Plant cell walls are mainly composed of
a. dna.
b. chitin.
c. cellulose.
d. microtubules.

Which of the following is an effect of the uneven heating of the earth by the sun? a sea breeze a land breeze a convection current all of the above

Tropical rain forests support more species per unit area than any other terrestrial ecosystem. what is one way rain forests are important to the health of the biosphere?

Fill in the blank 1. digestion occurs in the small intestine through the action of enzymes. 2. urea, excess water, and other waste materials are eliminated in a water fluid called 3. can cause infections by injecting dna or rna into hosts. 4. the human immune system produces in response to a vaccine, which later can bind to and destroy a pathogen if it invades. 5. are structures that link bone to bone at a joint 6. in the heart, blood flows from the right atrium to the right ventricle, where it is pumped to the the words i can use are: lymphocytes gliding pivot gas urine absorption dermis fulcrum lungs chemical viruses capillary vertebrae esophagus tendons antibodies synapses ligaments kidneys pathogens