Unit 2 Cell Structure and Function

Biology - AP

Unit 2: Cell Structure and Function

Living things

Cell is life’s basic unit of structure and function

As cells increase in volume, the surface area-to-volume ratio decreases, and the exchange of materials becomes less efficient. The surface area and volumes of cells can be calculated using typical geometry formulas.

The surface area-to-volume ratio concept can also be applied to organisms. As organisms increase in size, their ratio will decrease and this can affect properties of the organism, such as heat-exchange with their surroundings. Small organisms lose heat at much higher rates than larger organisms as a result of their efficient exchange of heat.

Types of cells and organelles

Light microscopes are used to study stained or living cells. They can magnify the size of an organism up to 1,000 times.

Electron microscopes are used to study detailed structures of a cell that cannot be easily seen or observed by light microscopy.

There are two distinct types of cells: prokaryotic cells and eukaryotic cells.

-Prokaryotic cell-

It is a lot smaller than a eukaryotic cell and simpler. Bacteria and archaea are examples of prokaryotes.

The inside of the cell is filled with a substance called cytoplasm.

The genetic material in a prokaryote is one continuous, circular DNA molecule that is found free in the cell in the nucleoid.

Most prokaryotes have a cell wall composed of peptidoglycans that surrounds a lipid layer called the plasma membrane.

Prokaryotes also have small ribosomes.

Some bacteria may also have one or more flagella, which are used for motility and they might have a thick capsule outside their cell wall for extra protection.

Prokaryotes do not have any membrane-bound organelles. Their only membrane is the plasma membrane

-Eukaryotic cell-

Eukaryotic cells are more complex. Fungi, protists, plants, and animals are examples of eukaryotes.

Eukaryotic cells have many smaller structures called organelles. Some of these organelles are the same structures seen in prokaryotic cells, but many are uniquely eukaryotic.

-Plasma Membrane-

It is the outer envelope of the cell, made up of mostly phospholipids and proteins.

The plasma membrane is important because it regulates the movement of substances into and out of the cell. The membrane is semipermeable.

Many proteins are associated with the cell membrane. Some of these proteins are loosely associated with the lipid bilayer (peripheral proteins). They are located on the inner or outer surface of the membrane.

Others are firmly bound to the plasma membrane (integral proteins). These proteins are amphipathic.

Some integral proteins extend all the way through the membrane (transmembrane proteins).

This arrangement of phospholipids and proteins is known as the fluid- mosaic model.

Adhesion proteins form junctions between adjacent cells.

Receptor proteins such as hormones, serve as docking sites for arrivals at the cell.

Transport proteins form pumps that use ATP to actively transport solutes across the membrane.

Channel proteins form channels that selectively allow the passage of certain ions or molecules.

Cell surface markers such as glycoproteins, and some lipids, such as glycolipids, are exposed on the extracellular surface and play a role in cell recognition and adhesion. .

Carbohydrate side chains are found only on the outer surface of the plasma membrane

-The Nucleus-

The nucleus is usually the largest organelle in the cell. The nucleus not only directs what goes on in the cell, but is also responsible for the cell’s ability to reproduce. It’s the home of the hereditary information—DNA—which is organized into large structures called chromosomes. The most visible structure within the nucleus is the nucleolus, which is where rRNA is made and ribosomes are assembled.

-Ribosomes-

The ribosomes are sites of protein synthesis. Their job is to manufacture all the proteins required by the cell or secreted by the cell. Ribosomes are round structures composed of two subunits, the large subunit and the small subunit. The structure is composed of ribosomal RNA (rRNA) and proteins. Ribosomes can be either free floating in the cell or attached to another structure called the endoplasmic reticulum (ER)

-Endoplasmic Reticulum (ER)-

The ER is a continuous channel that extends into many regions of the cytoplasm and provides mechanical support and transportation. The rough ER compartmentalises the cell.

The region of the ER that lacks ribosomes is called the smooth ER. The smooth ER makes lipids, hormones, and steroids and breaks down toxic chemicals.

-Golgi Complex-

After the ribosomes on the rough ER have completed synthesizing proteins, the Golgi complex modify, process, and sort the products.

They’re the packaging and distribution centers for materials destined to be sent out of the cell. They package the final products in little sacs called vesicles, which carry products to the plasma membrane.

-Mitochondria-

They’re power stations responsible for converting energy from organic molecules into useful energy for the cell. The most common energy molecule in the cell is adenosine triphosphate (ATP).

It consists of an inner portion and an outer portion. The inner mitochondrial membrane forms folds known as cristae and separates the innermost area (the matrix) from the inter-membrane space. The outer membrane separates the inter-membrane space from the cytoplasm.

-Lysosomes-

They have sacs that carry digestive enzymes, which they use to break down old, worn-out organelles, debris, or large ingested particles.

Lysosomes are made when vesicles containing specific enzymes from the trans Golgi fuse with vesicles made during endocytosis. Lysosomes are also essential during programmed cell death called apoptosis.

-Vacuoles-

They are fluid-filled sacs that store water, food, wastes, salts, or pigments. Vacuoles serve multiple functions in plant cells.

-Peroxisomes-

Peroxisomes are organelles that detoxify various substances, producing hydrogen peroxide (H2O2) as a byproduct. They have enzymes that break down hydrogen peroxide into oxygen and water.

-Cytoskeleton-

The shape of a cell is determined by a network of protein fibers called the cytoskeleton. The most important fibers are microtubules and microfilaments.

Microtubules are made up of the protein tubulin, participate in cellular division and movement.

Microfilaments are important for movement. These thin, rodlike structures are composed of the protein actin. Actin monomers are joined together and broken apart as needed to allow microfilaments to grow and shrink.

-Cilia and Flagella-

Cilia and flagella have locomotive properties in single-celled organisms. The beating motion of cilia and flagella structure allows it to move.

-Plant Cells Versus Animal Cells-

Plant cells, unlike animal cells, have a cell wall (made of cellulose). A cell wall is a rigid layer just outside the plasma membrane that provides support for the cell.

Plant cells possess chloroplasts, which have a double outer membrane. Chloroplasts contain chlorophyll, which gives plants their characteristic green color.

Cytoplasm within a plant cell is usually taken up by a large vacuole which is the central vacuole.

Plant cells also differ from animal cells in that plant cells do not contain centrioles.

Transport: traffic across membranes

The ability of molecules to move across the cell membrane depends on:

the semipermeability of the plasma membrane

the size and charge of particles that want to get through

Small substances cross the membrane without any resistances since “like dissolves like.” The lipid bilayer has hydrophilic outside and hydrophobic on the inside so only hydrophobic things can pass that central zone. If a substance is hydrophilic, the bilayer won’t let it pass without assistance, called facilitated transport

Aquaporins are water-specific channels. Glucose and ions such as Na+ and K+ are also transported across the plasma membrane via membrane proteins. Membranes may become polarised as these ions move across them.

-Passive Transport: Simple and Facilitated Diffusion-

If there is a high concentration of something in one area, it will move to spread out and diffuse into an area with a lower concentration. The substance moves down a concentration gradient. This is called diffusion.

When the molecule that is diffusing is hydrophobic, the diffusion is called simple diffusion because the small non-polar molecule can just drift right through the membrane without trouble.

When the diffusion requires the help of a channel-type protein, it is called facilitated diffusion.

Anytime that a substance is moving by diffusion, it is called passive transport because there is no outside energy required to power the movement.

-Osmosis-

The only difference is that in diffusion the membrane is usually permeable to solute, and in osmosis it is not.

In plants, the cell wall is important to protect it against osmotic changes, while the cell membrane can shrink away from the wall (a process called plasmolysis) if it loses water and can expand and squeeze tightly against the cell wall if it takes in water.

Tonicity is used to describe osmotic gradients.

If an environment is isotonic to the cell, the solute concentration is the same inside and outside.

A hypertonic solution has more total dissolved solutes than the cell, while a hypotonic solution has less.

Water potential (Ψ) is the measure of potential energy in water and describes the eagerness of water to flow from an area of high water potential to an area of low water potential.

It is affected by: pressure potential (Ψp) and solute potential (Ψs)

Solute Potential of a Solution Ψs = −iCRT where: i = ionization constant C = molar concentration R = pressure constant T = temperature in Kelvin (°C + 273)

Adding a solute lowers the water potential of a solution, causing water to be less likely to leave this solution and more likely to flow into it. The more solute molecules present, the more negative the solute potential is.

-Active Transport-

Movement against the natural flow is called active transport.

Some proteins in the plasma membrane are powered by ATP.

An example of active transport is a special protein called the sodium-potassium pump. It ushers out three sodium ions (Na+) and brings in two potassium ions (K+) across the cell membrane. This pump depends on ATP to get ions across that would otherwise remain in regions of higher concentration.

Primary active transport occurs when ATP is directly utilised to transport something.

Secondary active transport occurs when something is actively transported using the energy captured from the movement of another substance flowing down its concentration gradient.

-Endocytosis-

When the particles that want to enter a cell are just too large, the cell uses a portion of the cell membrane to engulf the substance. The cell membrane forms a pocket, pinches in, and eventually forms either a vacuole or a vesicle. This process is called endocytosis.

Three types of endocytosis : pinocytosis, phagocytosis, and receptor- mediated endocytosis.

Pinocytosis: the cell ingests liquids.

Phagocytosis: the cell takes in solids.

Receptor-mediated endocytosis: involves cell surface receptors that work in tandem with endocytic pits that are lined with a protein called clathrin. When a particle, or ligand, binds to one of these receptors, the ligand is brought into the cell by the invagination, or “folding in” of the cell membrane. A vesicle then forms around the incoming ligand and carries it into the cell’s interior.

-Bulk Flow-

Bulk flow is the one-way movement of fluids brought about by pressure.

Example: movement of blood through a blood vessel and the movement of fluids in xylem and phloem of plants are examples of bulk flow.