Chapter 3: The CellLong Questions with Answers

Cell membrane is composed of proteins and lipids and small quantities of carbohydrates. Its structure is described as the fluid-mosaic model.

Lipid bilayer (fluid)

The lipids make a fluid-like bilayer. The lipid molecules can move sideways, so the membrane is flexible, like a fluid.

Proteins (mosaic)

Protein molecules are submerged in the bilayer; some pass right through it. The lipids and proteins can move laterally, so the pattern or "mosaic" is not constant.

Carbohydrates and cholesterol

Carbohydrates are joined with proteins (glycoproteins) or lipids (glycolipids) on the outer side; cholesterol in the bilayer adds stability.

Function

The membrane is selectively permeable: it lets some molecules through and blocks others.

The cells of bacteria, fungi, plants and algae have a rigid non-living wall around the cell membrane. It provides shape, strength, protection and support to the protoplasm.

Middle lamella

Holds together the primary walls of adjacent cells; contains magnesium, calcium and pectin.

Primary wall

Just above the cell membrane; mainly cellulose, hemicellulose and pectin; cellulose fibres crisscross to form a strong wall.

Secondary wall

Made by some cells, e.g. xylem, on the inner side of the primary wall; mainly cellulose, lignin and other chemicals.

In other organisms

Algae: cellulose; prokaryotes: peptidoglycan; fungi: chitin. Plasmodesmata (pores) connect neighbouring plant cells.

Nuclear envelope

A double membrane around the nucleus; semi-permeable with many small nuclear pores, which control traffic between nucleus and cytoplasm.

Nucleoplasm

The inner jelly-like material of the nucleus.

Nucleolus

One or more small bodies in the nucleoplasm where ribosomes are assembled.

Chromatin and chromosomes

Fine thread-like chromatin of DNA and proteins condenses into thick chromosomes when the cell divides. DNA contains genes which control all the activities of the cell and transmit characteristics to the next generation (hereditary material).

Position: centre in animal cells, pushed to a side in plant cells. Prokaryotes lack a nucleus.

Lysosome

Small membrane-bound vesicles containing digestive enzymes, mostly in animal cells; discovered by de Duve; they bud off from the Golgi apparatus. They fuse with food vacuoles and digest food, break cellular wastes, engulf and digest damaged organelles, and store some molecules.

Endoplasmic reticulum

A network of membrane-bounded channels throughout the cytoplasm of eukaryotic cells.

Discovery

In 1898, the Italian physician Camillo Golgi discovered a set of flattened sacs in the cytoplasm.

Structure

Flattened sacs called cisternae are stacked over each other to make the Golgi apparatus; found in both plant and animal cells. Vesicles arrive at one face (from RER) and leave from the other.

Function

It modifies proteins coming from rough ER and packs them into Golgi vesicles; the vesicles are kept in the cell or sent to the exterior as secretions. Lysosomes are formed from it.

Structure

Function

Chloroplasts carry out photosynthesis: with their pigments they capture light energy and convert it into chemical energy, which they use to prepare glucose. They are present in the green parts of plants and in algae.

Most mature plant cells have a single, large central vacuole formed by the fusion of many small vacuoles. Its membrane is the tonoplast and it contains cell sap, a watery solution of salts.

Turgor pressure maintains the shape of cells and keeps soft plant parts upright; when water is lost, cells become flaccid and the plant wilts.

Within cells

Each organelle carries out a distinct function: mitochondria generate energy, ER synthesizes proteins and lipids, lysosomes break down wastes; together they keep the cell alive.

Across cells

In multicellular organisms each type of cell performs a specific role:

This intercellular specialization allows organisms to perform a wide range of functions, though cells then depend on one another.

Cells formed by cell division are all similar. Afterwards they undergo specialization or differentiation, getting different sizes, structures and metabolic features suited to a function.

All of them come from unspecialized stem cells (first the zygote).

Mitochondria

Double membrane; smooth outer membrane; inner membrane folded into cristae (more surface for respiration); matrix inside; own DNA and ribosomes. They perform aerobic respiration and release energy as ATP: the "powerhouse" of the cell.

Plastids

Cytoskeleton

Microtubules (tubulin): shape, tracks, spindle, cilia and flagella. Microfilaments (actin): movement, muscle contraction. Intermediate filaments (keratin, vimentin): anchor organelles, cell junctions.

Centrioles

Barrel-shaped, 9 triplets of microtubules, in animals and most protists; a pair at right angles = centrosome; form spindle fibres at division; as basal bodies they form cilia and flagella.

Cilia and flagella

Cilia short and many, flagella long and few; 9 + 2 microtubules in eukaryotes; prokaryotic flagella of flagellin. Function: movement.

A stem cell is an unspecialized cell that has the ability to make a variety of specialized cell types. All cells of a multicellular organism arise from a single stem cell, the zygote, which divides into cell lines that differentiate into skin, muscle, nerve, blood cells, etc.

Adult stem cells

They regenerate damaged tissues under the right conditions, which makes them important in medicine.