The Cell: Structure and Functions

The cell

It’s the basic, structural and functional unit of life

Each cell is capable of performing all the essential life functions.

The process by which cells assume specialized functions and structure is known as differentiation.

Cell structure

Cells conform to the same basic structure.

All cells are composed of an external lining membrane known as the plasma membrane that serves as an interface between the internal and external environment.

The cell membrane surrounds a number of organelles that are embedded in a fluid medium known as the cytoplasm.

The cell membrane

• All cell membranes conform to the same basic model: the fluid mosaic model.

The model was proposed by Singer and Nicholoson in 1970.

The cell membrane

Cell membrane

The Cell: Structure and Functions

The cell membrane is made up of a lipid by layer with a hydrophilic polar end and a hydrophobic non polar end.

The hydrophilic end is made up of glycerol attached to a nitrogenous compound by a phosphate bond. The nitrogenous cpds include chlorine, ethanolamine or serine. They are positively charged. The phosphate gp is negatively charged.

The Cell: Structure and Functions

The non polar end is made up of two chains of fatty acids, one of them is straight and saturated while the other is kinked and non saturated.

The kinked fatty acids increase flexibility and fluidity Between the fatty acids is cholesterol molecules in the ratio of 1:1 with the phospholipids.

• The cholesterol prevents close packing and increases fluidity.

They regulate the fluidity and increase stability of the cell membrane.

Proteins in the cell membrane

Associated with the cell membrane are three types of proteins: extrinsic, intrinsic and transmembrane proteins.

These make up ½ the total mass of the cell membrane.

Extrinsic proteins are found on the outside surface, intrinsic on the internal surface of the cell membrane while transmembrane proteins traverse the whole thickness of the cell membrane

They act as pores for passage of ions

Associated with the proteins and phospholipids are carbohydrates forming glycoproteins and glycolipids.

These are involved in cell recognition, formation of intracellular adhesions and absorption of molecules to the cell surface.

Transport across the cell membrane

• Molecules enter and leave
• Passive diffusion
• Facilitated diffusion
• Active transport

Bulk transport

Passive diffusion

• Requires a concentration gradient.

Doesn’t involve metabolic energy.

Molecules move from a region of high concentration gradient to a region of low concentration gradient across a semi permeable membrane It is for non polar molecules like ethanol, and polar molecules that are uncharged like oxygen, carbondioxide, urea

Facilitated diffusion

• Similar to simple diffusion but requies a carrier protein
• It is for larger hydrophillic molecules like amino acids, glucose and fatty acids

The binding to carriers is specific but reverscible

Active transport

Active transport

• Requires energy in form of ATP
• Occurs against a concentration gradient

It involves pumps

Bulk transport

Includes endocytosis and exocytosis (phagocytosis and pinocytosis)

Cell organelles: Nucleus

Nucleus

Largest of all cell organelles. Has a nuclear membrane that has pores Contains the genetic material of the cell inform of DNA and proteins(nucleoproteins). The proteins include the DNA associated proteins: histones, and the non DNA associated proteins.

Nucleoli

Densely stained structures found inside the nucleus. They are sites for ribosomal RNA synthesis.

Endoplasmic reticulum

• Rough endoplasmic reticulum
• Made up of interconnecting network of tubules, sacs and vesicles covered by ribosomes
• Sites for synthesis and packaging of proteins
• Smooth endoplasmic reticulum:

not covered by ribosomes. Sites for lipid synthesis, and synthesis of cell membrane.

Golgi apparatus

Consists of s system of stacked, saucer shaped cisternae with the concave surface facing the nucleus Involved in intracellular transport by packaging proteins into vesicles and releasing them by exocytosis.

Golgi apparatus

Mitochondria

Mitochondria

Cigar shaped highly motile organelles composed of a smooth outer membrane and a folded inner membrane (cristae) Sites for energy production in the cell Tend to be plenty in highly metabolic tissues like liver and muscle cells.

Lysosomes

Lysosomes

Oval membrane bound organelles with numerous enzymes involved in breakdown of phagocytosed bacteria and other particles.

The cell cycle

Refers to events that occur in a cell’s lifetime The cycle is composed of a dividing phase (M phase) and a non dividing phase (Inter phase) There are three types of cells according to the cell cylcle: the continuously dividing cells that are always dividing e.g skin cells and cells of digestive system

The Cell: Structure and Functions

The facultative dividers: these live the cell cycle and enter the Go phase but retain the ability to divide e.g liver cells The terminally differentiated cells: leave the cell cycle permanently and lose the ability to divide e.g cells of the nervous system

The cell cycle

Interphase

• Composed of G1, S and G2 phases
• G1 phase: cells perform their functions
• S phase: cells replicate their DNA

G2 phase: cells prepare for division.

Dividing phase: MITOSIS

Composed of prophase, metaphase, anaphase and telophase Prophase: chromosomes become visible, initially thread like, but later appear to be composed of two sister chromatids joined at centromere, nucleoli disappears, process ends with disappearence of nuclear membrane

The Cell: Structure and Functions

Metaphase: chromosomes arrange themselves at equator of the spindle, microtubules span from opposite centrioles, sister chromatids begin separating and moving towards opposite poles

Anaphase: sister chromatids continue moving towards opposite poles

Telophase: sister cromatids reach opposite poles, spindles diappear, nucleoli and nuclear membrane reappears, cell membrane divides (cytokinesis) to form two daughter cells

Importance of cell division

• Repair
• Growth

reproduction

Meiosis

• Occurs in two phases, meoisis one and two
• Crossing over occurs leading to exchange of genetic material
• Occurs only in gametes

Product is four daughter cells with half the chromosome number of parent cell.

Meiosis