Biology Chapter 3: Cellular Form and Function

Cellular Form and Function

• Chapter 3 covers membranes, proteins, transport, membrane potential, and organelles.
• The objectives include explaining cell theory, describing cell shapes and types, outlining cell components, describing plasma membrane structure, mechanisms of material transport through cell membranes, and listing and describing organelles.
• Studying cellular form and function is important for understanding structural and functional properties of organisms, human body workings, disease mechanisms, and developing therapy and treatments.
• Cell theory states that the cell is the simplest unit of life (structural and functional), organisms are composed of cells, cells arise from preexisting cells, and an organism's structure and function are due to cell activities.
• All living cells share similarities, evolving from a common ancestor approximately 3.5-3.8 billion years ago. They follow the laws of chemistry and physics, have similar basic chemistry and macromolecules (participating in similar chemical reactions). Nucleic acids (genes), proteins (functional and structural), and carbohydrates and lipids are common to all cells.
• Cells vary in appearance (form) and function due to differences in size, shape, chemical requirements (e.g., ATP hydrolysis rate), specialized functions (hormones, glycogen, fat factories), and mechanical work and electricity generation.
• Cell size is limited by the surface area to volume ratio. A higher ratio of surface area to volume facilitates nourishment and waste removal, and requires a sufficient membrane area for efficient transport.
• Major components of a cell include cytoplasm, organelles (cytoskeleton, inclusions – stored or foreign particles), and cytosol (fluid). Plasma membranes, composed primarily of proteins (2%) and lipids (75% phospholipids, 20% cholesterol, 5% glycolipids), form the boundary and control transport.

Major Membrane Protein Functions

• Membrane proteins have diverse functions like transport, enzymatic activity, signal transduction, cell-cell recognition, and attachment.
• Transport proteins can act as channels (allowing passage of specific substances), carriers (changing shape for movement), or actively pumping substances against a gradient. This active transport requires energy.

Membrane Transport Mechanisms

• Membranes exhibit selective permeability, allowing some substances to pass through the phospholipid bilayer more easily than others.
• Passive transport includes filtration, diffusion (through channels or as simple diffusion), and osmosis.
• Active transport, including vesicular transport and active transport, needs energy to move substances against concentration gradients.

Specific Membrane Transport Processes

• Filtration is driven by hydrostatic pressure (e.g., blood capillaries).
• Diffusion occurs down concentration gradients (higher to lower concentrations), through permeability of cell membranes, influenced by temperature, molecular weight, concentration gradient, and surface area.
• Osmosis refers to the movement of water across a selectively permeable membrane towards a higher concentration of non-permeable solutes. Aquaporins play a role, enabling the selective movement of water. Osmotic and hydrostatic pressures are important determinants in this process. Tonicity describes the fluid's capacity to affect cell volume; reverse osmosis is a pressure-driven process.
• Carrier-mediated transport involves transmembrane proteins that bind solutes, change shape, and release them on the other side. Carrier saturation and transport maximum (T) are characteristics of this process (e.g., diabetes). Uniports, symports, and antiports are types of carrier proteins.
• Facilitated diffusion uses membrane proteins to move substances down their concentration gradient without direct ATP input.
• Primary active transport directly uses ATP to move substances against their concentration gradient (e.g., Na+/K+ pump).
• Secondary active transport indirectly uses ATP through an established gradient (e.g., sodium-glucose transporter; SGLT).

Vesicular Transport

• Vesicular transport (bulk transport) includes processes where large quantities of materials are moved in or out of the cell using vesicles. These processes require energy and include endocytosis (into the cell), exocytosis (out of the cell), encompassing phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Organelles

• Endoplasmic Reticulum (ER): Rough ER is studded with ribosomes involved in protein synthesis and phospholipid production. Smooth ER lacks ribosomes and plays roles in detoxification, metabolism, and calcium storage.
• Ribosomes: Sites of protein synthesis, found in the cytosol, nucleus, rough ER, and mitochondria. Ribosomes can assemble proteins in long polypeptide chains based on the RNA code.
• Golgi Complex: Modifies, sorts, and packages proteins and lipids.
• Mitochondria: Sites of cellular respiration, producing ATP. They contain their own DNA, machinery for their replication and RNA production, and special ribosomes.

Additional Cellular Processes

• Proteolysis: Degradation of proteins by proteasomes or other enzymes, crucial for removing, recycling, and regulating the cell processes.
• Ubiquitin Signaling: Ubiquitin protein tags other proteins for destruction by the proteasome.
• Transcytosis: Transport of particles through cells, which might involve endocytosis, movement through the cytoplasm, and exocytosis.

Summary of Cell Theory and Form and Function

• Cells are the fundamental units of life with diverse forms and functions, influencing whole organism structure/function.
• Cell form is directly linked to their specific function and the tasks they perform in the body.
• Understanding both forms (structures) and functions is critical for a complete understanding of cellular processes, influencing the organism's function and performance as a whole.