Biology To revise

What are the features of sieve cells in Phloem

What are the features of sieve cells in Phloem

They are long and narrow tubes which connect to form the sieve tube

They lack nuclei and reduced organelles to maximise space for translocation

They are porous to allow flow of materials

They have think, rigid walls to withstand the hydrostatic pressure

What are the features of companion cells

Connected to sieve tube elements

Contain numerous mitochondria for energy to fuel active transport of materials between sieve tube and source/sink.

Contains transport proteins to assist in loading and unloading of sugars in sieve tubes

What are plasmodesmata

Channels that connect plant cells, allowing for transport of materials between them.

What are the properties of the phloem sieve tube

Transports organic nutrients bidirectionally

Composed of sieve elements and companion cells which connect to form a tube

Contains sieve plates for fluid flow which are highly perforated

Companion cells aid with loading and unloading of organic materials

Movement is mediated by hydrostatic pressure gradient from xylem

What is saltatory conduction

Saltatory conduction is a process in which electrical signals, called action potentials, jump or "leap" from one node of Ranvier to another along a myelinated nerve fiber. This allows for faster and more efficient transmission of signals in the nervous system.

What is the myelin sheath

Myelin is a fatty substance that wraps around the axon of a nerve cell. It acts as an insulator, preventing the leakage of electrical signals. The myelin sheath is not continuous, and there are small gaps called nodes of Ranvier between the myelin segments.

What is action potential and how does it work

An action potential is an electrical signal that travels along the nerve fibre. When an action potential is generated at one node of Ranvier, it rapidly depolarises the membrane, causing the adjacent node to reach the threshold for generating its own action potential.

Which enzymes are found in the small intestine

membrane bound enzymes eg.maltase

What did Meselton and Stahls experiment show

Meselson and Stahl's experiment showed that DNA replication is semi-conservative.

How did Meselton and Stahls experiment results show that DNA replication is semi-conservative

The first round of replication produced DNA molecules with a hybrid density, indicating that each DNA molecule consisted of one strand with heavy nitrogen (N-15) and one strand with light nitrogen (N-14). This supported the semi-conservative model.

Where does the light dependant reaction occur?

thylakoid membrane

Where does the light independent reaction occur?

Stroma

What occurs in photoactivation 1

It happens in photosystem 2
2 photons of light are absorbed
This promotes 2 electrons to an electron acceptor
These electrons are replaced by photolysis

What is photolysis

The splitting of water using light energy

What happens to the electron acceptor in LDR?

It is oxidised
Releasing electrons to a proton pump in the electron transport chain
It uses the energy from this to pump protons from the stroma to the thylakoid space
The electrons are then passed through an acceptor to photosystem 1, in photoactivation 2

What occurs in photo activation 2

The electrons donated are used to reduce NADP to NADPH

What happens in chemiosmosis

the H+ diffuses back across the inner membrane through ATP synthase complexes, driving the phosphorylation of ADP to ATP

What occurs in the Calvin Cycle (light independent reactions)

Firstly, CO2 is fixed by RuBisCo (5C). The carbon atom from the CO2 molecule is added to the 5C molecule to form an unstable 6C molecule which then splits in half to form two 3C molecules (Gp3). Then, ATP is introduced from the previous light-dependent reactions. The ATP loses a phosphate group to the 3C molecule - ADP is formed as a result and the 3C molecules become triosephosphate. The triosephosphate is then reduced by NADPH to form GALP. One Carbon atom from GALP is removed in every cycle - this carbon will contribute to the creation of the desired 6C carbohydrate (therefore, 6 cycles of the calvin cycle are needed to form the carbohydrate). We have 5 carbons remaining - the Ribulose-5-phosphate is then regenerated. Lastly, ATP will donate a phosphate group to the 5C Ribulose which will then form RuBP, which brings us back to square one. The cycle then repeats.

What is Pr and Pfr

Pr is the inactive form of Pr

Pr is converted to Pfr in red light

Pfr is converted to Pr in far red light

Pfr slowly breaks down to Pr overnight, meaning Pfr is dominant during the day

What affect does Pr and Pfr have on plant flowering

In long day plants Pfr activates flowering

In short day plants Pfr inhibits flowering