Cell Structure and Function Lecture Notes PDF

The Cell Dr Nicola King In these lectures we will cover  Initial Learning Outcomes Define cell Name and describe the composition of extracellular materials List the 3 major regions of a generalised cell and their functions  Cell Theory The cell is the smallest unit of...

The Cell Dr Nicola King In these lectures we will cover  Initial Learning Outcomes Define cell Name and describe the composition of extracellular materials List the 3 major regions of a generalised cell and their functions  Cell Theory The cell is the smallest unit of life. When you define the properties of cells, you define the properties of life All organisms are made of one or more cells. Cells are the structural and functional building blocks of an organism. Different cell types have different functions within an organism, and the activity of an organism depends on the activities of individual cells and of all the cells together Cells only arise from other cells. Mostly by mitosis Have trillions of cells in total >250 cell types Vary greatly in size shape and function All cells have the same basic parts and some common features  The 3 Main Parts of a Human Cell Plasma membrane: outer boundary with selective permeability (next week) Cytoplasm: ICF containing organelles, small structures that perform specific cell functions Nucleus: organelle that controls cellular activities  Extracellular Materials Although we are a composite of cells, cells don’t comprise the total body contents. There’s stuff outside cells as well ECF including interstitial fluid (IF), blood plasma and cerebrospinal fluid. ECF dissolves and transports substances in the body IF – fluid in tissues that bathes all our cells  Cellular Secretions – including substances that aid in digestion and some that act as lubricants Extracellular matrix – most cells are in contact with a jelly like substance composed of proteins and polysaccharides Secreted by the cells, these molecules self-assemble into an organised mesh in the extracellular space, where they serve as a universal “cell glue” that binds body cells together. Extracellular Matrix  The Cytoplasm – Learning Objectives Describe the composition of the cytosol Discuss the structure and function of the mitochondria Discuss the structure and function of ribosomes, the endoplasmic reticulum, and the Golgi apparatus, including functional interrelationships among these organelles Compare the functions of lysosomes and peroxisomes Name and describe the structure and function of cytoskeletal elements  Cytoplasm Contains 3 Major Elements Cytosol – viscous, semitransparent fluid in which the other cytoplasmic elements are suspended. Complex mixture Inclusions – chemical substances that may or may not be present depending on cell type (e.g. glycogen granules in liver and muscle cells) Organelles – metabolic machinery of the cell. Each one carries out a specific function for the cell. Most are surrounded by a membrane to maintain an internal environment different from that of the surrounding cytosol.  Mitochondria Typically threadlike membranous organelles Squirm, elongate, and change shape almost continuously Power plants Number in different cells depends on cell’s activity Enclosed by 2 membranes Outer membrane – smooth and featureless Inner membrane – folds inward, forming shelflike cristae that protrude into the matrix, the gel-like substance within the mitochondrion The Electron Transport Chain Mitochondrial DNA  Subunits ND1,2,3,4,4L,5 and 6 from complex I  cyt b from complex III  COX I, II and III from complex IV,  ATPase 6 and 8 from complex V)  Ribosomes Small dark staining granules composed of proteins and a variety of RNAs 2 globular units that fit together like the body and cap of an acorn Site of protein synthesis 2 ribosomal populations: Free ribosomes float freely in the cytosol. Make soluble proteins that function in the cytoplasm as well as those imported into the mitochondria and some other organelles Membrane bound ribosomes are attached to membranes forming the rough endoplasmic reticulum. Synthesise proteins destined either for incorporation into cell membranes or lysosomes or for export from the cell Ribosomes Found floating in cytoplasm and attached to RER 37-62% RNA, rest proteins 80S ribosomes comprised of small (40S) subunits and substantial (60S) subunits The S stands for Svedberg unit, which relates to the sedimentation rate.  Endoplasmic Reticulum Extensive system of interconnected tubes and parallel sacs called cisterns Cisterns filled with fluid Coiling and twisting throughout the cytosol, ER is continuous with outer nuclear membrane and accounts for about half of the cell’s membranes Comes in 2 types  Rough ER External surface studded with ribosomes Proteins made in these ribosomes make their way into the cisterns where they are packaged into vesicles to travel to the Golgi apparatus for further processing Ribosomes on RER manufacture all proteins secreted from the cell Cell’s membrane factory  Smooth ER Continuous with the RER consists of tubules arranged in a looping network Enzymes play no role in protein synthesis. Instead catalyse reactions involved with following tasks: Metabolise lipids, synthesise cholesterol and phospholipids and synthesise the lipid components of lipoproteins (in liver cells) Synthesise steroid-based hormones Detoxify drugs, certain pesticides, and cancer causing chemicals (in liver and kidney) Break down glycogen to form glucose (especially in liver cells) Store calcium ions in most cell types (note sarcoplasmic reticulum in skeletal and cardiac muscle)  Golgi Apparatus Stacked and flattened membranous sacs, shaped like hollow dinner plates associated with swarms of tiny membrane vesicles Principal “traffic director” for cellular proteins Major role is to modify, concentrate, and package the proteins and lipids made at the RER and destined for export from the  Peroxisomes Spherical membranous sacs containing a variety of powerful enzymes, the most important of which are oxidases and catalases Numerous in kidney and liver Detoxify harmful substances e.g. alcohol and formaldehyde Neutralise free radicals to hydrogen peroxide Which is then converted to water by the catalases  Lysosomes Also spherical contain activated hydrolytic enzymes Can digest almost all kinds of biological molecules Work best in acidic conditions Digests particles taken into the cell Degrades stressed or dead cells and worn out non functioning organelles (autophagy) Perform metabolic functions such as glycogen breakdown and release Normally lysosome = stable But can be injured (oxygen deprivation, excess vit A) Lysosome ruptures – cell digests itself! Process called autolysis  Cytoskeleton Acts as a cell’s “bones,” “muscles,” and “ligaments” by supporting cellular structures and providing the machinery to generate various cell movements Composed of: Microfilments Intermediate filaments Microtubules  Microfilaments Semiflexible strands of the protein, actin Involved in cell motility or changes in cell shape (except muscle cells) In muscle cells the interaction of actin with myosin brings about contraction Constantly breaking down and re-forming from smaller subunits whenever and wherever their services are required As they do so they push or pull on the cell membrane thus changing the cell shape  Intermediate Filaments Tough insoluble protein fibres that resemble woven ropes Most stable and permanent of the cytoskeletal elements and strongly resist tension Attach to desmosomes Main job is to act as internal cables to resist pulling forces exerted on the cell Protein composition varies in different cell types  Microtubules Have largest diameter – hollow tubes made of tubulin Radiate from a small region of cytoplasm near the nucleus called the centrosome Highly dynamic – constantly growing, disassembling, then reassembling at same or different sites Determine overall shape of cell and distribution of organelles Mitochondria, lysosomes and secretory vesicles attach to the microtubules Tiny protein machines called motor proteins continually move and reposition the organelles Cilia Microvilli  Nucleus – learning Objectives Outline the structure and function of the nuclear envelope, nucleolus and chromatin Largest of the organelles – same shape as cell  Nuclear Envelope Double membrane barrier separated by a fluid filled sac Outer membrane continuous with RER Inner membrane lined by a network of lamins (rod-like proteins) that maintain nucleus shape Punctuated by nuclear pores Encloses a jellylike fluid called nucleoplasm in which other nuclear elements are suspended  Nucleoli Dark-staining spherical bodies where ribosomal subunits are assembled Normally 1 or 2 per nucleus Aggregations of all of the components needed to synthesise and assemble ribosomal subunits They centre around the DNA that codes for ribosomal RNA (rRNA). As rRNA molecules are synthesised they combine with proteins to form the 2 kinds of ribosomal subunits These then leave via the nuclear pore into the cytoplasm where they join to become mature ribosomes  Chromatin System of bumpy threads weaving through the nucleoplasm Contain 30% DNA, 60% histone proteins, 10% RNA Functional units are the nucleosomes, which consist of flattened disc-shaped cores or clusters of 8 histone proteins connected like beads on a string by a DNA molecule DNA winds twice around each nucleosome and continues on to the next cluster via linker DNA segments Histones – physical means for packing DNA but also important in gene regulation When cell is preparing to divide the chromatin threads coil and condense enormously to form short barlike bodies called chromosomes Let’s have a 10 minute break Mentimeter quiz in part 2