Plants physiology PLANTS AND FOOD Photosynthesis Green plants

Plants physiology PLANTS AND FOOD

Photosynthesis 光合作用 �Green plants and algae （藻类） use light energy to make glucose （葡萄糖） and oxygen （氧气） from carbon dioxide （二氧化碳） and water. Temperature, carbon dioxide concentration and light intensity can affect the rate of photosynthesis.

photosynthesis �Plants make their own food using photosynthesis. The food produced is the sugar called glucose. Food produced by plants is important, not only for the plants themselves, but for other organisms that feed on the plants. �Algae can also make their own food by the process of photosynthesis. Plant biomass will increase as a result of photosynthesis. �During photosynthesis, plants produce glucose and oxygen from carbon dioxide and water, using light energy from the Sun.

The following equations summarise what happens in photosynthesis:

Organization of the plant � Photosynthesis is an endothermic reaction as it requires light energy to react carbon dioxide and water to produce glucose and oxygen. � The light energy required is absorbed by a green pigment called chlorophyll in the leaves. Chlorophyll is located in chloroplasts in plant cells. � Plant leaves are the main organ for photosynthesis. � The carbon dioxide required for photosynthesis comes from the air. It enters leaves through the stomata. Water enters the plant through the roots, and is transported to the leaves in the xylem. � Oxygen is formed as a product. Some is used for respiration. During the day, provided the rate of photosynthesis is high enough, plant and algae give out oxygen. � Some of the glucose produced by photosynthesis is used for respiration. � Glucose is the starting point for making the materials that plants need to live. These materials are used to make cell walls and other cell components and will enable the plant to growth and increase in biomass.

The glucose not used for respiration is used in the following ways:

intensity �There are several ways of measuring the rate of photosynthesis in the lab. These include: �the rate of oxygen production (number of bubbles or volume of oxygen gas given off in a set time) �the rate of carbon dioxide uptake �the rate of glucose production �Several factors can affect the rate of photosynthesis: �light intensity �carbon dioxide concentration �temperature

Light intensity �Without enough light, a plant cannot photosynthesise very quickly - even if there is plenty of water and carbon dioxide. �Increasing the light intensity increases the rate of photosynthesis, until some other factor - a limiting factor - becomes in short supply.

another factor becomes limiting. At very high light intensities, photosynthesis is slowed, but these light intensities do not occur in nature.

Carbon dioxide concentration �Carbon dioxide is one of the reactants in photosynthesis. If the concentration of carbon dioxide is increased, the rate of photosynthesis will therefore increase. At some point, another factor may become limiting and this is shown by the plateau (flattened section) of the graph.

Temperature �The chemical reactions involved in photosynthesis are controlled by enzymes. As with any other enzyme-controlled reaction, the rate of photosynthesis is affected by temperature. �At low temperatures, the rate of photosynthesis is limited by the number of collisions between enzymes and substrate. As temperature increases the number of collisions increases, therefore the rate of photosynthesis increases. However, at high temperatures, enzymes are denatured and this will decrease the rate of photosynthesis.

photosynthesis - Higher �If we look in more detail at the law of limiting factors, for instance, for the relationship between light intensity and the rate of photosynthesis:

Or for carbon dioxide concentration:

�One example of how factors might interact: �The graph shows how carbon dioxide concentration and temperature interact with the effect of light intensity on photosynthesis: �the rate of photosynthesis increases until factors becoming limiting �if carbon dioxide concentration is increased, the rate increases further, and then another factor becomes limiting �the rate can be increased further if the temperature is increased �the rate increases again until another factor becomes limiting

This graph shows the effect of temperature as a limiting factor at two different carbon dioxide concentrations of carbon dioxide:

and gaseous exchange � Leaves are adapted for photosynthesis and gaseous exchange. � They are adapted for photosynthesis by having a large surface area, and contain openings, called stomata to allow carbon dioxide into the leaf and oxygen out. Although these design features are good for photosynthesis, they can result in the leaf losing a lot of water. The cells inside the leaf have water on their surface. Some of this water evaporates, and the water vapour can then escape from inside the leaf. � When water evaporates from the leaves, resulting in more water being drawn up from the roots, it is called transpiration. � To reduce water loss the leaf is coated in a waxy cuticle to stop the water vapour escaping through the epidermis. Leaves usually have fewer stomata on their top surface to reduce this water loss. � Leaves enable photosynthesis to occur. Photosynthesis is the process by which leaves absorb light and carbon dioxide to produce glucose (food) for plants to grow. Leaves are adapted to perform their function, eg they have a large surface area to absorb sunlight. � Plants have two different types of 'transport' tissue, xylem and phloem. These specialised tissues move substances in and around the plant.

Functions of leaves �The function of a leaf is photosynthesis - to absorb light and carbon dioxide to produce glucose (food). The equation for photosynthesis is:

�Leaves are also involved in gas exchange. Carbon dioxide enters the leaf and oxygen and water vapour leave the plant through the stomata. Leaves are adapted in several ways to help them perform their functions.

Features of leaves Adaption Purpose Large surface area To absorb more light Thin Short distance for carbon dioxide to diffuse into leaf cells Chlorophyll Absorbs sunlight to transfer energy into chemicals Network of veins To support the leaf and transport water, mineral ions and sucrose (sugar)

Structure of a leaf

promote efficient photosynthesis. Functions of tissues of the leaf Adaption Purpose Epidermis is thin and transparent To allow more light to reach the palisade cells Thin cuticle made of wax To protect the leaf from infection and prevent water loss without blocking out light Palisade cell layer at top of leaf To absorb more light and increase the rate of photosynthesis Spongy layer Air spaces allow gases to diffuse through the leaf

Gas exchange �When a plant is carrying out photosynthesis carbon dioxide needs to move from the air into the leaf. It does this by diffusing through small pores called stomata. �At the same time oxygen moves out of the leaf through the stomata. This movement of gases in opposite directions is called gas exchange. �Water vapour also diffuses out of the stomata. The stomata are surrounded by guard cells, which control their opening and closing. Cells in the leaf are loosely packed.

Absorbing light energy �Light absorption happens in the palisade mesophyll tissue of the leaf. Palisade cells are column shaped and packed with many chloroplasts. They are arranged closely together so that a lot of light energy can be absorbed.

Mineral deficiency