Cell Structure and Function notes

Cell Theory and General Cell Organisation

A cell is the smallest unit that can perform all basic activities of life. Organisms may consist of one cell, such as bacteria and Amoeba, or many cells, such as plants and animals. In multicellular organisms, cells become specialised for particular functions.

Most cells have a plasma membrane, cytoplasm and genetic material. Prokaryotic cells do not have a membrane-bound nucleus, while eukaryotic cells contain a true nucleus surrounded by a nuclear envelope.

• Cell theory states that all living organisms are made of one or more cells.
• The cell is the basic structural and functional unit of life.
• New cells arise from pre-existing cells by cell division.
• Prokaryotic cells have genetic material in a nucleoid region.
• Eukaryotic cells have a membrane-bound nucleus and membrane-bound organelles.
• Prokaryotic and eukaryotic cells both generally contain ribosomes.
• A nucleoid is not found in eukaryotic cells such as goblet cells.

Animal and Plant Cells

Animal and plant cells are eukaryotic, so both possess a true nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes and a plasma membrane. Their structures are similar because they perform many common cellular functions.

Plant cells have a cellulose cell wall, plastids and a large central vacuole. Animal cells lack a cell wall and plastids. Centrioles are usually present in animal cells, whereas typical higher plant cells lack centrioles.

The plasma membrane lies inside the cell wall in a plant cell. A lomasome is a membranous structure present outside the plasma membrane but inside the cell wall.

• Plant cell wall is mainly composed of cellulose, hemicellulose and pectin.
• Animal cells have a flexible plasma membrane but no cellulose cell wall.
• Plant cells contain chloroplasts for photosynthesis.
• The large central vacuole of a plant cell maintains turgor and stores substances.
• Animal cells commonly contain centrioles and small temporary vacuoles.
• Fungi resemble plants in lacking centrioles, but fungal cell walls are made mainly of chitin, not cellulose.
• Lomasomes occur between the plasma membrane and cell wall.
• The plasma membrane is mainly a phospholipid bilayer containing proteins.

Prokaryotic and Eukaryotic Cells

Prokaryotic cells are smaller and simpler in organisation. Bacteria are the main examples. Their DNA is usually a circular chromosome located in the nucleoid. They do not possess a membrane-bound nucleus, mitochondria, chloroplasts, endoplasmic reticulum or Golgi apparatus.

Eukaryotic cells occur in protists, fungi, plants and animals. Their DNA is present in linear chromosomes inside a nucleus. They contain specialised organelles surrounded by membranes.

• Bacterial cell walls contain peptidoglycan, also called murein.
• Peptidoglycan is a distinctive component of the bacterial cell wall.
• The bacterial plasma membrane lies inside the cell wall.
• A prokaryotic nucleoid is not enclosed by a nuclear membrane.
• Eukaryotic chromosomes are associated with histone proteins.
• Ribosomes occur in both prokaryotic and eukaryotic cells.
• Prokaryotic ribosomes are 70S, while cytoplasmic eukaryotic ribosomes are 80S.
• The 70S ribosomes of mitochondria and chloroplasts support their evolutionary relationship with prokaryotes.

Plasma Membrane, Cell Wall and Cell Connections

The plasma membrane follows the fluid mosaic model. It consists of a phospholipid bilayer in which proteins are embedded or attached. Phospholipids have hydrophilic heads facing the watery surfaces and hydrophobic tails facing inward.

The membrane controls the movement of substances into and out of the cell. It is selectively permeable. Small non-polar molecules may pass through the lipid layer, while ions and many polar molecules require transport proteins.

Plant cells have a cell wall outside the plasma membrane. The wall gives mechanical support and prevents the cell from bursting when water enters. Adjacent plant cells communicate through plasmodesmata.

• Phospholipids form the basic structural framework of the plasma membrane.
• Membrane proteins function as channels, carriers, receptors and enzymes.
• Cholesterol helps maintain membrane stability and fluidity in animal cell membranes.
• Diffusion is the movement of particles from higher concentration to lower concentration.
• Osmosis is the movement of water through a selectively permeable membrane.
• Active transport uses cellular energy, usually ATP, to move substances against a concentration gradient.
• The plant cell wall provides support but is freely permeable to many small substances.
• Plasmodesmata connect the cytoplasm of neighbouring plant cells.

Cytoplasmic Organelles

The cytoplasm contains the cytosol, organelles and stored materials. Each organelle performs a specialised role, but organelles work together as an integrated system.

The endoplasmic reticulum forms an internal membrane network. Rough endoplasmic reticulum has ribosomes and synthesises proteins for secretion or membranes. Smooth endoplasmic reticulum lacks ribosomes and is involved in lipid synthesis, detoxification and steroid hormone production.

The Golgi apparatus modifies, sorts and packages materials. Its convex cis face receives vesicles from the endoplasmic reticulum, while its concave trans face, also called the maturation face, gives rise to outgoing vesicles.

• Ribosomes are non-membranous structures that synthesise proteins.
• Rough endoplasmic reticulum synthesises proteins for export, lysosomes and cell membranes.
• Smooth endoplasmic reticulum produces lipids and steroid hormones.
• Smooth endoplasmic reticulum also helps detoxify drugs and poisons in some cells.
• The Golgi apparatus modifies, sorts and packages proteins and lipids.
• The inner concave surface of the Golgi apparatus is the trans face.
• Lysosomes contain hydrolytic enzymes for intracellular digestion and recycling.
• Peroxisomes contain enzymes involved in oxidation reactions and hydrogen peroxide breakdown.

Mitochondria, Chloroplasts and ATP Production

Mitochondria are the main sites of aerobic respiration and ATP production in eukaryotic cells. Each mitochondrion has an outer membrane and a folded inner membrane. The folds are called cristae. The internal fluid is the matrix.

Chloroplasts occur in photosynthetic plant cells and algae. They have an outer membrane, inner membrane, stroma and thylakoid membranes. A stack of thylakoids is called a granum. The light-dependent reactions occur on thylakoid membranes, while the Calvin cycle occurs in the stroma.

The chemiosmotic theory explains ATP synthesis through a proton gradient. Electron transport creates a difference in hydrogen ion concentration across a membrane. The return flow of protons through ATP synthase drives ATP formation.

• Mitochondria contain their own circular DNA and 70S ribosomes.
• Chloroplasts contain their own DNA and ribosomes.
• Mitochondrial inner membrane folds are called cristae.
• The chloroplast fluid surrounding grana is called stroma.
• The number of mitochondria is not constant. It varies with cell type and energy demand.
• A proton gradient is the basis of chemiosmotic ATP synthesis in mitochondria and chloroplasts.
• ATP synthase uses the flow of protons to join ADP and inorganic phosphate.
• Mitochondria and chloroplasts are semi-autonomous organelles.

Cytoskeleton, Centrioles and Cell Division

The cytoskeleton is a network of protein fibres that maintains cell shape, positions organelles and assists movement. Microfilaments are the thinnest fibres and are composed mainly of actin. Microtubules are made of tubulin and form spindle fibres.

Centrioles occur in the centrosome of animal cells. A centriole is a cylindrical structure made of microtubule triplets. Before mitosis, the centriole pair duplicates. By completion of G2 phase, an animal cell has four centrioles, and four centrioles are present during metaphase.

Mitosis produces two genetically similar daughter cells. During metaphase, chromosomes are arranged at the equator. Spindle fibres attach to centromeres and separate sister chromatids during anaphase.

• Microfilaments are composed mainly of actin.
• Microtubules are composed of tubulin.
• Intermediate filaments provide mechanical strength to cells.
• Centrioles help organise spindle fibres during animal cell division.
• Number of centrioles after G2 phase is four.
• Number of centrioles during metaphase of mitosis is four.
• Colchicine prevents spindle formation and arrests mitosis at metaphase.
• If spindle function is stopped by colchicine, chromatids do not migrate to opposite poles.
• Phragmoplast helps form the cell plate during cytokinesis in plant cells and is associated with Golgi-derived vesicles.

Chromosomes, DNA and Gene Expression

A chromosome is a DNA molecule associated with proteins. Before DNA replication, a chromosome contains one chromatid. After replication, it consists of two identical sister chromatids joined at a centromere. The number of chromosomes remains constant after replication, but the amount of DNA doubles.

DNA replication is semi-conservative. Each original strand acts as a template for a new complementary strand. DNA polymerase adds nucleotides in the 5' to 3' direction. Because the two DNA strands are antiparallel, synthesis is continuous on one strand and discontinuous on the other.

The discontinuous strand is called the lagging strand. Short pieces called Okazaki fragments form on it. DNA ligase joins these fragments by forming bonds in the sugar-phosphate backbone. Phosphoric acid, H3PO4, forms ester linkages with the hydroxyl groups of pentose sugars in nucleotides.

A gene is a segment of DNA that carries information for a functional product. A trait controlled by a gene located on the X chromosome is called sex-linked. The gene for blue opsin is located on chromosome 7.

• A replicated chromosome has two sister chromatids joined at a centromere.
• DNA molecules are made of nucleotides containing deoxyribose, phosphate and nitrogenous bases.
• H3PO4 forms ester linkages with the OH groups of pentose sugars.
• DNA polymerase adds complementary nucleotides during replication.
• DNA ligase attaches Okazaki fragments.
• Okazaki fragments are formed on the lagging strand.
• The lagging strand is synthesised discontinuously.
• A gene is a hereditary unit present at a particular locus on a chromosome.
• A trait controlled by a gene on the X chromosome is sex-linked.
• The gene for blue opsin is present on chromosome 7.

Cell Regulation, Lac Operon and Related Biological Concepts

Cells regulate gene expression so that proteins are produced only when required. In bacteria, the lac operon of Escherichia coli controls the use of lactose. The lac I gene produces a repressor protein.

In the absence of lactose, the repressor binds to the operator and prevents transcription of structural genes. When lactose is present, it acts as an inducer by inactivating the repressor. The structural genes can then be transcribed. This system is negative and inducible because a repressor normally prevents transcription.

Uncontrolled cell division can produce a tumour. If tumour cells spread from their original location to other parts of the body through blood or lymph, the process is called metastasis.

Some additional biological terms are linked with animal development and classification. A pseudocoelom develops from the blastocoel. In annelids, the circulatory system is arranged segmentally. Virulent Streptococcus pneumoniae has a smooth, capsulated colony form.

• The lac I gene codes for the repressor protein.
• The lac operon is negative because a repressor can block transcription.
• The lac operon is inducible because lactose permits transcription by inactivating the repressor.
• The operator is the DNA region to which the repressor binds.
• A tumour may result from uncontrolled cell proliferation.
• Metastasis is the spread of cancer cells from one body region to another.
• A pseudocoelom develops from the blastocoel.
• The circulatory system of annelids is segmentally arranged.
• Virulent Streptococcus pneumoniae occurs in the smooth, or S, form.