Suggestions
Use up and down arrows to review and enter to select.Please wait while we process your payment
If you don't see it, please check your spam folder. Sometimes it can end up there.
If you don't see it, please check your spam folder. Sometimes it can end up there.
Please wait while we process your payment
By signing up you agree to our terms and privacy policy.
Don’t have an account? Subscribe now
Create Your Account
Sign up for your FREE 7-day trial
Already have an account? Log in
Your Email
Choose Your Plan
Individual
Group Discount
Save over 50% with a SparkNotes PLUS Annual Plan!
Purchasing SparkNotes PLUS for a group?
Get Annual Plans at a discount when you buy 2 or more!
Price
$24.99 $18.74 /subscription + tax
Subtotal $37.48 + tax
Save 25% on 2-49 accounts
Save 30% on 50-99 accounts
Your Plan
Payment Details
Payment Summary
SparkNotes Plus
You'll be billed after your free trial ends.
7-Day Free Trial
Not Applicable
Renews April 4, 2024 March 28, 2024
Discounts (applied to next billing)
DUE NOW
US $0.00
SNPLUSROCKS20 | 20% Discount
This is not a valid promo code.
Discount Code (one code per order)
SparkNotes PLUS Annual Plan - Group Discount
Qty: 00
SparkNotes Plus subscription is $4.99/month or $24.99/year as selected above. The free trial period is the first 7 days of your subscription. TO CANCEL YOUR SUBSCRIPTION AND AVOID BEING CHARGED, YOU MUST CANCEL BEFORE THE END OF THE FREE TRIAL PERIOD. You may cancel your subscription on your Subscription and Billing page or contact Customer Support at custserv@bn.com. Your subscription will continue automatically once the free trial period is over. Free trial is available to new customers only.
Choose Your Plan
For the next 7 days, you'll have access to awesome PLUS stuff like AP English test prep, No Fear Shakespeare translations and audio, a note-taking tool, personalized dashboard, & much more!
You’ve successfully purchased a group discount. Your group members can use the joining link below to redeem their group membership. You'll also receive an email with the link.
Members will be prompted to log in or create an account to redeem their group membership.
Thanks for creating a SparkNotes account! Continue to start your free trial.
Please wait while we process your payment
Your PLUS subscription has expired
Please wait while we process your payment
Please wait while we process your payment
Vitamin A is a term used to describe a family of essential, fat-soluble compounds structurally related to, and sharing the biological activity of, the lipid alcohol retinol. Vitamin A includes provitamin A carotenoids that are dietary precursers of retinol.
Vitamin A is vital to normal vision. In the retina, vitamin A is responsible for the transduction of light into neural signals. Vitamin A has an important role in cellular differentiation, responsible for the integrity of epithelial tissues. Vitamin A is involved in embryonic development. It is thought that vitamin A has a role in immune function, both cell-mediated immunity and humoral immunity. Vitamin A deficiency is associated with a decrease in resistance to infection. Interventions with vitamin A have demonstrated an association with decreased severity of measles and diarrhea.
Most of dietary preformed vitamin A is absorbed into the intestine, and absorption remains high with increased intake. Vitamin A is packaged along with lipids into chylomicrons for transport through the lymph and plasma to the liver. The liver is the principal storage site of vitamin A and the majority of retinol oxidation and catabolism takes place here. Too little dietary fat and factors that interfere with lipolysis or emulsification may decrease absorption of vitamin A.
A deficiency of vitamin A leads to epithelial keratinization, appetite changes resulting in poor growth, and xerophthalmia. Each year approximately 3 to 10 million children in developing countries become xerophthalmic and 250,000 to 500,000 become blind.
Overconsumption of vitamin A by ten times the RDA results in a condition called hypervitaminosis A. Symptoms include headache, vomiting, diplopia, alopecia, dry mucous membranes, desquamation, bone and joint pain, liver damage, hemorrhage, and coma. Too much dietary vitamin A may be teratogenic. Spontaneous abortions and birth defects have occurred with the consumption of the 13-cis RA form of vitamin A, such as found in the acne drug Accutane.
The units of measure for vitamin A are micrograms retinol equivalents (mcg RE). This is equivalent to 1 mcg of retinol. International unit (IU) is used as a unit of measure also. One IU equals 0.3 mcg retinol. The Recommended Dietary Allowances (RDA) set by the National Academy of Sciences are as follows: 375 mcg RE for infants, 400-700 mcg for children, 1000 mcg for adult males, 800 mcg for adult females and pregnant women, and 1200-1300 mcg for lactating women.
Please wait while we process your payment