Cell Biology

• Compare a prokaryotic cell and a eukaryotic cell ( 2 marks)

  • A prokaryotic cell has no nucleus, while a eukaryotic cell does.
  • Prokaryotic cells are smaller and have their DNA in a loop or plasmid.
• What is the function of mitochondria in a cell?

  To carry out aerobic respiration, releasing energy for the cell.
• Describe how a light microscope and electron microscope differ and how this has increased our understanding of cells.(6 marks)

  Light microscopes use light to form an image and have a lower resolution and magnification than electron microscopes. Electron microscopes use electrons and can magnify up to 2 million times, allowing us to see sub-cellular structures in much more detail, such as ribosomes and mitochondria. This has increased our understanding of cell structure and function. For example, we can now study how organelles are organised inside cells and how they contribute to processes like respiration and protein synthesis.
• Explain how the structure of specialised cells is related to their function. Use examples.

  (6 marks)

  • Specialised cells have adaptations that enable them to perform specific functions efficiently.
  • Sperm cells have a tail (flagellum) for movement and numerous mitochondria to provide energy for swimming.
  • Root hair cells have a large surface area to absorb water and minerals from the soil effectively.
  • Muscle cells contain many mitochondria to supply energy for contraction.
  • Nerve cells are long and have branched connections to transmit electrical impulses across the body.
  • These structural features are directly linked to the roles each cell plays in the organism.
  • Such specialisations ensure that biological processes occur efficiently.
• Describe the process of mitosis and explain its importance in the life of an organism.

  (6 marks)

  • Mitosis is a type of cell division that results in two genetically identical daughter cells.
  • Before mitosis, the cell replicates its DNA so that each new cell will have a complete set of chromosomes.
  • The chromosomes condense and align in the center of the cell.
  • Spindle fibers pull the sister chromatids apart to opposite poles of the cell.
  • The cell then divides its cytoplasm (cytokinesis), forming two new cells.
  • Mitosis is crucial for growth, repair of damaged tissues, and asexual reproduction in some organisms.
• Explain how substances are transported into and out of cells by diffusion, osmosis, and active transport. (6 marks)

  
  

  • Diffusion is the movement of particles from an area of high concentration to an area of low concentration, such as oxygen entering cells.
  • Osmosis is the movement of water molecules across a partially permeable membrane from a dilute solution to a concentrated solution.
  • Active transport moves substances against their concentration gradient, from low to high concentration, using energy from respiration.
  • For example, root hair cells use active transport to absorb mineral ions from the soil.
  • These processes are essential for maintaining proper cell function and homeostasis.
  • They allow cells to obtain necessary substances and remove waste products.
• Describe the differences between animal and plant cells and explain how these differences relate to their functions.

  (6 marks)

  • Both animal and plant cells have a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes.
  • Plant cells also have a cell wall, which provides structure and support.
  • They have chloroplasts to carry out photosynthesis and produce food for the plant.
  • Plant cells contain a large vacuole that maintains turgor pressure to keep the cell firm.
  • These features allow plant cells to make their own food and support the plant structure, unlike animal cells.
• Explain how stem cells could be used in medicine and discuss one potential risk.

  (6 marks)

  • Stem cells are unspecialised cells that can differentiate into any type of cell.
  • They can be used to replace damaged tissues, such as nerve cells in spinal injuries or insulin-producing cells in diabetes.
  • Embryonic stem cells can become any type of cell, whereas adult stem cells are more limited.
  • They offer the potential to cure diseases that currently have no effective treatments.
  • One risk is that the stem cells may cause tumours or be rejected by the body.
  • There are also ethical concerns around the use of embryonic stem cells.
• Describe how cell division by mitosis is involved in growth and repair.

  (6 marks)

  • Mitosis is a type of cell division that produces two identical daughter cells.
  • Before mitosis, the DNA is replicated so each new cell gets an exact copy.
  • The chromosomes line up and are pulled to opposite ends of the cell.
  • Then the nucleus divides, followed by the cytoplasm, creating two genetically identical cells.
  • Mitosis is used for growth, as it increases the number of cells.
  • It also helps in repair, replacing damaged or dead cells with identical new ones.
• Describe how practical techniques using a light microscope can be used to observe and compare cells.

  (6 marks)

  • First, prepare the slide by placing the specimen (e.g. onion cells) on a microscope slide.
  • Add a drop of iodine solution to stain the cells and make the structures easier to see.
  • Carefully place a cover slip on top to avoid air bubbles.
  • Place the slide on the stage and use the lowest magnification objective lens.
  • Use the coarse adjustment knob to focus the image, then the fine adjustment for clarity.
  • To compare cells, use a higher magnification to view details like the nucleus or cell wall in plant cells.
• Explain how changes in surface area to volume ratio affect the ability of cells and organisms to exchange substances efficiently.

  (6 marks)

  • As an organism gets larger, its surface area to volume ratio decreases.
  • This makes it harder for substances like oxygen and nutrients to diffuse into all cells quickly.
  • Small organisms or single-celled organisms have a large surface area compared to their volume, so diffusion is efficient.
  • Larger organisms need specialised exchange surfaces like lungs or villi in the intestine.
  • These surfaces have thin walls, large surface areas, and good blood supply to maintain steep concentration gradients.
  • These adaptations help ensure that efficient diffusion still occurs in larger organisms.
• Explain how cell differentiation differs in animals and plants, and why it is important.

  (6 marks)

  • Cell differentiation is the process where cells become specialised for a specific function.
  • In animals, most cells differentiate early in development and lose the ability to become other cell types.
  • In plants, cells can continue to differentiate throughout their life, especially in regions like the meristem.
  • Differentiation is important because it allows cells to perform specialised functions, like muscle contraction or nutrient absorption.
  • Specialised cells form tissues, which then form organs and organ systems.
  • This organisation enables complex organisms to survive and function efficiently.
• Describe a method to investigate the effect of sugar concentration on the mass of potato tissue.

  (6 marks)

  • Cut equal-sized pieces of potato using a cork borer and measure their initial mass using a balance.
  • Prepare several solutions of different sugar concentrations (e.g. 0%, 10%, 20%).
  • Place one potato piece into each solution and leave them for a fixed time (e.g. 24 hours).
  • Remove, dry gently with paper towel, and measure the final mass.
  • Calculate the change in mass for each piece.
  • Plot the results on a graph to see how sugar concentration affects osmosis in the potato cells.
• Describe how you would prepare an uncontaminated culture of microorganisms in the lab.

  (6 marks)

  • Sterilise all Petri dishes, inoculating loops, and nutrient agar to kill unwanted microbes.
  • Pour sterile agar into the Petri dish and allow it to set.
  • Dip the inoculating loop into the bacteria solution and gently spread it over the agar.
  • Replace the lid quickly and secure it with tape (but don’t seal all the way to allow oxygen in).
  • Incubate at 25°C in schools to prevent growth of harmful pathogens.
  • All steps help avoid contamination and ensure safe, reliable results.
• Describe a method to investigate the effect of antiseptics or antibiotics on bacterial growth.

  (6 marks)

  • Soak paper discs in different antiseptics or antibiotics.
  • Place them on an agar plate inoculated with bacteria (e.g. E. coli) using a sterile loop.
  • Include a control disc soaked in water.
  • Tape the lid, incubate the plate at 25°C for 48 hours.
  • Measure the diameter of clear zones (zones of inhibition) around the discs.
  • The larger the clear zone, the more effective the antiseptic or antibiotic is.
• Why is the large surface area of the villi important for absorption in the small intestine?

  (2 marks)

  • The large surface area of the villi increases the rate of absorption of nutrients by allowing more contact with digested food.
• What is osmosis?

  • Osmosis is the movement of water molecules from an area of higher concentration to an area of lower concentration through a semi-permeable membrane.