Science GCSE Photosynthesis

Photosynthesis / How plants make food

Photosynthesis captures energy for life on Earth. Many chemicals are made to allow life processes to occur in plants. These chemicals can move in and out of cells by the process of diffusion. Osmosis is a specific type of diffusion.

Photosynthesis

Photosynthesis is a process used by plants in which energy from sunlight is used to convert carbon dioxide and water into molecules needed for growth. These molecules include sugars, enzymes and chlorophyll.

Light energy is absorbed by the green chemical chlorophyll. This energy allows the production of glucose by the reaction between carbon dioxide and water. Oxygen is also produced as a waste product.

This reaction can be summarised in the word equation:

Carbon dioxide + water  glucose + oxygen

Photosynthesis - Higher tier

The chemical equation for photosynthesis is:

6CO₂ + 6H₂O  C₆H₁₂O₆ + 6O₂

Glucose from photosynthesis

Glucose is made up of carbon, hydrogen and oxygen atoms. Glucose made by the process of photosynthesis may be used in three ways:

1. It can be converted into chemicals required for growth of plant cells such as cellulose
2. It can be converted into starch, a storage molecule, that can be converted back to glucose when the plant requires it
3. It can be broken down during the process of respiration, releasing energy stored in the glucose molecules

Plants cells contain a number of structures that are involved in the process of photosynthesis:

Diagram of a plant cell involved in production of glucose from photosynthesis

• Chloroplasts - containing chlorophyll and enzymes needed for reactions in photosynthesis.
• Nucleus - containing DNA carrying the genetic code for enzymes and other proteins used in photosynthesis
• Cell membrane - allowing gas and water to pass in and out of the cell while controlling the passage of other molecules
• Vacuole - containing cell sap to keep the cell turgid
• Cell wall - strengthens the cell
• Cytoplasm - enzymes and other proteins used in photosynthesis made here

Diffusion

Dissolved substances have to pass through the cell membrane to get into or out of a cell. Diffusion is one of the processes that allows this to happen.

Diffusiondiffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration occurs when particles spread. They move from a region where they are in high concentration to a region where they are in low concentration. Diffusion happens when the particles are free to move. This is true in gases and for particles dissolved in solutions. Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. This is how the smell of cooking travels around the house from the kitchen, for example.

The movement of oxygen and carbon dioxide in and out of leaves during photosynthesis occurs by diffusion.

The higher concentration of carbon dioxide in the air diffuses into the leaf for photosynthesis.

Oxygen produced during photosynthesis builds up to higher concentrations and diffuses out of the leaf into the air which has a lower concentration of oxygen.

Osmosis

Water can move across cell membranes because of osmosis. For osmosis to happen you need:

• Two solutions with different concentrations
• A partially permeable membrane to separate them

Partially permeable membranes let some substances pass through them, but not others. The animation shows an example of osmosis.

Osmosis is the movement of water from a less concentrated solution to a more concentrated solution through a partially permeable membrane.

This is shown in the animation above. Eventually the level on the more concentrated side of the membrane rises, while the one on the less concentrated side falls. When the concentration is the same on both sides of the membrane, the movement of water molecules will be the same in both directions. At this point, the net exchange of water is zero and there is no further change in the liquid levels.

Osmosis is important to plants. They gain water by osmosis through their roots. Water moves into plant cells by osmosis, making them turgid [turgid: Having turgor; enlarged and swollen with water ] or stiff so they that able to hold the plant upright.

Minerals from the soil - Higher tier

The concentration of minerals in the soil is very low. They dissolve in water and move around the soil in solution. Root hair cells are adapted to absorb the water out of soil byosmosis – they have a large surface area, thin walls and are close to the xylem cells used for transporting water up the plant. Minerals such as nitrate ions cannot be absorbed by osmosis (which is the movement of water only) or diffusion(because the minerals are in very low concentration). The root hair cells have carrier molecules on their surface that pick up the minerals and move them into the cell against the concentration gradient. This requires energy, and is calledactive transport.

Factors affecting photosynthesis

Three factors can limit the speed of photosynthesis: light intensity, carbon dioxide concentration and temperature.

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  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 will boost the speed of photosynthesis.
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  Carbon dioxide concentration
  Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.
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  Temperature
  If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.

If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.

In practice, any one of these factors could limit the rate of photosynthesis.

Maximising growth

Farmers can use their knowledge of these limiting factors to increase crop growth in greenhouses. They may use artificial light so that photosynthesis can continue beyond daylight hours, or in a higher-than-normal light intensity. The use of paraffin lamps inside a greenhouse increases the rate of photosynthesis because the burning paraffin produces carbon dioxide, and heat too.

Fieldwork techniques

Energy from light drives photosynthesis. The effect of light levels on plants can be investigated using a number of techniques.

The diagram below shows how apparatus can be set up to investigate the effect of changing light intensity on the rate of photosynthesis.

Diagram of photosynthesis experiment equipment

The distance separating the lamp and the pondweed can be altered and the resulting light intensity measured using a light meter.

Quadrat

A quadrat can be used to survey the plants present in a square metre.

The quadrat is a square grid that is placed on the ground. Any plants or animals present within the area of the quadrat are recorded. The organisms can be named using an identification key that provides pictures and/or descriptions of the organisms.

Quadrats are positioned randomly to remove bias and allow for reliable comparison between locations.

If appropriate, data may be recorded at regular intervals along a transect. A transect is a straight line that runs through a location upon which data on organism distribution can be recorded. An identification key can then be used to identify species present, so a link may be formed between light level and species present.

Source: http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_21c/life_processes/plantfoodrev_print.shtml