Cell Structure and Function

GDC Learning Outcomes

• Describe dental, oral, craniofacial, and general anatomy, and explain its relevance to patient care
• Describe appropriate physiology and explain its relevance to patient care

Learning Outcomes

• Understand the structure and components of the cell
• Label the cell and describe the function of its components
• Explain mitosis and meiosis cell division
• Understand abnormalities of cell growth and function

Features of the Cell

• Cells are the smallest functional units of the body
• All organisms consist of cells
• Cells perform all vital chemical processes
• Humans are multicellular
• Cells are differentiated and adapted for different tasks

Order of Development

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

Cell Structure

• Plasma membrane is a key component
• Cytoplasm fills the cell
• Nucleus houses genetic material
• Mitochondria produce energy
• Ribosomes synthesize proteins
• Golgi apparatus processes and packages proteins
• Rough and smooth endoplasmic reticulum is involved in protein and lipid synthesis
• Lysosomes break down waste
• Microfilaments provide structure
• Microtubules facilitate movement and transport
• Centrioles organize microtubules
• Peroxisomes break down fatty acids

Nucleus Features

• A specialized organelle that functions as the information processor and administrative center of the cell
• Most cells have one nucleus
• Skeletal muscle cells have several nuclei
• Red blood cells lack a nucleus
• The nucleus is the largest feature in a cell

Nuclear Envelope

• A double membrane surrounds the nucleus
• The outer layer connects to the endoplasmic reticulum
• Pores in the membrane enable substance passage

Nucleus Functions

• Stores genetic material (DNA)
• Directs all metabolic activities like growth, metabolism, protein synthesis and reproduction through cell division
• Is involved in cell division
• Instructs the synthesis of proteins/ribosomes & RNA

Plasma Membrane

• The plasma membrane or cell membrane is partially permeable
• It's composed of 2 layers
• The membrane consists of phospholipids and is embedded
• Also contains proteins and carbohydrate molecules

Phospholipids

• Phospholipids form a bilayer
• They are hydrophilic at their phosphate ends, facing outwards
• They are hydrophobic along lipid tail regions, orientated inwards

Plasma Membrane Components

• Some proteins are embedded in the bilayer, while others attach to the surface
• Some play a role in selective transport of substances across the membrane
• Acts as surface receptors for substances such as hormones or act as enzymes

Membrane Functions

• Contains cell contents
• Carbohydrate molecules provide immunological identity
• Regulates transport in and out of cell
• Acts as receptors for hormones
• Directs cell or organelle motility

Cytoplasm

• The cytoplasm is a gelatinous, semi-transparent fluid that fills the cells
• Metabolic reactions occur in the cytoplasm
• Cytoplasm is mainly water with solutes including glucose, proteins, & ions
• Cellular chemicals, such as sodium, phosphates, amino acids, ATP, and oil droplets are present

Mitochondria

• The mitochondria are the "powerhouse of the cell"
• Consists of outer and inner membranes
• Cristae are folded inner membranes which allow a maximum surface area for chemical reactions to occur

Mitochondria Function

• Involved with cellular aerobic respiration
• Process by which chemical energy is made available in the cell
• Energy is produced by ATP from the catabolism of carbohydrates, fats and proteins
• Quantity depends on the cell & its function, greatest in physically and metabolically active cells

Rough Endoplasmic Reticulum (RER)

• Consists of interconnected membranous canals
• Encloses flattened sacs called cisternae
• Is an extension of the nuclear membrane
• Has a large surface area for chemical reactions
• Covered in ribosomes which make proteins

RER Functions

• Produces proteins especially enzymes
• Transports materials throughout the cell
• Collects and store synthesized material
• Provides a structural skeleton to maintain a cells’ shape

Smooth Endoplasmic Reticulum (SER)

• Has similar features to the RER bur no ribosomes
• Is concerned with synthesis of lipids & steroid hormones
• Large amounts are found in cells which secrete steroids i.e. adrenal cortex in the kidney
• Contains enzymes for detoxifying chemicals including drugs and pesticides

Golgi Apparatus

• Is composed of stacks of flattened membranous sacs called cisternae
• Sacs are fluid filled and pinch off smaller membranous sacs, called vesicles
• Typically, there is only one Golgi apparatus in each animal cell

Golgi Apparatus Functions

• Packages & processes proteins received from the ER into vesicles prior to secretion
• Delivers vesicles to their destinations e.g., lysosomes or cell membrane
• Plays a key role in the secretory pathway
• Receives glycoproteins i.e. mucin, by adding the protein's carbohydrate part
• Produces secretory enzymes i.e. digestive enzymes
• Transports & storing lipids
• Secretes carbohydrates in cell wall production

Lysosomes

• Secretory vesicle formed by the Golgi apparatus
• Contain enzymes for degrading proteins & membranes in the cell
• Aids degradation of materials ingested by the cell
• Are kept apart from the cell to prevent destruction of cell contents
• Kept inactive by an alkaline environment within

Lysosome Functions

• Digests materials consumed from the environment e.g. bacteria
• Release enzymes outside the cell (exocytosis) to break down other cells
• Involved with autophagy, the degradation of its own components through the lysosomal machinery
• Involved with autolysis, the destruction of a cell through the action of its own enzymes
• Self-destructs cells by apoptosis (programmed cell death) if damaged beyond repair
• Destroys invading viruses & bacteria
• Breaks down excess or worn-out cell parts
• A membrane bound organelle that contains digestive enzymes

Centrosome

• Directs organisation of microtubules within the cell
• Comprises a pair of centrioles
• Plays a part during cell division
• The 2 centrioles are cylindrical structures, orientated at right angles to each other
• They act as organisers of the nuclear spindle during cell division

Microfilaments

• Composed mainly of actin, the most abundant cellular protein
• Are fine, thread-like proteins fibres which are 3-6 nm in diameter
• Carries out cellular movements including gliding, contraction & cytokinesis
• Association with the protein myosin is responsible for muscle contraction

Microtubules

• Cylindrical tubes 20-25 nm in diameter
• Composed of subunits of the protein tubulin
• Involved in intracellular transport, e.g. movement of mitochondria
• Form spindle fibres for separating chromosomes during mitosis
• Acts as a scaffold to determine cell shape
• When arranged in geometric patterns inside flagellae and cilia, they perform locomotion

Peroxisome

• A membrane-bound organelle
• Contains oxidative enzymes i.e. catalase
• Has a major function with the breakdown of fatty acid molecules
• Is important in delaying cell ageing

Mitosis

• A cell duplicates chromosomes in its cell nucleus to generate two genetically identical daughter nuclei
• Each daughter nucleus contains 46 chromosomes
• The process of mitosis is complex and highly regulated

Mitosis Stages

• Interphase
• Prophase
• Metaphase
• Anaphase
• Telophase

Interphase

• Cell grows
• DNA is synthesized
• Chromosomes replicate into pairs of chromatids
• 2 pairs of centrioles appear

Early Prophase

• Each chromosome has two chromatids joined at a centromere
• The mitotic spindle is composed of microtubules and proteins and forms in the cytoplasm
• The two pairs of centrioles move away from one another toward opposite ends of the cell

Late Prophase

• Nuclear envelope breaks up
• Microtubules reach from each cell pole to the cell's equator
• Chromosomes and chromatids migrate towards cell center

Metaphase

• Nuclear membrane disappears completely
• Two pairs of centrioles align at opposite poles of the cell
• Chromosomes and chromatids align at the metaphase plate at right angles to the spindle poles

Anaphase

• Paired chromatids in each chromosome begin to move apart
• Once the sister chromatids separate from one another, each is a "full" chromosome, referred to as daughter chromosomes
• The daughter chromosomes move to the poles at opposite ends of the cell

Telophase

• Nuclei begin to form at opposite poles
• Chromatin fibres of chromosomes uncoil
• Cytokinesis, (the division of the original cell's cytoplasm) , begins before the end of mitosis and completes shortly after telophase
• The end result after cytokinesis is two distinct daughter cells.

Meiosis

• Occurs in formation of reproductive cells: gametes, ova, and spermatozoa
• Ova grow to maturity in the ovaries and spermatozoa in the testes
• After division each of the daughter cells has only 23 chromosomes
• Full complement of 46 chromosomes returns when the ovum is fertilized

Stages of Meiosis - Meiosis I

• Chromosomes in the diploid cell replicate itself
• Homologous chromosomes pair up
• The stages of meiosis are as for mitosis
• The homologous pairs swap some DNA
• Homologous pairs then divides to produce 2 daughter cells, each with 23 only

Stages of Meiosis - Meiosis II

• Meiosis II is similar to mitosis and produces 4 daughter haploid cells

Transport of Substances Across Cell Membranes

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

Cell Transport

• Diffusion
• Osmosis
• Active Transport
• Phagocytosis
• Pinocytosis

Diffusion

• The process by which a substance moves from a region of high concentration to a region of low concentration
• Variable Rate: Concentration gradient, distance, the size of area and structure in path

Osmosis

• The passage of water down its concentration gradient across a semi permanent membrane
• Occurs when solute molecules cannot pass through membrane pores by diffusion,
• Check out Osmosis on YouTube

Active Transport

• The transport of substances “up” their concentration gradient
• Requires chemical energy (ATP) to drive protein molecules in the membrane that move substances
• For example the sodium-potassium pump: maintains homeostasis of electrolytes sodium and potassium

Phagocytosis

• The process by which the cell engulfs particles that are too large for diffusion or active transport

Pinocytosis

• Cell drinking
• Is very similar to phagocytosis
• Vesicles are produced called 'pinocytotic vesicles'
• Used for intake of liquids rather than solids
• Both pinocytosis and phagocytosis are methods used for materials that enter the cell in bulk by endocytosis
• Reverse process materials are removed from the cell i.e. waste products by exocytosis
• Check out cell membrane, Exocytosis & Endocytosis on YouTube

Hyperplasia

• The proliferation of cells within an organ or tissue beyond what's normally seen
• Caused by constant cell division

Hypoplasia

• Underdevelopment or incomplete development of tissue or an organ
• Refers to an inadequate or below- normal number of cells
• Is a congenital condition
• Hyperplasia generally refers to excessive cell growth later in life

Atrophy

• The partial or complete wasting away of a part of the body

Hypertrophy

• The increase in volume of an organ or tissue due to the enlargement of its component cells
• Hypertrophy and hyperplasia are distinct, these processes frequently occur together

Metaplasia

• The transformation of one type of cell into another
• Caused by some sort of abnormal stimulus;
• If said Stimulus is ceased tissues return to their normal pattern state

Dysplasia

• Refers to an abnormality in maturation of cells within a tissue
• Consists of an expansion of immature cells, with a decline in the number of locations of mature cells

Neoplasia

• Abnormal proliferation of cells typically causing a lump or tumor
• The Growth of a cell clones exceeds and uncoordinates with the normal tissue surrounding the cells
• May be benign, pre-malignant, or malignant

Test your knowledge of cell biology. This quiz covers cell structures, functions, and their organization into tissues, organs, and systems. Questions cover topics such as organelles, the nucleus, and cell types.