Cell Structure and Function

Definition of a cell:

basic structural and functional unit of life

the smallest units that display the characteristics of life, i.e. reproduction, metabolism, response to stimuli

General Subdivisions of a Cell• Plasma Membrane = selectively permeable

boundary between the cell and the environment

• Nucleus = regulatory center of the cell

• Cytoplasm = everything between the plasma membrane and the nucleus (fluid + organelles)

Plasma MembraneStructure

• phospholipd bilayer

• proteins embedded in, and attached to, the inner (intracellular) and outer (extracellular) surfaces

• phospholipids are liquid at body temperature, so proteins float around in the membrane

Plasma MembraneFunction

• Main responsibility: ensure the composition of extracellular fluid is not the same as the composition of the intracellular fluid

• Selectively permeable barrier controls what enters and leaves the cell

Plasma MembraneFunction• Water-soluble substances (salts, nutrients) cross

membrane with aid of protein channels, which are selective about what can pass through

• Lipids can pass directly through bilayer by diffusion (movement from area of high pressure to area of low pressure)

• Attachment site of cytoskeleton, the internal support of the cell

Plasma Membrane

Nucleus

Nuclear Envelope (membrane)

• Phospholipid bilayer with nuclear pores

• Controls what enters/leaves the nucleus

– things only go in or out by passing through Protein channels, which are selective

• Encloses all the chromosomes

NucleusChromatin • All the chromosomes, which are long strands

of the molecule DNA

• DNA regulates all cell activities, yet never leaves the nucleus;

• produces RNA, short messenger molecules that exit through nuclear pores

• RNA carries instructions out into the cytoplasm

Nucleus

Nucleolus• Site of ribosome synthesis

• Compartment in the nucleus where ribsomes are assembled

• Ribosomes are then moved out into cytoplasm through nuclear pores

• Ribosomes and RNA work together outside the nucleus, to build all the proteins in the cell

Nuclear Compartment

RibosomesRibosomes = site of protein synthesis

--assembled in the nucleolus

--exported into the cytoplasm

Free – unbound in the fluid cytoplasm, produceproteins for use in the cell

Bound – attached to the endoplasmic reticulum(ER), produce proteins for export, or for theplasma membrane

Cytoplasm

Fluid portion = “cytoplasm”

• Water with dissolved salts, nutrients and enzymes (= proteins that do a particular job)

• Site of many metabolic reactions

Cytoskeleton

• Network of protein fibers extending throughout the fluid cytoplasm

• Support and structure for the cell

• Very dynamic, always remodeling itself

• Critical for cells to divide and copy themselves

Cell Structure

Organelles• Separate compartments within the cytoplasm

formed by membranes

Mitochondrion – “thread granule”

• major source of cell’s energy

• energy is taken from sugar, stored in molecule called ATP (adenosine triphosphate)

• requires oxygen to make this exchange (aerobic metabolism)

• contained within double membrane

Mitochondrion

OrganellesCytomembrane System

• system of tubes and chambers formed by membranes

• extensively distributed throughout the fluid cytoplasm

• involved in synthesis, modification, processing & packaging of cellular lipids and proteins

Cytomembrane System

Endoplasmic Reticulum (ER) = “within the cytoplasm network”

• system of tubes and sacs formed by membranes (an enclosed space)

Cytomembrane System

• Two Types

• Rough = with bound ribosomes– modifies proteins produced by the

ribosomes

• Smooth = without bound ribosomes – doesn’t modify proteins

– functions in lipid synthesis, drug detoxification, carbohydrate metabolism

Cytomembrane System

Golgi Apparatus

• series of flattened sacs formed by membranes,

• functions in final protein processing prior to use by the cell

• proteins get shuttled from the ER to one end of the Golgi

Cytomembrane SystemGolgi Apparatus

• in each sac, different modifications are made (proteins get individually tailored)

• proteins get sorted and shipped off to their destination (like the post office of the cell)

Cytomembrane SystemVesicles • small membrane-bound structures that transport

proteins and lipids around the cell

• little transporters that shuttle their contents from one organelle to another

• when they contact the appropriate organelle, they fuse with its outer membrane and dump their contents inside

• same for plasma membrane, allowing the export of materials from the cell (exocytosis)

Vesicles move from ER to Golgi

Cytomembrane SystemLysosome

• membrane-bound vesicle that contains digestive enzymes or toxic chemicals

• merges with vesicles containing food particles, invading bacteria

• harsh chemicals and enzymes degrade the food or bacteria, without harming rest of cell