Cell Structure & Function

BIO 231

Anatomy and Physiology I

Generalized Cell

Plasma Membrane

Phospholipid bilayer

Proteins imbedded into bilayer

Ion channels

Receptors

Carbohydrate chains of glycolipids & glycoproteins

Function in cell-to-cell

Fluid Mosasic Model

Plasma Membrane

Microvilli

Apical surface of cells specialized for rapid diffusion

Tiny fingerlike

Dramatically increase surface area

Tight Junctions

Protein molecules of adjacent cells fuse together - encircle the cell - impermeable junction

Epithelial cells of digestive tract

Desmosomes

Anchoring junctions - scattered over the surface of adjacent cells

In cells of skin & heart muscle

Gap junctions

“Nexus” - cells come very close together - link by protein channels called connexons

Found in heart & smooth muscle - electrically connects the cells

Molecules Crossing Membranes

Small molecules like O₂ and CO₂ can pass easily through lipid bilayer

Larger molecules can’t easily pass through

Molecules Crossing Membranes

Cell membrane is a barrier to of molecules

H₂O passes through pores or channels as do some ions

Larger molecules & some ions use carriers

Facilitated Diffusion

Osmosis

Diffusion of water across a cell membrane

Tonicity

The ability of a solution to change the tone or shape of cells by altering their internal water volume

Isotonic - cells retain normal shape - same amount of water and solute on both sides of the membrane

Hypotonic - cells swell - less water & solutes in side than out and water enters the cell

Hypertonic - cells shrink (crenate) - more water, less solute inside than out - water leaves the cell

Primary Active Transport

Secondary Active Transport

Molecules Crossing Membranes

Largest molecules are to big for carriers

Membrane invaginates or forms a pouch

Contains extracellular fluid and the large molecules to cross the membrane

The pouch then pinches off - get a vesicle

Called

Phagocytosis

Form pseudopod

Bulk-phase

A bit of infolding of the membrane surrounds a droplet containing extracellular molecules - vesicle forms

Receptor-Mediated Endocytosis

Molecules bind to specific receptors - clathrin coated vesicles forms

e.g. Cholesterol

Exocytosis

Reverse of endocytosis

Mechanism for secretion of cellular products

Certain hormones and neurotransmitters

Cilia and Flagella

Make cells move

Hairlike - move in undulating fashion

Cilia

Apical surface - respiratory tract lining

Lining of uterine or fallopian tubes

Flagella - whiplike

Propels sperm toward the egg

Cilia and Flagella

Cytoplasm

Jellylike matrix found in the cell

Structural elements (cytoskeleton)

Microtubules (tubulin)

Microfilaments

e.g. actin - in - cause movement

Cytoskeleton

Mitochondrion

Powerhouse of the cell

Double membrane

Outer smooth

Inner folded - (folds)

Contain enzymes involved in oxidative metabolism

Contains their own DNA

Endoplasmic Reticulum

Piles of flattened sheets of membrane - tubular

Types

Rough Endoplasmic Reticulum

Granular -

Smooth Endoplasmic Reticulum

No ribosomes

Endoplasmic Reticulum

Smooth Endoplasmic Reticulum

Site of synthesis of new membrane fatty acids

In muscle - stores Ca⁺⁺

Sarcoplasmic reticulum

Involved in in some cells

Hepatic cells

Rough Endoplasmic Reticulum

Site of synthesis of proteins destined to be

Secreted

Inserted into plasma membrane

Ribosomes

Two subunits

RNA +

Free in cytoplasm, polysomes or attached to ER

Golgi Apparatus

Series of flattened membranous sacs

Proteins produced by RER arrive in vesicles

Carbohydrate residues added, removed or replaced

Golgi Apparatus

Modified proteins are repackaged into vesicles

Concerned with intracellular trafficking of proteins

via destination

Golgi Apparatus

Lysosomes

Formed by Golgi - membrane bound

Contain digestive or hydrolytic enzymes

Lysosomes

Primary lysosome - only digestive enzymes

Fuse with endocytotic vesicles

Secondary lysosome

May also encapsulate worn-out organelles

Autophagy

Cell Nucleus

Found in all cells except mature

Contains DNA - determines structure and function of the cell

Bounded by double membrane - nuclear envelope

Nuclear pores - (80 - 100 nM in diameter)

Oversees functioning of the cell

Cell Nucleus

Chromatin

Masses of threads of DNA & protein

Most protein is histones

Cause DNA to spool - nucleosomes

Structure of DNA

Formed by joining nucleotides

Phosphate

Sugar - Deoxyribose

Base

Nucleotides of DNA

DNA - Double Helix

Sugar & phosphate - backbone

Bases extend to the sides

Two strands of nucleotides

Held together by weak hydrogen bonds

Complimentary Base Pairing

DNA Structure

RNA

Sugar is ribose

Bases of RNA

Adenine

Cytosine

Uracil - replaces thymine of DNA

Guanine

DNA and RNA Differences

Types of RNA

Ribosomal RNA

Precursor mRNA

Messenger RNA

Transfer RNA

Protein Synthesis

DNA - Triplet code

Three bases code for a AA

Transcription &Translation

Transcription

1. mRNA takes message from DNA to Ribosomes

2. DNA serves as a template

3. mRNA - codon (code words for a particular AA)

RNA Synthesis

Translation

4. Order of codons - brings about a particular order of AA

5. tRNA combines with AA

6. tRNA then brings them to the ribosomes

Translation

7. tRNA has anticodon - compliment of codon

8. Ribosome moves along mRNA

9. Growing polypeptide is always attached by means of only one tRNA to the strand of mRNA

Translation