2b Cell Structure¶

Part of 2 Structure and Functions in Living Organisms.

Cell structure links what cells contain to what they can do. In this course the key comparisons are between plant and animal cells, and between unspecialised cells, specialised cells and stem cells.

Learning Objectives¶

Sub-cellular Structures¶

The following structures are found in both plant and animal cells:

Plant cells also contain these additional structures:

Plant Cells and Animal Cells¶

• Plant and animal cells share structures such as the nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
• Plant cells also have a cellulose cell wall, chloroplasts and a large permanent vacuole. Animal cells do not have any of these.
• These differences reflect function. Plants need support and photosynthesis; animals rely on a nervous system for co-ordination and store carbohydrates as glycogen rather than starch.

Biology-Only Content¶

This content is required for Biology-only students and is not required for Combined Science students.

Specialised Cells¶

Cells become specialised through a process called differentiation, in which cells gain new sub-cellular structures suited to their particular role. In animals, most cells differentiate only once. In plants, many cells retain the ability to differentiate throughout the life of the plant.

Specialised animal cells:

• Sperm cells are adapted to carry the male's DNA to the egg. They have a streamlined head with a long tail for swimming, many mitochondria to power movement, and an acrosome at the tip that contains enzymes to penetrate the outer layers of the egg.
• Nerve cells (neurones) transmit electrical impulses rapidly. A long axon carries the impulse over long distances, dendrites branch out to connect with many other neurones, and many mitochondria at the nerve endings supply energy for producing neurotransmitters.
• Muscle cells are adapted to contract and cause movement. Specialised proteins (actin and myosin) slide over each other to shorten the cell. Muscle cells contain many mitochondria for the energy needed during contraction and can store glycogen as a fuel reserve.

Specialised plant cells:

• Root hair cells are adapted to absorb water and mineral ions from the soil. The root hair extension greatly increases the surface area for absorption. A large permanent vacuole draws water in by osmosis, and mitochondria provide energy for the active transport of mineral ions.
• Xylem cells are adapted for transporting water and mineral ions upwards. After forming, a strengthening material called lignin is deposited in spirals, causing the cells to die and become hollow. These empty tubes join end-to-end to form a continuous column for transport. Lignin also contains bordered pits, allowing water to move sideways into adjacent tissues.
• Phloem cells transport sucrose and amino acids (the products of photosynthesis) to all parts of the plant. As the cell matures, the end walls partially break down to form sieve plates with holes, allowing substances to flow from cell to cell. Although phloem cells lose many organelles, companion cells alongside them have many mitochondria to supply the energy the phloem cells need to survive.

Stem Cells¶

A stem cell is an undifferentiated cell that can divide to produce many more cells, some of which then differentiate into specialised cell types. They are essential for development, growth and repair.

Types of stem cells:

1. Embryonic stem cells form when a fertilised egg (zygote) first divides. They can differentiate into any cell type in the body. Scientists can culture them and direct their differentiation, making them potentially useful for replacing insulin-producing cells in diabetes, neural cells in Alzheimer's disease, or nerve cells in spinal cord injuries.
2. Adult stem cells are found mainly in bone marrow. They can produce many cell types, particularly blood cells, but cannot differentiate into as wide a range as embryonic stem cells. Bone marrow transplants using adult stem cells are already used to treat blood cancers such as leukaemia.
3. Meristems in plants are found in root and shoot tips. They can differentiate into any plant cell type and retain this ability throughout the plant's life. They are used to produce clones of plants with desirable features or to conserve rare species.

Benefits and risks of stem-cell therapies:

Specialisation and Stem Cells¶

• Differentiation is the process by which a cell develops structures that suit a particular job. Specialised cells include sperm cells, nerve cells and root hair cells.
• Stem cells can divide and then differentiate into specialised cell types. This makes them valuable in medicine for replacing damaged tissues.
• Stem-cell use also has limitations, including ethical concerns, possible immune rejection and the risk that transplanted cells may not behave as intended.

Common Confusions¶

• Stem cells vs all cells: Not every cell can differentiate repeatedly. Stem cells are unusual because they keep the ability to divide and specialise.
• Cell wall vs cell membrane: The cell membrane controls exchange with the surroundings. The cell wall mainly provides support and does not replace the membrane.
• Xylem vs phloem: Xylem transports water upwards; phloem transports dissolved sugars and amino acids in both directions.

Key Terms¶

• Nucleus: the organelle that contains the cell's genetic material and controls cell activities.
• Cytoplasm: the jelly-like substance filling the cell in which chemical reactions occur.
• Ribosome: the site of protein synthesis.
• Mitochondrion: the organelle where most aerobic respiration occurs.
• Chloroplast: the organelle that contains chlorophyll and carries out photosynthesis.
• Vacuole: a fluid-filled space in plant cells that contains cell sap and supports the cell.
• Differentiation: the process by which a cell becomes specialised for a particular function.
• Stem cell: an undifferentiated cell that can divide and differentiate into specialised cell types.
• Lignin: a material deposited in xylem cell walls, strengthening them and causing the cells to die and become hollow.
• Sieve plate: the perforated end wall of a phloem cell through which substances can flow.
• Meristem: a region of actively dividing stem cells in plants, found at root and shoot tips.

Connected Pages¶

• 2a Level of Organisation
• 2c Biological Molecules
• 2h Transport
• 3b Inheritance
• IGCSE Biology Key Terms Glossary