Comprehensive Biology Revision: Cells and Body Systems

Characteristics of Living Organisms

• Criteria for Life (MRS GREN): The determination of whether an entity is alive is based on seven fundamental biological processes, often remembered by the acronym MRS GREN.

• Movement: The ability of living organisms or specific parts to change position or location. This occurs in response to stimuli or to perform essential biological functions.

• Respiration: The process by which living things produce energy. This is achieved by breaking down food molecules.

• Sensitivity: The capacity of living things to respond to their environment and external stimuli.

  • Examples include plants growing toward a light source (phototropism) or animals reacting to sounds.

• Growth: The process where living things increase in size. This involves an increase in either the number of cells or the size of individual cells.

• Reproduction: The ability to produce offspring or new individuals. This process is essential for the continuation of a species and for maintaining genetic diversity.

• Excretion: The removal of waste products from the organism. This helps maintain internal balance (homeostasis) by eliminating harmful substances.

• Nutrition: The process of obtaining and using nutrients for growth and energy production. It encompasses digestion, absorption, and metabolism.

Introduction to Cells

• Definition: Cells are the primary building blocks of life. They vary significantly in shape and size.

• Scale and Visibility:

  • Most cells are microscopic and require a microscope for observation.

  • Some cells are large enough to be seen with the naked eye, such as eggs.

• Size Extremes:

  • Smallest Cell: Mycoplasma gallicepticum, a disease-causing bacteria found in birds, which can be as small as $0.3\,\text{nanometers}$.

  • Largest Cell: The ostrich egg, which can reach a diameter of up to $6\,\text{inches}$.

• Cell Variety Examples:

  • Bacteria

  • Red blood cells

  • Nerve cells

  • Fat cells

  • Skeletal muscle cells

  • Smooth muscle cells

  • Columnar epithelial cells

  • Algae

  • Skin cells

Discovery of Cells and Cell Theory

• Robert Hooke (1665): Hooke first recorded the existence of cells while improving microscope designs. He observed a piece of cork and noted box-like structures, which he identified as cell walls.

• Development of Cell Theory: Following Hooke's discovery, German scientists Theodor Schwann, Matthias Schleiden, and Rudolph Virchow consecutively developed the classical Cell Theory.

• Tenets of Cell Theory:

  1. All organisms are made up of cells: An organism can be unicellular (e.g., an amoeba) or multicellular (e.g., a human body containing trillions of cells).

  2. The basic unit of life: To be classified as a life form, an entity must consist of at least one cell. All life functions occur within the cell.

  3. Biogenesis: Cells cannot exist spontaneously or emerge from non-living matter. All cells originate from pre-existing cells.

Plant vs. Animal Cells

• Shared Organelles: Both plant and animal cells contain:

  • Nucleus

  • Cytoplasm

  • Mitochondria (or Mitochondrion)

  • Cell membrane

  • Vacuoles (though sizes vary)

• Distinct Plant Cell Organelles:

  • Cell Wall: Provides structural support.

  • Chloroplasts: Site of photosynthesis.

  • Large Vacuole: Used for storage and maintaining turgor pressure.

• Comparison of Structures:

  • Plant Cell: Nucleus, Cytoplasm, Cell membrane, Cell wall, Mitochondrion, Large vacuole, Chloroplast.

  • Animal Cell: Nucleus, Cytoplasm, Cell membrane, Mitochondria, Vacuole (generally smaller).

Specialised Cells in Animals

• Definition: A specialised cell has unique structures designed to perform specific functions effectively. Cells vary in shape, size, number of organelles, and types of organelles.

• Red Blood Cells:

  • Structure: Small, round, and concave in shape with no nucleus.

  • Components: Contains haemoglobin, which binds to oxygen.

  • Function: Designed to transport the maximum amount of oxygen possible through the body.

• Nerve Cells:

  • Structure: Features branching structures (dendrites/axons) to receive and transmit information.

  • Function: Relays information throughout the body and communicates via electrical impulses.

• Cardiac Muscle Cells:

  • Structure: Cells are branched and interconnected; they typically contain one nucleus per cell.

  • Function: Facilitates the involuntary movement required to keep the heart pumping.

Specialised Cells in Plants

• Root Hair Cell:

  • Structure: Features thin cell walls and tiny hair-like projections to increase surface area. They lack chloroplasts because they are located underground.

  • Function: Designed to extend into the soil to absorb water and nutrients.

• Palisade Cell:

  • Structure: Contains a high density of chloroplasts.

  • Function: Maximizes the process of photosynthesis to produce glucose (sugar) as food for the plant.

Digestive Systems and Nutrient Absorption

• Absorption Process: Nutrients from physically and chemically broken-down food must pass through the walls of the digestive tract to enter the body.

• The Small Intestine: Over $90\%\,\text{of absorption}$ occurs here. Nutrients dissolved in water pass through the intestinal lining into the blood plasma for transport to other cells.

• Surface Area Maximization: To maximize total absorption, the intestinal lining features structures called villi and microvilli.

  • Microvilli: Small finger-like projections that enhance nutrient absorption and increase the surface area of the intestine.

• Dietary Comparisons and Digestive Structures:

  • Vultures (Carnivores): Eat meat, which is low in fibre and easier to digest. Proteins and fats are broken down efficiently by enzymes such as Pepsin.

  • Koalas (Herbivores): Feed primarily on Eucalyptus leaves, which are high in insoluble fibre (Cellulose). Cellulose is difficult to digest and requires more time in the tract. Eucalyptus leaves also contain toxins.

  • Human: Average body length of $167\,\text{cm}$, featuring a standard digestive tract including oesophagus, stomach, liver, pancreas, small intestine, large intestine, and rectum.

The Role of the Caecum

• General Function: The caecum stores bacteria that assist in digesting food and aids in the absorption of water and nutrients.

• Carnivores: Possess a small caecum because meat is relatively easy to digest.

• Herbivores (Koalas): Require a large caecum to digest tough, fibrous, cellulose-rich plant matter. The bacteria in a koala's caecum also help break down the toxins found in Eucalyptus leaves.

The Circulatory System

• Heart Chambers and Blood Movement:

  • Right Atrium: Receives deoxygenated blood from the body via the Vena Cava.

  • Right Ventricle: Pumps deoxygenated blood to the lungs through the Pulmonary Artery.

  • Left Atrium: Receives oxygenated blood from the lungs via the Pulmonary Vein.

  • Left Ventricle: Pumps oxygenated blood to the rest of the body through the Aorta. This chamber has tougher (thicker) walls because it must pump blood throughout the entire body.

• Blood Vessels:

  • Artery: Carries blood (usually oxygenated) away from the heart.

  • Vein: Carries blood (usually deoxygenated) toward the heart and transports wastes.

  • Capillaries: Small vessels where blood passes nutrients to cells and collects waste. Capillaries contain red blood cells.

Gas Exchange and the Respiratory System

• Interaction of Systems: The circulatory system meets the respiratory system at the junction of capillaries and alveoli.

• Alveoli: Provide oxygen to the capillaries to be distributed to body cells.

• Capillaries: Transport carbon dioxide ($CO_2$) to the alveoli to be exhaled by the lungs.

• Efficiency: Thin walls in both capillaries and alveoli facilitate efficient gas exchange ($O_2\,\text{in}$, $CO_2\,\text{out}$).