Cell Structure and Function

What color are cells in the body?• clear

Functions of the Cell

• Basic unit of life

• Protection and support

• Movement

• Communication

• Cell metabolism and energy release

• Inheritance

Cell Characteristics

• Plasma (cell) Membrane– Outer cell boundary

• Cytoplasm – Composed of cytoplasmic organelles

• ER, ribosomes, centrioles, mitochondria, Golgi complex, lysosomes and vacuoles

• Organelles– Specialized structures that perform specific

functions

Plasma Membrane

• Intracellular versus extracellular

• Selectively permeable

• Fluid-mosaic model: regulate the passage of nutrients and waste

• Composed of proteins and lipids

Nucleus

• DNA dispersed throughout; controls and integrates the function of the entire cell

• Consists of :– Nuclear envelope: Separates nucleus from cytoplasm and regulates

movement of materials in and out

– Chromatin: Condenses to form chromosomes during cell division

– Nucleolus: Assembly site of large and small ribosomal units where rRNA is manufactured

Cytoplasm• Cellular material outside

nucleus but inside plasma membrane

• Cytosol: Fluid portion• Cytoskeleton: Supports

the cell– Microtubules

– Microfilaments

– Intermediate filaments

• Cytoplasmic inclusions

Organelles

• Small specialized structures for particular functions

• Most have membranes that separates the interior of organelles from cytoplasm

• Related to specific structure and function of the cell

Ribosomes

• Sites of protein synthesis

• Composed of a large and small subunit

• Types– Free

– Attached to endoplasmic reticulum

• Found in both eukaryotes and prokaryotes

Endoplasmic Reticulum

• Extension of outer nuclear membrane

• Types– Rough

• Attached ribosomes• Proteins produced and modified

for secretion and for internal use

– Smooth• No attached ribosomes• Manufacture lipids

• Cisternae: Interior spaces isolated from rest of cytoplasm

• Transports nutrients to the nucleus and provides some cell structure

Golgi Apparatus

• Modification, packaging, distribution of proteins and lipids for secretion or internal use produced by ER

• Flattened membrane sacs stacked on each other

Function of Golgi Apparatus

Mitochondria

• Provide energy for cell• Major site of ATP

synthesis• Membranes

– Crista: Infoldings of inner membrane

– Matrix: Substance located in space formed by inner membrane

Centrioles• In specialized zone

near nucleus: Centrosome

• Each unit consists of microtubules

• Before cell division, centrioles divide, move to ends of cell and become spindle fibers in eukaryotic cells

Cilia• Appendages

projecting from cell surfaces

• Capable of movement• Moves materials over

the cell surface

Flagella

• Similar to cilia but longer

• Usually only one exists per cell

• Move the cell itself in wavelike fashion

• Example: Sperm cell

Microvilli

• Extension of plasma membrane

• Increase the cell surface

• Normally many on each cell

• One tenth to one twentieth size of cilia

• Do not move

Prokaryotes

• Simple cells which lack a membrane bound nucleus

• Initial or primitive cell type found on earth

• Less complex than eukaryotes– Have no organelles (microtubules or centrioles)– Examples include bacteria

• Division is by binary fission

Eukaryotes

• Evolved from prokaryotes; they have a true nucleus

• Structurally and biochemically more complex– Examples include protozoa, fungi, algae, plant

and animal cells

• Golgi apparatus is the structure in eukaryotic cells which secretes substances

Viruses

• Acellular: neither prokaryotic or eukaryotic• Contain nucleic acid and either RNA or DNA

but not both• Depend on the energy of the host cell• Has protective coat which makes them hard

to control; antibiotics have no effect• Bacteriophage: virus which invades bacteria• Examples of viral diseases: measles,

chickenpox, mumps, mono, hepatitis, AIDs

Overview of Cell Metabolism

Movement through the Plasma Membrane

• Four different modes– Directly through the plasma membrane

• Small, uncharged, hydrophobic molecules• Osmosis

– Membrane channels• Rapid movement of water, charged small molecules

– Carrier molecules• Large polar molecules that are not lipid soluble• Glucose

– Vesicles• Large non-lipid soluble molecules, small pieces of matter,

whole cells• Small sac surrounded by membrane

Movement through the Plasma Membrane

• Diffusion

• Osmosis

• Filtration

• Mediated transport mechanisms– Facilitated diffusion– Active transport– Secondary active transport

Diffusion• Movement of solutes from an area of higher

concentration to lower concentration in solution

• Only applies to small, hydrophobic, uncharged molecules

• Factors that influence rate of diffusion– Temperature of solution

– Distance

– Size of diffusing molecules

– Concentration or density gradient• Difference between two points

– Viscosity• How easily a liquid flows

Osmosis• Diffusion of water (solvent) across a selectively

permeable membrane (water most abundant compound in cell)

• Important because large volume changes caused by water movement disrupt normal cell function

• Cell shrinkage or swelling– Isotonic: equal concentration on both sides

• cell neither shrinks nor swells

– Hypertonic: Less water/more solute than other side• cell shrinks (crenation)

– Hypotonic: More water/less solute than other side• cell swells (lysis)

Osmosis

• Osmotic pressure – force required to prevent the movement of water a solution by osmosis across a selectively permeable membrane

• The greater the concentration of the solution (the more hypertonic the solution) the greater the osmotic pressure

• The greater the osmotic pressure the greater the tendency for water to move into the solution

Osmosis

Filtration

• Works like a sieve

• Depends on pressure difference on either side of partition

• Moves from side of greater pressure to lower– Example: In kidneys in urine formation

Osmosis