Cells and movement across membranes



Animals and plants are multicellular organisms  – they consist of many cells that work together.                                                                Animal cell structure                                                                                  The main parts of an animal cell are the nucleus, cell membrane, cytoplasm and mitochondria.                                                                    Plant cell structure                                                                                    Plant cells contain the same features as animal cells. They also have some additional ones:

  • chloroplasts
  • cell wall made of cellulose
  • large central vacuole
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Summary of features found in cells and their funct

PartFunctionFound in Cell membrane Controls the movement of substances into and out of the cell Plant and animal cells Cytoplasm Jelly-like substance, where chemical reactions happen Plant and animal cells Nucleus Carries genetic information and controls what happens inside the cell Plant and animal cells Mitochondria Where respiration takes place, releasing energy for the cell Plant and animal cells Large central vacuole Contains a liquid called cell sap, which keeps the cell firm Plant cells only Cell wall Made of a tough substance called cellulose, which supports the cell Plant cells only Chloroplasts Contains green pigment chlorophyll, which absorbs light energy. This is where photosynthesis occurs. Plant cells only

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The microscope in the picture is a light microscope. It uses focused light passed through the object and two lenses to magnify an image of the object. Lenses If looking at an object under the ×4 objective lens, light will have passed through the object, the ×4 objective lens, and the ×10 eyepiece lens before it gets to your eye. This makes the image 4 × 10 = 40 times bigger.Stage It has a hole in the centre which light can be focused up through the object. Using light microscopy it is possible to magnify using a ×40 objective lens which magnifies the image 400 times. Using an objective lens with higher magnification than this will make the image bigger but will not improve image clarity, this means the image will look bigger but less detailed. To see things at a higher magnification, and with a higher level of detail, then electron microscopy canbe used.

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Making a glass slide

The easiest cells to look at in the lab are animal cells, from your own cheek. To look at the cells under the microscope you must first prepare a slide of the cells to place on the microscope stage.

  1. Rub a clean cotton bud gently on the inside of your cheek.
  2. Smear the sample across a clean glass slide.
  3. Cells are transparent, place a few drops of a dye called methylene blue onto the smear so the cells will be visible under the microscope.
  4. Use a mounted needle to gently lower a glass coverslip onto the sample on the slide, take care not to form air bubbles under the coverslip.
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Before you undertake any practical activity it is important that you carry out a risk assessment. To do this you must consider three things: the hazard, the risk, and the control measure.                                                                                                         Carrying out a risk assessment       HazardRiskControl measure Name the chemical or apparatus in the experiment and give a reason it is a hazard. Methylene blue. It is an irritant. Give a step in the method where there is a risk of this hazard causing harm. If methylene blue comes into contact with the skin or eyes during the procedure, then it could cause irritation. What you can do to minimise the risk? Wear laboratory gloves. Wear safety glasses. Use low concentrations of the chemical.

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Multicellular organisms begin life as a single fertilised egg cell called a zygote. The zygote has a nucleus containing a full set of genes. When the zygote divides by mitosis, the full set of genes are copied and this process continues until a ball of cells called an embryo is formed. At this point cells begin to become adapted to specific functions. This is called differentiation and is controlled by genes.

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nerve cell is long and insulated with a fatty layer to carry electrical impulses around the body.

Diagram showing the shape of a nerve cell

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sperm cell has a tail so that it can swim to the egg.

Diagram showing the shape of a sperm cell

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palisade cell is packed with chloroplasts for photosynthesis.

Diagram showing the shape of a palisade cell

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xylem cell

xylem cell is a long, thin, straw-like waterproof tube which carries water from plant roots to leaves.

Diagram showing the shape of xylem cells

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red blood cells

Red blood cells are a biconcave shape, have no nucleus, and contain haemoglobin to carry oxygen around the body.

Diagram showing the shape of red blood cells

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cells tissues and organs

Cells, tissues and organs

Diagram of the hierarchy of cells, tissues, organs, organ systems and organisms, in that order.

Cells with a similar function are grouped together in tissues. A collection of different tissues carrying out a particular function is called an organ. Several different organs working together to perform specific functions are called an organ system. Organ systems working together form an organism.

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Concentration gradients

The idea of concentrations and the gradients within them is important when understanding the movement of particles or molecules across cell membranes.                                                                                              Concentration                                                                                                When sucrose is dissolved in water, the solute is sucrose and water is the solvent. Together they form a solution.                                The more sucrose particles there are in a certain volume of the solution, the more concentrated the solution is.

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Solution one Low solute concentration

Diagram showing solute concentration. A beaker filled with water labelled Solution one; Low solute concentration; High water concentration. Particles in the beaker are labelled as Solute.

Solution one

Low solute concentration

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Solution two High solute concentration

Diagram showing solute concentration. A beaker filled with water labelled Solution two; High solute concentration; Low water concentration. Particles in the beaker are labelled as Solute.

Solution two

High solute concentration

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Concentration gradients 2

A solution with a low solute concentration has a high water concentration. Pure water has the highest water concentration.

A concentration gradient exists when there is a region of high concentration leading to a region of low concentration:

  • going from high to low concentration is going down the concentration gradient
  • going from low to high concentration is going against the concentration gradient
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Diffusion is the movement of particles from a region where they are in high concentration to a region where they are in low concentration, and is one of the ways substances can move across the cell membrane, into or out of the cell.

Particles diffuse down a concentration gradient. This is known as passive transport.

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Diffusion experiment

A beaker filled with water. A purple lump sits at the bottom of the beaker labelled as Crystals of potassium permanganate.Diffusion experiment Potassium permanganate is placed into a beaker of water

A beaker with the label After 15 minutes. Layers of purple sit in the beaker. The layers get lighter in colour from bottom to top. The layers are labelled A, B, C and D.Diffusion experiment Particles diffuse from an area of high concentration to an area of low concentrationA beaker with the label After 1 hour. The contents of the beaker are completely purple. The contents are labelled Potassium permanganate solution after 1 hour - equilibrium. Diffusion experiment The contents of the beaker are now all the same concentration. The solution is now said to be in equilibrium

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Examples of diffusion in living organisms

Examples of diffusion in living organisms                                                                                  Products of digestion, dissolved in water, can pass across the wall of the small intestine by diffusion. Their concentration is higher in the small intestine than their concentration in the blood, so there is a concentration gradient from the intestine to the blood. Oxygen and carbon dioxide, dissolved in water, are exchanged by diffusion in the lungs: oxygen moves down a concentration gradient from the air in the alveoli to the blood carbon dioxide moves down a concentration gradient from the blood to the air in the alveoli the dissolved substances will only continue to diffuse while there is a concentration gradient. Blood flow continuously takes oxygen away from the lungs. This helps to maintain the concentration gradient. Gas exchange in the lungs happening in the alveoli

Diagram showing deoxygenated blood entering the alveoli and oxygenated blood leaving through the other side.

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Diagram of a selectively permeable membrane. Large solute molecules (coloured blue) are unable to pass through the membrane. Small water molecules (coloured red) are able to pass through the membrane.Osmosis is the diffusion of water molecules, from a region of higher concentration to a region of lower concentration, through a selectively permeable membrane. A dilute solution contains a high concentration of water molecules, while a concentrated solution contains a low concentration of water molecules. Osmosis is a passive process. Selectively permeable membranes allow some substances to pass through them, but not others.

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Osmosis experiment 1

A beaker's contents separated by a selectively permeable membrane. Solution 1, higher water concentration, lower sugar concentration. Solution 2, lower water concentration, higher sugar concentration.Osmosis experiment

The beaker contains water and sugar molecules

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Osmosis experiment 2

A beaker's contents separated by a selectively permeable membrane. Solution 1, higher water concentration. Solution 2, lower water concentration. Arrow across the membrane labelled Diffusion of water.Osmosis experiment

Water molecules pass through from solution one to solution two

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Osmosis in cells

Eventually, the concentration either side of the membrane will be the same. At this point, there will be equal movement of water molecules in both directions. The solution is said to be in equilibrium – there is therefore no net movement in one direction.

Osmosis in cells

The results of osmosis are different in plant and animal cells.

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plant cells osmosis

Plant cells have a strong cellulose cell wall on the outside of the cell membrane. This supports the cell and stops it bursting when it gains water by osmosis. A plant cell in a dilute solution (higher water concentration than the cell contents) Water enters the cell by osmosis. The cytoplasm pushes against the cell wall and the cell becomes turgid.A plant cell in a concentrated solution (lower water concentration than the cell contents) Water leaves the cell by osmosis. The cytoplasm shrinks and pulls away from the cell wall. This process is called plasmolysis. The cell becomes flaccid and the plant wilts.  Turgid plant cells play an important part in supporting the plant.

Diagram of cytoplasm pulling away from the cell wall. There is a space between the cytoplasm and the cell wall. The cell wall is arching inwards.

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animal cell osmosis

Animal cells do not have a cell wall. They change size and shape when put into solutions that are at a different concentration to the cell contents. For example, red blood cells:

  • gain water, swell and burst in a more dilute solution (this is called haemolysis)
  • lose water and shrink in a more concentrated solution (they become crenated or wrinkled)

These things do not happen inside the body. Osmoregulationinvolving the kidneys ensures that the concentration of the blood stays about the same as the concentration of the cell contents.

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Active transport

Active transport is the movement of dissolved molecules into or out of a cell through the cell membrane, from a region of lower concentration to a region of higher concentration. The particles move against the concentration gradient, using the energy released during respiration. Sometimes dissolved molecules are at a higher concentration inside the cell than outside, but because the organism needs these molecules, they still have to be absorbed. Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient.

Active transport diagram with the labels: Carrier molecule, Outside cell, Inside cell and Cell membrane.

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examples of active transport

Examples of active transport include: uptake of glucose byepithelial cells in the villi of the small intestine uptake of ions from soil water by root hair cells in plants Because energy is required for this process to occur, anything that prevents oxygen or glucose uptake by the cell will also prevent respiration. Without respiration, energy is not released from glucose and active transport cannot occur. Active transport is the biological process involving molecules passing through a cell membrane into the cell where there are a bunch of molecules. Active transport concerns different concentrations of molecules from low concentrations of molecules into high. As you can see, there are far more molecules inside the cells than there are outside the cells. When a molecule moves into a cell with a high concentration of molecules, the particle is going against what is called the concentration gradient. ate that last Venus bar? It took a lot of energy to do it, didn’t it?  In order to get molecules from an area of low concentration into an area of high concentration, it requires energy.

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Comparing diffusion, osmosis and active transport

Comparing diffusion, osmosis and active transport

DiffusionOsmosisActive transport Down a concentration gradient Against a concentration gradient Energy needed Substance moved Dissolved solutes Water Dissolved solutes Notes Gases and dissolved gases also diffuse Partially permeable membrane needed Carrier protein needed

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Osmosis experiments

Visking tubing is an artificial selectively permeable membrane:

  • smaller molecules like water and glucose pass through its microscopic holes
  • larger molecules like starch and sucrose cannot pass through it

This slideshow shows a typical experiment using Visking tubing and sucrose solution.

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os ex 1

A capillary tube leading to visking tubing, filled with sucrose solution in a beaker filled with water. An arrow pointing upwards signifies liquid rising. The water is blue and the solution is red.Visking tubing experiment

The Visking tubing is the selectively permeable membrane. Water moves by osmosis from the high water concentration (dilute solution) in the beaker into the low water concentration (concentrated solution) in the Visking tubing across the membrane, increasing the volume of liquid in the Visking tubing, which forces liquid up the capillary tube.

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os ex 2

A capillary tube leading to visking tubing, filled with sucrose solution in a beaker filled with water. An arrow pointing downwards signifies liquid falling. The water is red and the solution is blue.The Visking tubing is the selectively permeable membrane. Water moves by osmosis from the high water concentration (dilute solution) in the Visking tubing into the low water concentration (concentrated solution) in the beaker across the membrane, decreasing the volume of liquid in the Visking tubing, which lowers the level of liquid in the capillary tube.

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osmosis in potatoes

Osmosis in potatoes Cylinders or discs of fresh potato are often used to investigate osmosis in living cells. To carry out this type of experiment, you need to: cut equal-sized pieces of potato blot with tissue paper and weigh put pieces into different concentrations of sucrose solution for a few hours remove, blot with tissue paper and reweigh The percentage change in mass can be calculated for each piece of potato.

\text{percentage change in mass} = \frac{\text{end mass - start mass}}{\text{start mass}} \times 100

A piece of potato has a mass of 2.5 g at the start and 3.0 g at the end. percentage change in mass = (3.0 – 2.5) ÷ 2.5 × 100 = 0.5 ÷ 2.5 × 100 = +20% The plus sign shows A piece of potato has a mass of 2.5 g at the start and 2.0 g at the end. percentage change in mass = (2.0 – 2.5) ÷ 2.5 × 100 = –0.5 ÷ 2.5 × 100 = –20% The minus sign shows that it has lost mass. It will have lost water by osmosis.

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Enzymes An enzyme is a protein that functions as a biological catalyst – a substance that speeds up a chemical reaction without being changed by the reaction.

Structure of enzymes Different enzymes contain up to 20 different amino acids linked together to form a chain which then folds into the globular enzyme shape. Enzymes have active sites which only match specific substrates.                       Lock and key model Enzymes are folded into complex shapes that allow smaller molecules to fit into them. The place where these molecules fit is called the active site. In the lock and key model, the shape of the active site matches the shape of its substrate molecules. This makes enzymes highly specific – each type of enzyme can catalyse only one type of reaction (or just a few types of reactions). The diagram shows how this works. In this example, the enzyme splits one molecule into two smaller ones, but other enzymes join small molecules together to make a larger one.

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lock and key

1. Substrate collides with active site of enzyme and becomes attached. 2. Enzyme catalyses breakdown of substitute. Enzyme substrate complex is formed. 3. Products released from active site. If the shape of the enzyme changes, its active site may no longer work. We say that the enzyme has been denatured. Enzymes can be denatured by high temperatures or extremes of pH.

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effect on temperature

Effect of temperature

As with ordinary chemical reactions, the rate of an enzyme-catalysed reaction increases as the temperature increases. However, at high temperatures the rate decreases again because the enzyme becomes denatured and can no longer function as a biological catalyst.

Line graph showing Rate of enzyme activity by Temperature (C°). Points are labelled from 1 - 4 with the peak rate being point 3, labelled as Optimum temperature. At low temperatures the enzymes and substrates have low kinetic energy. This results in the particles colliding less often, which means there will be fewer successful collisions between the substrate and the enzyme’s active site. As the temperature increases, the kinetic energy increases, leading to more collisions and enzyme substrate complexes formed per unit time. This increases the rate of reaction. At the optimum temperature the maximum number of enzyme-substrate complexes form per unit time. If the temperature continues to increase past the optimum, the increased kinetic energy breaks the weak hydrogen bonds holding the enzyme’s unique active site shape. Enzyme–substrate complexes can no longer form as the substrates no longer fit into the active site. The enzyme is denatured.

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effect of PH

Effect of pH

Changes in pH alter the shape of an enzyme’s active site. Different enzymes work best at different pH values.

The optimum pH for an enzyme depends on where it normally works. For example, intestinal enzymes have an optimum pH of about 7.5, but stomach enzymes have an optimum pH of about 2.

Line graph showing Increasing enzyme activity by pH levels. pH 8, the peak of the line, is labelled as Optimum pH.

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