Beyond: Advanced is here to provide A Level cell structure revision for KS5 Biology, exploring the functions of a range of animal organelles.
Cell Structures of Animal Organelle
The cell membrane consists of a phospholipid bilayer that is approximately 7nm thick. The hydrophilic heads of the phospholipids form the inner and outer surface of the membrane and the hydrophobic tails form the inside of the membrane. This means that the surface of the membrane can interact with water inside and outside the cell, but water-soluble substances cannot diffuse through the hydrophobic core.
The structure of the cell membrane is described by the fluid mosaic model. The cell membrane
is made up of many structures that are constantly moving within the bilayer (they are fluid). Cholesterol molecules are embedded between the phospholipids to prevent too much movement.
Channel proteins and carrier proteins are found within the bilayer. These allow large molecules and ions to be transported across the cell membrane, making the cell membrane partially permeable.
Receptor proteins, glycoproteins and glycolipids are scattered throughout the membrane. Glycoproteins are proteins with a carbohydrate attached to them, while glycolipids are lipids with a carbohydrate attached to them.
Cell membranes are physical barriers. They prevent the movement of foreign bodies and other molecules into and out of the cell. Cell membranes are partially permeable: they allow some molecules to pass through but not others. Substances can be transported across the cell membrane by diffusion, osmosis and active transport.
Membranes around organelles such as mitochondria physically separate the cytoplasm of the cell and the organelle.
Glycoproteins and glycolipids have several functions in the human body, including:
• responding to insulin in liver cells, resulting in the cell absorbing glucose from the bloodstream;
• establishing blood type;
• immune responses;
• responding to neurotransmitters involved in nervous responses.
The nucleus is encased within a double membrane called a nuclear envelope. The nuclear envelope has spaces within it called nuclear pores. These nuclear pores allow substances such as RNA to move between the cytoplasm of the cell and the nucleus.
DNA associates with proteins called histones. This complex coils tightly to form chromosomes which are found in the nucleus. DNA is too large to fit through the nuclear pores, preventing it from leaving the nucleus.
Chromatin, a substance consisting of DNA and protein, is scattered throughout the nucleus.
The nucleolus is a structure consisting of DNA, RNA and proteins. Some cells contain more than one nucleolus.
The nucleus controls the functions of the cell. It does this by controlling DNA transcription.
The functions of the nucleus include controlling gene expression, protein synthesis and storing DNA (hereditary material).
Protein synthesis and ribosome production occur in the nucleolus.
Ribosomes are very small structures composed of a large subunit and a small subunit. They consist of ribosomal proteins and ribosomal RNA (rRNA). A single eukaryotic cell can contain over 10 million ribosomes.
Ribosomes are often associated with another organelle called the rough endoplasmic reticulum, but otherwise float freely within the cytoplasm. Ribosomes are not surrounded by a separate membrane.
Ribosomes are responsible for synthesising proteins from messenger RNA (mRNA) during translation. In this process, the codons in mRNA are used to produce a specific sequence of amino acids. This amino acid chain is then folded into a protein.
Rough Endoplasmic Reticulum (RER)
The rough endoplasmic reticulum (RER) is a network of channel-like structures filled with fluid. They are linked to the nuclear membrane by small tubules.
The RER has ribosomes attached along the outer surface. It has a large surface area to increase the rate of protein synthesis.
The RER works in conjunction with the attached ribosomes to process and produce the 3D structures of proteins. It is also the site of glycoprotein synthesis. Cells that make lots of protein typically contain extremely high amounts of RER.
Smooth Endoplasmic Reticulum (SER)
The structure of the smooth endoplasmic reticulum (SER) is very similar to the rough endoplasmic reticulum (RER), but there are no ribosomes along the outer sur face. The SER is typically attached to RER and linked to the nuclear membrane.
The SER has a large surface area to increase the rate of synthesis of lipids and other molecules.
The SER is responsible for storing, synthesising and processing lipids, phospholipids and cholesterol. Within skeletal muscle cells the SER stores other substances such as calcium ions.
Within some endocrine glands, the SER contains enzymes that detoxify harmful substances. For example, in liver cells, the SER is involved in breaking down carcinogens.
The Golgi complex is composed of a Golgi apparatus and several small vesicles.
The Golgi apparatus is a membrane-bound channel-like structure filled with fluid. The vesicles are detached fluid-filled pockets found at the edges of the complex.
The Golgi apparatus is responsible for processing and packaging lipids and proteins. It also produces lysosomes.
The lipids and proteins produced by the Golgi apparatus are stored by the Golgi vesicles. The vesicles can transport these substances out of the cell through the cell membrane.
Lysosomes are types of Golgi vesicle with no obvious internal structure. They are surrounded by a membrane and contain digestive enzymes called lysozymes. The pH inside the lysosome is acidic compared to the alkaline cytoplasm of the cell. This acidic pH is necessary for lysozymes to function.
Lysozymes are responsible for digesting invading cells, old, unwanted parts of cells and complex biomolecules.
The membrane ensures that the lysozymes are kept separate from the cytoplasm of the cell to prevent self-digestion.
Mitochondria are oval-shaped structures with an outer membrane and inner membrane. The inner membrane is highly folded, forming finger-like structures called cristae. Cristae increase the surface area available for chemical reactions to take place.
The matrix is formed from the folding of the inner cell membrane and contains some of the enzymes involved in aerobic respiration.
Mitochondria are the site of aerobic respiration, which produces adenosine triphosphate (ATP), a molecule that is essential for cellular activity. Cells that require large amounts of energy contain many mitochondria.
Cell Structure Material from Beyond: Advanced
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