National 5 Biology SQA

Cell Structure

Cell Types

• All living organisms are composed of cells which may be unicellular (one celled) like bacteria, or multicellular (many-celled), like plants and animals.
• Cells can be broadly classified into two categories: prokaryotic and eukaryotic cells.
• Prokaryotic cells are the simpler of the two, lacking a nucleus and membrane-bound organelles. They are typically found in bacteria and archaea.
• Eukaryotic cells are more complex, with a nucleus and membrane-bound organelles. These cells are typically found in plants, animals, and fungi.

Cell Structure

• The plasma membrane is the outer covering of the cell that regulates the entry and exit of substances.
• The cell wall is present in plant cells, fungi, and bacteria. It maintains shape and prevents the cell from bursting.
• The nucleus is the control centre of the cell, responsible for producing mRNA and controlling cell activities.
• Cytoplasm is the watery substance where most cell activities take place.
• Mitochondria are the site of aerobic respiration producing ATP, the cell’s energy currency.
• Ribosomes are the site of protein synthesis.
• Endoplasmic reticulum (ER) is involved with protein and lipid metabolism.
• In plants, chloroplasts are the site of photosynthesis and vacuoles are storage organelles for water, food, and waste.

Differences Between Plant and Animal Cells

• Both animal and plant cells are eukaryotic, which is to say, they contain a nucleus and other membrane-bound organelles.
• Animal cells contain centrioles important in cell division, and may also possess lysosomes which break down waste materials.
• Plant cells have a rigid cell wall, large permanent vacuole and chloroplasts for photosynthesis which are all absent in animal cells.

Specialised Cells

• Organisms contain a variety of cell types that have unique structures fit for their specific function.
• For instance, red blood cells carry oxygen due to their haemoglobin content, and their biconcave shape provides a large surface area for gas exchange.
• Muscle cells have numerous mitochondria for energy and protein fibres for contraction.
• Root hair cells in plants have a large surface area to absorb water and mineral nutrients efficiently from the soil.
• Guard cells in plants control the opening and closing of stomata, regulating gas exchange.