GCSE_Biology

My Revision Planner

• Lists the units and topics covered in the revision notes, including:

  • Cells

  • Photosynthesis and plants

  • Food and energy

  • Enzymes and digestion

  • The respiratory system and cell respiration

  • Coordination and control

  • Ecological relationships and energy flow

  • Osmosis and plant transport

  • The circulatory system

  • DNA, cell division, and genetics

  • Reproduction, fertility, and contraception

  • Variation and natural selection

  • Health, disease, defence mechanisms, and treatments

Cells

• The cell is the basic building block of animals and plants; bacteria are formed of single cells.

• Cell: the basic building block of all living organisms.

Animal, Plant, and Bacterial Cells

• Plants and animals are formed of millions of cells, while bacteria are formed of only one cell.

• Table 1.1 summarizes the main features of animal, plant, and bacterial cells.

• The only structures present in all three types of cells are cell membranes and cytoplasm.

Using a Microscope to Examine Plant and Animal Cells

• Making a slide of plant cells:

  • Peel a small section of onion tissue and place on the center of a microscope slide.

  • Add water using a drop pipette to the onion tissue to stop it drying out.

  • Gently lower a coverslip onto the onion tissue to avoid trapping air bubbles.

  • Set the slide onto the stage of the microscope and examine using low power first and then high power.

• The coverslip should be lowered one end first onto the onion tissue to avoid trapping air bubbles.

• Making a slide of animal cells is described in an earlier section.

• When looking at onion cells under a microscope, you will probably see the cell walls, cytoplasm, nuclei (if stained, with iodine for example), and possibly the vacuole.

• Plant cells are much more regularly shaped than animal cells and are usually much larger as well.

• Always use low power first when using a microscope.

Biological Drawings

• Good quality biological drawings are:

  • Made in pencil with firm and continuous lines.

  • Reasonable size, making good use of the available space.

  • In the same proportions as, and a good representation of, the cell(s) being observed.

  • Labeled using separate ruled lines spread out around the drawing, with each line starting as a bullet point on the structure and ending with a clearly written label.

  • Given a title and the magnification used (if appropriate).

• Only draw what you see, not what you think is there.

• If the number of cells to draw is not specified, do not draw too many - accuracy is often sacrificed for quantity.

Magnification

• The magnification of a microscope is the magnification of the eyepiece lens multiplied by the magnification of the objective lens.

• Magnification of a photograph or a drawing:

  • Magnification = \frac{\text{size of image}}{\text{actual size}}

  • The size of an image is the size of the cell or structure in the photograph or drawing.

  • The actual length (size) of the cell or structure being examined can be calculated using the formula:

    • l = A \times M

    • Where:

      • l = size (length) of the image

      • A = Actual size (length)

      • M = Magnification

    • Therefore:

      • Magnification = \frac{\text{size of image}}{\text{actual size}}

      • Actual \, size = \frac{\text{size of image}}{\text{magnification}}

• Micrometre: the unit of measurement usually used to measure cells; there are one million micrometres in a metre and one thousand micrometres in a millimetre.

• 1 \,\mu m = 1 \times 10^{-6} m

• When calculating the actual length of a cell (structure), it is important that the same units are used for the length of the image and the actual length of the cell or structure.

Using a Scale Bar

• A scale bar gives the actual length of a section of a photograph (or drawing); they are normally used for calculating the magnification.

Estimating Size

• Students are expected to be able to estimate sizes where appropriate.

Electron Microscopes

• Electron microscopes use beams of electrons to form an image, allowing for a much greater resolution than light microscopes.

• Electron microscopes allow us to:

  • See structures that we were previously not aware of.

  • See the internal detail of cell structures such as the nucleus and chloroplasts.

• Resolution: describes the ability of a microscope to preserve detail when magnifying.

Stem Cells

• Stem cells: cells that have the ability to divide and produce different types of cells.

Animals Have Two Types of Stem Cell

• Embryonic stem cells from embryos or the umbilical cord can form the full range of cells in the body.

• Adult stem cells can divide to form cells of the same general type, e.g., stem cells in the bone marrow only form the different types of blood cell.

• In plants, stem cells originate in the meristems (the rapidly dividing zones at shoot and root apices, or tips).

• Plant cells retain the ability to divide and so can be used in cloning techniques.

Stem Cells in Medicine

• Bone marrow transplants can be used to treat leukaemia; stem cells in the bone marrow can produce different blood cell types in the right proportions.

Peer Review

• Peer review is where scientific research is checked by other scientists of at least equal standing to validate stem cell research.

Diffusion

• Diffusion: the random movement of molecules from a region of high concentration to a region of low concentration.

• Factors affecting the rate of diffusion:

  • Temperature: The higher the temperature, the faster the diffusion.

  • Surface area: The greater the surface area, the faster the rate.

  • Concentration gradient: The greater the concentration gradient, the greater the rate of diffusion.

Multicelled Organisms and Specialisation

• In multicelled organisms, cells and regions of the body become specialised.

  Structure	Description
  Cell	Basic building block of living organisms, e.g., animal cell.
  Tissue	Groups of cells with similar structures and functions, e.g., skin.
  Organ	Groups of different tissues working together, e.g., brain.
  Organ system	Organs organized into organ systems, e.g., the nervous system.
  Organism	Different organ systems make up the organism, e.g., human.

• The Need for Exchange Surfaces in Multicelled Organisms:

  • As organisms get larger, their surface area/volume (SA/V) ratio decreases.

  • Surface area represents the area of body surface across which diffusion can take place.

  • Volume represents the volume of cells in the organism that need to be supplied with nutrients and oxygen.

  • Large multicelled organisms need to increase the surface area across which molecules can diffuse using specialised exchange surfaces.

Photosynthesis and Plants

Photosynthesis

• Photosynthesis: a process in plants in which light energy is trapped by chlorophyll to make food (sugars and starch) using carbon dioxide and water.

• The word equation for photosynthesis is:

  • carbon dioxide + water → glucose + oxygen

• The balanced chemical equation is:

  • 6CO2 + 6H2O → C6H{12}O6 + 6O2

• Endothermic reactions: reactions that require energy to be absorbed (taken in) to work.

Photosynthesis Experiments

• The glucose produced during photosynthesis is usually converted into starch for storage.

• A starch test can indicate whether photosynthesis has taken place.

• Destarching a plant involves leaving it in darkness for 48 hours to remove any existing starch before starting investigations.

Carrying out the test for starch

| Step                                  | Method                                                                                                                                                                                                                                       | Reason                                                                                       |
| :------------------------------------ | :-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :------------------------------------------------------------------------------------------- |
| 1.  Put the leaf in boiling water            | This kills the leaf and stops further reactions.                                                                                                                                                                                                | -                                                                                            |
| 2.  Boil the leaf in ethanol (alcohol)        | This removes chlorophyll (green color) from the leaf.                                                                                                                                                                                       | -                                                                                            |
|3. Dip the leaf in boiling water again    | This makes the leaf soft and less brittle.                                                                                                                                                                                                     | -                                                                                            |
|4. Spread the leaf on a white tile and add iodine | If starch is present, the iodine will turn from yellow-brown to blue-black.                                                                                                                                                                          | -                                                                                            |

To show that light is needed for photosynthesis:
* Destarch a plant.
* Partially cover a leaf on a plant with foil.
* Put the plant in bright light for at least 6 hours.
* Test the leaf for starch.

• To show that chlorophyll is needed for photosynthesis:

  • Destarch a variegated plant.

  • Put the plant in bright light for at least 6 hours.

  • Test the leaf for starch.

To show that carbon dioxide is needed for photosynthesis:
* Destarch a plant.
* Set up the sodium hydroxide to absorb carbon dioxide from the air inside the plastic bag.
* Leave the plant in bright light for at least 6 hours.
* Test one of the leaves for starch; a negative starch test shows it is needed.

To show that oxygen is produced:
* Using apparatus similar to that in Figure 2.3, it is possible to demonstrate that oxygen is produced in photosynthesis.
* Investigate the effect of light intensity on photosynthesis.

Limiting Factors in Photosynthesis

• Environmental factors (light, carbon dioxide and temperature) affect the rate of photosynthesis.

• Limiting factor: an environmental factor that limits the rate of photosynthesis due to that factor being present at a sub-optimal level.

• Light and carbon dioxide are necessary for photosynthesis, so the more of these are present, the faster the photosynthesis reaction takes place (up to a maximum).

• Hydrogencarbonate indicator is red in normal atmospheric carbon dioxide levels (0.04%). It turns yellow in increased carbon dioxide levels but purple in decreased carbon dioxide levels.
  *The point at which the rates of photosynthesis and respiration are equal the point is referred to as the compensation point.
  *Describe how to use hydrogencarbonate indicator to find the amount of light required to reach the compensation point (the point at which the rates of photosynthesis are equal) in pondweed.
  *Describe how to use hydrogencarbonate indicator to find the amount of light required to reach the compensation point (the point at which the rates of photosynthesis are equal) in pondweed.

Leaf Structure

• 
  

• Leaves are the plant organs in which photosynthesis occurs.
  

  Structure	Description
  Upper epidermis	No chloroplasts
  Palisade mesophyll layer	Cells with many chloroplasts
  Spongy mesophyll layer	Cells with air spaces between them
  Cuticle	waxy layer that prevents evaporation
  Lower epidermis	Stoma
  Guard	Guard cell
  Adaptations for Light Absorption

  • Large surface area
    Cuticle (waxy layer that prevents evaporation
    Thin, transparent cuticle
    *The presence of many tightly packed palisade mesophyll cells, end-on to the upper surface, with many chloroplasts rich in chlorophyll

  Adaptations for Gas Exchange

  The spongy mesophyll cells have a large surface area for gas exchange
  Intercellular spaces in the spongy mesophyll allow carbon dioxide to enter and oxygen to
  leave the photosynthesising cells, which are mainly concentrated in the palisade layer
  Stomata that allow carbon dioxide and oxygen to enter the leaf

  Food and Energy

  Food Tests

  Food type	Test	Method	Result (if food type present)
  Starch	Starch test	Add iodine solution.	Iodine turns from yellow-brown to blue-black.
  Sugar	Benedict's	Add Benedict's solution and heat in a water bath.	Solution changes from blue to brick-red.
  Protein	Biuret	Add sodium hydroxide, then copper sulfate and shake.	Colourless solution turns from blue to purple.
  Fat	Ethanol	Shake with ethanol, then add water; the color must remains clear.	Colourless ethanol becomes a cloudy emulsion.

  Biological Molecules

  • Carbohydrates, proteins, and fats are very important biological molecules, each containing carbon, hydrogen, and oxygen; protein also contains nitrogen.

  • Carbohydrate: biological molecule formed of sugar sub-units.

  Carbohydrates

  • Simple carbohydrates (sugars like glucose and lactose) are fast-release energy stores.

  • Complex carbohydrates (starch, glycogen, cellulose) are made of many glucose units linked together; starch and glycogen are storage molecules, while cellulose is a structural carbohydrate that provides support in plant cell walls.

  Proteins

  • Proteins are long chains of amino acids bonded together, which can be structural or functional.

  • Protein: a biological molecule formed of sub-units of amino acids.

  Fats

  • Fatty acids

  • The basic sub-unit of a fat consists of one molecule of glycerol and three fatty acid molecules, are high in energy (1 gram of fat contains approximately twice as much energy as 1 gram of carbohydrate or protein) so are excellent storage molecules.

  • Fat: the basic unit of what fats are ormed of is glycerol and three fatty acids.

  • Note: Fats are also called lipids.

  Enzymes and Digestion

  • Enzyme: a biological catalyst that speeds up reactions without being used up in the reaction itself.

  How Enzymes Work

  • In enzyme action, the substrate fits snugly into the active site of the enzyme (lock and key model).

  • Enzyme specificity means each enzyme works on only one substrate.

  Factors Affecting Enzyme Action

  • Temperature, pH, and enzyme concentration affect enzyme action.

  • The maximum rate of enzyme activity is the optimum.

  Factors that affect the action of
  Temperature
  maximum enzyme The Shape of the enzyme's
  activity occurs at the active site 1s Changed when
  optimum temperature The phiS not the Same as the
  hig her temperatu e the optimumn pH of the enzyme denatured
  enzyme e Substrate
  The shape of the active not a PerfeCt fit
  site and The Substrate The number of rate levels off as enzyme concentration
  Is always Complementary. Available Substrate increases molecules because the number
  of molecules rate of beComes limiting in concentration there is more

  Inhibitors

  • Inhibitor molecules inhibit (reduce) normal enzyme activity.

  Digestion and Absorption in the Digestive System

  • Digestion: breaking down large, insoluble food molecules into small, soluble molecules.

  • Absorption: small, soluble food molecules are transferred from the gut to the blood system (in the ileum).

  Enzymes in the Digestive System

  Enzyme	Food Digested (Substarte)	Products of Digestion
  Carbohydrate (amylase)	Starch	Glucose
  Protease	Protein	Amino acids
  Lipase	Fat	Glycerol and fatty acids

  Absorption in the Ileum

  The ileum is adapted for absorption in a number of ways:
  • a very large surface area due to its length, presence of folds (or twists) and villi
  • a good blood supply
  • thin and permeable membranes.
  Villi are microscopic finger-like extensions on the inner surface of the ileum.
  . There are lacteals that absorb fats into the blood to increase the surface area.

  Commercial Enzymes

  • Commercial processes often use enzymes (e.g., in biological washing powders). Commercial enzymes are widely used in the food industry.
    Commercial enzymes amylases are used to break down starch into sweeter and more soluble sugars in many products.

  The Respiratory System and Cell Respiration

  • Respiration: the release of energy from food.

  • Aerobic respiration: respiration in the presence of oxygen.

  • Anaerobic respiration: respiration in the absence of oxygen.

  Respiration (Cell Respiration)

  • Respiration is a biological process that continually releases energy from food (usually glucose); the energy is used for heat, movement, growth, reproduction, and active transport.

  • The word equation for aerobic respiration is:

    • glucose + oxygen → carbon dioxide + water + energy

  • The balanced chemical equation for aerobic respiration is:
    C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy

  • In mammalian muscle: glucose —> lactic acid + energy

  • in yeast glucose — alcohol + carbon dioxide + energy

  • Anaerobic respiration releases less energy than aerobic respiration.

  Demonstrating Anaerobic Respiration in Yeast

  • The rate of anaerobic respiration with affect different factors of how Anaerobic is repierated in yeast.

  The Respiratory System

  • In humans and many other animals, a specialized respiratory system is needed to ensure that enough oxygen can get into the body for respiratory requirements.

  Breathing

  Breathing is the process that brings fresh air, rich in oxygen, into the
  lungs and expels air rich in carbon dioxide.
  Aircapelled Air enters
  from balloons balloons In nalation Air expelleal
  from ba loons In the bell are there is the
  Baltoons Air erders rubber Sheet in the lungs and rubber sheet is from the

  Breathing and humans

  Description of the breathing in humans
  Intercostal muscles contract causing
  Rins to move up and out.
  the chest cavity (thorax)The volume of increases air enters the lungs this
  causing the pressure to causes pressure 1ss Below The falls
  The volume of the thorax decreases this increases
  The pressure Which forces alrOut of the

  The Effect of Exercise on the Depth and Rate of Breathing

  *Breathing is a process which brings air rich in oxygen into the lungs and thus supplies the oxygen the body needs. It also removes carbon dioxide produced during respiration from the body.
  *It brings more oxygen into the lungs as well extra oxygen is required.

  Respiratory Surfaces

  • Respiratory surface: the parts of living organisms across which respiratory gases can be exchanged between the environment (atmosphere) and the organism's cells.

  Gas exchange that takes place between the lungs consists of many microscopic air sacs called alveoli across a large surface

  Good diffusion gradients blood Low in oxygen cells cell increasing further increasing and high diffusion the diffusion the surface gradient gradient increasing InCarbon increases alveolus means
  membrane with increase for air with Very large number cells
  surfaces respiratory the good the that and Inanimals membranes gradient.
  **Plants does not have active breathing
  **Double Award students do not need to be able to be able to identify by and for increased
  concentration diagram.

  Coordination and Control

  The Nervous System

  • Stimuli affect receptors in the body, which may cause effectors to produce responses.

  • Neurons link receptors to the central nervous system (CNS), consisting of the brain and spinal cord.

  • Neurons carry nerve impulses.
    1 . The roles of the new are summarizes this.

  Voluntary and effectors are actions

  There are two main nervous sections are voluntary as as summarize reflex and as the summary the
  Conscious control there 1s no There 15 conscious the spinal of the in a actions Brain Involved of brain control with time is cord cord the of as or
  Speed action This 1s Slower FASter
  Neurone The small