Different substances, such as sucrose and amino acids, can move in different directions in the phloem sieve tubes.
Which statement explains this?
1 )
Active transport occurs in some phloem sieve tubes and mass flow occurs in other phloem sieve tubes.
2 )
Both active transport and mass flow occur in each individual phloem sieve tube.
3 )
Mass flow occurs in both directions at the same time in each individual phloem sieve tube.
Mass flow occurs in different directions in different phloem sieve tubes at the same time.
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