b3bdiffusion(扩散运动).ppt

1、B3b Diffusion,1. Describe diffusion as the movement of a substance from a region of high to low concentration. Describe how molecules enter and leave cells by diffusion through the cell membrane (higher tier) Explain that diffusion is the net movement of particles from an area of high concentration

2、to an area of low concentration and is a consequence of the random movement of individual particles Explain that the rate of diffusion is affected by: distance, concentration difference (gradient), surface area 2. Describe how small digested food molecules are absorbed into the blood in the small in

3、testine by diffusion 3. Describe gaseous exchange within alveoli between air and blood (higher tier) Explain how the alveoli are adapted for efficient gaseous exchange: permeable, moist, large surface area, good blood supply, wall one cell thick Explain how the small intestine is adapted for the abs

4、orption of food: long, large surface area (villi and microvilli), permeable surface, good blood supply 4. Describe how food and oxygen reach the foetus, and carbon dioxide and other wastes are removed, by diffusion through the placenta (higher tier) Explain how the placenta is adapted to increase th

5、e rate of diffusion (higher tier) Explain how transmitter substances diffuse across synapses to carry signals from one neurone to another 5. Describe how carbon dioxide and oxygen diffuse in and out of plants through the leaves (higher tier) Explain how leaves are adapted to increase the rate of dif

6、fusion of carbon dioxide and oxygen 6. Explain the loss of water from leaves in terms of the diffusion of water molecules,What is diffusion?,Diffusion and concentration,Why can the student smell the sock from a distance?,The sock can be smelt because sweat and other molecules are moving away from it

7、 and spreading out in the air. This is called diffusion.,Where is the smell strongest?,The smell is strongest at the sock. The smell becomes weaker further away from the sock.,Moving molecules,In which states are molecules able to diffuse?,Molecules in liquids and gases are constantly moving and bum

8、ping into each other. This means that they tend to spread out.,gas (e.g. steam),liquid (e.g. water),solid (e.g. ice),Diffusion in action: one gas,Diffusion in action: two gases,Changing concentrations,The rate of diffusion depends on several factors, such as the distance the particles have to travel

9、 and the difference in concentration.,Molecules diffuse until they are evenly spaced apart and equilibrium is reached.,During diffusion molecules move from an area of high concentration to an area of low concentration. They are said to move down a concentration gradient.,Diffusion is a passive proce

10、ss which means that no energy is needed.,Diffusion: true or false?,Why is diffusion important to life?,Diffusion is the main way in which substances move over short distances in organisms. What substances need to move?,Oxygen, food and waste products are some of the substances that move by diffusion

11、.,In animals, how do these vital substances get to where they are needed?,The substances are transported in the bloodstream, from where they can diffuse in and out of cells.,Diffusion and breathing,Breathing involves the exchange of gases in the lungs; a process that occurs by diffusion. What happen

12、s when you breathe in?,Oxygen in inhaled air diffuses through the lungs and into the bloodstream. The oxygen is then transported throughout the body.,Carbon dioxide is the waste gas produced by respiration. Carbon dioxide diffuses from body tissues into the bloodstream and is exhaled via the lungs.,

13、Where does gas exchange take place in the lungs?,Structures of the respiratory system,Cross-section through an alveolus,deoxygenated blood (from body tissues),oxygenated blood (to body tissues),air in/out,Alveoli are the tiny air sacs at the end of the bronchioles, in which gas exchange occurs.,How

14、are alveoli adapted?,Alveoli have several adaptations that help to make gas exchange very efficient:,They are very thin only one cell thick. They are covered by a network of fine capillaries, enabling gases to pass almost directly between the lungs and bloodstream. They are moist, encouraging gas mo

15、lecules to easily dissolve. They have a large combined surface area, allowing large amounts of gases to be exchanged with each breath.,What happens in the alveoli?,Diffusion and digestion,Digestion breaks down large food molecules into smaller molecules such as glucose, amino acids and fatty acids t

16、hat can be easily absorbed.,Carbohydrates, proteins and fats are made up of large molecules that cannot be readily absorbed by the body.,In which part of the digestive system is most food absorbed?,Small food molecules are usually absorbed in the small intestine, diffusing across the intestine wall

17、and into the bloodstream.,How is the small intestine adapted?,The wall of the small intestine is lined with many tiny finger-like projections called villi. These are very thin and increase the surface area of the small intestine, both of which increase the speed of diffusion.,Diffusion and the small

18、 intestine,How does the initial concentration of dissolved food molecules in the small intestine compare with the concentration of the blood in the villi?,The concentration of dissolved food molecules is higher in the small intestine than in the blood entering the villus.,This means that the dissolv

19、ed food molecules diffuse from the small intestine into the blood, moving from higher to lower concentration.,Diffusion and the placenta,The placenta is an organ that develops in the uterus of female mammals during pregnancy. The umbilical cord connects the placenta to the fetus.,The placenta enable

20、s nutrients and oxygen to pass from the mother to the fetus by diffusion, and waste substances to diffuse from the fetus back to the mother.,The placenta can filter out certain molecules and bacteria, but is unable to stop many harmful substances such as alcohol, chemicals and some types of virus fr

21、om reaching the fetus.,Diffusion and the placenta,How does the placenta work?,Diffusion and nerves impulses,A synapse is a junction between two neurones across which electrical signals must pass.,Neurotransmitter molecules diffuse from vesicles towards the neurotransmitter receptors, moving from an

22、area of high concentration to low concentration.,Diffusion in the leaf,Carbon dioxide and oxygen diffuse in and out of the leaf of a plant. The leaves are specially adapted to aid this movement of gas. Plants also lose water from the leaf which is movement of water through diffusion.,wide and flat t

23、his create a large surface area to absorb as much light as possible,thin this allows gases to reach cells easily,stomata these are pores on the underside of leaves through which gases move in and out.,veins these carry water to the cells and carry glucose away and also support leaves,How are leaves

24、adapted for photosynthesis?,To increase photosynthesis, leaves have certain key features:,Structure of a leaf activity,Take a look inside a leaf,How do gases enter and leave plants?,On the underside of leaves are small holes, or pores, called stomata. A single hole is called a stoma. Each stoma is surrounded by two guard cells.,When guard cells gain water, they curve outwards. This opens the stoma, allowing