Cell Structure and Function

• The session aims to provide a broad understanding of cells' structures and components.
• The learning outcomes include labeling cells, explaining cell division (mitosis and meiosis), and understanding cell growth abnormalities.

Features of the Cell

• The cell is the smallest unit of body.
• Organisms comprise cells.
• Cells are differentiated and adapted for different task.
• All vital chemical processes occur inside cells.
• Humans are multi-cellular organisms

Order of Development

• Cells form tissues.
• Tissues form organs.
• Organs form systems.

Cell Structure

• Cells consist of plasma membrane, cytoplasm, nucleus, mitochondria, ribosomes, Golgi apparatus, microtubules, centrioles, lysosomes, microfilaments, rough and smooth endoplasmic reticulum, and peroxisomes.

Nucleus

• A specialized organelle functions as the information processor and administrative center of the cell.
• Usually one nucleus per cell, but skeletal muscle cells can have several.
• Erythrocytes (red blood cells) don't have a Nucleus
• It is the largest feature in a cell.

Nuclear Envelope

• A double membrane bound outer layer continuous with the Endoplasmic Reticulum (ER).
• Pores in the membrane allow the passage of substances.

Nucleus Functions

• Stores genetic material (DNA).
• Directs metabolic activities, including growth, metabolism, protein synthesis, and reproduction (cell division).
• Involved in cell division.
• Instructs synthesis of proteins, ribosomes, and RNA.

Plasma Membrane (Cell Membrane)

• Cell membranes: partially permeable, made of 2 layers.
• Cell membranes contain: phospholipids (fatty substances) that are embedded, also consists of proteins and carbohydrate molecules.
• Phospholipids are hydrophilic at phosphate ends and hydrophobic along lipid tail regions.
• Selective transport of substances.
• Some proteins are embedded in the bilayer, while others attach to the surface.
• Acts as surface receptors for hormones or as enzymes.
• Carbohydrate molecules provide immunological identity.
• Regulates transport in and out of cell.
• Allows directed cell or organelle motility.

Cytoplasm

• Gelatinous, semi-transparent fluid fills most cells to allow metabolic reactions to occur
• It is mainly water, with solutes including glucose, proteins, and ions
• It contains cellular chemicals like sodium (Na), phosphates, chlorides, amino acids, ATP, and storage material like oil droplets.

Mitochondrion

• The powerhouse of the cell.
• Made from outer and inner membranes.
• Cristae are folded inner membranes that allow maximum surface area for chemical reactions.
• Involved with cellular aerobic (O2) respiration process by which chemical energy is made available in the cell.
• Energy = ATP (Adenosine triphosphate).
• ATP form from catabolism of carbohydrates, fats and proteins in the cell
• Occur in varying numbers, most in physically and metabolically active cells (e.g., skeletal muscles).

Rough Endoplasmic Reticulum (RER)

• Interconnecting membranous canals enclose flattened sacs called cisternae.
• Extension of outer nuclear membrane.
• Covered in ribosomes to make proteins, leading to chemical reactions.
• Provides a pathway for transport of materials.
• Collects, stores synthesized material.
• Provides a structural skeleton for cell shape, allowing proteins and enzymes production.

Smooth Endoplasmic Reticulum (SER)

• Similar features to RER, but no ribosomes.
• Found in large amounts in cells which secrete steroids e.g. adrenal cortex in the kidney
• Synthesizes lipids and steroid hormones and contains enzymes detoxifying chemicals e.g. drugs and pesticides.

Golgi Apparatus

• Features stacks of flattened membranous sacs (cisternae), fluid-filled sacs pinch off smaller membranous sacs called vesicles
• Packages & processes proteins received from ER into vesicles for secretion, and delivers to destinations e.g., lysosomes or cell membrane
• Plays a role in the secretory pathway.
• Receives glyco-proteins such as mucin required adding the carbohydrate to the protein, Produces secretory, transports and stores lipids, and secretes carbohydrates in e.g. by forming lysosomes.

Lysosomes

• Formed as a secretory vesicle by the Golgi apparatus.
• Vesicles contains enzymes degrade proteins and membranes.
• Aids degradation of materials ingested, due to hydrolytic enzymes.
• Kept separate from the cell to prevent destruction, and inactive by an alkaline environment.
• Breaks down digested materials e.g. bacteria, and allows autophagy of a cell through the lysosomal machinery and autolysis ( the destruction of a cell through the action of its own enzymes)

Centrosome

• Directs organization of microtubules within the cell.
• Contains a pair of centrioles which are small clusters of microtubules.
• Plays a role in cell division.
• 2 centrioles consist of cylindrical structures, located at right angles.
• Cylindrical structures are held at right angles to one another.

Microfilaments

• Preodominantly composed of a contractile protein called actin, which is therefore the most abundant protein inside cells
• Fine, thread fibers, 3-6 nm in diameter.
• Responsible for muscle contraction with the protein myosin, and cell gliding, contraction, and cytokinesis (division of cytoplasm following division of a nucleus).

Microtubules

• Cylindrical tubes, 20-25 nm in diameter.
• They are composed of subunits of the protein tubulin
• Function as a scaffold to determine cell shape, and allowing intracellular transport of material ( movement of mitochondria).
• Act as the spindle fibres to separate the chromosomes during mitosis
• Arranged in geometric patterns when inside flagellae and cilia in order to assist locomotion

Peroxisome

• Has a membrane bound organelle.
• An organelle that contains oxidative enzymes such as catalase.
• Used for breaking down is fatty acid molecules, and delays cell ageing.

Mitosis

• Cell duplicates the chromosomes in its nucleus to generate two genetically identical daughter nuclei, having 46 chromosomes each.
• A complex and highly regulated process.
• Duplicated chromosomes (chromatids) line up with spindle fibers attaching to the center.
• Interphase: Cell grows and develops, replicating chromosomes into pairs of chromatids and duplicating centrioles.
• Prophase: Chromosomes become visible, nuclear membrane disappears, and mitotic spindle forms with chromatids joining at the centromere.
• Metaphase: Nuclear membrane disappears completely, centromeres align at opposite poles, where chromosomes align at the metaphase plate at right angles to the spindle poles.
• Anaphase: Paired chromatids move apart, becoming daughter chromosomes towards opposite poles.
• Telophase: Nuclei form at opposite poles, chromatin uncoils, and cytokinesis divides the cytoplasm, creating two distinct daughter cells.

Meiosis

• Division occurs in the formation of reproductive cells (gametes – ova and spermatozoa).
• Mature ova grow in the ovaries (female), spermatozoa in testes (male).
• After division, daughter cells contain 23 chromosomes; fertilization restores the full complement of 46.
• Stages of Meiosis 1
• Homologous chromosome pair divide into two daughter cells.
• Each with 23 chromosomes (haploid). Stages of Meiosis 2
• Equivalent to mitosis where 4 daughter haploid cells are produced

Cellular Transport

• Passive transport (diffusion or osmosis)
• Active transport (uses ATP).
• Bulk transport (pinocytosis or phagocytosis).

Diffusion

• Substance moves from high to low concentration.
• Its rate depends on concentration gradient, distance, area size, structure in path, and molecule size.

Osmosis

• Occurs when water passes equilibrium membrane.
• Solute molecules large cannot pass the pores by diffusion
• Youtube - Osmosis

Active Transport

• Transports substances up their concentration gradient (lower to higher), driven by chemical energy (ATP).
• The sodium-potassium pump maintains homeostasis of electrolytes sodium and potassium

Phagocytosis

• The cell can obtain particles too, large to be absorbed by diffusion or active transport.

Pinocytosis

• Similar to phagocytosis, cell drinking intake of liquids through “pinocytotic vesicles”.
• Both pinocytosis & phagocytosis are methods by which materials are taken into the cell in bulk.
• Materials also removed from the cell e.g. waste products (EXOCYTOSIS).

Abnormal Cell Growth

• Hyperplasia: proliferation of cells within an organ or tissue, constant cell division.
• Hypoplasia: underdevelopment or incomplete development of a tissue or organ due to cell number.
• Atrophy: partial or complete wasting away of a part of the body.
• Hypertrophy: increase in the volume of an organ or tissue due to enlargement of its component cells.
• Metaplasia: transformation of one type of cell into another and if the stimulus is removed or ceases.
• Dysplasia: indicates an abnormality in maturation of cells within a tissue.
• Neoplasia: abnormal proliferation of cells causing a lump or tumor that is pre-malignant or malignant.