active transport

• Secondary active transport

– Na+ concentration gradient drives glucose transport

– ATP energy pumps Na+ back out

Figure 3–20 Secondary Active Transport.

Carriers and Vesicles• Vesicular Transport (or bulk transport)

– Materials move into or out of cell in vesicles

• Endocytosis (endo- = inside) is active transport using ATP:

– receptor mediated

– pinocytosis

– phagocytosis

• Exocytosis (exo- = outside)

– Granules or droplets are released from the cell

• Endocytosis

– Receptor-mediated endocytosis:

• Receptors (glycoproteins) bind target molecules (ligands)

• Coated vesicle (endosome) carries ligands and receptors into the

Figure 3–21 Receptor-Mediated Endocytosis.

Carriers and Vesicles• Endocytosis

– Pinocytosis

• Endosomes “drink” extracellular fluid

– Phagocytosis

• Pseudopodia (psuedo- = false, pod- = foot)

• Engulf large objects in phagosomes

– Exocytosis

• Is the reverse of endocytosis

Organelles and the Cytoplasm

• Membranous Organelles

– Membrane flow

• A continuous exchange of membrane parts by vesicles:

– all membranous organelles (except mitochondria)

– allow adaptation and change

unit_1_cell

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