Cells: The Living Unit

Cells are the structural unit of all living things 50 – 60 trillion cells in an adult human body Approximately 200 different types (with

different sizes, shapes, and functions) Cells range from 2µm (1/12,000 inch) to over 3

feet Cells have the same basic parts

(generalized/composite cell)◦ Nucleus—controls cell activity, centrally located◦ Cytoplasm—area containing necessary organelles◦ Plasma membrane—outer cell boundary

Overview of the Cellular Basis of life –

The Fluid Mosaic Model:◦ Double-layer (bilayer) of phospholipids with

interspersed proteins that are constantly changing

◦ polar, phosphate-containing head contacts HOH◦ non-polar, fatty acid tail avoids HOH◦ bilayer has tail in contact with heads outward◦ allows membrane to self-assemble into a closed

structure when forming/repairing◦ 10% of externally-facing phospholipids have sugar

groups attached (glycolipids)

The Plasma Membrane: Structure –

Branching sugar groups are on some external proteins that stick far out in the extracellular space (glycocalyx) and function as highly specific biological markers

cholesterol is present for stability integral (transmembrane) proteins span the entire

width of the membrane and help in transporting materials thru the membrane (either as channels or carriers)

peripheral proteins are not embedded in the membrane but attached to the ends of the integral proteins (usually enzymes or perform other mechanical functions)

Fluid Mosaic Model Continued

Microvilli – ◦ Minute, finger-like extensions of the plasma

membrane extending from an exposed cell surface

◦ increase the surface area of the membrane◦ often found on absorption cells (intestinal cells)

Specialization of the Plasma Membrane:

Membrane junctions – ◦ With the exception of blood cells (red and white)

and reproductive cells (sperm and egg) most other cells are bound closely together

◦ 2 factors help in binding: adhesive glycoprotein in the glycocalyx plasma membranes of adjacent cells fit together in

tongue-and-groove fashion

Specialization of the Plasma Membrane:

Membrane Transport:◦ Extracellular (interstitial) fluid is composed of

HOH, aa, sugars, fatty acids, vitamins, hormones, neurotransmitters, salts, and waste.

◦ Membrane is selectively (differentially) permeable (allows only some things to pass thru-keeps beneficial things inside & harmful things out)

The Plasma Membrane: Functions

Passive processes◦ Does not require cellular energy◦ Move from [high] to [low] (along/down their

[gradient])◦ Smaller molecules move faster◦ Warmer temperatures make faster reactions◦ Eventually the molecules will be evenly

distributed thru the whole environment and diffusion stops (equilibrium)

Plasma Membrane: Functions

Diffusion:◦ Diffusion thru a plasma membrane will only occur

if the molecule is lipid soluble and small enough to go thru the membrane

pores by itself (simple diffusion) nonpolar, lipid soluble molecules only O2, CO2, fats and alcohol small polar and charged particles can pass through

HOH-filled channels made by channel proteins

Plasma Membrane: Functions

Diffusion continued:◦ Molecules too big to go through on their own can

get help from carrier molecules (facilitated diffusion)

◦ not exactly sure how this happens◦ very selective (a carrier will only bind with a

specific substrate)◦ limited by the number of carriers present in the

membrane

Transport

Osmosis The movement of HOH is osmosis

◦ cannot move thru lipids bilayer but they can move thru pores

◦ if the [solute] is different on both sides, the [HOH] will be different as well

◦ (more [solute] – less [HOH])◦ (less [solute] – more [HOH])◦ (both move toward equilibrium)

Transport

The [total] of all solutes in a solution is the solution’s osmolarity

osmotic imbalances cause cells to shrink (loss of HOH) or swell (gain of HOH) until [solute] reaches equilibrium or the cells breaks

ability of a cell to alter internal HOH volume is tonicity

Osmosis Continued:

isotonic solutions have equal [solute] as cells – no net movement of HOH

hypertonic solutions have higher [solute] than cells – HOH moves out of the cell causing shrinkage (crenation)

hypertonic solutions have lower [solute] than cells – HOH moves into the cell causing swelling

distilled HOH is the ultimate hypotonic solution

Tonicity

HOH and solutes are forced through a membrane or capillary wall by fluid (hydrostatic) pressure

Depends on a pressure gradient that pushes solute-containing fluid (filtrate) from high pressure to low pressure

Not selective – only molecules too large to go thru are held back

Occurs in kidney – first step of urine formation

Filtration

Create a comic strip for the steps of kidney filtration. Put the steps in your words using vocabulary terms.

Activity

 Requires ATP to transport substances Molecules may be too large, unable to

dissolve in bilayer, or moving against a [gradient]

Active Processes

solute pumping:◦ moves aa and ions (Na+, K+, Ca++) against a

[gradient]◦ carrier protein uses ATP to change shape to carry

solutes

Vesicular (bulk) transport:◦ moving large particles/macromolecules thru

plasma membranes

Active Processes

◦ moving substances out of the cell◦ hormone secretion, mucus secretion,

neurotransmitter release, waste ejection◦ cell product is enclosed in a membranous sac

(vesicle) that migrates to the cell membrane, fuses with it, and ruptures into the interstitial fluid

Exocytosis

moving substances into a cell plasma membranes infolds, a vesicle forms,

pinches off the cell membrane, moves into the cytoplasm, and contents are used by the cell

3 types◦ Phagocytosis (“eating cell”) – intake of solid material

that fuses with a lysosome for digestion◦ Pinocytosis (“drinking cell”) – intake of liquid material

(important in nutrient absorption)◦ Receptor – mediated endocytosis is very selective –

plasma membrane proteins (receptors) bind only with certain substances

Endocytosis

Cellular material between plasma membrane and nucleus major functional area

Most cellular activities take place here 3 parts:

◦ Cytosol is the viscous, semitransparent fluid where the other elements are found including a variety of solutes (proteins, salts, sugars, etc.)

◦ Organelles are the metabolic machinery that have specific functions

◦ Inclusions are chemical substances present in some cells (stored nutrients, glycogen granules, lipid droplets, melanin, etc.)

Cytoplasm

Specialized cellular compartments with specific functions

Each organelle is surrounded by its own plasma membrane

Cytoplasm Organelles

Sausage-shaped Movable and change shape continuously Provide ATP for the cell Abundant in liver and muscle cells Contains DNA and RNA for self-replication

by fission (very similar to bacteria)

Mitochondria

Contain oxidase that uses O2 to detoxify toxic substances (alcohol and formaldehyde)

Destroy free radicals (highly reactive chemicals with unpaired electrons that can destroy

protein, lipid, and nucleic acid structures) self-replicating by fission

Peroxisomes

Small Made of proteins and ribosomal RNA 2 globular subunits that fit together Site of protein synthesis Some float in the cytoplasm

◦ free ribosomes◦ make soluble proteins

Some are attached to the rough endoplasmic reticulum◦ membrane-bound ribosomes◦ makes proteins for export or use in plasma membrane

Can switch between synthesis locations

Ribosomes

Extensive system of interconnected tubes and parallel membranes that make fluid-filled cavities (cisternae)

ER membranes are continuous with the nuclear membrane

2 types: Rough and smooth

Endoplasmic Reticulum

Surface is studded with ribosomes Proteins made are transported thru the

cisternae to be secreted Abundant in the following cells:

◦ secretory cells◦ antibody-producing cells◦ liver cells

Considered to be the cell’s “membrane factory”◦ makes integral proteins◦ phospholipids◦ cholesterol

Rough ER

Continuous of the rough ER Tubules in a branching network No role in protein synthesis Responsible for reactions involving lipid

metabolism and synthesis◦cholesterol◦ lipoproteins◦ steroid-based sex hormones◦ fat absorption/transport

Helps in the detoxification of drugs Important in storing/releasing Ca++ during

muscle contractions (called the sarcoplasmic reticulum)

Smooth ER

Flattened, stacked, membranous sacs Associated with numerous vesicles Transport vesicles budded off from the

rough ER fuse with the Golgi Modifies, concentrates, and packages the

proteins and membranes made in the rough ER

Some proteins are “tagged” for specific delivery, sorted, packaged, and shipped

Golgi Apparatus/body

Lysosomes – Spherical membranous vesicles Contains digestive enzymes Abundant in phagocytes Digest a wide variety of biological

molecules Works best in acidic conditions (pH 5)

Lysosomes

Digesting particles ingested by endocytosis (especially bacteria, viruses, toxins, etc.)

Degrading worn-out or nonfunctional organelles Breakdown of stored glycogen Breakdown of non-useful tissue

◦ embryonic digit web◦ uterine lining during menstruation◦ bone tissue to release Ca++

Autolysis happens when the lysosomal membrane breaks down and digests the cell (low O2, injury, high vitamin A)

Lysosomes

Series of rods going thru the cytoplasm Supports cellular structures Helps generate cell movement Not covered by a membrane 3 types:

Cytoskeleton

Microtubules◦ hollow tubes ◦ all originate near the nucleus in an area called the

centrosome◦ determines the overall shape of the cell and

distribution of organelles ◦ (some organelles are attached to microtubules

and are pulled thru the cytoplasm)◦ constantly grows out from the cell center,

breaking up, & reassembling

Cytoskeleton

Microfilaments◦ thin strands of contractile proteins ◦ dense cross-linked network in a unique

arrangement for each cell◦ responsible for cell motility or changes in cell

shape

Cytoskeleton

Intermediate filaments◦ tough, insoluble protein fibers that vary with cell

type◦ diameter intermediate between microtubules and

microfilament◦ most stable and permanent of cytoskeletal

elements◦ high tensile strength to resist pulling forces on the

cell◦ helps form chromosomes

Cytoskeleton

Whiplike extensions from the exposed surface of certain cells

Cilia moves substances in one direction across cells

Flagella are usually singular and used for movement

Cilia and Flagella

Control center of the cell Most cells have only one(except skeletal

muscle cells, some liver cells, and bone destruction cells which are multinucleate)(and red blood cells that are anucleate)

cannot reproduce and only live 3-4 months Largest organelle in the cell 3 distinct regions

Nucleus

Double layer membrane surrounding nucleus

Outer membrane is continuous with rough ER

At various points, the two membrane layers fuse and forms a nuclear pore

to allow for the import of proteins and export of messenger and ribosomal RNA

Nuclear Envelope

Spherical bodies within the nucleus Not membrane-bound Assembly site for ribosomal subunits (very

large and numerous in cells making tissue proteins)

Nucleoli

System of bumpy threads in the nucleoplasm

Made of DNA and histone (a globular protein)

During non-dividing times, the chromatin is extended and not easily seen

During cell division, the chromatin condenses to form visible Chromosomes

Chromatin

Cell Growth and Reproduction – The Cell Life Cycle:

Series of changes a cell goes thru from the time it is formed until it reproduces

2 major periods – interphase and mitotic phase

Cell Cycle

Period from cell formation to cell division Cell is carrying out all its routine activities Also prepares for cell division Divided into 3 subphases: G1, S, G2

Interphase

G1 (growth 1)◦ Cells are metabolically active ◦ Time length is variable◦ Cells that will not divide are in the G0 phase◦ Centrioles begin replication

G1

S (synthetic/synthesis) DNA replicates itself Must occur before cell can divide Replication must be exact to avoid

mutations Trigger is unknown Once replication starts it continues until it’s

finished

S

G2 (growth 2) Cell makes more organelles and membrane

to allow for division

G2

Some cells divide almost continuously (skin and intestinal lining)

Some cells divide more slowly (liver cells) Some cells do not divide at all (nervous,

skeletal, & cardiac muscle) The amount of nutrients a cell requires is

directly related to its volume Surface area does not increase proportionally

with volume and a growing cell will eventually “outgrow” its surface area

Mitotic Phase

Cell division creates daughter cells that have a favorable surface area: volume ration

Two events:◦ Mitosis (nuclear division)◦ Cytokinesis (cytoplasmic division)

Mitotic Phase

4 phases◦ P◦ M◦ A◦ T

Mitosis

Prophase – first and longest phase of mitosis◦ chromatin coils and condenses to form

chromosomes (each chromosome is two identical chromatin threads held together by a centromere)

◦ microtubules from the centriole lengthen and push the 2 centrioles apart (toward the poles)

◦ nuclear membrane disappears when centrioles are at the poles, some microtubules anchor the centrioles to the plasma membrane and other microtubules attach to the centromere of each chromosome

Phrophase

Metaphase – chromosomes cluster at the equator

Chromosomes align along the equator or center of the cell.

Spindle Fibers are attached to centromeres for each pair of sister chromatids.

Metaphase

Anaphase – shortest phase◦ microtubules begin to shorten◦ one chromatid of each chromosome is pulled to

its pole (chromatid is now called a chromosome)

Anaphase

Telophase – like prophase in reverse◦ identical set of chromosomes at each pole◦ chromosomes uncoil into chromatin◦ nuclear membrane & nucleoli reappear◦ until Cytokinesis, the cell is binucleate

Telophase

Plasma membrane over the center of the cell is drawn inward to form a cleavage furrow

Microfilaments make the furrow deepen Cytoplasmic mass is pinched into 2 parts Each resulting daughter cell has their own

plasma membrane, organelles, and nucleus

Cytokinesis

Cells become specialized very early in development

Cells in various regions of the embryo are exposed to different chemical signals that

trigger them into specific pathways of development

Development Aspects of Cells

As development continues cells release chemicals that influence the development of

neighboring cells by triggering processes that switch some of their genes off

Secret of cell specialization is the kinds of proteins made

Cell specialization therefore leads to structural variation

Development Aspects of Cells

sometimes attributed to cumulative effects of chemicals (both natural and man-made)

progressive disorders in the immune system results in a weakening and eventual failure of cell responses

cessation of mitosis and cell aging are pre-programmed into our genetic makeup (genetic theory)

Cell Aging Has 3 Theories