Unicellular Organism – consists of only one cell - that one cell carries out all functions required to maintain the life of the organism Multicellular Organism – consists of more than one cell - Cells are specialized to perform certain functions (cell specialization/differentiation)

 Cells in a multi- cellular organism become specialized by turning different genes on and off  This is known as DIFFERENTIATION

1. Neuron (nerve cell) -sends info from one part of the body to another - very long and thin

2. Red Blood Cells -carry oxygen throughout body -flattened disks so that they can easily flow through blood vessels

3. Sperm Cell -fertilizes egg -strong tail for quick swimming, distinct head for entering egg cell

4. Muscle cell  Muscle cells contain protein filaments of actin and myosin that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce force and motion.  maintaining and changing posture, locomotion, as well as movement of internal organs, such as the contraction of the heart and the movement of food through the digestive system via peristalsis.

5. Rods Rods handle vision in low light. Detects Black and White

6. Cones Handle color vision and detail. Detect Color

7. Brain Cells The structures within the brain are made up of about 100 billion neurons. Neurons relay messages about what you're thinking, feeling, or doing. Support cells called glia. Neurons can not function without the glia cells

8. Skin Cells The outer layer of your skin that you see and feel and with which you sense the world is called the epidermis. Within the epidermis are layers of four different kinds of skin cells: keratinocytes, melanocytes, Merkel cells, and Langerhans cells.. Function is to protect and cool the body

Cell differentiation is a result of different gene expression.

 Most cells in your body have the same DNA  Cells use only the DNA that they need; the rest is inactive  For example, skin cells use the DNA that codes for melanin (a skin pigment that protects from UV light)  Heart cells do not need melanin, so it is “turned off” in heart cells

 stem cells – cells of humans that can develop into different cell types

 2 kinds:embryonic and adult  some researchers believe that they can be a source of replacement cells to treat disorders (ex. Parkinson’s disease and burn injuries) Embryonic stem cells Adult stem cells

- Cells communicate with one another by secreting and receiving chemicals -Chemicals released by one cell can influence the activity of another cell

-Sensory cells to nerve cells to muscle cells

Hormone – communication chemicals that travel through the blood (ex. insulin, estrogen) -Secreted by endocrine glands

- Acts much slower than nerve impulses but effects last much longer - Only influence the action of target cells that have specific receptors

1.Negative Feedback – the end product stops the production of the hormone

2. Positive Feedback – the last step triggers more release of the initial hormone or other hormones - triggers a series of events