Cells: The Living Unit

Cells – general • 4 aspects of cell theory –

– A cell is the basic structural & functional unit of life

• Cell properties directly relate to the properties of life

– The activity of an organism depends on the activities of individual and collective cells

– Principle of complementarity –• Activities of cells are dictated by specific structures of

cells

– Continuity (the smooth working) of life is based on a cellular level

• Cells come from pre-existing cells

Cells – general cont. • Characteristics of cells –

– Cells vary greatly in their size, shape, and function

– All cells are composed primarily of carbon, hydrogen, nitrogen, and oxygen (CHON)

– All cells have the same basic parts & some common functions

– All general cells contain a plasma membrane, cytoplasm, and nucleus

Cell diversity

Plasma membrane structure

• Plasma membrane –– Defines the extent of the cell – Separates bodies major fluid components

• Intracellular fluid within cells• Extracellular fluid outside cells

• The plasma membrane is composed of a double layer of phosopholipids embedded with smaller amounts of cholesterol and glycolipids and proteins

Plasma membrane structure cont.

• Fluid mosaic model =– Membrane structure composed of a double

layer (bilayer) of lipid molecules with protein molecules dispersed in it.

– Proteins which “float” within the bilayer form an ever changing mosaic pattern

• Composed of…– Double layer of phospholipids (lying tail to tail

with their polar heads exposed to the water inside and outside the cell) embedded with cholesterol, proteins, and glycolipids

Plasma membrane structure cont.

• Surfaces of the plasma membrane –– Outside surfaces of the cell contain more lipids

• Contains glycolipids (sugar lipid)• Helps to make the outer surface more polar

– Inside contains integral proteins, phospholipids, & cytoskeleton

• Phospholipids –– Modified lipid containing phosphorus– 2 components –

• Hydrophilic –– Polar “head” – Has a charge– Water loving

• Hydrophobic – – Nonpolar “tail”– Does not have a charge– Water hating

P.M.

Plasma membrane structure cont.

• Plasma membrane proteins –– Integral proteins are firmly inserted into the plasma

membrane• Have both hydrophobic and hydrophilic regions • Some protrude from only one side of the plasma

membrane – Usually involved in receptors for hormones or chemical

messengers or act as chemical relay messengers • Transmembrane proteins span the entire plasma

membrane – Mainly involved in transport– Channel proteins –

» Have a pore in which water soluble molecules pass – Carrier proteins –

» Bind to substances to move them through the membrane

P.M

Plasma membrane structure cont.

– Peripheral proteins are not embedded in the plasma membrane, but attach to integral proteins or to phospholipids

• Can be removed w/o disturbing the plasma membrane • Contain filaments that helps support the membrane on

the cytoplasmic side • Some involved in changing the cells shape (during

division)

– The glycocalyx is the fuzzy, sticky, carbohydrate-rich area surrounding the cell

• “sugar covering”• Provides a highly specific biological markers

– Recognize self from non-self

P.M.

Specialization of the plasma membrane

• Microvilli – – “shaggy hairs” – Fingerlike extensions of the plasma membrane

that increase the surface area of the cell – Most often found in cells with an absorptive

function – Kidneys and intestines – Contains actin –

• Contractile protein that helps stiffen the microvilli to keep them erect

Specialization of the plasma membrane cont.

• Membrane junctions –– Many cells are close together and in

tight communities • Glycoproteins act as the cells “adhesive” • Wavy contours of cells edges allow cells to

stick together in a tongue and grove fashion • Special membrane junctions are formed

– Tight Junctions– Desmosomes– Gap Junctions

Specialization of the plasma membrane cont.

– Tight junctions• Integral proteins on

adjacent cells fuse together to form impermeable junctions

• Prevents molecules from passing through the extracellular space between cells

Specialization of the plasma membrane cont.

• Membrane junctions cont. -– Desmosomes –

• Anchoring junctions • Scattered like rivets to prevent the cells separation • Held together by linker proteins that extend from the

plaque on the cytoplasmic face• Linker proteins span from one cell to the other &

attach themselves to the other cells plaque• Reduces the chances of ripping the cell when tension

is applied

Specialization of the plasma membrane cont.

• Membrane junctions cont. –– Gap junctions –

• Communication junction • Allows chemical substances to pass between

adjacent cells • Connected by connexons – hollow tubules

that allow the cells to communicate because their cytoplasm's are so close

• Allows small molecules to pass between two cells

• In electrically charged tissues

Gap junctions

P.M. Functions – general

• Cells are “bathed” in interstitial fluid –– Contains important substances cells need to

survive– Cells extract nutrients to remain healthy – Where the cells eject their waste

• The plasma membrane is a selectively permeable barrier, regulating how substances pass into and out of the cell – Allows some substances to pass while excluding

others

P.M. Functions cont. • Solute –

– Substance being dissolved within a solution– Ex. Sugar, salt, calcium, etc.

• Solvent – – Substance in which solutes are dissolved– “vat” in which substances are dissolved – Typically water

• Concentration gradient – – The difference in solute concentration inside

and outside the cell

Passive processes • Passive processes –

– Does not require energy (ATP)– Move substances down (or with) a

concentration gradient• Substances move from areas of higher concentration

to areas of lower concentration – Types –

• Diffusion• Simple diffusion • Facilitated diffusion• Osmosis • Filtration

Passive processes

• Diffusion –– The tendency of molecules/ions to scatter

evenly throughout the environment – Substances move directly through the

plasma membrane if its…• Lipid soluble• Small enough to pass through membrane

channels• Assisted by carrier proteins

– Goal of diffusion – to reach equilibrium – where molecules are moving equally in all directions – there is no net movement – molecules are balanced on each side

Passive processes cont.

• Diffusion cont –– Move from areas of higher concentration to

areas of lower concentration – move down the concentration gradient

• Molecules move very quickly & erratically • Bounce off each other • They don’t like to be close they naturally move to

areas where numbers are lower (move from high to low)

• The greater the concentration difference the faster the diffusion process

• Movement occurs due to the kinetic energy of the molecules

– The smaller the molecule the faster the diffusion– Warmer the temp the faster the diffusion

Diffusion

Passive processes cont.

• Diffusion cont. –– Simple diffusion –

• Unassisted diffusion of lipid-soluble or very small particles

– Substances that are nonpolar– Oxygen, carbon dioxide, alcohol, fat-soluble vit

• Down the concentration gradient

– Facilitated diffusion - • Substances are moved through the plasma membrane

by binding to protein carriers or by membrane channels • Transport proteins change shape to allow substances

(glucose & simple sugars) through• Transported down the concentration gradient

Passive processes

Passive processes cont. • Osmosis –

– Diffusion of water – Water is polar, but it is a small molecule that

can slip through the plasma membrane as the membrane changes shape

– Occurs until solute concentrations are balanced – Movement occurs due to solute concentrations

• Water moves when solute concentrations differ • Requires less energy to move water than a solute • Osmolarity – the total number of all solute particles

w/in a solution • Osmotic pressure – the cells ability to resist further

(net) water movement – when solute concentrations are equal

Osmosis

Passive processes cont• Osmosis cont. –

– Tonicity –• A solutions ability to change the tone or shape of

cells by altering their internal water volume • The number of nonpenetrating solutes

– Isotonic –• Solutions with the same concentration of

nonpenetrating solutes on either side of the cell or the total solute concentration

• Solutes are equal inside and outside the cell • Cells keep their “normal” shape because there is no

net movement of water • Body fluids = isotonic

Passive process cont.• Osmosis cont. –

– Hypertonic –• Solutions with high concentrations of solutes • Cells loose water trying to equalize the solutes• Loosing water causes them to shrink or crenate

– Hypotonic –• Solutions that contain fewer solutes• The solution bathing the cell is more dilute than the

cell itself • Cells take on water as it tries to equalize the lower

number of solutes on the inside of the cell • Cells take on so much water they eventually burst or

lyse • Rehydrating – sports drinks, cola, and apple juice are

hypotonic increasing the amount of water cells absorb, causing rehydration

Tonicities

Passive processes cont.

• Filtration –– A pressure-driven process that forces water

and solutes through a membrane or capillary wall by fluid or hydrostatic pressure

– Passive processes – Involves a pressure gradient that pushes the

solute fluid (filtrate) from higher-pressure areas to lower-pressure areas

– Not a selective process – Anything that is small enough will be pushed,

by pressure, through the plasma membrane

Active processes

• Cells use energy, ATP, to move substances against their concentration gradient

• Active transport – – Uses solute pumps to move substances against

their concentration gradient – Energy is needed either directly or indirectly – Provide movement for substances who can’t

pass by diffusion – Ions such as Na+ and K+ need a protein

transport (solute pump) to move across the membrane against their concentration (moving them uphill)

Active processes cont.

• Active transport cont. –– Primary active transport –

• Energy provided directly by the hydrolysis of ATP• Causes the transport protein to change shape which

pumps the bound solute across the membrane • Sodium potassium pump • K+ higher inside the cell• Na+ higher outside the cell • They leak across the plasma membrane along their

concentration gradient – the pump is needed to keep K+ higher inside and Na+ higher outside

Active processes cont.

• Active transport cont. –– Secondary active transport –

• The driving forces of primary transport can indirectly drive the transport of other solutes

• As sodium is transported it drags other solutes with it – they are cotransported (sugars and amino acids)

• Even though solutes are cotransported which is passive – they would not be able to be transported w/o the energy required movement of sodium

Active transport

Active transport cont.

• Active transport cont –– Vesicular transport –

• Means by which large particles (macromolecules) & fluids are transport across the plasma membrane

• Energized by ATP • Two types –

– Endocytosis – Exocytosis

Active transport cont.

• Vesicular transport –– Exocytosis –

• “out of the cell”• Used to move substances from inside the

cell to the extracellular environment • Hormone secretion, neurotransmitting,

ejection of waste, mucus secretion • Substance to be secreted is enclosed in a

membranous sac called a vesicle

Exocytosis

Active transport cont.

• Endocytosis – – “into the cell”– Move substances into the cell using protein-

coated vesicles• Clathrin-coated• Non-clathrin-coated (Caveolae or coatomer)

– Moved into the cell by extensions of the plasma membrane

Active transport cont.

• Endocytosis cont –– 3 Types of Clathrin-coated vesicles

• Phagocytosis• Pinocytosis• Receptor-mediated endocytosis

– Phagocytosis –• “cell eating”

– Cytoplasmic extensions = pseudopods – protrude from cell and cover/contain large/solid material

• Formed vesicle = phagosome – Fuses with a lysosome to digest the contents

Active transport cont.

• Endocytosis cont –– Pinocytosis (fluid phase endocytosis)

• Infolding of the plasma membrane• Contains extracellular fluid w/ dissolved

molecules• Allows the cell to “sample” the contents of

the extracellular fluid– Important for cells that absorb nutrients

Active transport cont. • Receptor-mediated endocytosis –

– Most common method for specific uptake– Enzymes, insulin, hormones, and iron– Flu viruses utilize this method to attack our

cells– Receptors = membrane proteins that only

bind with certain substances – Create a vesicle = coated pit– Contents dissolved/utilized within the cell

Enodcytosis

Active transport cont.

• Non-clathrin-coated vesicles– Caveolae – inpocketings of plasma

membrane• Caveolin proteins• Capture specific molecules (folate, tetanus toxin)• Close association with lipid rafts, important for

cell signaling

– Coatomer (COP1 and COP2) proteins• Vesicular trafficking

Membrane potentials

• A membrane potential is a voltage across the cell membrane that occurs due to a separation of oppositely charged particles.

• In a resting stage a cell exhibits a resting membrane potential – the cell is polarized – the inside of the cell is more negative than the outside – Charge only exists at the membrane– Charges within in the cell are neutral

• Determined by…– Concentration gradient of potassium (K+) and sodium

(Na+)– More K+ within cells– More Na+ in the extracellular fluid – K+ diffuses out of the cell – cells is impermeable to Na+– Now more positive charges are outside the cell giving it

its negative charge

Cellular environmental interactions

• Membrane receptors –– Diverse groups of integral proteins and

glycoproteins that serve as binding sites – Function in:

• Contact signaling –– Touching of cells– Method for recognizing other cells– Bacteria and viruses utilize this method

• Electrical signaling –– Responding to the changes in voltage– Neural and muscle tissue

• Chemical signaling –– Job of most membrane receptors – Nervous & endocrine systems

Developmental aspects of cells

• Aging –– Due to wear-and-tear – Accumulation of free radicals – May be a result of autoimmune responses &

progressive weakening of the immune system• Apoptosis –

– Programmed cell death – Cell suicide – Cancer = cells fail to go through apoptosis – Strokes & heart attacks increase the rate of

apoptosis

• The rest of the material is not necessary for the test.

Components of the cell • The cytoplasm is the cellular material between

the cell membrane and the nucleus, and is the site of most cellular activity.

• Major elements of the cytoplasm –– Cytosol –

• Fluid in which the other cytoplasmic elements are suspended

• Largely water w/ salts, proteins, sugars & other solutes – Cytoplasmic organelles –

• Major components/workings of the cell – Cytoplasmic inclusions –

• Not a functional unit• Chemical substances • Ex: lipid droplet in a fat cell

Components of the cell

• Organelles –– Mitochondria –

• Sausage-shaped organelle • Powerhouse of the cell• Produce ATP• The more mitochondria – the more energy a

particular cell needs • Large quantities in liver and muscle cells

– Ribosome – • Consist of proteins & ribosomal RNA• Site of protein synthesis

Components of the cell cont. • Endoplasmic reticulum –

– Continuous with the nuclear membrane – “network within the cell”– Two types –

• Rough endoplasmic reticulum –– Contains ribosomes making it appear rough – Manufactures all proteins secreted from the cell– Produces components of the membranes – Liver & secretory cells

• Smooth endoplasmic reticulum –– Continuation of the RER – Lipid & cholesterol synthesis – Synthesis of steroid-based hormones (sex hormones) – Absorbtion, synthesis & transort of fats– Detoxification of drugs– Breakdown of glycogen to form free glucose

Components cont. • Golgi apparatus –

– Moves & directs cellular proteins– Modifies, concentrates, & packages proteins– Proteins bud off in vesicles from the ER & bind

with the golgi in order to be modified – Golgi vessicles are created allowing proteins to

be transferred to the plasma membrane in order to be excreted into the extracellular space

• Lysosomes –– Involved in digestion & phagocytosis (bacteria,

viruses, & toxins)– Degrading nonfunctional/worn-out organelles– Break down nonuseful tissues

Components cont. • Peroxisomes –

– Digestion of alcohol • Cytoskeleton –

– Cells skeleton – Supports cell structure & generates cell movements

• Centrosomes & centrioles –– Organizes mircrotubules – Arranged at right angles to each other– Organize mitotic spindles – Form base of cilia and flagella

• Cellular extensions –– Cilia –

• Occur in large numbers• Move together in order to move substances • Propels other substances

– Flagella –• Single• Whip like motion • Sperm• Propels itself

Nucleus • Control center of the cell• Nuclear envelope –

– Binds the nucleus – Double layer membrane – Continuous with rough ER

• Nuclear pores – – Regulates entry and exit of large particles

• Nucleoli – – Site of ribosome production– Contain DNA

• Chromatin – – DNA (genetic materials) & histone proteins

• Nucleosomes –– Eight histone proteins & DNA molecule

• Chromosomes – – Condensed chromatin – Utilized when cells are going to divide