Comprehensive Biology Notes on Cell Biology and Organisation

Eukaryotic and Prokaryotic Cell Structures

• Eukaryotic cells are complex cells found in plants and animals that contain a nucleus and membrane-bound organelles.

• Animal cell components include:

  • Nucleus: Contains the genetic material and controls the activities of the cell.

  • Nuclear envelope: Present around the nucleus.

  • Mitochondria: The site of ATP synthesis (aerobic respiration).

  • Ribosomes: Involved in protein synthesis.

  • Cell membrane: Controls the movement of substances in and out of cells.

  • Rough Endoplasmic Reticulum (RER): Involved in protein synthesis.

  • Smooth Endoplasmic Reticulum (SER): Involved in lipid synthesis.

  • Golgi apparatus: Responsible for protein modification for transport and packaging.

• Plant cell components (in addition to those found in animals):

  • Cellulose cell wall: Provides structural support to the cell.

  • Chloroplasts: Contain chlorophyll for photosynthesis.

  • Permanent vacuole: Stores cell sap and pigments.

• Prokaryotic cells are simpler, smaller organisms like bacteria that lack a nucleus and membrane-bound organelles.

• Prokaryotic characteristics include:

  • No nucleus: DNA is not enclosed; instead, they have circular DNA.

  • Plasmids: Small rings of extra DNA.

  • Cell wall: Present but not made of cellulose.

  • Lack of membrane-bound organelles (e.g., no mitochondria or chloroplasts).

  • Ribosomes: These have no membrane.

  • Replication: They divide via binary fission.

Specialized Cells and Differentiation

• Differentiation is the process by which a cell changes to become specialized for its job.

• Animal cell specialization:

  • Most animal cells differentiate at an early stage.

  • Sperm cells: Specialized for reproduction.

  • Nerve cells: Specialized for rapid signaling.

• Plant cell specialization:

  • Many types of plant cells retain the ability to differentiate throughout the plant's entire life.

  • Xylem cells: Specialized for transporting water and minerals.

  • Phloem cells: Specialized for transporting food (sugars).

Microscopy and Magnification

• Light Microscope: Uses light to create an image. It is limited by lower magnification and resolution compared to electron microscopes.

• Electron Microscope: Uses an electron beam to observe specimens in much greater detail.

• Resolution: The smallest distance between $2$ points where they are still seen as separate entities.

• Magnification: The number of times larger the image appears compared to the actual size of the object.

• Magnification Formula:  $Magnification = \frac{\text{Size of image}}{\text{Size of object}}$

Aseptic Techniques and Culturing Microorganisms

• Aseptic techniques are used to prevent contamination during the culturing of microorganisms.

• Sterilization: Equipment such as inoculating loops should be sterilized by flaming.

• Petri dish preparation: The lid should be taped down and the dish stored upside-down. Storing it upside-down prevents condensation from dripping onto and disrupting the culture.

• Incubation: In school laboratories, cultures should be incubated at $25\,^{\circ}C$. This specific temperature is used to prevent the growth of bacteria that could be pathogenic to humans.

• Testing Effectiveness: To test the effectiveness of antibiotics, one can measure the area of the zone of inhibition (the clear area where bacteria have not grown).

Cell Division and the Cell Cycle

• Chromosomes: These are DNA molecules combined with proteins. They carry genes, which are sections of DNA that code for specific proteins.

• Human somatic cells contain $46$ chromosomes, organized into $23$ pairs.

• The Cell Cycle describes the stages of cell growth and division:

  • Interphase: The cell grows in size and synthesizes new organelles. DNA replicates to form $2$ sets of chromosomes.

  • Mitosis: The $2$ sets of chromosomes are separated to opposite poles of the cell.

  • Cytoplasm Division: The cytoplasm and cell membranes divide to produce two genetically identical daughter cells.

• Purpose: Cell division is essential for the growth and repair of multicellular organisms.

Stem Cells and Cloning

• Stem Cell: An unspecialized cell capable of self-renewal and differentiating into other cell types.

• Animal Stem Cells:

  • Embryonic Stem Cells: Can differentiate into most types of human cells.

  • Bone Marrow Stem Cells: Can only differentiate into some types of cells, such as blood cells.

• Therapeutic Cloning: An embryo is cloned with the same genetic makeup as the patient. This ensures there is no rejection by the patient's immune system. It can be used to treat conditions like diabetes and paralysis.

• Plant Stem Cells (Meristem Cells):

  • Found in the meristems of plants and can differentiate into any kind of plant cell throughout the plant’s life.

  • Meristem cloning allows for the production of large numbers of disease-resistant plants and the protection of rare species from extinction.

Transport Mechanisms in Cells

• Diffusion: The movement of molecules from a region of high concentration to a region of low concentration down their concentration gradient. This is a passive process.

  • Factors affecting diffusion rate include Temperature, Surface Area, and the Concentration Gradient.

  • Examples: Oxygen ($O_2$) diffusing into red blood cells (RBCs).

• Osmosis: A form of passive diffusion involving the movement of water molecules from a region of high water potential to a region of low water potential down their water potential gradient across a partially permeable membrane.

  • Plant tissue in a hypotonic solution (high water potential) will increase in mass.

  • Plant tissue in a hypertonic solution (low water potential) will decrease in mass.

• Active Transport: The movement of molecules from a low concentration to a high concentration against their concentration gradient. This process requires energy from respiration.

  • Examples: Absorption of sugar in the small intestine; absorption of ions in plant roots.

Adaptations for Exchange and Mass Transport

• Small organisms often rely on simple diffusion, but larger organisms have a surface area to volume (SA:V) ratio that is too small for this to be sufficient.

• Need for Mass Transport Systems: Required to efficiently move substances like oxygen, carbon dioxide, and urea.

• General Adaptations for Exchange:

  • Large surface area (e.g., Alveoli in lungs, cells of the small intestine).

  • Thin membrane or thin cell walls (e.g., root cells) providing a short diffusion distance.

  • Good ventilation and a constant blood supply to maintain concentration gradients.

Principles of Biological Organisation

• Cells: The basic building blocks of all living organisms. Cells with similar structures and functions work together.

• Tissues: A group of similar cells working together to perform a specific function.

• Organs: A group of different tissues which work together to perform a specific biological function.

• Organ Systems: A group of organs working together (e.g., Digestive system, Circulatory system, Respiratory system).

• Organism: The highest level of organization, consisting of multiple organ systems.

The Human Digestive System and Enzymes

• Function: To break down and absorb food.

• Enzymes: Biological catalysts that work best at an optimum temperature and pH. They follow the 'Lock and Key' theory, where each enzyme has a specific shape for a specific substrate.

  • Carbohydrase (including amylase): Breaks down carbohydrates.

  • Protease: Breaks down proteins.

  • Lipase: Breaks down lipids.

• Bile: An alkaline substance produced to neutralize stomach acid and ensure the optimum pH for enzymes in the small intestine. It also emulsifies lipids, aiding their breakdown.

The Human Circulatory and Respiratory Systems

• The Heart: A double circulatory system where the right ventricle pumps blood to the lungs and the left ventricle pumps blood to the rest of the body.

  • Main blood vessels: Pulmonary vein, pulmonary artery, aorta, vena cava, and coronary arteries.

  • Pacemaker: A group of cells that control the resting heart rate.

• Blood Vessels:

  • Arteries: Carry blood away from the heart.

  • Veins: Carry blood towards the heart.

  • Capillaries: Allow for the exchange of materials between blood and tissues.

• Blood Components: Plasma, white blood cells, red blood cells, and platelets.

• The Respiratory System: Centered on the lungs and alveoli.

  • Alveoli are adapted for exchange with a large surface area, thin walls, a moist lining, and a dense capillary network.

  • Oxygen enters the blood vessels at the alveoli and is transported to the heart via the pulmonary vein.

Health, Disease, and Risk Factors

• Health is a state caused by the interaction of different factors. Diseases can be classified as:

  • Communicable: Caused by pathogens like viruses or bacteria.

  • Non-communicable: Not infectious, such as heart disease or cancer.

• Disease Interactions: Different diseases can interact; for example, viruses can trigger certain types of cancers.

• Cancer: Often caused by carcinogens, which include specific chemicals, ionising radiation, and certain viruses.

• Risk Factors: Various factors increase the chance of developing a disease, including diet, exercise levels, drug and alcohol consumption, environmental factors, and stress.