Cells and control revision notes

Potential Benefits of Stem Cell Use

Stem cells hold significant promise for treating various diseases and injuries. They can differentiate into specialized cells, allowing for the repair or replacement of damaged tissues. Examples include treating spinal cord injuries, Parkinson's disease, Alzheimer's disease, heart disease, stroke, burns, and diabetes. Stem cells may also be used to grow organs for transplantation, reducing reliance on organ donors and the risk of immune rejection. Additionally, they are valuable for drug testing and studying disease mechanisms in a laboratory setting without harming patients.

Potential Risks and Ethical Concerns

Despite the benefits, stem cell use carries risks. One major concern is the potential for stem cells, especially embryonic stem cells, to form tumours (teratomas) if their growth and differentiation are not precisely controlled. There's also a risk of immune rejection if the patient's body identifies the transplanted cells as foreign, which can be mitigated by using a patient's own adult stem cells, but remains a concern for embryonic or donor-derived cells. Transmission of diseases, although rare with proper screening, is another potential risk if cells are not sourced and handled appropriately. Ethical concerns surround the destruction of human embryos to obtain embryonic stem cells, prompting debate about the moral status of an embryo. While adult stem cells avoid this particular ethical issue, they are harder to obtain and have more limited differentiation potential.

Cornea and Lens: Focusing Light

The cornea is the transparent frontal part of the eye, responsible for the majority of light refraction (bending light rays) to focus them onto the retina. It has a fixed curvature. The lens is situated behind the iris and pupil; it is a flexible, biconvex structure. Its curvature can change due to the action of ciliary muscles, allowing for accommodation – the adjustment of focus for objects at different distances. Both work together to ensure a clear image is projected onto the retina.

Iris: Light Regulation

The iris is the coloured, muscular part of the eye surrounding the pupil. It acts like the aperture of a camera. In bright light, the circular muscles of the iris contract, while the radial muscles relax, making the pupil smaller (constriction) to limit the amount of light entering. In dim light, the radial muscles contract, and the circular muscles relax, dilating the pupil to allow more light in. This reflex action protects the retina from damage and optimises vision in varying light conditions.

Rod and Cone Cells: Light Detection

The retina at the back of the eye contains two types of photoreceptor cells: rods and cones. Rod cells are far more numerous (around 120 million per eye) and highly sensitive to low light intensities, enabling scotopic (black-and-white) vision and peripheral vision. They are most concentrated around the periphery of the retina. Cone cells (around 6 million per eye) are less sensitive but responsible for photopic (colour) vision and high visual acuity. They are concentrated in the fovea, the central part of the retina. There are three types of cone cells, each sensitive to different wavelengths of light (red, green, and blue).

What is cell differentiation?

Cell differentiation is the process where a less specialised cell becomes a more specialised cell type, acquiring distinct structures and functions. This process is essential for the development and maintenance of multicellular organisms.

Importance in multicellular organisms

In multicellular organisms, differentiation allows cells to develop specific roles. This division of labour enables the organism to perform complex functions efficiently. It leads to the formation of specialised tissues and organs, each undertaking particular tasks vital for survival.

Embryonic development

During embryonic development, cells start as unspecialised stem cells. Through differentiation, they become various cell types like nerve cells, muscle cells, and skin cells. This ordered specialisation is crucial for the formation of a complete and functional organism.

Specialised cell functions

Specialised cells, such as nerve cells with their elongated structure for signal transmission or red blood cells lacking a nucleus for more oxygen transport, arise from differentiation. Their unique structures are adapted for their specific functions, underpinning the organism's physiological processes.

What is Mitosis?

Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. It ensures genetic consistency between parent and daughter cells.

Mitosis in Growth

Multicellular organisms grow by increasing the number of their cells through mitosis. From a single fertilized egg (zygote), repeated mitotic divisions form a complex organism, adding cells to become larger and more developed.

Mitosis in Repair and Replacement

Mitosis is essential for repairing damaged tissues and replacing dead or worn-out cells. For instance, when you cut yourself, new skin cells are generated via mitosis to heal the wound. Red blood cells and skin cells are constantly replaced through this process.

Mitosis in Asexual Reproduction

In asexual reproduction, a single parent organism produces genetically identical offspring. Mitosis is the cellular basis for this process in organisms like bacteria, yeast, and some plants (e.g., runners in strawberries) and fungi. The offspring are clones of the parent.