Chapter 3The Cell

It modifies proteins coming from rough ER and packs them into Golgi vesicles, which are kept in the cell (e.g. lysosomes bud off from it) or sent out as secretions.

Their digestive enzymes break down food in food vacuoles, cellular wastes and damaged organelles, recycling materials; they can also store molecules for later use.

Smooth endoplasmic reticulum (SER). (In liver cells, peroxisomes also neutralise toxic substances.)

Lipid metabolism, transport of materials from one part of the cell to another, and detoxifying harmful chemicals; in muscle cells it helps in contraction.

Wastes, worn-out organelles and undigested molecules would pile up in the cell; food in food vacuoles would not be digested, and the cell would be damaged or die. (Lysosomal storage diseases in humans are due to this.)

Cristae are folds of the inner mitochondrial membrane that increase the surface area for respiration, so more reactions can occur and more ATP is made.

Chromatin is the fine thread-like material of DNA and proteins in the nucleus. When a cell starts dividing, its chromatin condenses and takes the shape of thick chromosomes. They are the same material in two forms.

Neurons: they transmit nerve impulses; dendrites receive and the axon carries impulses away from the cell body.

They are green cells specialized for photosynthesis, with many chloroplasts; their shape and arrangement allow maximum absorption of light.

An unspecialized cell that has the ability to make a variety of specialized cell types, e.g. the zygote; bone marrow stem cells make blood cells.

Vacuoles store waste products as well as water and nutrients; contractile vacuoles collect and pump out extra water and wastes.

The liquid part of the cytoplasm, including molecules and small particles like ribosomes, but not membrane-bound organelles.

It oversees cellular activities by directing the production of proteins; its DNA carries the genes that control all activities.

Prokaryotic cells are simple and do not have membrane-bound organelles; eukaryotic cells are more complex and have membrane-bound organelles and a nucleus.

Cilia are short and numerous; flagella are longer and fewer. Both (in eukaryotes) have a 9 + 2 arrangement of microtubules and help in movement.

Robert Hooke discovered cells in cork; Robert Brown observed the nucleus in 1831.

It provides shape, strength, protection and support to the protoplasm.

Middle lamella, primary wall and secondary wall.

Cellulose, hemicellulose and pectin; cellulose fibres crisscross to form a strong wall.

A membrane that allows some molecules to pass through it while it blocks many others.

Carbohydrates joined with proteins (glycoproteins) or with lipids (glycolipids) on the cell membrane.

Pores in plant cell walls connecting the cytoplasm of neighbouring cells, for communication and transport.

Medium for organelles; transport of materials; site of metabolic reactions like glycolysis; also stores food and wastes.

A double membrane around the nucleus; semi-permeable with many nuclear pores.

It contains genes which control all cell activities and is responsible for transmitting characteristics to the next generation.

It is made of DNA only and floats in the cytoplasm; there is no nuclear envelope.

Microtubules (tubulin), microfilaments (actin) and intermediate filaments (keratin, vimentin).

Ribosomes are not bounded by membranes, so prokaryotes, which lack membrane-bound organelles, still have them.

RER has ribosomes and makes proteins; SER lacks ribosomes and works in lipid metabolism, transport and detoxification.

Camillo Golgi (1898). Cisternae are the flattened sacs stacked to make the Golgi apparatus.

Christian René de Duve. Lysosomes bud off from the Golgi apparatus.

They produce energy (ATP) by aerobic respiration, breaking glucose with oxygen.

Thylakoids are sac-like structures carrying the pigments; a stack of thylakoids is a granum (plural grana), inside the chloroplast.

Their bright carotenoid pigments colour petals and fruits, helping in pollination and in the dispersal of fruits and seeds.

A barrel-shaped organelle of 9 triplets of microtubules. A pair at right angles makes the centrosome.

Centrioles located near the cell membrane of cells having cilia or flagella; they form cilia and flagella.

The outward pressure of the central vacuole on the cytoplasm and cell wall, which makes plant cells turgid and maintains their shape.

Rigid cellulose wall for support; chloroplasts for making food (also a large vacuole for turgor, plasmodesmata for transport).

No rigid wall, so they change shape (white blood cells); lysosomes for cleanup (also centrioles for spindles, flagella for movement).

The process after cell division in which cells get different sizes, structures and metabolic features and become specialized for a function.

Guard cells (leaf epidermis) regulate the opening and closing of stomata; root hair cells (root epidermis) increase surface area to absorb water and minerals.

Skeletal: long, striated, voluntary, attached to bones. Smooth: spindle-shaped, non-striated, involuntary, in walls of internal organs.

Dendrites are short extensions that receive impulses and carry them to the cell body; the axon is a long extension carrying impulses away from it.

The biconcave disc shape provides more surface area to absorb and release oxygen.

Storage of glycogen, iron and some vitamins; detoxification; making clotting proteins; recycling old red blood cells; secreting bile.

The specialization of different parts of a system to perform specific tasks more efficiently, within cells (organelles) and across cells (tissues).

In the skin (wound healing), liver (repair), bone marrow (blood and immune cells) and gut.

Mitochondria make most of the cell's ATP by aerobic respiration. Without them, only a little ATP comes from glycolysis in the cytoplasm.

Cells needing much energy (muscle, nerve, liver) are affected most. This shows division of labour: organelles depend on each other.

The nucleus holds the DNA (genes) that carry the instructions for every protein; ribosomes (assembled in the nucleolus) read these instructions to make proteins.

It is important because proteins do almost every job in the cell; the nucleus controls cell activities by directing the production of proteins, and ribosomes are where it happens.

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.