Photosynthesis

Question 1

What is Photosynthesis?

Answer 1

the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar.

Question 2

Why Photosynthesis?

Answer 2

To make food (organic material) form
the raw material (inorganic materials)
found in the environment.

Question 3

Inorganic Materials

Answer 3

Inorganic materials = carbon dioxide, water

Question 4

Organic Materials

Answer 4

Organic materials = glucose, sucrose, starch, proteins, lipids

Question 5

Overview

Answer 5

Carbon dioxide and water are taken in by plants
◦ Plants absorb light energy and convert it to a usable form.
◦ ATP (adenosine triphosphate)
◦ Energy is used to “fix” carbon dioxide into sugar molecules
◦ Chemical energy
◦ Sugar is converted to starch and stored for use by the plant, and by animals when they eat plants.

Question 6

Holozoic

Answer 6

digestion assimilation and absorption occurs
inside the body. Eg. Most animals and some protists such
as the Amoeba sp.

Question 7

Saprophytic

Answer 7

digestion occurs outside the body as
enzymes are passed out from the body onto the food. Eg.
Mould feeding on dead and decaying matter, flies
feeding on food.

Question 8

Parasitic

Answer 8

Digestion is unnecessary as digested food is

absorbed directly from the host. Eg. Tapeworm dodder.

Question 9

Worded Equation

Answer 9

sunlight
chlorophyll
carbon dioxide + water sunlight → glucose + water + oxygen
enzyme

Question 10

Reactions

Answer 10

Light-dependent reaction (requires light)

Light- independent reaction (does not require light)

Question 11

Light-dependent reaction (requires light)

Answer 11

◦ Happen ONLY in sunlight
◦ Hence they depend on light!
◦ Light is absorbed by chlorophyll molecules
◦ The energy generates molecules of ATP

The light energy is used to split the hydrogen bonds holding the water molecules together; to form oxygen and hydrogen molecules.

The oxygen gas is passed out of the leaf as waste.

The hydrogen is used up in the light independent reaction.

Question 12

Light Independent Reactions (formerly the “dark reactions”)

Answer 12

◦ Happen in sunlight, and in the dark.
◦ Hence “independent of light”
◦ Carbon dioxide and hydrogen combine in a chemical reaction which requires enzymes
◦ Monosaccharides (eg. glucose) are manufactured.

Question 13

Glucose

Answer 13

◦ Some is used in respiration to provide energy need by the plants.

◦ Some is converted to sucrose for transport around the plant.

◦ Some is used to make other substances need for the plants survival eg. hormones, vitamins, lipids,
proteins ect.

◦ The excess is converted to starch for storage.

Question 14

Arrangement of Leaves

Answer 14

Leaves are arranged at right angle to each other so as to prevent folding over each other. This ensures that every leaf has equal opportunity to capture sunlight (even if it is at different times of the day).

Question 15

Colour

Answer 15

Leaves are usually green because they contain chlorophyll which is chemical used to absorb sunlight. (leaves change colour as the chlorophyll begins to decay and is replaced by carotenoids).

Question 16

Shape and Size of leaves (surface area)

Answer 16

Leaves are flat with a large surface of area for maximum absorption of sunlight and carbon dioxide.

Question 17

Thickness

Answer 17

Leaves are usually less than 1mm thick, which allows diffusion of gases (carbon dioxide and oxygen) into and out of the leaves to occur quickly and easily.

Question 18

Veins

Answer 18

They help to support leaves as well as they carry water and minerals to the cells of the leaves and transport sucrose from the leaves.

Question 19

Stomata

Answer 19

Both surfaces of the leaf have stomata but the lower surface contains far more than the upper. These allow for gaseous exchange.

Question 20

Waxy Cuticle

Answer 20

A waxy water proof layer that protects the leaf and allows light through for photosynthesis, also prevents water loss from the surface of the leaf.

Question 21

Upper Epidermis

Answer 21

Thin layer of transparent cells (rectangular and arranged horizontally) that do not contain chlorophyll and allows light to pass through. Protects the internal structure of the leaf.

Question 22

Palisade Mesophyll

Answer 22

Layer of tightly packed cells (rectangular and arranged vertically) contains numerous chloroplast molecules to absorb light for photosynthesis.

Question 23

Spongy Mesophyll

Answer 23

Layer of rounded cells with some chloroplast, arranged loosely with many air spaces between them to allow diffusion of carbon dioxide and oxygen through the leaf.

Question 24

Vascular bundle (leaf vein)

Answer 24

Made up of the xylem vessels (outside) and phloem tubes (inside). Xylem transport water and dissolved
minerals from the root to the leaf. While phloem transport dissolve food substances from the leaves to the
rest of the plant.

Question 25

Lower Epidermis

Answer 25

Protects the leaves and contains stomata along it length. Controls the movement of water into and out of the leaf. Is the site of gaseous exchange.

Question 26

Stomata

Answer 26

Pores that are made from two surrounding sausage-shaped guard cells. They allow gases in and out of the leaf and open and close to control water loss form the leaf.

Question 27

Guard Cells

Answer 27

◦ When guard cells are turgid (swollen) they form a sausage shape, leaving a opening in the middle (stoma)

◦ When guard cells are flaccid (shrunken) they stick together, closing the stoma

◦ The change in shape of the guard cells is due to the presence or absence of water.

Question 28

FACTORS AFFECTING THE RATE OF PHOTOSYNTHESIS

Answer 28

Light intensity, carbon dioxide concentration, temperature, water availability and humidity

Question 29

Limiting Factor

Answer 29

The limiting factor is any molecule that when in short supply causes the rate of reaction to slow down.

Question 30

Light Intensity

Answer 30

INCREASING THE LIGHT INTENSITY WILL BOOST THE SPEED OF PHOTOSYNTHESIS. HOWEVER, AT HIGH LIGHT INTENSITIES THE RATE BECOMES CONSTANT

EXPLANATION:
LIGHT ENERGY ABSORBED BY CHLOROPHYLL IS CONVERTED TO ATP AND H+

AT VERY LOW LIGHT LEVELS THE PLANT WILL BE RESPIRING ONLY NOT PHOTOSYNTHESISING.

AS THE LIGHT INTENSITY INCREASES, THE RATE OF PHOTOSYNTHESIS INC REASES. HOWEVER, THE RATE WILL NOT INCREASE BEYOND A CERTAIN LEVEL OF LIGHT INTENSITY. AS SOME OTHER FACTOR HAS NOW BECOME THE LIMITING FACTOR.

AT HIGH LIGHT INTENSITIES THE RATE BECOMES CONSTANT, EVEN WITH FURTHER INCREASES IN LIGHT INTENSITY, THERE ARE NO INCREASES IN THE RATE.

THE PLANT IS UNABLE TO HARVEST THE LIGHT AT THESE HIGH INTENSITIES AND THE CHLOROPHYLL SYSTEM CAN BE DAMAGED BY VERY INTENSE LIGHT LEVELS.

Question 31

Carbon Dioxide Concentration

Answer 31

AN INCREASE IN THE CARBON DIOXIDE CONCENTRATION INCREASES THE RATE AT WHICH CARBON IS INCORPORATED INTO CARBOHYDRATE IN THE LIGHT-INDEPENDENT REACTION AND SO THE RATE OF PHOTOSYNTHESIS GENERALLY INCREASES UNTIL LIMITED BY ANOTHER FACTOR.

EXPLANATION:
THE RATE OF PHOTOSYNTHESIS INCREASES LINEARLY WITH INCREASING CO2 CONCENTRATION (FROM POINT A TO B ).

THE RATE FALLS GRADUALLY, AND AT A CERTAIN CO2 CONCENTRATION IT STAYS CONSTANT (FROM POINT B TO C). HERE A RISE IN CO2 LEVELS HAS NO EFFECT AS THE OTHER FACTORS SUCH AS LIGHT INTENSITY BECOME LIMITING.

Question 32

Temperature

Answer 32

◦ When the temperature rises the rate of photosynthesis rises also. There is an optimum temperature at which the rate of photosynthesis is maximum. Beyond this temperature, the reaction quickly comes to a halt.

◦ Explanation
At low temperature, the enzyme does not have enough energy to meet many substrate molecules, so the
reaction is slowed.

◦ When the temperature rises, the particles in the reaction move quicker and collide more, so the rate of photosynthesis rises also.

◦ At the optimum temperature, the enzyme is most efficient and the rate if maximum.

◦ At temperatures above 40°C the rate slows down. This is because the enzymes involved in the chemical reactions of photosynthesis are temperature sensitive and destroyed (denatured) at higher temperatures.

Question 33

Humidity

Answer 33

◦ Generally higher temperatures are better than cold temperatures for photosynthesis, as the enzymes function best in warmer conditions.

◦ However if it is humid, and the air is saturated with water vapour, photosynthesis is limited.

◦ This is because the water vapour being expelled cannot leave and go into the air, as there isn’t a concentration difference for it to diffuse across. So the water vapour sits in the leaf, stopping photosynthesis.

Question 34

Availability of Water

Answer 34

◦ The availability of water affects the rate of photosynthesis.

◦ If they lack it, they wilt. When they have a deficiency of water, their stomata

close, so CO2 cannot diffuse into the leaves.
◦ So CO2 is also the limiting factor, although it is as a result of lack of water. Without water molecules to split, hydrogen for combination with carbon dioxide to form glucose will be in short supply.

◦ A lack of water slows down the rate of photosynthesis