Mitochondria and Other Cellular Structures

Mitochondria are oval-shaped organelles bound by a double membrane called the envelope. The inner membrane is folded to form projections called cristae, which increase the surface area for chemical reactions. The matrix inside the mitochondria contains enzymes necessary for aerobic respiration and ATP production. Mitochondria also contain small circular pieces of DNA and ribosomes, making them semi-autonomous organelles.

Definition: Mitochondria are often referred to as the "powerhouses" of the cell due to their crucial role in energy production through aerobic respiration.

Centrioles are hollow cylinders containing a ring of microtubules arranged at right angles to each other. They are involved in producing spindle fibres during cell division, playing a crucial role in the separation of chromosomes.

Ribosomes are composed of two subunits and serve as the site of protein production in the cell. They can be found free in the cytoplasm or attached to the rough endoplasmic reticulum.

Lysosomes are membrane-bound organelles that contain hydrolytic enzymes. They play a vital role in breaking down waste materials, worn-out organelles, and are used by cells of the immune system. Lysosomes are also involved in apoptosis (programmed cell death).

Vocabulary: Apoptosis - A regulated process of programmed cell death that occurs in multicellular organisms.

The cell surface membrane, also known as the plasma membrane, surrounds all cells and controls the exchange of materials between the internal cell environment and the external environment. It is partially permeable and formed from a phospholipid bilayer.

Highlight: Understanding the structure and function of the cell surface membrane is crucial for A Level Biology Cell structure topics, as it plays a key role in cellular transport and communication.

Plant-Specific Organelles and Structures

Chloroplasts are unique to plant cells and some algae. They are surrounded by a double membrane and contain membrane-bound compartments called thylakoids that house chlorophyll. These thylakoids are arranged in stacks called grana, which are interconnected by lamellae. Chloroplasts are the site of photosynthesis, with the light-dependent stage occurring in the thylakoids and the light-independent stage (Calvin cycle) taking place in the stroma.

Example: The structure of eukaryotic cells for a level revision often emphasizes the importance of chloroplasts in understanding plant cell function and energy production.

Plant cells also have a cell wall, which is freely permeable to most substances and provides structural support to the cell. In plants, the cell wall is primarily composed of cellulose. Narrow threads of cytoplasm, called plasmodesmata, connect neighboring plant cells through the cell wall.

Vocabulary: Plasmodesmata - Channels that traverse the cell walls of plant cells, allowing for direct communication between adjacent cells.

Plant cells typically have a large central vacuole surrounded by a selectively permeable membrane called the tonoplast. This vacuole plays crucial roles in maintaining cell turgor, storing nutrients, and sequestering waste products.

Highlight: The central vacuole is a key feature that distinguishes plant cells from animal cells in A Level Biology Cell structure notes PDF resources.

Additional Cellular Structures

Vesicles are membrane-bound sacs used for transport and storage within the cell. They play a crucial role in the trafficking of molecules between different cellular compartments.

Centrioles are hollow fibres made from microtubules. Two centrioles positioned at right angles to each other form a centrosome, which is important in cell division.

Microtubules are part of the cell's cytoskeleton, providing support and facilitating movement within the cell. They are also components of other cellular structures such as cilia and flagella.

Definition: The cytoskeleton is a network of protein filaments that provides structural support and enables cellular movement.

Microvilli are projections of the cell membrane that increase the surface area for absorption. They are particularly important in cells involved in absorption, such as those lining the small intestine.

Cilia are hair-like projections made from microtubules that allow for the movement of substances over the cell surface. They play crucial roles in various physiological processes, including mucus clearance in the respiratory tract.

Flagella are longer microtubule-based structures that contract to provide cell movement. They are found in certain types of cells, such as sperm cells, and are essential for their motility.

Example: The structure and function of flagella in sperm cells is a common topic in A Level Biology Cell structure exam questions.

Prokaryotic Cell Structure

Prokaryotic cells are much smaller than eukaryotic cells and have several distinct features. They lack membrane-bound organelles and a true nucleus. Instead of a nucleus, prokaryotes have a single circular DNA molecule that is free in the cytoplasm and not associated with proteins.

Definition: Prokaryotic cells are simpler, typically single-celled organisms that lack a membrane-bound nucleus and organelles.

The cell wall of prokaryotes is composed of peptidoglycan, which provides rigidity and protection. Many prokaryotic cells also have a capsule, a protective slimy layer that helps the cell retain moisture and adhere to surfaces.

Prokaryotes often contain one or more plasmids, which are small circular pieces of DNA separate from the main chromosomal DNA. These plasmids can carry genes that provide advantages to the cell, such as antibiotic resistance.

Highlight: Understanding the differences between prokaryotic and eukaryotic cells is crucial for A Level Biology Cell structure AQA examinations.

Many prokaryotic cells possess flagella, tail-like structures that rotate to move the cell. This feature allows bacteria to move towards nutrients or away from harmful substances.

Pili are hair-like appendages found on the surface of many prokaryotic cells. They can be involved in attachment to surfaces or in the transfer of genetic material between cells.

Vocabulary: Pili - Short, hair-like structures on the surface of some prokaryotic cells, involved in adherence and genetic transfer.

Understanding the structure of prokaryotic cells is essential for comprehending their functions and roles in various biological processes, including their interactions with eukaryotic organisms.

Eukaryotic Cell Structure

The eukaryotic cell is a complex structure containing various organelles and components essential for life processes. This section focuses on the key structures found in eukaryotic cells, which is crucial knowledge for A Level Biology Cell structure AQA exams.

Definition: Eukaryotic cells are cells that contain a nucleus and membrane-bound organelles, typically found in complex multicellular organisms.

The nucleus is a defining feature of eukaryotic cells. It is surrounded by a double membrane called the nuclear envelope, which contains approximately 3000 nuclear pores. These pores are vital for the movement of molecules in and out of the nucleus, including mRNA, ribosomes, enzymes, and signaling molecules. The nucleus houses chromatin (which contains DNA) and the nucleolus, where ribosome production occurs.

Highlight: The nuclear pores play a crucial role in cellular communication, allowing for the exchange of genetic information and regulatory molecules between the nucleus and the cytoplasm.

The rough endoplasmic reticulum (RER) is composed of a series of flattened sacs formed from continuous folds of membrane. Its surface is studded with ribosomes, giving it a "rough" appearance. The RER is responsible for folding and processing proteins made on the ribosomes.

Vocabulary: Rough endoplasmic reticulum $RER$ - A network of membranes in eukaryotic cells involved in protein synthesis and modification.

The smooth endoplasmic reticulum $SER$ is a system of membrane-bound sacs without ribosomes on its surface. It plays a crucial role in producing and processing lipids, and is also involved in the processing and storage of carbohydrates and steroids.

Example: The smooth endoplasmic reticulum function in animal cells includes the synthesis of steroid hormones in certain endocrine cells.

The Golgi apparatus consists of a series of fluid-filled, flattened, and curved sacs with vesicles surrounding the edges. Its primary functions include processing and packaging proteins and lipids, as well as producing lysosomes. The Golgi vesicles transport these processed molecules to their required destinations within or outside the cell.

Highlight: The Golgi apparatus function is crucial for cellular secretion and intracellular transport, making it an essential component of the endomembrane system.