Biology Topic 6

What is photosynthesis?

a series of chemical reactions catalysed by enzymes

What is the equation for photosynthesis?

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O

Where does photosynthesis occur?

In chloroplasts → chlorophyll (green pigment) in chloroplasts traps energy transferred by light

What kind of reaction is photosynthesis?

An endothermic reaction → the products in photosynthesis have more energy than the reactants

What happens to the glucose in photosynthesis?

They are linked together to form a polymer, starch, which stays in the chloroplasts until photosynthesis stops

What happens to starch?

It is then broken down into simpler substances which are moved to the cytoplasm to make sucrose

What is the role of plants and algae?

They are photosynthetic plants that produce their own biomass by trapping energy from the Sun, therefore are the producers of food and biomass

What are the three limiting factors of photosynthesis?

Temperature, Carbon Dioxide Concentration, Light Intensity

What is the effect of temperature on the rate of photosynthesis?

• the reactions in photosynthesis are catalysed by enzymes

• the enzymes work better at warmer temperatures

• however the enzymes have optimum temperatures and if the temperature continues to rise after its optimum, the rate of photosynthesis will decrease

What is the effect of carbon dioxide concentration on the rate of photosynthesis?

• carbon dioxide is needed for photosynthesis

• increasing the carbon dioxide concentration also increases the rate of photosynthesis

• however after a while, increasing the concentration won’t affect the rate and so another factor is the limiting factor

What is the effect of light intensity on the rate of photosynthesis?

• photosynthesis requires the energy transferred by light

• the greater the light intensity, the more energy it transfers which increases the rate of photosynthesis

What is the relation between light intensity and distance from light source?

Light intensity is inversely proportional to the distance from the light source and the change in light intensity can be calculated with 1/d squared

What are two experiments you can do to test the affect of light intensity on the rate of photosynthesis?

Algal balls and pondweed

What indicator do you use with algal ball experiment?

Hydrogen carbonate indicator, monitors the change in pH of the solution

What is the independent variable in the algal ball experiment?

The distance at which each bottle of algal balls will be placed from the light source

What is the dependent variable in the algal ball experiment?

The rate of photosynthesis which is determined but the colour of the indicator

What are the control variables in the algal ball experiment?

• same amount of algal balls in each bottle

• the amount of time allowed for photosynthesis

How do you set up the pondweed experiment?

Placing pondweed in a test tube of water

What is the independent variable in the pondweed experiment?

The distance at which the light source is away from the pondweed

What is the dependent variable in the pondweed experiment?

The number of bubble of oxygen that appear in a certain amount of time

What are the control variables in the pondweed experiment?

• the kind of pondweed

• the temperature of the water

• no other light sources to interfere with the results

Why do plants need water?

• is required as a reactant for many chemical reactions e.g. photosynthesis

• it contains dissolved minerals and ions which are needed for healthy growth

• it keeps the cells turgid and helps maintain their shape

• evaporation of water provides cooling

What is transpiration?

The flow of water into a root, up the stem and out of the leaves

What are the six steps in transpiration?

1. Water moves into the roots (root hair cells) by osmosis

2. Water moves into the xylem, a continuous tube that starts in the roots

3. Water moves up the xylem in a continuous chain because water molecules have weak forces of attraction between them

4. Water evaporates from the xylem vessels and forms water vapour

5. The loss of water from the xylem pulls up more water from the roots to replace it

6. The water molecules diffuse out of the air spaces in the lead through the stomata

What are the adaptations of root hair cells?

• large surface area/no chloroplasts → helps increase the rate of absorption and allows more water and minerals to be absorbed

• lots of mitochondria → provides energy for active transport

• thin walls → less diffusion distance

What are the adaptions of the xylem?

• consists of dead cells → the cells lose their end walls so that there is a continuous hollow tube for water to flow through

• no cytoplasm → creates more space for water to travel

• vessels are lined with lignin → provides strength and support to the plant and withstand the water pressure

What is translocation?

The transportation of substances such as glucose (in the form of sucrose) in the phloem to places where they are needed e.g. flower formation, seed growth, tubers

What are the adaptions of the phloem?

• sieve tube cells → cells that are alive and have a reduced cytoplasm + no nucleus to allow more central room, in between the phloem and allow sucrose to pass through

• companion cells → located on the side walls of the phloem and contain mitochondrion which provides the energy required for active transport

What are the differences between xylem and phloem?

What is the structure and functions of a leaf?

• are often broad and flat giving them a large surface area to maximize light absorption

• leaf veins contain xylem veins which carry water

• leaf veins contain phloem veins which carry sucrose

• contains stomata to enable gas exchange

• are thin so there is a short diffusion distance between stomata and chloroplasts

• waxy cuticle protects plant and prevent water loss

How does light affect the stomata?

• in the light, water enters guard cells by osmosis and they become turgid resulting in the stomatal pore opening

• at night, water flows out the guard cells by osmosis and they become flaccid resulting in the stomatal pore closing

What are the four factors that affect the rate of transpiration?

Humidity, Wind speed, Light Intensity, Temperature

How does humidity affect the rate of transpiration?

• when there is low humidity (low concentration of water in the atmosphere) there is a steep diffusion gradient so there is a high rate of transpiration

• when there is high humidity (high concentration of water in atmosphere) there is a shallow diffusion gradient so there is a low rate of transpiration

How does wind speed affect the rate of transpiration?

• when the wind speed is weak, there is a shallow diffusion gradient because the wind doesn’t move away that a lot of water molecules so there is a low rate of transpiration

• when the wind speed is strong, there is a steep diffusion gradient because the wind moves away the water molecules so there is a high rate of transpiration

How does light intensity affect the rate of transpiration?

• when the light intensity is high, it encourages the stomata to open which increases the rate of transpiration

• when the light intensity is low, the stomata closes which decreases the rate of transpiration

How does temperature affect the rate of transpiration?

• when the temperature is high, it increases the rate of evaporation of water molecules from the xylem so that they can diffuse through the stomata which increases the rate of transpiration

• when the temperature is low, water doesn't evaporate as fast from the xylem and so the rate of transpiration is not increased

What are the adaptations of xerophytic plants?

• they are plants that live in conditions where freshwater is limited

• very few stomata

• sunken stomata

• hairs surrounding stomata

• needle-shaped or small leaves

• waxy cuticle

What are the adaptations of cacti?

• their leaves become spines that can no longer photosynthesise

• their stem has a waxy cuticle and a is large in diameter to store water

• their roots are shallow and deep to allow it access to all available water

What are the adaptations of marram grass?

• they live on sand dunes

• leaves can roll up to reduce the exposure of surfaces to the wind

• this also provides deep grooves which protect the stomata

• exposed surface has no stomata and a thick cuticle

• inner surface of leaves have hair to trap air and reduce water loss

What are the adaptations of conifers?

• they live in regions where ground water freezes for a part of the year

• they have needle-like leaves which gives them a small surface area and reduces wind resistance and allows snow to fall off easily

• stomata are sunk into pits or grooves to reduce the rate of diffusion of water out of the leaves

• leaf veins are protected firn the cold and surrounded by a waxy cuticle that sheds water and snow easily

What is a stimulus?

A change in the environment that causes a response from an organism

What is a tropism?

Responding to a stimulus by growing towards (positive tropism) or away from it (negative tropism)

What is phototropism?

A tropism caused by light

What are plant shoots?

• they are positively phototropic so that the plant gets enough light for photosynthesis

• positive phototropism is caused by plant hormones called auxins

What does auxin do in plant shoots?

• it is produced in the tips of a shoot

• they cause elongation in the cells of plant shoots

• if light is coming from on directions, it causes the auxin to move to the shaded side so there is an unequal distribution of auxin

• this causes the cells on the shaded side to elongate causing the shoot to bend towards the light

What is gravitropism?

A tropism caused by gravity

What are plant roots?

• they are positively gravitropic to anchor the plant in place and help reach substances in soil

• it is also caused by auxin

What does auxin do in plant roots?

• it is also found in the tips of roots

• however in roots, auxin inhibits the growth of cells

• auxin accumulates at the bottom side of the root due to gravity which causes the cells at the top to elongate more

• this causes the roots to root down deeper into the soil

What are artificial auxins?

• they make some plants grow uncontrollably which can kill them

• selective weedkillers contain artificial auxins to kill plants with broad leaves (e.g. dandelions) but not those with narrow leaves (e.g. wheat)

• therefore farmers can kill weeds in a wheat fiel without affecting the crop

What are rooting powders?

• chemicals that help plant cuttings grow roots and become new plants

• they contain auxin, which causes plant cuttings to develop roots quicky

• large numbers of identical plants can be produced this waya

What is gibberellins?

• a plant hormone that is naturally released inside a seed to start germination

• some seeds need a period of darkness or cold before they can germinate but plant growers can use gibberellins to make these seeds germinate without this

What is photoperiodism?

• the response of an organisms to the number of daylight hours in a day

• some plants use this to flower at a certain time

• flower growers can override photoperiodism by spraying the plant with giberellins

What are some other advantages of gibberellins?

• allows plants to produce seeds and fruits without being pollinated

• can be sprayed on plants to produce bigger fruits

What is ethene used for?

To ripen fruit

Why is ethene used?

• fruits that are not ripe (e.g. bananas) are easier to transport without damaging them than ripe fruits

• the unripe fruit can also be kept for longer in cold storage without going off

• fruit producers will pick up the unripe fruit and then when its needed use a plant hormone called ethene to ripen it

• this makes sure that fruit reaches the shops in just-ripened condition