In Depth on... Bone Tissue!

First off, the study of bone structure and the treatment of disorders relating to bones is osteology. 
There are 206 bones in the human body!! I'll get into where they are a bit later...

Bones are good for support, protection, movement, mineral homeostasis (we mainly discussed Ca2+) and red blood cell production. 


Bones are classified as (and examples of each...):

1. long 
1. humerus
2. radius
3. ulnar
4. metacarpals
5. femur
6. tibia
7. fibula
8. metatarsals
9. clavicle
2. short
1. carpals
2. tarsals
3. patella*
4. calcaneus
3. irregular
1. vertebra
2. illium
3. ischium
4. pubis
5. bones of pelvic girdle
4. flat
1. skull
2. sternum
3. scapula
4. ribs
5. sesamoid*
1. patella

Next we talked about the structure of long bones.  Long bones have a compact layer of bone that surrounds some spongy bone.  
The ends of a long bone are called the epiphyses.  On the superficial layer of the epiphyses,   Next come the metaphyses.  The metaphyses includes the epiphyseal plates.  Between the two ends of epiphyses and metaphyses is the diaphysis.  
The membranes of the long bone include the endostuem and the periosteum.  The endostuem (endo-in) is the lining of the marrow cavities.  The periosteum (perio-outer) is a tough membrane covering only the bone, not any cartilage and is composed of two layers.  The two layers of the periosteum are the outer fibrous layer and the inner osteogenic layer.  The outer fibrous layer is composed of dense irregular tissue.  The inner osteogenic layer is bone cells and blood vessels that nourish or help with repairs. 


The matrix of the bone is abundant with mineral salts.  The primary one is calcium phosphate (plus some other stuff, aka hydroxyapitite).  The salts are deposited in the membrane, then they crystalize and harden.  This process is known as crystalization or mineralization.  Mature bones contain much more matrix than immature bone.  


Osteogenic cells develop into osteoblasts which form the matrix that is made of osteocytes.  'blasts get stuck in the matrix and become 'cytes'  Osteoclasts are the cells that reabsorb the old bones/break down old bone matrix.  


There are two types of bone: compact and spongy.
Compact bone is really, really organized and is composed of tubular osteons (more on those in a second).  Osteocytes lie in the lacunae (tiny chambers arranged in concentric rings around a central [Hasverian] canal).  Compact bone lies over spongy bone and composes most of the bone tissue of the diaphysis.  It protects, supports and resists stress.   An osteon is a concentric ring (lamellae) of calcified matrix surrounding a vertically oriented blood vessels.  Osteocytes are found in spaces called lacunae.  Osteocytes communicate through canaliculi filled with extracellular fluid that connect one cell to the next.  Interstitial lamellae are the older osteons that have been partially removed by osteoclasts during tissue remodeling (a little more detail a bit later..).  


Spongy bone is not made of osteons.  It is not hard and solid like compact bone.  Instead, spongy bone consists of trabeculae that surrounds many red marrow filled spaces.  It forms most of the structure of short, flat and irregular bones as well as the  epiphyses of long bones.  Spongy bone is very light and supports and protects the red bone marrow.  


Blood and Nerve supply of bones:
Periosteal (perio-outer) arteries supply the periosteum.  Nutrient arteries supply the diaphyis of the long bone  and red marrow.  Metaphyseal and epiphyseal arteries supply the red marrow and spongy bone of the epiphysis.  Nerves follow blood supply into the bone tissue and periosteum where they sense damage and transmit pain messages.  


To grow in length: cartilage grows on the epiphyseal side of the epiphyseal plate.  Then osteoclasts break down the cartilage on the diaphyseal side and eventually the plate closes and is completely bone. 
To grow in width, periosteal cells differentiate into osteoblasts and secrete collagen and matrix to form bony ridges and then a tunnel around periosteal blood vessel.  Concentric lamellae fill in the tunnel to form an osteon. 


Factors that affect bone growth:

1. the parathyroid horomone and calcitonin regulate calcium salt storage
2. growth hormone (from the pituitary gland) stimulates the liver to releast growth factors that stimulate the growth plate.  
3. thyroxin makes sure growth hormone stimulates growth in proper proportions (deficiencies delay bone growth)
4. steroids (testosterone and estrogen) stimulate growth spurts in puberty by stimulating growth plates (cartilage cells)
5. Vitamins A, C, D deficiencies make bones weak and develop slower.  

 Bone Remodeling: 
remodeling is the ongoing replacement of old osteons/interstitial lamellae.  Old bone is constantly destroyed by osteoclasts and new bone is built by osteoblasts.  Orthodontics uses this practice by forcing the deep teeth cells to be broken down and moved and rebuilt in the right spot.  


Bone fractures:
A fracture is any break in the bone.  
Open fractures break the bone and go through the skin.
Simple fractures break the bone but not the skin.
Comminuted fracture is when the bone splinters at site of impact
Greenstick fractures only occur in children.  One side of the bone breaks and the other just bends.  
Impacted fracture occurs when one end of the break is pushed up into the other.  
Stress fractures are many microfissures in the bone.  


The bone healing process occurs in steps.  The first step is bleeding.  Then a clot forms.  Fibroblasts then secrete cartilage and collagen fibers that form a callus.  Osteoblasts transform cartilage into new bone.  


Calcium homeostasis is very important for the body because calcium is involved in a lot of systems.  Bone is the major reservoir of Calcium.  Bone will take in calcium when blood levels are too high.  
when we wake up our blood levels of calcium are low, so we release parathyroid hormone to get some more calcium out of our bones.  then we eat which should bring calcium levels above the norm, so the PTH slows down it's releasing.  Because levels are now too high, calcitonin is released from the thyroid and sends calcium back into the bone.   Calcium ions help nerve and muscle cells function, they aid in blood clotting and in enzyme functions for biochemical reactions.  Small changes in calcium levels can be deadly.  


Exercise and Bone Tissue:
Bones get stronger in response to mechanical stress by increasing deposition of mineral salts and production of collagen fibers.  If you don't have any mechanical stressor, then bones weaken through demineralization and collagen reduction.  This often occurs while being in a cast, to astronauts in outer space and people who are bedridden.  Weight-bearing activities like walking or moderate weight-lifting can help build and retain bone mass.  


Osteoporosis is more common in women.  The rate of bone formation/deposition increases until age 20-30.  Then there is an equal rate of deposition and resorbtion until age 40-50.  After that, resorbtion rates exceed deposition rates.  This causes the total amount of bone mass to decrease.  Men lose approximately 25% while women lose 35% Estrogen therapy can help women slow down the onset of osteoporosis

integumentary system

The integumentary system is composed of the skin, hair, nails, oil and sweat glands. The skin is the largest organ of the body. It can be about 22 pounds and has a very large surface area. Dermatology is the medical specialty for diagnosis of problems. The epidermis is the superficial layer. The dermis is deep to the epidermis. The hypodermis, while not a part of this skin is still discussed with the integumentary system.


Functions of the integumentary system include:

Body temperature (sweat)
Protection (physical barrier)
Sensation (somatic receptors)
Excretion (salts)
Synthesis of Vitamin D (vision)
Immunity (secrete oils that inhibit bacterial growth) Ø
Blood reservoir (10% of total blood flow) Ø .


The epidermis consists of 5 layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum and stratum corneum.

1. 1. Stratum Corneum
1. 20-30 layers
2. Dead cells w/ keratin
2. 2. Stratum Lucidum
1. Clear, lots of keratin
2. ONLY ON fingertips, palms, soles
3. Stratum Granuolsum
1. Granules - Water repellant
2. Cells get flatter
4. Stratum Spinosum
1. Strength & Flexibility
5. Stratum Basale
1. Corrugated border
6. Stem cells become all other cells of Epidermis

The dermis is composed of two regions: papillary region which is composed of areolar tissue and the reticular region which is composed of dense irregular connective tissue.  The dermis is a layer of connective tissue composed of collagen and elastic fibers (give skin toughness and elasticity, respectively), hair follicles, glands and nerves.  It is highly vascular.

The superficial region of the dermis is the papillary region. It has finger-like projections called dermal papillae.  It also has pain and touch receptors.  These are called free nerve endings and Meissner corpuscles.

The deep region of the dermis, reticular layer, makes up 80% of the dermis. This layer has blood vessels, sweat glands (a watery secretion), sebacious glands (secrete sebrum/oil) and Pacinian corpuscles. 

Different skin colors happen for different reasons.
Jaundice makes the skin a yellowish color.  It occurs when there is a buildup of bilirubin in the bloor because of a liver disease.
Cyanosis occurs when there isn't enough oxygen in the blood.  It makes the skin turn a bluish color
Erythema is a redness of the skin due to enlargement of capillaries in the dermis.  This occurs during inflammation or infection.
Vitiligo  is the partial loss of melanocytes in patches (supposedly the reason Michael Jackson bleach his skin was to hide the vitiligo.)
Freckles and age spots are the accumulation of melanin in patches.
Moles are a benign raised collection of melanocytes.
Albinism is the lack of ability to produce melanin.
Tan is the stimulus of melanocytes from UV radiation to produce more melanin

Skin Cancer
UV radiation breaks DNA, inflammation makes free radicals that promote mutations and blocks cellular response to DNA damage.
Basal Cell carcinoma is most common in fair-skinned white people with light eyes.  It is most common on the head and neck regions and invades the stratum basale into the dermis and hypodermis.
Squamous cell carcinoma is most commonly found in outdoor workers.  It is found in the stratum spinosum and metastasizes to lymph nodes.  It is also found in the head and neck regions.

Malignant melanoma is caused by severe sun exposure with blistering.  It is cancer of the melanocyte.  It metastasizes to lymphnodes and blood vessels.  It may appear as a mole that is asymmetrical, that has irregular borders, color variegation, a diameter greater than 6 mm and it changes over time.

Oncogenes and Tumor-Suppressor Genes
Oncogenes are genes that normally direct protein synthesis and cell growth.  In cancer, this cell is turned on permanently.
Tumor-Suppressor genes normally encode the protein that stops cell proliferation.  It might also be called an anti-oncogene.  In cancer, these cells don't turn on.

A piece of telomere is lost each time a cell divides.  Eventually, when enough of the telomere has been lost, the cell will no longer divide.  Cancerous cells have an enzyme that repairs the telomeres and makes the cell immortal.

Bed sores occur from prolonged pressure on the body.  It kills both the dermis and the epidermis.

Homeostatic imbalances of the skin include athlete's foot (a fungus), boils (inflammation of hair follicles) and contact dermatitis (an allergic reactions).

There are three different types of burns.
1st degree burn only burns the epidermis.  This makes the skin red and swollen.
2nd degree burn burns the epidermis and part of the dermis.  When this occurs, fluid filled blisters occur and separate the two layers of skin.
3rd degree burns destroy the dermis, epidermis and the epidermal derivatives.  The damaged area may become numb due to the loss of nerves.
The head is approximately 9%.  The trunk is 36% (18% for each side).  Each side of each arm is 4.5%.  The legs are each 18%

Appendages of the skin include
-hair and follicles
Hairs may also be referred to as pili and are present on all skin surfaces except our palms, and digits and soles.
The function is to shield the eyes and filter for the respiratory tract.
The hair follicle is the site of hair production with the skin.  Hair is composed of hard keritinized epithelial cells.  Melanocytes, like in skin coloring, also give hair its color.
The arrector muscle is a smooth muscle (involuntary) that is attached to the hair follicle.  It is the reason we get goose bumps.  
Other components include the root (the part of hair within the follicle), the shaft (the part that sticks out of the skin) and the bulb (where hair grows).
Hair growth occurs in cycles.  It has a growth stage and a resting stage.  The growth stage lasts for 2-6 years and the matrix cells at the base of hair root produce the length.  The resting stage lasts for 3 months and the matrix cells are inactive and follicle atrophies.  Old hair falls out as growth stage begins.  Normal hair loss is 70-100 hairs a day.
-nail
Nails are scale like modifications of the epidermis.  They consist of tightly packed keretinized cells.  The nail body is the part that is visible.  The nail root is buried under the skin.  The lunula is the white area on the nail and is caused by a thickened stratum basale.  The eponychium (cuticle) is a stratum corneum layer.
-exocrine glands
--sweat glands
--sebaceous glands

Epidermal wound healing occurs when we get a minor cut or burn.  The basal cells will migrate across the wound.  Contact inhibition will stop the migration.  The epidermal growth factor simulates basal cells to divide and replace the ones that have moved into the wound.  The thickness is a result of mitosis.

Deep wound healing has three stages.  The first stage, inflammatory stage, is when the blood clot unites the wound edges.  Fibroblasts secrete collagen matrix as a frame work for the epithelial cells.  Vasodilation and increased permeability of blood vessels deliver phagocytes.  The second stage, migratory stage, is when the epithelial cells bridge across the wound under the scab.  Fibroblasts begin scar tissue and damaged blood cells begin to grow.  During this phase the wound is called granulation tissue.  Maturation, the third stage, occurs when the scab falls off.  The epidermis has returned to normal thickness and the collagen fibers return to a normal state.  The fibroblasts being to disappear and the blood vessels return to normal.

Age and skin
Age mostly affects the dermis.  Collagen fiber, fibroblast, Langerhans cell and melanocyte numbers will decrease, elastic fibers will become less elastic, and phagocytes will be less effective.  Wrinkling, slower growth of hair and nails are also a part of aging.  Sebaceous glands are atrophic.  Walls of the blood vessels thicken so there is less nutrients available.  This causes people to get thinner.
Anti-aging treatments:
microdermabrasion, chemical peel, laser resurfacing, dermal fillers, Botuliism toxin injection and non-surgical facelifts
-sun screens and sun blocks help minimize photo damage from UV exposure

Cell Junctions

Most epithelial cells are tightly bound together.  Some muscle and nerve cells can be found joined together.   They perform together when joined as functional units.  There are five different types of cell junctions.

1. Tight junctions: the plasma membrane proteins are fused together. 
1. these junctions look as if someone took a sewing machine and sewed them together.
2. it looks like a web of transmembrane proteins.  
3. they tend to seal off passageways between adjacent cells
1. commonly found in the epithelial tissue of the stomach, urinary bladder and intestines.
2. They are used to slow down the passage of substances between cells and leaking into the blood or other tissues.
2. Adherons: proteins of the integral membrane are connected to the membranes of another cell.  
3. Desmosomes: attach to the intermediate filaments inside the cytoplasm.  
1. These junctions contain plaque and cadherins that extend into the intercellular space to attach adjacent cells together.  
1. The plaque attaches to intermediate filaments that contain keratin (a protein)
2. this prevents the epidermal cells from separating under stress.
4. Hemidesmosomes: only connect to extracellular material
5. Gap: The plasma membrane channels fuse together.  
1. Gap Junctions connect neighboring cells via tiny fluid-filled tunnels called connexons. 
1. Plasma membranes of gap junctions are separated by a very narrow intercellular gap
1. this allows for communication of cells within a tissue
2. ions, nutrients, waste, chemical and electrical signals travel through the connexons from one cell to another. 

(for the test, remember the ones with more information)

Tissues... Part II

Connective tissue is everywhere! It can be found in all parts of the body or as a part of various organs!  The primary function is for protection, support and binding different tissues together.
 For the most part, connective tissues are highly vascular.  The exceptions include tendons and ligaments.  Connective tissues are composed of many, many different cells.  Between the cells of connective tissue, there is a large amount of noncellular, nonliving material.

Before getting into the different types of connective tissue, I'm going to explain the extracellular matrix.  The matrix is produced by the cells around it and then extruded.  There are two components of the extracellular matrix: ground substance and fibers.  The ground substance is made of interstitial fluid, cell adhesion proteins and proteoglycans.  The ground substance, depending on its location and function, can be in any state of matter.  The ground substance functions as a medium for nutrients and things to diffuse between the blood capillaries and the cells. The fibers provide support and come in different forms: collagen, elastic or reticular fibers.  Fibers make diffusion a little more difficult because there is less pliability.

Areolar (connective tissue proper: loose connective tissue)
description: Areolar tissue has a gel-like matrix.  All three fiber types are present: fibroblasts, macrophages and some red blood cells.
function: This tissue wraps and cushions organs  The macrophages phagocytize bacteria.  This tissue plays an important role in inflammation and conveys tissue fluid.
location:  It is most commonly found underneath the epithelia, because forms the lamina propria of mucous membranes, packages organs and surrounds our capillaries.

Adipose (connective tissue proper; loose connective tissue)
description: This tissue is in a matrix form similar to areolar tissue, but it is very sparse.  These fat cells have pushed the nucleus to the outer edge of the cell.
function: Adipose tissue provides reserve fuel.  It insulates against heat loss and supports and protects our organs.
location: We can find adipose tissue under the skin, around our kidneys and eye balls, within our abdomen and in breasts.

Reticular (connective tissue proper, loose connective tissue)
description: The reticular tissue is full of reticular fibers! They are in a typically loose ground substance.
function: The reticular fibers form a soft internal skeleton that supports other cell types.
location: You'll find reticular tissue in the lymphoid organs.

Dense Regular (connective tissue proper, dense connective tissue)
description: This tissue is full of parallel collagen fibers.  There are a few elastin fibers.  The major cell type is the fibroblast.
function: this tissue, dense regular, attaches muscles to bones or vice versa.  It is very, very strong and withstands a great tensile strength when a force is applied in one direction.
location: You'll find this tissue making up tendons and ligaments and aponeuroses.  (remember tendons and ligaments are mostly avascular.)


Dense Irregular (connective tissue proper, dense connective tissue)

description: This irregular tissue is also mostly collagen fibers.  There are some elastic fibers.  The major cell is the fibroblast.  

function: The dense irregular tissue can withstand tension in many directions.  It also provides structural strength.

location: This tissue is found as the dermis of the skin.  It is also the submucosa of the digestive tract and the fibrous capsules of the organs and joints.  

Hyaline (cartilage)

description: Hyaline cartilage is amorphous but firm matrix.  The matrix, made from collagen fibers, in imperceptible.  

function: This cartilage supports and reinforces.  There are some built in cushioning properties.  It also resists compressive stress.

location: Most of the embryonic skeleton is made from hyaline cartilage.  In mature humans, it will be the covering of long bones in joint cavities, form the costal cartilages of the ribs, and will also be in the nose, trachea and larynx.  

Elastic cartilage 

description: this is an elastic version of the hyaline cartilage, the major difference is that there are more elastic fibers here.  

function: this cartilage maintains the shape of a structure, but allows for great flexibility. 

location: It's in the ear and the epiglottis.  

Fibrocartilage

description: This cartilage is a less firm form of hyaline cartilage.  It's thick collagen fibers are predominate.  

function: They are able to provide tensile strength and absorb compressive shock.

location: The fibrocartilage is located in the intervertebral discs, the pubic symphysis and the discs of the knee joint.  

Bone (osseous tissue)

description: Bones are made of hard calcified matrix containing many collagen fibers.  Osteocytes lie in lacunae.  Bones are very vascularized. 

function: Bones support and protect. They provide levers for muscles to act on.  They store calcium and other minerals and fat.  Marrow inside the bone is for blood cell formation.  

location: Bones!  

Blood 

description: Red and white blood cells are in a fluid matrix.  

function: Blood transports oxygen and carbon dioxide, nutrients, wastes and some other stuff.  

location:  Blood is typically contained within blood vessels.  

Nervous Tissue is composed of two major cell populations.  They are the neuroglia and neurons.  The neuroglia are the special supporting cells that protect, support and insulate the delicate neurons.  Neurons are highly specialized cells.  They receive stimuli and then conduct impulses to all parts of the body.  They are located in the brain, spinal cord and nerves.  

Tissues: part I

Holy freakin cow there are a LOT of different tissues. Good thing I'm here to tell you all about them :)
First off, I'll have you know, that a tissue is a group of cells that have a common embryonic origin.  Their may be different types of cells that all come together to perform a common function.  They can be solid, like bones; semisolid, like fats; or liquid, like blood.  Tissue cells are separated by nonliving, intercellular materials they secrete. Tissues, as I mentioned already, have a common origin from the embryonic cell which has three layers, endoderm, mesoderm and ectoderm. 

There are four main types of tissues: epithelial, connective, nervous and muscular.  Each of these four tissues has many derivatives, each with their own function, shape and arrangement. 

Epithelium tissue's origin is from all three embryonic layers.  It is the covering and lining tissue of all outer openings and areas exposed to the environment.  Glandular (it has glands) epithelium secrets pieces of the gland.  These glands include thyroid, adrenal and sweat glands.  There are five characteristics that make epithelial tissue its very own.  1. Polarity - there is always one side that isn't touching anything else. 2. Cellularity and specialized contacts - each has it's own function. 3. Supported by connective tissue - it's basically glued to the body by the connective tissue. 4. It's avascular - there aren't any blood vessels in the epithelium. 5. Regeneration - since it lines like every cavity and ALL of our skin, it's constantly being sloshed off, which means it must go through mitosis, like always. 
Epithelial cells are classified in two ways and always have both in their name.
The first way is by shape. They can be:
when identifying shape, one must look at the most superficial layer.  some layers may look like another shape when it is deeper.  

• squamous (squished) 
• cuboidal (cube-like)
• columnar (columns)
• transitional (they really don't have a standard shape)

The next way they can be classified is by arrangement:

• simple (only one layer)
• stratified (many organized layers)
• psudostratified (looks like many layers, but in reality it's not)

now, to put each of the shapes and arrangements together and to identify their function and location...

the simple layers...
simple squamous epithelium
description: this layer, the simple squamous, is a single layer of flattened cells with disk shaped nuclei in the center of the cell.
function: this layer is really great because it is what allows nutrients to flow easily by methods of diffusion and filtration when protection doesn't matter.  This layer also secretes lubricating substances in serosae (a smooth membrane consisting of a thin layer of cells which secrete serous fluid). 
location: parts of the body that don't need much protection include the kidneys, the air sacs in our lungs, the lining of our hearts, blood vessels and lymphatic vessels. as well as the lining of the ventral body cavity. 
simple cuboidal epithelium
description: the simple cuboidal cells are cubelike that are in a single layer.  they have a central nucleus that is large and spherical.
function: the only function of the simple cuboidal cells are secretion and absorbtion
location: they are found in the kidney tubules; the ducts and secretory portions of small glands and the lovely ovary surface.
simple columnar epithelium
description:  This interesting layer is made up of a single layer of column shaped cells.  they may or may not be ciliated. this layer might also contain goblet cells (the ones that secrete mucus)
function: The simple columnar epithelium are good for absorption, secretion of mucus, enzymes and other stuff.
location: The ones that are not ciliated, line the digestive tract all the way from the stomach to the anal canal, the gall bladder and the excretory ducts of some glands.  The ciliated kind line small bronchi, the uterine tubes and some regions of the uterus.

 the stratified layers...
stratified squamous epithelium
description: this is my favorite type of tissue.  The stratified squamous epithelium is a really thick membrane composed of lots of layers.  The basal layers look like cuboidal cells (because they are active and working on mitosis) but when they die and head toward the surface, that's when we identify them and say that they are squamous. 
function: The function of the stratified squamous epithelium is to protect the underlying tissues that could get hurt.  The top ones just fall off it we scratch our arm or drink something or go poo, but because we have living ones underneath that constantly undergo mitosis, we're still alive and kickin'.
location: the nonkeretinized type lines the mouth, esophogus and the vagina.  The keretinized type is the outermost layer of our skin! by the way, just remember that in this layer, there aren't any blood vessels, so if you bleed, then you've scraped farther than this layer - kinda cool right?

stratified cuboidal epithelium 
description: this cube shaped epithelium usually appears in two layers. 
function: it has one and only function: protection.
location: It is present in the largest ducts of sweat glands, mammary glands and salivary glands. 
stratified columnar epithelium
description: the stratified columnar epithelium is usually several layers thick, and like the stratified squamous tissue, the stratified columnar tissue is also cuboidal at the basal layers. 
function: Just two functions here: protection and secretion.
location: this tissue is rather rare. it is only found in the male urethra, and maybe a few large ducts. 

the random layers...
pseudostratified columnar epithelium
description: This "random layer" really does belong with the single layered cells, but I felt it deserved to go somewhere else.  The cells differ in height.  Some of them don't even reach the surface.  Their nuclei are not similar.  The cells may contain goblet cells and cilia. 
function: The pseudostratified columnar epithelium are good for secretion, and propulsion of that mucus. 
location:  If they are lame (because they are the unciliated kind), they're probably found in the sperm-carrying ducts of the guys in our society.  If they're pretty cool (they have cilia) you'll be able to find them in the trachea, as well as most of the upper respiratory tract. 
transitional epithelium 
description: this tissue looks like both squamous and cuboidal tissues.  some of the basal cells might look cuboidal or columnar.  the surface cells might be dome shaped or squamous shaped. 
function: this tissues allows organs to stretch
location: this tissue lines the ureters, the bladder and part of the urethra.

Movement through the membranes.

The cell membrane is selectively permeable.  Molecules that need/want into the cell have to use either an active or passive process.  Usually molecules will follow the [gradient], meaning that they go from a higher concentration to a lower concentration.  Sometimes, a cell needs to get something out (Na+) and will push it against the gradient.  These processes help maintain homeostasis.

Passive Processes - we discussed four of them.  These processes do not require ATP (aka energy) to do.

Simple Diffusion occurs when a small noncharged lipid soluble molecule wants to cross the membrane.  Movement always occurs down the [gradient] until equilibrium is met. 
Facilitated diffusion requires a protein to help it get across the plasma membrane.  This process allows larger molecules to get to the other side. Amino acids, glucose, K⁺, Cl^- are some of the molecules that use facilitated diffusion.  This process also occurs down the [gradient].
Osmosis is simple diffusion for water.  Water moves from a high concentration to a low concentration.  Tonicity is the ability of a solution to exert an osmotic pressure upon a membrane.  When a cell gains water (high concentration of solute), the cell is in a hypotonic state.  When a cell loses water (high conctration of water), it is because it is in a hypertonic state and there is a hypotonic state outside of the cell.  Osmotic pressure is the force exerted on a membrane due to the osmotic movement of water.  
Filtration only occurs in single layers of cells.  Examples include blood pressure (aka hydrostatic pressure) and molecules leaving the blood. 

Active Processes require the use of ATP. They often move substances against their [gradient].  Sometimes they are called pumps or revolving doors.  40% of ATP is used on active transport.
Endocytosis is when stuff leaves the cell
Exocytosis is when stuff enters the cell. 

Meiosis and Apoptosis

Meiosis creates gametes.  It starts with 1 diploid cell and divides into two haploid cells (meiosis I). these two haploid cells then divide into 4 gamete cells (meiosis II). Each phase of meiosis follows the order of the stages of mitosis (PMAT) but exactly what each stage does varies slightly.  This will be covered in more depth during the reproductive system.

Apoptosis is the programmed death of a cell. It occurs in a cell that does not divide or differentiate.  Apoptosis is a protective mechanism as well as a continuous process.  The process begins with a receptor that turns on capsases.  Typically takes less than an hour.