Cells & Photosynthesis Section 4 Cells & Photosynthesis

Light Carbon dioxide + Water Glucose + Oxygen

Sunlight & Starch Production Cells in green plants make their own food in a process known as………. photosynthesis

Carbohydrates Carbohydrate Carbon (C) Hydrogen (H) Oxygen (O) Plants make glucose. Some of this is used immediately as an energy source and the rest is converted to starch for storage. Starch is a large molecule so it cannot leave the plant cell. Glucose and starch are carbohydrates made up of the following elements: These are: Carbon (C) Hydrogen (H) Oxygen (O) Carbohydrate

Carbohydrates You now know that glucose is converted to starch for storage Presence of starch shows that plants have first made glucose by photosynthesis

How is starch made? Small, soluble glucose molecules Large, insoluble starch molecule

Chloroplasts Light energy from the sun is trapped by the green pigment chlorophyll. Chlorophyll is found in disc-shaped structures called chloroplasts in green leaves. chloroplasts

Variegated leaf White area: No chlorophyll Green area: Contains chlorophyll Variegated leaf

carbon dioxide absorbed by the air Raw Materials sunlight carbon dioxide absorbed by the air water absorbed from soil

Carbon Dioxide + Water Glucose + Oxygen Word equation for photosynthesis Oxygen is a waste product and given off as a gas Light energy Carbon Dioxide + Water Glucose + Oxygen Raw Materials Products

The Two Stages of Photosynthesis Light energy Carbon Dioxide + Water Glucose + Oxygen Photosynthesis occurs in two stages Photolysis - The splitting of water Carbon Fixation

Light energy from the sun trapped by chlorophyll Light energy converted to chemical energy Passed to second stage Released to the air as oxygen gas

Carbon Dioxide + Water Glucose + Oxygen Light energy Carbon Dioxide + Water Glucose + Oxygen Light energy CO2 + H2O C6H12O6 + O2

Stage 1: Photolysis Water is split up into the two elements that it is made from – hydrogen and oxygen Oxygen is a by-product of plants and is given off as a gas Hydrogen is carried to the second stage by a hydrogen carrier Light energy from the sun used to build up ADP + Pi into ATP for use in the second stage Photolysis is controlled by a series of enzymes

Stage 1: Photolysis Water is split into hydrogen and oxygen. Hydrogen is carried to the second stage (carbon fixation) by a hydrogen carrier and is used in this stage. Oxygen is a by-product that is given off as a gas. ATP is built up using light energy. This ATP is used in carbon fixation.

Stage 2: Carbon Fixation Hydrogen that is produced in photolysis combines with carbon dioxide to form the carbohydrate, glucose Hydrogen is carried to the second stage by a hydrogen acceptor molecule Light energy Carbon Dioxide + Water Glucose + Oxygen CO2 + H2O C6H12O6 + O2 Light energy

Stage 2: Carbon Fixation Carbon dioxide and hydrogen are able to combine using the energy from the ATP made in the first stage photolysis ATP (from stage 1) Hydrogen (from stage 1) ADP +Pi Stage 2: Carbon Fixation. This reaction is controlled by a series of enzymes Carbon Dioxide (enters by diffusion from the air) Glucose (C + H + O)

Diffusion Carbon dioxide gas diffuses into the leaf from the air Oxygen gas that is produced inside the leaf diffuses out Carbon dioxide Oxygen

Storage of Carbohydrates Glucose molecules can link to each other in different ways This results in different types of carbohydrates being formed E.g. Starch and cellulose

Starch How plants store excess glucose Large and insoluble Chains of glucose units that form are coiled up and becomes tangled to form a dense starch grain Easily broken down to be used as a source of energy

Cellulose Used as building materials Forms parallel chains Cell walls in plants Not soluble in water Forms parallel chains Not easily broken down (tough fibres!)

Cellulose Fibres Fibres arranged in sheets that run at right angles to each other Criss-cross arrangement adds strength to cell wall Small molecules are soluble, e.g. glucose, water and carbon dioxide and can pass into cell

Measuring rate of photosynthesis The rate of photosynthesis can be measured in the following ways: Measuring the increase in dry mass of a plant over a period of time Measuring the volume of oxygen given off over a period of time Measuring the volume of carbon dioxide taken in over a period of time

Measuring rate of photosynthesis with Elodea (pondweed) Upside down test tube (to collect oxygen gas) Thermometer Solution of sodium carbonate (a source of CO2) funnel elodea lamp How could the light intensity be altered? How could the temperature be altered (and kept constant)? How could the CO2 concentration be altered? How can the rate of photosynthesis be measured?

Elodea http://www.kscience.co.uk/animations/photolab.htm

Limiting Factors A limiting factor is a factor which slows down the rate of photosynthesis when it is in short supply. There are three limiting factors that affect the rate of photosynthesis: Light intensity Carbon dioxide concentration Temperature (because photosynthesis is an enzyme-controlled reaction and enzymes are affected by temperatures)

Light intensity as a limiting factor At A, low light intensity is limiting the rate of photosynthesis. At B, light intensity is high, so another factor must be in short supply (carbon dioxide concentration or temperature). B A Light intensity

Carbon dioxide concentration as a limiting factor At A, low carbon dioxide concentration is limiting the rate of photosynthesis. At B, carbon dioxide concentration is high, so another factor must be in short supply (light intensity or temperature). B A Carbon dioxide concentration

Temperature as a limiting factor At A, low temperature is limiting the rate of photosynthesis. At B, temperature is high, so another factor must be in short supply (carbon dioxide concentration or light intensity). B A 20 40 Temperature (°C)

Limiting Factors Light Intensity At this point, something other than light intensity is the limiting factor As a general rule, whatever is on the x-axis is the limiting factor on the slope of the graph At this point, light intensity is the limiting factor Light Intensity

What’s the limiting factor? At A, low ________________ is the limiting factor. At B, ___________ or ______________ is the limiting factor. B A

Making the most of photosynthesis – Reducing limiting factors By reducing the limiting factors for photosynthesis, plant growers can grow larger plants with higher yield of fruit, flowers or grain. By covering plants with polythene tunnels, or growing them in greenhouses, the temperature is raised and the plants are protected from damaging wind and pests If a paraffin stove is burned, this increases the temperature and has the added benefit of providing them with extra carbon dioxide. Artificial lights provide extra light and warmth for plants