PP1201 Cell Structure and Function

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Cell

The basic structural and functional unit of any living thing.

Three major regions of a cell

1. Plasma Membrane, 2. Nucleus, 3. Cytoplasm (Including organelles)

Function of the plasma membrane

The Plasma Membrane separates the extracellular fluid from the cytoplasm, separates organelles from the cytoplasm, regulates transport across membranes, and creates communication between cells.

Chemical composition of the plasma membrane

The Plasma Membrane is composed of a phospholipid bilayer, dotted with proteins.

Function of membrane proteins

The membrane proteins create intercellular junctions, regulate transport across cells, act as receptors, enzymes, and channels.

Integral Proteins

Proteins that are firmly inserted into the lipid bilayer.

Peripheral Proteins

Membrane proteins that are loosely attached to an integral protein and can be removed without disrupting the bilayer.

Function of membrane lipids

The lipids in the phospholipid bilayer are hydrophobic, and when attached to hydrophilic heads naturally attract to each other to form the plasma membrane, which keeps water and other molecules from crossing in and out of the cell.

Function of membrane carbohydrates

The carbohydrate glycocalyx, which sits on the bilayer's outer surface, acts as a biological marking system that identifies the cell.

Selective Permeability

The process in which the cell decides which molecules enter the cell and which molecules enter the cell. Selective permeability is largely determined by proteins.

Simple Diffusion

The process in which nonpolar and lipid-soluable substances diffuse directly through the lipid bilayer.

Facilitated Diffusion

The passive transport of certain molecules, such as glucose, across the cell membranes, using either protein carriers or protein channels.

Passive Transport

The transport of substances across the cell membrane without energy input from the cell.

Active Transport

The transport of substances across the cell membrane using metabolic energy (ATP) from the cell.

Primary Active Transport

Active transport where hydrolysis of ATP phosphorylyzes the transport protein, causing it to change its shape and 'pump' the substance through the membrane.

Secondary Active Transport

The process in which energy is stored from a primary active transport 'pump', and is used to simultaneously 'carry along' other substances through the membrane.

Endocytosis

The process by which a cell absorbs a substance by engulfing them inside a vesicle.

Types of Endocytosis

Phagocytosis: Engulfing a relatively large piece of organic matter, such as a bacterium.
Pinocytosis: Engulfing an amount of non-specific small particles.
Receptor-Mediated Endocytosis: The process in which specific molecules bind to certain receptor proteins, which are then brought inside the cell.

Exocytosis

The process by which a cell expels or releases substances.

Cytosol

The viscous fluid in which cytoplasmic elements are suspended. Composed of mostly water, as well as proteins, salts, sugars, and other solutes.

Mitochondria

Continually shape-changing organelles that provide most of the ATP for a cell. Enclosed by two membranes - a smooth outer membrane, and an inner one folded into cristae. Food fuels are broken down into water and carbon dioxide by enzymes that reside on the surface of the cristae, or inside the matrix (The fluid inside the cristae). The energy released by breaking down these molecules is used to convert ADP into ATP. This is referred to as 'aerobic cellular respiration'.

Ribosomes

Granules formed of proteins and RNAs that play a role in protein synthesis. Can float freely throughout the cytoplasm but is often attached to the rough endoplasmic reticulum.

Rough Endoplasmic Reticulum

A complex, formed of a membrane folded into cisternae, in which ribosomes are embedded. All proteins that a cell produces are manufactured by the ribosomes embedded in the rough ER.

Smooth Endoplasmic Reticulum

Although connected with the rough ER, plays no role in protein synthesis. Contains enzymes that play a vital role in many various bodily processes.

Golgi Apparatus

An organelle that modifies, concentrates, and packages proteins that are made in the rough ER. Protein-filled vesicles formed from the rough ER merge with the apparatus.

Lyosomes

Organelles containing acidic digestive enzymes, used to dispose of harmful bacteria and cell debris.
Membrane utilizes proton pumps in order to maintain an acidic solution. Releases neutralized material back into the cell while retaining the harmful hydrolases.

If membrane is broken, cell digests itself (autolysis).

Peroxisomes

Membranous sacs containing oxidases and catalases. Are used to neutralize free radicals by converting them to hydrogen peroxide, which is then converted to water by catalase enzymes. Particularly present in liver and kidney cells.

Also play a significant role in energy metabolism.

Cytoskeleton

An elaborate network of rods running through the cytosol, which enables the cell to move and form in certain ways.

Components of the cytoskeleton

Microfilaments, intermediate filaments, microtubules. None are covered by membranes.

Microtubules

Hollow tubes made out of protein subunits called tubulins. Mostly radiate from the centrosome. Used to determine the overall shape of the cell and the distribution of organelles. Propels mitochondria, lyosomes, and vesicles using motor proteins and motor molecules.

Microfilaments

Strands of the protein 'actin' that form a relatively dense network called the terminal web, which attaches to the inside of the plasma membrane. Strengthens the cell surface and resists compression. Also involved in cell movement and changes in shape, by contracting (pulled by 'unconventional myosin', or propelling crawling movements.
Constantly breaking down and reforming, except in muscle cells.

Intermediate Filaments

Tough fibers that attach to desmosomes and resist pulling forces on the cell. Structure varies in different cells, and thus are given different names depending on the cell in which they reside.

Nuclear Envelope

A double-membrane barrier that forms the outer layer of the nucleus. The inner layer is lined by nuclear lamina, a network of filaments that maintain the shape and act as a scaffold to organize DNA.
The two membrane layers connect at various points to form the edges of nuclear pores, which regulate the entry and exit of molecules and large particles.

Nucleoli

Spherical bodies inside the nucleus that assist in the creation of ribosomal subunits and ribosomal RNA.

Chromatin

A system of threads that exists throughout the nucleoplasm, consisting of DNA, histone proteins, and RNA chains. Made out of nucleosomes connected by a strand of DNA.

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