Biology Edexcel Powerpoint

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This is a powerpoint on biology for all edexcel students.

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DefinitionsOrganelles Tiny structures in a cell that have a specific function. Mammals Warm blooded animals which have hair on their skin and the women can produce milk for their children.

CytoplasmThe cytoplasm is the liquid that keeps the cell together and keeps it functioning. It is where different reactions take place and its where all the different organelles, structures that have a specific role, for example ribosomes. The cytoplasm holds fats, also known as lipids, proteins and enzymes as well as the organelles. Enzymes control the rate of which reactions in the cytoplasm happen and some are suspended in one position but some move freely around in the cytoplasm. Cell MembraneThe cell membrane is a thin layer around the cytoplasm and it holds everything together and is almost like a bouncer, allowing certain things into and out of the cell. Generally oxygen, food and water are allowed in, waste products are allowed to go and harmful products are kept out of the cell. Cells under a microscopeThis is a transverse section and it is where you cut across the structure. This is a longitudinal section and it is where you cut across the length structure. If you were to put these cuts under a microscope you should find little structures called cells. Plant cells are easy to work with as all you have to do is cut the leaf and you should see a cell. With animal tissue however it is not that easy. Animal tissue is flexible and soft so is harder to cut thin so you have to dip the structure in melted wax and when the wax dries you should be able to cut thin sections. Dyes are also used to show the animal cell structure more clearly. Photographs of cells under seen under a microscope are photomicrographs. Plant cells are bigger than animal cells so you need to be more zoomed in to clearly view animal cells.

Chemical Components of Cells. Cell structure.There are no generic plant or animal cells however there generalisations we can make. When we refer to plant or animal cells we usually refer to the generalised cells because these generalised cells hold a majority of the things every animal or plant cell would have. All cells have a cell membrane which is like a tiny wall that keeps outside things out and holds the cytoplasm together. The cytoplasm is where most reactions take place and the nucleus is almost the command centre or the brains of the cell. Most but not all cells have a nucleus. RespirationDiffusionCells need to get food substances so they can digest the food and respire, and cells need water to stay alive and they also need to expel carbon dioxide, because in large quantities it is harmful to the cell. This is done by the process of diffusion. Diffusion is the idea that molecules (whether they gas or liquid) will spread to an area of high concertation to an area of low concertation. The final goal of diffusion is to have all the molecules equally spaced out and for them to be evenly distributed. This can happen with cells, molecules can enter a cell if there is a low concertation inside the cell and a high concertation on the outside of the cell. Active TransportDiffusion is not the only way substances enter or leave a cell. If this was the case harmful substance could easily enter the cell and helpful substances could leave. Cells can go one step further and work against the concertation difference through a method called active transport. One example of this is the villi and microvilli in the small intestine, they take in glucose through this active transport method. The energy from respiration causes this active transport to happen. PhotosynthesisCells and organisms need food to fuel essential process that keep them alive and for development and growth. Animals take in food and digest them and with the exception of a few rare plants, plants do not eat, they photosynthesise. A plant gets its simple raw materials from two places, the air and the soil and synthesises them into complex carbohydrates. The plants get water which contains hydrogen from the soil and carbon dioxide from the air which contains carbon and oxygen. These raw materials combined with nitrogen and sulphur from the soil allows the plant to synthesise, build up, complex carbohydrates like glucose. The reason in plants it is called photosynthesis is because the energy comes from the sun and is converted by chlorophyll into chemical energy, what the plant uses to drive the synthesis to make carbohydrates.Main PartsVacuoleWhile most textbooks will tell you only a plant cell has a vacuole this is not true. An animal cell can have a vacuole but it is a lot smaller than a plants vacuole and its not permanent and grows in size when it is needed. A plants vacuole is much larger and is permanent. Simple SummaryPart/OrganelleFunctionRibosomesProduce proteins.MitochondriaRelease energy.Nucleus Controls what goes on in the cell.Cell MembraneDictates what goes in and out of the cell. CytoplasmThe jelly that holds the cell together and where reactions take place.EnzymesDictate what reactions happen and at what speed and when. VacuoleHolds water but is not as good as a plants permanent vacuoleParts of a plant cell, GCSE level. Both plant and animal cells have nucleus, cytoplasm, cell membranes and mitochondria so we will not go over them. Click on the part you want to see and it will take you there. Cell WallThe cell wall on a plant gives the plant cell extra support. The cell wall allows water and other liquids into the cell. Plants do have a cell membrane just like an animal plant. Permanent VacuoleThe permanent vacuole in a plant cell is central in the dell and is very big compared to the animal cell vacuole. It holds a solution known as cell sap and this is a weak solution of salts and sugars in water. The permanent vacuole ensures that the plant cells stays full of water. Due to the enormous size of the vacuole the cytoplasm and cell membrane are pushed close to one another and this results in a firmer, more robust cell. ChloroplastsChloroplasts are the cells food and energy hub. Chloroplasts containing a green substance known as chlorophyll and photosynthesis occurs here. This is where sunlight is converted to energy and this keeps the plant cell going. Simple SummaryPart/OrganelleFunctionCell WallGives extra support to the cell.Permanent VacuoleStores a solution of water, sugars and salts.Chloroplasts This is where photosynthesis happens. Cell Division. Cells increase in number by dividing. When a cell divides into two separate cells the cells are said to be daughters. While it is possible for both daughter cells to divide again this is not usually the case as one of the daughter cells changes in shape and structure and becomes specialised, and loses the ability to divide. The other cell however is fine and can divide again. In animal cells the cell starts to enlarge. Then the nucleus divides and the cytoplasm connecting the to eventually breaks off and they become separate cells. Organelles like mitochondria and ribosomes are equally divided between the two cellsSpecialised Cells

Ciliated Cells (Found more in animals)Forms the lining of the nose and windpipe. Tiny cytoplasmic hairs are celled cilia.The cilia constantly flick in a wave like motion. This motion will carry mucus to the nose away from the lungs.The mucus will trap and hold bacteria and dust in it.

Nerve Cells (Found only in animals)Conducts electrical impulses.Wired all around the body and they lead to the brain. Some are very long and groups connect body parts like the foot to the spinal cord.Chemical reactions allow electrical impulses to travel through the nerve fibre.When you tap your knee or arm the nerve cells notice this and sends messages to the brain.

Root Hair Cells (Found only in plants)Absorbs water and minerals from the soil. The finger like projection increases the surface area and the rate of absorption.The cell membrane controls what substances enter and do not enter the cell. In the thousands a massive surface area is created and they form the outside layer of roots.

White Blood Cells (Found only in animals)Found in the blood stream with red blood cells. One function of the white blood cell is to engulf and digest bad bacteria. Is flexible and can change its shape. Can move into other tissues and go through the blood vessel.

Phloem Cells (Found only in animals)The phloem cell transports food in plants. They are long cells and join end to end.When they meet a wall separates the cells, but the wall has holes in it.It is believed that the cytoplasm's communicate to one another and food is passed through the tiny walls.The tiny walls are called sieve plates.

Please note!This is in no way a complete list.There are lots of other specialised cells out there.Survival of cells. There are lots of single cell organisms that carry out all the process they need to in order to live. This is not the case for large animals and plants as many of the cells left on their own would die. This is because the cells help one another in the body. For example a muscle cell cant get its own oxygen or food so on its own it would die. Epithelial TissueA thin layer of tissue which usually lies on top of other tissues or organs.They protect said tissues from physical or chemical damage. There are different types of epithelial tissues. Some examples of epithelial tissue in humans are:In the mouth.WindpipeFood canal

Muscle TissuesIn muscle tissue you can also expect to find other types of cells like blood vessels and nerves. They are formed like a sheet and are elastic like so they can easily go back into place after being stretched. They are not found just in the arms or legs. Muscles can be found elsewhere, for example in the stomach muscle movement will crush food.

The glands and tubesGlands are ball like structures that secrete chemicals and hormones. One example of this is the salivary gland. After these glands have secreted what they need to the product is carried in small tubes to other organs in the body.

OrgansOrgans are simply a group of tissues working together in unison with one task or goal in mind. They have special functions and there are lots of examples in humans. For example the stomach has epithelial tissue, gland cells and muscle tissues, and the stomach is provided with food and oxygen by the blood stream. The organs do not stop there. The heart, lungs, eyes, brain and intestines are all examples of organs in the human body. Plants also have organs and the main ones are the root, stem and leaves. The tissues that make up the leaf are the epidermis, palisade tissue, spongey tissue and the phloem and xylem. Systems and Organisms.Systems are larger than organs, they are organs that work together that have similar functions and goals. For example the nervous system is made up of the brain, spinal cord and nerves. In humans you also have the skeletal and circulatory systems and in plants you have the shoot made up of the stem, leaves and buds. An organism is a created when you have organs and systems working together to make an independent, living thing.

TissuesCellsOrgansSystemsOrganismsTissue CultureThere is a scientific technique called tissue culture and this is where you get a single cell in a dish and the cell divides and a one cell thick tissue is formed on the bottom of the dish. The cells do not become specialised. This is done so drugs and medicines can be tested on the cells to look for any possible harmful impacts of a drug. Another reason this may be done is so scientists can examine and explore how cells divide and learn more about cells and cell division.

Mammal cells usually stop dividing after 20 generations or so. Practical Work with Plant Cells. When you want to look at a plants cell you usually take a sample from the epidermal tissue which is on the outside of the plant and rhubarb or onions are particularly good when it comes to this. The sample will be soaked with weak iodine and this will stain the nucleus yellow and the starch grains will be blue. If you used the epidermis of rhubarb the cell sap in the vacuole should be seen as a red colour. To see chloroplasts you need to carefully pull a leaf from a moss plant and once that leaf is soaked in water you should see the chloroplasts, however you do need a powerful microscope. Practical Work with Animal CellsThere used to be a few ways to quickly get animal cells to examine in the classroom and they were, from us. A previous practice involved getting cheek cells from a human and examining the cell, however this was banned due to the threat it posed to spreading AIDS, even though the threat was very minimal. Today there are two ways could get animal cells. The first method is you swabbing your cheeks and gums with a cotton bud and after a through cleaning and after placing some methylene blue solution on the cells you should be able to view them. The second method involves sellotape being put onto a washed wrist, peeled off, and then examined. You should be able to see the cells and maybe a nucleus and putting the same methylene blue solution onto the cells you should be able to see a stained nucleus. PhysiologyCell physiology is a name given to all the functions, roles and needs a cell takes on, this can also expand larger and go to the tissues, organs, systems and an organism itself. Some examples of human physiology are digestion, blood circulation and contraction of muscles. Some examples of plant physiology are water absorption and the production of food. An organisms physiology to an extent is the physiology of its cells. For example cells in humans need oxygen and food and we have to breathe and eat. Over the next few PowerPoints we will examine cell physiology. WaterAbout 75% of a cell is water and the cell will die if its water percentage falls lower than this. Water is useful because it is used in a lot of chemical reactions and enzymes and organelles can move about in and work easily with water. For example water in plant cells are useful for photosynthesis and water in animal cells is used to break help down food molecules. Water also have some helpful and useful properties, for example water has a high capacity for heat. This means it can take a lot of heat in and it wont heat up a lot. This is useful because it ensures proteins in the cytoplasm wont get harmed. One downside to water is that it freezes at 0 degrees Celsius so if the temperature goes below 0 degrees Celsius the cell will be damaged because ice will form in the cytoplasm. ProteinsSome proteins contribute to the cells make up and aid the cells various structures. (EG: the cell membrane, mitochondria and ribosomes.) These are called structural proteins because they help make up the cell. There is a second type of protein and they are enzymes. Enzymes are found throughout the cell and they control the chemical reactions what keep the cell alive. The nucleus controls how many enzymes are made and so the nucleus is comparable to brain or a control centre for the cell. All proteins contain carbon, hydrogen, oxygen and nitrogen. This can be split even further into amino acidsAmino AcidsAmino acids make up proteins and there are 20 different amino acids found in animal cells. A protein then is made up of hundreds of these amino acids combined together in different unique ways. The chain isnt a straight line and they form shapes. The shapes are dependent on hoe the amino acids connect and link. The shape of the protein can change how it reacts to other substances. When you heat a protein up the links start to break down. This happens at around 50 degrees Celsius and the protein will not cool and form into its original shape. When this happens the protein is said to be denatured. Due to this the protein will lose its original qualities. DenaturingWhen a protein is denatured links between its amino acids break and the protein loses its shape and characteristics and when it cools it will have different properties and different shapes. One example of this is egg white. Egg white is a protein and when you denature and let it cool you will have a new protein with new characteristics. Before being denatured egg white is a runny, clear liquid but after it turns into a white solid and it can not change back. Cells can be damaged through this process. Cells exposed to heats of over 50 degrees Celsius can experience structural and enzyme failure due to the cells proteins being damaged. Lipids Lipids are oils or fats and substances related or derived from them. Fats are formed from carbon, hydrogen and oxygen only. A fat molecule is made up of smaller organic molecules called fatty acids and one molecule of glycerol. Lipids form parts of the cell and nucleus membranes and fat stored in the cytoplasm serve as energy reserves. CarbohydratesCarbohydrates arent one thing, there are lots of different types of carbohydrates and a very common one is glucose, however you do also have starch and cellulose. They are sugars and they can be very large molecules. Glucose is a simple sugar and is very common and two glucose molecules can combine to form maltose. Glucose can link together to make long chains. SaltsVarious salts are present in a cell but they are ions. Ions are helpful to a cell but you have to have them in moderation. Too little or too many ions can influence and change the physiology of the cell. Some ions help a cell react to electrical input and this can help the cell respond to stimuli or in the case of a nerve cell pass on electrical signals. VitaminsVitamins are substances that cells need to perform chemical reactions and to keep everything in check. Plants can create their own vitamins but human need to eat vitamins because animal cells can't create vitamins however if you give an animal cell the building blocks to create vitamins it can. With a lack of vitamins the cell physiology is messed up and the whole organism can suffer if there is a lack of needed vitamins. Synthesis and conversionCells are able to make up (synthesise) or convert proteins, lipids and carbohydrates. All cells can make proteins if they have the amino acids and they can build up fat from glycerol and the fatty acids needed. Animal cells can change carbohydrates to lipids and vice versa. Animals can make proteins but an animal cells needs to have the amino acids where as plant cells can make their own amino acids from sugars and salts. Enzymes and other factorsEnzymes and HeatEnzymes are proteins and can not be exposed to temperatures above 50 degrees or else theyll become denatured however heat can be very beneficial for enzymes and a cell. Chemical reactions happen quicker the hotter the temperature is. It has been noted that a rise of 10 degrees Celsius will double the rate of a chemical reaction. This holds true for enzymes. Above 50 degrees Celsius an enzyme is subject to deformation and this means that they dont speed up chemical reactions. This may be a factor in a cells death if exposed to too much heat. Scientists can take advantage of this in experiments. It is called control and it tests if a protein is an enzyme. If this substance still carries out chemical reactions after being heated to 50 degrees it cant be an enzyme. Enzymes and pH. There is not one type of enzyme and all the different types of enzymes work well at different levels of pH. For example an enzyme in your stomach works best at a pH of 2, but amylase in your saliva would fail to function at this pH level. In cells most enzymes work best at 7 or neutral. This is called the optimal pH level but optimal can also apply to other aspects, for example you also have optimal temperature. Unlike heat if an enzyme is exposed to a pH level it doesnt like its not permanently damaged unless its at the extreme ends of the scale. Enzymes are specific. This means that enzymes are suited to only work with one substance and will usually only work with that one substance. This ensures that an enzyme which breaks down substances doesnt start to break down needed and intact substances. You can almost think of the enzyme as a lock and it will only work with its specific key which is the substance. This means that if a reaction happens in stages with the substance changing various times a new enzyme is needed. Most enzymes end in ase and they tend to be named on what they work with. For example the enzyme that works with proteins is named protease. Amount of EnzymesNot only the temperature and pH affect how well and how fast reactions occur, the number of enzymes themselves affect how fast a reaction occurs. With more enzymes you have faster reactions. This is why the nucleus is the brain of the cell, it dictates how many enzymes are made. Intracelluar and Extracelluar EnzymesMost enzymes produced in a cell will stay inside the ecll and be in the cytoplasm doing its job. These are called intercellular enzymes. Some however will be secreted and go outside of the cell. These are extracellular enzymes. Fungi and bacteria take advantage of this and release enzymes so food can be digested. Mould growing on bread will release enzymes and the mould will absorb the sugars the enzymes produce. The same thing happens in animals in the digestive system, enzymes are released to digest and break down food. Enzymes in IndustriesFor years enzymes have been used in industries and by companies. In baking and brewing enzymes in yeast are exploited. The enzymes convert sugar to alcohol and carbon dioxide and it is this what gives bread a fluffy full effect and in brewing it gives beers and wines a sparking effect. In both these examples the enzymes were from living cells but with cheese making enzymes from calves stomachs were used and it clotted milk in the early stages of the cheese making process. The organic enzyme is called Rennin but an engineered one is commonly used now and is called Chymosin. Why Use EnzymesThere are three main reasons we use enzymes in situations like these and these are:They work at lower temperatures and are money efficient.They are not corrosive and easy to work with compared to the alternatives to enzymes.They are specific. Because enzymes only work with one substance you know what will happen and everything can be controlled and manipulated. Cellular RespirationMore or less all the processes that happen in a cell need energy to make them happen. Cells get energy from food and that is most commonly glucose. The process from energy being produced from food is called respiration and it is a chemical process. This can be easily confused for breathing which is also called respiration so to be specific you would normally use phrases like cellular respiration, internal respiration or tissue respiration. There are two types of respiration, aerobic and anaerobic and both are useful at different times. Aerobic RespirationFor aerobic respiration you need to have oxygen. This is where the food and oxygen combine in a process known as oxidation and the food is now oxidised. All food molecules have carbon, hydrogen and oxygen atoms and this process takes that carbon and turns it into carbon dioxide and turns that hydrogen into water. At the same time energy is released and this can be shown in a single equation. Aerobic Respiration EquationC6H12 06 + 602 6CO2 + 6H20 + 2830kJ

EnzymesGlucoseOxygenCarbonDioxideWaterEnergy!This is the foodNotes on the EquationSo to get 2830kJ you need 180 grams of glucose.It doesn't happen in one large chunk, it happens over a long period of time.This slowly releases the energy.Some of the energy released is heat, this is inevitable. This happens in the mitochondria.

Anaerobic Respiration With anaerobic respiration you do not use oxygen. In the chemical reaction you only have glucose and enzymes to work with, no oxygen involved. A common example of this anaerobic respiration is yeast and sugars. The yeast uses the sugars to respire and the sugar isnt completely broken down or oxidised. This is called fermentation and is shown in this chemical equation on the next slide. Carbon dioxide is produced but no water is produced. Animals can use anaerobic respiration and usually do at periods of intense physical activity, e.g. exercise.Anaerobic Respiration EquationC6H12 06 2C2H5OH + 2CO2 + 118kJ

EnzymesGlucoseEnergy!This is the foodAlcoholCarbon DioxideThe Second Stage Aerobic RespirationAs you can see not as much energy is produced by anaerobic respiration. This is because the alcohol holds a lot of the energy and the yeast can tap into that energy. Animals on the other hand can get hold of that energy. This alcohol in animals is called pyruvic acid and after anaerobic respiration has taken place aerobic respiration will break down the acid into carbon dioxide and water. During exercise this pyruvic acid can build up into lactic acid and is taken away from the muscles and goes into the bloodstream. When the lactic acid reaches the liver it some of it will be oxidised. Once exercise has ended the animal is still going to breathe heavily and this will continue until all the lactic acid has been oxidised. This accumulation of lactic acid and oxygen needed to pay off the acid is considered to have created an oxygen debt. It is thought that this lactic acid build up causes muscle fatigue. MetabolismThe chemical changes happening in a cell or an organism has a name; metabolism. The minimum energy needed to keep the cell or organism alive, without movement or growth, is basal metabolism. Basal metabolism covers key aspect and functions which keep us alive such as respiration, secretion and digestion. A process in an organism which breaks substances down can be known as catabolism and a process which creates or builds new substances can be known as anabolism. Respiration is considered a catabolic process and building new proteins is considered an anabolic process. Cell MembraneWhether this diffusion into or out of a cell happens is down to the cell membrane. Remember that the cell membrane acts as a bouncer and allows things into or out of the cell. As a rule of thumb however small molecules like water, carbon dioxide and oxygen are easily allowed to pass through the cell membrane. This is very beneficial for the cell. For example if a cell oxidises food the concertation of oxygen in the cell will decrease and oxygen will diffuse into the cell or if carbon dioxide builds up in the cell due to respiration the carbon dioxide due to diffusion will leave the cell. Both of these benefit the cell and are helpful for the cell. Diffusion Rate FactorsThere are factors that can affect the rate of diffusion into or out of a cell. These factors are temperature, pressure, distance it has to diffuse, concertation and the size of the molecules. The cell can also impact the rate of diffusion. One example is the thickness of the cell membranes or cell walls. Generally the thicker the cell membranes and cell walls the slower the rate of diffusion. Another factor is the concertation gradient or the difference between the high and low concertation numbers. The higher the gradient the higher the rate of diffusion. It can also be down to the molecule itself, water diffuses slower than amino acids do. Enzymes can also speed up the rate of diffusion. Surface AreaSurface is one of the biggest factors in determining how fast diffusion takes place. The greater the surface area, the greater the rate of diffusion. Some places in the human body require rapid diffusion, for example the intestines. The cells in the intestines are special and maximise the surface area because the cells are in long finger like shapes that boost surface area. It gets better though as these cells, called villi, has smaller versions of themselves projecting out from the cell membrane and these are called microvilli. Endo- and Exocytosis.Cells can take in or expel solids or liquids through the cell membrane. Endocytosis (taking in a solid or liquid) happens with a particular white blood cell called phagocytes. Phagocytes engulf and digest bacteria and this engulfing has a special name specific to phagocytes and it is called phagocytosis. Exocytosis happens in glands and this is where cells form vacuoles that hold a digestive enzymes. The enzyme filled vacuole makes its way to the cell membrane where it is secreted and leaves the cell. Diagram.

OsmosisOsmosis occurs when you have a partially permeable membrane and two different solutions, a concentrated solution and a dilute solution. (Concentrated meaning pure). When the two are separated by the membrane water from the diluted solution moves over to the concentrated solution meaning the concentrated solution rises and the diluted solution falls. This is similar to diffusion but only applies to water and it can go both ways. In some cases the partially permeable membrane can act as a sieve and other times it wont. There is a visual representation on the following slide.Visual Representation. In this case the membrane doesnt act like a sieve as most of the water will go left to right. The larger sugar molecules will go through but very very slowly. In this case the membrane does act like a sieve as most of the water will go right to left. The larger sugar molecules will go through again but very very slowly. This is why it acts like a sieve, it stops the sugar molecules going through, or at least makes them go through very slowly. In Animal CellsExplanationThe cell started out with a low concentration of water but water osmotically diffused into the cell. This in turn makes the cell swell up and increase in size. If too much water enters a cell it will burst and if too much leaves a cell it will die as we have previously talked about before. Plant cells can take it even further. When they are full of water they are ridged and sturdy. The water filled vacuole presses against the cell wall and membrane and the vacuole is exerting turgor pressure on the cell wall. It can also be said that the cell is turgid. If the stem and leaf is turgid the plant looks healthy strong and alive. The opposite is a state of flaccid and this is characterised by a limp, weak and droopy plant. Remember!Carbohydrates are made up from carbon, hydrogen and oxygen. Glucose which is produced by photosynthesis is C6H12O6 . The waste product produced is Oxygen.Glucose is only one of many carbohydrates that could be created.

Flow diagramSunlight is converted to chemical energy by the chlorophyll.Water and carbon dioxide with the chemical energy is converted to glucose and oxygen.The chemical energy is used for photosynthesis. Photosynthesis Equation6CO2 + 6H2O C6H12O6 + 6O2

Light EnergyCarbon DioxideWaterGlucoseOxygenPhotosynthesis ProcessThe The ProcessThe process of photosynthesis happens in the leaves. In plants water is soaked up by the roots and is transported to the leaves. The stomata in the leaf absorbs carbon dioxide. Here the cell synthesises the water and carbon dioxide into glucose, a sugar and the waste product oxygen is also created. Gaseous ExchangePlants and animal cells need to take in and expel gasses and they do this through a process called gaseous exchange. The needs of plant and animal cells is reversed (Plants need carbon dioxide and for animals it is a waste product) so the gaseous exchange is reversed. Gaseous exchange doesnt just happen in plants in the leaf, in humans gas exchange happens in the alveoli where oxygen is exchanged with carbon dioxide in the red blood cells. Adaptations of LeavesBroad and flat shape allows for quicker diffusion. Leaves are thin so the gas doesnt have to travel far to reach the inside cells. Inside the leaf it is spacious so gas can easily move about. There are a lot of stomata so gas can easily get into the leaf.Chloroplasts are on top where the most sunlight will hit. The web like water network in a leaf means that all cells will receive water.