Cells and control Revision Notes

Embryonic Stem Cell Function

Embryonic stem cells are found in the early embryo and are totipotent or pluripotent. This means they can differentiate into almost every cell type in an organism. Their primary function during development is to form all the different tissues and organs of the individual. In medicine, their potential lies in repairing damaged tissues (e.g., nerve damage) or growing new organs.

Animal Stem Cell Function (Adult)

Adult stem cells are found in differentiated tissues throughout an animal's body. They are typically multipotent, meaning they can only differentiate into a limited number of cell types specific to the tissue they reside in. Their function is to replace dead or damaged cells and maintain tissue health. Examples include bone marrow stem cells replacing blood cells and intestinal stem cells renewing the gut lining.

Plant Meristem Function

Meristems are regions of actively dividing, undifferentiated cells in plants, similar to stem cells in animals. They are responsible for plant growth. Apical meristems at root and shoot tips enable primary growth (length). Lateral meristems, like the cambium, allow secondary growth (width). Meristems produce new cells that differentiate into various plant tissues (e.g., xylem, phloem, epidermal cells).

Benefits of Stem Cell Use

Stem cells offer significant potential in regenerative medicine. They can be used to replace damaged or diseased cells and tissues, treating conditions like Parkinson's disease, spinal cord injuries, and diabetes. They are valuable for testing new drugs, providing a model to observe drug effects on human cells before human trials. Stem cells also allow scientists to study disease mechanisms, providing insight into how diseases develop and progress. Therapeutic cloning, which involves creating an embryo to harvest stem cells, can produce genetically identical cells, reducing immune rejection risks.

Risks of Stem Cell Use

Despite their potential, stem cells pose risks. A major concern is immune rejection: if stem cells are not from the patient, their body's immune system may attack them. There's a risk of uncontrolled cell division, leading to tumour formation, as stem cells are designed to grow and divide rapidly. Ethical issues surrounding the use of embryonic stem cells are significant, as their harvesting involves the destruction of an embryo, which some consider the destruction of a human life. Additionally, there are practical challenges in successfully culturing stem cells and ensuring they differentiate into the desired cell types without mutation or contamination.

Reflex Arc Components

A reflex arc is a neural pathway controlling a reflex. It typically involves three types of neurones: sensory, relay, and motor. This pathway allows for rapid, involuntary responses to stimuli, bypassing conscious brain processing for speed.

Neurone Function in the Reflex Arc

Sensory neurones transmit impulses from receptors to the central nervous system (spinal cord). Relay neurones (interneurones) in the spinal cord connect sensory and motor neurones. Motor neurones carry impulses from the spinal cord to effectors (muscles or glands) to produce a response.

Pathway Summary

Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector → Response. This sequence ensures a quick, protective action without requiring brain interpretation first.

Importance in Growth

Multicellular organisms grow by increasing the number of cells, not just the size of individual cells. Mitosis ensures that new cells produced for growth are genetically identical to the parent cell, maintaining the organism's characteristics. This controlled cell division is essential from the zygote stage, leading to the development of a fully formed organism.

Importance in Repair and Replacement

When tissues are damaged (e.g., a cut in the skin, a broken bone), mitosis produces new cells to repair the injury. Old or worn-out cells, such as those lining the digestive tract or red blood cells, are constantly replaced through mitotic division, ensuring the body's systems remain functional.

Importance in Asexual Reproduction

Many organisms, including some plants, fungi, and single-celled organisms, reproduce asexually using mitosis. This process creates genetically identical offspring (clones) from a single parent. Examples include budding in yeast, vegetative propagation in plants, and binary fission in amoebas. This allows for rapid population growth in stable environments.